:::::::::::::: 1993-001 :::::::::::::: Title: Performance Evaluation of Two-Shadow Speculative Concurrency Control Author: A. Bestavros, S. Braoudakis, E. Panagos, Boston University Date: February 1993 Abstract: Speculative Concurrency Control (SCC) is a new concurrency control approach especially suited for real-time database applications. It relies on the use of redundancy to ensure that serializable schedules are discovered and adopted as early as possible, thus increasing the likelihood of the timely commitment of transactions with strict timing constraints. In a recent publication by two of the authors, SCC-nS, a generic algorithm that characterizes a family of SCC-based algorithms was described, and its correctness established by showing that it only admits serializable histories. In this paper, we evaluate the performance of the Two-Shadow SCC algorithm (SCC-2S), a member of the SCC-nS family, which is notable for its minimal use of redundancy. In particular, we show that SCC-2S (as a representative of SCC-based algorithms) provides significant performance gains over the widely used Optimistic Concurrency Control with Broadcast Commit (OCC-BC), under a variety of operating conditions and workloads. :::::::::::::: 1993-002 :::::::::::::: Author: Azer Bestavros, Boston University Title: Speculative Concurrency Control for Real-Time Databases Date: January 1993 Abstract: In this paper, we propose a new class of Concurrency Control Algorithms that is especially suited for real-time database applications. Our approach relies on the use of (potentially) redundant computations to ensure that serializable schedules are found and executed as early as possible, thus, increasing the chances of a timely commitment of transactions with strict timing constraints. Due to its nature, we term our concurrency control algorithms Speculative. The aforementioned description encompasses many algorithms that we call collectively Speculative Concurrency Control (SCC) algorithms. SCC algorithms combine the advantages of both Pessimistic and Optimistic Concurrency Control (PCC and OCC) algorithms, while avoiding their disadvantages. On the one hand, SCC resembles PCC in that conflicts are detected as early as possible, thus making alternative schedules available in a timely fashion in case they are needed. On the other hand, SCC resembles OCC in that it allows conflicting transactions to proceed concurrently, thus avoiding unecessary delays that may jeopardize their timely commitment. :::::::::::::: 1993-003 :::::::::::::: Title: Quadsim Student Manual Author: Marwan Shaban, Boston University Date: April 1993 Abstract: Quadsim is an intermediate code simulator. It allows you to "run" programs that your compiler generates in intermediate code format. Its user interface is similar to most debuggers in that you can step through your program, instruction by instruction, set breakpoints, examine variable values, and so on. The intermediate code format used by Quadsim is that described in [Aho 86]. If your compiler generates intermediate code in this format, you will be able to take intermediate-code files generated by your compiler, load them into the simulator, and watch them "run." You are provided with functions that hide the internal representation of intermediate code. You can use these functions within your compiler to generate intermediate code files that can be read by the simulator. Quadsim was inspired and greatly influenced by [Aho 86]. The material in chapter 8 (Intermediate Code Generation) of [Aho 86] should be considered background material for users of Quadsim. :::::::::::::: 1993-004 :::::::::::::: Title: Proceedings of Sixth International Workshop on Unification Author: Wayne Snyder, Boston University Date: April 1993 Abstract: The Proceedings of the Sixth International Workshop on Unification contains short papers presented at the workshop which took place at the Dagstuhl conference center in Germany, in June 1992. :::::::::::::: 1993-005 :::::::::::::: Title: Mermera: Non-coherent Distributed Shared Memory for Parallel Computing Name: Himanshu Shekhar Sinha, Boston University Date: May 1993 Abstract: MERMERA: NON-COHERENT DISTRIBUTED SHARED MEMORY FOR PARALLEL COMPUTING} Ph.D. Thesis Himanshu Shekhar Sinha Computer Science Department Boston University The proliferation of inexpensive workstations and networks has prompted several researchers to use such distributed systems for parallel computing. Attempts have been made to offer a shared-memory programming model on such distributed memory computers. Most systems provide a shared-memory that is {\em coherent} in that all processes that use it agree on the order of all memory events. This dissertation explores the possibility of a significant improvement in the performance of some applications when they use {\em non-coherent} memory. First, a new formal model to describe existing non-coherent memories is developed. I use this model to prove that certain problems can be solved using asynchronous iterative algorithms on shared-memory in which the coherence constraints are substantially relaxed. In the course of the development of the model I discovered a new type of non-coherent behavior called {\em Local Consistency}. Second, a programming model, {\sc Mermera}, is proposed. It provides programmers with a choice of hierarchically related non-coherent behaviors along with one coherent behavior. Thus, one can trade-off the ease of programming with coherent memory for improved performance with non-coherent memory. As an example, I present a program to solve a linear system of equations using an asynchronous iterative algorithm. This program uses all the behaviors offered by {\sc Mermera}. Third, I describe the implementation of {\sc Mermera} on a BBN Butterfly TC2000 and on a network of workstations. The performance of a version of the equation solving program that uses all the behaviors of {\sc Mermera} is compared with that of a version that uses coherent behavior only. For a system of 1000 equations the former exhibits at least a 5-fold improvement in convergence time over the latter. The version using coherent behavior only does not benefit from employing more than one workstation to solve the problem while the program using non-coherent behavior continues to achieve improved performance as the number of workstations is increased from 1 to 6. This measurement corroborates our belief that non-coherent shared memory can be a performance boon for some applications. :::::::::::::: 1993-006 :::::::::::::: Title: An Implementation of Mermera: A Shared Memory System that Mixes Coherence with Non-coherence Author: Abdelsalam Heddaya and Himanshu Sinha Date: June 1993 Abstract: Coherent shared memory is a convenient, but inefficient, method of inter-process communication for parallel programs. By contrast, message passing can be less convenient, but more efficient. To get the benefits of both models, several non-coherent memory behaviors have recently been proposed in the literature. We present an implementation of Mermera, a shared memory system that supports both coherent and non-coherent behaviors in a manner that enables programmers to mix multiple behaviors in the same program~\cite{HeddayaS93}. A programmer can debug a Mermera program using coherent memory, and then improve its performance by selectively reducing the level of coherence in the parts that are critical to performance. Mermera permits a trade-off of coherence for performance. We analyze this trade-off through measurements of our implementation, and by an example that illustrates the style of programming needed to exploit non-coherence. We find that, even on a small network of workstations, the performance advantage of non-coherence is compelling. Raw non-coherent memory operations perform 20-40~times better than non-coherent memory operations. An example aplication program is shown to run 5-11~times faster when permitted to exploit non-coherence. We conclude by commenting on our use of the Isis Toolkit of multicast protocols in implementing Mermera. Keywords: Distributed Shared Memory, Weak consistency, Parallel Computing, Asynchronous Iterative Methods, Isis. :::::::::::::: 1993-007 :::::::::::::: Title: Using Warp to Control Network Contention in Mermera Author: Abdelsalam Heddaya, Kihong Park, and Himanshu Sinha, Boston University Date: June 1993 Abstract: Parallel computing on a distributed system, such as a network of workstations, can saturate the communication network, leading to excessive message delays and consequently poor application performance. Current operating systems offer only partial support for flow control protocols that can help insulate application performance from extraneous traffic on the shared network. We examine empirically the consequences of integrating one such protocol, called Warp control~\cite{Park93}, into Mermera, a software shared memory system that supports parallel computing on distributed systems~\cite{HeddayaS93hicss}. Preliminary performance measurements are reported for an asynchronous iterative program to solve a system of linear equations, under varying levels of network contention. The experiments were conducted on a network of seven Sun Sparc~1+ workstations, using an auxiliary traffic generator. These measurements show that Warp succeeds in stabilizing the network behavior when there is high contention, increasing the effective throughput available to the application, and consequently decreasing its completion time. In some cases, however, Warp control does not achieve the performance attainable by fixed size buffering when using a statically optimal buffer size. Based on the nature of Warp and the underlying communication layers, we offer explanations for our results. Our use of Warp to regulate the allocation of network bandwidth emphasizes the possibility for integrating it with the allocation of other resources, such as CPU cycles and disk bandwidth, so as to optimize overall system throughtput, and enable fully-shared execution of parallel programs. Keywords: Distributed non-coherent shared memory, network contention, flow control, iterative methods, Isis. :::::::::::::: 1993-008 :::::::::::::: Title: Fixed Point vs. First-Order Logic on Finite Ordered Structures with Unary Relations Name: A. J. Kfoury and M. Wymann-Boeni, Boston University Date: August 1993 Abstract: We prove that first order logic is strictly weaker than fixed point logic over every infinite classes of finite ordered structures with additional unary relations: Over these classes there is always an inductive unary relation which cannot be defined by a first-order formula, even when every inductive sentence (i.e., closed formula) can be expressed in first-order over this particular class. Our proof first establishes a property valid for every unary relation definable by first-order logic over these classes which is peculiar to classes of ordered structures with unary relations. In a second step we show that this property itself can be expressed in fixed point logic and can be used to construct a non-elementary unary relation. :::::::::::::: 1993-009 :::::::::::::: Title: A Characterization of First-Order Definable Subsets on Classes of Finite Total Orders Author: A.J. Kfoury and M. Wymann-Boeni, Boston University Date: August 1993 Abstract: We give an explicit and easy-to-verify characterization for subsets in finite total orders (infinitely many of them in general) to be definable by the same first-order formula over any class of finite total orders. From this characterization we derive immediately that Beth's definability theorem does not hold in any class of finite total orders, as well as that McColm's first conjecture is true for all classes of finite total orders. Another consequence is a natural 0-1 law for definable subsets on finite total orders expressed as a statement about the possible densities of first-order definable subsets. :::::::::::::: 1993-010 :::::::::::::: Title: Learning Unions of Rectangles with Queries Author: Zhixiang Chen and Steve Homer, Boston University Date: September 1993 Abstract: We investigate the efficient learnability of unions of $k$ rectangles in the discrete plane $\{1,\ldots,n\}^{2}$ with equivalence and membership queries. We exhibit a learning algorithm that learns any union of $k$ rectangles with $O(k^{3}\log n)$ queries, while the time complexity of this algorithm is bounded by $O(k^{5}\log n)$. We design our learning algorithm by finding ``corners'' and ``edges'' for rectangles contained in the target concept and then constructing the target concept from those ``corners'' and ``edges''. Our result provides a first approach to on-line learning of nontrivial subclasses of unions of intersections of halfspaces with equivalence and membership queries. :::::::::::::: 1993-011 :::::::::::::: Title: Typability and Type Checking in the Second-Order Lambda-Calculus Are Equivalent and Undecidable Author: J. B. Wells,Boston University Date: September 1993 Abstract: We consider the problems of typability and type checking in the Girard/Reynolds second-order polymorphic typed lambda calculus, for which we use the short name ``System F'' and which we use in the ``Curry style'' where types are assigned to pure lambda terms. These problems have been considered and proven to be decidable or undecidable for various restrictions and extensions of System F and other related systems, and lower-bound complexity results for System F have been achieved, but they have remained ``embarrassing open problems'' for System F itself. We first prove that type checking in System F is undecidable by a reduction from semi-unification. We then prove typability in System F is undecidable by a reduction from type checking. Since the reverse reduction is already known, this implies the two problems are equivalent. The second reduction uses a novel method of constructing lambda terms such that in all type derivations, specific bound variables must always be assigned a specific type. Using this technique, we can require that specific subterms must be typable using a specific, fixed type assignment in order for the entire term to be typable at all. Any desired type assignment may be simulated. We develop this method, which we call ``constants for free'', for both the lambda-K and lambda-I calculi. :::::::::::::: 1993-012 :::::::::::::: Title: Building Responsive Systems from Physically-correct Specifications Author: Azer Bestavros,Boston University Date: October 1993 Abstract: Predictability -- the ability to foretell that an implementation will not violate a set of specified reliability and timeliness requirements -- is a crucial, highly desirable property of responsive embedded systems. This paper overviews a development methodology for responsive systems, which enhances predictability by eliminating potential hazards resulting from physically-unsound specifications. The backbone of our methodology is the Time-constrained Reactive Automaton (TRA) formalism, which adopts a fundamental notion of space and time that restricts expressiveness in a way that allows the specification of only reactive, spontaneous, and causal computation. Using the TRA model, unrealistic systems -- possessing properties such as clairvoyance, caprice, infinite capacity, or perfect timing -- cannot even be specified. We argue that this ``ounce of prevention'' at the specification level is likely to spare a lot of time and energy in the development cycle of responsive systems -- not to mention the elimination of potential hazards that would have gone, otherwise, unnoticed. The TRA model is presented to system developers through the Cleopatra programming language. Cleopatra features a C-like imperative syntax for the description of computation, which makes it easier to incorporate in applications already using C. It is event-driven, and thus appropriate for embedded process control applications. It is object-oriented and compositional, thus advocating modularity and reusability. Cleopatra is semantically sound; its objects can be transformed, mechanically and unambiguously, into formal TRA automata for verification purposes, which can be pursued using model-checking or theorem proving techniques. Since 1989, an ancestor of Cleopatra has been in use as a specification and simulation language for embedded time-critical robotic processes. :::::::::::::: 1993-013 :::::::::::::: Title: A Minimal GB Parser Author: Marwan Shaban, Boston University Date: October 1993 Abstract: We describe a GB parser implemented along the lines of those written by Fong [Fong91] and Dorr [Dorr87]. The phrase structure recovery component is an implementation of Tomita's generalized LR parsing algorithm (described in [Tomi86]), with recursive control flow (similar to Fong's implementation). The major principles implemented are government, binding, bounding, trace theory, case theory, theta-theory, and barriers. The particular version of GB theory we use is that described by Haegeman [Haeg91]. The parser is minimal in the sense that it implements the major principles needed in a GB parser, and has fairly good coverage of linguistically interesting portions of the English language. :::::::::::::: 1993-014 :::::::::::::: Title: Multi-version Speculative Concurrency Control with Delayed Commit Author: Azer Bestavros and Biao Wang, Boston University Date: October 1993 Abstract: This paper presents an algorithm which extends the relatively new notion of speculative concurrency control by delaying the commitment of transactions, thus allowing other conflicting transactions to continue execution and commit rather than restart. This algorithm propagates uncommitted data to other outstanding transactions thus allowing more speculative schedules to be considered. The algorithm is shown always to find a serializable schedule, and to avoid cascading aborts. Like speculative concurrency control, it considers strictly more schedules than traditional concurrency control algorithms. Further work is needed to determine which of these speculative methods performs better on actual transaction loads. :::::::::::::: 1993-015 :::::::::::::: Title: How good are genetic algorithms at finding large cliques: an experimental study Author: Bob Carter and Kihong Park, Boston University Date: November 1993 Abstract: This paper investigates the power of genetic algorithms at solving the MAX-CLIQUE problem. We measure the performance of a standard genetic algorithm on an elementary set of problem instances consisting of embedded cliques in random graphs. We indicate the need for improvement, and introduce a new genetic algorithm, the {\em multi-phase annealed GA}, which exhibits superior performance on the same problem set. As we scale up the problem size and test on ``hard'' benchmark instances, we notice a degraded performance in the algorithm caused by premature convergence to local minima. To alleviate this problem, a sequence of modifications are implemented ranging from changes in input representation to systematic local search. The most recent version, called {\em union GA}, incorporates the features of union cross-over, greedy replacement, and diversity enhancement. It shows a marked speed-up in the number of iterations required to find a given solution, as well as some improvement in the clique size found. We discuss issues related to the SIMD implementation of the genetic algorithms on a Thinking Machines CM-5, which was necessitated by the intrinsically high time complexity ($O(n^3)$) of the serial algorithm for computing one iteration. Our preliminary conclusions are: (1) a genetic algorithm needs to be heavily customized to work ``well'' for the clique problem; (2) a GA is computationally very expensive, and its use is only recommended if it is known to find larger cliques than other algorithms; (3) although our customization effort is bringing forth continued improvements, there is no clear evidence, at this time, that a GA will have better success in circumventing local minima. :::::::::::::: 1993-016 :::::::::::::: Title: An Algebraic Characterization of First-Order Definability Author: A.J. Kfoury and M. Wymann-Boeni Date: November 1993 Abstract: We give a variable-free relational calculus which defines exactly all first-order definable relations in a arbitrary structure. We then show that, over an arbitrary class $\C$ of finite ordered structures with signature $\{ \LE, R_1, \ldots, R_\alpha \}$, the unary relations uniformly defined by this calculus over $\C$ are characterized by a another simplified variable-free calculus which we call $\Q$. $\Q$ is the least set of formal expressions such that: \begin{eqnarray*} \Q &\supseteq&\ \{ \varnothing, R_1,\ldots, R_\alpha \}\ \cup\\ & &\ \{ (Q\PLUS x)\ |\ Q\in\Q, x\in\omega \cup \{\infty\} \}\ \cup \ \{ (Q\MINUS x)\ |\ Q\in\Q, x\in\omega \cup \{\infty\} \}\ \cup \\ & &\ \{ (\NOT Q)\ |\ Q\in\Q\}\ \cup \ \{ (Q_1\AND Q_2)\ |\ Q_1,Q_2\in\Q\}\ \cup \ \{ (Q_1\OR Q_2)\ |\ Q_1,Q_2\in\Q\}\ .\ \end{eqnarray*} where $\PLUS$ and $\MINUS$ are ``shift'' operators defined in Section 3. \end{abstract} :::::::::::::: 1993-017 :::::::::::::: Title: A Direct Algorithm for Type Inference in the Rank 2 Fragment of the Second-Order Lambda-Calculus Author: Joe Wells,Boston University Date: November 1993 Abstract: We study the problem of type inference for a family of polymorphic type disciplines containing the power of Core-ML. This family comprises all levels of the stratification of the second-order lambda-calculus by ``rank'' of types. We show that typability is an undecidable problem at every rank k >= 3 of this stratification. While it was already known that typability is decidable at rank <= 2, no direct and easy-to-implement algorithm was available. To design such an algorithm, we develop a new notion of reduction and show how to use it to reduce the problem of typability at rank 2 to the problem of acyclic semi-unification. A by-product of our analysis is the publication of a simple solution procedure for acyclic semi-unification. :::::::::::::: 1993-018 :::::::::::::: Title: A General Theory of Semi-Unification Author: Said Jahama and A. J. Kfoury Date: December 1993 Abstract: Various restrictions on the terms allowed for substitution give rise to different cases of semi-unification. Semi-unification on finite and regular terms has already been considered in the literature. We introduce a general case of semi-unification where substitutions are allowed on non-regular terms, and we prove the equivalence of this general case to a well-known undecidable data base dependency problem , thus establishing the undecidability of general semi-unification. We present a unified way of looking at the various problems of semi-unification. We give some properties that are common to all the cases of semi-unification. We also the principality property and the solution set for those problems. We prove that semi-unification on general terms has the principality property. Finally, we present a recursive inseparability result between semi-unification on regular terms and semi-unification on general terms. :::::::::::::: 1993-019 :::::::::::::: Title: Type Reconstruction in the Presence of Polymorphic Recursion and Recursive Types Author: Said Jahama, Boston University Date: December 1993 Abstract: We establish the equivalence of type reconstruction with polymorphic recursion and recursive types is equivalent to regular semi-unification which proves the undecidability of the corresponding type reconstruction problem. We also establish the equivalence of type reconstruction with polymorphic recursion and positive recursive types to a special case of regular semi-unification which we call positive regular semi-unification. The decidability of positive regular semi-unification is an open problem. :::::::::::::: 1993-020 :::::::::::::: Title: AIDA-based Distributed File System Authors: Azer Bestavros and Mohammad Makarechian Date: December 1993 Abstract: This paper describes a prototype implementation of a Distributed File System (DFS) based on the Adaptive Information Dispersal Algorithm (AIDA). Using AIDA, a file block is encoded and dispersed into smaller blocks stored on a number of DFS nodes distributed over a network. The implementation devises file creation, read, and write operations. In particular, when reading a file, the DFS accepts an optional timing constraint, which it uses to determine the level of redundancy needed for the read operation. The tighter the timing constraint, the more nodes in the DFS are queried for encoded blocks. Write operations update all blocks in all DFS nodes--with future implementations possibly including the use of read and write quorums. This work was conducted under the supervision of Professor Azer Bestavros (best@cs.bu.edu) in the Computer Science Department as part of Mohammad Makarechian's Master's project. :::::::::::::: 1994-001 :::::::::::::: Title: On the Performance of Polynomial-time CLIQUE Algorithms on Very Large Graphs Author: Steve Homer and Marcus Peinado, Boston University Date: January 1994 Abstract: The performance of a randomized version of the subgraph-exclusion algorithm (called Ramsey) for CLIQUE by Boppana and Halld\'{o}rsson is studied on very large graphs. We compare the performance of this algorithm with the performance of two common heuristic algorithms, the greedy heuristic and a version of simulated annealing. These algorithms are tested on graphs with up to 10,000 vertices on a workstation and graphs as large as 70,000 vertices on a Connection Machine. Our implementations establish the ability to run clique approximation algorithms on very large graphs. We test our implementations on a variety of different graphs. Our conclusions indicate that on randomly generated graphs minor changes to the distribution can cause dramatic changes in the performance of the heuristic algorithms. The Ramsey algorithm, while not as good as the others for the most common distributions, seems more robust and provides a more even overall performance. In general, and especially on deterministically generated graphs, a combination of simulated annealing with either the Ramsey algorithm or the greedy heuristic seems to perform best. This combined algorithm works particularly well on large Keller and Hamming graphs and has a competitive overall performance on the DIMACS benchmark graphs. :::::::::::::: 1994-002 :::::::::::::: Title: On Learning Counting Functions With Queries Author: Zhixiang Chen and Steven Homer Date: February 1994 Abstract: We investigate the problem of learning disjunctions of counting functions, which are general cases of parity and modulo functions, with equivalence and membership queries. We prove that, for any prime number $p$, the class of disjunctions of integer-weighted counting functions with modulus $p$ over the domain $Z^{n}_{q}$ (or $Z^{n}$) for any given integer $q \ge 2$ is polynomial time learnable using at most $n+1$ equivalence queries, where the hypotheses issued by the learner are disjunctions of at most $n$ counting functions with weights from $Z_{p}$. The result is obtained through learning linear systems over an arbitrary field. In general a counting function may have a composite modulus. We prove that, for any given integer $q \ge 2$, over the domain $Z_{2}^{n}$, the class of read-once disjunctions of Boolean-weighted counting functions with modulus $q$ is polynomial time learnable with only one equivalence query, and the class of disjunctions of $\log \log n$ Boolean-weighted counting functions with modulus $q$ is polynomial time learnable.tions, which are general cases Finally, we present an algorithm for learning graph-based counting functions.ies. :::::::::::::: 1994-003 :::::::::::::: Title: Mapping parallel iterative algorithms onto workstation networks Author: Abdelsalam Heddaya and Kihong Park, Boston University Date: February 1994 Abstract: For communication-intensive parallel applications, the maximum degree of concurrency achievable is limited by the communication throughput made available by the network. In previous work, we showed experimentally that the performance of certain parallel applications running on a workstation network can be enhanced significantly if a congestion control protocol is used to enhance network performance. In this paper, we characterize and analyze the communication requirements of a large class of supercomputing applications that fall under the category of fixed-point problems, amenable to solution by parallel iterative methods. This results in a set of interface and architectural features sufficient for the efficient implementation of the application over a large-scale distributed system. In particular, we propose a direct link between the application and network layer, supporting congestion control actions at both ends. This in turn enhances the system's responsiveness to network congestion, improving performance. Preliminary results of a prototype system are summarized showing the efficacy of our scheme to support large-scale parallel computations. We conclude with a description of a full implementation in progress. KEYWORDS: Parallel iterative methods, congestion control, communication architecture. :::::::::::::: 1994-004 :::::::::::::: Title: A Hybrid GLR Algorithm for Parsing with Epsilon Grammars Author: Marwan Shaban, Boston University Date: March 22, 1994 Abstract: We give a hybrid algorithm for parsing $\epsilon$-grammars based on Tomita's non-$\epsilon$-grammar parsing algorithm (\cite{tomi86}) and Nozohoor-Farshi's $\epsilon$-grammar recognition algorithm (\cite{fars91}). The hybrid parser handles the same set of grammars handled by Nozohoor-Farshi's recognizer. The algorithm's details and an example of its use are given. We also discuss the deployment of the hybrid algorithm within a GB parser, and the reason an $\epsilon$-grammar parser is needed in our GB parser. :::::::::::::: 1994-005 :::::::::::::: Title: Structure Sharing and Parallelization in a GB Parser Author: Marwan Shaban, Boston University Date: March 22, 1994 Abstract: By utilizing structure sharing among its parse trees, a GB parser can increase its efficiency dramatically. Using a GB parser which has as its phrase structure recovery component an implementation of Tomita's algorithm (as described in \cite{tomi86}), we investigate how a GB parser can preserve the structure sharing output by Tomita's algorithm. In this report, we discuss the implications of using Tomita's algorithm in GB parsing, and we give some details of the structure-sharing parser currently under construction. We also discuss a method of parallelizing a GB parser, and relate it to the existing literature on parallel GB parsing. Our approach to preserving sharing within a shared-packed forest is applicable not only to GB parsing, but anytime we want to preserve structure sharing in a parse forest in the presence of features. :::::::::::::: 1994-006 :::::::::::::: Title: Adding Polymorphic Abstraction to ML (Detailed Abstract) Author: A. J. Kfoury and J. B. Wells, Boston University Date: May 1994 Abstract: The ML programming language restricts type polymorphism to occur only in the ``let-in'' construct and requires every occurrence of a formal parameter of a function (a lambda abstraction) to have the same type. Milner in 1978 refers to this restriction (which was adopted to help ML achieve automatic type inference) as a serious limitation. We show that this restriction can be relaxed enough to allow universal polymorphic abstraction without losing automatic type inference. This extension is equivalent to the rank-2 fragment of system F. We precisely characterize the additional program phrases (lambda terms) that can be typed with this extension and we describe typing anomalies both before and after the extension. We discuss how macros may be used to gain some of the power of rank-3 types without losing automatic type inference. We also discuss user-interface problems in how to inform the programmer of the possible types a program phrase may have. :::::::::::::: 1994-007 :::::::::::::: Title: Timeliness via Speculation for Real-Time Databases Author: Azer Bestavros and Spyridon Braoudakis, Boston University Date: May 1994 Abstract: Various concurrency control algorithms differ in the time when conflicts are detected, and in the way they are resolved. In that respect, the Pessimistic and Optimistic Concurrency Control (PCC and OCC) alternatives represent two extremes. PCC locking protocols detect conflicts as soon as they occur and resolve them using {\em blocking}. OCC protocols detect conflicts at transaction commit time and resolve them using {\em rollbacks} (restarts). For real-time databases, blockages and rollbacks are hazards that increase the likelihood of transactions missing their deadlines. We propose a {\em Speculative} Concurrency Control (SCC) technique that minimizes the impact of blockages and rollbacks. SCC relies on the use of added system resources to {\em speculate} on potential serialization orders and to ensure that if such serialization orders materialize, the hazards of blockages and roll-backs are minimized. We present a number of SCC-based algorithms that differ in the level of speculation they introduce, and the amount of system resources (mainly memory) they require. We show the performance gains (in terms of number of satisfied timing constraints) to be expected when a representative SCC algorithm (SCC-2S) is adopted. :::::::::::::: 1994-008 :::::::::::::: Title: Towards Physically-Correct Specifications of Embedded Real-Time Systems Author: Azer Bestavros, Boston University Date: May 1994 Abstract: Predictability (the ability to foretell that an implementation will not violate a set of specified reliability and timeliness requirements) is a crucial, highly desirable property of responsive embedded systems. This paper overviews a development methodology for responsive systems, which enhances predictability by eliminating potential hazards resulting from physically-unsound specifications. The backbone of our methodology is a formalism that restricts expressiveness in a way that allows the specification of only reactive, spontaneous, and causal computation. Unrealistic systems (possessing properties such as clairvoyance, caprice, infinite capacity, or perfect timing) cannot even be specified. We argue that this ``ounce of prevention'' at the specification level is likely to spare a lot of time and energy in the development cycle of responsive systems -- not to mention the elimination of potential hazards that would have gone, otherwise, unnoticed. :::::::::::::: 1994-009 :::::::::::::: Title: A lower-bound result on the power of a genetic algorithm Author: Kihong Park, Computer Science Dept., Boston University Date: October 12, 1994 Abstract: This paper presents a lower-bound result on the computational power of a genetic algorithm in the context of combinatorial optimization. We describe a new genetic algorithm, the merged genetic algorithm, and prove that for the class of monotonic functions, the algorithm finds the optimal solution, and does so with an exponential convergence rate. The analysis pertains to the ideal behavior of the algorithm where the main task reduces to showing convergence of probability distributions over the search space of combinatorial structures to the optimal one. We take exponential convergence to be indicative of efficient solvability for the sample-bounded algorithm, although a sampling theory is needed to better relate the limit behavior to actual behavior. The paper concludes with a discussion of some immediate problems that lie ahead. :::::::::::::: 1994-010 :::::::::::::: Title: On the effectiveness of genetic search in combinatorial optimization Author: Bob Carter and Kihong Park, Computer Science Dept, Boston University Date: November 10, 1994 Abstract: In this paper, we study the efficacy of genetic algorithms in the context of combinatorial optimization. In particular, we isolate the effects of cross-over, treated as the central component of genetic search. We show that for problems of nontrivial size and difficulty, the contribution of cross-over search is marginal, both synergistically when run in conjunction with mutation and selection, or when run with selection alone, the reference point being the search procedure consisting of just mutation and selection. The latter can be viewed as another manifestation of the Metropolis process. Considering the high computational cost of maintaining a population to facilitate cross-over search, its marginal benefit renders genetic search inferior to its singleton-population counterpart, the Metropolis process, and by extension, simulated annealing. This is further compounded by the fact that many problems arising in practice may inherently require a large number of state transitions for a near-optimal solution to be found, making genetic search infeasible given the high cost of computing a single iteration in the enlarged state-space. :::::::::::::: 1994-011 :::::::::::::: Title: Concurrency Control Protocols for Real-Time Databases, Phd Thesis Author: Spyridon Braoudakis (Major Advisor: Azer Bestavros) Date: November 12, 1994 Abstract: Concurrency control methods developed for traditional database systems are not appropriate for real-time database systems (RTDBS), where, in addition to database consistency requirements, satisfying timing constraints is an integral part of the correctness criterion. Most real-time concurrency control protocols considered in the literature combine time-critical scheduling with traditional concurrency control methods to conform to transaction timing constraints. These methods rely on either transaction {\em blocking} or {\em restarts}, both of which are inappropriate for real-time concurrency control because of the {\em unpredictability} they introduce. Moreover, RTDBS performance objectives differ from those of conventional database systems in that maximizing the number of transactions that complete before their deadlines becomes the decisive performance objective, rather than merely maximizing concurrency (or throughput). Recently, Speculative Concurrency Control (SCC) was proposed as a categorically different approach to concurrency control for RTDBS. SCC relies on the use of {\em redundant} processes ({\em shadows}), which {\em speculate} on alternative schedules, once conflicts that threaten the consistency of the database are detected. SCC algorithms utilize added system resources to ensure that correct (serializable) executions are discovered and adopted as early as possible, thus increasing the likelihood of the timely commitment of transactions. This dissertation starts by reviewing the Order-Based SCC (SCC-OB) algorithm which associates almost as many shadows as there are serialization orders of transactions. After demonstrating SCC-OB's excessive use of redundancy, a host of novel SCC-based protocols is introduced. Conflict-Based SCC (SCC-CB) reduces the number of shadows that a running transaction needs to keep by maintaining one shadow per uncommitted conflicting transaction. It is shown that the quadratic number of shadows maintained by SCC-CB is optimal, covering {\em all} serialization orders produced by SCC-OB. SCC-CB's correctness is established by showing that it admits only serializable histories. Next, the trade-off between the number of shadows and timeliness is considered. A generic SCC algorithm (SCC-kS) that operates under a limited redundancy assumption is presented; it allows no more than a constant number $k$ of shadows to coexist on behalf of any uncommitted transaction. Next, a novel technique is proposed that incorporates additional information such as {\em deadline}, {\em priority} and {\em criticalness} within the SCC methodology. SCC with Deferred Commit (SCC-DC) utilizes this additional information to improve the timeliness through the controlled {\em deferment} of transaction commitments. A probabilistic Value Induced Shadow Allocation (VISA) policy is developed which aims at preserving the most {\em valuable} shadows for each system transaction. The thesis of this dissertation is that SCC-based algorithms offer a new dimension, {\em redundancy}, to improve the {\em timeliness} of RTDBS. SCC-based algorithms are efficient (quadratic number of shadows is optimal), scalable (redundancy can be traded-off for timeliness), and easily amendable (deadline and priority information can be incorporated). :::::::::::::: 1994-012 :::::::::::::: Title: OS Support for Portable Bulk Synchronous Parallel Programs Author: Abdelsalam Heddaya and Amr F. Fahmy (Harvard) Date: December 5, 1994 Abstract: For parallel programs to become portable, they must be executable with uniform efficiency on a variety of hardware platforms, which is not the case at present. In 1990, Valiant proposed Bulk-Synchronous Parallelism (BSP) as a model on which portable parallel programs can be built. We argue that shared-memory BSP is efficiently implementable on a wide variety of parallel hardware, and that BSP forms a useful basis for providing an even higher level programming interface based on Sequential Consistency (SC). A list of memory and thread management features needed to support BSP and SC parallel programs are given, under the assumption that the parallel computer is space-shared among multiple parallel task, rather than time-shared. Known techniques to realize efficiently the most important of these features are sketched. :::::::::::::: 1994-013 :::::::::::::: Title: An Algorithm for Inferring Quasi-Static Types Author: Alberto Oliart Date: November 1994 Abstract: This report presents an algorithm, and its implementation, for doing type inference in the context of Quasi-Static Typing (QST) ["Quasy-static Typing." Satish Thatte Proc. ACM Symp. om Principles of Programming Languages, 1988]. The package infers types a la ``QST'' for the simply typed lambda-calculus. :::::::::::::: 1994-014 :::::::::::::: Title: New Notions of Reduction and Non-Semantic Proofs of Beta-Strong Normalization in Typed Lambda-Calculi Author: A. J. Kfoury and J. B. Wells Date: December 19, 1994 Abstract: Two new notions of reduction for terms of the lambda-calculus are introduced and the question of whether a lambda-term is beta-strongly normalizing is reduced to the question of whether a lambda-term is merely normalizing under one of the new notions of reduction. This leads to a new way to prove beta-strong normalization for typed lambda-calculi. Instead of the usual semantic proof style based on Girard's ``candidats de r\'eductibilit\'e'', termination can be proved using a decreasing metric over a well-founded ordering in a style more common in the field of term rewriting. This new proof method is applied to the simply-typed lambda-calculus and the system of intersection types. :::::::::::::: 1994-015 :::::::::::::: Title: Search by Shape Examples: Modeling Nonrigid Deformation Author: S. Sclaroff and A. P. Pentland Date: October, 1994 Abstract: We describe our work on shape-based image database search using the technique of modal matching. Modal matching employs a deformable shape decomposition that allows users to select example objects and have the computer efficiently sort the set of objects based on the similarity of their shape. Shapes are compared in terms of the types of nonrigid deformations (differences) that relate them. The modal decomposition provides deformation ``control knobs'' for flexible matching and thus allows for selecting weighted subsets of shape parameters that are deemed significant for a particular category or context. We demonstrate the utility of this approach for shape comparison in 2-D image databases; however, the general formulation is applicable to signals of any dimensionality. :::::::::::::: 1994-016 :::::::::::::: Title: Physically-Based Combinations of Views: Representing Rigid and Nonrigid Motion Author: S. Sclaroff and A. P. Pentland Date: November, 1994 Abstract: Nonrigid motion can be described as morphing or blending between extremal shapes, e.g., heart motion can be described as transitioning between the systole and diastole states. Using physically-based modeling techniques, shape similarity can be measured in terms of forces and strain. This provides a physically-based coordinate system in which motion is characterized in terms of physical similarity to a set of extremal shapes. Having such a low-dimensional characterization of nonrigid motion allows for the recognition and the comparison of different types of nonrigid motion. :::::::::::::: 1995-001 :::::::::::::: Title: Proceedings of the Workshop on Versioning in Hypertext Systems Author: David Durand, Anja Haake (GMD-IPSI, Germany), David Hicks (Texas A&M), Fabio Vitali (CIRFID - University of Bologna) Date: February 7, 1995 Abstract: This report (ftp://cs-ftp.bu.edu/techreports/misc/95-001/Home.html) contains 9 papers presented at a workshop on version management and hypertext, as well as a summary introduction by the organizers. These papers address requirements, solutions, and research issues related to the management of hypertext databases. Version management is not only a key application requirement in some domains (like design journals and electronic manuals) but provides a way to preserve the integrity of links in a changing hyperbase. :::::::::::::: 1995-002 :::::::::::::: Title: Application-Level Document Caching in the Internet Author: Azer Bestavros, Robert L. Carter, Mark E. Crovella, Carlos R. Cunha, Abdelsalam Heddaya and Sulaiman A. Mirdad Date: February 15, 1995 Abstract: With the increasing demand for document transfer services such as the World Wide Web comes a need for better resource management to reduce the latency of documents in these systems. To address this need, we report on the potential for document caching at the application level in document transfer services. We collected traces of over 250 executions of Mosaic, reflecting actual user requests for WWW documents. Using those traces, we study the tradeoffs between caching at three levels in the system, and the potential for use of application-level information in the caching system. Our traces show that while a high hit rate in terms of URLs is achievable, a much lower hit rate is possible in terms of bytes, because most profitably-cached documents are small. We considered the performance of caching when applied at the level of individual user sessions, at the level of individual hosts, and at the level of a collection of hosts on a single LAN. We show that the performance gain achievable by caching at the session level (which is straightforward to implement) is nearly all of that achievable at the LAN level (where caching is more difficult to implement). However, when resource requirements are considered, LAN level caching becomes much more desirable, since it can achieve a given level of caching performance using a much smaller amount of cache space. Finally, we consider the use of organizational boundary information as an example of the potential for use of application-level information in caching. We show that while it is desirable to cache local documents at the LAN level, the opposite is true at the session level, where remote documents are more profitably cached. :::::::::::::: 1995-003 :::::::::::::: Title: Demand-based Document Dissemination for the World-Wide Web Author: Azer Bestavros Date: February 15, 1995 Abstract: We analyzed the logs of our departmental HTTP server {\tt http://cs-www.bu.edu} as well as the logs the more popular Rolling Stones HTTP server {\tt http://www.stones.com}. These servers have very different purposes; the former caters primarily to local clients, whereas the latter caters exclusively to remote clients all over the world. In both cases, our analysis showed that remote HTTP accesses were confined to a very small subset of documents. Using an analytical model of server popularity and file access profiles, we show that by disseminating the most popular documents on servers (proxies) closer to the clients, network traffic could be reduced considerably, while server loads are balanced. We argue that this process could be generalized so as to provide for an automated demand-based duplication of documents. In that respect, we sketch the DDD-WWW protocol to implement this Demand-based Document Dissemination on the WWW. We believe that such server-based information dissemination protocols will be more effective at reducing {\em both} network bandwidth and document retrieval times than client-based caching protocols \cite{bestavros:95c}. :::::::::::::: 1995-004 :::::::::::::: Title: Equational Axiomatization of Bicoercibility for Polymorphic Types Author: Jerzy Tiuryn, Institute of Informatics, Warsaw University Date: February 16, 1995 Abstract: Two polymorphic types \sigma and \tau are said to be bicoercible if there is a coercion from \sigma to \tau and conversely. We give a complete equational axiomatization of bicoercible types and prove that the relation of bicoercibility is decidable. :::::::::::::: 1995-005 :::::::::::::: Title: Speculative Concurrency Control with Deferred Commitment for Real-Time Databases Author: Azer Bestavros and Spyridon Braoudakis Date: February 20, 1995 Abstract: A problem with Speculative Concurrency Control algorithms and other common concurrency control schemes using forward validation is that committing a transaction as soon as it finishes validating, may result in a value loss to the system. Haritsa showed that by making a lower priority transaction wait after it is validated, the number of transactions meeting their deadlines is increased, which may result in a higher value-added to the system. SCC-based protocols can benefit from the introduction of such delays by giving optimistic shadows with high value-added to the system more time to execute and commit instead of being aborted in favor of other validating transactions, whose value-added to the system is lower. In this paper we present and evaluate an extension to SCC algorithms that allows for commit deferments. :::::::::::::: 1995-006 :::::::::::::: Title: Using Speculation to Reduce Server Load and Service Time on the WWW Author: Azer Bestavros Date: February 21, 1995 Abstract: Speculative service implies that a client's request for a document is serviced by sending, in addition to the document requested, a number of other documents that the server speculates will be requested by the client in the near future. This speculation is based on statistical information that the server maintains for each document it serves. The notion of speculative service is analogous to prefetching, which is used to improve cache performance in distributed/parallel shared memory systems, with the exception that servers (not clients) control when and what to prefetch. Using trace simulations based on the logs of our departmental HTTP server http://cs-www.bu.edu, we show that both server load and service time could be reduced considerably, if speculative service is used. This is above and beyond what is currently achievable using client-side caching and server-side dissemination. We identify a number of parameters that could be used to fine-tune the level of speculation performed by the server. :::::::::::::: 1995-007 :::::::::::::: Title: Addendum to ``New Notions of Reduction and Non-Semantic Proofs of Beta Strong Normalization in Typed Lambda Calculi'' Author: A. J. Kfoury and J. B. Wells Date: March 1995 Abstract: This is an addendum to our technical report BUCS TR-94-014 of December 19, 1994. It clarifies some statements, adds information on some related research, includes a comparison with research be de Groote, and fixes two minor mistakes in a proof. :::::::::::::: 1995-008 :::::::::::::: Title: Modal Matching for Correspondence and Recognition Author: S. Sclaroff and A. P. Pentland Date: March 1995 Abstract: Modal matching is a new method for establishing correspondences and computing canonical descriptions. The method is based on the idea of describing objects in terms of generalized symmetries, as defined by each object's eigenmodes. The resulting modal description is used for object recognition and categorization, where shape similarities are expressed as the amounts of modal deformation energy needed to align the two objects. In general, modes provide a global-to-local ordering of shape deformation and thus allow for selecting which types of deformations are used in object alignment and comparison. In contrast to previous techniques, which required correspondence to be computed with an initial or prototype shape, modal matching utilizes a new type of finite element formulation that allows for an object's eigenmodes to be computed directly from available image information. This improved formulation provides greater generality and accuracy, and is applicable to data of any dimensionality. Correspondence results with 2-D contour and point feature data are shown, and recognition experiments with 2-D images of hand tools and airplanes are described. :::::::::::::: 1995-009 :::::::::::::: Title: A New Version of Toom's Proof Author: Peter Gacs Date: March 27, 1995 Abstract: There are several proofs now for the stability of Toom's example of a two-dimensional stable cellular automaton and its application to fault-tolerant computation. Simon and Berman simplified and strengthened Toom's original proof: the present report is simplified exposition of their proof. :::::::::::::: 1995-010 :::::::::::::: Title: Characteristics of WWW Client-based Traces Author: Carlos Cunha, Azer Bestavros, and Mark Crovella Date: April 1, 1995 (modified July 18, 1995) Abstract: The explosion of WWW traffic necessitates an accurate picture of WWW use, and in particular requires a good understanding of client requests for WWW documents. To address this need, we have collected traces of actual executions of NCSA Mosaic, reflecting over half a million user requests for WWW documents. In this paper we present a descriptive statistical summary of the traces we collected, which identifies a number of trends and reference patterns in WWW use. In particular, we show that many characteristics of WWW use can be modelled using power-law distributions, including the distribution of document sizes, the popularity of documents as a function of size, the distribution of user requests for documents, and the number of references to documents as a function of their overall rank in popularity (Zipf's law). In addition, we show how the power-law distributions derived from our traces can be used to guide system designers interested in caching WWW documents. --- Our client-based traces are available via FTP from ftp://cs-ftp.bu.edu/techreports/1995-010-www-client-traces.tar.gz :::::::::::::: 1995-011 :::::::::::::: Title: A Prefetching Protocol Using Client Speculation for the WWW Author: Azer Bestavros and Carlos Cunha Date: April 28, 1995 Abstract: The growing traffic of WWW related services requires the development of efficient protocols for reducing traffic, balancing load, and improving service time. One way of achieving these effects is via caching or replication. Studies like [Bestavros et al, 1995] show that simple demand-driven caching is not enough, and that aggressive caching policies have to be adopted. One such policy is prefetching. In an earlier paper, the potential of speculation (server-initiated prefetching) in distributed information systems (such as the WWW) was investigated and shown to be effective in reducing service time and server load. This speculation was based on statistical information that the server maintains for each document it serves. In this paper we study the performance of a client-initiated prefetching protocol, whereby speculation is based on past user-specific access patterns. We propose a technique whereby the history of a user is analyzed to predict his/her future accesses. Our technique does not make a distinction between embbed links and traversal links. In particular, embedded links are treated as a special case of traversal links with the probability of traversal being 1. We show that performance gains are possible to obtain by identifying common access patterns. Our study was conducted using client-based traces obtained from our departmental labs over a period of 100 days [Cunha et al, 1995]. :::::::::::::: 1995-012 :::::::::::::: Title: Object-Oriented Animation on the World Wide Web Author: Patrick Cai and Azer Bestavros Date: May 8, 1995 Abstract: We propose that video/audio animation be considered as a first-class object on the World Wide Web. Animation is a very "bandwidth-efficient" alternative to using video streams, especially for presentations involving mathematical objects and interactions. We present an object-oriented model that supports drawing-based and frame-based animation. Based on that model, we describe an extension of the HyperText Markup Language to support these capabilities. BU-NCSA Mosanim, a modified version of the NCSA Mosaic for X(v2.5), was developed and is available for distribution via anonymous FTP to demonstrate the concepts and potentials of animation in presentations and interactive game playing over the web. :::::::::::::: 1995-013 :::::::::::::: Title: Simulation of Hardware Dynamic Scheduling on the DLX Architecture Author: Azer Bestavros and Yueh-Lin Liu Date: June 6, 1995 Abstract: We describe our extention of the existing DLX simulator (DLXsim), available from the University of California at Berkeley, which allows the simulation of two hardware dynamic scheduling techniques. There are two DLXsim-like interactive simulators developed as part of this project. DLXscore simulates the operation of a DLX architecture equipped with scoreboarding hardware. DLXscore provides the status of instructions, scoreboard tables, and statistics. DLXtomasulo simulates the operation of a DLX architecture equipped with a hardware implementation of Tomasulo's algorithm. DLXtomasulo provides the status of instructions, reservation stations, and statistics. Both programs allow the user to configure the number of functional units and the latency of floating point operations. :::::::::::::: 1995-014 :::::::::::::: Title: Dynamic Server Selection in the Internet Author: Mark E. Crovella and Robert L. Carter Date: June 30, 1995 Abstract: As distributed information services like the World Wide Web become increasingly popular on the Internet, problems of scale are clearly evident. A promising technique that addresses many of these problems is service (or document) replication. However, when a service is replicated, clients then need the additional ability to find a ``good'' provider of that service. In this paper we report on techniques for finding good service providers without a priori knowledge of server location or network topology. We consider the use of two principal metrics for measuring distance in the Internet: hops, and round-trip latency. We show that these two metrics yield very different results in practice. Surprisingly, we show data indicating that the number of hops between two hosts in the Internet is {\em not\/} strongly correlated to round-trip latency. Thus, the distance in hops between two hosts is not necessarily a good predictor of the expected latency of a document transfer. Instead of using known or measured distances in hops, we show that the extra cost at runtime incurred by dynamic latency measurement is well justified based on the resulting improved performance. In addition we show that selection based on dynamic latency measurement performs much better in practice that any static selection scheme. Finally, the difference between the distribution of hops and latencies is fundamental enough to suggest differences in algorithms for server replication. We show that conclusions drawn about service replication based on the distribution of hops need to be revised when the distribution of latencies is considered instead. :::::::::::::: 1995-015 :::::::::::::: Title: Explaining World Wide Web Traffic Self-Similarity Author: Mark E. Crovella and Azer Bestavros Date: August 29, 1995 Abstract: Recently the notion of self-similarity has been shown to apply to wide-area and local-area network traffic. In this paper we examine the mechanisms that give rise to self-similar network traffic. We present an explanation for traffic self-similarity by using a particular subset of wide area traffic: traffic due to the World Wide Web (WWW). Using an extensive set of traces of actual user executions of NCSA Mosaic, reflecting over half a million requests for WWW documents, we show evidence that WWW traffic is self-similar. Then we show that the self-similarity in such traffic can be explained based on the underlying distributions of WWW document sizes, the effects of caching and user preference in file transfer, the effect of user ``think time'', and the superimposition of many such transfers in a local area network. To do this we rely on empirically measured distributions both from our traces and from data independently collected at over thirty WWW sites. :::::::::::::: 1995-016 :::::::::::::: Title: World Wide Web Image Search Engines Author: Stan Sclaroff Date: 27 May 1995 Abstract: We propose the development of a world wide web image search engine that crawls the web collecting information about the images it finds, computes the appropriate image decompositions and indices, and stores this extracted information for searches based on image content. Indexing and searching images need not require solving the image understanding problem. Instead, the general approach should be to provide an arsenal of image decompositions and discriminants that can be precomputed for images. At search time, users can select a weighted subset of these decompositions to be used for computing image similarity measurements. While this approach avoids the search-time-dependent problem of labeling what is important in images, it still holds several important problems that require further research in the area of query by image content. We briefly explore some of these problems as they pertain to shape. (white paper presented at the NSF Workshop on Visual Information Management, MIT, June 1995) :::::::::::::: 1995-017 :::::::::::::: Title: Deformable Prototypes for Encoding Shape Categories in Image Databases Author: Stan Sclaroff Date: Sept 12, 1995 Abstract: We describe a method for shape-based image database search that uses deformable prototypes to represent categories. Rather than directly comparing a candidate shape with all shape entries in the database, shapes are compared in terms of the types of nonrigid deformations (differences) that relate them to a small subset of representative prototypes. To solve the shape correspondence and alignment problem, we employ the technique of {\em modal matching}, an information-preserving shape decomposition for matching, describing, and comparing shapes despite sensor variations and nonrigid deformations. In modal matching, shape is decomposed into an ordered basis of orthogonal principal components. We demonstrate the utility of this approach for shape comparison in 2-D image databases. :::::::::::::: 1995-018 :::::::::::::: Title: Deterministic Computations Whose Hisrtory is Independent of the Order of Updating Author: Peter Gacs Date: November 18, 1995 Abstract: Consider a network of processors (sites) in which each site has finitely many neighbors. Each site has some transition function computing its next state from the states of the neighbors. These transitions (updates) are applied in arbitrary order, one or many at a time. If the state of site x at time t is r(x,t) then let us define the sequence r'(x,0),r'(x,1),... by taking the sequence r(x,0),r(x,1),... and deleting each repetition, i.e. each element equal to the preceding one. The system of transition functions is said to support asynchrony if the sequence r'(x,i), (while it lasts, in case it is finite) depends only on the initial configuration, not on the order of updates. This paper gives a simple characterization of transition functions supporting asynchrony. The characterization says that it is equivalent to the following seemingly weaker commutativity condition: For any configuration, for any pair x,y of neighbors, if the updating would change both s(x) and s(y) then the result of updating first x and then y is be the same as the result of doing this in the reverse order. :::::::::::::: 1995-019 :::::::::::::: Title: The Undecidability of Mitchell's Subtyping Relationship Author: J. B. Wells Date: December 10, 1995 Abstract: Mitchell defined and axiomatized a subtyping relationship (also known as containment , coercibility , or subsumption over the types of System F (with "arrow" and "forall"). This subtyping relationship is quite simple and does not involve bounded quantification. Tiuryn and Urzyczyn quite recently proved this subtyping relationship to be undecidable. This paper supplies a new undecidability proof for this subtyping relationship. First, a new syntax-directed axiomatization of the subtyping relationship is defined. Then, this axiomatization is used to prove a reduction from the undecidable problem of semi-unification to subtyping. The undecidability of subtyping implies the undecidability of type checking for System F extended with Mitchell's subtyping, also known as F plus eta. :::::::::::::: 1996-001 :::::::::::::: Title: AIDA-based Real-Time Fault-Tolerant Broadcast Disks Author: Azer Bestavros Date: January 6, 1996 Abstract: The proliferation of mobile computers and wireless networks requires the design of future distributed real-time applications to recognize and deal with the significant asymmetry between downstream and upstream communication capacities, and the significant disparity between server and client storage capacities. Recent research work proposed the use of Broadcast Disks as a scalable mechanism to deal with this problem. In this paper, we propose a new broadcast disks protocol, based on our Adaptive Information Dispersal Algorithm (AIDA). Our protocol is different from previous broadcast disks protocols in that it improves communication timeliness, fault-tolerance, and security, while allowing for a finer control of multiplexing of prioritized data (broadcast frequencies). We start with a general introduction of broadcast disks. Next, we propose broadcast disk organizations that are suitable for real-time applications. Next, we present AIDA and show its fault-tolerance and security properties. We conclude the paper with the description and analysis of AIDA-based broadcast disks organizations that achieve both timeliness and fault-tolerance, while preserving downstream communication capacity. :::::::::::::: 1996-002 :::::::::::::: Title: An Admission Control Paradigm for Real-Time Databases Author: Azer Bestavros and Sue Nagy Date: January 11, 1996 Abstract: We propose and evaluate an admission control paradigm for RTDBS, in which a transaction is submitted to the system as a pair of processes: a primary task, and a recovery block. The execution requirements of the primary task are not known a priori, whereas those of the recovery block are known a priori. Upon the submission of a transaction, an Admission Control Mechanism is employed to decide whether to admit or reject that transaction. Once admitted, a transaction is guaranteed to finish executing before its deadline. A transaction is considered to have finished executing if exactly one of two things occur: Either its primary task is completed (successful commitment), or its recovery block is completed (safe termination). Committed transactions bring a profit to the system, whereas a terminated transaction brings no profit. The goal of the admission control, and scheduling protocols (e.g., concurrency control, I/O scheduling, memory management) employed in the system is to maximize system profit. We describe a number of admission control strategies and contrast (through simulations) their relative performance. :::::::::::::: 1996-003 :::::::::::::: Title: Advances in Real-Time Database Systems Research: Special Section on RTDBS of ACM SIGMOD Record 25(1), March 1996. Author: Azer Bestavros Date: January 15, 1996 Abstract: A Real-Time DataBase System (RTDBS) can be viewed as an amalgamation of a conventional DataBase Management System (DBMS) and a real-time system. Like a DBMS, it has to process transactions and guarantee ACID database properties. Furthermore, it has to operate in real-time, satisfying time constraints imposed on transaction commitments. A RTDBS may exist as a stand-alone system or as an embedded component in a larger multidatabase system. The publication in 1988 of a special issue of ACM SIGMOD Record on Real-Time DataBases signaled the birth of the RTDBS research area---an area that brings together researchers from both the database and real-time systems communities. Today, almost eight years later, I am pleased to present in this special section of ACM SIGMOD Record a review of recent advances in RTDBS research. There were 18 submissions to this special section, of which eight papers were selected for inclusion to provide the readers of ACM SIGMOD Record with an overview of current and future research directions within the RTDBS community. In this paper, I will summarize these directions and provide the reader with pointers to other publications for further information. :::::::::::::: 1996-004 :::::::::::::: Title: On the Fractal Nature of WWW and Its Application to Cache Modeling Author: Virgilio A. F. Almeida and Adriana Oliveira Date: February 5, 1996 Abstract: The World Wide Web (WWW or Web) is growing rapidly on the Internet. Web users want fast response time and easy access to a enormous variety of information across the world. Thus, performance is becoming a main issue in the Web. Fractals have been used to study fluctuating phenomena in many different disciplines, from the distribution of galaxies in astronomy to complex physiological control systems. The Web is also a complex, irregular, and random system. In this paper, we look at the document reference pattern at Internet Web servers and use fractal-based models to understand aspects (e.g. caching schemes) that affect the Web performance. :::::::::::::: 1996-005 :::::::::::::: Title: Distributed Parallel Computing in Mermera: Mixing Noncoherent Shared Memories Author: A. Heddaya and H.S. Sinha (GTE Labs) Date: March 7, 1996 Abstract: Programmers of parallel processes that communicate through shared globally distributed data structures (DDS) face a difficult choice. Either they must explicitly program DDS management, by partitioning or replicating it over multiple distributed memory modules, or be content with a high latency coherent (sequentially consistent) memory abstraction that hides the DDS' distribution. We present Mermera, a formalism and system that enables a smooth spectrum of noncoherent shared memory behaviors to coexist between the above two extremes. Our approach allows us to define known noncoherent memories in a new simple way, to identify new memory behaviors, and to characterize generic mixed-behavior computations. The latter are useful for programming using multiple behaviors that complement each others' advantages, and for programming by step-wise refinement. On the practical side, we show that the large class of programs that use asynchronous iterative methods (AIM) can run correctly on slow memory, one of the weakest, and hence most efficient and fault-tolerant, noncoherence conditions. An example AIM program to solve linear equations, is developed to illustrate the need for concurrently mixing memory behaviors, and the performance gains attainable via noncoherence. Other program classes tolerate weak memory consistency by synchronizing in such a way as to yield executions indistinguishable from coherent ones. AIM computations on noncoherent memory yield noncoherent, yet correct, computations. We present performance data that illustrate the benefits of noncoherence, in terms of raw memory performance, as well as application speed. Keywords: Distributed parallel computing, noncoherent shared memory, asynchronous iterative algorithms, network of workstations. :::::::::::::: 1996-006 :::::::::::::: Title: Measuring Bottleneck Link Speed in Packet-Switched Networks Authors: Robert L. Carter and Mark E. Crovella Date: March 15, 1996 Abstract: The quality of available network connections can often have a large impact on the performance of distributed applications. For example, document transfer applications such as FTP, Gopher and the World Wide Web suffer increased response times as a result of network congestion. For these applications, the document transfer time is directly related to the available bandwidth of the connection. Available bandwidth depends on two things: 1) the underlying capacity of the path from client to server, which is limited by the bottleneck link; and 2) the amount of other traffic competing for links on the path. If measurements of these quantities were available to the application, the current utilization of connections could be calculated. Network utilization could then be used as a basis for selection from a set of alternative connections or servers, thus providing reduced response time. Such a dynamic server selection scheme would be especially important in a mobile computing environment in which the set of available servers is frequently changing. In order to provide these measurements at the application level, we introduce two tools: bprobe, which provides an estimate of the uncongested bandwidth of a path; and cprobe, which gives an estimate of the current congestion along a path. These two measures may be used in combination to provide the application with an estimate of available bandwidth between server and client thereby enabling application-level congestion avoidance. In this paper we discuss the design and implementation of our probe tools, specifically illustrating the techniques used to achieve accuracy and robustness. We present validation studies for both tools which demonstrate their reliability in the face of actual Internet conditions; and we give results of a survey of available bandwidth to a random set of WWW servers as a sample application of our probe technique. We conclude with descriptions of other applications of our measurement tools, several of which are currently under development. :::::::::::::: 1996-007 :::::::::::::: Title: Dynamic Server Selection using Bandwidth Probing in Wide-Area Networks Authors: Robert L. Carter and Mark E. Crovella Date: February 2, 1996 Abstract: Replication is a commonly proposed solution to problems of scale associated with distributed services. However, when a service is replicated, each client must be assigned a server. Prior work has generally assumed that assignment to be static. In contrast, we propose dynamic server selection, and show that it enables application-level congestion avoidance. To make dynamic server selection practical, we demonstrate the use of three tools. In addition to direct measurements of round-trip latency, we introduce and validate two new tools: bprobe, which estimates the maximum possible bandwidth along a given path; and cprobe, which estimates the current congestion along a path. Using these tools we demonstrate dynamic server selection and compare it to previous static approaches. We show that dynamic server selection consistently outperforms static policies by as much as 50%. Furthermore, we demonstrate the importance of each of our tools in performing dynamic server selection. :::::::::::::: 1996-008 :::::::::::::: Title: Responsive Web Computing: Resource Management, Protocol Techniques, and Applications (A research statement) Authors: Azer Bestavros, Marina Chen, Mark Crovella, Abdelsalam Heddaya, Stan Sclaroff, and James Cowie (Cooperating Systems Corporation) Date: March 21, 1996 Abstract: The exploding demand for services like the World Wide Web reflects the potential that is presented by globally distributed information systems. The number of WWW servers world-wide has doubled every 3 to 5 months since 1993, outstripping even the growth of the Internet. At each of these self-managed sites, the Common Gateway Interface (CGI) and Hypertext Transfer Protocol (HTTP) already constitute a rudimentary basis for contributing local resources to remote collaborations. However, the Web has serious deficiencies that make it unsuited for use as a true medium for metacomputing --- the process of bringing hardware, software, and expertise from many geographically dispersed sources to bear on large scale problems. These deficiencies are, paradoxically, the direct result of the very simple design principles that enabled its exponential growth. There are many symptoms of the problems exhibited by the Web: disk and network resources are consumed extravagantly; information search and discovery are difficult; protocols are aimed at data movement rather than task migration, and ignore the potential for distributing computation. However, all of these can be seen as aspects of a single problem: as a distributed system for metacomputing, the Web offers unpredictable performance and unreliable results. The goal of our project is to use the Web as a medium (within either the global Internet or an enterprise intranet) for metacomputing in a reliable way with performance guarantees. We attack this problem one four levels: (1) Resource Management Services: Globally distributed computing allows novel approaches to the old problems of performance guarantees and reliability. Our first set of ideas involve setting up a family of real-time resource management models organized by the Web Computing Framework with a standard Resource Management Interface (RMI), a Resource Registry, a Task Registry, and resource management protocols to allow resource needs and availability information be collected and disseminated so that a family of algorithms with varying computational precision and accuracy of representations can be chosen to meet realtime and reliability constraints. (2) Middleware Services: Complementary to techniques for allocating and scheduling available resources to serve application needs under realtime and reliability constraints, the second set of ideas aim at reduce communication latency, traffic conjestion, server work load, etc. We develop customizable middleware services to exploit application characteristics in traffic analysis to drive new server/browser design strategies (e.g., exploit self-similarity of Web traffic), derive document access patterns via multiserver cooperation, and use them in speculative prefetching, document caching, and aggressive replication to reduce server load and bandwidth requirements. (3) Communication Infrastructure: Finally, to achieve any guarantee of quality of service or performance, one must get at the network layer that can provide the basic guarantees of bandwidth, latency, and reliability. Therefore, the third area is a set of new techniques in network service and protocol designs. (4) Object-Oriented Web Computing Framework A useful resource management system must deal with job priority, fault-tolerance, quality of service, complex resources such as ATM channels, probabilistic models, etc., and models must be tailored to represent the best tradeoff for a particular setting. This requires a family of models, organized within an object-oriented framework, because no one-size-fits-all approach is appropriate. This presents a software engineering challenge requiring integration of solutions at all levels: algorithms, models, protocols, and profiling and monitoring tools. The framework captures the abstract class interfaces of the collection of cooperating components, but allows the concretization of each component to be driven by the requirements of a specific approach and environment. :::::::::::::: 1996-009 :::::::::::::: Title: Proceedings of the ECSCW'95: Workshop on the Role of Version Control in CSCW Applications Editors: David Hicks, Anja Haake, David Durand, and Fabio Vitali Date: April 26, 1996 Abstract: The workshop entitled "The Role of Version Control in Computer Supported Cooperative Work Applications" was held on September 10, 1995 in Stockholm, Sweden in conjunction with the ECSCW'95 conference. Version control, the ability to manage relationships between successive instances of artifacts, organize those instances into meaningful structures, and support navigation and other operations on those structures, is an important problem in CSCW applications. It has long been recognized as a critical issue for inherently cooperative tasks such as software engineering, technical documentation, and authoring. The primary challenge for versioning in these areas is to support opportunistic, open-ended design processes requiring the preservation of historical perspectives in the design process, the reuse of previous designs, and the exploitation of alternative designs. This report contains a summary in which the workshop organizers report the major results of the workshop. The summary is followed by a section that contains the position papers that were accepted to the workshop. The position papers provide more detailed information describing recent research efforts of the workshop participants as well as current challenges that are being encountered in the development of CSCW applications. A list of workshop participants is provided at the end of the report. :::::::::::::: 1996-010 :::::::::::::: Title: Client-Based Logging: A New Paradigm For Distributed Transaction Management Authors: Thimios Panagos Date: June 13, 1996 Abstract: The proliferation of inexpensive workstations and networks has created a new era in distributed computing. At the same time, non-traditional applications such as computer-aided design (CAD), computer-aided software engineering (CASE), geographic- information systems (GIS), and office-information systems (OIS) have placed increased demands for high-performance transaction processing on database systems. The combination of these factors gives rise to significant challenges in the design of modern database systems. In this thesis, we propose novel techniques whose aim is to improve the performance and scalability of these new database systems. These techniques exploit client resources through client-based transaction management. Client-based transaction management is realized by providing logging facilities locally even when data is shared in a global environment. This thesis presents several recovery algorithms which utilize client disks for storing recovery related informa- tion (i.e., log records). Our algorithms work with both coarse and fine-granularity locking and they do not require the merging of client logs at any time. Moreover, our algorithms support fine-granularity locking with multiple clients permitted to con- currently update different portions of the same database page. The database state is recovered correctly when there is a complex crash as well as when the updates performed by different clients on a page are not present on the disk version of the page, even though some of the updating transactions have committed. This thesis also presents the implementation of the proposed algorithms in a memory-mapped storage manager as well as a detailed performance study of these algorithms using the OO1 database benchmark. The performance results show that client- based logging is superior to traditional server-based logging. This is because client-based logging is an effective way to reduce dependencies on server CPU and disk resources and, thus, prevents the server from becoming a performance bottleneck as quickly when the number of clients accessing the database increases. :::::::::::::: 1996-011 :::::::::::::: Title: Characterizing Reference Locality in the WWW Authors: Virgilio Almeida, Azer Bestavros, Mark Crovella, and Adriana de Oliveira Date: June 21, 1996 Abstract: As the World Wide Web (Web) is increasingly adopted as the infrastructure for large-scale distributed information systems, issues of performance modeling become ever more critical. In particular, locality of reference is an important property in the performance modeling of distributed information systems. In the case of the Web, understanding the nature of reference locality will help improve the design of middleware, such as caching, prefetching, and document dissemination systems. For example, good measurements of reference locality would allow us to generate synthetic reference streams with accurate performance characteristics, would allow us to compare empirically measured streams to explain differences, and would allow us to predict expected performance for system design and capacity planning. In this paper we propose models for both temporal and spatial locality of reference in streams of requests arriving at Web servers. We show that simple models based only on document popularity (likelihood of reference) are insufficient for capturing either temporal or spatial locality. Instead, we rely on an equivalent, but numerical, representation of a reference stream: a stack distance trace. We show that temporal locality can be characterized by the marginal distribution of the stack distance trace, and we propose models for typical distributions and compare their cache performance to our traces. We also show that spatial locality in a reference stream can be characterized using the notion of self-similarity. Self-similarity describes long-range correlations in the dataset, which is a property that previous researchers have found hard to incorporate into synthetic reference strings. We show that stack distance strings appear to be stongly self-similar, and we provide measurements of the degree of self-similarity in our traces. Finally, we discuss methods for generating synthetic Web traces that exhibit the properties of temporal and spatial locality that we measured in our data. Keywords: Self-similarity; Long-range dependence; Distance strings; Reference locality; Caching; Performance modeling. :::::::::::::: 1996-012 :::::::::::::: Title: Management of Communicable Memory and Lazy Barriers for Bulk Synchronous Parallelism in BSPk Authors: Amr Fahmy (Harvard University) and Abdelsalam Heddaya Date: July 2, 1996 Abstract: Communication and synchronization stand as the dual bottlenecks in the performance of parallel systems, and especially those that attempt to alleviate the programming burden by incurring overhead in these two domains. We formulate the notions of communicable memory and lazy barriers to help achieve efficient communication and synchronization. These concepts are developed in the context of BSPk, a toolkit library for programming networks of workstations---and other distributed memory architectures in general---based on the Bulk Synchronous Parallel (BSP) model. BSPk, whose design is the subject of this paper, emphasizes efficiency in communication by minimizing local memory-to-memory copying, and in barrier synchronization by not forcing a process to wait unless it needs remote data. Both the message passing (MP) and distributed shared memory (DSM) programming styles are supported in BSPk, for the former helps processes exchange short-lived unnamed data values, while the latter permits communication through long-lived named variables. :::::::::::::: 1996-013 :::::::::::::: Title: Real-Time Databases: Issues and Applications (RTDB'96 Workshop Report) Authors: Azer Bestavros, Kwei-Jay Lin (University of California Irvine), and Sang Son (University of Virginia) Date: July 3, 1996 Abstract: This report summarizes the technical presentations and discussions that took place during RTDB'96: the First International Workshop on Real-Time Databases, which was held on March 7 and 8, 1996 in Newport Beach, California. The main goals of this project were to (1) review recent advances in real-time database systems research, (2) to promote interaction among real-time database researchers and practitioners, and (3) to evaluate the maturity and directions of real-time database technology. :::::::::::::: 1996-014 :::::::::::::: Title: TCP Boston --- A Fragmentation-tolerant TCP Protocol for ATM Networks Authors: Azer Bestavros and Gitae Kim Date: July 15, 1996 Abstract: The popularity of TCP/IP coupled with the premise of high speed communication using Asynchronous Transfer Mode (ATM) technology have prompted the network research community to propose a number of techniques to adapt TCP/IP to ATM network environments. ATM offers Available Bit Rate (ABR) and Unspecified Bit Rate (UBR) services for best-effort traffic, such as conventional file transfer. However, recent studies have shown that TCP/IP, when implemented using ABR or UBR, leads to serious performance degradations, especially when the utilization of network resources (such as switch buffers) is high. Proposed techniques---switch-level enhancements, for example---that attempt to patch up TCP/IP over ATMs have had limited success in alleviating this problem. The major reason for TCP/IP's poor performance over ATMs has been consistently attributed to packet fragmentation, which is the result of ATM's 53-byte cell-oriented switching architecture. In this paper, we present a new transport protocol, TCP Boston, that turns ATM's 53-byte cell-oriented switching architecture into an advantage for TCP/IP. At the core of TCP Boston is the Adaptive Information Dispersal Algorithm (AIDA), an efficient encoding technique that allows for dynamic redundancy control. AIDA makes TCP/IP's performance less sensitive to cell losses, thus ensuring a graceful degradation of TCP/IP's performance when faced with congested resources. In this paper, we introduce AIDA and overview the main features of TCP Boston. We present detailed simulation results that show the superiority of our protocol when compared to other adaptations of TCP/IP over ATMs. In particular, we show that TCP Boston improves TCP/IP's performance over ATMs for both network-centric metrics (e.g., effective throughput) and application-centric metrics (e.g., response time). Keywords: ATM networks; TCP/IP; Adaptive Information Dispersal Algorithm; congestion control; performance evaluation. :::::::::::::: 1996-015 :::::::::::::: Title: Ergodicity and mixing rate of one-dimensional cellular automata Author: Kihong Park, Computer Science Department, Boston University Date: July 22, 1996 Abstract: One-and two-dimensional cellular automata which are known to be fault-tolerant are very complex. On the other hand, only very simple cellular automata have actually been proven to lack fault-tolerance, i.e., to be mixing. The latter either have large noise probability $\eps$ or belong to the small family of two-state nearest-neighbor monotonic rules which includes local majority voting. For a certain simple automaton $L$ called the soldiers rule, this problem has intrigued researchers for the last two decades since $L$ is clearly more robust than local voting: in the absence of noise, $L$ eliminates any finite island of perturbation from an initial configuration of all 0's or all 1's. The same holds for a 4-state monotonic variant of $L$, $K$, called two-line voting. We will prove that the probabilistic cellular automata $K_\eps$ and $L_\eps$ asymptotically lose all information about their initial state when subject to small, strongly biased noise. The mixing property trivially implies that the systems are ergodic. The finite-time information-retaining quality of a mixing system can be represented by its relaxation time $\Relax(\cdot)$, which measures the time before the onset of significant information loss. This is known to grow as $(1/\eps)^c$ for noisy local voting. The impressive error-correction ability of $L$ has prompted some researchers to conjecture that $\Relax(L_\eps)=2^{c/\eps}$. We prove the tight bound $2^{c_1\log^2 1/\eps} < \Relax(L_\eps) < 2^{c_2\log^2 1/\eps}$ for a biased error model. The same holds for $K_\eps$. Moreover, the lower bound is independent of the bias assumption. The strong bias assumption makes it possible to apply sparsity/renormalization techniques, the main tools of our investigation, used earlier in the opposite context of proving fault-tolerance. :::::::::::::: 1996-016 :::::::::::::: Title: On the relationship between file sizes, transport protocols, and self-similar network traffic Author: Kihong Park, Gitae Kim, and Mark Crovella, Computer Science Department, Boston University Date: July 30, 1996 Abstract: Recent measurements of local-area and wide-area traffic have shown that network traffic exhibits variability at a wide range of scales---self-similarity. In this paper, we examine a mechanism that gives rise to self-similar network traffic and present some of its performance implications. The mechanism we study is the transfer of files or messages whose size is drawn from a heavy-tailed distribution. We examine its effects through detailed transport-level simulations of multiple TCP streams in an internetwork. First, we show that in a ``realistic'' client/server network environment---i.e., one with bounded resources and coupling among traffic sources competing for resources---the degree to which file sizes are heavy-tailed can directly determine the degree of traffic self-similarity at the link level. We show that this causal relationship is not significantly affected by changes in network resources (bottleneck bandwidth and buffer capacity), network topology, the influence of cross-traffic, or the distribution of interarrival times. Second, we show that properties of the transport layer play an important role in preserving and modulating this relationship. In particular, the reliable transmission and flow control mechanisms of TCP (Reno, Tahoe, or Vegas) serve to maintain the long-range dependency structure induced by heavy-tailed file size distributions. In contrast, if a non-flow-controlled and unreliable (UDP-based) transport protocol is used, the resulting traffic shows little self-similar characteristics: although still bursty at short time scales, it has little long-range dependence. If flow-controlled, unreliable transport is employed, the degree of traffic self-similarity is positively correlated with the degree of throttling at the source. Third, in exploring the relationship between file sizes, transport protocols, and self-similarity, we are also able to show some of the performance implications of self-similarity. We present data on the relationship between traffic self-similarity and network performance as captured by performance measures including packet loss rate, retransmission rate, and queueing delay. Increased self-similarity, as expected, results in degradation of performance. Queueing delay, in particular, exhibits a drastic increase with increasing self-similarity. Throughput-related measures such as packet loss and retransmission rate, however, increase only gradually with increasing traffic self-similarity as long as reliable, flow-controlled transport protocol is used. :::::::::::::: 1996-017 :::::::::::::: Title: Load Profiling in Distributed Real-Time Systems Author: Azer Bestavros Date: August 1, 1996 Abstract: Load balancing is often used to ensure that nodes in a distributed systems are equally loaded. In this paper, we show that for real-time systems, load balancing is not desirable. In particular, we propose a new load-profiling strategy that allows the nodes of a distributed system to be unequally loaded. Using load profiling, the system attempts to distribute the load amongst its nodes so as to maximize the chances of finding a node that would satisfy the computational needs of incoming real-time tasks. To that end, we describe and evaluate a distributed load-profiling protocol for dynamically scheduling time-constrained tasks in a loosely-coupled distributed environment. When a task is submitted to a node, the scheduling software tries to schedule the task locally so as to meet its deadline. If that is not feasible, it tries to locate another node where this could be done with a high probability of success, while attempting to maintain an overall load profile for the system. Nodes in the system inform each other about their state using a combination of multicasting and gossiping. The performance of the proposed protocol is evaluated via simulation, and is contrasted to other dynamic scheduling protocols for real-time distributed systems. Based on our findings, we argue that keeping a diverse availability profile and using passive bidding (through gossiping) are both advantageous to distributed scheduling for real-time systems. :::::::::::::: 1996-018 :::::::::::::: Title: Performance Analysis of a WWW Server Author: Virgilio Almeida (UFMG and BU), Jussara Almeida (UFMG), and Cristina Murta (UFMG) Date: August 5, 1996 Abstract: The WWW has experienced a phenomenal growth and has become the most popular Internet application. As a consequence of its large popularity, the Internet has suffered from various performance problems, such as network congestion and overloaded servers. These days, it is not uncommon to find servers refusing connections because they are overloaded. Performance has always been a key issue in the design and operation of on-line systems. With regard to Internet, performance is also critical, because users want fast and easy access to all objects (e.g., documents, graphics, audio, and video) available on the net. Thus, it is important to understand WWW performance issues. This paper focuses on the performance analysis of Web servers. Using a synthetic benchmark (WebStone) and standard operating systems monitoring tools, it analyzes three different Web server software running on top of a Windows NT platform and performing some typical WWW tasks. It also discusses the main steps needed to carry out a WWW performance analysis effort and shows relations between the workload characteristics and system resource usage. :::::::::::::: 1996-019 :::::::::::::: Title: Beta-Reduction as Unification Author: A.J. Kfoury, Computer Science, Boston University Date: July 8, 1996 Abstract: We define a unification problem ^UP with the property that, given a pure lambda-term M, we can derive an instance Gamma(M) of ^UP from M such that Gamma(M) has a solution if and only if M is beta-strongly normalizable. There is a type discipline for pure lambda-terms that characterizes beta-strong normalization; this is the system of intersection types (without a ``top'' type that can be assigned to every lambda-term). In this report, we use a lean version LAMBDA of the usual system of intersection types. Hence, ^UP is also an appropriate unification problem to characterize typability of lambda-terms in LAMBDA. It also follows that ^UP is an undecidable problem, which can in turn be related to semi-unification and second-order unification (both known to be undecidable). :::::::::::::: 1996-020 :::::::::::::: Title: An Infinite Pebble Game and Applications Authors: A.J. Kfoury, Computer Science, Boston University and A.P. Stolboushkin, Mathematics, UCLA Date: August 15, 1996 Abstract: We generalize the well-known pebble game to infinite dag's, and we use this generalization to give new and shorter proofs of results in different areas of computer science (as diverse as ``logic of programs'' and ``formal language theory''). Our applications here include a proof of a theorem due to Salomaa, asserting the existence of a context-free language with infinite index, and a proof of a theorem due to Tiuryn and Erimbetov, asserting that unbounded memory increases the power of logics of programs. The original proofs by Salomaa, Tiuryn, and Erimbetov, are fairly technical. The proofs by Tiuryn and Erimbetov also involve advanced techniques of model theory, namely, back-and-forth constructions based on a variant of Ehrenfeucht-Fraisse games. By contrast, our proofs are not only shorter, but also elementary. All we need is essentially finite induction and, in the case of the Tiuryn-Erimbetov result, the compactness and completeness of first-order logic. :::::::::::::: 1996-021 :::::::::::::: Title: A Linearization of the Lambda Calculus and Consequences Author: A.J. Kfoury, Computer Science, Boston University Date: August 19, 1996 Abstract: If every lambda-abstraction in a lambda-term M binds at most one variable occurrence, then M is said to be "linear". Many questions about linear lambda-terms are relatively easy to answer, e.g. they all are beta-strongly normalizing and all are simply-typable. We extend the syntax of the standard lambda-calculus L to a non-standard lambda-calculus L^ satisfying a linearity condition generalizing the notion in the standard case. Specifically, in L^ a subterm Q of a term M can be applied to several subterms R1,...,Rk in parallel, which we write as (Q. R1 \wedge ... \wedge Rk). The appropriate notion of beta- reduction beta^ for the calculus L^ is such that, if Q is the lambda- abstraction (\lambda x.P) with m\geq 0 bound occurrences of x, the reduction can be carried out provided k = max(m,1). Every M in L^ is thus beta^-SN. We relate standard beta-reduction and non-standard beta^-reduction in several different ways, and draw several consequences, e.g. a new simple proof for the fact that a standard term M is beta-SN iff M can be assigned a so-called ``intersection'' type (``top'' type disallowed). :::::::::::::: 1996-022 :::::::::::::: Title: Typability is Undecidable for F+Eta Author: J. B. Wells Date: March 9, 1996 Abstract: System F is the well-known polymorphically-typed lambda calculus with universal quantifiers. F+eta is System F extended with the eta rule, which says that if term M can be given type tau and M eta-reduces to N , then N can also be given the type tau. Adding the eta rule to System F is equivalent to adding the subsumption rule using the subtyping (containment) relation that Mitchell defined and axiomatized [Mit88]. The subsumption rule says that if M can be given type tau and tau is a subtype of type sigma, then M can be given type sigma. Mitchell's subtyping relation involves no extensions to the syntax of types, i.e., no bounded polymorphism and no supertype of all types, and is thus unrelated to the system "F-sub". Typability for F+eta is the problem of determining for any term M whether there is any type tau that can be given to it using the type inference rules of F+eta. Typability has been proven undecidable for System F [Wel94] (without the eta rule), but the decidability of typability has been an open problem for F+eta. Mitchell's subtyping relation has recently been proven undecidable [TU95,Wel95b], implying the undecidability of "type checking" for F+eta. This paper reduces the problem of subtyping to the problem of typability for F+eta, thus proving the undecidability of typability. The proof methods are similar in outline to those used to prove the undecidability of typability for System F, but the fine details differ greatly. :::::::::::::: 1996-023 :::::::::::::: Title: Pinwheel Scheduling for Fault-tolerant Broadcast Disks in Real-time Database Systems Author: Sanjoy Baruah (U of Vermont) and Azer Bestavros (Boston U) Date: August 22, 1996 Abstract: The design of programs for broadcast disks which incorporate real-time and fault-tolerance requirements is considered. A generalized model for real-time fault-tolerant broadcast disks is defined. It is shown that designing programs for broadcast disks specified in this model is closely related to the scheduling of pinwheel task systems. Some new results in pinwheel scheduling theory are derived, which facilitate the efficient generation of real-time fault-tolerant broadcast disk programs. :::::::::::::: 1996-024 :::::::::::::: Title: WebWave: Globally Load Balanced Fully Distributed Caching of Hot Published Documents Author: Abdelsalam Heddaya and Sulaiman Mirdad Date: October 10, 1996 Abstract: Document publication service over such a large network as the Internet challenges us to harness available server and network resources to meet fast growing demand. In this paper, we show that large-scale dynamic caching can be employed to globally minimize server idle time, and hence maximize the aggregate throughput of the whole service. Given the distributed nature of the system, a successful caching mechanism must satisfy three properties: (1) that it maximize the global throughput of the system, (2) that it be completely distributed in the sense of operating only on the basis of local information, and (3) that it require no naming service that introduces a scalability bottleneck. In this paper, we develop a precise definition, which we call "tree load-balance", of what it means for a mechanism to satisfy these three goals, and present two algorithms that achieve them. Both algorithms compute the request rate that should be allocated to each cache server, so that global throughput is maximized. The first algorithm, WebFold, is a centralized one that is provably optimal with respect to throughput. The second algorithm, WebWave, whose optimality is evidenced by simulation, is a fully distributed diffusion-based protocol. Both algorithms assume that cache copies are placed on the routing tree that connects the cached document's home server with its clients. As a consequence, document requests can find cache copies without resorting to a cache directory of any kind. The results herein apply only to immutable documents; we do not consider the cache consistency problem. :::::::::::::: 1996-025 :::::::::::::: Title: Measuring the Behavior of a World-Wide Web Server Author: Jussara Almeida, Virgilio Almeida, and David Yates Date: October 29, 1996 Abstract: Server performance has become a crucial issue for improving the overall performance of the World-Wide Web. This paper describes Webmonitor, a tool for evaluating and understanding server performance, and presents new results for a realistic workload. Webmonitor measures activity and resource consumption, both within the kernel and in HTTP processes running in user space. Webmonitor is implemented using an efficient combination of sampling and event-driven techniques that exhibit low overhead. Our initial implementation is for the Apache World-Wide Web server running on the Linux operating system. We demonstrate the utility of Webmonitor by measuring and understanding the performance of a Pentium-based PC acting as a dedicated WWW server. Our workload uses a file size distribution with a heavy tail. This captures the fact that Web servers must concurrently handle some requests for large audio and video files, and a large number of requests for small documents, containing text or images. Our results show that in a Web server saturated by client requests, over 90% of the time spent handling HTTP requests is spent in the kernel. Furthermore, keeping TCP connections open, as required by TCP, causes a factor of 2-9 increase in the elapsed time required to service an HTTP request. Data gathered from Webmonitor provide insight into the causes of this performance penalty. Specifically, we observe a significant increase in resource consumption along three dimensions: the number of HTTP processes running at the same time, CPU utilization, and memory utilization. These results emphasize the important role of operating system and network protocol implementation in determining Web server performance. :::::::::::::: 1996-026 :::::::::::::: Title: Blocking Java Applets at the Firewall Authors: David M. Martin Jr. (BU), Sivaramakrishnan Rajagopalan (Bellcore), and Aviel D. Rubin (Bellcore). Date: November 14, 1996 Abstract: This paper explores the problem of protecting a site on the Internet against hostile external Java applets while allowing trusted internal applets to run. With careful implementation, a site can be made resistant to current Java security weaknesses as well as those yet to be discovered. In addition, we describe a new attack on certain sophisticated firewalls that is most effectively realized as a Java applet. :::::::::::::: 1996-027 :::::::::::::: Title: Proceedings of the Real-Time Systems Symposium WIP Session Author: Azer Bestavros (Editor) Date: December 4, 1996 Abstract: This technical report includes 14 short papers presented during the WIP session of the 17th Real-Time Systems Symposium, held in Washington DC on December 4-6, 1996. The title and authors are included below. ------ (1) A Specialized Specification and Verification System for Timed Automata Myla Archer and Constance Heitmeyer Naval Research Laboratory, USA Abstract: Assuring the correctness of specifications of real-time systems can involve significant human effort. The use of a mechanical theorem prover to encode such specifications and to verify their properties could significantly reduce this effort. A barrier to routinely encoding and mechanically verifying specifications has been the need first to master the specification language and logic of a general theorem proving system. Our approach to overcoming this barrier is to provide mechanical support for producing specifications and verifying proofs, specialized for particular mathematical models and proof techniques. We are currently developing a mechanical verification system called TAME (Timed Automata Modeling Environment), which provides this specialized support using SRI's Prototype Verification System (PVS). Our system is intended to permit steps in reasoning similar to those in hand proofs that use model-specific techniques. TAME has recently been used to detect errors in a realistic example. ------ (2) Scheduling Slack in MetaH Pam Binns Honeywell Technology Center, USA Abstract: A real-time implementation for allocating slack to aperiodic proceesses in MetaH is nearing completion. The slack scheduling algorithm is based on the slack stealer originally proposed in "An Optimal Algorithm for Scheduling Soft-Aperiodic Tasks in Fixed-Priority Preemptive Systems" with practical extensions to allow for support of process criticalities, multiple process streams (of different criticalities) competing for pooled slack and inclusion of run-time overheads in the slack functions. Areas in need of future work are also identified. ------ (3) AFTER: A case tool to assist in Fine-tuning of embedded real-time systems Gaurav Arora and David Stewart University of Maryland, USA Abstract: AFTER (Assist in Fine-Tuning of Embedded Real-time systems) is an interactive analysis and predictor tool for embedded systems. It helps designers quickly identify timing problems and systematically fine-tune an application during and after the implementation phase of a product's lifecycle. The tool begins with raw timing data collected from an embedded system. It analyzes the data to provide a temporal image of the current implementation, highlighting actual and potential problems. The user then interacts with AFTER to obtain predictions on what overall effect can be expected if small adjustments are made to configuration parameters or to the timing properties of specific software components. The tool integrates and extends prior research in scheduling, task monitoring, and operating system design for real-time systems. ------ (4) Genericity and Upgradability in Ultra-Dependable Real-Time Architectures Andy Wellings, Ljerka Beus-Dukis, Alan Burns, and David Powell LAAS-CNRS, France and University of York, UK Abstract: We report on the ideas currently being developed within the European GUARDS project to develop a generic upgradable architecture for real-time dependable systems. After a brief introduction and overview of the architecture, we outline the GUARDS approach for scheduling real-time replicated computation. ------ (5) Challenges in Engineering Distributed Shipboard Control System L.Welch, B.Ravindran, R.Harrison, L.Madden, M.W.Masters and W.Mills Naval Surface Warfare Center and University of Texas at Arlington, USA Abstract: In response to the need to develop high capacity, scalable computer systems for shipboard use, a program called the High Performance Distributed Computing Program (HiPer-D), was created. HiPer-D is intended to provide the technical design concepts and engineering data needed to enable the Navy to capitalize on commercial computing products. The program, conducted jointly by the Defense Advanced Research Projects Agency (DARPA) and the Aegis Shipbuilding Program, consists of simultaneous top down engineering studies and large-scale critical experiments using new computer technology. ------ (6) Issues for realizing a scalable Real Time Kernel for function-distributed Multiprocessors Hiroaki Takada, Cai-Dong Wang, and ken Sakamura University of Tokyo, Japan Abstract: In multiprocessor systems, the worst-case execution time of a task that exclusively accesses a shared resource is unavoidably prolonged as the number of contending processors is increased. In case of function-distributed multiprocessors, because many of the tasks can be processed within a processor, it is advantageous that their worst-case behavior are independent of the number of processors in the system. This paper summarizes the required properties on scalable real-time kernels and discusses their realization techniques. What we have solved so far are described, and the remaining problem to be solved is presented. ------ (7) The design and implementation of the CPU power regulator for multimedia operating systems Giun-Haur Huang, Shie-Kai Ni, and Tei-Wei Kuo National Chung Cheng University, Taiwan Abstract: This paper describes a Windows NT/95 utility, the CPU Power Regulator (CPR), which improves the capability of Windows NT/95 in servicing time-critical applications. CPR considers a distance model [4] to service time-critical applications such as multimedia softwares and electronic games in a timely fashion. Distinct from the past work [7, 8, 9], CPR adopts a user-level control mechanism to manage the resource allocations on Windows NT/95 and makes no modifications to the operating system and application softwares. The performance of CPR was verified by a collection of simulation experiments of randomly generated and realistic workloads. CPR not only introduces very low system overheads but also largely reduces the phenomenon of non-timely resource allocation for applications. The experimental results also demonstrate the capability and flexibility of CPR in multiplexing CPU cycles to provide different degrees of quality-of-service to time-critical applications. The results of this work present a low-cost software solution to transform an ordinary operating system into a multimedia operating system. ------ (8) An approach for monitoring intrusion removal in Real Time Systems Vishal Jain, Madalene Spezialetti, and Rajiv Gupta University of Pittsburgh and Trinity College, USA Abstract: To assist in the development of a real-time application, monitoring is used to collect execution timing information for the application. In this paper we propose a strategy that accurately reports timing information by accounting for intrusion introduced by monitoring. In addition, by allowing processes that miss deadlines to run to completion, our approach provides the user with times by which the execution of these processes exceeds their deadlines. This information can be used to guide the user in restructuring the application to meet timing requirements. ------ (9) Empirical Evaluation of Task and Resource Scheduling in Dynamic Real-Time Systems Ken Tew and Panos Chrysantis and Daniel Mosse University of Pittsburgh, USA Abstract: This work-in-progress reports on our on-going empirical evaluation of a two-tiered resource allocation scheme assuming independent jobs, that is, jobs have no precedence constraints. The first tier extends the temporal density approach, while the second tier uses an Earliest Deadline First (EDF) approach to schedule jobs at a site. However, job scheduling at sites is constrained by the precedence relation between the loading and execution of a job. In addition to CPU scheduling, we also take care of the time it takes to load a task onto memory from a disk (or from another processor over the network). We assume that loading (i.e., disk scheduling) follows an EDF non-preemptive discipline whereas the execution (i.e., CPU scheduling) follows a preemptive EDF. ------ (10) Scalability based admission control of real-time channels Ramesh Yerraballi and Ravi Mukkamala Midwestern State University and Old Dominion University, USA Abstract: This paper reports our continuing efforts and initial results with the problem of admission control in real-time networks. This problem was first addressed by the Tenet group, and, their approach was based on the assumption that the link level scheduling was EDD (Earliest Due Date) based. Our work departs from this assumption by addressing the problem in the context of any arbitrary dynamic/fixed priority link level scheduling. Our approach is based on extending a result we have derived in a different context, viz., Task Scalability. It involves assessing the current capacity of a link in terms of its ability to accommodate (scale to) new channels. This assessment (called the admittance measure) is then heuristically compared against the traffic requirements of the newly requested channel to decide its admissibility. A simulation study was performed to study the effectiveness of our approach in improving both utilization of the link and admissibility of channels. Further, we demonstrate the relevance of our heuristic by observing that it reduces to the Tenet schedulability test, for the case of EDD. ------ (11) Optimization of scheduling on real-time parallel computer systems Leyuan Shi and Philip Q. Hwang University of Wisconsin and Defence Mapping Agency, USA Abstract: We describe our ongoing work in the field of optimal scheduling for real-time systems. We are primarily concerned with optimal task allocation and job scheduling for parallel computer systems. Many real-time task allocation and job scheduling problems are proven to be NP-hard. Recently, we proposed a randomized optimization framework for efficiently solving such NP-hard problems. The proposed method, the Nested Partitions (NP) method, has been proved to converge to global optimal solutions and it is also highly matched to emerging massively parallel processing capabilities. ------ (12) Dynamic Scheduling of Hard Real-Time Applications in Open System Environment Z. Deng, J. W.-S. Liu, and J. Sun University of Illinois at Urbana Champaign, USA Abstract: This paper focuses on the problem of providing run-time support to real-time applications and non-real-time applications in an open system. It describes a two-level hierarchical priority-driven scheme for scheduling independently developed applications. The scheme allows the developer of each real-time application to validate the schedulability of the application independently of other applications. Once a real-time application is created and accepted by the open system, its schedulability is guaranteed regardless of the behaviors of other applications that execute concurrently in the system. ------ (13) In Search for an efficient Real-Time Atomic Commit Protocol Yousef Al-Houmaily and Panos Chrysantis University of Pittsburgh, USA Abstract: The purpose of this paper is to report on the first step in our quest for an efficient atomic commit protocol in real-time databases. This includes the development of RT-IYV (real-time implicit yes-vote), a new real-time atomic commit protocol. In contrast to other real-time commit protocols that provide for semantic atomicity, RT-IYV is designed to ensure the traditional notion of transaction atomicity. RT-IYV (1) eliminates the voting phase from 2PC hence, reducing the number of sequential coordination messages and forced log writes during normal processing, and (2) supports transactions' forward recovery hence, enabling partially executed transactions to resume their execution after a failure. To illustrate its performance advantages, we compare RT-IYV with the recently proposed OPT (optimistic commit protocol) which is also designed to support the standard transaction atomicity in real-time databases. ------ (14) Distributed Real-Time Dataflow: An Execution Paradigm for Image Processing and Anti-Submarine Warfare Applications Steve Goddard and Kevin Jeffay University of North Carolina, USA Abstract: The purpose of this paper is to report on the first step in our quest for an efficient atomic commit protocol in real-time databases. This includes the development of RT-IYV (real-time implicit yes-vote), a new real-time atomic commit protocol. In contrast to other real-time commit protocols that provide for semantic atomicity, RT-IYV is designed to ensure the traditional notion of transaction atomicity. RT-IYV (1) eliminates the voting phase from 2PC hence, reducing the number of sequential coordination messages and forced log writes during normal processing, and (2) supports transactions' forward recovery hence, enabling partially executed transactions to resume their execution after a failure. To illustrate its performance advantages, we compare RT-IYV with the recently proposed OPT (optimistic commit protocol) which is also designed to support the standard transaction atomicity in real-time databases. :::::::::::::: 1997-001 :::::::::::::: Title: Exploiting Redundancy for Timeliness in TCP Boston Author: Azer Bestavros and Gitae Kim Date: January 24, 1997 Abstract: While ATM bandwidth-reservation techniques are able to offer the guarantees necessary for the delivery of real-time streams in many applications (e.g. live audio and video), they suffer from many disadvantages that make them inattractive (or impractical) for many others. These limitations coupled with the flexibility and popularity of TCP/IP as a best-effort transport protocol have prompted the network research community to propose and implement a number of techniques that adapt TCP/IP to the Available Bit Rate (ABR) and Unspecified Bit Rate (UBR) services in ATM network environments. This allows these environments to smoothly integrate (and make use of) currently available TCP-based applications and services without much (if any) modifications. However, recent studies have shown that TCP/IP, when implemented over ATM networks, is susceptible to serious performance limitations. In a recently completed study, we have unveiled a new transport protocol, TCP Boston, that turns ATM's 53-byte cell-oriented switching architecture into an advantage for TCP/IP. In this paper, we demonstrate the real-time features of TCP Boston that allow communication bandwidth to be traded off for timeliness. We start with an overview of the protocol. Next, we analytically characterize the dynamic redundancy control features of TCP Boston. Next, We present detailed simulation results that show the superiority of our protocol when compared to other adaptations of TCP/IP over ATMs. In particular, we show that TCP Boston improves TCP/IP's performance over ATMs for both network-centric metrics ({\em e.g.}, effective throughput and percent of missed deadlines) and real-time application-centric metrics (e.g., response time and jitter). :::::::::::::: 1997-002 :::::::::::::: Title: The Network Effects of Prefetching Author: Mark Crovella and Paul Barford Date: February 7, 1997 Prefetching has been shown to be an effective technique for reducing user perceived latency in distributed systems. In this paper we show that even when prefetching adds no extra traffic to the network, it can have serious negative performance effects. Straightforward approaches to prefetching increase the burstiness of individual sources, leading to increased average queue sizes in network switches. However, we also show that applications can avoid the undesirable queueing effects of prefetching. In fact, we show that applications employing prefetching can significantly improve network performance, to a level much better than that obtained without any prefetching at all. This is because prefetching offers increased opportunities for traffic shaping that are not available in the absence of prefetching. Using a simple transport rate control mechanism, a prefetching application can modify its behavior from a distinctly ON/OFF entity to one whose data transfer rate changes less abruptly, while still delivering all data in advance of the user's actual requests. :::::::::::::: 1997-003 :::::::::::::: Title: Visible Volume: A Robust Measure for Protein Structure Characterization. Author: Loredana Lo Conte and Temple F. Smith Date: March 20, 1997 Abstract: We propose a new characterization of protein structure based on the natural tetrahedral geometry of the beta carbon and a new geometric measure of structural similarity, called visible volume. In our model, the side-chains are replaced by an ideal tetrahedron, the orientation of which is fixed with respect to the backbone and corresponds to the preferred rotamer directions. Visible volume is a measure of the non-occluded empty space surrounding each residue position after the side-chains have been removed. It is a robust, parameter-free, locally-computed quantity that accounts for all spatial constraints that are of relevance to the corresponding position in the native structure. When computing visible volume, we ignore the nature of both the residue observed at each site and the ones surrounding it. We focus instead on the space that, together, these residues could occupy. By doing so, we are able to quantify a new kind of invariance beyond the apparent variations in a protein family, namely, the conservation of the physical space that is available at structurally equivalent positions for 3-D side-chain packing. Visible volume has the unique property of estimating how much space can be used at each site for different combinations of side-chains to fit in. This property, and the relation of visible volume to the degree of exposure of a residue position, qualify it as a powerful tool in a variety of applications, from the detailed analysis of protein structure to the definition of better scoring functions for threading purpose. :::::::::::::: 1997-004 :::::::::::::: Title: Determining WWW User's Next Access and Its Application to Pre-fetching Authors: Carlos R. Cunha and Carlos F. B. Jaccoud Date: March 24, 1997 Abstract: World-Wide Web (WWW) services have grown to levels where significant delays are expected to happen. Techniques like pre-fetching are likely to help users to personalize their needs, reducing their waiting times. However, pre-fetching is only effective if the right documents are identified and if user's move is correctly predicted. Otherwise, pre-fetching will only waste bandwidth. Therefore, it is productive to determine whether a revisit will occur or not, before starting pre-fetching. In this paper we develop two user models that help determining user's next move. One model uses Random Walk approximation and the other is based on Digital Signal Processing techniques. We also give hints on how to use such models with a simple pre-fetching technique that we are developing. This is an extended version of the article with the same title presented in the International Symposium on Computers and Communication'97, Alexandria, Egypt, 1-3 July, 1997. :::::::::::::: 1997-005 :::::::::::::: Title: ImageRover: A Content-Based Image Browser for the World Wide Web Authors: Stan Sclaroff, Leonid Taycher, and Marco La Cascia Date: March 31, 1997 Abstract: ImageRover is a search by image content navigation tool for the world wide web. To gather images expediently, the image collection subsystem utilizes a distributed fleet of WWW robots running on different computers. The image robots gather information about the images they find, computing the appropriate image decompositions and indices, and store this extracted information in vector form for searches based on image content. At search time, users can iteratively guide the search through the selection of relevant examples. Search performance is made efficient through the use of an approximate, optimized k-d tree algorithm. The system employs a novel relevance feedback algorithm that selects the Lm distance metrics appropriate for a particular query. :::::::::::::: 1997-006 :::::::::::::: Title: Generating Representative Web Workloads for Network and Server Performance Evaluation Authors: Barford, Paul and Crovella, Mark Date: May 5, 1997 (revised November 4, 1997) Abstract: One role for workload generation is as a means for understanding how servers and networks respond to variation in load. This enables management and capacity planning based on current and projected usage. This paper applies a number of observations of Web server usage to create a realistic Web workload generation tool which mimics a set of real users accessing a server. The tool, called SURGE (Scalable URL Reference Generator) generates references matching empirical measurements of 1) server file size distribution; 2) request size distribution; 3) relative file popularity; 4) embedded file references; 5) temporal locality of reference; and 6) idle periods of individual users. This paper reviews the essential elements required in the generation of a representative Web workload. It also addresses the technical challenges to satisfying this large set of simultaneous constraints on the properties of the reference stream, the solutions we adopted, and their associated accuracy. Finally, we present evidence that SURGE exercises servers in a manner significantly different from other Web server benchmarks. :::::::::::::: 1997-007 :::::::::::::: Title: Real-Time Mutable Broadcast Disks Authors: Sanjoy Baruah and Azer Bestavros Date: May 5, 1997 Abstract: There is an increased interest in using broadcast disks to support mobile access to real-time databases. However, previous work has only considered the design of real-time immutable broadcast disks, the contents of which do not change over time. This paper considers the design of programs for real-time mutable broadcast disks --- broadcast disks whose contents are occasionally updated. Recent scheduling-theoretic results relating to pinwheel scheduling and pfair scheduling are used to design algorithms for the efficient generation of real-time mutable broadcast disk programs. :::::::::::::: 1997-008 :::::::::::::: Title: Active Blobs Authors: Sclaroff, Stan and Isidoro, John Date: April 30, 1997 Abstract: Active blobs, a new region-based approach to nonrigid motion tracking is described. Active blobs employ a view-based representation; each object is defined in terms of a deformable, active blob of color pixels. Shape is defined in terms of a triangulated finite element model that captures object shape plus a color texture map that captures object appearance. Active blobs also provide normalization with respect to some photometric variations. Nonrigid shape registration and motion recovery is achieved by posing the problem as an energy-based, robust minimization procedure. The active blob formulation is robust to occlusions, shadows, and specular highlights. :::::::::::::: 1997-009 :::::::::::::: Title: Load Profiling for Efficient Route Selection in Multi-Class Networks Authors: Azer Bestavros and Ibrahim Matta Date: May 14, 1997 Abstract: High-speed networks, such as ATM networks, are expected to support diverse Quality of Service (QoS) constraints, including real-time QoS guarantees. Real-time QoS is required by many applications such as those that involve voice and video communication. To support such services, routing algorithms that allow applications to reserve the needed bandwidth over a Virtual Circuit (VC) have been proposed. Commonly, these bandwidth-reservation algorithms assign VCs to routes using the least-loaded concept, and thus result in balancing the load over the set of all candidate routes. In this paper, we show that for such reservation-based protocols---which allow for the exclusive use of a preset fraction of a resource's bandwidth for an extended period of time---load balancing is not desirable as it results in resource fragmentation, which adversely affects the likelihood of accepting new reservations. In particular, we show that load-balancing VC routing algorithms are not appropriate when the main objective of the routing protocol is to increase the probability of finding routes that satisfy incoming VC requests, as opposed to equalizing the bandwidth utilization along the various routes. We present an on-line VC routing scheme that is based on the concept of ``load profiling'', which allows a distribution of ``available'' bandwidth across a set of candidate routes to match the characteristics of incoming VC QoS requests. We show the effectiveness of our load-profiling approach when compared to traditional load-balancing and load-packing VC routing schemes. :::::::::::::: 1997-010 :::::::::::::: Title: Concurrency Admission Control Management in ACCORD Authors: Susan Nagy and Azer Bestavros Date: May 15, 1997 Abstract: We propose and evaluate admission control mechanisms for ACCORD, an Admission Control and Capacity Overload management Real-time Database framework---an architecture and a transaction model---for hard deadline RTDB systems. The system architecture consists of admission control and scheduling components which provide early notification of failure to submitted transactions that are deemed not valuable or incapable of completing on time. In this paper, we focus on our Concurrency Admission Control Manager (CACM), which ensures that admitted transactions do not overburden the system by requiring a level of concurrency that is not sustainable. The transaction model consists of two components: a primary taskand a compensating task. The execution requirements of the primary task are notknown a priori, whereas those of the compensating task are known a priori. Upon the submission of a transaction, the Admission Control Mechanismsare employed to decide whether to admitor rejectthat transaction. Once admitted, a transaction is guaranteed to finishexecuting before its deadline. A transaction is considered to have finished executing if exactly one of two things occur: Either its primary task is completed (successful commitment), or its compensating task is completed (safe termination). Committed transactions bring a profit to the system, whereas a terminated transaction brings no profit. The goal of the admission control and scheduling protocols (e.g., concurrency control, I/O scheduling, memory management) employed in the system is to maximize system profit. In that respect, we describe a number of concurrency admission control strategies and contrast (through simulations) their relative performance. :::::::::::::: 1997-011 :::::::::::::: Title: Reliability, Availability, Dependability and Performability: A User-centered View Authors: Abdelsalam Heddaya (BU) and Abdelsalam Helal (MCC) Date: May 22, 1997 Abstract: Reliability and availability have long been considered twin system properties that could be enhanced by distribution. Paradoxically, the traditional definitions of these properties do not recognize the positive impact of recovery---as distinct from simple repair and restart---on reliability, nor the negative effect of recovery, and of internetworking of clients and servers, on availability. As a result of employing the standard definitions, reliability would tend to be underestimated, and availability overestimated. We offer revised definitions of these two critical metrics, which we call service reliability and service availability, that improve the match between their formal expression, and intuitive meaning. A fortuitous advantage of our approach is that the product of our two metrics yields a highly meaningful figure of merit for the overall dependability of a system. But techniques that enhance system dependability exact a performance cost, so we conclude with a cohesive definition of performability that rewards the system for performance that is delivered to its client applications, after discounting the following consequences of failure: service denial and interruption, lost work, and recovery cost. :::::::::::::: 1997-012 :::::::::::::: Title: On the Interaction Between an Operating System and Web Server Author: David J. Yates, Virgilio Almeida, and Jussara M. Almeida Date: July 16, 1997 Abstract: This paper examines how and why web server performance changes as the workload at the server varies. We measure the performance of a PC acting as a standalone web server, running Apache on top of Linux. We use two important tools to understand what aspects of software architecture and implementation determine performance at the server. The first is a tool that we developed, called WebMonitor, which measures activity and resource consumption, both in the operating system and in the web server. The second is the kernel profiling facility distributed as part of Linux. We vary the workload at the server along two important dimensions: the number of clients concurrently accessing the server, and the size of the documents stored on the server. Our results quantify and show how more clients and larger files stress the web server and operating system in different and surprising ways. Our results also show the importance of fixed costs (i.e., opening and closing TCP connections, and updating the server log) in determining web server performance. :::::::::::::: 1997-013 :::::::::::::: Title: Evaluation of a Load Profiling Approach to Routing Guaranteed Bandwidth Flows Authors: Ibrahim Matta and Azer Bestavros Date: July 30, 1997 Abstract: To support the diverse Quality of Service (QoS) requirements of real-time (e.g. audio/video) applications in integrated services networks, several routing algorithms that allow for the reservation of the needed bandwidth over a Virtual Circuit (VC) established on one of several candidate routes have been proposed. Traditionally, such routing is done using the least-loaded concept, and thus results in balancing the load across the set of candidate routes. In a recent study, we have established the inadequacy of this load balancing practice and proposed the use of load profiling as an alternative. Load profiling techniques allow the distribution of ``available'' bandwidth across a set of candidate routes to match the characteristics of incoming VC QoS requests. In this paper we thoroughly characterize the performance of VC routing using load profiling and contrast it to routing using load balancing and load packing. We do so both analytically and via extensive simulations of multi-class traffic routing in Virtual Path (VP) based networks. Our findings confirm that for routing guaranteed bandwidth flows in VP networks, load balancing is not desirable as it results in VP bandwidth fragmentation, which adversely affects the likelihood of accepting new VC requests. This fragmentation is more pronounced when the granularity of VC requests is large. Typically, this occurs when a common VC is established to carry the aggregate traffic flow of many high-bandwidth real-time sources. For VP-based networks, our simulation results show that our load-profiling VC routing scheme performs better or as well as the traditional load-balancing VC routing in terms of revenue under both skewed and uniform workloads. Furthermore, load-profiling routing improves routing fairness by proactively increasing the chances of admitting high-bandwidth connections. :::::::::::::: 1997-014 :::::::::::::: Title: Image Digestion and Relevance Feedback in the ImageRover WWW Search Engine Authors: Leonid Taycher, Marco La Cascia, and Stan Sclaroff Date: August 14, 1997 Abstract: ImageRover is a search by image content navigation tool for the world wide web. The staggering size of the WWW dictates certain strategies and algorithms for image collection, digestion, indexing, and user interface. This paper describes two key components of the ImageRover strategy: image digestion and relevance feedback. Image digestion occurs during image collection; robots digest the images they find, computing image decompositions and indices, and storing this extracted information in vector form for searches based on image content. Relevance feedback occurs during index search; users can iteratively guide the search through the selection of relevant examples. ImageRover employs a novel relevance feedback algorithm to determine the weighted combination of image similarity metrics appropriate for a particular query. ImageRover is available and running on the web site. :::::::::::::: 1997-015 :::::::::::::: Title: Admission Control and Scheduling for High-Performance WWW Servers Authors: Azer Bestavros and Naomi Katagai Date: August 21, 1997 Abstract: In this paper we examine a number of admission control and scheduling protocols for high-performance web servers based on a 2-phase policy for serving HTTP requests. The first ``registration'' phase involves establishing the TCP connection for the HTTP request and parsing/interpreting its arguments, whereas the second ``service'' phase involves the service/transmission of data in response to the HTTP request. By introducing a delay between these two phases, we show that the performance of a web server could be potentially improved through the adoption of a number of scheduling policies that optimize the utilization of various system components (e.g. memory cache and I/O). In addition, to its premise for improving the performance of a single web server, the delineation between the registration and service phases of an HTTP request may be useful for load balancing purposes on clusters of web servers. We are investigating the use of such a mechanism as part of the Commonwealth testbed being developed at Boston University. :::::::::::::: 1997-016 :::::::::::::: Title: Discovering Spatial Locality in WWW Access Patterns using Data Mining of Document Clusters in Server Logs Authors: Azer Bestavros Date: September 10, 1997 Abstract: In this paper, we introduce the notion of a ``document cluster'' in WWW space as a generalization of the notion of a ``cache line'' in linear memory address space. Through the analysis of Web server logs, we show evidence of the spatial locality of reference in WWW access patterns and present an implementation of an efficient data mining algorithm that discovers document clusters. We show preliminary simulation results that quantify the benefits of using document clusters for file allocation on server disks, as well as for purposes of prefetching into server cache/main memory. :::::::::::::: 1997-017 :::::::::::::: Title: To queue or not to queue?: When FCFS is better than PS in a distributed system Authors: Mor Harchol-Balter, Mark E. Crovella, and Cristina D. Murta Date: October 31, 1997 Abstract: We examine the question of whether to employ the first-come-first-served (FCFS) discipline or the processor-sharing (PS) discipline at the nodes in a distributed server system. We are interested in the case in which service times are drawn from a heavy-tailed distribution, and so have very high variability. Traditional wisdom in such a situation would prefer the PS discipline, because it allows small tasks to avoid being delayed behind large tasks in a queue. However, we show that system performance can actually be significantly better under FCFS queueing, if a particular kind of task assignment is used. By task assignment, we mean an algorithm that inspects incoming tasks and assigns them to hosts for service. The policy we propose is called SITA-E: Size Interval Task Assignment with Equal Load; it is a static policy that does not incorporate feedback knowledge of the state of the hosts. Surprisingly, under SITA-E, FCFS queueing typically outperforms the PS discipline by a factor of about two, as measured by mean waiting time and mean slowdown (waiting time of task divided by its service time). We analyze the FCFS/SITA-E policy and compare it to the processor-sharing case; in addition we compare it in simulation to a number of other policies. We show that the benefits of SITA-E are present even in small-scale distributed systems (four or more hosts), and that SITA-E can in many cases be more effective than a dynamic policy that takes into account the current load at each host. Finally we discuss issues in employing this policy in distributed Web servers. :::::::::::::: 1997-018 :::::::::::::: Title: Task Assignment in a Distributed System: Improving Performance by Unbalancing Load Authors: Mark Crovella, Mor Harchol-Balter, Cristina Murta Date: October 29, 1997 Abstract: We consider the problem of task assignment in a distributed system (such as a distributed Web server) in which task sizes are drawn from a heavy-tailed distribution. Many task assignment algorithms are based on the heuristic that balancing the load at the server hosts will result in optimal performance. We show this conventional wisdom is less true when the task size distribution is heavy-tailed (as is the case for Web file sizes). We introduce a new task assignment policy, called Size Interval Task Assignment with Variable Load (SITA-V). SITA-V purposely operates the server hosts at different loads, and directs smaller tasks to the lighter-loaded hosts. The result is that SITA-V provably decreases the mean task slowdown by significant factors (up to 1000 or more) where the more heavy-tailed the workload, the greater the improvement factor. We evaluate the tradeoff between improvement in slowdown and increase in waiting time in a system using SITA-V, and show conditions under which SITA-V represents a particularly appealing policy. We conclude with a discussion of the use of SITA-V in a distributed Web server, and show that it is attractive because it has a simple implementation which requires no communication from the server hosts back to the task router. :::::::::::::: 1997-019 :::::::::::::: Title: Color Region Grouping and Shape Recognition with Deformable Models Authors: Lifeng Liu and Stan Sclaroff Date: November 24, 1997 Abstract: A new deformable shape-based method for color region segmentation is described. The method includes two stages: over-segmentation using a traditional color region segmentation algorithm, followed by deformable model-based region merging via grouping and hypothesis selection. During the second stage, region merging and object identification are executed simultaneously. A statistical shape model is used to estimate the likelihood of region groupings and model hypotheses. The prior distribution on deformation parameters is precomputed using principal component analysis over a training set of region groupings. Once trained, the system autonomously segments deformed shapes from the background, while not merging them with similarly colored adjacent objects. Furthermore, the recovered parametric shape model can be used directly in object recognition and comparison. Experiments in segmentation and image retrieval are reported. :::::::::::::: 1997-020 :::::::::::::: Title: Head Tracking via Robust Registration in Texture Map Images Authors: Marco La Cascia, John Isidoro, and Stan Sclaroff Date: November 24, 1997 Abstract: A novel method for 3D head tracking in the presence of large head rotations and facial expression changes is described. Tracking is formulated in terms of color image registration in the texture map of a 3D surface model. Model appearance is recursively updated via image mosaicking in the texture map as the head orientation varies. The resulting dynamic texture map provides a stabilized view of the face that can be used as input to many existing 2D techniques for face recognition, facial expressions analysis, lip reading, and eye tracking. Parameters are estimated via a robust minimization procedure; this provides robustness to occlusions, wrinkles, shadows, and specular highlights. The system was tested on a variety of sequences taken with low quality, uncalibrated video cameras. Experimental results are reported. :::::::::::::: 1997-021 :::::::::::::: Title: Proceedings of the 18th Real-Time Systems Symposium WIP Session Author: Bestavros, Azer (Editor) Date: December 1, 1997 Abstract: This technical report includes 10 short papers presented during the WIP session of the 18th Real-Time Systems Symposium, held in Washington DC on December 3-5, 1997. The title and authors are included below. ------ (1) CPU Reservations and Time Constraints: Efficient, Predictable Scheduling of Independent Activities Michael B. Jones, Microsoft Research, Microsoft Corporation Daniela Rosu and Marcel-Catalin Rosu, Georgia Institute of Technology Abstract: Workstations and personal computers are increasingly being used for applications with real-time characteristics such as speech understanding and synthesis, media computations and I/O, and animation, often concurrently executed with traditional non-real-time workloads. This paper presents a system that can schedule multiple independent activities so that: - activities can obtain minimum guaranteed execution rates with application-specified reservation granularities via CPU Reservations, - CPU Reservations, which are of the form "reserve X units of time out of every Y units", provide not just an average case execution rate of X/Y over long periods of time, but the stronger guarantee that from any instant of time, by Y time units later, the activity will have executed for at least X time units, - applications can use Time Constraints to schedule tasks by deadlines, with on-time completion guaranteed for tasks with accepted constraints, and - both CPU Reservations and Time Constraints are implemented very efficiently. In particular, - CPU scheduling overhead is bounded by a constant and is not a function of the number of schedulable tasks. Other key scheduler properties are: - activities cannot violate other activities' guarantees, - time constraints and CPU reservations may be used together, separately, or not at all (which gives a round-robin schedule), with well-defined interactions between all combinations, and - spare CPU time is fairly shared among all activities. The Rialto operating system, developed at Microsoft Research, achieves these goals by using a precomputed schedule, which is the fundamental basis of this work. ------ (2) Characterizing Group Communication Middleware for a Real-time Distributed System L. M. Feeney, P. Bernadat, F. Travostino The Open Group Research Institute Abstract: This paper presents our current work in characterizing the behavior of a real-time dependable distributed system, which must exhibit predictable behavior under load and in the presence of partial failures. We focus on measuring the end-to-end properties of the middleware which implements the real-time process group service, specifically its membership and message latency. The paper also describes the tools and techniques we have developed, along with some of the practical issues that arise in instrumenting a real-time distributed system. ------ (3) Real-Time Monitoring of the EIVIS Distributed Video-Server on Windows NT M. Gergeleit and M. Mock GMD - German National Research Center for Information Technology Abstract: JewelNT is a fine-grained, trace-based real-time monitoring tool for Windows NT. It hooks into the NT kernel and provides full information about NT?s thread scheduling combined with application-level timing information. JewelNT allows monitoring a number of NT machines remotely controlled from one central desktop. JewelNT has been initially developed for the evaluation and performance tuning of the distributed EIVIS video server, a European ESPRIT project. ------ (4) Achieving Predictability and Responsiveness of Fault Recovery Operations in Real-Time Systems. Pedro Mejia-Alvarez, CINVESTAV-IPN, Seccion de Computacion, Mexico Juan A. de la Puente, Universidad Politecnica de Madrid, Spain. Abstract: The dependability of real-time software can be improved by enhancing the robustness of the scheduler in predicting and controlling the occurrence of timing failures during recovery. This may be achieved by developing strategies which allow the scheduler to dynamically control the manner in which real-time applications tasks and its time-critical recovery operations are handled in time. In this paper, an scheme is presented to provide scheduling guarantees for a variety of fault tolerant techniques. Bounds of execution are developed and an study case examined to analyze these techniques in its ability to recover from transient faulty situations. A criterion for providing responsiveness for fault-tolerant scheduler is discussed and some approaches were developed. A responsiveness table RTAB, has been developed for assisting the scheduler during recovery of transient faults. This table is based on different criterion for responsiveness of recovery. An analytical characterization of the table, for supporting on-line scheduling has been developed. Some of the issues involved in using this table to support run-time scheduling decisions are illustrated with a hypothetical application example. The advantages of the RTAB approach over previously proposed scheduling policies for aperiodic tasks include the support for run-time customization and guaranteed scheduling stability during recovery. ------ (5) Compositional Reasoning about Real-Time Asynchronous Communication with Time-Outs D. Peticolas and F.A. Stomp, University of California, Davis Abstract: This paper describes ongoing work in developing a compositional trace-based semantics and proof system for a real-time language. The semantics models distributed processes communicating over asynchronous FIFO communication channels. Sending processes can specify time-out periods for individual messages. Messages not received within their time-out period are `lost'. Program behavior is modeled as traces of events, including events (such as asynchronous messages) which occur after termination. The proof system uses specification triples with explicit variables for time and program traces. ------ (6) Exploring Consistency of Read-Only Transactions in Real-Time Systems Kwok-Wa Lam, Sang H. Son* and Sheung-Lun Hung City University of Hong Kong, Hong Kong. University of Virginia, U.S.A. Abstract: In this paper, we describe our current work on exploring the consistency of read-only transactions (ROT) in real-time systems. A ROT is a transaction that only reads, but does not update any data items. Since there is a significant proportion of ROTs in several real-time systems, it is important to investigate how to process ROTs efficiently with separate algorithms. We identify three different consistency requirements for ROTs. Particularly, we define a weaker form of consistency, view consistency, which allows ROTs to perceive different serialization order of update transactions, thus permitting non-serializable execution of transactions. However, ROTs are still ensured to see consistent data. Based on view consistency, we present two algorithms which let ROTs read the most recent and consistent data without interfering with update transactions. The recency of data read by a ROT could be important in some real-time applications. ------ (7) Dynamic Timing Constraints - Relaxing Overconstraining Specifications of Real-Time Systems Gerhard Fohler Malardalens University, S-72123 Vasteras, Sweden Abstract: Standard timing constraints, such as deadlines and periods can overconstrain specifications and lack expressive power. Only few tasks have "natural" periods and deadlines. Most are artifacts, derived during system design. Knowledge of more flexibility is abandoned in the process, thus overconstraining the specification. In this paper, we propose dynamic timing constraints, which represent conditions for the temporal correctness rather than fixed values for constraints such as period and deadline. This is achieved by so-called timing entities, which combine a functional unit, such as a task, with a feasibility function for testing the feasibility of the timing of the unit. This representation allows the system specification to provide information about feasibility and various options of time related design decisions. We outline how dynamic timing constraints can be used with standard scheduling algorithms, indicate modifications to these algorithms, and novel approaches fully utilizing the benefits of dynamic timing constraints. ------ (8) Exploring the Importance of Preprocessing Operations in Real-Time Multiprocessor Scheduling Jan Jonsson, Chalmers University of Technology, Sweden Abstract: Recent real-time scheduling research has mainly been focused on generating mature scheduling theories. Therefore, the important field of preprocessing operations has been left fairly unexplored. Most real-time scheduling techniques in use today assume that the constraints (e.g. local task deadlines, degree of task replication, or task clustering) on the constituent tasks are entirely known beforehand. In such cases, no preprocessing is typically applied. However, when the constraints are relaxed, preprocessing operations can be applied for increasing the likelihood of succeeding with a scheduling attempt. In addition, preprocessing operations are vital in quality-of-service negotiations for adaptive real-time systems since changing some of the task constraints may result in a higher system reward. In this paper, we define a set of preprocessing operations that we believe is representative for real-time multiprocessor scheduling. We also give a rationale for using these operations, and present results from some preliminary work that corroborate our conjecture. In conjunction to this, we present an evaluation framework for objective studies of different preprocessing operations. ------ (9) Compiler Support for Non-intrusive Monitoring and Debugging of Real-Time Systems in the CRL Environment P. V. Petrov, A. D. Stoyen New Jersey Institute of Technology Abstract: In this work we approach the problem of monitoring and debugging real-time distributed systems by performing static analysis and transformations to eliminate obtrusion to the monitored system. Our work extends the CRL testbed compiler and run-time environment to support monitoring and logging for the purpose of post-mortem debugging. The main contribution of this work is the innovative use of compiler transformations and idle slots for monitoring and logging. ------ (10) Optimization of Real-Time MRL Rule-Based Systems with the EQL Optimizer Albert Mo Kim Cheng University of Houston--University Park Houston, Texas, USA Abstract: In our earlier work, we developed an efficient algorithm for optimizing a class of EQL rule-based systems so that they can meet specified response time constraints. In this paper, we show that this EQL optimizer with minor modifications can be used to optimize a class of real-time MRL rule-based systems. As a more expressive superset of EQL, MRL allows existentially quantified as well as universally quantified variables (simple or macro), making it comparable in expressive power to that of OPS5 and CLIPS (two of the most popular commercially available rule-based system languages) while maintaining predictable response time behavior. :::::::::::::: 1997-022 :::::::::::::: Title: A Framework for Local Anonymity in the Internet Author: David M. Martin Jr. Date: December 1997 Abstract: We describe and evaluate options for providing anonymous IP service, argue for the further investigation of local anonymity, and sketch a framework for the implementation of locally anonymous networks. :::::::::::::: 1998-001 :::::::::::::: Title: Aggregating Congestion Information Over Sequences of TCP Connections Authors: Azer Bestavros and Olivier Hartmann Date: January 5, 1998 Abstract: In this paper we present an extension of the TCP stack that allows a sequence of TCP connections between the same machines to share the congestion window. Our Linux implementation of this scenario shows significant improvement in performance, particularly when the individual connections are short-lived. Such a behavior is common on the web, due to the nature of the HTTP protocol and the distribution of file sizes. :::::::::::::: 1998-002 :::::::::::::: Author: Peter Gacs Title: Reliable Cellular Automata with Self-Organization Date: Jan. 15, 1998 Abstract: In a probabilistic cellular automaton in which all local transitions have positive probability, the problem of keeping a bit of information for more than a constant number of steps is nontrivial, even in an infinite automaton. Still, there is a solution in 2 dimensions, and this solution can be used to construct a simple 3-dimensional discrete-time universal fault-tolerant cellular automaton. This technique does not help much to solve the following problems: remembering a bit of information in 1 dimension; computing in dimensions lower than 3; computing in any dimension with non-synchronized transitions. Our more complex technique organizes the cells in blocks that perform a reliable simulation of a second (generalized) cellular automaton. The cells of the latter automaton are also organized in blocks, simulating even more reliably a third automaton, etc. Since all this (a possibly infinite hierarchy) is organized in ``software'', it must be under repair all the time from damage caused by errors. A large part of the problem is essentially self-stabilization recovering from a mess of arbitrary-size and content caused by the faults. The present paper constructs an asynchronous one-dimensional fault-tolerant cellular automaton, with the further feature of ``self-organization''. The latter means that unless a large amount of input information must be given, the initial configuration can be chosen to be periodical with a small period. :::::::::::::: 1998-003 :::::::::::::: Title: Distributed Packet Rewriting and its Application to Scalable Server Architectures Date: Feb 1, 1998 Authors: Azer Bestavros, Mark Crovella, Jun Liu, and David Martin Abstract: To construct high performance Web servers, system builders are increasingly turning to distributed designs. An important challenge that arises in distributed Web servers is the need to direct incoming connections to individual hosts. Previous methods for connection routing have employed a centralized node which handles all incoming requests. In contrast, we propose a distributed approach, called Distributed Packet Rewriting (DPR), in which all hosts of the distributed system participate in connection routing. We argue that this approach promises better scalability and fault-tolerance than the centralized approach. We describe our implementation of four variants of DPR and compare their performance. We show that DPR provides performance comparable to centralized alternatives, measured in terms of throughput and delay under the SPECweb96 benchmark. Finally, we argue that DPR is particularly attractive both for small scale systems and for systems following the emerging trend toward increasingly intelligent I/O subsystems. :::::::::::::: 1998-004 :::::::::::::: Title: Combining Textual and Visual Cues for Content-based Image Retrieval on the World Wide Web Authors: Marco La Cascia, Sarathendu Sethi, and Stan Sclaroff Date: February 9, 1998 Abstract: Some WWW image engines allow the user to form a query in terms of text keywords. To build the image index, keywords are extracted heuristically from HTML documents containing each image, and/or from the image URL and file headers. Unfortunately, text-based image engines have merely retro-fitted standard SQL database query methods, and it is difficult to include images cues within such a framework. On the other hand, visual statistics ({\em e.g.}, color histograms) are often insufficient for helping users find desired images in a vast WWW index. By truly unifying textual and visual statistics, one would expect to get better results than either used separately. In this paper, we propose an approach that allows the combination of visual statistics with textual statistics in the vector space representation commonly used in query by image content systems. Text statistics are captured in vector form using latent semantic indexing (LSI). The LSI index for an HTML document is then associated with each of the images contained therein. Visual statistics ({\em e.g.}, color, orientedness) are also computed for each image. The LSI and visual statistic vectors are then combined into a single index vector that can be used for content-based search of the resulting image database. By using an integrated approach, we are able to take advantage of possible statistical couplings between the topic of the document (latent semantic content) and the contents of images (visual statistics). This allows improved performance in conducting content-based search. This approach has been implemented in a WWW image search engine prototype. :::::::::::::: 1998-005 :::::::::::::: Title: Preserving Bandwidth Through A Lazy Packet Discard Policy in ATM Networks Authors: Gitae Kim and Azer Bestavros Date: February 9, 1998 Abstract: A number of recent studies have pointed out that TCP's performance over ATM networks tends to suffer, especially under congestion and switch buffer limitations. Switch-level enhancements and link-level flow control have been proposed to improve TCP's performance in ATM networks. Seletive Cell Discard (SCD) and Early Packet Discard (EPD) ensure that partial packets are discarded from the network "as early as possible", thus reducing wasted bandwidth. While such techniques improve the achievable throughput, their effectiveness tends to degrade in multi-hop networks. In this paper, we introduce Lazy Packet Discard (LPD), an AAL-level enhancement that improves effective throughput, reduces response time, and minimizes wasted bandwidth for TCP/IP over ATM. In contrast to the SCD and EPD policies, LPD delays as much as possible the removal from the network of cells belonging to a partially communicated packet. We outline the implementation of LPD and show the performance advantage of TCP/LPD, compared to plain TCP and TCP/EPD through analysis and simulations. :::::::::::::: 1998-006 :::::::::::::: Title: Active Voodoo Dolls: A Vision Based Input Device for Non-rigid Control Authors: John Isidoro and Stan Sclaroff Date: 16 February 1998 Abstract: A vision based technique for non-rigid control is presented that can be used for animation and video game applications. The user grasps a soft, squishable object in front of a camera that can be moved and deformed in order to specify motion. Active Blobs, a non-rigid tracking technique is used to recover the position, rotation and non-rigid deformations of the object. The resulting transformations can be applied to a texture mapped mesh, thus allowing the user to control it interactively. Our use of texture mapping hardware allows us to make the system responsive enough for interactive animation and video game character control. :::::::::::::: 1998-007 :::::::::::::: Title: Improved Tracking of Multiple Humans with Trajectory Predcition and Occlusion Modeling Authors: Romer Rosales and Stan Sclaroff Date: March 2, 1998 Abstract: A combined 2D, 3D approach is presented that allows for robust tracking of moving bodies in a given environment as observed via a single, uncalibrated video camera. Tracking is robust even in the presence of occlusions. Low-level features are often insufficient for detection, segmentation, and tracking of non-rigid moving objects. Therefore, an improved mechanism is proposed that combines low-level (image processing) and mid-level (recursive trajectory estimation) information obtained during the tracking process. The resulting system can segment and maintain the tracking of moving objects before, during, and after occlusion. At each frame, the system also extracts a stabilized coordinate frame of the moving objects. This stabilized frame is used to resize and resample the moving blob so that it can be used as input to motion recognition modules. The approach enables robust tracking without constraining the system to know the shape of the objects being tracked beforehand; although, some assumptions are made about the characterstics of the shape of the objects, and how they evolve with time. Experiments in tracking moving people are described. :::::::::::::: 1998-008 :::::::::::::: Title: Determining Acceptance Possibility for a Quantum Computation is Hard for PH Author: Stephen Fenner, University of Southern Maine Frederic Green, Clark University Steven Homer, Boston University Randall Pruim, Boston University Date: April 2, 1998 Abstract: It is shown that determining whether a quantum computation has a non-zero probability of accepting is at least as hard as the polynomial time hierarchy. This hardness result also applies to determining in general whether a given quantum basis state appears with nonzero amplitude in a superposition, or whether a given quantum bit has positive expectation value at the end of a quantum computation. :::::::::::::: 1998-009 :::::::::::::: Title: Slack Stealing Job Admission Control Scheduling Authors: Alia Atlas and Azer Bestavros Date: May 2, 1998 Abstract: In this paper, we present Slack Stealing Job Admission Control (SSJAC)---a methodology for scheduling periodic firm-deadline tasks with variable resource requirements, subject to controllable Quality of Service (QoS) constraints. In a system that uses Rate Monotonic Scheduling, SSJAC augments the slack stealing algorithm of Thuel et al with an admission control policy to manage the variability in the resource requirements of the periodic tasks. This enables SSJAC to take advantage of the 31\% of utilization that RMS cannot use, as well as any utilization unclaimed by jobs that are not admitted into the system. Using SSJAC, each task in the system is assigned a resource utilization threshold that guarantees the minimal acceptable QoS for that task (expressed as an upper bound on the rate of missed deadlines). Job admission control is used to ensure that (1) only those jobs that will complete by their deadlines are admitted, and (2) tasks do not interfere with each other, thus a job can only monopolize the slack in the system, but not the time guaranteed to jobs of other tasks. We have evaluated SSJAC against RMS and Statistical RMS (SRMS). Ignoring overhead issues, SSJAC consistently provides better performance than RMS in overload, and, in certain conditions, better performance than SRMS. In addition, to evaluate optimality of SSJAC in an absolute sense, we have characterized the performance of SSJAC by comparing it to an inefficient, yet optimal scheduler for task sets with harmonic periods. :::::::::::::: 1998-010 :::::::::::::: Title: Statistical Rate Monotonic Scheduling Authors: Alia Atlas and Azer Bestavros Date: May 2, 1998 Abstract: In this paper we present Statistical Rate Monotonic Scheduling (SRMS), a generalization of the classical RMS results of Liu and Layland that allows scheduling periodic tasks with highly variable execution times and statistical QoS requirements. Similar to RMS, SRMS has two components: a feasibility test and a scheduling algorithm. The feasibility test for SRMS ensures that using SRMS' scheduling algorithms, it is possible for a given periodic task set to share a given resource (e.g. a processor, communication medium, switching device, etc.) in such a way that such sharing does not result in the violation of any of the periodic tasks QoS constraints. The SRMS scheduling algorithm incorporates a number of unique features. First, it allows for fixed priority scheduling that keeps the tasks' value (or importance) independent of their periods. Second, it allows for job admission control, which allows the rejection of jobs that are not guaranteed to finish by their deadlines as soon as they are released, thus enabling the system to take necessary compensating actions. Also, admission control allows the preservation of resources since no time is spent on jobs that will miss their deadlines anyway. Third, SRMS integrates reservation-based and best-effort resource scheduling seamlessly. Reservation-based scheduling ensures the delivery of the minimal requested QoS; best-effort scheduling ensures that unused, reserved bandwidth is not wasted, but rather used to improve QoS further. Fourth, SRMS allows a system to deal gracefully with overload conditions by ensuring a fair deterioration in QoS across all tasks---as opposed to penalizing tasks with longer periods, for example. Finally, SRMS has the added advantage that its schedulability test is simple and its scheduling algorithm has a constant overhead in the sense that the complexity of the scheduler is not dependent on the number of the tasks in the system. We have evaluated SRMS against a number of alternative scheduling algorithms suggested in the literature (e.g. RMS and slack stealing), as well as refinements thereof, which we describe in this paper. Consistently throughout our experiments, SRMS provided the best performance. In addition, to evaluate the optimality of SRMS, we have compared it to an inefficient, yet optimal scheduler for task sets with harmonic periods. :::::::::::::: 1998-011 :::::::::::::: Title: Multiplexing VBR Traffic Flows with Guaranteed Application-level QoS Using Statistical Rate Monotonic Scheduling Authors: Alia Atlas and Azer Bestavros Date: May 2, 1998 Abstract: Quality of Service (QoS) guarantees are required by an increasing number of applications to ensure a minimal level of fidelity in the delivery of application data units through the network. Application-level QoS does not necessarily follow from any transport-level QoS guarantees regarding the delivery of the individual cells (e.g. ATM cells) which comprise the application's data units. The distinction between application-level and transport-level QoS guarantees is due primarily to the fragmentation that occurs when transmitting large application data units (e.g. IP packets, or video frames) using much smaller network cells, whereby the partial delivery of a data unit is useless; and, bandwidth spent to partially transmit the data unit is wasted. The data units transmitted by an application may vary in size while being constant in rate, which results in a variable bit rate (VBR) data flow. That data flow requires QoS guarantees. Statistical multiplexing is inadequate, because no guarantees can be made and no firewall property exists between different data flows. In this paper, we present a novel resource management paradigm for the maintenance of application-level QoS for VBR flows. Our paradigm is based on Statistical Rate Monotonic Scheduling (SRMS), in which (1) each application generates its variable-size data units at a fixed rate, (2) the partial delivery of data units is of no value to the application, and (3) the QoS guarantee extended to the application is the probability that an arbitrary data unit will be successfully transmitted through the network to/from the application. :::::::::::::: 1998-012 :::::::::::::: Title: The Statistical Rate Monotonic Scheduling Workbench Authors: Alia Atlas and Azer Bestavros Date: May 2, 1998 Abstract: The SRMS Workbench is a software system developed to demonstrate the notion of Statistical QoS employed in SRMS [AtlasBestavros:1998]. The SRMS Workbench includes: (1) the SRMS schedulability analyzer (QoS negotiator), and (2) a SRMS simulator (Basic SRMS + all extensions). These two components are packaged into a Java Applet that can be executed remotely on any Java-capable Internet browser. For comparison, other scheduling algorithms, including RMS [LiuLayland:1973] and SSJAC [AtlasBestavros:1998] are included. Through a simple GUI, the SRMS Workbench allows users to specify a set of periodic tasks, each with (a) its own period, (b) the distributional characteristics of its periodic resource requirements (e.g. Poisson, Pareto, Normal, Exponential, Gamma, etc.), (c) its desired QoS as a lower bound on the percentage of deadlines to be met, and (d) a criticality/importance index indicating the value of the task (relative to other tasks in the task set). Once the task set is specified, the SRMS Workbench allows the user to check for schedulability under SRMS. If the task set is schedulable, the SRMS Workbench generates the appropriate allowance for each task and allows the user to create an animated simulation of the task system, which can be executed and profiled. If the task set is not schedulable, the SRMS Workbench informs the user of that fact and suggests (as part of the QoS negotiation) an alternative set of feasible QoS requirements that reflects the specified criticality/importance index of the tasks in the task set. The SRMS Workbench is available on the Web at http://www.cs.bu.edu/groups/realtime/SRMSworkbench :::::::::::::: 1998-013 :::::::::::::: Title: Design and Implementation of SRMS in Kurt Linux Authors: Alia Atlas and Azer Bestavros Date: September 2, 1998 Abstract: Statistical Rate Monotonic Scheduling (SRMS) is a generalization of the classical RMS results of Liu and Layland \cite{ll:sched} for periodic tasks with highly variable execution times and statistical QoS requirements. The main tenet of SRMS is that the variability in task resource requirements could be smoothed through aggregation to yield guaranteed QoS. This aggregation is done over time for a given task and across multiple tasks for a given period of time. Similar to RMS, SRMS has two components: a feasibility test and a scheduling algorithm. SRMS feasibility test ensures that it is possible for a given periodic task set to share a given resource without violating any of the statistical QoS constraints imposed on each task in the set. The SRMS scheduling algorithm consists of two parts: a job admission controller and a scheduler. The SRMS scheduler is a simple, preemptive, fixed-priority scheduler. The SRMS job admission controller manages the QoS delivered to the various tasks through admit/reject and priority assignment decisions. In particular, it ensures the important property of task isolation, whereby tasks do not infringe on each other. In this paper we present the design and implementation of SRMS within the KURT Linux Operating System \cite{hspn:kurt,sphan:cots,srin:kurt}. KURT Linux supports conventional tasks as well as real-time tasks. It provides a mechanism for transitioning from normal Linux scheduling to a mixed scheduling of conventional and real-time tasks, and to a focused mode where only real-time tasks are scheduled. We overview the technical issues that we had to overcome in order to integrate SRMS into KURT Linux and present the API we have developed for scheduling periodic real-time tasks using SRMS. :::::::::::::: 1998-014 :::::::::::::: Title: An Omniscient Scheduling Oracle for Systems with Harmonic Periods Authors: Alia Atlas and Azer Bestavros Date: September 2, 1998 Abstract: Most real-time scheduling problems are known to be NP-complete. To enable accurate comparison between the schedules of heuristic algorithms and the optimal schedule, we introduce an omniscient oracle. This oracle provides schedules for periodic task sets with harmonic periods and variable resource requirements. Three different job value functions are described and implemented. Each corresponds to a different system goal. The oracle is used to examine the performance of different on-line schedulers under varying loads, including overload. We have compared the oracle against Rate Monotonic Scheduling, Statistical Rate Monotonic Scheduling, and Slack Stealing Job Admission Control Scheduling. Consistently, the oracle provides an upper bound on performance for the metric under consideration. :::::::::::::: 1998-015 :::::::::::::: Title: Principality and Decidable Type Inference for Finite-Rank Intersection Types Authors: A. J. Kfoury and J. B. Wells Date: 6 November 1998 Abstract: Principality of typings is the property that for each typable term, there is a typing from which all other typings are obtained via some set of operations. Type inference is the problem of finding a typing for a given term, if possible. We define an intersection type system which has principal typings and types exactly the strongly normalizable $\lambda$-terms. More interestingly, every finite-rank restriction of this system (using Leivant's first notion of rank) has principal typings and also has decidable type inference. This is in contrast to System~F where the finite rank restriction for every finite rank at 3 and above has neither principal typings nor decidable type inference. This is also in contrast to earlier presentations of intersection types where the status of these properties is not known for the finite-rank restrictions at 3 and above. Furthermore, the notion of principal typings for our system involves only one operation, substitution, rather than several operations (not all substitution-based) as in earlier presentations of principality for intersection types (of unrestricted rank). A unification-based type inference algorithm is presented using a new form of unification, $\beta$-unification. :::::::::::::: 1998-016 :::::::::::::: Title: A Performance Evaluation of Hyper Text Transfer Protocols Authors: Paul Barford and Mark Crovella Date: 10/23/98 Abstract: Version 1.1 of the Hyper Text Transfer Protocol (HTTP) was principally developed as a means for reducing both document transfer latency and network traffic. The rationale for the performance enhancements in HTTP/1.1 is based on the assumption that the network is the bottleneck in Web transactions. In practice, however, the Web server can be the primary source of document transfer latency. In this paper, we characterize and compare the performance of HTTP/1.0 and HTTP/1.1 in terms of throughput at the server and transfer latency at the client. Our approach is based on considering a broader set of bottlenecks in an HTTP transfer; we examine how bottlenecks in the network, CPU, and in the disk system affect the relative performance of HTTP/1.0 versus HTTP/1.1. We show that the network demands under HTTP/1.1 are somewhat lower than HTTP/1.0, and we quantify those differences in terms of packets transferred, server congestion window size and data bytes per packet. We show that when the CPU is the bottleneck, there is relatively little difference in performance between HTTP/1.0 and HTTP/1.1. Surprisingly, we show that when the disk system is the bottleneck, performance using HTTP/1.1 can be much worse than with HTTP/1.0. Based on these observations, we suggest a connection management policy for HTTP/1.1 that can improve throughput, decrease latency, and keep network traffic low when the disk system is the bottleneck. :::::::::::::: 1998-017 :::::::::::::: Title: Deformable Shape Detection and Description via Model-Based Region Grouping Authors: Lifeng Liu and Stan Sclaroff Date: December 4, 1998 Abstract: A method for deformable shape detection and recognition is described. Deformable shape templates are used to partition the image into a globally consistent interpretation, determined in part by the minimum description length principle. Statistical shape models enforce the prior probabilities on global, parametric deformations for each object class. Once trained, the system autonomously segments deformed shapes from the background, while not merging them with adjacent objects or shadows. The formulation can be used to group image regions based on any image homogeneity predicate; e.g., texture, color, or motion. The recovered shape models can be used directly in object recognition. Experiments with color imagery are reported. Note: This TR supercedes BUCS-TR-1997-019 :::::::::::::: 1998-018 :::::::::::::: Title: Fast, Reliable Head Tracking under Varying Illumination Date: December 4, 1998 Authors: Marco La Cascia and Stan Sclaroff Abstract: An improved technique for 3D head tracking under varying illumination conditions is proposed. The head is modeled as a texture mapped cylinder. Tracking is formulated as an image registration problem in the cylinder's texture map image. To solve the registration problem in the presence of lighting variation and head motion, the residual error of registration is modeled as a linear combination of texture warping templates and orthogonal illumination templates. Fast and stable on-line tracking is then achieved via regularized, weighted least squares minimization of the registration error. The regularization term tends to limit potential ambiguities that arise in the warping and illumination templates. It enables stable tracking over extended sequences. Tracking does not require a precise initial fit of the model; the system is initialized automatically using a simple 2-D face detector. The only assumption is that the target is facing the camera in the first frame of the sequence. The warping templates are computed at the first frame of the sequence. Illumination templates are precomputed off-line over a training set of face images collected under varying lighting conditions. Experiments in tracking are reported. :::::::::::::: 1998-019 :::::::::::::: Title: 3D Trajectory Recovery for Tracking Multiple Objects and Trajectory Guided Recognition of Actions Date: December 4, 1998 Authors: Romer Rosales and Stan Sclaroff Abstract: A mechanism is proposed that integrates low-level (image processing), mid-level (recursive 3D trajectory estimation), and high-level (action recognition) processes. It is assumed that the system observes multiple moving objects via a single, uncalibrated video camera. A novel extended Kalman filter formulation is used in estimating the relative 3D motion trajectories up to a scale factor. The recursive estimation process provides a prediction and error measure that is exploited in higher-level stages of action recognition. Conversely, higher-level mechanisms provide feedback that allows the system to reliably segment and maintain the tracking of moving objects before, during, and after occlusion. The 3D trajectory, occlusion, and segmentation information are utlized in extracting stabilized views of the moving object. Trajectory-guided recognition (TGR) is proposed as a new and efficient method for adaptive classification of action. The TGR approach is demonstrated using ``motion history images'' that are then recognized via a mixture of Gaussian classifier. The system was tested in recognizing various dynamic human outdoor activities; e.g., running, walking, roller blading, and cycling. Experiments with synthetic data sets are used to evaluate stability of the trajectory estimator with respect to noise. :::::::::::::: 1998-020 :::::::::::::: Title: Recognition of Human Action Using Moment-Based Features Author: Romer Rosales Date: December 4, 1998 Abstract: The performance of different classification approaches is evaluated using a view-based approach for motion representation. The view-based approach uses computer vision and image processing techniques to register and process the video sequence. Two motion representations called Motion Energy Images and Motion History Image are then constructed. These representations collapse the temporal component in a way that no explicit temporal analysis or sequence matching is needed. Statistical descriptions are then computed using moment-based features and dimensionality reduction techniques. For these tests, we used 7 Hu moments, which are invariant to scale and translation. Principal Components Analysis is used to reduce the dimensionality of this representation. The system is trained using different subjects performing a set of examples of every action to be recognized. Given these samples, K-nearest neighbor, Gaussian, and Gaussian mixture classifiers are used to recognize new actions. Experiments are conducted using instances of eight human actions (i.e., eight classes) performed by seven different subjects. Comparisons in the performance among these classifiers under different conditions are analyzed and reported. Our main goals are to test this dimensionality-reduced representation of actions, and more importantly to use this representation to compare the advantages of different classification approaches in this recognition task. :::::::::::::: 1998-021 :::::::::::::: Leonid Sigal, Vassilis Athitsos, and Stan Sclaroff. "Estimation and Prediction of Evolving Color Distributions for Skin Segmentation Under Varying Illumination" Abstract: A novel approach for real-time skin segmentation in video sequences is described. The approach enables reliable skin segmentation despite wide variation in illumination during a tracking sequence. An explicit second order Markov model is used to predict the evolution of the skin color (HSV) histogram over time. Histograms are dynamically updated based on feedback from the current segmentation and based on predictions of the Markov model. The evolution of the skin color distribution at each frame is parameterized by translation, scaling and rotation in color space. Consequent changes in the geometric parameterization of the distribution are propagated by warping and resampling of the histogram. The parameters of the discrete-time dynamic Markov model are estimated using Maximum Likelihood Estimation, and also evolve over time. The likelihood of each pixel being skin or background can be measured directly from the probability density function of the histogram. Connected components analysis and size filtering are used to extract the patches of skin from the segmented image. Segmentation accuracy is evaluated using labeled ground-truth video sequences taken from popular movies, and results are encouraging. :::::::::::::: 1998-022 :::::::::::::: 3.) Stan Sclaroff and John Isidoro, "Active Blobs: Region-Based, Deformable Appearance Models." :::::::::::::: 1998-023 :::::::::::::: Title: Changes in Web Client Access Patterns: Characteristics and Caching Implications Authors: Paul Barford, Azer Bestavros, Adam Bradley, and Mark Crovella Date: December 4, 1998 Abstract: Understanding the nature of the workloads and system demands created by users of the World Wide Web is crucial to properly designing and provisioning Web services. Previous measurements of Web client workloads have been shown to exhibit a number of characteristic features; however, it is not clear how those features may be changing with time. In this study we compare two measurements of Web client workloads separated in time by three years, both captured from the same computing facility at Boston University. The older dataset, obtained in 1995, is well-known in the research literature and has been the basis for a wide variety of studies. The newer dataset was captured in 1998 and is comparable in size to the older dataset. The new dataset has the drawback that the collection of users measured may no longer be representative of general Web users; however using it has the advantage that many comparisons can be drawn more clearly than would be possible using a new, different source of measurement. Our results fall into two categories. First we compare the statistical and distributional properties of Web requests across the two datasets. This serves to reinforce and deepen our understanding of the characteristic statistical properties of Web client requests. We find that the kinds of distributions that best describe document sizes have not changed between 1995 and 1998, although specific values of the distributional parameters are different. Second, we explore the question of how the observed differences in the properties of Web client requests, particularly the popularity and temporal locality properties, affect the potential for Web file caching in the network. We find that for the computing facility represented by our traces between 1995 and 1998, (1) the benefits of using size-based caching policies have diminished; and (2) the potential for caching requested files in the network has declined. :::::::::::::: 1999-001 :::::::::::::: Title: Load Balancing a Cluster of Web Servers using Distributed Packet Rewriting Authors: Luis Aversa and Azer Bestavros Date: January 6, 1999 Abstract: In this paper, we propose and evaluate an implementation of a prototype scalable web server. The prototype consists of a load-balanced cluster of hosts that collectively accept and service TCP connections. The host IP addresses are advertised using the Round Robin DNS technique, allowing any host to receive requests from any client. Once a client attempts to establish a TCP connection with one of the hosts, a decision is made as to whether or not the connection should be redirected to a different host---namely, the host with the lowest number of established connections. We use the low-overhead Distributed Packet Rewriting (DPR) technique to redirect TCP connections. In our prototype, each host keeps information about connections in hash tables and linked lists. Every time a packet arrives, it is examined to see if it has to be redirected or not. Load information is maintained using periodic broadcasts amongst the cluster hosts. :::::::::::::: 1999-002 :::::::::::::: Title: Trajectory Guided Tracking and Recognition of Actions Authors: Romer Rosales and Stan Sclaroff Date: March 9, 1999 Abstract: A combined 2D, 3D approach is presented that allows for robust tracking of moving people and recognition of actions. It is assumed that the system observes multiple moving objects via a single, uncalibrated video camera. Low-level features are often insufficient for detection, segmentation, and tracking of non-rigid moving objects. Therefore, an improved mechanism is proposed that integrates low-level (image processing), mid-level (recursive 3D trajectory estimation), and high-level (action recognition) processes. A novel extended Kalman filter formulation is used in estimating the relative 3D motion trajectories up to a scale factor. The recursive estimation process provides a prediction and error measure that is exploited in higher-level stages of action recognition. Conversely, higher-level mechanisms provide feedback that allows the system to reliably segment and maintain the tracking of moving objects before, during, and after occlusion. The 3D trajectory, occlusion, and segmentation information are utilized in extracting stabilized views of the moving object that are then used as input to action recognition modules. Trajectory-guided recognition (TGR) is proposed as a new and efficient method for adaptive classification of action. The TGR approach is demonstrated using ``motion history images'' that are then recognized via a mixture-of-Gaussians classifier. The system was tested in recognizing various dynamic human outdoor activities: running, walking, roller blading, and cycling. Experiments with real and synthetic data sets are used to evaluate stability of the trajectory estimator with respect to noise. (This technical report supercedes TR's [98-020] and [98-007]) :::::::::::::: 1999-003 :::::::::::::: Title: Connection Scheduling in Web Servers Authors: Mark E. Crovella, Robert Frangioso, and Mor Harchol-Balter Date: March 31, 1999 Abstract: Under high loads, a Web server may be servicing many hundreds of connections concurrently. In traditional Web servers, the question of the order in which concurrent connections are serviced has been left to the operating system. In this paper we ask whether servers might provide better service by using non-traditional service ordering. In particular, for the case when a Web server is serving static files, we examine the costs and benefits of a policy that gives preferential service to short connections. We start by assessing the scheduling behavior of a commonly used server (Apache running on Linux) with respect to connection size and show that it does not appear to provide preferential service to short connections. We then examine the potential performance improvements of a policy that does favor short connections (shortest-connection-first). We show that mean response time can be improved by factors of four or five under shortest-connection-first, as compared to an (Apache-like) size-independent policy. Finally we assess the costs of shortest-connection-first scheduling in terms of unfairness ( i.e., the degree to which long connections suffer). We show that under shortest-connection-first scheduling, long connections pay very little penalty. This surprising result can be understood as a consequence of heavy-tailed Web server workloads, in which most connections are small, but most server load is due to the few large connections. We support this explanation using analysis. :::::::::::::: 1999-004 :::::::::::::: Title: Measuring Web Performance in the Wide Area Author: Paul Barford and Mark Crovella Date: April 23, 1999 Abstract: One of the most vexing questions facing researchers interested in the World Wide Web is why users often experience long delays in document retrieval. The Internet's size, complexity, and continued growth make this a difficult question to answer. We describe the Wide Area Web Measurement project (WAWM) which uses an infrastructure distributed across the Internet to study Web performance. The infrastructure enables simultaneous measurements of Web client performance, network performance and Web server performance. The infrastructure uses a Web traffic generator to create representative workloads on servers, and both active and passive tools to measure performance characteristics. Initial results based on a prototype installation of the infrastructure are presented in this paper. :::::::::::::: 1999-005 :::::::::::::: Title: Fast, Reliable Head Tracking under Varying Illumination: An Approach Based on Registration of Texture-Mapped 3D Models by: Marco La Cascia, Stan Sclaroff, and Vassilis Athitsos supercedes: BU-TR-98-018 and BU-TR-97-020 Abstract: An improved technique for 3D head tracking under varying illumination conditions is proposed. The head is modeled as a texture mapped cylinder. Tracking is formulated as an image registration problem in the cylinder's texture map image. The resulting dynamic texture map provides a stabilized view of the face that can be used as input to many existing 2D techniques for face recognition, facial expressions analysis, lip reading, and eye tracking. To solve the registration problem in the presence of lighting variation and head motion, the residual error of registration is modeled as a linear combination of texture warping templates and orthogonal illumination templates. Fast and stable on-line tracking is achieved via regularized, weighted least squares minimization of the registration error. The regularization term tends to limit potential ambiguities that arise in the warping and illumination templates. It enables stable tracking over extended sequences. Tracking does not require a precise initial fit of the model; the system is initialized automatically using a simple 2D face detector. The only assumption is that the target is facing the camera in the first frame of the sequence. The formulation is tailored to take advantage of texture mapping hardware available in many workstations, PC's, and game consoles. The non-optimized implementation runs at about 15 frames per second on a SGI O2 graphic workstation. Extensive experiments evaluating the effectiveness of the formulation are reported. The sensitivity of the technique to illumination, regularization parameters, errors in the initial positioning and internal camera parameters are analyzed. Examples and applications of tracking are reported. :::::::::::::: 1999-006 :::::::::::::: Title: Non-Rigid Shape from Image Streams Authors: Stan Sclaroff and Jonathan Alon Date: July 27, 1999 Abstract: We present a framework for estimating 3D relative structure (shape) and motion given objects undergoing nonrigid deformation as observed from a fixed camera, under perspective projection. Deforming surfaces are approximated as piece-wise planar, and piece-wise rigid. Robust registration methods allow tracking of corresponding image patches from view to view and recovery of 3D shape despite occlusions, discontinuities, and varying illumination conditions. Many relatively small planar/rigid image patch trackers are scattered throughout the image; resulting estimates of structure and motion at each patch are combined over local neighborhoods via an oriented particle systems formulation. Preliminary experiments have been conducted on real image sequences of deforming objects and on synthetic sequences where ground truth is known. :::::::::::::: 1999-007 :::::::::::::: Title: Combinations of Deformable Shape Prototypes Authors: Saratendu Sethi and Stan Sclaroff Date: July 27, 1999 We propose to investigate a model-based technique for encoding non-rigid object classes in terms of object prototypes. Objects from the same class can be parameterized by identifying shape and appearance invariants of the class to devise low-level representations. The approach presented here creates a flexible model for an object class from a set of prototypes. This model is then used to estimate the parameters of low-level representation of novel objects as combinations of the prototype parameters. Variations in the object shape are modeled as non-rigid deformations. Appearance variations are modeled as intensity variations. In the training phase, the system is presented with several example prototype images. These prototype images are registered to a reference image by a finite element-based technique called Active Blobs. The deformations of the finite element model to register a prototype image with the reference image provide the shape description or shape vector for the prototype. The shape vector for each prototype, is then used to warp the prototype image onto the reference image and obtain the corresponding texture vector. The prototype texture vectors, being warped onto the same reference image have a pixel by pixel correspondence with each other and hence are ``shape normalized''. Given sufficient number of prototypes that exhibit appropriate in-class variations, the shape and the texture vectors define a linear prototype subspace that spans the object class. Each prototype is a vector in this subspace. The matching phase involves the estimation of a set of combination parameters for synthesis of the novel object by combining the prototype shape and texture vectors. The strengths of this technique lie in the combined estimation of both shape and appearance parameters. This is in contrast with the previous approaches where shape and appearance parameters were estimated separately. :::::::::::::: 1999-008 :::::::::::::: Title: Optimal Scheduling of Secondary Content for Aggregation in Video-on-Demand Systems Authors: P. Basu, A. Narayanan, W. Ke, T.D.C. Little, and A. Bestavros Date: July 27, 1999 Abstract: Dynamic service aggregation techniques can exploit skewed access popularity patterns to reduce the costs of building interactive VoD systems. These schemes seek to cluster and merge users into single streams by bridging the temporal skew between them, thus improving server and network utilization. Rate adaptation and secondary content insertion are two such schemes. In this paper, we present and evaluate an optimal scheduling algorithm for inserting secondary content in this scenario. The algorithm runs in polynomial time, and is optimal with respect to the total bandwidth usage over the merging interval. We present constraints on content insertion which make the overall QoS of the delivered stream acceptable, and show how our algorithm can satisfy these constraints. We report simulation results which quantify the excellent gains due to content insertion. We discuss dynamic scenarios with user arrivals and interactions, and show that content insertion reduces the channel bandwidth requirement to almost half. We also discuss differentiated service techniques, such as N-VoD and premium no-advertisement service, and show how our algorithm can support these as well. (This report is cross listed as BU ECE Department Technical Report: TR-12-16-98) :::::::::::::: 1999-009 :::::::::::::: Title: Popularity-Aware GreedyDual-Size Web Proxy Caching Algorithms Authors: Shudong Jin and Azer Bestavros Date: August 21, 1999 Abstract: Web caching aims to reduce network traffic, server load, and user-perceived retrieval delays by replicating ``popular'' content on proxy caches that are strategically placed within the network. While key to effective cache utilization, popularity information (e.g. relative access frequencies of objects requested through a proxy) is seldom incorporated directly in cache replacement algorithms. Rather, other properties of the request stream (e.g. temporal locality and content size), which are easier to capture in an on-line fashion, are used to indirectly infer popularity information, and hence drive cache replacement policies. Recent studies suggest that the correlation between these secondary properties and popularity is weakening due in part to the prevalence of efficient client and proxy caches (which tend to mask these correlations). This trend points to the need for proxy cache replacement algorithms that directly capture and use popularity information. In this paper, we (1) present an on-line algorithm that effectively captures and maintains an accurate popularity profile of Web objects requested through a caching proxy, (2) propose a novel cache replacement policy that uses such information to generalize the well-known GreedyDual-Size algorithm, and (3) show the superiority of our proposed algorithm by comparing it to a host of recently-proposed and widely-used algorithms using extensive trace-driven simulations and a variety of performance metrics. :::::::::::::: 1999-010 :::::::::::::: Title: A Fully Distributed Location Management Scheme for Large PCS Authors: Karunaharan Ratnam (Northeastern University), Ibrahim Matta (Boston University), and Sampath Rangarajan (Bell Laboratories) Date: August 24, 1999 Abstract: In [previous papers] we presented the design, specification and proof of correctness of a fully distributed location management scheme for PCS networks and argued that fully replicating location information is both appropriate and efficient for small PCS networks. In this paper, we analyze the performance of this scheme. Then, we extend the scheme in a hierarchical environment so as to scale to large PCS networks. Through extensive numerical results, we show the superiority of our scheme compared to the current IS-41 standard. :::::::::::::: 1999-011 :::::::::::::: Title: BU Computer Science 1998 Proxy Trace Author: Adam D. Bradley Date: September 7, 1999 Abstract: In a recent paper (Changes in Web Client Access Patterns: Characteristics and Caching Implications by Barford, Bestavros, Bradley, and Crovella) we performed a variety of analyses upon user traces collected in the Boston University Computer Science department in 1995 and 1998. A sanitized version of the 1995 trace has been publicly available for some time; the 1998 trace has now been sanitized, and is available from: http://www.cs.bu.edu/techreports/1999-011-usertrace-98.gz ftp://ftp.cs.bu.edu/techreports/1999-011-usertrace-98.gz This memo discusses the format of this public version of the log, and includes additional discussion of how the data was collected, how the log was sanitized, what this log is and is not useful for, and areas of potential future research interest. :::::::::::::: 1999-012 :::::::::::::: Title: Adaptive Reliable Multicast Authors: Jaehee Yoon, Azer Bestavros, and Ibrahim Matta Date: September 15, 1999 Abstract: An increasing number of applications, such as distributed interactive simulation, live auctions, distributed games and collaborative systems, require the network to provide a reliable multicast service. This service enables one sender to reliably transmit data to multiple receivers. Reliability is traditionally achieved by having receivers send negative acknowledgments (NACKs) to request from the sender the retransmission of lost (or missing) data packets. However, this Automatic Repeat reQuest (ARQ) approach results in the well-known NACK implosion problem at the sender. Many reliable multicast protocols have been recently proposed to reduce NACK implosion. But, the message overhead due to NACK requests remains significant. Another approach, based on Forward Error Correction (FEC), requires the sender to encode additional redundant information so that a receiver can independently recover from losses. However, due to the lack of feedback from receivers, it is impossible for the sender to determine how much redundancy is needed. In this paper, we propose a new reliable multicast protocol, called ARM for Adaptive Reliable Multicast. Our protocol integrates ARQ and FEC techniques. The objectives of ARM are (1) reduce the message overhead due to NACK requests, (2) reduce the amount of data transmission, and (3) reduce the time it takes for all receivers to receive the data intact (without loss). During data transmission, the sender periodically informs the receivers of the number of packets that are yet to be transmitted. Based on this information, each receiver predicts whether this amount is enough to recover its losses. Only if it is not enough, that the receiver requests the sender to encode additional redundant packets. Using ns simulations, we show the superiority of our hybrid ARQ-FEC protocol over the well-known Scalable Reliable Multicast (SRM) protocol. :::::::::::::: 1999-013 :::::::::::::: Title: Search Space Reduction in QoS Routing Authors: Liang Guo and Ibrahim Matta Date: October 8, 1999 Abstract: To provide real-time service or engineer constrained-based paths, networks require the underlying routing algorithm to be able to find low-cost paths that satisfy given Quality-of-Service (QoS) constraints. However, the problem of constrained shortest (least-cost) path routing is known to be NP-hard, and some heuristics have been proposed to find a near-optimal solution. However, these heuristics either impose relationships among the link metrics to reduce the complexity of the problem which may limit the general applicability of the heuristic, or are too costly in terms of execution time to be applicable to large networks. In this paper, we focus on solving the delay-constrained minimum-cost path problem, and present a fast algorithm to find a near-optimal solution. This algorithm, called DCCR (for Delay-Cost-Constrained Routing), is a variant of the k-shortest path algorithm. DCCR uses a new adaptive path weight function together with an additional constraint imposed on the path cost, to restrict the search space. Thus, DCCR can return a near-optimal solution in a very short time. Furthermore, we use the method proposed by Blokh and Gutin to further reduce the search space by using a tighter bound on path cost. This makes our algorithm more accurate and even faster. We call this improved algorithm SSR+DCCR (for Search Space Reduction+DCCR). Through extensive simulations, we confirm that SSR+DCCR performs very well compared to the optimal but very expensive solution. * This technical report revises TR NU-CCS-98-09. :::::::::::::: 1999-014 :::::::::::::: Title: Temporal Locality in Web Request Streams: Sources, Characteristics, and Caching Implications Authors: Shudong Jin and Azer Bestavros Date: October 10, 1999 Abstract: Temporal locality of reference in Web request streams emerges from two distinct phenomena: the popularity of Web objects and the {\em temporal correlation} of requests. Capturing these two elements of temporal locality is important because it enables cache replacement policies to adjust how they capitalize on temporal locality based on the relative prevalence of these phenomena. In this paper, we show that temporal locality metrics proposed in the literature are unable to delineate between these two sources of temporal locality. In particular, we show that the commonly-used distribution of reference interarrival times is predominantly determined by the power law governing the popularity of documents in a request stream. To capture (and more importantly quantify) both sources of temporal locality in a request stream, we propose a new and robust metric that enables accurate delineation between locality due to popularity and that due to temporal correlation. Using this metric, we characterize the locality of reference in a number of representative proxy cache traces. Our findings show that there are measurable differences between the degrees (and sources) of temporal locality across these traces, and that these differences are effectively captured using our proposed metric. We illustrate the significance of our findings by summarizing the performance of a novel Web cache replacement policy---called GreedyDual*---which exploits both long-term popularity and short-term temporal correlation in an adaptive fashion. Our trace-driven simulation experiments (which are detailed in an accompanying Technical Report) show the superior performance of GreedyDual* when compared to other Web cache replacement policies. :::::::::::::: 1999-015 :::::::::::::: Title: Estimation and Prediction of Evolving Color Distributions for Skin Segmentation Under Varying Illumination Authors: Leonid Sigal and Stan Sclaroff Date: December 1, 1999 Abstract: A novel approach for real-time skin segmentation in video sequences is described. The approach enables reliable skin segmentation despite wide variation in illumination during tracking. An explicit second order Markov model is used to predict evolution of the skin color (HSV) histogram over time. Histograms are dynamically updated based on feedback from the current segmentation and based on predictions of the Markov model. The evolution of the skin color distribution at each frame is parameterized by translation, scaling and rotation in color space. Consequent changes in geometric parameterization of the distribution are propagated by warping and re-sampling the histogram. The parameters of the discrete-time dynamic Markov model are estimated using Maximum Likelihood Estimation, and also evolve over time. Quantitative evaluation of the method was conducted on labeled ground-truth video sequences taken from popular movies. :::::::::::::: 1999-016 :::::::::::::: Title: Recursive Estimation of Motion and Planar Structure Authors: Jonathan Alon and Stan Sclaroff Date: December 1, 1999 Abstract: A specialized formulation of Azarbayejani and Pentland's framework for recursive recovery of motion, structure and focal length from feature correspondences tracked through an image sequence is presented. The specialized formulation addresses the case where all tracked points lie on a plane. This planarity constraint reduces the dimension of the original state vector, and consequently the number of feature points needed to estimate the state. Experiments with synthetic data and real imagery illustrate the system performance. The experiments confirm that the specialized formulation provides improved accuracy, stability to observation noise, and rate of convergence in estimation for the case where the tracked points lie on a plane. :::::::::::::: 1999-017 :::::::::::::: Title: Inferring Body Pose without Tracking Body Parts Authors: Romer Rosales and Stan Sclaroff Date: December 1, 1999 Abstract: A novel approach for estimating articulated body posture and motion from monocular video sequences is proposed. Human pose is defined as the instantaneous two dimensional configuration (i.e., the projection onto the image plane) of a single articulated body in terms of the position of a predetermined set of joints. First, statistical segmentation of the human bodies from the background is performed and low-level visual features are found given the segmented body shape. The goal is to be able to map these, generally low level, visual features to body configurations. The system estimates different mappings, each one with a specific cluster in the visual feature space. Given a set of body motion sequences for training, unsupervised clustering is obtained via the Expectation Maximation algorithm. Then, for each of the clusters, a function is estimated to build the mapping between low-level features to 3D pose. Currently this mapping is modeled by a neural network. Given new visual features, a mapping from each cluster is performed to yield a set of possible poses. From this set, the system selects the most likely pose given the learned probability distribution and the visual feature similarity between hypothesis and input. Performance of the proposed approach is characterized using a new set of known body postures, showing promising results. :::::::::::::: 1999-018 :::::::::::::: Title: SomeCast: A Paradigm for Real-Time Adaptive Reliable Multicast Authors: Jaehee Yoon, Azer Bestavros, and Ibrahim Matta Date: December 10, 1999 Abstract: SomeCast is a novel paradigm for the reliable multicast of real-time data to a large set of receivers over the Internet. SomeCast is receiver-initiated and thus scalable in the number of receivers, the diverse characteristics of paths between senders and receivers (e.g. maximum bandwidth and round-trip-time), and the dynamic conditions of such paths (e.g. congestion-induced delays and losses). SomeCast enables receivers to dynamically adjust the rate at which they receive multicast information to enable the satisfaction of real-time QoS constraints (e.g. rate, deadlines, or jitter). This is done by enabling a receiver to join SOME number of concurrent multiCAST sessions, whereby each session delivers a portion of an encoding of the real-time data. By adjusting the number of such sessions dynamically, client-specific QoS constraints can be met independently. The SomeCast paradigm can be thought of as a generalization of the AnyCast (e.g. Dynamic Server Selection) and ManyCast (e.g. Digital Fountain) paradigms, which have been proposed in the literature to address issues of scalability of UniCast and MultiCast environments, respectively. In this paper we overview the SomeCast paradigm, describe an instance of a SomeCast protocol, and present simulation results that quantify the significant advantages gained from adopting such a protocol for the reliable multicast of data to a diverse set of receivers subject to real-time QoS constraints. :::::::::::::: 1999-019 :::::::::::::: Title: BU/NSF Workshop on Internet Measurement Instrumentation and Characterization Authors: Azer Bestavros, John Byers, and Mark Crovella (PIs and co-organizers) Paul Barford, Ibrahim Matta, and Michael Mitzenmacher (co-organizers) Date: December 15, 1999 Abstract: Because of its growth in size, scope, and complexity---as well as its increasingly central role in society---the Internet has become an important object of study and evaluation. Many significant innovations in the networking community in recent years have been directed at obtaining a more accurate understanding of the fundamental behavior of the complex system that is the Internet. These innovations have come in the form of better models of components of the system, better tools which enable us to measure the performance of the system more accurately, and new techniques coupled with performance evaluation which have delivered better system utilization. The continued development and improvement of our understanding of the properties of the Internet is essential to guide designers of hardware, protocols, and applications for the next decade of Internet growth. As a research community, an important next step involves an comprehensive look at the challenges that lie ahead in this area. This includes an an evaluation of both the current unsolved challenges and the upcoming challenges the Internet will present us with in the near future, and a discussion of the promising new techniques that innovators in the field are currently developing. To this end, the Networking Research Group at Boston University, with support from the National Science Foundation, organized a one-day workshop which was held at Boston University on Monday, August 30, 1999. This report summarizes the technical presentations and discussions that took place during that workshop. :::::::::::::: 2000-001 :::::::::::::: Title: Faithful Translations between Polyvariant Flows and Polymorphic Types Author: Torben Amtoft (Boston University) and Franklyn Turbak (Wellesley College) Abstract: Recent work has shown equivalences between various type systems and flow logics. Ideally, the translations upon which such equivalences are based should be faithful in the sense that information is not lost in round-trip translations from flows to types and back or from types to flows and back. Building on the work of Nielson & Nielson and of Palsberg & Pavlopoulou, we present the first faithful translations between a class of finitary polyvariant flow analyses and a type system supporting polymorphism in the form of intersection and union types. Additionally, our flow/type correspondence solves several open problems posed by Palsberg & Pavlopoulou: (1) it expresses call-string based polyvariance (such as k-CFA) as well as argument based polyvariance; (2) it enjoys a subject reduction property for flows as well as for types; and (3) it supports a flow-oriented perspective rather than a type-oriented one. :::::::::::::: 2000-002 :::::::::::::: Title: Determining Acceptance Possibility for a Quantum Computation is Hard for the Polynomial Hierarchy Authors: Fenner, Stephen; Green, Frederic; Homer, Steven and Pruim, Randall Date: Jan 20, 2000 Abstract: It is shown that determining whether a quantum computation has a non-zero probability of accepting is at least as hard as the polynomial time hierarchy. This hardness result also applies to determining in general whether a given quantum basis state appears with nonzero amplitude in a superposition, or whether a given quantum bit has positive expectation value at the end of a quantum computation. This result is achieved by showing that the complexity class NQP of Adleman, Demarrais, and Huang, a quantum analog of NP, is equal to the counting class $co-C equals P$. :::::::::::::: 2000-003 :::::::::::::: Title: On the Complexity of Quantum ACC Authors: Green, Frederic; Homer, Steven; and Pollett, Christopher Date: Jan 20, 2000 Abstract: For any q > 1, let MOD_q be a quantum gate that determines if the number of 1's in the input is divisible by q. We show that for any q,t > 1, MOD_q is equivalent to MOD_t (up to constant depth). Based on the case q=2, Moore has shown that quantum analogs of AC^(0), ACC[q], and ACC, denoted QAC^(0)_wf, QACC[2], QACC respectively, define the same class of operators, leaving q > 2 as an open question. Our result resolves this question, implying that QAC^(0)_wf = QACC[q] = QACC for all q. We also prove the first upper bounds for QACC in terms of related language classes. We define classes of languages EQACC, NQACC (both for arbitrary complex amplitudes) and BQACC (for rational number amplitudes) and show that they are all contained in TC^(0). To do this, we show that a TC^(0) circuit can keep track of the amplitudes of the state resulting from the application of a QACC operator using a constant width polynomial size tensor sum. In order to accomplish this, we also show that TC^(0) can perform iterated addition and multiplication in certain field extensions. :::::::::::::: 2000-004 :::::::::::::: Title: On the Origin of Power Laws in Internet Topologies Authors: Alberto Medina, Ibrahim Matta, and John Byers Date: January 21, 2000 Abstract: Recent empirical studies have shown that Internet topologies exhibit power laws of the form $y = x^\alpha$ for the following relationships: (P1) outdegree of node (domain or router) versus rank; (P2) number of nodes versus outdegree; (P3) number of node pairs within a neighborhood versus neighborhood size (in hops); and (P4) eigenvalues of the adjacency matrix versus rank. However, causes for the appearance of such power laws have not been convincingly given. In this paper, we examine four factors in the formation of Internet topologies. These factors are (F1) preferential connectivity of a new node to existing nodes; (F2) incremental growth of the network; (F3) distribution of nodes in space; and (F4) locality of edge connections. In synthetically generated network topologies, we study the relevance of each factor in causing the aforementioned power laws as well as other properties, namely diameter, average path length and clustering coefficient. Different kinds of network topologies are generated: (T1) topologies generated using our parametrized generator, we call BRITE; (T2) random topologies generated using the well-known Waxman model; (T3) Transit-Stub topologies generated using GT-ITM tool; and (T4) regular grid topologies. We observe that some generated topologies may not obey power laws P1 and P2. Thus, the existence of these power laws can be used to validate the accuracy of a given tool in generating representative Internet topologies. Power laws P3 and P4 were observed in nearly all considered topologies, but different topologies showed different values of the power exponent $\alpha$. Thus, while the presence of power laws P3 and P4 do not give strong evidence for the representativeness of a generated topology, the value of $\alpha$ in P3 and P4 can be used as a litmus test for the representativeness of a generated topology. We also find that factors F1 and F2 are the key contributors in our study which provide the resemblance of our generated topologies to that of the Internet. * BRITE (Boston university Representative Internet Topology gEnerator) is available at http://www.cs.bu.edu/fac/matta/software.html :::::::::::::: 2000-005 :::::::::::::: Title: BRITE: A Flexible Generator of Internet Topologies Authors: Alberto Medina, Ibrahim Matta, and John Byers Date: January 21, 2000 (Revised January 15, 2001) Abstract: BRITE is a parameterized topology generation tool, which can be used to flexibly control various parameters (such as connectivity and growth models) and study various properties of generated network topologies (such power laws, path length and clustering coefficient). BRITE can be used to study the relevance of possible causes for properties recently observed in Internet topologies. Different combinations of possible causes can be tested. In this version, we consider four of them: (1) preferential connectivity of a new node to existing nodes; (2) incremental growth of the network; (3) geographical distribution of nodes; and (4) locality of edge connections. We use BRITE in [BU-CS-TR-2000-0004] to study the origin of power laws and other metrics in Internet topologies. BRITE (Boston university Representative Internet Topology gEnerator) is available on the Web at http://www.cs.bu.edu/faculty/matta/Research/BRITE/ :::::::::::::: 2000-006 :::::::::::::: Title: Efficient Hash-Consing of Recursive Types Author: Jeffrey Considine Date: January 29, 2000 Abstract: Efficient storage of types within a compiler is necessary to avoid large blowups in space during compilation. Recursive types in particular are important to consider, as naive representations of recursive types may be arbitrarily larger than necessary through unfolding. Hash-consing has been used to efficiently store non-recursive types. Deterministic finite automata techniques have been used to efficiently perform various operations on recursive types. We present a new system for storing recursive types combining hash-consing and deterministic finite automata techniques. The space requirements are linear in the number of distinct types. Both update and lookup operations take polynomial time and linear space and type equality can be checked in constant time once both types are in the system. :::::::::::::: 2000-007 :::::::::::::: Title: Type Inference For Recursive Definitions Author: Assaf J. Kfoury (Boston University) and Santiago M. Pericas-Geertsen (Boston University) Date: March 6, 2000 Abstract: We consider type systems that combine universal types, recursive types, and object types. We study type inference in these systems under a rank restriction, following Leivant's notion of rank. To motivate our work, we present several examples showing how our systems can be used to type programs encountered in practice. We show that type inference in the rank-k system is decidable for k <= 2 and undecidable for k >= 3. (Similar results based on different techniques are known to hold for System F, without recursive types and object types.) Our undecidability result is obtained by a reduction from a particular adaptation (which we call ``regular'') of the semi-unification problem and whose undecidability is, interestingly, obtained by methods totally different from those used in the case of standard (or finite) semi-unification. :::::::::::::: 2000-008 :::::::::::::: Title: QoS Controllers for the Internet Authors: Ibrahim Matta and Azer Bestavros Date: March 12, 2000 Abstract: In this position paper, we review basic control strategies that machines acting as "traffic controllers" could deploy in order to improve the management of Internet services. Such traffic controllers are likely to spur the widespread emergence of advanced applications, which have (so far) been hindered by the inability of the networking infrastructure to deliver on the promise of Quality-of-Service (QoS). :::::::::::::: 2000-009 :::::::::::::: Title: Index trees for efficient deformable shape-based retrieval Author: Lifeng Liu and Stan Sclaroff Image and Vision Computing Group, Computer Science Department, Boston University Date: March 22, 2000 Abstract: An improved method for deformable shape-based image indexing and retrieval is described. A pre-computed index tree is used to improve the speed of our previously reported on-line model fitting method; simple shape features are used as keys in a pre-generated index tree of model instances. In addition, a coarse to fine indexing scheme is used at different levels of the tree to further improve speed while maintaining matching accuracy. Experimental results show that the speedup is significant, while accuracy of shape-based indexing is maintained. A method for shape population-based retrieval is also described. The method allows query formulation based on the population distributions of shapes in each image. Results of population-based image queries for a database of blood cell micrographs are shown. :::::::::::::: 2000-010 :::::::::::::: Title: Deciding Isomorphisms of Simple Types in Polynomial Time Author: Jeffrey Considine Date: April 2, 2000 Abstract: The isomorphisms holding in all models of the simply typed lambda calculus with surjective and terminal objects are well studied - these models are exactly the Cartesian closed categories. Isomorphism of two simple types in such a model is decidable by reduction to a normal form and comparison under a finite number of permutations (Bruce, Di Cosmo, and Longo 1992). Unfortunately, these normal forms may be exponentially larger than the original types so this construction decides isomorphism in exponential time. We show how using space-sharing/hash-consing techniques and memoization can be used to decide isomorphism in practical polynomial time (low degree, small hidden constant). Other researchers have investigated simple type isomorphism in relation to, among other potential applications, type-based retrieval of software modules from libraries and automatic generation of bridge code for multi-language systems. Our result makes such potential applications practically feasible. :::::::::::::: 2000-011 :::::::::::::: Title: GreedyDual* Web Caching Algorithm: Exploiting the Two Sources of Temporal Locality in Web Request Streams Authors: Shudong Jin and Azer Bestavros Date: April 4, 2000 Abstract: The relative importance of long-term popularity and short-term temporal correlation of references for Web cache replacement policies has not been studied thoroughly. This is partially due to the lack of accurate characterization of temporal locality that enables the identification of the relative strengths of these two sources of temporal locality in a reference stream. In [JB99], we have proposed such a metric and have shown that Web reference streams differ significantly in the the prevelance of these two sources of temporal locality. These findings underscore the importance of a Web caching strategy that can adapt in a dynamic fashion to the prevelance of these two sources of temporal locality. In this paper, we propose a novel cache replacement algorithm, GreedyDual*, which is a generalization of GreedyDual-Size. GreedyDual* uses the metrics proposed in [JB99] to adjust the relative worth of long-term popularity versus short-term temporal correlation of references. Our trace-driven simulation experiments show the superior performance of GreedyDual* when compared to other Web cache replacement policies proposed in the literature. :::::::::::::: 2000-012 :::::::::::::: Title: Differentiated Predictive Fair Service for TCP Flows Author: Ibrahim Matta and Liang Guo Computer Science Department, Boston University Date: May 17, 2000 Abstract: The majority of the traffic (bytes) flowing over the Internet today have been attributed to the Transmission Control Protocol (TCP). This strong presence of TCP has recently spurred further investigations into its congestion avoidance mechanism and its effect on the performance of short and long data transfers. At the same time, the rising interest in enhancing Internet services while keeping the implementation cost low has led to several service-differentiation proposals. In such service-differentiation architectures, much of the complexity is placed only in access routers, which classify and mark packets from different flows. Core routers can then allocate enough resources to each class of packets so as to satisfy delivery requirements, such as predictable (consistent) and fair service. In this paper, we investigate the interaction among short and long TCP flows, and how TCP service can be improved by employing a low-cost service-differentiation scheme. Through control-theoretic arguments and extensive simulations, we show the utility of isolating TCP flows into two classes based on their lifetime/size, namely one class of short flows and another of long flows. With such class-based isolation, short and long TCP flows have separate service queues at routers. This protects each class of flows from the other as they possess different characteristics, such as burstiness of arrivals/departures and congestion/sending window dynamics. We show the benefits of isolation, in terms of better predictability and fairness, over traditional shared queueing systems with both tail-drop and Random-Early-Drop (RED) packet dropping policies. The proposed class-based isolation of TCP flows has several advantages: (1) the implementation cost is low since it only requires core routers to maintain per-class (rather than per-flow) state; (2) it promises to be an effective traffic engineering tool for improved predictability and fairness for both short and long TCP flows; and (3) stringent delay requirements of short interactive transfers can be met by increasing the amount of resources allocated to the class of short flows. :::::::::::::: 2000-013 :::::::::::::: Title: Robust Identification of Shared Losses Using End-to-End Unicast Probes Authors: Khaled Harfoush, Azer Bestavros, and John Byers Date: May 30, 2000 Abstract: Current Internet transport protocols make end-to-end measurements and maintain per-connection state to regulate the use of shared network resources. When two or more such connections share a common endpoint, there is an opportunity to correlate the end-to-end measurements made by these protocols to better diagnose and control the use of shared resources. We develop packet probing techniques to determine whether a pair of connections experience shared congestion. Correct, efficient diagnoses could enable new techniques for aggregate congestion control, QoS admission control, connection scheduling and mirror site selection. Our extensive simulation results demonstrate that the conditional (Bayesian) probing approach we employ provides superior accuracy, converges faster, and tolerates a wider range of network conditions than recently proposed memoryless (Markovian) probing approaches for addressing this opportunity. :::::::::::::: 2000-014 :::::::::::::: Title: Utility-Based Decision-Making in Wireless Sensor Networks Author: John Byers and Gabriel Nasser (B.U.) Date: June 1, 2000 Abstract: We consider challenges associated with application domains in which a large number of distributed, networked sensors must perform a sensing task repeatedly over time. For the tasks we consider, there are three significant challenges to address. First, nodes have resource constraints imposed by their finite power supply, which motivates computations that are energy-conserving. Second, for the applications we describe, the utility derived from a sensing task may vary depending on the placement and size of the set of nodes who participate, which often involves complex objective functions for nodes to target. Finally, nodes must attempt to realize these global objectives with only local information. We present a model for such applications, in which we define appropriate global objectives based on utility functions and specify a cost model for energy consumption. Then, for an important class of utility functions, we present distributed algorithms which attempt to maximize the utility derived from the sensor network over its lifetime. The algorithms and experimental results we present enable nodes to adaptively change their roles over time and use dynamic reconfiguration of routes to load balance energy consumption in the network. :::::::::::::: 2000-015 :::::::::::::: Title: Estimating Human Body Pose from a Single Image via the Specialized Mappings Architecture Authors: Romer Rosales and Stan Sclaroff Date: June 10, 2000 Abstract: A non-linear supervised learning architecture, the Specialized Mapping Architecture (SMA) and its application to articulated body pose reconstruction from single monocular images is described. The architecture is formed by a number of specialized mapping functions, each of them with the purpose of mapping certain portions (connected or not) of the input space, and a feedback matching process. A probabilistic model for the architecture is described along with a mechanism for learning its parameters. The learning problem is approached using a maximum likelihood estimation framework; we present Expectation Maximization (EM) algorithms for two different instances of the likelihood probability. Performance is characterized by estimating human body postures from low level visual features, showing promising results :::::::::::::: 2000-016 :::::::::::::: Title: Unicast-based Characterization of Network Loss Topologies Authors: Khaled Harfoush, Azer Bestavros, and John Byers Date: July 3, 2000 Abstract: Current Internet transport protocols make end-to-end measurements and maintain per-connection state to regulate the use of shared network resources. When a number of such connections share a common endpoint, that endpoint has the opportunity to correlate these end-to-end measurements to better diagnose and control the use of shared resources. A valuable characterization of such shared resources is the ``loss topology''. From the perspective of a server with concurrent connections to multiple clients, the loss topology is a logical tree rooted at the server in which edges represent lossy paths between a pair of internal network nodes. We develop an end-to-end unicast packet probing technique and an associated analytical framework to: (1) infer loss topologies, (2) identify loss rates of links in an existing loss topology, and (3) augment a topology to incorporate the arrival of a new connection. Correct, efficient inference of loss topology information enables new techniques for aggregate congestion control, QoS admission control, connection scheduling and mirror site selection. Our extensive simulation results demonstrate that our approach is robust in terms of its accuracy and convergence over a wide range of network conditions. :::::::::::::: 2000-017 :::::::::::::: Title: TCP Congestion Control and Heavy Tails Author: Liang Guo, Mark Crovella, and Ibrahim Matta Computer Science Department, Boston University Date: July 3, 2000 Abstract: Long-range dependence has been observed in many recent Internet traffic measurements. Previous studies have shown that there is a close relationship between heavy-tailed distribution of various traffic parameters and the long-range dependent property. In this paper, we use a simple Markov chain model to argue that when the loss rate is relatively high, TCP's adaptive congestion control mechanism indeed generates traffic with heavy-tailed OFF, or idle, periods, and therefore introduces long-range dependence into the overall traffic. Moreover, the degree of such long-range dependence, measured by the Hurst parameter, increases as the loss rate increases, agreeing with many previous measurement-based studies. In addition, we observe that more variable initial retransmission timeout values for different packets introduces more variable packet inter-arrival times, which increases the burstiness of the overall traffic. Finally, we show that high loss conditions can lead to a heavy-tailed distribution of transmission times even for constant-sized files. This means that file size variability need not be the only cause of heavy-tailed variability in transmission durations. :::::::::::::: 2000-018 :::::::::::::: Title: On the Marginal Utility of Deploying Measurement Infrastructure Authors: Paul Barford, Azer Bestavros, John Byers, and Mark Crovella Computer Science Department, Boston University Date: July 3, 2000 Abstract: The cost and complexity of deploying measurement infrastructure in the Internet for the purpose of analyzing its structure and behavior is considerable. Basic questions about the {\em utility} of increasing the number of measurements and/or measurement sites have not yet been addressed which has lead to a ``more is better'' approach to wide-area measurements. In this paper, we quantify the marginal utility of performing wide-area measurements in the context of Internet topology discovery. We characterize topology in terms of nodes, links, node degree distribution, and end-to-end flows using statistical and information-theoretic techniques. We classify nodes discovered on the routes between a set of 8 sources and 1277 destinations to differentiate nodes which make up the so called ``backbone'' from those which border the backbone and those on links between the border nodes and destination nodes. This process includes reducing nodes that advertise multiple interfaces to single IP addresses. We show that the utility of adding sources goes down significantly after 2 from the persperspective of interface, node, link and node degree discovery. We show that the utility of adding destinations is constant for interfaces, nodes, links and node degree indicating that it is more important to add destinations than sources. Finally, we analyze paths through the backbone and show that shared link distributions approximate a power law indicating that a small number of backbone links in our study are very heavily utilized. :::::::::::::: 2000-019 :::::::::::::: Title: Cachability of Web Objects Author: Xiaohui Zhang Computer Science Department, Boston University Date: August 8, 2000 Abstract: Much work on the performance of Web proxy caching has focused on high-level metrics such as hit rate and byte hit rate, but has ignored all the information related to the cachability of Web objects. Uncachable objects include those fetched by dynamic requests, objects with uncachable HTTP status code, objects with the uncachable HTTP header, objects with an HTTP 1.0 cookie, and objects without a last-modified header. Although some researchers filter the Web traces before they use them for analysis or simulation,many do not have a comprehensive understanding of the cachability of Web objects. In this paper we evaluate all the reasons that a Web object might be uncachable. We use traces from NLANR. Since these traces do not contain HTTP header information, we replay them using request generator to get the response header information. We find that between 15% and 40% of Web objects in our traces can not be cached by a Web proxy server . We use a LRU simulator to show the performance gap when the cachability is either considered or not. We show the characteristics of the cachable data set and find that all its characteristics are fairly similar to that of total data set. Finally, we present some additional results for the cachable and total data set: (1) The main reasons for uncachability are: dynamic requests, responses without last-modified header, responses with HTTP "302 Moved Temporarily" status code, and responses with a HTTP/1.0 cookie. (2) The cachability of Web objects can not be ignored in simulation because uncachable objects comprise a huge percentage of the total trace. Simulations without cachability consideration will be misleading. :::::::::::::: 2000-020 :::::::::::::: Title: Type Inference for Variant Object Types Author: Michele Bugliesi and Santiago M. Pericas-Geertsen Date: October 16, 2000 Abstract: Existing type systems for object calculi are based on invariant subtyping. Subtyping invariance is required for soundness of static typing in the presence of method overrides, but it is often in the way of the expressive power of the type system. Flexibility of static typing can be recovered in different ways: in first-order systems, by the adoption of object types with variance annotations, in second-order systems by resorting to Self types. Type inference is known to be P-complete for first-order systems of finite and recursive object types, and NP-complete for a restricted version of Self types. The complexity of type inference for systems with variance annotations is yet unknown. This paper presents a new object type system based on the notion of Split types, a form of object types where every method is assigned two types, namely, an update type and a select type. The subtyping relation that arises for Split types is variant and, as a result, subtyping can be performed both in width and in depth. The new type system generalizes all the existing first-order type systems for objects, including systems based on variance annotations. Interestingly, the additional expressive power does not affect the complexity of the type inference problem, as we show by presenting an O(n^3) inference algorithm. :::::::::::::: 2000-021 :::::::::::::: Title: What are polymorphically-typed ambients? Authors: Torben Amtoft, Assaf Kfoury, Santiago Pericas-Geertsen Date: October 19, 2000 Abstract: The Ambient Calculus was developed by Cardelli and Gordon as a formal framework to study issues of mobility and migrant code. We consider an Ambient Calculus where ambients transport and exchange programs rather that just inert data. We propose different senses in which such a calculus can be said to be polymorphically typed, and design accordingly a polymorphic type system for it. Our type system assigns types to embedded programs and what we call behaviors to processes; a denotational semantics of behaviors is then proposed, here called trace semantics, underlying much of the remaining analysis. We state and prove a Subject Reduction property for our polymorphically typed calculus. Based on techniques borrowed from finite automata theory, type-checking of fully type-annotated processes is shown to be decidable; the time complexity of our decision procedure is exponential (this is a worst-case in theory, arguably not encountered in practice). Our polymorphically-typed calculus is a conservative extension of the typed Ambient Calculus originally proposed by Cardelli and Gordon. :::::::::::::: 2000-022 :::::::::::::: Title: 3D Hand Pose Reconstruction Using Specialized Mappings Authors: Romer Rosales, Vassilis Athitsos, and Stan Sclaroff Date: December 4, 2000 Abstract: A system for recovering 3D hand pose from monocular color sequences is proposed. The system employs a non-linear supervised learning framework, the specialized mappings architecture (SMA), to map image features to likely 3D hand poses. The SMA's fundamental components are a set of specialized forward mapping functions, and a single feedback matching function. The forward functions are estimated directly from training data, which in our case are examples of hand joint configurations and their corresponding visual features. The joint angle data in the training set is obtained via a CyberGlove, a glove with 22 sensors that monitor the angular motions of the palm and fingers. In training, the visual features are generated using a computer graphics module that renders the hand from arbitrary viewpoints given the 22 joint angles. We test our system both on synthetic sequences and on sequences taken with a color camera. The system automatically detects and tracks both hands of the user, calculates the appropriate features, and estimates the 3D hand joint angles from those features. Results are encouraging given the complexity of the task. :::::::::::::: 2000-023 :::::::::::::: Title: An Integrated Approach for Segmentation and Estimation of Planar Structures Authors: Joni Alon and Stan Sclaroff Date: December 4, 2000 Abstract: Standard structure from motion algorithms recover 3D structure of points. If a surface representation is desired, for example a piece-wise planar representation, then a two-step procedure typically follows: in the first step the plane-membership of points is first determined manually, and in a subsequent step planes are fitted to the sets of points thus determined, and their parameters are recovered. This paper presents an approach for automatically segmenting planar structures from a sequence of images, and simultaneously estimating their parameters. In the proposed approach the plane-membership of points is determined automatically, and the planar structure parameters are recovered directly in the algorithm rather than indirectly in a post-processing stage. Simulated and real experimental results show the efficacy of this approach. :::::::::::::: 2000-024 :::::::::::::: Title: Region Segmentation via Deformable Model-Guided Split and Merge Authors: Lifeng Liu and Stan Sclaroff Date: December 4, 2000 Abstract: An improved method for deformable shape-based image segmentation is described. Image regions are merged together and/or split apart, based on their agreement with an a priori distribution on the global deformation parameters for a shape template. The quality of a candidate region merging is evaluated by a cost measure that includes: homogeneity of image properties within the combined region, degree of overlap with a deformed shape model, and a deformation likelihood term. Perceptually-motivated criteria are used to determine where/how to split regions, based on the local shape properties of the region group's bounding contour. A globally consistent interpretation is determined in part by the minimum description length principle. Experiments show that the model-based splitting strategy yields a significant improvement in segmention over a method that uses merging alone. :::::::::::::: 2000-025 :::::::::::::: Title: The Cyclone Server Architecture: Streamlining Delivery of Popular Content Authors: Stan Rost, John Byers, Azer Bestavros (Boston University) Date: 12/15/2000 Abstract: We propose a new technique for efficiently delivering popular content from information repositories with bounded file caches. Our strategy relies on the use of fast erasure codes (a.k.a. forward error correcting codes) to generate encodings of popular files, of which only a small sliding window is cached at any time instant, even to satisfy an unbounded number of asynchronous requests for the file. Our approach capitalizes on concurrency to maximize sharing of state across different request threads while minimizing cache memory utilization. Additional reduction in resource requirements arises from providing for a lightweight version of the network stack. In this paper, we describe the design and implementation of our Cyclone server as a Linux kernel subsystem. :::::::::::::: 2000-026 :::::::::::::: Title: Fine-Grained Layered Multicast Authors: John Byers, Boston University Michael Luby, Digital Fountain, Inc. Michael Mitzenmacher, Harvard University Date: 1/12/01 Abstract: Traditional approaches to receiver-driven layered multicast have advocated the benefits of cumulative layering, which can enable coarse-grained congestion control that complies with TCP-friendliness equations over large time scales. In this paper, we quantify the costs and benefits of using non-cumulative layering and present a new, scalable multicast congestion control scheme which provides a fine-grained approximation to the behavior of TCP additive increase / multiplicative decrease (AIMD). In contrast to the conventional wisdom, we demonstrate that fine-grained rate adjustment can be achieved with only modest increases in the number of layers and aggregate bandwidth consumption, while using only a small constant number of control messages to perform either additive increase or multiplicative decrease. :::::::::::::: 2000-027 :::::::::::::: Title: An Infrastructure for the Dynamic Distribution of Web Applications and Services Authors: Enrique Duvos and Azer Bestavros Abstract: This paper presents the design and implementation of an infrastructure that enables any Web application, regardless of its current state, to be stopped and uninstalled from a particular server, transferred to a new server, then installed, loaded, and resumed, with all these events occurring "on the fly" and totally transparent to clients. Such functionalities allow entire applications to fluidly move from server to server, reducing the overhead required to administer the system, and increasing its performance in a number of ways: (1) Dynamic replication of new instances of applications to several servers to raise throughput for scalability purposes, (2) Moving applications to servers to achieve load balancing or other resource management goals, (3) Caching entire applications on servers located closer to clients. :::::::::::::: 2000-028 :::::::::::::: "Scaling Phenomena in Small-World Networks," Byers, Crovella, Ye. :::::::::::::: 2001-001 :::::::::::::: Title: Robust Identification of Shared Losses Using End-to-End Unicast Probes (ERRATA) Authors: Khaled Harfoush, Azer Bestavros and John Byers Date: January 8, 2001 Abstract: We present corrections to Fact 3 and (as a consequence) to Lemma 1 of BUCS Technical Report BUCS-TR-2000-013 (also published in IEEE ICNP'2000). These corrections result in slight changes to the formulae used for the identifications of shared losses, which we quantify. :::::::::::::: 2001-002 :::::::::::::: Title: Program representation size in an intermediate language with intersection and union types Authors: Allyn Dimock, Ian Westmacott, Robert Muller, Franklyn Turbak, J. B. Wells, and Jeffrey Considine Date: March 15, 2001 Abstract: The CIL compiler for core Standard ML compiles whole programs using a novel typed intermediate language (TIL) with intersection and union types and flow labels on both terms and types. The CIL term representation duplicates portions of the program where intersection types are introduced and union types are eliminated. This duplication makes it easier to represent type information and to introduce customized data representations. However, duplication incurs compile-time space costs that are potentially much greater than are incurred in TILs employing type-level abstraction or quantification. In this paper, we present empirical data on the compile-time space costs of using CIL as an intermediate language. The data shows that these costs can be made tractable by using sufficiently fine-grained flow analyses together with standard hash-consing techniques. The data also suggests that non-duplicating formulations of intersection (and union) types would not achieve significantly better space complexity. :::::::::::::: 2001-003 :::::::::::::: Title: BRITE: Universal Topology Generation from a User's Perspective Authors: Alberto Medina, Anukool Lakhina, Ibrahim Matta, John Byers Date: April 1, 2001 Abstract: Effective engineering of the Internet is predicated upon a detailed understanding of issues such as the large-scale structure of its underlying physical topology, the manner in which it evolves over time, and the way in which its constituent components contribute to its overall function. Unfortunately, developing a deep understanding of these issues has proven to be a challenging task, since it in turn involves solving difficult problems such as mapping the actual topology, characterizing it, and developing models that capture its emergent behavior. Consequently, even though there are a number of topology models, it is an open question as to how representative the topologies they generate are of the actual Internet. Our goal is to produce a topology generation framework which improves the state of the art and is based on design principles which include representativeness, inclusiveness, and interoperability. Representativeness leads to synthetic topologies that accurately reflect many aspects of the actual Internet topology (e.g. hierarchical structure, degree distribution, etc.). Inclusiveness combines the strengths of as many generation models as possible in a single generation tool. Interoperability provides interfaces to widely-used simulation applications such as ns and SSF as well as visualization applications. We call such a tool a "universal topology generator". In this paper we discuss the design, implementation and usage of the BRITE universal topology generation tool that we have built. We also describe the BRITE Analysis Engine, BRIANA, which is an independent piece of software designed and built upon BRITE design goals of flexibility and extensibility. The purpose of BRIANA is to act as a repository of analysis routines along with a user--friendly interface that allows its use on different topology formats. KEYWORDS: Topology generation, graph models, network topology, growth models, annotated topologies, simulation environments. :::::::::::::: 2001-004 :::::::::::::: Title: Automatic 3D Registration of Lung Surfaces in Computed Tomography Scans Authors: Margrit Betke, Harrison Hong, Jane P. Ko Date: April 24, 2001 Abstract: We developed an automated system that registers chest CT scans temporally. Our registration method matches corresponding anatomical landmarks to obtain initial registration parameters. The initial point-to-point registration is then generalized to an iterative surface-to-surface registration method. Our ``goodness-of-fit'' measure is evaluated at each step in the iterative scheme until the registration performance is sufficient. We applied our method to register the 3D lung surfaces of 11 pairs of chest CT scans and report promising registration performance. :::::::::::::: 2001-005 :::::::::::::: Title: The War Between Mice and Elephants Author: Liang Guo, Ibrahim Matta Date: May 7, 2001 Abstract: Recent measurement based studies reveal that most of the Internet connections are short in terms of the amount of traffic they carry (mice), while a small fraction of the connections are carrying a large portion of the traffic (elephants). A careful study of the TCP protocol shows that without help from an Active Queue Management (AQM) policy, short connections tend to lose to long connections in their competition for bandwidth. This is because short connections do not gain detailed knowledge of the network state, and therefore they are doomed to be less competitive due to the conservative nature of the TCP congestion control algorithm. Inspired by the Differentiated Services (Diffserv) architecture, we propose to give preferential treatment to short connections inside the bottleneck queue, so that short connections experience less packet drop rate than long connections. This is done by employing the RIO (RED with In and Out) queue management policy which uses different drop functions for different classes of traffic. Our simulation results show that: (1) in a highly loaded network, preferential treatment is necessary to provide short TCP connections with better response time and fairness without hurting the performance of long TCP connections; (2) the proposed scheme still delivers packets in FIFO manner at each link, thus it maintains statistical multiplexing gain and does not misorder packets; (3) choosing a smaller default initial timeout value for TCP can help enhance the performance of short TCP flows, however not as effectively as our scheme and with the risk of congestion collapse; (4) in the worst case, our proposal works as well as a regular RED scheme, in terms of response time and goodput. Keywords: Traffic Engineering, Congestion Control, TCP Performance, Fairness. :::::::::::::: 2001-006 :::::::::::::: Title: TCP-friendly SIMD Congestion Control and Its Convergence Behavior Authors: Shudong Jin, Liang Guo, Ibrahim Matta, and Azer Bestavros Date: May 8, 2001 Abstract: The increased diversity of Internet application requirements has spurred recent interest in flexible congestion control mechanisms. Window-based congestion control schemes use increase rules to probe available bandwidth, and decrease rules to back off when congestion is detected. The parameterization of these control rules is done so as to ensure that the resulting protocol is TCP-friendly in terms of the relationship between throughput and packet loss rate. In this paper, we propose a novel window-based congestion control algorithm called SIMD (Square-Increase/Multiplicative-Decrease). Contrary to previous memory-less controls, SIMD utilizes history information in its control rules. It uses multiplicative decrease but the increase in window size is in proportion to the {\em square} of the time elapsed since the detection of the last loss event. Thus, SIMD can efficiently probe available bandwidth. Nevertheless, SIMD is TCP-friendly as well as TCP-compatible under RED, and it has much better convergence behavior than TCP-friendly AIMD and binomial algorithms proposed recently. :::::::::::::: 2001-007 :::::::::::::: Title: Retrieval by Shape Population: An Index Tree Approach Authors: Lifeng Liu and Stan Sclaroff Date: June 5, 2001 Based on our previous work in deformable shape model-based object detection, a new method is proposed that uses index trees for organizing shape features to support content-based retrieval applications. In the proposed strategy, different shape feature sets can be used in index trees constructed for object detection and shape similarity comparison respectively. There is a direct correspondence between the two shape feature sets. As a result, application-specific features can be obtained efficiently for shape-based retrieval after object detection. A novel approach is proposed that allows retrieval of images based on the population distribution of deformed shapes in each image. Experiments testing these new approaches have been conducted using an image database that contains blood cell micrographs. The precision vs. recall performance measure shows that our method is superior to previous methods. :::::::::::::: 2001-008 :::::::::::::: Title: Estimating 3D Body Pose using Uncalibrated Cameras Author: Romer Rosales, Matheen Siddiqui, Jonathan Alon and Stan Sclaroff Date: June 5, 2001 An approach for estimating 3D body pose from multiple, uncalibrated views is proposed. First, a mapping from image features to 2D body joint locations is computed using a statistical framework that yields a set of several body pose hypotheses. The concept of a ``virtual camera'' is introduced that makes this mapping invariant to translation, image-plane rotation, and scaling of the input. As a consequence, the calibration matrices (intrinsics) of the virtual cameras can be considered completely known, and their poses are known up to a single angular displacement parameter. Given pose hypotheses obtained in the multiple virtual camera views, the recovery of 3D body pose and camera relative orientations is formulated as a stochastic optimization problem. An Expectation-Maximization algorithm is derived that can obtain the most likely (self-consistent) combination of body pose hypotheses. Performance of the approach is evaluated with synthetic sequences as well as real video sequences of human motion. :::::::::::::: 2001-009 :::::::::::::: Title: Surface Reconstruction from Multiple Views using Rational B-Splines Author: Matheen Siddiqui and Stan Sclaroff Date: June 5, 2001 A method for reconstructing 3D rational B-spline surfaces from multiple views is proposed. The method takes advantage of the projective invariance properties of rational B-splines. Given feature correspondences in multiple views, the 3D surface is reconstructed via a four step framework. First, corresponding features in each view are given an initial surface parameter value (s,t), and a 2D B-spline is fitted in each view. After this initialization, an iterative minimization procedure alternates between updating the 2D B-spline control points and re-estimating each feature's (s,t). Next, a non-linear minimization method is used to upgrade the 2D B-splines to 2D rational B-splines, and obtain a better fit. Finally, a factorization method is used to reconstruct the 3D B-spline surface given 2D B-splines in each view. This surface recovery method can be applied in both the perspective and orthographic case. The orthographic case allows the use of additional constraints in the recovery. Experiments with real and synthetic imagery demonstrate the efficacy of the approach for the orthographic case. :::::::::::::: 2001-010 :::::::::::::: Title: Inference and Labeling of Metric-Induced Network Topologies Authors: Azer Bestavros, John Byers, and Khaled Harfoush Date: June 5, 2001 Abstract: The deployment of distributed network-aware applications over the Internet requires an accurate representation of the conditions of underlying network resources. To be effective, this representation must be possible at multiple resolutions relative to a metric of interest. In this paper, we propose an approach for the construction of such representations using end-to-end measurements. We instantiate our approach by considering packet loss rates as an example metric. To that end, we present an analytical framework for the inference of Internet loss topologies. From the perspective of a server the loss topology is a logical tree rooted at the server with clients at its leaves, in which edges represent lossy paths---paths exhibiting observable loss rates higher than a specified resolution---between a pair of internal network nodes. We show how end-to-end unicast packet probing techniques could be used to (1) infer a loss topology, and (2) identify the loss rates of links in an existing loss topology. We report on simulation, implementation, and Internet deployment results that show the effectiveness of our approach and its robustness in terms of its accuracy and convergence. :::::::::::::: 2001-011 :::::::::::::: Title: On Class-based Isolation of UDP, Short-lived and Long-lived TCP Flows Author: Selma Yilmaz and Ibrahim Matta Date: June 12, 2001 Abstract: The congestion control mechanisms of TCP make it vulnerable in an environment where flows with different congestion-sensitivity compete for scarce resources. With the increasing amount of unresponsive UDP traffic in today's Internet, new mechanisms are needed to enforce fairness in the core of the network. We propose a scalable Diffserv-like architecture, where flows with different characteristics are classified into separate service queues at the routers. Such class-based isolation provides protection so that flows with different characteristics do not negatively impact one another. In this study, we examine different aspects of UDP and TCP interaction and possible gains from segregating UDP and TCP into different classes. We also investigate the utility of further segregating TCP flows into two classes, which are class of short and class of long flows. Results are obtained analytically for both Tail-drop and Random Early Drop (RED) routers. Class-based isolation have the following salient features: (1) better fairness, (2) improved predictability for all kinds of flows, (3) lower transmission delay for delay-sensitive flows, and (4) better control over Quality of Service (QoS) of a particular traffic type. :::::::::::::: 2001-012 :::::::::::::: Title: DNS-based Internet Client Clustering and Characterization Authors: Azer Bestavros and Sumit Mehrotra Date: June 5, 2001 Abstract: This paper proposes a novel protocol which uses the Internet Domain Name System (DNS) to partition Web clients into disjoint sets, each of which is associated with a single DNS server. We define an L-DNS cluster to be a grouping of Web Clients that use the same Local DNS server to resolve Internet host names. We identify such clusters in real-time using data obtained from a Web Server in conjunction with that server's Authoritative DNS---both instrumented with an implementation of our clustering algorithm. Using these clusters, we perform measurements from four distinct Internet locations. Our results show that L-DNS clustering enables a better estimation of proximity of a Web Client to a Web Server than previously proposed techniques. Thus, in a Content Distribution Network, a DNS-based scheme that redirects a request from a web client to one of many servers based on the client's name server coordinates (e.g., hops/latency/loss-rates between the client and servers) would perform better with our algorithm. :::::::::::::: 2001-013 :::::::::::::: Title: Open Issues on TCP for Mobile Computing Author: Vassilis Tsaoussidis (Northeastern University) and Ibrahim Matta (Boston University) Date: July 3, 2001 Abstract: We discuss the design principles of TCP within the context of heterogeneous wired/wireless networks and mobile networking. We identify three shortcomings in TCP's behavior: (i) the protocol's error detection mechanism, which does not distinguish different types of errors and thus does not suffice for heterogeneous wired/wireless environments, (ii) the error recovery, which is not responsive to the distinctive characteristics of wireless networks such as transient or burst errors due to handoffs and fading channels, and (iii) the protocol strategy, which does not control the tradeoff between performance measures such as goodput and energy consumption, and often entails a wasteful effort of retransmission and energy expenditure. We discuss a solution-framework based on selected research proposals and the associated evaluation criteria for the suggested modifications. We highlight an important angle that did not attract the required attention so far: the need for new performance metrics, appropriate for evaluating the impact of protocol strategies on battery-powered devices. Keywords: TCP, congestion control, wireless links, mobile computing, energy efficiency. :::::::::::::: 2001-014 :::::::::::::: Title: How does TCP generate Pseudo-self-similarity? Author: Liang Guo, Mark Crovella, and Ibrahim Matta Computer Science Department, Boston University Date: July 12, 2001 Abstract: Long-range dependence has been observed in many recent Internet traffic measurements. In addition, some recent studies have shown that under certain network conditions, TCP itself can produce traffic that exhibits dependence over limited timescales, even in the absence of higher-level variability. In this paper, we use a simple Markovian model to argue that when the loss rate is relatively high, TCP's adaptive congestion control mechanism indeed generates traffic with OFF periods exhibiting power-law shape over several timescales and thus introduces pseudo-long-range dependence into the overall traffic. Moreover, we observe that more variable initial retransmission timeout values for different packets introduces more variable packet inter-arrival times, which increases the burstiness of the overall traffic. We can thus explain why a single TCP connection can produce a time-series that can be misidentified as self-similar using standard tests. Keywords: Congestion Control, Long-Range Dependence, Self-Similarity. Revises Technical Report BUCS-TR-2000-017. :::::::::::::: 2001-015 :::::::::::::: Title: A Spectrum of TCP-friendly Window-based Congestion Control Algorithms Author: Shudong Jin, Liang Guo, Ibrahim Matta, and Azer Bestavros {jins, guol, matta, best}@cs.bu.edu Computer Science Department Boston University Date: February 2, 2001 Revised on April 27, 2001 Posted on July 12, 2001 Abstract: The increased diversity of Internet application requirements has spurred recent interests in transport protocols with flexible transmission controls. In window-based congestion control schemes, increase rules determine how to probe available bandwidth, whereas decrease rules determine how to back off when losses due to congestion are detected. The parameterization of these control rules is done so as to ensure that the resulting protocol is TCP-friendly in terms of the relationship between throughput and loss rate. In this paper, we define a new spectrum of window-based congestion control algorithms that are TCP-friendly as well as TCP-compatible under RED. Contrary to previous memory-less controls, our algorithms utilize history information in their control rules. Our proposed algorithms have two salient features: (1) They enable a wider region of TCP-friendliness, and thus more flexibility in trading off among smoothness, aggressiveness, and responsiveness; and (2) they ensure a faster convergence to fairness under a wide range of system conditions. SIMD is one instance of this spectrum of algorithms, in which the congestion window is increased super-linearly with time since the detection of the last loss. Compared to recently proposed TCP-friendly AIMD and binomial algorithms, we demonstrate the superiority of SIMD in: (1) adapting to sudden increases in available bandwidth, while maintaining competitive smoothness and responsiveness; and (2) rapidly converging to fairness and efficiency. Keywords: Congestion Control, TCP-friendly, Fairness, Convergence. :::::::::::::: 2001-016 :::::::::::::: Title: Measuring Bottleneck Bandwidth of Targeted Path Segments Author: Khaled Harfoush, Azer Bestavros, and John Byers {harfoush, best, byers}@cs.bu.edu Computer Science Department Boston University Date: July 31, 2001 Abstract: Accurate measurement of network bandwidth is crucial for flexible Internet applications and protocols which actively manage and dynamically adapt to changing utilization of network resources. These applications must do so to perform tasks such as distributing and delivering high-bandwidth media, scheduling service requests and performing admission control. Extensive work has focused on two approaches to measuring bandwidth: measuring it hop-by-hop, and measuring it end-to-end along a path. Unfortunately, best-practice techniques for the former are inefficient and techniques for the latter are only able to observe bottlenecks visible at end-to-end scope. In this paper, we develop and simulate end-to-end probing methods which can measure bottleneck bandwidth along arbitrary, targeted subpaths of a path in the network, including subpaths shared by a set of flows. As another important contribution, we describe a number of practical applications which we foresee as standing to benefit from solutions to this problem, especially in emerging, flexible network architectures such as overlay networks, ad-hoc networks, peer-to-peer architectures and massively accessed content servers. :::::::::::::: 2001-017 :::::::::::::: Title: Proceedings of the Sixth International Web Content Caching and Distribution Workshop (WCW'01) Author: Azer Bestavros and Michael Rabinovich Date: June 20-22, 2001 Posted August 2, 2001 Abstract: The International Web Content Caching and Distribution Workshop (WCW) is a premiere technical meeting for researchers and practitioners interested in all aspects of content caching, distribution and delivery on the Internet. This year's meeting will be held on the Boston University Campus and will build on the successes of the five previous WCW meetings. This technical report includes all the technical papers presented at WCW'01.  :::::::::::::: 2001-018 :::::::::::::: Title: STAIR: Practical AIMD Multirate Multicast Congestion Control Authors: John Byers and Gu-In Kwon Date: September 3, 2001 Abstract: Existing approaches for multirate multicast congestion control are either friendly to TCP only over large time scales or introduce unfortunate side effects, such as significant control traffic, wasted bandwidth, or the need for modifications to existing routers. We advocate a layered multicast approach in which steady-state receiver reception rates emulate the classical TCP sawtooth derived from additive-increase, multiplicative decrease (AIMD) principles. Our approach introduces the concept of dynamic {\em stair} layers to simulate various rates of additive increase for receivers with heterogeneous round-trip times (RTTs), facilitated by a minimal amount of IGMP control traffic. We employ a mix of cumulative and non-cumulative layering to minimize the amount of excess bandwidth consumed by receivers operating asynchronously behind a shared bottleneck. We integrate these techniques together into a congestion control scheme called STAIR which is amenable to those multicast applications which can make effective use of arbitrary and time-varying subscription levels. :::::::::::::: 2001-019 :::::::::::::: Title: Generating Good Degree Distributions for Sparse Parity Check Codes using Oracles Author: Jeffrey Considine Date: October 1, 2001 Abstract: Fast forward error correction codes are becoming an important component in bulk content delivery. They fit in naturally with multicast scenarios as a way to deal with losses and are now seeing use in peer to peer networks as a basis for distributing load. In particular, new irregular sparse parity check codes have been developed with provable average linear time performance, a significant improvement over previous codes. In this paper, we present a new heuristic for generating codes with similar performance based on observing a server with an oracle for client state. This heuristic is easy to implement and provides further intuition into the need for an irregular heavy tailed distribution. :::::::::::::: 2001-020 :::::::::::::: Title: Gismo: A Generator of Internet Streaming Media Objects and Workloads Authors: Shudong Jin and Azer Bestavros Date: October 10, 2001 Abstract: This paper presents a tool called GISMO (Generator of Internet Streaming Media Objects and workloads). GISMO enables the specification of a number of streaming media access characteristics, including object popularity, temporal correlation of requests, seasonal access patterns, user session durations, user inter-activity times, and variable bit-rate (VBR) self-similarity and marginal distributions. The embodiment of these characteristics in GISMO enables the generation of realistic and scalable request streams for use in the benchmarking and comparative evaluation of Internet streaming media delivery techniques. To demonstrate the usefulness of GISMO, we present a case study that shows the importance of various workload characteristics in determining the effectiveness of proxy caching and server patching techniques in reducing bandwidth requirements. :::::::::::::: 2001-021 :::::::::::::: Title: 3D Hand Pose Estimation by Finding Appearance-Based Matches in a Large Database of Training Views Authors: Vassilis Athitsos and Stan Sclaroff Date: October 22, 2001 Abstract: Ongoing work towards appearance-based 3D hand pose estimation from a single image is presented. Using a 3D hand model and computer graphics a large database of synthetic views is generated. The views display different hand shapes as seen from arbitrary viewpoints. Each synthetic view is automatically labeled with parameters describing its hand shape and viewing parameters. Given an input image, the system retrieves the most similar database views, and uses the shape and viewing parameters of those views as candidate estimates for the parameters of the input image. Preliminary results are presented, in which appearance-based similarity is defined in terms of the chamfer distance between edge images. :::::::::::::: 2001-022 :::::::::::::: Title: An Appearance-Based Framework for 3D Hand Shape Classification and Camera Viewpoint Estimation Authors: Vassilis Athitsos and Stan Sclaroff Date: October 22, 2001 Abstract: An appearance-based framework for 3D hand shape classification and simultaneous camera viewpoint estimation is presented. Given an input image of a segmented hand, the most similar matches from a large database of synthetic hand images are retrieved. The ground truth labels of those matches, containing hand shape and camera viewpoint information, are returned by the system as estimates for the input image. Database retrieval is done hierarchically, by first quickly rejecting the vast majority of all database views, and then ranking the remaining candidates in order of similarity to the input. Four different similarity measures are employed, based on edge location, edge orientation, finger location and geometric moments. :::::::::::::: 2001-023 :::::::::::::: Title: Accelerating Internet Streaming Media Delivery using Network-Aware Partial Caching Authors: Shudong Jin and Azer Bestavros Date: October 30, 2001 Abstract: Internet streaming applications are adversly affected by network conditions such as high packet loss rates and long delays. This paper aims at mitigating such effects by leveraging the availability of client-side caching proxies. We present a novel caching architecture (and associated cache management algorithms) that turn edge caches into accelerators of streaming media delivery. A salient feature of our caching algorithms is that they allow partial caching of streaming media objects and joint delivery of content from caches and origin servers. The caching algorithms we propose are both network-aware and stream-aware; they take into account the popularity of streaming media objects, their bit-rate requirements, and the available bandwidth between clients and servers. Using realistic models of Internet bandwidth (derived from proxy cache logs and measured over real Internet paths), we have conducted extensive simulations to evaluate the performance of various caching management alternatives. Our experiments demonstrate that network-aware caching algorithms can significantly reduce service delay and improve overall stream quality. Also, our experiments show that partial caching is particularly effective when bandwidth variability is not very high. :::::::::::::: 2001-024 :::::::::::::: Title: Basis Token Consistency: A Practical Mechanism for Strong Web Cache Consistency Authors: Adam D. Bradley and Azer Bestavros Date: November 1, 2001 Abstract: With web caching and cache-related services like CDNs and edge services playing an increasingly significant role in the modern internet, the problem of the weak consistency and coherence provisions in current web protocols is becoming increasingly significant and drawing the attention of the standards community. Toward this end, we present definitions of consistency and coherence for web-like environments, that is, distributed client-server information systems where the semantics of interactions with resources are more general than the read/write operations found in memory hierarchies and distributed file systems. We then present a brief review of proposed mechanisms which strengthen the consistency of caches in the web, focusing upon their conceptual contributions and their weaknesses in real-world practice. These insights motivate a new mechanism, which we call ``Basis Token Consistency'' or BTC; when implemented at the server, this mechanism allows any client (independent of the presence and conformity of any intermediaries) to maintain a self-consistent view of the server's state. This is accomplished by annotating responses with additional per-resource application information which allows client caches to recognize the obsolescence of currently cached entities and identify responses from other caches which are already stale in light of what has already been seen. The mechanism requires no deviation from the existing client-server communication model, and does not require servers to maintain any additional per-client state. We discuss how our mechanism could be integrated into a fragment-assembling Content Management System (CMS), and present a simulation-driven performance comparison between the BTC algorithm and the use of the Time-To-Live (TTL) heuristic. :::::::::::::: 2001-025 :::::::::::::: Title: Scalability of Multicast Delivery for Non-sequential Streaming Access Author: Shudong Jin and Azer Bestavros Date: October 30, 2001 Abstract: Multicast is considered a panacea for scalable streaming media delivery over the Internet. To enable asynchronous service over a multicast infrastructure, two categories of techniques have been proposed: stream merging and periodic broadcasting. The scalability of these techniques stems from the fact that for sequential streaming access, the required server bandwidth grows {\em logarithmically} with request arrival rates for stream merging techniques, and {\em logarithmically} with the inverse of start-up delay for periodic multicasting techniques. Recent studies raise doubts as to the appropriateness of the sequential access model (in which access to a stream proceeds uninterrupted from beginning to end). A non-sequential access model (allowing access to start at random points in the stream) is more accurate as it allows the modeling of partial access and client inter-activity. In this paper, we analytically and experimentally (re-)evaluate the scalability of multicast delivery under a non-sequential access model. We show that under such a realistic model, the required server bandwidth for any protocol providing immediate service grows at least as fast as the {\em square root} of the request arrival rate, and that the required server bandwidth for any protocol providing delayed service grows {\em linearly} with the inverse of the start-up delay. We also investigate the impact of limited client bandwidth on scalability. We present practical protocols, which provide immediate service to non-sequential requests (subject to limited client bandwidth), and which are near-optimal in that the required server bandwidth is very close to its lower bound. :::::::::::::: 2001-026 :::::::::::::: Title: How does TCP generate Pseudo-self-similarity? (ERRATA) Authors: Liang Guo, Mark Crovella, and Ibrahim Matta Computer Science Department, Boston University Date: November 7, 2001 Abstract: In this note we clarify and amend a number of points made in BUCS Technical Report BUCS-TR-2001-014 (also published in MASCOTS 2001). We address the relationship to Technical Report UMass-CMPSC-00-55 by Figueiredo, Liu, Misra, and Towsley. :::::::::::::: 2002-001 :::::::::::::: Title: Specialized Mappings Architecture with Applications to Vision-Based Estimation of Articulated Body Pose Author: Romer Rosales Abstract: A fundamental task of vision systems is to infer the state of the world given some form of visual observations. From a computational perspective, this often involves facing an ill-posed problem; e.g., information is lost via projection of the 3D world into a 2D image. Solution of an ill-posed problem requires additional information, usually provided as a model of the underlying process. It is important that the model be both computationally feasible as well as theoretically well-founded. In this thesis, a probabilistic, nonlinear supervised computational learning model is proposed: the Specialized Mappings Architecture (SMA). The SMA framework is demonstrated in a computer vision system that can estimate the articulated pose parameters of a human body or human hands, given images obtained via one or more uncalibrated cameras. The SMA consists of several specialized forward mapping functions that are estimated automatically from training data, and a possibly known feedback function. Each specialized function maps certain domains of the input space (e.g., image features) onto the output space (e.g., articulated body parameters). A probabilistic model for the architecture is first formalized. Solutions to key algorithmic problems are then derived: simultaneous learning of the specialized domains along with the mapping functions, as well as performing inference given inputs and a feedback function. The SMA employs a variant of the Expectation-Maximization algorithm and approximate inference. The approach allows the use of alternative conditional independence assumptions for learning and inference, which are derived from a forward model and a feedback model. Experimental validation of the proposed approach is conducted in the task of estimating articulated body pose from image silhouettes. Accuracy and stability of the SMA framework is tested using artificial data sets, as well as synthetic and real video sequences of human bodies and hands. :::::::::::::: 2002-002 :::::::::::::: Title: Securing Bulk Content Almost for Free Authors: John Byers, Mei Chin Cheng, Jeffrey Considine, Gene Itkis, Alex Yeung Date: January 22, 2002 Abstract: Content providers often consider the costs of security to be greater than the losses they might incur without it; many view ``casual piracy'' as their main concern. Our goal is to provide a low cost defense against such attacks while maintaining rigorous security guarantees. Our defense is integrated with and leverages fast forward error correcting codes, such as Tornado codes, which are widely used to facilitate reliable delivery of rich content. We tune one such family of codes - while preserving their original desirable properties - to guarantee that none of the original content can be recovered whenever a key subset of encoded packets is missing. Ultimately we encrypt only these key codewords (only 4% of all transmissions), making the security overhead negligible. :::::::::::::: 2002-003 :::::::::::::: Title: Deanonymizing Users of the SafeWeb Anonymizing Service Authors: David Martin, Andrew Schulman Date: Feb 11, 2002 Abstract The SafeWeb anonymizing system has been lauded by the press and loved by its users; self-described as "the most widely used online privacy service in the world," it served over 3,000,000 page views per day at its peak. SafeWeb was designed to defeat content blocking by firewalls and to defeat Web server attempts to identify users, all without degrading Web site behavior or requiring users to install specialized software. In this article we describe how these fundamentally incompatible requirements were realized in SafeWeb's architecture, resulting in spectacular failure modes under simple JavaScript attacks. These exploits allow adversaries to turn SafeWeb into a weapon against its users, inflicting more damage on them than would have been possible if they had never relied on SafeWeb technology. By bringing these problems to light, we hope to remind readers of the chasm that continues to separate popular and technical notions of security. :::::::::::::: 2002-004 :::::::::::::: Title: Small-World Internet Topologies: Possible Causes and Implications on Scalability of End-System Multicast Author: Shudong Jin and Azer Bestavros Date: January 30, 2002 Abstract: Recent work has shown the prevalence of small-world phenomena in many networks. Small-world graphs exhibit a high degree of clustering, yet have typically short path lengths between arbitrary vertices. Internet AS-level graphs have been shown to exhibit small-world behaviors. In this paper, we show that both Internet AS-level and router-level graphs exhibit small-world behavior. We attribute such behavior to two possible causes--namely the high variability of vertex degree distributions (which were found to follow approximately a power law) and the preference of vertices to have local connections. We show that both factors contribute with different relative degrees to the small-world behavior of AS-level and router-level topologies. Our findings underscore the inefficacy of the Barabasi-Albert model in explaining the growth process of the Internet, and provide a basis for more promising approaches to the development of Internet topology generators. We present such a generator and show the resemblance of the synthetic graphs it generates to real Internet AS-level and router-level graphs. Using these graphs, we have examined how small-world behaviors affect the scalability of end-system multicast. Our findings indicate that lower variability of vertex degree and stronger preference for local connectivity in small-world graphs results in slower network neighborhood expansion, and in longer average path length between two arbitrary vertices, which in turn results in better scaling of end system multicast. :::::::::::::: 2002-005 :::::::::::::: Title: PeriScope: An Active Internet Probing and Measurement API Author: Khaled Harfoush, Azer Bestavros, and John Byers {harfoush, best, byers}@cs.bu.edu Computer Science Department Boston University Date: January 30, 2002 Abstract: Growing interest in inference and prediction of network characteristics is justified by its importance for a variety of network-aware applications. One widely adopted strategy to characterize network conditions relies on active, end-to-end probing of the network. Active end-to-end probing techniques differ in (1) the structural composition of the probes they use (e.g., number and size of packets, the destination of various packets, the protocols used, etc.), (2) the entity making the measurements (e.g. sender vs. receiver), and (3) the techniques used to combine measurements in order to infer specific metrics of interest. In this paper, we present PeriScope, a Linux API that enables the definition of new probing structures and inference techniques from user space through a flexible interface. PeriScope requires no support from clients beyond the ability to respond to ICMP ECHO REQUESTs and is designed to minimize user/kernel crossings and to ensure various constraints (e.g., back-to-back packet transmissions, fine-grained timing measurements) We show how to use PeriScope for two different probing purposes, namely the measurement of shared packet losses between pairs of endpoints and for the measurement of subpath bandwidth. Results from Internet experiments for both of these goals are also presented. :::::::::::::: 2002-006 :::::::::::::: :::::::::::::: 2002-007 :::::::::::::: Title: Informed Content Delivery Across Adaptive Overlay Networks Authors: John Byers, Jeffrey Considine, Michael Mitzenmacher, Stanislav Rost Date: 3/4/2002 Abstract: Overlay networks have emerged as a powerful and highly flexible method for delivering content. We study how to optimize throughput of large, multipoint transfers across richly connected overlay networks, focusing on the question of what to put in each transmitted packet. We first make the case for transmitting encoded content in this scenario, arguing for the digital fountain approach which enables end-hosts to efficiently restitute the original content of size n from a subset of any n symbols from a large universe of encoded symbols. Such an approach affords reliability and a substantial degree of application-level flexibility, as it seamlessly tolerates packet loss, connection migration, and parallel transfers. However, since the sets of symbols acquired by peers are likely to overlap substantially, care must be taken to enable them to collaborate effectively. We provide a collection of useful algorithmic tools for efficient estimation, summarization, and approximate reconciliation of sets of symbols between pairs of collaborating peers, all of which keep messaging complexity and computation to a minimum. Through simulations and experiments on a prototype implementation, we demonstrate the performance benefits of our informed content delivery mechanisms and how they complement existing overlay network architectures. :::::::::::::: 2002-008 :::::::::::::: Title: End-to-End Inference of Loss Nature in a Hybrid Wired/Wireless Environment Authors: Jun Liu, Ibrahim Matta, and Mark Crovella (Boston University) Date: March 14, 2002 Abstract: End-to-End differentiation between wireless and congestion loss can equip TCP control so it operates effectively in a hybrid wired/wireless environment. Our approach integrates two techniques: packet loss pairs (PLP) and Hidden Markov Modeling (HMM). A packet loss pair is formed by two back-to-back packets, where one packet is lost while the second packet is successfully received. The purpose is for the second packet to carry the state of the network path, namely the round trip time (RTT), at the time the other packet is lost. Under realistic conditions, PLP provides strong differentiation between congestion and wireless type of loss based on distinguishable RTT distributions. An HMM is then trained so observed RTTs can be mapped to model states that represent either congestion loss or wireless loss. Extensive simulations confirm the accuracy of our HMM-based technique in classifying the cause of a packet loss. We also show the superiority of our technique over the Vegas predictor, which was recently found to perform best and which exemplifies other existing loss labeling techniques. :::::::::::::: 2002-009 :::::::::::::: Title: Scheduling Flows with Unknown Sizes: Approximate Analysis Author: Liang Guo and Ibrahim Matta Date: March 21, 2002 Abstract: Previous studies have shown that giving preferential treatment to short jobs helps reduce the average system response time, especially when the job size distribution possesses the heavy-tailed property. Since it has been shown that the TCP flow length distribution also has the same property, it is natural to let short TCP flows enjoy better service inside the network. Analyzing such discriminatory system requires modification to traditional job scheduling models since usually network traffic managers do not have detailed knowledge about individual flows such as their lengths. The Multi-Level (ML) queue, proposed by Kleinrock, can be used to characterize such system. In an ML queueing system, the priority of a flow is reduced as the flow stays longer. We present an approximate analysis of the ML queueing system to obtain a closed-form solution of the average system response time function. We show that the response time of short flows can be significantly reduced without penalizing long flows. :::::::::::::: 2002-010 :::::::::::::: Title: Surface Reconstruction from Multiple Views using Rational B-Splines and Knot Insertion Authors: Matheen Siddiqui and Stan Sclaroff Date: March 25, 2002 Abstract: A method for reconstruction of 3D rational B-spline surfaces from multiple views is proposed. Given corresponding features in multiple views, though not necessarily visible in all views, the surface is reconstructed. First 2D B-spline patches are fitted to each view. The 3D B-splines and projection matricies can then be extracted from the 2D B-splines using factorization methods. The surface fit is then further refined via an iterative procedure. Finally, a hierarchal fitting scheme is proposed to allow modeling of complex surfaces by means of knot insertion. Experiments with real imagery demonstrate the efficacy of the approach. :::::::::::::: 2002-011 :::::::::::::: Title: Automatic Detection of Relevant Head Gestures in American Sign Language Communication Authors: Ugur Murat Erdem and Stan Sclaroff Date: May 3, 2002 Abstract: An automated system for detection of head movements is described. The goal is to label relevant head gestures in video of American Sign Language (ASL) communication. In the system, a 3D head tracker recovers head rotation and translation parameters from monocular video. Relevant head gestures are then detected by analyzing the length and frequency of the motion signal's peaks and valleys. Each parameter is analyzed independently, due to the fact that a number of relevant head movements in ASL are associated with major changes around one rotational axis. No explicit training of the system is necessary. Currently, the system can detect ``head shakes." In experimental evaluation, classification performance is compared against ground-truth labels obtained from ASL linguists. Initial results are promising, as the system matches the linguists' labels in a significant number of cases. Keywords: Computer human interaction, gesture classification, visual motion, image and video indexing. :::::::::::::: 2002-012 :::::::::::::: Title: Differentiated Control of Web Traffic: A Numerical Analysis Authors: Liang Guo and Ibrahim Matta Computer Science Department Boston University Date: May 10, 2002 Abstract: Internet measurements show that the size distribution of Web-based transactions is usually very skewed; a few large requests constitute most of the total traffic. Motivated by the advantages of scheduling algorithms which favor short jobs, we propose to perform differentiated control over Web-based transactions to give preferential service to short web requests. The control is realized through service semantics provided by Internet Traffic Managers, a Diffserv-like architecture. To evaluate the performance of such a control system, it is necessary to have a fast but accurate analytical method. To this end, we model the Internet as a time-shared system and propose a numerical approach which utilizes Kleinrock's conservation law to solve the model. The numerical results are shown to match well those obtained by packet-level simulation, which runs orders of magnitude slower than our numerical method. Keywords: Heavy-tailed Distributions, TCP Congestion Control, Traffic Engineering. :::::::::::::: 2002-013 :::::::::::::: Title: On the Scalability-Performance Tradeoffs in MPLS and IP Routing Authors: Selma Yilmaz and Ibrahim Matta Computer Science Department Boston University Date: May 10, 2002 Abstract: MPLS (Multi-Protocol Label Switching) has recently emerged to facilitate the engineering of network traffic. This can be achieved by directing packet flows over paths that satisfy multiple requirements. MPLS has been regarded as an enhancement to traditional IP routing, which has the following problems: (1) all packets with the same IP destination address have to follow the same path through the network; and (2) paths have often been computed based on static and single link metrics. These problems may cause traffic concentration, and thus degradation in quality of service. In this paper, we investigate by simulations a range of routing solutions and examine the tradeoff between scalability and performance. At one extreme, IP packet routing using dynamic link metrics provides a stateless solution but may lead to routing oscillations. At the other extreme, we consider a recently proposed Profile-based Routing (PBR), which uses knowledge of potential ingress-egress pairs as well as the traffic profile among them. Minimum Interference Routing (MIRA) is another recently proposed MPLS-based scheme, which only exploits knowledge of potential ingress-egress pairs but not their traffic profile. MIRA and the more conventional widest-shortest path (WSP) routing represent alternative MPLS-based approaches on the spectrum of routing solutions. We compare these solutions in terms of utility, bandwidth acceptance ratio as well as their scalability (routing state and computational overhead) and load balancing capability. While the simplest of the per-flow algorithms we consider, the performance of WSP is close to dynamic per-packet routing, without the potential instabilities of dynamic routing. Keywords: Multi-Protocol Label Switching, IP Routing, Constraint-Based Routing, Multicommodity Flow Algorithms, Simulation. :::::::::::::: 2002-014 :::::::::::::: Title: A Hierarchical Characterization of a Live Streaming Media Workload Authors: Eveline Veloso, Virgilio Almeida, Wagner Meira (Federal University of Minas Gerais, Brazil), and Azer Bestavros, Sudong Jin (Boston University, Massachusetts, USA) Date: May 10, 2002 Abstract: We present what we believe to be the first thorough characterization of live streaming media content delivered over the Internet. Our characterization of over five million requests spanning a 28-day period is done at three increasingly granular levels, corresponding to clients, sessions, and transfers. Our findings support two important conclusions. First, we show that the nature of interactions between users and objects is fundamentally different for live versus stored objects. Access to stored objects is user driven, whereas access to live objects is object driven}. This reversal of active/passive roles of users and objects leads to interesting dualities. For instance, our analysis underscores a Zipf-like profile for user interest in a given object, which is to be contrasted to the classic Zipf-like popularity of objects for a given user. Also, our analysis reveals that transfer lengths are highly variable and that this variability is due to the stickiness of clients to a particular live object, as opposed to structural (size) properties of objects. Second, based on observations we make, we conjecture that the particular characteristics of live media access workloads are likely to be highly dependent on the nature of the live content being accessed. In our study, this dependence is clear from the strong temporal correlations we observed in the traces, which we attribute to the synchronizing impact of live content on access characteristics. Based on our analyses, we present a model for live media workload generation that incorporates many of our findings, and which we implement in GISMO. Keywords: Live streaming content delivery; streaming media characterization; synthetic workload generation. :::::::::::::: 2002-015 :::::::::::::: Title: On the Geographic Location of Internet Resources Authors: Anukool Lakhina, John Byers, Mark Crovella, and Ibrahim Matta Date: May 21, 2002 Abstract: One relatively unexplored question about the Internet's physical structure concerns the geographical location of its components: routers, links and autonomous systems (ASes). We study this question using two large inventories of Internet routers and links, collected by different methods and about two years apart. We first map each router to its geographical location using two different state-of-the-art tools. We then study the relationship between router location and population density; between geographic distance and link density; and between the size and geographic extent of ASes. Our findings are consistent across the two datasets and both mapping methods. First, as expected, router density per person varies widely over different economic regions; however, in economically homogeneous regions, router density shows a strong superlinear relationship to population density. Second, the probability that two routers are directly connected is strongly dependent on distance; our data is consistent with a model in which a majority (up to 75-95\%) of link formation is based on geographical distance (as in the Waxman topology generation method). Finally, we find that ASes show high variability in geographic size, which is correlated with other measures of AS size (degree and number of interfaces). Among small to medium ASes, ASes show wide variability in their geographic dispersal; however, all ASes exceeding a certain threshold in size are maximally dispersed geographically. These findings have many implications for the next generation of topology generators, which we envisage as producing router-level graphs annotated with attributes such as link latencies, AS identifiers and geographical locations. :::::::::::::: 2002-016 :::::::::::::: Title: Effectiveness of Loss Labeling in Improving TCP Performance in Wired/Wireless Networks Authors: Dhiman Barman and Ibrahim Matta Computer Science Department Boston University Date: May 22, 2002 Abstract: The current congestion-oriented design of TCP hinders its ability to perform well in hybrid wireless/wired networks. We propose a new improvement on TCP NewReno (NewReno-FF) using a new loss labeling technique to discriminate wireless from congestion losses. The proposed technique is based on the estimation of average and variance of the round trip time using a filter called Flip Flop filter that is augmented with history information. We show the comparative performance of TCP NewReno, NewReno-FF, and TCP Westwood through extensive simulations. We study the fundamental gains and limits using TCP NewReno with varying Loss Labeling accuracy (NewReno-LL) as a benchmark. Lastly our investigation opens up important research directions. First, there is a need for a finer grained classification of losses (even within congestion and wireless losses) for TCP in heterogeneous networks. Second, it is essential to develop an appropriate control strategy for recovery after the correct classification of a packet loss. Keywords: TCP; Congestion Control; Error Control; Loss Labeling (Classification); Wireless Links; Simulation. :::::::::::::: 2002-017 :::::::::::::: Title: Safe Composition of Web Communication Protocols for Extensible Edge Services Authors: Adam Bradley, Azer Bestavros, and Assaf Kfoury Date: May 22, 2002 Abstract: As new multi-party edge services are deployed on the Internet, application-layer protocols with complex communication models and event dependencies are increasingly being specified and adopted. To ensure that such protocols (and compositions thereof with existing protocols) do not result in undesirable behaviors (e.g., livelocks) there needs to be a methodology for the automated checking of the ``safety'' of these protocols. In this paper, we present ingredients of such a methodology. Specifically, we show how SPIN, a tool from the formal systems verification community, can be used to quickly identify problematic behaviors of application-layer protocols with non-trivial communication models---such as HTTP with the addition of the ``100 Continue'' mechanism. As a case study, we examine several versions of the specification for the Continue mechanism; our experiments mechanically uncovered multi-version interoperability problems, including some which motivated revisions of HTTP/1.1 and some which persist even with the current version of the protocol. One such problem resembles a classic degradation-of-service attack, but can arise between well-meaning peers. We also discuss how the methods we employ can be used to make explicit the requirements for hardening a protocol's implementation against potentially malicious peers, and for verifying an implementation's interoperability with the full range of allowable peer behaviors. Keywords: Formal verification, HTTP, Interoperability, Model checking, Protocol composition. :::::::::::::: 2002-018 :::::::::::::: Title: Unicast Routing: Cost-Performance Tradeoffs Author: Selma Yilmaz and Ibrahim Matta Date: July 5, 2002 Abstract: The objective of unicast routing is to find a path from a source to a destination. Conventional routing has been used mainly to provide connectivity. It lacks the ability to provide any kind of service guarantees and smart usage of network resources. Improving performance is possible by being aware of both traffic characteristics and current available resources. This paper surveys a range of routing solutions, which can be categorized depending on the degree of the awareness of the algorithm: (1) QoS/Constraint-based routing solutions are aware of traffic requirements of individual connection requests; (2) Traffic-aware routing solutions assume knowledge of the location of communicating ingress-egress pairs and possibly the traffic demands among them; (3) Routing solutions that are both QoS-aware as (1) and traffic-aware as (2); (4) Best-effort solutions are oblivious to both traffic and QoS requirements, but are adaptive only to current resource availability. The best performance can be achieved by having all possible knowledge so that while finding a path for an individual flow, one can make a smart choice among feasible paths to increase the chances of supporting future requests. However, this usually comes at the cost of increased complexity and decreased scalability. In this paper, we discuss such cost-performance tradeoffs by surveying proposed heuristic solutions and hybrid approaches. :::::::::::::: 2002-019 :::::::::::::: Title: Fast Approximate Reconciliation of Set Differences Authors: John W. Byers, Jeffrey Considine and Michael Mitzenmacher Date: 7/11/02 Abstract: We present new, simple, efficient data structures for approximate reconciliation of set differences, a useful standalone primitive for peer-to-peer networks and a natural subroutine in methods for exact reconciliation. In the approximate reconciliation problem, peers A and B respectively have subsets of elements S(A) and S(B) of a large universe U. Peer A wishes to send a short message M to peer B with the goal that B should use M to determine as many elements in the set S(B) - S(A) as possible. To avoid the expense of round trip communication times, we focus on the situation where a single message M is sent. We motivate the performance tradeoffs between message size, accuracy and computation time for this problem with a straightforward approach using Bloom filters. We then introduce approximation reconciliation trees, a more computationally efficient solution that combines techniques from Patricia tries, Merkle trees, and Bloom filters. We present an analysis of approximation reconciliation trees and provide experimental results comparing the various methods proposed for approximate reconciliation. :::::::::::::: 2002-020 :::::::::::::: Title: Graph Wavelets for Spatial Traffic Analysis Authors: Mark Crovella and Eric Kolaczyk Date: July 15, 2002 Abstract: A number of problems in network operations and engineering call for new methods of traffic analysis. While most existing traffic analysis methods are fundamentally temporal, there is a clear need for the analysis of traffic across multiple network links --- that is, for spatial traffic analysis. In this paper we give examples of problems that can be addressed via spatial traffic analysis. We then propose a formal approach to spatial traffic analysis based on the wavelet transform. Our approach generalizes the traditional wavelet transform so that it can be applied to data elements connected via an arbitrary topology. We explore the necessary and desirable properties of this approach (graph wavelets) and consider some of its possible realizations. We then apply graph wavelets to measurements from an operating network. Our results show that graph wavelets are very useful for our motivating problems; for example, they can be used to form highly summarized views of an entire network's traffic load, to gain insight into a network's global traffic response to a link failure, and to localize the extent of a failure event within the network. :::::::::::::: 2002-021 :::::::::::::: Title: Sampling Biases in IP Topology Measurements Authors: Anukool Lakhina, John W. Byers, Mark Crovella, and Peng Xie Date: July 15, 2002 Abstract: Considerable attention has been focused on the properties of graphs derived from Internet measurements. Router-level topologies collected via traceroute studies have led some authors to conclude that the router graph of the Internet is a scale-free graph, or more generally a power-law random graph. In such a graph, the degree distribution of nodes follows a distribution with a power-law tail. In this paper we argue that the evidence to date for this conclusion is at best insufficient. We show that graphs appearing to have power-law degree distributions can arise surprisingly easily, when sampling graphs whose true degree distribution is not at all like a power-law. For example, given a classical Erdos-Renyi sparse, random graph, the subgraph formed by a collection of shortest paths from a small set of random sources to a larger set of random destinations can easily appear to show a degree distribution remarkably like a power-law. We explore the reasons for how this effect arises, and show that in such a setting, edges are sampled in a highly biased manner. This insight allows us to distinguish measurements taken from the Erdos-Renyi graphs from those taken from power-law random graphs. When we apply this distinction to a number of well-known datasets, we find that the evidence for sampling bias in these datasets is strong. :::::::::::::: 2002-022 :::::::::::::: Title: On the Intrinsic Locality Properties of Web Reference Streams Authors: Rodrigo Fonseca, Virgilio Almeida, Mark Crovella, and Bruno Abrahao Date: July 15, 2002 Abstract: There has been considerable work done in the study of Web reference streams: sequences of requests for Web objects. In particular, many studies have looked at the locality properties of such streams, because of the impact of locality on the design and performance of caching and prefetching systems. However, a general framework for understanding why reference streams exhibit given locality properties has not yet emerged. In this paper we take a first step in this direction. We propose a framework for describing how reference streams are transformed as they pass through the Internet, based on three operations: aggregation, disaggregation, and filtering. We also propose metrics to capture the temporal locality of reference streams in this framework. We argue that these metrics (marginal entropy and interreference coefficient of variation) are more natural and more useful than previously proposed metrics for temporal locality; and we show that these metrics provide insight into the nature of reference stream transformations in the Web. :::::::::::::: 2002-023 :::::::::::::: Title: A Self-initializing Eyebrow Tracker for Binary Switch Emulation Authors: Jonathan Lombardi and Margrit Betke, Boston University Date: 9/20/2002 Abstract: We designed the "Eyebrow-Clicker," a camera-based human computer interface system that implements a new form of binary switch. When the user raises his or her eyebrows, the binary switch is activated and a selection command is issued. The Eyebrow-Clicker thus replaces the "click" functionality of a mouse. The system initializes itself by detecting the user's eyes and eyebrows, tracks these features at frame rate, and recovers in the event of errors. The initialization uses the natural blinking of the human eye to select suitable templates for tracking. Once execution has begun, a user therefore never has to restart the program or even touch the computer. In our experiments with human-computer interaction software, the system successfully determined 93% of the time when a user raised his eyebrows. :::::::::::::: 2002-024 :::::::::::::: Title: Cache-and-Relay Streaming Media Delivery for Asynchronous Clients Authors: Shudong Jin and Azer Bestavros Date: September 20, 2002 Abstract: We consider the problem of delivering popular streaming media to a large number of asynchronous clients. We propose and evaluate a cache-and-relay end-system multicast approach, whereby a client joining a multicast session caches the stream, and if needed, relays that stream to neighboring clients which may join the multicast session at some later time. This cache-and-relay approach is fully distributed, scalable, and efficient in terms of network link cost. In this paper we analytically derive bounds on the network link cost of our cache-and-relay approach, and we evaluate its performance under assumptions of limited client bandwidth and limited client cache capacity. When client bandwidth is limited, we show that although finding an optimal solution is NP-hard, a simple greedy algorithm performs surprisingly well in that it incurs network link costs that are very close to a theoretical lower bound. When client cache capacity is limited, we show that our cache-and-relay approach can still significantly reduce network link cost. We have evaluated our cache-and-relay approach using simulations over large, synthetic random networks, power-law degree networks, and small-world networks, as well as over large real router-level Internet maps. :::::::::::::: 2002-025 :::::::::::::: Title: Smooth Multirate Multicast Congestion Control Authors: Gu-In Kwon and John W. Byers Date: September 25, 2002 Abstract: A significant impediment to deployment of multicast services is the daunting technical complexity of developing, testing and validating congestion control protocols fit for wide-area deployment. Protocols such as pgmcc and TFMCC have recently made considerable progress on the single rate case, i.e. where one dynamic reception rate is maintained for all receivers in the session. However, these protocols have limited applicability, since scaling to session sizes beyond tens of participants necessitates the use of multiple rate protocols. Unfortunately, while existing multiple rate protocols exhibit better scalability, they are both less mature than single rate protocols and suffer from high complexity. We propose a new approach to multiple rate congestion control that leverages proven single rate congestion control methods by orchestrating an ensemble of independently controlled single rate sessions. We describe SMCC, a new multiple rate equation-based congestion control algorithm for layered multicast sessions that employs TFMCC as the primary underlying control mechanism for each layer. SMCC combines the benefits of TFMCC (smooth rate control, equation-based TCP friendliness) with the scalability and flexibility of multiple rates to provide a sound multiple rate multicast congestion control policy. :::::::::::::: 2002-026 :::::::::::::: Title: Scalable Peer-to-Peer Indexing with Constant State Authors: Jeffrey Considine and Thomas Florio Date: September 25, 2002 Abstract: We present a distributed indexing scheme for peer to peer networks. Past work on distributed indexing traded off fast search times with non-constant degree topologies or network-unfriendly behavior such as flooding. In contrast, the scheme we present optimizes all three of these performance measures. That is, we provide logarithmic round searches while maintaining connections to a fixed number of peers and avoiding network flooding. In comparison to the well known scheme Chord, we provide competitive constant factors. Finally, we observe that arbitrary linear speedups are possible and discuss both a general brute force approach and specific economical optimizations. :::::::::::::: 2002-027 :::::::::::::: Title: A Spectrum of TCP-friendly Window-based Congestion Control Algorithms Author: Shudong Jin, Liang Guo, Ibrahim Matta, and Azer Bestavros {jins, guol, matta, best}@cs.bu.edu Computer Science Department Boston University Date: February 2, 2001 Revised on April 27, 2001 Posted on July 12, 2001 Abstract: This technical report revises BUCS-TR-2001-015 and is a longer version of a paper to appear in IEEE/ACM Transactions on Networking. The increased diversity of Internet application requirements has spurred recent interests in transport protocols with flexible transmission controls. In window-based congestion control schemes, increase rules determine how to probe available bandwidth, whereas decrease rules determine how to back off when losses due to congestion are detected. The parameterization of these control rules is done so as to ensure that the resulting protocol is TCP-friendly in terms of the relationship between throughput and loss rate. In this paper, we define a new spectrum of window-based congestion control algorithms that are TCP-friendly as well as TCP-compatible under RED. Contrary to previous memory-less controls, our algorithms utilize history information in their control rules. Our proposed algorithms have two salient features: (1) They enable a wider region of TCP-friendliness, and thus more flexibility in trading off among smoothness, aggressiveness, and responsiveness; and (2) they ensure a faster convergence to fairness under a wide range of system conditions. SIMD is one instance of this spectrum of algorithms, in which the congestion window is increased super-linearly with time since the detection of the last loss. Compared to recently proposed TCP-friendly AIMD and binomial algorithms, we demonstrate the superiority of SIMD in: (1) adapting to sudden increases in available bandwidth, while maintaining competitive smoothness and responsiveness; and (2) rapidly converging to fairness and efficiency. Keywords: Congestion Control, TCP-friendly, Fairness, Convergence. :::::::::::::: 2002-028 :::::::::::::: Title: A Short History of Computational Complexity Author: Lance Fortnow, NEC Research and Steven Homer, BU Date: October 30, 2002 Abstract: A brief history of the major issues and developments in computational complexity theory over the past 30 years is presented. This paper will appear in the volume entitled, "A History of Mathematical Logic", edited by D. van Dalen, J. Dawson and A. Kanamori, and published by Elsevier. :::::::::::::: 2002-029 :::::::::::::: Title: Simple Load Balancing for Distributed Hash Tables Authors: John Byers, Jeffrey Considine, and Michael Mitzenmacher Date: November 1, 2002 Abstract: Distributed hash tables have recently become a useful building block for a variety of distributed applications. However, current schemes based upon consistent hashing require both considerable implementation complexity and substantial storage overhead to achieve desired load balancing goals. We argue in this paper that these goals can be achieved more simply and more cost-effectively. First, we suggest the direct application of the ``power of two choices'' paradigm, whereby an item is stored at the less loaded of two (or more) random alternatives. We then consider how associating a small constant number of hash values with a key can naturally be extended to support other load balancing methods, including load-stealing or load-shedding schemes, as well as providing natural fault-tolerance mechanisms. :::::::::::::: 2002-030 :::::::::::::: Title: Validating Arbitrarily Large Network Protocol Compositions with Finite Computation Authors: Adam D. Bradley, Azer Bestavros, and Assaf J. Kfoury Date: November 1, 2002 Abstract: Formal tools like finite-state model checkers have proven useful in verifying the correctness of systems of bounded size and for hardening single system components against arbitrary inputs. However, conventional applications of these techniques are not well suited to characterizing emergent behaviors of large compositions of processes. In this paper, we present a methodology by which arbitrarily large compositions of components can, if sufficient conditions are proven concerning properties of small compositions, be modeled and completely verified by performing formal verifications upon only a finite set of compositions. The sufficient conditions take the form of reductions, which are claims that particular sequences of components will be causally indistinguishable from other shorter sequences of components. We show how this methodology can be applied to a variety of network protocol applications, including two features of the HTTP protocol, a simple active networking applet, and a proposed web cache consistency algorithm. We also doing discuss its applicability to framing protocol design goals and to representing systems which employ non-model-checking verification methodologies. Finally, we briefly discuss how we hope to broaden this methodology to more general topological compositions of network applications. Keywords: Protocol Verification, Formal Methods, Model Checking, Language Reduction, Protocol Design :::::::::::::: 2002-031 :::::::::::::: Title: Cluster-based Optimizations for Distributed Hash Tables Author: Jeffrey Considine Date: November 1, 2002 Abstract: We consider the problem of performing topological optimizations of distributed hash tables. Such hash tables include Chord and Tapestry and are a popular building block for distributed applications. Optimizing topologies over one dimensional hash spaces is particularly difficult as the higher dimensionality of the underlying network makes close fits unlikely. Instead, current schemes are limited to heuristically performing local optimizations finding the best of small random set of peers. We propose a new class of topology optimizations based on the existence of clusters of close overlay members within the underlying network. By constructing additional overlays for each cluster, a significant portion of the search procedure can be performed within the local cluster with a corresponding reduction in the search time. Finally, we discuss the effects of these additional overlays on spatial locality and other load balancing schemes. :::::::::::::: 2003-001 :::::::::::::: Title: On the Size Distribution of Autonomous Systems Authors: Marwan Fayed, Paul Krapivsky, John Byers, Mark Crovella, David Finkel(WPI), Sid Redner Date: January 17, 2003 Abstract: This paper explores reasons for the high degree of variability in the sizes of ASes that have recently been observed, and the processes by which this variable distribution develops. AS size distribution is important for a number of reasons. First, when modeling network topologies, an AS size distribution assists in labeling routers with an associated AS. Second, AS size has been found to be positively correlated with the degree of the AS (number of peering links), so understanding the distribution of AS sizes has implications for AS connectivity properties. Our model accounts for AS births, growth, and mergers. We analyze two models: one incorporates only the growth of hosts and ASes, and a second extends that model to include mergers of ASes. We show analytically that, given reasonable assumptions about the nature of mergers, the resulting size distribution exhibits a power law tail with the exponent independent of the details of the merging process. We estimate parameters of the models from measurements obtained from Internet registries and from BGP tables. We then compare the models solutions to empirical AS size distribution taken from Mercator and Skitter datasets, and find that the simple growth-based model yields general agreement with empirical data. Our analysis of the model in which mergers occur in a manner independent of the size of the merging ASes suggests that more detailed analysis of merger processes is needed. :::::::::::::: 2003-002 :::::::::::::: Title: Geometric Generalizations of the Power of Two Choices Authors: John Byers, Jef Considine, and Michael Mitzenmacher Abstract: A well-known paradigm for load balancing in distributed systems is the ``power of two choices,'' whereby an item is stored at the less loaded of two (or more) random alternative servers. We investigate the power of two choices in natural settings for distributed computing where items and servers reside in a geometric space and each item is associated with the server that is its nearest neighbor. This is in fact the backdrop for distributed hash tables such as Chord, where the geometric space is determined by clockwise distance on a one-dimensional ring. Theoretically, we consider the following load balancing problem. Suppose that servers are initially hashed uniformly at random to points in the space. Sequentially, each item then considers d candidate insertion points also chosen uniformly at random from the space, and selects the insertion point whose associated server has the least load. For the one-dimensional ring, and for Euclidean distance on the two-dimensional torus, we demonstrate that when n data items are hashed to n servers, the maximum load at any server is log log n / log d + O(1) with high probability. While our results match the well-known bounds in the standard setting in which each server is selected equiprobably, our applications do not have this feature, since the sizes of the nearest-neighbor regions around servers are non-uniform. Therefore, the novelty in our methods lies in developing appropriate tail bounds on the distribution of nearest-neighbor region sizes and in adapting previous arguments to this more general setting. In addition, we provide simulation results demonstrating the load balance that results as the system size scales into the millions. :::::::::::::: 2003-003 :::::::::::::: Title: On the Convergence of Statistical Techniques for Inferring Network Traffic Demands Author: Alberto Medina, Kave Salamatian, Nina Taft, Ibrahim Matta, Yolanda Tsang, Christophe Diot Date: February 6, 2003 Abstract: Accurate knowledge of traffic demands in a communication network enables or enhances a variety of traffic engineering and network management tasks of paramount importance for operational networks. Directly measuring a complete set of these demands is prohibitively expensive because of the huge amounts of data that must be collected and the performance impact that such measurements would impose on the regular behavior of the network. As a consequence, we must rely on statistical techniques to produce estimates of actual traffic demands from partial information. The performance of such techniques is however limited due to their reliance on limited information and the high amount of computations they incur, which limits their convergence behavior. In this paper we study strategies to improve the convergence of a powerful statistical technique based on an Expectation-Maximization iterative algorithm. First we analyze modeling approaches to generating starting points. We call these starting points {\it informed priors} since they are obtained using actual network information such as packet traces and SNMP link counts. Second we provide a very fast variant of the EM algorithm which extends its computation range, increasing its accuracy and decreasing its dependence on the quality of the starting point. Finally, we study the convergence characteristics of our EM algorithm and compare it against a recently proposed Weighted Least Squares approach. :::::::::::::: 2003-004 :::::::::::::: Title: Cryptographic Tamper Evidence Author: Gene Itkis, Boston University Date: February 11, 2003 Abstract: We propose a new notion of cryptographic tamper evidence. A tamper-evident signature scheme provides an additional procedure Div which detects tampering: given two signatures, Div can determine whether one of them was generated by the forger. Surprisingly, this is possible even after the adversary has inconspicuously learned some --- or even all --- the secrets in the system. In this case, it might be impossible to tell which signature is generated by the legitimate signer and which by the forger. But at least the fact of the tampering will be made evident. We define several variants of tamper-evidence, differing in their power to detect tampering. In all of these, we assume an equally powerful adversary: she adaptively controls all the inputs to the legitimate signer (i.e., all messages to be signed and their timing), and observes all his outputs; she can also adaptively expose all the secrets at arbitrary times. We provide tamper-evident schemes for all the variants and prove their optimality. We stress that our mechanisms are purely cryptographic: the tamper-detection algorithm Div is stateless and takes no inputs except the two signatures (in particular, it keeps no logs), we use no infrastructure (or other ways to conceal additional secrets), and we use no hardware properties (except those implied by the standard cryptographic assumptions, such as random number generators). Our constructions are based on arbitrary ordinary signature schemes and do not require random oracles. :::::::::::::: 2003-005 :::::::::::::: Title: On the Emergence of Highly Variable Distributions in the Autonomous System Topology Author: Fayed, Marwan; Krapivsky, Paul; Byers, John; Crovella, Mark; Finkel, David; Redner, Sid Date: March 1, 2003 Abstract: Recent studies have noted that vertex degree in the autonomous system (AS) graph exhibits a highly variable distribution \cite{fff,MP01}. The most prominent explanatory model for this phenomenon is the Barabasi-Albert (B-A) model [BA99,AB00]. A central feature of the B-A model is preferential connectivity --- meaning that the likelihood a new node in a growing graph will connect to an existing node is proportional to the existing node's degree. In this paper we ask whether a more general explanation than the B-A model, and absent the assumption of preferential connectivity, is consistent with empirical data. We are motivated by two observations: first, AS degree and AS size are highly correlated [CHEN02]; and second, highly variable AS size can arise simply through exponential growth. We construct a model incorporating exponential growth in the size of the Internet, and in the number of ASes. We then show via analysis that such a model yields a size distribution exhibiting a power-law tail. In such a model, if an AS's link formation is roughly proportional to its size, then AS degree will also show high variability. We instantiate such a model with empirically derived estimates of growth rates and show that the resulting degree distribution is in good agreement with that of real AS graphs. :::::::::::::: 2003-006 :::::::::::::: Title: Skin Color-Based Video Segmentation under Time-Varying Illumination Authors: Leonid Sigal and Stan Sclaroff Date: March 28, 2003 Abstract: A novel approach for real-time skin segmentation in video sequences is described. The approach enables reliable skin segmentation despite wide variation in illumination during tracking. An explicit second order Markov model is used to predict evolution of the skin-color (HSV) histogram over time. Histograms are dynamically updated based on feedback from the current segmentation and predictions of the Markov model. The evolution of the skin-color distribution at each frame is parameterized by translation, scaling and rotation in color space. Consequent changes in geometric parameterization of the distribution are propagated by warping and re-sampling the histogram. The parameters of the discrete-time dynamic Markov model are estimated using Maximum Likelihood Estimation, and also evolve over time. The accuracy of the new dynamic skin color segmentation algorithm is compared to that obtained via a static color model. Segmentation accuracy is evaluated using labeled ground-truth video sequences taken from staged experiments and popular movies. An overall increase in segmentation accuracy of up to 24% is observed in 17 out of 21 test sequences. In all but one case the skin-color classification rates for our system were higher, with background classification rates comparable to those of the static segmentation. :::::::::::::: 2003-007 :::::::::::::: Title: The Specialized Mappings Architecture Authors: Romer Rosales and Stan Sclaroff Date: March 28, 2003 Abstract: A probabilistic, nonlinear supervised learning model is proposed: the Specialized Mappings Architecture (SMA). The SMA employs a set of several mapping functions that are estimated automatically from training data. Each specialized function maps certain domains of the input space (e.g., image features) onto the output space (e.g., articulated body parameters). One important advantage of the SMA is that it can model ambiguous, one-to-many mappings that may yield multiple valid output hypotheses. Once learned, the mapping functions generate a set of output hypotheses for a given input via a statistical inference procedure. The SMA inference procedure incorporates an inverse mapping or feedback function, which enables the SMA to evaluate the likelihood of each hypothesis. Possible feedback functions include computer graphics rendering routines that can generate images for given hypotheses. The SMA employs a variant of the Expectation-Maximization algorithm for simultaneous learning of the specialized domains along with the mapping functions, and approximate strategies for inference. The framework is demonstrated in a computer vision system that can estimate the articulated pose parameters of a human body or human hands, given image silhouettes. The accuracy and stability of the SMA are also tested using synthetic images of human bodies and hands, where ground truth is known. :::::::::::::: 2003-008 :::::::::::::: Title: Discovering Clusters in Motion Time-Series Data Authors: Jonathan Alon, Stan Sclaroff, George Kollios, and Vladimir Pavlovic Date: March 28, 2003 Abstract: A new approach is proposed for clustering time-series data. The approach can be used to discover groupings of similar object motions that were observed in a video collection. A finite mixture of hidden Markov models (HMMs) is fitted to the motion data using the expectation-maximization (EM) framework. Previous approaches for HMM-based clustering employ a k-means formulation, where each sequence is assigned to only a single HMM. In contrast, the formulation presented in this paper allows each sequence to belong to more than a single HMM with some probability, and the hard decision about the sequence class membership can be deferred until a later time when such a decision is required. Experiments with simulated data demonstrate the benefit of using this EM-based approach when there is more "overlap" in the processes generating the data. Experiments with real data show the promising potential of HMM-based motion clustering in a number of applications. :::::::::::::: 2003-009 :::::::::::::: Title: Estimating 3D Hand Pose from a Cluttered Image Authors: Vassilis Athitsos and Stan Sclaroff Date: April 1, 2003 Abstract: A method is proposed that can generate a ranked list of plausible three-dimensional hand configurations that best match an input image. Hand pose estimation is formulated as an image database indexing problem, where the closest matches for an input hand image are retrieved from a large database of synthetic hand images. In contrast to previous approaches, the system can function in the presence of clutter, thanks to two novel clutter-tolerant indexing methods. First, a computationally efficient approximation of the image-to-model chamfer distance is obtained by embedding binary edge images into a high-dimensional Euclidean space. Second, a general-purpose, probabilistic line matching method identifies those line segment correspondences between model and input images that are the least likely to have occurred by chance. The performance of this clutter-tolerant approach is demonstrated in quantitative experiments with hundreds of real hand images. :::::::::::::: 2003-010 :::::::::::::: Title: Database Indexing Methods for 3D Hand Pose Estimation Authors: Vassilis Athitsos and Stan Sclaroff Date: April 1, 2003 Abstract: Estimation of 3D hand pose is useful in many gesture recognition applications, ranging from human-computer interaction to automated recognition of sign languages. In this paper, 3D hand pose estimation is treated as a database indexing problem. Given an input image of a hand, the most similar images in a large database of hand images are retrieved. The hand pose parameters of the retrieved images are used as estimates for the hand pose in the input image. Lipschitz embeddings of edge images into a Euclidean space are used to improve the efficiency of database retrieval. In order to achieve interactive retrieval times, similarity queries are initially performed in this Euclidean space. The paper describes ongoing work that focuses on how to best choose reference images, in order to improve retrieval accuracy. :::::::::::::: 2003-011 :::::::::::::: Title: How well can TCP infer network state? Authors: Dhiman Barman and Ibrahim Matta Date: May 16, 2003 Abstract: The Transmission Control Protocol (TCP) has been the protocol of choice for many Internet applications requiring reliable connections. The design of TCP has been challenged by the extension of connections over wireless links. We ask a fundamental question: {\em What is the basic predictive power of TCP of network state, including wireless error conditions?} The goal is to improve or readily exploit this predictive power to enable TCP (or variants) to perform well in generalized network settings. To that end, we use Maximum Likelihood Ratio tests to evaluate TCP as a detector/estimator. We quantify how well network state can be estimated, given network response such as distributions of packet delays or TCP throughput that are conditioned on the type of packet loss. Using our model-based approach and extensive simulations, we demonstrate that congestion-induced losses and losses due to wireless transmission errors produce sufficiently different statistics upon which an efficient detector can be built; distributions of network loads can provide effective means for estimating packet loss type; and packet delay is a better signal of network state than short-term throughput. We demonstrate how estimation accuracy is influenced by different proportions of congestion versus wireless losses and penalties on incorrect estimation. Keywords: TCP; Congestion Control; Error Control; Binary Hypothesis Testing; Maximum Likelihood Ratio Test; Gaussian Distribution; Wireless Links; Simulation. :::::::::::::: 2003-012 :::::::::::::: Title: Systematic Verication of Safety Properties of Arbitrary Network Protocol Compositions Using CHAIN Authors: Adam Bradley, Azer Bestavros, and Assaf Kfoury Date: May 16, 2003 Abstract: Formal correctness of complex multi-party network protocols can be difficult to verify. While models of specific fixed compositions of agents can be checked against design constraints, protocols which lend themselves to arbitrarily many compositions of agents--such as the chaining of proxies or the peering of routers--are more difficult to verify because they represent potentially infinite state spaces and may exhibit emergent behaviors which may not materialize under particular fixed compositions. We address this challenge by developing an algebraic approach that enables us to reduce arbitrary compositions of network agents into a behaviorally-equivalent (with respect to some correctness property) compact, canonical representation, which is amenable to mechanical verification. Our approach consists of an algebra and a set of property-preserving rewrite rules for the Canonical Homomorphic Abstraction of Infinite Network protocol compositions (CHAIN). Using CHAIN, an expression over our algebra (i.e., a set of configurations of network protocol agents) can be reduced to another behaviorally-equivalent expression (i.e., a smaller set of configurations). Repeated applications of such rewrite rules produces a canonical expression which can be checked mechanically. We demonstrate our approach by characterizing deadlock-prone configurations of HTTP agents, as well as establishing useful properties of an overlay protocol for scheduling MPEG frames, and of a protocol for Web intra-cache consistency. :::::::::::::: 2003-013 :::::::::::::: Title: On the Efficiency and Fairness of Transmission Control Loops: A Case for Exogenous Losses Authors: Mina Guirguis, Azer Bestavros, and Ibrahim Matta Date: May 16, 2003 Abstract: We postulate that exogenous losses--which are typically regarded as introducing undesirable ``noise'' that needs to be filtered out or hidden from end points--can be surprisingly beneficial. In this paper we evaluate the effects of exogenous losses on transmission control loops, focusing primarily on efficiency and convergence to fairness properties. By analytically capturing the effects of exogenous losses, we are able to characterize the transient behavior of TCP. Our numerical results suggest that ``noise'' resulting from exogenous losses should not be filtered out blindly, and that a careful examination of the parameter space leads to better strategies regarding the treatment of exogenous losses inside the network. Specifically, we show that while low levels of exogenous losses do help connections converge to their fair share, higher levels of losses lead to inefficient network utilization. We draw the line between these two cases by determining whether or not it is advantageous to hide, or more interestingly introduce, exogenous losses. Our proposed approach is based on classifying the effects of exogenous losses into long-term and short-term effects. Such classification informs the extent to which we control exogenous losses, so as to operate in an efficient and fair region. We validate our results through simulations. :::::::::::::: 2003-014 :::::::::::::: Title: User-Level Sandboxing: a Safe and Efficient Mechanism for Extensibility Author: Richard West and Jason Gloudon Date: June 1, 2003 Abstract: Extensible systems allow services to be configured and deployed for the specific needs of individual applications. This paper describes a safe and efficient method for user-level extensibility that requires only minimal changes to the kernel. A sandboxing technique is described that supports multiple logical protection domains within the same address space at user-level. This approach allows applications to register sandboxed code with the system, that may be executed in the context of any process. Our approach differs from other implementations that require special hardware support, such as segmentation or tagged translation look-aside buffers (TLBs), to either implement multiple protection domains in a single address space, or to support fast switching between address spaces. Likewise, we do not require the entire system to be written in a type -safe language, to provide fine-grained protection domains. Instead, our user-level sandboxing technique requires only paged-based virtual memory support, and the requirement that extension code is written either in a type-safe language, or by a trusted source. Using a fast method of upcalls, we show how our sandboxing technique for implementing logical protection domains provides significant performance improvements over traditional methods of invoking user-level services. Experimental results show our approach to be an efficient method for extensibility, with inter-protection domain communication costs close to those of hardware-based solutions leveraging segmentation. :::::::::::::: 2003-015 :::::::::::::: Title: ROMA: Reliable Overlay Multicast with Loosely Coupled TCP Connections Author: Gu-In Kwon and John Byers Date: July 1, 2003 Abstract: We consider the problem of architecting a reliable content delivery system across an overlay network using TCP connections as the transport primitive. We first argue that natural designs based on store-and-forward principles that tightly couple TCP connections at intermediate end-systems impose fundamental performance limitations, such as dragging down all transfer rates in the system to the rate of the slowest receiver. In contrast, the ROMA architecture we propose incorporates the use of loosely coupled TCP connections together with fast forward error correction techniques to deliver a scalable solution that better accommodates a set of heterogeneous receivers. The methods we develop establish chains of TCP connections, whose expected performance we analyze through equation-based methods. We validate our analytical findings and evaluate the performance of our ROMA architecture using a prototype implementation via extensive Internet experimentation across the PlanetLab distributed testbed. :::::::::::::: 2003-016 :::::::::::::: Title: Stochastic Mesh-Based Multiview Reconstruction Authors: John Isidoro and Stan Sclaroff Date: July 1, 2003 Abstract: A method for reconstruction of 3D polygonal models from multiple views is presented. The method uses sampling techniques to construct a texture-mapped semi-regular polygonal mesh of the object in question. Given a set of views and segmentation of the object in each view, constructive solid geometry is used to build a visual hull from silhouette prisms. The resulting polygonal mesh is simplified and subdivided to produce a semi-regular mesh. Regions of model fit inaccuracy are found by projecting the reference images onto the mesh from different views. The resulting error images for each view are used to compute a probability density function, and several points are sampled from it. Along the epipolar lines corresponding to these sampled points, photometric consistency is evaluated. The mesh surface is then pulled towards the regions of higher photometric consistency using free-form deformations. This sampling-based approach produces a photometrically consistent solution in much less time than possible with previous multi-view algorithms given arbitrary camera placement. :::::::::::::: 2003-017 :::::::::::::: Title: Stochastic Refinement of the Visual Hull to Satisfy Photometric and Silhouette Consistency Constraints Authors: John Isidoro and Stan Sclaroff Date: 7/18/03 Abstract: An iterative method for reconstructing a 3D polygonal mesh and color texture map from multiple views of an object is presented. In each iteration, the method first estimates a texture map given the current shape estimate. The texture map and its associated residual error image are obtained via maximum a posteriori estimation and reprojection of the multiple views into texture space. Next, the surface shape is adjusted to minimize residual error in texture space. The surface is deformed towards a photometrically-consistent solution via a series of 1D epipolar searches at randomly selected surface points. The texture space formulation has improved computational complexity over standard image-based error aproaches, and allows computation of the reprojection error and uncertainty for any point on the surface. Moreover, shape adjustments can be constrained such that the recovered model's silhouette matches those of the input images. Experiments with real world imagery demonstrate the validity of the approach. :::::::::::::: 2003-018 :::::::::::::: Title: Segmenting Foreground Objects from a Dynamic Textured Background via a Robust Kalman Filter Authors: Jing Zhong and Stan Sclaroff Date: July 18, 2003 Abstract: The algorithm presented in this paper aims to segment the foreground objects in video (e.g., people) given time-varying, textured backgrounds. Examples of time-varying backgrounds include waves on water, clouds moving, trees waving in the wind, automobile traffic, moving crowds, escalators, etc. We have developed a novel foreground-background segmentation algorithm that explicitly accounts for the non-stationary nature and clutter-like appearance of many dynamic textures. The dynamic texture is modeled by an Autoregressive Moving Average Model (ARMA). A robust Kalman filter algorithm iteratively estimates the intrinsic appearance of the dynamic texture, as well as the regions of the foreground objects. Preliminary experiments with this method have demonstrated promising results. :::::::::::::: 2003-019 :::::::::::::: Title: Dynamic Window-Constrained Scheduling for Real-Time Media Streaming Authors: Richard West, Karsten Schwan Christian Poellabauer Date: August 29, 2003 Abstract: This paper describes an algorithm for scheduling packets in real-time multimedia data streams. Common to these classes of data streams are service constraints in terms of bandwidth and delay. However, it is typical for real-time multimedia streams to tolerate bounded delay variations and, in some cases, finite losses of packets. We have therefore developed a scheduling algorithm that assumes streams have window-constraints on groups of consecutive packet deadlines. A window-constraint defines the number of packet deadlines that can be missed in a window of deadlines for consecutive packets in a stream. Our algorithm, called Dynamic Window-Constrained Scheduling (DWCS), attempts to guarantee no more than x out of a window of y deadlines are missed for consecutive packets in real-time and multimedia streams. Using DWCS, the delay of service to real-time streams is bounded even when the scheduler is overloaded. Moreover, DWCS is capable of ensuring independent delay bounds on streams, while at the same time guaranteeing minimum bandwidth utilizations over tunable and finite windows of time. We show the conditions under which the total demand for link bandwidth by a set of real-time (i.e., window-constrained) streams can exceed 100% and still ensure all window-constraints are met. In fact, we show how it is possible to guarantee worst-case per-stream bandwidth and delay constraints while utilizing all available link capacity. Finally, we show how best-effort packets can be serviced with fast response time, in the presence of window-constrained traffic. Keywords: Real-time (window-constrained) scheduling and communications :::::::::::::: 2003-020 :::::::::::::: Title: Adaptive Routing of QoS-constrained Media Streams over Scalable Overlay Topologies Authors: Gerald Fry and Richard West Abstract: Current research on Internet-based distributed systems emphasizes the scalability of overlay topologies for efficient search and retrieval of data items, as well as routing amongst peers. However, most existing approaches fail to address the transport of data across these logical networks in accordance with quality of service (QoS) constraints. Consequently, this paper investigates the use of scalable overlay topologies for routing real-time media streams between publishers and potentially many thousands of subscribers. Specifically, we analyze the costs of using k-ary n-cubes for QoS-constrained routing. Given a number of nodes in a distributed system, we calculate the optimal k-ary n-cube structure for minimizing the average distance between any pair of nodes. Using this structure, we describe a greedy algorithm that selects paths between nodes in accordance with the real-time delays along physical links. We show this method improves the routing latencies by as much as 67%, compared to approaches that do not consider physical link costs. We are in the process of developing a method for adaptive node placement in the overlay topology, based upon the locations of publishers, subscribers, physical link costs and per-subscriber QoS constraints. One such method for repositioning nodes in logical space is discussed, to improve the likelihood of meeting service requirements on data routed between publishers and subscribers. Future work will evaluate the benefits of such techniques more thoroughly. :::::::::::::: 2003-021 :::::::::::::: Title: Structural Analysis of Network Traffic Flows Authors: Anukool Lakhina, Konstantina Papagiannaki, Mark Crovella, Christophe Diot, Eric Kolaczyk, and Nina Taft Abstract: Network traffic arises from the superposition of Origin-Destination (OD) flows. Hence, a thorough understanding of OD flows is essential for modeling network traffic, and for addressing a wide variety of problems including traffic engineering, traffic matrix estimation, capacity planning, forecasting and anomaly detection. However, to date, OD flows have not been closely studied, and there is very little known about their properties. We present the first analysis of complete sets of OD flow timeseries, taken from two different backbone networks (Abilene and Sprint-Europe). Using Principal Component Analysis (PCA), we find that the set of OD flows has small intrinsic dimension. In fact, even in a network with over a hundred OD flows, these flows can be accurately modeled in time using a small number (10 or less) of independent components or dimensions. We also show how to use PCA to systematically decompose the structure of OD flow timeseries into three main constituents: common periodic trends, short-lived bursts, and noise. We provide insight into how the various constitutents contribute to the overall structure of OD flows and explore the extent to which this decomposition varies over time. :::::::::::::: 2003-022 :::::::::::::: Title: Analysis of OD Flows (Raw Data) Authors: Anukool Lakhina, Konstantina Papagiannaki, Mark Crovella, Christophe Diot, Eric D. Kolaczyk and Nina Taft Abstract: In a recent paper, Structural Analysis of Network Traffic Flows, we analyzed the set of Origin Destination traffic flows from the Sprint-Europe and Abilene backbone networks. This report presents the complete set of results from analyzing data from both networks. The results in this report are specific to the Sprint-1 and Abilene datasets studied in the above paper. The following results are presented here: 1 Rows of Principal Matrix ($V$) ... 2 1.1 Sprint-1 Dataset ... 2 1.2 Abilene Dataset ... 9 2 Set of Eigenflows ... 14 2.1 Sprint-1 Dataset ... 14 2.2 Abilene Dataset ... 21 3 Classifying Eigenflows ... 26 3.1 Sprint-1 Dataset ... 26 3.2 Abilene Dataset ... 44 :::::::::::::: 2003-023 :::::::::::::: Title: BoostMap: A Method for Efficient Approximate Similarity Rankings Authors: Vassilis Athitsos, Jonathan Alon, Stan Sclaroff, George Kollios Date: November 24, 2003 Abstract: This paper introduces BoostMap, a method that can significantly reduce retrieval time in image and video database systems that employ computationally expensive distance measures, metric or non-metric. Database and query objects are embedded into a Euclidean space, in which similarities can be rapidly measured using a weighted Manhattan distance. Embedding construction is formulated as a machine learning task, where AdaBoost is used to combine many simple, 1D embeddings into a multidimensional embedding that preserves a significant amount of the proximity structure in the original space. Performance is evaluated in a hand pose estimation system, and a dynamic gesture recognition system, where the proposed method is used to retrieve approximate nearest neighbors under expensive image and video similarity measures. In both systems, BoostMap significantly increases efficiency, with minimal losses in accuracy. Moreover, the experiments indicate that BoostMap compares favorably with existing embedding methods that have been employed in computer vision and database applications, i.e., FastMap and Bourgain embeddings. :::::::::::::: 2003-024 :::::::::::::: Title: A Pragmatic Approach to DHT Adoption Authors: Jeffrey Considine, Michael Walfish, David G. Andersen Date: December 1, 2003 Abstract: Despite the peer-to-peer community's obvious wish to have its systems adopted, specific mechanisms to facilitate incremental adoption have not yet received the same level of attention as the many other practical concerns associated with these systems. This paper argues that ease of adoption should be elevated to a first-class concern and accordingly presents HOLD, a front-end to existing DHTs that is optimized for incremental adoption. Specifically, HOLD is backwards-compatible: it leverages DNS to provide a key-based routing service to existing Internet hosts without requiring them to install any software. This paper also presents applications that could benefit from HOLD as well as the trade-offs that accompany HOLD. Early implementation experience suggests that HOLD is practical. :::::::::::::: 2003-025 :::::::::::::: Title: Contour Generator Points for Threshold Selection and a Novel Photo-Consistency Measure for Space Carving Authors: John Isodoro and Stan Sclaroff Date: December 2, 2003 Abstract: Space carving has emerged as a powerful method for multiview scene reconstruction. Although a wide variety of methods have been proposed, the quality of the reconstruction remains highly-dependent on the photometric consistency measure, and the threshold used to carve away voxels. In this paper, we present a novel photo-consistency measure that is motivated by a multiset variant of the chamfer distance. The new measure is robust to high amounts of within-view color variance and also takes into account the projection angles of back-projected pixels. Another critical issue in space carving is the selection of the photo-consistency threshold used to determine what surface voxels are kept or carved away. In this paper, a reliable threshold selection technique is proposed that examines the photo-consistency values at contour generator points. Contour generators are points that lie on both the surface of the object and the visual hull. To determine the threshold, a percentile ranking of the photo-consistency values of these generator points is used. This improved technique is applicable to a wide variety of photo-consistency measures, including the new measure presented in this paper. Also presented in this paper is a method to choose between photo-consistency measures, and voxel array resolutions prior to carving using receiver operating characteristic (ROC) curves. :::::::::::::: 2003-026 :::::::::::::: Title: Exogenous-Loss Awareness in Queue Management: Toward Global Fairness Authors: Mina Guirguis, Azer Bestavros, and Ibrahim Matta Date: December 2, 2003 Abstract: For a given TCP flow, exogenous losses are those occurring on links other than the flow's bottleneck link. Exogenous losses are typically viewed as introducing undesirable ``noise'' into TCP's feedback control loop, leading to inefficient network utilization and potentially severe global unfairness. This has prompted much research on mechanisms for hiding such losses from end-points. In this paper, we show through analysis and simulations that low levels of exogenous losses are surprisingly beneficial in that they improve stability and convergence, without sacrificing efficiency. Based on this, we argue that exogenous loss awareness should be taken into account in any AQM design that aims to achieve global fairness. To that end, we propose an eXogenous-loss aware Queue Management (XQM) that actively accounts for and leverages exogenous losses. We use an equation based approach to derive the quiescent loss rate for a connection based on the connection's profile and its global fair share. In contrast to other queue management techniques, XQM ensures that a connection sees its quiescent loss rate, not only by complementing already existing exogenous losses, but also by actively hiding exogenous losses, if necessary, to achieve global fairness. We establish the advantages of exogenous-loss awareness using extensive simulations in which, we contrast the performance of XQM to that of a host of traditional exogenous-loss unaware AQM techniques. :::::::::::::: 2003-027 :::::::::::::: Title: Efficiently and Fairly Allocating Bandwidth at a Highly Congested Link Authors: Tao Wang, Ibrahim Matta, and Azer Bestavros Date: December 2, 2003 Abstract: We consider the problem of efficiently and fairly allocating bandwidth at a highly congested link to a diverse set of flows, including TCP flows with various Round Trip Times (RTT), non-TCP-friendly flows such as Constant-Bit-Rate (CBR) applications using UDP, misbehaving, or malicious flows. Though simple, a FIFO queue management is vulnerable. Fair Queueing (FQ) can guarantee max-min fairness but fails at efficiency. RED-PD exploits the history of RED's actions in preferentially dropping packets from higher-rate flows. Thus, RED-PD attempts to achieve fairness at low cost. By relying on RED's actions, RED-PD turns out not to be effective in dealing with non-adaptive flows in settings with a highly heterogeneous mix of flows. In this paper, we propose a new approach we call RED-NB (RED with No Bias). RED-NB does not rely on RED's actions. Rather it explicitly maintains its own history for the few high-rate flows. RED-NB then adaptively adjusts flow dropping probabilities to achieve max-min fairness. In addition, RED-NB helps RED itself at very high loads by tuning RED's dropping behavior to the flow characteristics (restricted in this paper to RTTs) to eliminate its bias against long-RTT TCP flows while still taking advantage of RED's features at low loads. Through extensive simulations, we confirm the fairness of RED-NB and show that it outperforms RED, RED-PD, and CHOKe in all scenarios. :::::::::::::: 2003-028 :::::::::::::: Title: Providing Soft Bandwidth Guarantees Using Elastic TCP-based Tunnels Authors: Mina Guirguis, Azer Bestavros, Ibrahim Matta, Niky Riga, Galia Damiant, and Yuting Zhang Date: December 2, 2003 Abstract: The best-effort nature of the Internet poses a significant obstacle to the deployment of many applications that require guaranteed bandwidth. In this paper, we present a novel approach that enables two edge/border routers---which we call Internet Traffic Managers (ITM)---to use an adaptive number of TCP connections to set up a tunnel of desirable bandwidth between them. The number of TCP connections that comprise this tunnel is elastic in the sense that it increases/decreases in tandem with competing cross traffic to maintain a target bandwidth. An origin ITM would then schedule incoming packets from an application requiring guaranteed bandwidth over that elastic tunnel. Unlike many proposed solutions that aim to deliver soft QoS guarantees, our elastic-tunnel approach does not require any support from core routers (as with IntServ and DiffServ); it is scalable in the sense that core routers do not have to maintain per-flow state (as with IntServ); and it is readily deployable within a single ISP or across multiple ISPs. To evaluate our approach, we develop a flow-level control-theoretic model to study the transient behavior of established elastic TCP-based tunnels. The model captures the effect of cross-traffic connections on our bandwidth allocation policies. Through extensive simulations, we confirm the effectiveness of our approach in providing soft bandwidth guarantees. We also outline our kernel-level ITM prototype implementation. :::::::::::::: 2003-029 :::::::::::::: Title: TCP Optimization through FEC, ARQ and Transmission Power Tradeoffs Authors: Dhiman Barman, Ibrahim Matta, Eitan Altman, and Rachid El Azouzi Date: December 3, 2003 Abstract: TCP performance degrades when end-to-end connections extend over wireless connections --- links which are characterized by high bit error rate and intermittent connectivity. Such link characteristics can significantly degrade TCP performance as the TCP sender assumes wireless losses to be congestion losses resulting in unnecessary congestion control actions. Link errors can be reduced by increasing transmission power, code redundancy (FEC) or number of retransmissions (ARQ). But increasing power costs resources, increasing code redundancy reduces available channel bandwidth and increasing persistency increases end-to-end delay. The paper proposes a TCP optimization through proper tuning of power management, FEC and ARQ in wireless environments (WLAN and WWAN). In particular, we conduct analytical and numerical analysis taking into account the three aforementioned factors, and evaluate TCP (and ``wireless-aware'' TCP) performance under different settings. Our results show that increasing power, redundancy and/or retransmission levels always improves TCP performance by reducing link-layer losses. However, such improvements are often associated with cost and arbitrary improvement cannot be realized without paying a lot in return. It is therefore important to consider some kind of net utility function that should be optimized, thus maximizing throughput at the least possible cost. :::::::::::::: 2003-030 :::::::::::::: Title: A Bayesian Approach for TCP to Distinguish Congestion from Wireless Losses Authors: Dhiman Barman and Ibrahim Matta Date: December 3, 2003 Abstract: (This Technical Report revises TR-BUCS-2003-011) The Transmission Control Protocol (TCP) has been the protocol of choice for many Internet applications requiring reliable connections. The design of TCP has been challenged by the extension of connections over wireless links. In this paper, we investigate a Bayesian approach to infer at the source host the reason of a packet loss, whether congestion or wireless transmission error. Our approach is ``mostly'' end-to-end since it requires only one {\em long-term average} quantity (namely, long-term average packet loss probability over the wireless segment) that may be best obtained with help from the network (e.g.\ wireless access agent). Specifically, we use Maximum Likelihood Ratio tests to evaluate TCP as a classifier of the type of packet loss. We study the effectiveness of {\em short-term} classification of packet errors (congestion vs. wireless), given stationary prior error probabilities and distributions of packet delays conditioned on the type of packet loss (measured over a larger time scale). Using our Bayesian-based approach and extensive simulations, we demonstrate that congestion-induced losses and losses due to wireless transmission errors produce sufficiently different statistics upon which an efficient online error classifier can be built. We introduce a simple queueing model to underline the conditional delay distributions arising from different kinds of packet losses over a heterogeneous wired/wireless path. We show how Hidden Markov Models (HMMs) can be used by a TCP connection to infer efficiently conditional delay distributions. We demonstrate how estimation accuracy is influenced by different proportions of congestion versus wireless losses and penalties on incorrect classification. :::::::::::::: 2003-031 :::::::::::::: Title: Automated Placement of Cameras in a Floorplan to Satisfy Task-Specific Constraints Authors: Ugur Murat Erdem and Stan Sclaroff Abstract: In many multi-camera vision systems the effect of camera locations on the task-specific quality of service is ignored. Researchers in Computational Geometry have proposed elegant solutions for some sensor location problem classes. Unfortunately, these solutions utilize unrealistic assumptions about the cameras' capabilities that make these algorithms unsuitable for many real-world computer vision applications: unlimited field of view, infinite depth of field, and/or infinite servo precision and speed. In this paper, the general camera placement problem is first defined with assumptions that are more consistent with the capabilities of real-world cameras. The region to be observed by cameras may be volumetric, static or dynamic, and may include holes that are caused, for instance, by columns or furniture in a room that can occlude potential camera views. A subclass of this general problem can be formulated in terms of planar regions that are typical of building floorplans. Given a floorplan to be observed, the problem is then to efficiently compute a camera layout such that certain task-specific constraints are met. A solution to this problem is obtained via binary optimization over a discrete problem space. In preliminary experiments the performance of the resulting system is demonstrated with different real floorplans. :::::::::::::: 2003-032 :::::::::::::: Title: itmBench: Generalized API for Internet Traffic Managers Authors: Gali Diamant, Leonid Veytser, Ibrahim Matta, Azer Bestavros, Mina Guirguis, Liang Guo, Yuting Zhang, Sean Chen Date: December 16, 2003 Abstract: Internet Traffic Managers (ITMs) are special machines placed at strategic places in the Internet. itmBench is an interface that allows users (e.g. network managers, service providers, or experimental researchers) to register different traffic control functionalities to run on one ITM or an overlay of ITMs. Thus {\em itmBench} offers a tool that is extensible and powerful yet easy to maintain. ITM traffic control applications could be developed either using a kernel API so they run in kernel space, or using a user-space API so they run in user space. We demonstrate the flexibility of {\em itmBench} by showing the implementation of both a kernel module that provides a differentiated network service, and a user-space module that provides an overlay routing service. Our itmBench Linux-based prototype is free software and can be obtained from http://www.cs.bu.edu/groups/itm/. :::::::::::::: 2004-001 :::::::::::::: Title: Integrated Chest Image Analysis System ``BU-MIA'' Authors: Margrit Betke, Boston University Jingbin Wang, Boston University Jane P. Ko, New York University Date: January 7, 2004 Abstract: We introduce ``BU-MIA,'' a Medical Image Analysis system that integrates various advanced chest image analysis methods for detection, estimation, segmentation, and registration. BU-MIA evaluates repeated computed tomography (CT) scans of the same patient to facilitate identification and evaluation of pulmonary nodules for interval growth. It provides a user-friendly graphical user interface with a number of interaction tools for development, evaluation, and validation of chest image analysis methods. The structures that BU-MIA processes include the thorax, lungs, and trachea, pulmonary structures, such as lobes, fissures, nodules, and vessels, and bones, such as sternum, vertebrae, and ribs. :::::::::::::: 2004-002 :::::::::::::: Title: Quantum Lower Bounds for Fanout Authors: M. Fang, . Fenner, F. Green, S. Homer, Y. Zhang Date: Jan 12, 2004 Abstract: We prove several new lower bounds for constant depth quantum circuits. The main result is that parity (and hence fanout) requires log depth circuits, when the circuits are composed of single qubit and arbitrary size Toffoli gates, and when they use only constantly many ancillae. Under this constraint, this bound is close to optimal. In the case of a non-constant number of ancillae , we give a tradeoff between the number of ancillae and the required depth. :::::::::::::: 2004-003 :::::::::::::: Title: Bounds on the Power of Constant-Depth Quantum Circuits Authors: S. Fenner, F. Green, S. Homer, Y. Zhang Date: Jan 12, 2004 Abstract: We show that if a language is recognized within certain error bounds by constant-depth quantum circuits over a finite family of gates, then it is computable in (classical) polynomial time. In particular, our results imply EQNC^0 is contained in P, where EQNC^0 is the constant-depth analog of the class EQP. On the other hand, we adapt and extend ideas of Terhal and DiVincenzo (quant-ph/0205133) to show that, for any family F of quantum gates including Hadamard and CNOT gates, computing the acceptance probabilities of depth-five circuits over F is just as hard as computing these probabilities for circuits over F. In particular, this implies that NQNC^0 is hard for the polynomial time hierarchy, where NQNC^0 is the constant-depth analog of the class NQP. This essentially refutes a conjecture of Green et al. that NQACC is contained in TC^0 (quant-ph/0106017). :::::::::::::: 2004-004 :::::::::::::: Title: Programming Examples Needing Polymorphic Recursion Authors: J. J. Hallett and A. J. Kfoury Date: January 22, 2004 Abstract: Inferring types for polymorphic recursive function definitions (abbreviated to polymorphic recursion) is a recurring topic on the mailing lists of popular typed programming languages. This is despite the fact that type inference for polymorphic recursion using for all-types has been proved undecidable. This report presents several programming examples involving polymorphic recursion and determines their typability under various type systems, including the Hindley-Milner system, an intersection-type system, and extensions of these two. The goal of this report is to show that many of these examples are typable using a system of intersection types as an alternative form of polymorphism. By accomplishing this, we hope to lay the foundation for future research into a decidable intersection-type inference algorithm. We do not provide a comprehensive survey of type systems appropriate for polymorphic recursion, with or without type annotations inserted in the source language. Rather, we focus on examples for which types may be inferred without type annotations. :::::::::::::: 2004-005 :::::::::::::: Title: Exploiting the Transients of Adaptation for RoQ Attacks on Internet Resources Authors: Mina Guirguis, Azer Bestavros, and Ibrahim Matta Date: January 30, 2004 Abstract: In this paper, we expose an unorthodox adversarial attack that exploits the transients of a system's adaptive behavior, as opposed to its limited steady-state capacity. We show that a well orchestrated attack could introduce significant inefficiencies that could potentially deprive a network element from much of its capacity, or significantly reduce its service quality, while evading detection by consuming an unsuspicious, small fraction of that element's hijacked capacity. This type of attack stands in sharp contrast to traditional brute-force, sustained high-rate DoS attacks, as well as recently proposed attacks that exploit specific protocol settings such as TCP timeouts. We exemplify what we term as Reduction of Quality (RoQ) attacks by exposing the vulnerabilities of common adaptation mechanisms. We develop control-theoretic models and associated metrics to quantify these vulnerabilities. We present numerical and simulation results, which we validate with observations from real Internet experiments. Our findings motivate the need for the development of adaptation mechanisms that are resilient to these new forms of attacks. :::::::::::::: 2004-006 :::::::::::::: Title: Boosting Nearest Neighbor Classifiers for Multiclass Recognition Authors: Vassilis Athitsos and Stan Sclaroff Date: February 13, 2004 Abstract: This paper introduces an algorithm that uses boosting to learn a distance measure for multiclass k-nearest neighbor classification. Given a family of distance measures as input, AdaBoost is used to learn a weighted distance measure, that is a linear combination of the input measures. The proposed method can be seen both as a novel way to learn a distance measure from data, and as a novel way to apply boosting to multiclass recognition problems, that does not require output codes. In our approach, multiclass recognition of objects is reduced into a single binary recognition task, defined on triples of objects. Preliminary experiments with eight UCI datasets yield no clear winner among our method, boosting using output codes, and k-nn classification using an unoptimized distance measure. Our algorithm did achieve lower error rates in some of the datasets, which indicates that, in some domains, it may lead to better results than existing methods. :::::::::::::: 2004-007 :::::::::::::: Title: StaXML: Static Typing of XML Document Fragments for Imperative Web Scripting Languages Authors: Adam Bradley, Assaf Kfoury, and Azer Bestavros Date: February 13, 2004 Abstract: We present a type system, StaXML, which employs the stacked type syntax to represent essential aspects of the potential roles of XML fragments to the structure of complete XML documents. The simplest application of this system is to enforce well-formedness upon the construction of XML documents without requiring the use of templates or balanced "gap plugging" operators; this allows it to be applied to programs written according to common imperative web scripting idioms, particularly the echoing of unbalanced XML fragments to an output buffer. The system can be extended to verify particular XML applications such as XHTML and identifying individual XML tags constructed from their lexical components. We also present StaXML for PHP, a prototype precompiler for the PHP4 scripting language which infers StaXML types for expressions without assistance from the programmer. :::::::::::::: 2004-008 :::::::::::::: Title: Diagnosing Network-Wide Traffic Anomalies Authors: Anukool Lakhina, Mark Crovella and Christophe Diot Date: February 24, 2004 Abstract: Anomalies are unusual and significant changes in a network's traffic levels, which can often involve multiple links. Diagnosing anomalies is critical for both network operators and end users. It is a difficult problem because one must extract and interpret anomalous patterns from large amounts of high-dimensional, noisy data. In this paper we propose a general method to diagnose anomalies. This method is based on a separation of the high-dimensional space occupied by a set of network traffic measurements into disjoint subspaces corresponding to normal and anomalous network conditions. We show that this separation can be performed effectively using Principal Component Analysis. Using only simple traffic measurements from links, we study volume anomalies and show that the method can: (1) accurately detect when a volume anomaly is occurring; (2) correctly identify the underlying origin-destination (OD) flow which is the source of the anomaly; and (3) accurately estimate the amount of traffic involved in the anomalous OD flow. We evaluate the method's ability to diagnose (i.e., detect, identify, and quantify) both existing and synthetically injected volume anomalies in real traffic from two backbone networks. Our method consistently diagnoses the largest volume anomalies, and does so with a very low false alarm rate. :::::::::::::: 2004-009 :::::::::::::: Title: Efficient End-Host Architecture for High Performance Communication Using User-level Sandboxing Authors: Xin Qi, Gabriel Parmer, Richard West, Jason Gloudon, Luis Hernandez Date: March 1, 2004 Abstract: Current low-level networking abstractions on modern operating systems are commonly implemented in the kernel to provide sufficient performance for general purpose applications. However, it is desirable for high performance applications to have more control over the networking subsystem to support optimizations for their specific needs. One approach is to allow networking services to be implemented at user-level. Unfortunately, this typically incurs costs due to scheduling overheads and unnecessary data copying via the kernel. In this paper, we describe a method to implement efficient application-specific network service extensions at user-level, that removes the cost of scheduling and provides protected access to lower-level system abstractions. We present a networking implementation that, with minor modifications to the Linux kernel, passes data between ``sandboxed'' extensions and the Ethernet device without copying or processing in the kernel. Using this mechanism, we put a customizable networking stack into a user-level sandbox and show how it can be used to efficiently process and forward data via proxies, or intermediate hosts, in the communication path of high performance data streams. Unlike other user-level networking implementations, our method makes no special hardware requirements to avoid unnecessary data copies. Results show that we achieve a substantial increase in throughput over comparable user-space methods using our networking stack implementation. :::::::::::::: 2004-010 :::::::::::::: Title: A Randomized Solution to BGP Divergence Author: Selma Yilmaz and Ibrahim Matta Date: March 1, 2004 Abstract: The Border Gateway Protocol (BGP) is an interdomain routing protocol that allows each Autonomous System (AS) to define its own routing policies independently and use them to select the best routes. By means of policies, ASes are able to prevent some traffic from accessing their resources, or direct their traffic to a preferred route. However, this flexibility comes at the expense of a possibility of divergence behavior because of mutually conflicting policies. Since BGP is not guaranteed to converge even in the absence of network topology changes, it is not {\em safe}. In this paper, we propose a randomized approach to providing safety in BGP. The proposed algorithm dynamically detects policy conflicts, and tries to eliminate the conflict by changing the local preference of the paths involved. Both the detection and elimination of policy conflicts are performed locally, {\em i.e.}\ by using only local information. Randomization is introduced to prevent synchronous updates of the local preferences of the paths involved in the same conflict. :::::::::::::: 2004-011 :::::::::::::: Title: A Two-step Statistical Approach for Inferring Network Traffic Demands (Revises Technical Report BUCS-2003-003) Author: Alberto Medina, Kave Salamatian, Nina Taft, Ibrahim Matta, and Christophe Diot Date: March 1, 2004 Abstract: Accurate knowledge of traffic demands in a communication network enables or enhances a variety of traffic engineering and network management tasks of paramount importance for operational networks. Directly measuring a complete set of these demands is prohibitively expensive because of the huge amounts of data that must be collected and the performance impact that such measurements would impose on the regular behavior of the network. As a consequence, we must rely on statistical techniques to produce estimates of actual traffic demands from partial information. The performance of such techniques is however limited due to their reliance on limited information and the high amount of computations they incur, which limits their convergence behavior. In this paper we study a two-step approach for inferring network traffic demands. First we elaborate and evaluate a modeling approach for generating good starting points to be fed to iterative statistical inference techniques. We call these starting points {\it informed priors} since they are obtained using actual network information such as packet traces and SNMP link counts. Second we provide a very fast variant of the EM algorithm which extends its computation range, increasing its accuracy and decreasing its dependence on the quality of the starting point. Finally, we evaluate and compare alternative mechanisms for generating starting points and the convergence characteristics of our EM algorithm against a recently proposed Weighted Least Squares approach. :::::::::::::: 2004-012 :::::::::::::: Title: Simultaneous Localization and Recognition of Dynamic Hand Gestures Authors: Jonathan Alon, Vassilis Athitsos, Quan Yuan, and Stan Sclaroff Date: March 8, 2004 Abstract: A framework for the simultaneous localization and recognition of dynamic hand gestures is proposed. At the core of this framework is a dynamic space-time warping (DSTW) algorithm, that aligns a pair of query and model gestures in both space and time. For every frame of the query sequence, feature detectors generate multiple hand region candidates. Dynamic programming is then used to compute both a global matching cost, which is used to recognize the query gesture, and a warping path, which aligns the query and model sequences in time, and also finds the best hand candidate region in every query frame. The proposed framework includes translation invariant recognition of gestures, a desirable property for many HCI systems. The performance of the approach is evaluated on a dataset of hand signed digits gestured by people wearing short sleeve shirts, in front of a background containing other non-hand skin-colored objects. The algorithm simultaneously localizes the gesturing hand and recognizes the hand-signed digit. Although DSTW is illustrated in a gesture recognition setting, the proposed algorithm is a general method for matching time series, that allows for multiple candidate feature vectors to be extracted at each time step. :::::::::::::: 2004-013 :::::::::::::: Title: A Virtual Deadline Scheduler for Window-Constrained Service Guarantees Authors: Yuting Zhang, Richard West and Xin Qi Date: March 23, 2004 Abstract: This paper presents a new approach to window-constrained scheduling, suitable for multimedia and weakly-hard real-time systems. We originally developed an algorithm, called Dynamic Window-Constrained Scheduling (DWCS), that attempts to guarantee no more than x out of y deadlines are missed for real-time jobs such as periodic CPU tasks, or delay-constrained packet streams. While DWCS is capable of generating a feasible window-constrained schedule that utilizes 100% of resources, it requires all jobs to have the same request periods (or intervals between successive service requests). We describe a new algorithm called Virtual Deadline Scheduling (VDS), that provides window-constrained service guarantees to jobs with potentially different request periods, while still maximizing resource utilization. VDS attempts to service m out of k job instances by their virtual deadlines, that may be some finite time after the corresponding real-time deadlines. Notwithstanding, VDS is capable of outperforming DWCS and similar algorithms, when servicing jobs with potentially different request periods. Additionally, VDS is able to limit the extent to which a fraction of all job instances are serviced late. Results from simulations show that VDS can provide better window-constrained service guarantees than other related algorithms, while still having as good or better delay bounds for all scheduled jobs. Finally, an implementation of VDS in the Linux kernel compares favorably against DWCS for a range of scheduling loads. :::::::::::::: 2004-014 :::::::::::::: Title: Learning Euclidean Embeddings for Indexing and Classification Authors: Vassilis Athitsos, Joni Alon, Stan Sclaroff, George Kollios Date: April 7, 2004 Abstract: BoostMap is a recently proposed method for efficient approximate nearest neighbor retrieval in arbitrary non- Euclidean spaces with computationally expensive and possibly non-metric distance measures. Database and query objects are embedded into a Euclidean space, in which similarities can be rapidly measured using a weighted Manhattan distance. The key idea is formulating embedding construction as a machine learning task, where AdaBoost is used to combine simple, 1D embeddings into a multidimensional embedding that preserves a large amount of the proximity structure of the original space. This paper demonstrates that, using the machine learning formulation of BoostMap, we can optimize embeddings for indexing and classification, in ways that are not possible with existing alternatives for constructive embeddings, and without additional costs in retrieval time. First, we show how to construct embeddings that are query-sensitive, in the sense that they yield a different distance measure for different queries, so as to improve nearest neighbor retrieval accuracy for each query. Second, we show how to optimize embeddings for nearest neighbor classification tasks, by tuning them to approximate a parameter space distance measure, instead of the original feature-based distance measure. :::::::::::::: 2004-015 :::::::::::::: Title: Automated Camera Layout to Satisfy Task-Specific and Floorplan-Specific Coverage Requirements Authors: Ugur Murat Erdem, Stan Sclaroff Date: April 15, 2004 Abstract: In many multi-camera vision systems the effect of camera locations on the task-specific quality of service is ignored. Researchers in Computational Geometry have proposed elegant solutions for some sensor location problem classes. Unfortunately, these solutions utilize unrealistic assumptions about the cameras' capabilities that make these algorithms unsuitable for many real-world computer vision applications: unlimited field of view, infinite depth of field, and/or infinite servo precision and speed. In this paper, the general camera placement problem is first defined with assumptions that are more consistent with the capabilities of real-world cameras. The region to be observed by cameras may be volumetric, static or dynamic, and may include holes that are caused, for instance, by columns or furniture in a room that can occlude potential camera views. A subclass of this general problem can be formulated in terms of planar regions that are typical of building floorplans. Given a foorplan to be observed, the problem is then to efficiently compute a camera layout such that certain task-specific constraints are met. A solution to this problem is obtained via binary optimization over a discrete problem space. In experiments the performance of the resulting system is demonstrated with different real foorplans. :::::::::::::: 2004-016 :::::::::::::: Title: Robust Tracking of Human Motion Author: Dan Buzan, Boston University Date: 04/23/2004 Abstract: This technical report presents a combined solution for two problems, one: tracking objects in 3D space and estimating their trajectories and second: computing the similarity between previously estimated trajectories and clustering them using the similarities that we just computed. For the first part, trajectories are estimated using an EKF formulation that will provide the 3D trajectory up to a constant. To improve accuracy, when occlusions appear, multiple hypotheses are followed. For the second problem we compute the distances between trajectories using a similarity based on LCSS formulation. Similarities are computed between projections of trajectories on coordinate axes. Finally we group trajectories together based on previously computed distances, using a clustering algorithm. To check the validity of our approach, several experiments using real data were performed. :::::::::::::: 2004-017 :::::::::::::: Title: Extraction and Clustering of Motion Trajectories in Video Author: Dan Buzan, Boston University Stan Sclaroff, Boston University George Kollios, Boston University Date: 04/23/2004 Abstract: A system is described that tracks moving objects in a video dataset so as to extract a representation of the objects' 3D trajectories. The system then finds hierarchical clusters of similar trajectories in the video dataset. Objects' motion trajectories are extracted via an EKF formulation that provides each object's 3D trajectory up to a constant factor. To increase accuracy when occlusions occur, multiple tracking hypotheses are followed. For trajectory-based clustering and retrieval, a modified version of edit distance, called longest common subsequence (LCSS) is employed. Similarities are computed between projections of trajectories on coordinate axes. Trajectories are grouped based, using an agglomerative clustering algorithm. To check the validity of the approach, experiments using real data were performed. :::::::::::::: 2004-018 :::::::::::::: Title: Group Key Manager on a Smart Card Authors: Hani Hamandi (Geotrust) and Gene Itkis (BU) Date: 4/27/04 Abstract: Group communication is as an important functionality, which needs to be supported by various communication technologies. Applications of group communication include IP (or application-level) multicast, wireless and/or ad-hoc networks, broadcast, conference calling, pay-per-view, and even such seemingly unrelated to networks areas as copy protection. For many, if not all, of these applications, security and trust play an important role. Securing group communication typically requires confidentiality and authentication, which typically rely on secret keys. Thus key management issues must be addressed. This paper describes an implementation of one approach to dynamic group key management, which is based on Logical Key Hierarchy or Subset-Cover approach [1,2]. Our approach achieves a dramatic reduction of the storage requirements for the Group Key Manager, and in particular allows all the secret key data to be stored on a smart-card. It also allows a number of subsequent improvements. :::::::::::::: 2004-019 :::::::::::::: Title: Interactive Password Schemes Authors: Gene Itkis (BU) and Arwa Maiss (BU) Date: 4/27/04 Abstract: Usual password schemes suffer from the flaw that they are easy to steal. An attacker who has correctly observed a login session (by peeping, wiretapping and/or by launching a "man-in-the-middle" attack, etc.) can easily impersonate the corresponding user. Available protection techniques require computations on hundreds digit integers that are so complex that they require special software and/or hardware. This project tries to combine the simplicity of the conventional password schemes with a protection technique that results in a different password being typed each session, but only requires simple computation performed in the user's head. :::::::::::::: 2004-020 :::::::::::::: Authors: Anukool Lakhina, Mark Crovella and Christophe Diot Title: Characterization of Network-Wide Anomalies in Traffic Flows Date: May 14, 2004 Abstract: Detecting and understanding anomalies in IP networks is an open and ill-defined problem. Toward this end, we have recently proposed the subspace method for anomaly diagnosis. In this paper we present the first large-scale exploration of the power of the subspace method when applied to flow traffic. An important aspect of this approach is that it fuses information from flow measurements taken throughout a network. We apply the subspace method to three different types of sampled flow traffic in a large academic network: multivariate timeseries of byte counts, packet counts, and IP-flow counts. We show that each traffic type brings into focus a different set of anomalies via the subspace method. We illustrate and classify the set of anomalies detected. We find that almost all of the anomalies detected represent events of interest to network operators. Furthermore, the anomalies span a remarkably wide spectrum of event types, including denial of service attacks (single-source and distributed), flash crowds, port scanning, downstream traffic engineering, high-rate flows, worm propagation, and network outage. :::::::::::::: 2004-021 :::::::::::::: Title: Safe Compositional Specification of Networking Systems Authors: Azer Bestavros, Adam D. Bradley, Assaf J. Kfoury, and Ibrahim Matta Date: May 14, 2004 Abstract: The Science of Network Service Composition has clearly emerged as one of the grand themes driving many of our research questions in the networking field today [NeXtworking 2003]. This driving force stems from the rise of sophisticated applications and new networking paradigms. By ``service composition'' we mean that the performance and correctness properties local to the various constituent components of a service can be readily composed into global (end-to-end) properties without re-analyzing any of the constituent components in isolation, or as part of the whole composite service. The set of laws that would govern such composition is what will constitute that new science of composition. The combined heterogeneity and dynamic open nature of network systems makes composition quite challenging, and thus programming network services has been largely inaccessible to the average user. We identify (and outline) a research agenda in which we aim to develop a specification language that is expressive enough to describe different components of a network service, and that will include type hierarchies inspired by type systems in general programming languages that enable the safe composition of software components. We envision this new science of composition to be built upon several theories (e.g., control theory, game theory, network calculus, percolation theory, economics, queuing theory). In essence, different theories may provide different languages by which certain properties of system components can be expressed and composed into larger systems. We then seek to lift these lower-level specifications to a higher level by abstracting away details that are irrelevant for safe composition at the higher level, thus making theories scalable and useful to the average user. In this paper we focus on services built upon an overlay management architecture, and we use control theory and QoS theory as example theories from which we lift up compositional specifications. :::::::::::::: 2004-022 :::::::::::::: Title: SEP: A Stable Election Protocol for clustered heterogeneous wireless sensor networks Authors: Georgios Smaragdakis, Ibrahim Matta, and Azer Bestavros Date: May 31, 2004 Abstract: We study the impact of heterogeneity of nodes, in terms of their energy, in wireless sensor networks that are hierarchically clustered. In these networks some of the nodes become cluster heads, aggregate the data of their cluster members and transmit it to the sink. We assume that a percentage of the population of sensor nodes is equipped with additional energy resources---this is a source of heterogeneity which may result from the initial setting or as the operation of the network evolves. We also assume that the sensors are randomly (uniformly) distributed and are not mobile, the coordinates of the sink and the dimensions of the sensor field are known. We show that the behavior of such sensor networks becomes very unstable once the first node dies, especially in the presence of node heterogeneity. Classical clustering protocols assume that all the nodes are equipped with the same amount of energy and as a result, they can not take full advantage of the presence of node heterogeneity. We propose SEP, a heterogeneous-aware protocol to prolong the time interval before the death of the first node (we refer to as {\em stability period}), which is crucial for many applications where the feedback from the sensor network must be reliable. SEP is based on weighted election probabilities of each node to become cluster head according to the remaining energy in each node. We show by simulation that SEP always prolongs the stability period compared to (and that the average throughput is greater than) the one obtained using current clustering protocols. We conclude by studying the sensitivity of our SEP protocol to heterogeneity parameters capturing energy imbalance in the network. We found that SEP yields longer stability region for higher values of extra energy brought by more powerful nodes. :::::::::::::: 2004-023 :::::::::::::: Title: DIP: Density Inference Protocol for wireless sensor networks and its application to density-unbiased statistics Authors: Niky Riga, Ibrahim Matta, and Azer Bestavros Date: May 31, 2004 Abstract: Wireless sensor networks have recently emerged as enablers of important applications such as environmental, chemical and nuclear sensing systems. Such applications have sophisticated spatial-temporal semantics that set them aside from traditional wireless networks. For example, the computation of temperature averaged over the sensor field must take into account local densities. This is crucial since otherwise the estimated average temperature can be biased by over-sampling areas where a lot more sensors exist. Thus, we envision that a fundamental service that a wireless sensor network should provide is that of estimating local densities. In this paper, we propose a lightweight probabilistic density inference protocol, we call DIP, which allows each sensor node to {\em implicitly} estimate its neighborhood size without the explicit exchange of node identifiers as in existing density discovery schemes. The theoretical basis of DIP is a probabilistic analysis which gives the relationship between the number of sensor nodes contending in the neighborhood of a node and the level of contention measured by that node. Extensive simulations confirm the premise of DIP: it can provide statistically reliable and accurate estimates of local density at a very low energy cost and constant running time. We demonstrate how applications could be built on top of our DIP-based service by computing density-unbiased statistics from estimated local densities. :::::::::::::: 2004-024 :::::::::::::: Title: On the Interaction between Data Aggregation and Topology Control in Wireless Sensor Networks Authors: Vijay Erramilli, Ibrahim Matta, and Azer Bestavros Date: June 18, 2004 Abstract: Wireless sensor networks are characterized by limited energy resources. To conserve energy, application-specific aggregation (fusion) of data reports from multiple sensors can be beneficial in reducing the amount of data flowing over the network. Furthermore, controlling the topology by scheduling the activity of nodes between active and sleep modes has often been used to uniformly distribute the energy consumption among all nodes by de-synchronizing their activities. We present an integrated analytical model to study the joint performance of in-network aggregation and topology control. We define performance metrics that capture the tradeoffs among delay, energy, and fidelity of the aggregation. Our results indicate that to achieve high fidelity levels under medium to high event reporting load, shorter and fatter aggregation/routing trees (toward the sink) offer the best delay-energy tradeoff as long as topology control is well coordinated with routing. :::::::::::::: 2004-025 :::::::::::::: Title: Bayesian Packet Loss Detection for TCP Author: Nahur Fonseca and Mark Crovella Date: July 6, 2004 Abstract: One of TCP's critical tasks is to determine which packets are lost in the network, as a basis for control actions (flow control and packet retransmission). Modern TCP implementations use two mechanisms: timeout, and fast retransmit. Detection via timeout is necessarily a time-consuming operation; fast retransmit, while much quicker, is only effective for a small fraction of packet losses. In this paper we consider the problem of packet loss detection in TCP more generally. We concentrate on the fact that TCP's control actions are necessarily triggered by *inference* of packet loss, rather than conclusive knowledge. This suggests that one might analyze TCP's packet loss detection in a standard inferencing framework based on probability of detection and probability of false alarm. This paper makes two contributions to that end: First, we study an example of more general packet loss inference, namely optimal Bayesian packet loss detection based on round trip time. We show that for long-lived flows, it is frequently possible to achieve high detection probability and low false alarm probability based on measured round trip time. Second, we construct an analytic performance model that incorporates general packet loss inference into TCP. We show that for realistic detection and false alarm probabilities (as are achievable via our Bayesian detector) and for moderate packet loss rates, the use of more general packet loss inference in TCP can improve throughput by as much as 25%. :::::::::::::: 2004-026 :::::::::::::: Title: dPAM: A Distributed Prefetching Protocol for Scalable Asynchronous Multicast in P2P Systems Authors: Abhishek Sharma, Azer Bestavros, and Ibrahim Matta Date: July 7, 2004 Abstract: We leverage the buffering capabilities of end-systems to achieve scalable, asynchronous delivery of streams in a peer-to-peer environment. Unlike existing cache-and-relay schemes, we propose a distributed {\em prefetching} protocol where peers prefetch and store portions of the streaming media ahead of their playout time, thus not only turning themselves to possible sources for other peers but their prefetched data can allow them to overcome the departure of their source-peer. This stands in sharp contrast to existing cache-and-relay schemes where the departure of the source-peer forces its peer children to go the original server, thus disrupting their service and increasing server and network load. Through mathematical analysis and simulations, we show the effectiveness of maintaining such asynchronous multicasts from several source-peers to other children peers, and the efficacy of prefetching in the face of peer departures. We confirm the scalability of our dPAM protocol as it is shown to significantly reduce server load. :::::::::::::: 2004-027 :::::::::::::: Title: On trip planning queries in spatial databases Author: Feifei Li, Dihan Cheng Date: July 1, 2004 Abstract: In this paper we discuss a new type of query in Spatial Databases, called Trip Planning Query (TPQ). Given a set of points P in space, where each point belongs to a category, and given two points s and e, TPQ asks for the best trip that starts at s, passes through exactly one point from each category, and ends at e. An example of a TPQ is when a user wants to visit a set of different places and at the same time minimize the total travelling cost, e.g. what is the shortest travelling plan for me to visit an automobile shop, a CVS pharmacy outlet, and a Best Buy shop along my trip from A to B? The trip planning query is an extension of the well-known TSP problem and therefore is NP-hard. The difficulty of this query lies in the existence of multiple choices for each category. In this paper, we first study fast approximation algorithms for the trip planning query in a metric space, assuming that the data set fits in main memory, and give the theory analysis of their approximation bounds. Then, the trip planning query is examined for data sets that do not fit in main memory and must be stored on disk. For the disk-resident data, we consider two cases. In one case, we assume that the points are located in Euclidean space and indexed with an Rtree. In the other case, we consider the problem of points that lie on the edges of a spatial network (e.g. road network) and the distance between two points is defined using the shortest distance over the network. Finally, we give an experimental evaluation of the proposed algorithms using synthetic data sets generated on real road networks. :::::::::::::: 2004-028 :::::::::::::: Title: GreedyDual-Join: Locality-Aware Buffer Management for Approximate Join Processing Over Data Streams Author: Ching Chang, Feifei Li, Azer Bestavros, and George Kollios Date: July 1, 2004 Abstract: We investigate adaptive buffer management techniques for approximate evaluation of sliding window joins over multiple data streams. In many applications, data stream processing systems have limited memory or have to deal with very high speed data streams. In both cases, computing the exact results of joins between these streams may not be feasible, mainly because the buffers used to compute the joins contain much smaller number of tuples than the tuples contained in the sliding windows. Therefore, a stream buffer management policy is needed in that case. We show that the buffer replacement policy is an important determinant of the quality of the produced results. To that end, we propose GreedyDual-Join (GDJ) an adaptive and locality-aware buffering technique for managing these buffers. GDJ exploits the temporal correlations (at both long and short time scales), which we found to be prevalent in many real data streams. We note that our algorithm is readily applicable to multiple data streams and multiple joins and requires almost no additional system resources. We report results of an experimental study using both synthetic and real-world data sets. Our results demonstrate the superiority and flexibility of our approach when contrasted to other recently proposed techniques. :::::::::::::: 2004-029 :::::::::::::: Title: M2RC: Multiplicative-increase/additive-decrease Multipath Routing Control for Wireless Sensor Networks Authors: Hany Morcos, Ibrahim Matta, and Azer Bestavros Date: July 14, 2004 Abstract: Routing protocols in wireless sensor networks (WSN) face two main challenges: first, the challenging environments in which WSNs are deployed negatively affect the quality of the routing process. Therefore, routing protocols for WSNs should recognize and react to node failures and packet losses. Second, sensor nodes are battery-powered, which makes power a scarce resource. Routing protocols should optimize power consumption to prolong the lifetime of the WSN. In this paper, we present a new adaptive routing protocol for WSNs, we call it M2RC. M2RC has two phases: mesh establishment phase and data forwarding phase. In the first phase,M2RC establishes the routing state to enable multipath data forwarding. In the second phase, M2RC forwards data packets from the source to the sink. Targeting hop-by-hop reliability, an M2RC forwarding node waits for an acknowledgement (ACK) that its packets were correctly received at the next neighbor. Based on this feedback, an M2RC node applies multiplicative-increase/additive-decrease (MIAD) to control the number of neighbors targeted by its packet broadcast. We simulated M2RC in the ns-2 simulator and compared it to GRAB, Max-power, and Min-power routing schemes. Our simulations show that M2RC achieves the highest throughput with at least 10-30 percent less consumed power per delivered report in scenarios where a certain number of nodes unexpectedly fail. :::::::::::::: 2004-030 :::::::::::::: Title: Friendly Virtual Machine: Leveraging a Feedback-Control Model for Application Adaptation Authors: Yuting Zhang, Azer Bestavros, Mina Guirguis, Ibrahim Matta, and Richard West Date: July 2004 Abstract: With the increased use of ``Virtual Machines'' (VMs) as vehicles that isolate applications running on the same host, it is necessary to devise techniques that enable multiple VMs to share underlying resources both fairly and efficiently. To that end, one common approach is to deploy complex resource management techniques in the %untrust% hosting infrastructure. Alternately, in this paper, we advocate the use of self-adaptation in the VMs themselves based on feedback about resource usage and availability. Consequently, we define a ``Friendly'' VM (FVM) to be a virtual machine that adjusts its demand for system resources, so that they are both efficiently and fairly allocated to competing FVMs. Such properties are ensured using one of many provably convergent control rules, such as AIMD. By adopting this distributed application-based approach to resource management, it is not necessary to make assumptions about the underlying resources nor about the requirements of FVMs competing for these resources. To demonstrate the elegance and simplicity of our approach, we present a prototype implementation of our FVM framework in User-Mode Linux (UML)---an implementation that consists of less than 500 lines of code changes to UML. We present an analytic, control-theoretic model of FVM adaptation, which establishes convergence and fairness properties. These properties are also backed up with experimental results using our prototype FVM implementation. :::::::::::::: 2004-031 :::::::::::::: Title: Approximately Uniform Random Sampling in Sensor Networks Authors: Boulat A. Bash, John W. Byers and Jeffrey Considine Date: July 19, 2004 Abstract: Recent work in sensor databases has focused extensively on distributed query problems, notably distributed computation of aggregates. Existing methods for computing aggregates broadcast queries to all sensors and use in-network aggregation of responses to minimize messaging costs. In this work, we focus on uniform random sampling across nodes, which can serve both as an alternative building block for aggregation and as an integral component of many other useful randomized algorithms. Prior to our work, the best existing proposals for uniform random sampling of sensors involve contacting all nodes in the network. We propose a practical method which is only approximately uniform, but contacts a number of sensors proportional to the diameter of the network instead of its size. The approximation achieved is tunably close to exact uniform sampling, and only relies on well-known existing primitives, namely geographic routing, distributed computation of Voronoi regions and von Neumann's rejection method. Ultimately, our sampling algorithm has the same worst-case asymptotic cost as routing a point-to-point message, and thus it is asymptotically optimal among request/reply-based sampling methods. We provide experimental results demonstrating the effectiveness of our algorithm on both synthetic and real sensor topologies. :::::::::::::: 2004-032 :::::::::::::: Title: A Note On the Statistical Difference of Small Direct Products Author: Leonid Reyzin Date: 9/21/04 Abstract: We demonstrate that if two probability distributions D and E of sufficiently small min-entropy have statistical difference \epsilon, then the direct-product distributions D^l and E^l have statistical difference at least roughly \epsilon\sqrt{l}, provided that l is sufficiently small, smaller than roughly \epsilon^{-4/3}. Previously known bounds did not work for few repetitions l, requiring l>\epsilon^{-2}. :::::::::::::: 2004-033 :::::::::::::: Title: Periodic Motion Detection and Estimation via Space-Time Sampling Authors: Ashwin Thangali and Stan Sclaroff Date: November 2, 2004 Abstract: A novel technique to detect and localize periodic movements in video is presented. The distinctive feature of the technique is that it requires neither feature tracking nor object segmentation. Intensity patterns along linear sample paths in space-time are used in estimation of period of object motion in a given sequence of frames. Sample paths are obtained by connecting (in space-time) sample points from regions of high motion magnitude in the first and last frames. Oscillations in intensity values are induced at time instants when an object intersects the sample path. The locations of peaks in intensity are determined by parameters of both cyclic object motion and orientation of the sample path with respect to object motion. The information about peaks is used in a least squares framework to obtain an initial estimate of these parameters. The estimate is further refined using the full intensity profile. The best estimate for the period of cyclic object motion is obtained by looking for consensus among estimates from many sample paths. The proposed technique is evaluated with synthetic videos where ground-truth is known, and with American Sign Language videos where the goal is to detect periodic hand motions. :::::::::::::: 2004-034 :::::::::::::: Title: Multi-scale 3D Scene Flow from Binocular Stereo Sequences Authors: Rui Li and Stan Sclaroff Date: November 2, 2004 Abstract: Scene flow methods estimate the three-dimensional motion field for points in the world, using multi-camera video data. Such methods combine multi-view reconstruction with motion estimation approaches. This paper describes an alternative formulation for dense scene flow estimation that provides convincing results using only two cameras by fusing stereo and optical flow estimation into a single coherent framework. To handle the aperture problems inherent in the estimation task, a multi-scale method along with a novel adaptive smoothing technique is used to gain a regularized solution. This combined approach both preserves discontinuities and prevents over-regularization -- two problems commonly associated with basic multi-scale approaches. Internally, the framework generates probability distributions for optical flow and disparity. Taking into account the uncertainty in the intermediate stages allows for more reliable estimation of the 3D scene flow than standard stereo and optical flow methods allow. Experiments with synthetic and real test data demonstrate the effectiveness of the approach. :::::::::::::: 2004-035 :::::::::::::: Title: Automatic 2D Hand Tracking in Video Sequences Author: Quan Yuan , Stan Sclaroff and Vassilis Athitsos Date: November 2, 2004 Abstract: In gesture and sign language video sequences, hand motion tends to be rapid, and hands frequently appear in front of each other or in front of the face. Thus, hand location is often ambiguous, and naive color-based hand tracking is insufficient. To improve tracking accuracy, some methods employ a prediction-update framework, but such methods require careful initialization of model parameters, and tend to drift and lose track in extended sequences. In this paper, a temporal filtering framework for hand tracking is proposed that can initialize and reset itself without human intervention. In each frame, simple features like color and motion residue are exploited to identify multiple candidate hand locations. The temporal filter then uses the Viterbi algorithm to select among the candidates from frame to frame. The resulting tracking system can automatically identify video trajectories of unambiguous hand motion, and detect frames where tracking becomes ambiguous because of occlusions or overlaps. Experiments on video sequences of several hundred frames in duration demonstrate the system's ability to track hands robustly, to detect and handle tracking ambiguities, and to extract the trajectories of unambiguous hand motion. :::::::::::::: 2004-036 :::::::::::::: Title : Handsignals Recognition From Video Using 3D Motion Capture Data Authors : Tai-Peng Tian and Stan Sclaroff Date: November 4, 2004 Abstract: Hand signals are commonly used in applications such as giving instructions to a pilot for airplane take off or direction of a crane operator by a foreman on the ground. A new algorithm for recognizing hand signals from a single camera is proposed. Typically, tracked 2D feature positions of hand signals are matched to 2D training images. In contrast, our approach matches the 2D feature positions to an archive of 3D motion capture sequences. The method avoids explicit reconstruction of the 3D articulated motion from 2D image features. Instead, the matching between the 2D and 3D sequence is done by backprojecting the 3D motion capture data onto 2D. Experiments demonstrate the effectiveness of the approach in an example app lication: recognizing six classes of basketball referee hand signals in video. :::::::::::::: 2005-001 :::::::::::::: Title: Scalable Coordination Techniques for Distributed Network Monitoring Authors: Manish Sharma and John Byers Date: January 20, 2005 Abstract: Emerging network monitoring infrastructures capture packet-level traces or keep per-flow statistics at a set of distributed vantage points. Today, distributed monitors in such an infrastructure do not coordinate monitoring effort, which both can lead to duplication of effort and can complicate subsequent data analysis. We argue that nodes in such a monitoring infrastructure, whether across the wide-area Internet, or across a sensor network, should coordinate effort to minimize resource consumption. We propose space-efficient data structures for use in gossip-based protocols to approximately summarize sets of monitored flows. With some fine-tuning of our methods, we can ensure that all flows observed by at least one monitor are monitored, and only a tiny fraction are monitored redundantly. Our preliminary results over a realistic ISP topology demonstrate the effectiveness of our techniques on monitoring tens of thousands of point-of-presence (PoP) level network flows. Our methods are competitive with optimal off-line coordination, but require significantly less space and network overhead than naive approaches. :::::::::::::: 2005-002 :::::::::::::: Title: Mining Anomalies Using Traffic Distributions Authors: Anukool Lakhina, Mark Crovella and Christophe Diot Date: February 10, 2005 Abstract: The increasing practicality of large-scale flow capture makes it possible to conceive of traffic analysis methods that detect and identify a large and diverse set of anomalies. However the challenge of effectively analyzing this massive data source for anomaly diagnosis is as yet unmet. We argue that the distributions of packet features (IP addresses and ports) observed in flow traces reveals both the presence and the structure of a wide range of anomalies. Using entropy as a summarization tool, we show that the analysis of feature distributions leads to significant advances on two fronts: (1) it enables highly sensitive detection of a wide range of anomalies, augmenting detections by volume-based methods, and (2) it enables automatic classification of anomalies via unsupervised learning. We show that using feature distributions, anomalies naturally fall into distinct and meaningful clusters. These clusters can be used to automatically classify anomalies and to uncover new anomaly types. We validate our claims on data from two backbone networks (Abilene and Geant) and conclude that feature distributions show promise as a key element of a fairly general network anomaly diagnosis framework. :::::::::::::: 2005-003 :::::::::::::: Title: Applied Type System with Stateful Views Authors: Hongwei Xi, Boston University Dengping Zhu, Boston University Yanka Li, Carnegie Mellon University Date: Feb. 10, 2005 Abstract: We present a type system that can effectively facilitate the use of types in capturing invariants in stateful programs that may involve (sophisticated) pointer manipulation. With its root in a recently developed framework Applied Type System (ATS), the type system imposes a level of abstraction on program states by introducing a novel notion of recursive stateful views and then relies on a form of linear logic to reason about such views. We consider the design and then the formalization of the type system to constitute the primary contribution of the paper. In addition, we mention a prototype implementation of the type system and then give a variety of examples that attests to the practicality of programming with recursive stateful views. :::::::::::::: 2005-004 :::::::::::::: Authors: Richard West, Gerald Fry and Gary Wong Title: Comparison of k-ary n-cube and de Bruijn Overlays in QoS-constrained Multicast Applications Date: February 23, 2005 Abstract: Research on the construction of logical overlay networks has gained significance in recent times. This is partly due to work on peer-to-peer (P2P) systems for locating and retrieving distributed data objects, and also scalable content distribution using end-system multicast techniques. However, there are emerging applications that require the real-time transport of data from various sources to potentially many thousands of subscribers, each having their own quality-of-service (QoS) constraints. This paper primarily focuses on the properties of two popular topologies found in interconnection networks, namely k-ary n-cubes and de Bruijn graphs. The regular structure of these graph topologies makes them easier to analyze and determine possible routes for real-time data than complete or irregular graphs. We show how these overlay topologies compare in their ability to deliver data according to the QoS constraints of many subscribers, each receiving data from specific publishing hosts. Comparisons are drawn on the ability of each topology to route data in the presence of dynamic system effects, due to end-hosts joining and departing the system. Finally, experimental results show the service guarantees and physical link stress resulting from efficient multicast trees constructed over both kinds of overlay networks. :::::::::::::: 2005-005 :::::::::::::: Title: An Efficient User-Level Shared Memory Mechanism for Application-Specific Extensions (revised and extended version of BUCS-TR-2003-014) Authors: Richard West, Jason Gloudon, Xin Qi and Gabriel Parmer Date: February 23, 2005 Abstract: This paper focuses on an efficient user-level method for the deployment of application-specific extensions, using commodity operating systems and hardware. A sandboxing technique is described that supports multiple extensions within a shared virtual address space. Applications can register sandboxed code with the system, so that it may be executed in the context of any process. Such code may be used to implement generic routines and handlers for a class of applications, or system service extensions that complement the functionality of the core kernel. Using our approach, application-specific extensions can be written like conventional user-level code, utilizing libraries and system calls, with the advantage that they may be executed without the traditional costs of scheduling and context-switching between process-level protection domains. No special hardware support such as segmentation or tagged translation look-aside buffers (TLBs) is required. Instead, our ``user-level sandboxing'' mechanism requires only paged-based virtual memory support, given that sandboxed extensions are either written by a trusted source or are guaranteed to be memory-safe (e.g., using type-safe languages). Using a fast method of upcalls, we show how our mechanism provides significant performance improvements over traditional methods of invoking user-level services. As an application of our approach, we have implemented a user-level network subsystem that avoids data copying via the kernel and, in many cases, yields far greater network throughput than kernel-level approaches. :::::::::::::: 2005-006 :::::::::::::: Title: Cuckoo: a Language for Implementing Memory- and Thread-safe System Services Authors: Richard West and Gary Wong Date: February 23, 2005 Abstract: This paper is centered around the design of a thread- and memory-safe language, primarily for the compilation of application-specific services for extensible operating systems. We describe various issues that have influenced the design of our language, called Cuckoo, that guarantees safety of programs with potentially asynchronous flows of control. Comparisons are drawn between Cuckoo and related software safety techniques, including Cyclone and software-based fault isolation (SFI), and performance results suggest our prototype compiler is capable of generating safe code that executes with low runtime overheads, even without potential code optimizations. Compared to Cyclone, Cuckoo is able to safely guard accesses to memory when programs are multithreaded. Similarly, Cuckoo is capable of enforcing memory safety in situations that are potentially troublesome for techniques such as SFI. :::::::::::::: 2005-007 :::::::::::::: Title: SymbolDesign: A User-centered Method to Design Pen-based Interfaces and Extend the Functionality of Pointer Input Devices Author: Margrit Betke, Oleg Gusyatin, and Mikhail Urinson Date: February 25, 2005 Abstract: A method called ``SymbolDesign'' is proposed that can be used to design user-centered interfaces for pen-based input devices. It can also extend the functionality of pointer input devices such as the traditional computer mouse or the Camera Mouse, a camera-based computer interface. Users can create their own interfaces by choosing single-stroke movement patterns that are convenient to draw with the selected input device and by mapping them to a desired set of commands. A pattern could be the trace of a moving finger detected with the Camera Mouse or a symbol drawn with an optical pen. The core of the SymbolDesign system is a dynamically created classifier, in the current implementation an artificial neural network. The architecture of the neural network automatically adjusts according to the complexity of the classification task. In experiments, subjects used the SymbolDesign method to design and test the interfaces they created, for example, to browse the web. The experiments demonstrated good recognition accuracy and responsiveness of the user interfaces. The method provided an easily-designed and easily-used computer input mechanism for people without physical limitations, and, with some modifications, has the potential to become a computer access tool for people with severe paralysis. :::::::::::::: 2005-008 :::::::::::::: Title: MosaicShape: Stochastic Region Grouping with Shape Prior Author: Jingbin Wang, Erdan Gu, and Margrit Betke Date: February 25, 2005 Abstract: A novel method that combines shape-based object recognition and image segmentation is proposed for shape retrieval from images. Given a shape prior represented in a multi-scale curvature form, the proposed method identifies the target objects in images by grouping oversegmented image regions. The problem is formulated in a unified probabilistic framework and solved by a stochastic Markov Chain Monte Carlo (MCMC) mechanism. By this means, object segmentation and recognition are accomplished simultaneously. Within each sampling move during the simulation process, probabilistic region grouping operations are influenced by both the image information and the shape similarity constraint. The latter constraint is measured by a partial shape matching process. A generalized parallel algorithm by Barbu and Zhu, combined with a large sampling jump and other implementation improvements, greatly speeds up the overall stochastic process. The proposed method supports the segmentation and recognition of multiple occluded objects in images. Experimental results are provided for both synthetic and real images. :::::::::::::: 2005-009 :::::::::::::: Authors: Vassilis Athitsos, Jonathan Alon, and Stan Sclaroff Title: Efficient Nearest Neighbor Classification Using a Cascade of Approximate Similarity Measures Date: March 16, 2005 Abstract: Nearest neighbor classification using shape context can yield highly accurate results in a number of recognition problems. Unfortunately, the approach can be too slow for practical applications, and thus approximation strategies are needed to make shape context practical. This paper proposes a method for efficient and accurate nearest neighbor classification in non-Euclidean spaces, such as the space induced by the shape context measure. First, a method is introduced for constructing a Euclidean embedding that is optimized for nearest neighbor classification accuracy. Using that embedding, multiple approximations of the underlying non-Euclidean similarity measure are obtained, at different levels of accuracy and efficiency. The approximations are automatically combined to form a cascade classifier, which applies the slower approximations only to the hardest cases. Unlike typical cascade-of-classifiers approaches, that are applied to binary classification problems, our method constructs a cascade for a multiclass problem. Experiments with a standard shape data set indicate that a two-to-three order of magnitude speed up is gained over the standard shape context classifier, with minimal losses in classification accuracy. :::::::::::::: 2005-010 :::::::::::::: Authors: Vassilis Athitsos, Marios Hadjieleftheriou, George Kollios, and Stan Sclaroff Title: Query-Sensitive Embeddings Date: March 16, 2005 Abstract: A common problem in many types of databases is retrieving the most similar matches to a query object. Finding those matches in a large database can be too slow to be practical, especially in domains where objects are compared using computationally expensive similarity (or distance) measures. This paper proposes a novel method for approximate nearest neighbor retrieval in such spaces. Our method is embedding-based, meaning that it constructs a function that maps objects into a real vector space. The mapping preserves a large amount of the proximity structure of the original space, and it can be used to rapidly obtain a short list of likely matches to the query. The main novelty of our method is that it constructs, together with the embedding, a query-sensitive distance measure that should be used when measuring distances in the vector space. The term ``query-sensitive'' means that the distance measure changes depending on the current query object. We report experiments with an image database of handwritten digits, and a time-series database. In both cases, the proposed method outperforms existing state-of-the-art embedding methods, meaning that it provides significantly better trade-offs between efficiency and retrieval accuracy. :::::::::::::: 2005-011 :::::::::::::: Title: Robust Sketching and Aggregation of Distributed Data Streams Author: Marios Hadjieleftheriou, John W. Byers, George Kollios Date: March 16, 2005 Abstract: The data streaming model provides an attractive framework for one-pass summarization of massive data sets at a single observation point. However, in an environment where multiple data streams arrive at a set of distributed observation points, sketches must be computed remotely and then must be aggregated through a hierarchy before queries may be conducted. As a result, many sketch-based methods for the single stream case do not apply directly, as either the error introduced becomes large, or because the methods assume that the streams are non-overlapping. These limitations hinder the application of these techniques to practical problems in network traffic monitoring and aggregation in sensor networks. To address this, we develop a general framework for evaluating and enabling robust computation of duplicate-sensitive aggregate functions (e.g., SUM and QUANTILE), over data produced by distributed sources. We instantiate our approach by augmenting the Count-Min and Quantile-Digest sketches to apply in this distributed setting, and analyze their performance. We conclude with experimental evaluation to validate our analysis. :::::::::::::: 2005-012 :::::::::::::: Title: Real Time Eye Tracking and Blink Detection with USB Cameras Authors: Michael Chau and Margrit Betke Date: April 28, 2005 Abstract: A human-computer interface (HCI) system designed for use by people with severe disabilities is presented. People that are severely paralyzed or afflicted with diseases such as ALS (Lou Gehrig's disease) or multiple sclerosis are unable to move or control any parts of their bodies except for their eyes. The system presented here detects the user's eye blinks and analyzes the pattern and duration of the blinks, using them to provide input to the computer in the form of a mouse click. After the automatic initialization of the system occurs from the processing of the user's involuntary eye blinks in the first few seconds of use, the eye is tracked in real time using correlation with an online template. If the user's depth changes significantly or rapid head movement occurs, the system is automatically reinitialized. There are no lighting requirements nor offline templates needed for the proper functioning of the system. The system works with inexpensive USB cameras and runs at a frame rate of 30 frames per second. Extensive experiments were conducted to determine both the system's accuracy in classifying voluntary and involuntary blinks, as well as the system's fitness in varying environment conditions, such as alternative camera placements and different lighting conditions. These experiments on eight test subjects yielded an overall detection accuracy of 95.3%. :::::::::::::: 2005-013 :::::::::::::: Title: "An Invariant Representation for Matching Trajectories across Uncalibrated Video Streams" Authors: Walter Nunziati (University of Florence), Stan Sclaroff, Alberto Del Bimbo (University of Florence) Abstract: We introduce a viewpoint invariant representation of moving object trajectories that can be used in video database applications. It is assumed that trajectories lie on a surface that can be locally approximated with a plane. Raw trajectory data is first locally approximated with a cubic spline via least squares fitting. For each sampled point of the obtained curve, a projective invariant feature is computed using a small number of points in its neighborhood. The resulting sequence of invariant features computed along the entire trajectory forms the view invariant descriptor of the trajectory itself. Time parametrization has been exploited to compute cross ratios without ambiguity due to point ordering. Similarity between descriptors of different trajectories is measured with a distance that takes into account the statistical properties of the cross ratio, and its symmetry with respect to the point at in nity. In experiments, an overall correct classification rate of about 95% has been obtained on a dataset of 58 trajectories of players in soccer video, and an overall correct classification rate of about 80% has been obtained on matching partial segments of trajectories collected from two overlapping views of outdoor scenes with moving people and cars. :::::::::::::: 2005-014 :::::::::::::: Title: Typed Abstraction of Complex Network Compositions Authors: Azer Bestavros and Adam D. Bradley and Assaf J. Kfoury and Ibrahim Matta Date: May 1, 2005 Abstract: The heterogeneity and open nature of network systems make analysis of compositions of components quite challenging, making the design and implementation of robust network services largely inaccessible to the average programmer. We propose the development of a novel type system and practical type spaces which reflect simplified representations of the results and conclusions which can be derived from complex compositional theories in more accessible ways, essentially allowing the system architect or programmer to be exposed only to the inputs and output of compositional analysis without having to be familiar with the ins and outs of its internals. Toward this end we present the TRAFFIC (Typed Representation and Analysis of Flows For Interoperability Checks) framework, a simple flow-composition and typing language with corresponding type system. We then discuss and demonstrate the expressive power of a type space for TRAFFIC derived from the network calculus, allowing us to reason about and infer such properties as data arrival, transit, and loss rates in large composite network applications. :::::::::::::: 2005-015 :::::::::::::: Title: Safe Compositional Specification of Networking Systems: TRAFFIC The Language and Its Type Checking Authors: Likai Liu, Assaf Kfoury, Azer Bestavros, Adam D. Bradley, Yarom Gabay, and Ibrahim Matta Date: May 12, 2005 Abstract: This paper formally defines the operational semantic for TRAFFIC, a specification language for flow composition applications proposed in BUCS-TR-2005-014, and presents a type system based on desired safety assurance. We provide proofs on reduction (weak-confluence, strong-normalization and unique normal form), on soundness and completeness of type system with respect to reduction, and on equivalence classes of flow specifications. Finally, we provide a pseudo-code listing of a syntax-directed type checking algorithm implementing rules of the type system capable of inferring the type of a closed flow specification. :::::::::::::: 2005-016 :::::::::::::: Title: An Invariant Representation for Matching Trajectories across uncalibrated video streams Authors: Walter Nunziati, Stan Sclaroff, and Alberto Del Bimbo Date: May 19, 2005 Abstract: We introduce a view-point invariant representation of moving object trajectories that can be used in video database applications. It is assumed that trajectories lie on a surface that can be locally approximated with a plane. Raw trajectory data is first locally?approximated with a cubic spline via least squares fitting. For each sampled point of the obtained curve, a projective invariant feature is computed using a small number of points in its neighborhood. The resulting sequence of invariant features computed along the entire trajectory forms the view? invariant descriptor of the trajectory itself. Time parametrization has been exploited to compute cross ratios without ambiguity due to point ordering. Similarity between descriptors of different trajectories is measured with a distance that takes into account the statistical properties of the cross ratio, and its symmetry with respect to the point at infinity. In experiments, an overall correct classification rate of about 95% has been obtained on a dataset of 58 trajectories of players in soccer video, and an overall correct classification rate of about 80% has been obtained on matching partial segments of trajectories collected from two overlapping views of outdoor scenes with moving people and cars. :::::::::::::: 2005-017 :::::::::::::: Title: View registration using interesting segments of planar trajectories Authors: Walter Nunziati, Jonathan Alon, Stan Sclaroff, and Alberto Del Bimbo Date: May 19, 2005 Abstract: We introduce a method for recovering the spatial and temporal alignment between two or more views of objects moving over a ground plane. Existing approaches either assume that the streams are globally synchronized, so that only solving the spatial alignment is needed, or that the temporal misalignment is small enough so that exhaustive search can be performed. In contrast, our approach can recover both the spatial and temporal alignment. We compute for each trajectory a number of interesting segments, and we use their description to form putative matches between trajectories. Each pair of corresponding interesting segments induces a temporal alignment, and defines an interval of common support across two views of an object that is used to recover the spatial alignment. Interesting segments and their descriptors are defined using algebraic projective invariants measured along the trajectories. Similarity between interesting segments is computed taking into account the statistics of such invariants. Candidate alignment parameters are verified checking the consistency, in terms of the symmetric transfer error, of all the putative pairs of corresponding interesting segments. Experiments are conducted with two different sets of data, one with two views of an outdoor scene featuring moving people and cars, and one with four views of a laboratory sequence featuring moving radio-controlled cars. :::::::::::::: 2005-018 :::::::::::::: Title: Foreground Object Segmentation from Binocular Stereo Video Authors: Kevin Law and Stan Sclaroff Date: May 19, 2005 Abstract: Moving cameras are needed for a wide range of applications in robotics, vehicle systems, surveillance, etc. However, many foreground object segmentation methods reported in the literature are unsuitable for such settings; these methods assume that the camera is fixed and the background changes slowly, and are inadequate for segmenting objects in video if there is significant motion of the camera or background. To address this shortcoming, a new method for segmenting foreground objects is proposed that utilizes binocular video. The method is demonstrated in the application of tracking and segmenting people in video who are approximately facing the binocular camera rig. Given a stereo image pair, the system first tries to find faces. Starting at each face, the region containing the person is grown by merging regions from an over-segmented color image. The disparity map is used to guide this merging process. The system has been implemented on a consumer-grade PC, and tested on video sequences of people indoors obtained from a moving camera rig. As can be expected, the proposed method works well in situations where other foreground-background segmentation methods typically fail. We believe that this superior performance is partly due to the use of object detection to guide region merging in disparity/color foreground segmentation, and partly due to the use of disparity information available with a binocular rig, in contrast with most previous methods that assumed monocular sequences. :::::::::::::: 2005-019 :::::::::::::: Title: Online and Offine Character Recognition Using Alignment to Prototypes Authors: Jonathan Alon, Vassilis Athitsos, and Stan Sclaroff Date: June 3, 2005 Abstract: Nearest neighbor classifiers are simple to implement, yet they can model complex non-parametric distributions, and provide state-of-the-art recognition accuracy in OCR databases. At the same time, they may be too slow for practical character recognition, especially when they rely on similarity measures that require computationally expensive pairwise alignments between characters. This paper proposes an efficient method for computing an approximate similarity score between two characters based on their exact alignment to a small number of prototypes. The proposed method is applied to both online and offline character recognition, where similarity is based on widely used and computationally expensive alignment methods, i.e., Dynamic Time Warping and the Hungarian method respectively. In both cases significant recognition speedup is obtained at the expense of only a minor increase in recognition error. :::::::::::::: 2005-020 :::::::::::::: Title: Fast and Accurate Gesture Spotting using Subgesture Reasoning and Pruning of Unlikely Dynamic Programming Paths Authors: Jonathan Alon, Vassilis Athitsos, and Stan Sclaroff Date: June 3, 2005 Abstract: Vision-based recognition of gestures in continuous video streams can facilitate more natural human-computer interaction. Gesture spotting is the challenging task of locating the start and end frames of the video stream that correspond to a gesture of interest, while at the same time rejecting non-gesture motion patterns. This paper proposes a new gesture spotting and recognition algorithm that is based on the widely used continuous dynamic programming (CDP) algorithm. Our first contribution is a pruning method that allows the system to evaluate a relatively small number of hypotheses compared to CDP. Pruning is implemented by a set of model-dependent classifiers, that are learned from training examples. In our experiments, the proposed CDP with pruning was an order of magnitude faster compared to the original CDP algorithm, and recognition accuracy improved by 7%. The second contribution of the proposed spotting algorithm is a subgesture reasoning process that models the fact that some gesture models can falsely match parts of other longer gestures. In our experiments, using the proposed subgesture modeling improved recognition accuracy by an additional 12%. :::::::::::::: 2005-021 :::::::::::::: Title: Detecting Instances of Shape Classes That Exhibit Variable Structure Authors: Vassilis Athitsos, Jingbin Wang, Stan Sclaroff, Margrit Betke Date: June 8, 2005 Abstract: This paper proposes a method for detecting shapes of variable structure in images with clutter. The term ``variable structure'' means that some shape parts can be repeated an arbitrary number of times, some parts can be optional, and some parts can have several alternative appearances. The particular variation of the shape structure that occurs in a given image is not known a priori. Existing computer vision methods, including deformable model methods, were not designed to detect shapes of variable structure; they may only be used to detect shapes that can be decomposed into a fixed, a priori known, number of parts. The proposed method can handle both variations in shape structure and variations in the appearance of individual shape parts. A new class of shape models is introduced, called Hidden State Shape Models, that can naturally represent shapes of variable structure. A detection algorithm is described that finds instances of such shapes in images with large amounts of clutter by finding globally optimal correspondences between image features and shape models. Experiments with real images demonstrate that our method can localize plant branches that consist of an a priori unknown number of leaves and can detect hands more accurately than a hand detector based on the chamfer distance. :::::::::::::: 2005-022 :::::::::::::: Title: Face identification by a cascade of rejection classifiers Authors: Quan Yuan, Ashwin Thangali, and Stan Sclaroff Date: June 10, 2005 Abstract: Nearest neighbor search is commonly employed in face recognition but it does not scale well to large dataset sizes. A strategy to combine rejection classifiers into a cascade for face identification is proposed in this paper. A rejection classifier for a pair of classes is defined to reject at least one of the classes with high confidence. These rejection classifiers are able to share discriminants in feature space and at the same time have high confidence in the rejection decision. In the face identification problem, it is possible that a pair of known individual faces are very dissimilar. It is very unlikely that both of them are close to an unknown face in the feature space. Hence, only one of them needs to be considered. Using a cascade structure of rejection classifiers, the scope of nearest neighbor search can be reduced significantly. Experiments on Face Recognition Grand Challenge (FRGC) version 1 data demonstrate that the proposed method achieves significant speed up and an accuracy comparable with the brute force Nearest Neighbor method. In addition, a graph cut based clustering technique is employed to demonstrate that the pairwise separability of these rejection classifiers is capable of semantic grouping. :::::::::::::: 2005-023 :::::::::::::: Title: Fast Head Tilt Detection for Human-Computer Interaction Authors: Benjamin N. Waber, John J. Magee, and Margrit Betke Boston University Date: July 7, 2005 Abstract: Accurate head tilt detection has a large potential to aid people with disabilities in the use of human-computer interfaces and provide universal access to communication software. We show how it can be utilized to tab through links on a web page or control a video game with head motions. It may also be useful as a correction method for currently available video-based assistive technology that requires upright facial poses. Few of the existing computer vision methods that detect head rotations in and out of the image plane with reasonable accuracy can operate within the context of a real-time communication interface because the computational expense that they incur is too great. Our method uses a variety of metrics to obtain a robust head tilt estimate without incurring the computational cost of previous methods. Our system runs in real time on a computer with a 2.53 GHz processor, 256 MB of RAM and an inexpensive webcam, using only 55% of the processor cycles. :::::::::::::: 2005-024 :::::::::::::: Title: Facial Feature Tracking and Occlusion Recovery in American Sign Language Authors: Thomas J. Castelli, Margrit Betke, and Carol Neidle, Boston University Date: July 7, 2005 Abstract: Facial features play an important role in expressing grammatical information in signed languages, including American Sign Language (ASL). Gestures such as raising or furrowing the eyebrows are key indicators of constructions such as yes-no questions. Periodic head movements (nods and shakes) are also an essential part of the expression of syntactic information, such as negation (associated with a side-to-side headshake). Therefore, identification of these facial gestures is essential to sign language recognition. One problem with detection of such grammatical indicators is occlusion recovery. If the signer's hand blocks his/her eyebrows during production of a sign, it becomes difficult to track the eyebrows. We have developed a system to detect such grammatical markers in ASL that recovers promptly from occlusion. Our system detects and tracks evolving templates of facial features, which are based on an anthropometric face model, and interprets the geometric relationships of these templates to identify grammatical markers. It was tested on a variety of ASL sentences signed by various Deaf native signers and detected facial gestures used to express grammatical information, such as raised and furrowed eyebrows as well as headshakes. :::::::::::::: 2005-025 :::::::::::::: Title: Articulated Pose Estimation in a Learned Smooth Space of Feasible Solutions Authors: Tai-Peng Tian, Rui Li, Stan Sclaroff Abstract: A learning based framework is proposed for estimating human body pose from a single image. Given a differentiable function that maps from pose space to image feature space, the goal is to invert the process: estimate the pose given only image features. The inversion is an ill-posed problem as the inverse mapping is a one to many process. Hence multiple solutions exist, and it is desirable to restrict the solution space to a smaller subset of feasible solutions. For example, not all human body poses are feasible due to anthropometric constraints. Since the space of feasible solutions may not admit a closed form description, the proposed framework seeks to exploit machine learning techniques to learn an approximation that is smoothly parameterized over such a space. One such technique is Gaussian Process Latent Variable Modelling. Scaled conjugate gradient is then used tond the best matching pose in the space of feasible solutions when given an input image. The formulation allows easy incorporation of various constraints, e.g. temporal consistency and anthropometric constraints. The performance of the proposed approach is evaluated in the task of upper-body pose estimation from silhouettes and compared with the Specialized Mapping Architecture. The estimation accuracy of the Specialized Mapping Architecture is at least one standard deviation worse than the proposed approach in the experiments with synthetic data. In experiments with real video of humans performing gestures, the proposed approach produces qualitatively better estimation results. :::::::::::::: 2005-026 :::::::::::::: Title: Mistreatment in Distributed Caching Groups: Causes and Implications Authors: Nikolaos Laoutaris, Georgios Smaragdakis, Azer Bestavros, Ioannis Stavrakakis Date: July 7, 2005 Abstract: Although cooperation generally increases the amount of resources available to a community of nodes, thus improving individual and collective performance, it also allows for the appearance of potential mistreatment problems through the exposition of one node's resources to others. We study such concerns by considering a group of independent, rational, self-aware nodes that cooperate using on-line caching algorithms, where the exposed resource is the storage of each node. Motivated by content networking applications -- including web caching, CDNs, and P2P -- this paper extends our previous work on the off-line version of the problem, which was limited to object replication and was conducted under a game-theoretic framework. We identify and investigate two causes of mistreatment: (1) cache state interactions (due to the cooperative servicing of requests) and (2) the adoption of a common scheme for cache replacement/redirection/admission policies. Using analytic models, numerical solutions of these models, as well as simulation experiments, we show that on-line cooperation schemes using caching are fairly robust to mistreatment caused by state interactions. When this becomes possible, the interaction through the exchange of miss-streams has to be very intense, making it feasible for the mistreated nodes to detect and react to the exploitation. This robustness ceases to exist when nodes fetch and store objects in response to remote requests, i.e., when they operate as Level-2 caches (or proxies) for other nodes. Regarding mistreatment due to a common scheme, we show that this can easily take place when the ``outlier'' characteristics of some of the nodes get overlooked. This finding underscores the importance of allowing cooperative caching nodes the flexibility of choosing from a diverse set of schemes to fit the peculiarities of individual nodes. To that end, we outline an emulation-based framework for the development of mistreatment-resilient distributed selfish caching schemes. :::::::::::::: 2005-027 :::::::::::::: Title: Computing a Uniform Scaling Parameter for 3D Registration of Lung Surfaces Authors: Vladimir Rodeski, William Mullally, Carissa Bellardine, Kenneth Lutchen, and Margrit Betke Date: July 7, 2005 Abstract: A difficulty in lung image registration is accounting for changes in the size of the lungs due to inspiration. We propose two methods for computing a uniform scale parameter for use in lung image registration that account for size change. A scaled rigid-body transformation allows analysis of corresponding lung CT scans taken at different times and can serve as a good low-order transformation to initialize non-rigid registration approaches. Two different features are used to compute the scale parameter. The first method uses lung surfaces. The second uses lung volumes. Both approaches are computationally inexpensive and improve the alignment of lung images over rigid registration. The two methods produce different scale parameters and may highlight different functional information about the lungs. :::::::::::::: 2005-028 :::::::::::::: Title: An Adaptive Policy Management Approach to BGP Convergence Author: Selma Yilmaz and Ibrahim Matta Date: July 13, 2005 Abstract: The Border Gateway Protocol (BGP) is the current inter-domain routing protocol used to exchange reachability information between Autonomous Systems (ASes) in the Internet. BGP supports policy-based routing which allows each AS to independently adopt a set of local policies that specify which routes it accepts and advertises from/to other networks, as well as which route it prefers when more than one route becomes available. However, independently chosen local policies may cause global conflicts, which result in protocol divergence. In this paper, we propose a new algorithm, called Adaptive Policy Management Scheme (APMS), to resolve policy conflicts in a distributed manner. Akin to distributed feedback control systems, each AS independently classifies the state of the network as either conflict-free or potentially-conflicting by observing its local history only (namely, route flaps). Based on the degree of measured conflicts, each AS dynamically adjusts its own path preferences---increasing its preference for observably stable paths over flapping paths. APMS also includes a mechanism to distinguish route flaps due to topology changes, so as not to confuse them with those due to policy conflicts. A correctness and convergence analysis of APMS based on the sub-stability property of chosen paths is presented. Implementation in the SSF network simulator is performed, and simulation results for different performance metrics are presented. The metrics capture the dynamic performance (in terms of instantaneous throughput, delay, routing load, etc.) of APMS and other competing solutions, thus exposing the often neglected aspects of performance. :::::::::::::: 2005-029 :::::::::::::: Title : Tracking Human Body Pose on a Learned Smooth Space Authors : Tai-Peng Tian, Rui Li and Stan Sclaroff Date: July 28, 2005 Abstract : Particle filtering is a popular method used in systems for tracking human body pose in video. One key difficulty in using particle filtering is caused by the curse of dimensionality: generally a very large number of particles is required to adequately approximate the underlying pose distribution in a high-dimensional state space. Although the number of degrees of freedom in the human body is quite large, in reality, the subset of allowable configurations in state space is generally restricted by human biomechanics, and the trajectories in this allowable subspace tend to be smooth. Therefore, a framework is proposed to learn a low-dimensional representation of the high-dimensional human poses state space. This mapping can be learned using a Gaussian Process Latent Variable Model (GPLVM) framework. One important advantage of the GPLVM framework is that both the mapping to, and mapping from the embedded space are smooth; this facilitates sampling in the low-dimensional space, and samples generated in the low-dimensional embedded space are easily mapped back into the original high-dimensional space. Moreover, human body poses that are similar in the original space tend to be mapped close to each other in the embedded space; this property can be exploited when sampling in the embedded space. The proposed framework is tested in tracking 2D human body pose using a Scaled Prismatic Model. Experiments on real life video sequences demonstrate the strength of the approach. In comparison with the Multiple Hypothesis Tracking and the standard Condensation algorithm, the proposed algorithm is able to maintain tracking reliably throughout the long test sequences. It also handles singularity and self occlusion robustly. :::::::::::::: 2005-030 :::::::::::::: Title: Some Considerations on a Calculus with Weak References Author: Kevin Donnelly and Assaf J. Kfoury Date: July 27, 2005 Abstract: Weak references are references that do not prevent the object they point to from being garbage collected. Most realistic languages, including Java, SML/NJ, and OCaml to name a few, have some facility for programming with weak references. Weak references are used in implementing idioms like memoizing functions and hash-consing in order to avoid potential memory leaks. However, the semantics of weak references in many languages are not clearly specified. Without a formal semantics for weak references it becomes impossible to prove the correctness of implementations making use of this feature. Previous work by Hallett and Kfoury extends $\gc$, a language for modeling garbage collection, to $\weak$, a similar language with weak references. Using this previously formalized semantics for weak references, we consider two issues related to well-behavedness of programs. Firstly, we provide a new, simpler proof of the well-behavedness of the syntactically restricted fragment of $\weak$ defined previously. Secondly, we give a natural semantic criterion for well-behavedness much broader than the syntactic restriction, which is useful as principle for programming with weak references. Furthermore we extend the result, proved in previously of $\gc$, which allows one to use type-inference to collect some reachable objects that are never used. We prove that this result holds of our language, and we extend this result to allow the collection of weakly-referenced reachable garbage without incurring the computational overhead sometimes associated with collecting weak bindings (e.g. the need to recompute a memoized function). Lastly we use extend the semantic framework to model the key/value weak references found in Haskell and we prove the Haskell is semantics equivalent to a simpler semantics due to the lack of side-effects in our language :::::::::::::: 2005-031 :::::::::::::: Title: A Formal Semantics for Weak References Author: Joseph J. Hallett and Assaf J. Kfoury Date: August 8, 2005 Abstract: A weak reference is a reference to an object that is not followed by the pointer tracer when garbage collection is called. That is, a weak reference cannot prevent the object it references from being garbage collected. Weak references remain a troublesome programming feature largely because there is not an accepted, precise semantics that describes their behavior (in fact, we are not aware of any formalization of their semantics). The trouble is that weak references allow reachable objects to be garbage collected, therefore allowing garbage collection to influence the result of a program. Despite this difficulty, weak references continue to be used in practice for reasons related to efficient storage management, and are included in many popular programming languages (Standard ML, Haskell, OCaml, and Java). We give a formal semantics for a calculus called that includes weak references and is derived from Morrisett, Felleisen, and Harper's . formalizes the notion of garbage collection by means of a rewrite rule. Such a formalization is required to precisely characterize the semantics of weak references. However, the inclusion of a garbage-collection rewrite-rule in a language with weak references introduces non-deterministic evaluation, even if the parameter-passing mechanism is deterministic (call-by-value in our case). This raises the question of confluence for our rewrite system. We discuss natural restrictions under which our rewrite system is confluent, thus guaranteeing uniqueness of program result. We define conditions that allow other garbage collection algorithms to co-exist with our semantics of weak references. We also introduce a polymorphic type system to prove the absence of erroneous program behavior (i.e., the absence of "stuck evaluation") and a corresponding type inference algorithm. We prove the type system sound and the inference algorithm sound and complete. :::::::::::::: 2005-032 :::::::::::::: Title: MusicCamera -- A Camera-based Music Making Tool for Physical Rehabilitation Date: December 8, 2005 Authors: Mikhail Gorman, Margrit Betke, Elliot Saltzman, and Amir Lahav Abstract: The therapeutic effects of playing music are being recognized increasingly in the field of rehabilitation medicine. People with physical disabilities, however, often do not have the motor dexterity needed to play an instrument. We developed a camera-based human-computer interface called ``MusicCamera'' to provide such people with a means to make music by performing therapeutic exercises. MusicCamera uses computer vision techniques to convert the movements of a patient's body part, for example, a finger, hand, or foot, into musical and visual feedback using the open software platform EyesWeb. It can be adjusted to a patient's particular therapeutic needs and provides quantitative tools for monitoring the recovery process and assessing therapeutic outcomes. We tested the potential of MusicCamera as a rehabilitation tool with six subjects who responded to or created music in various movement exercises. In these proof-of-concept experiments, MusicCamera has performed reliably and shown its promise as a therapeutic device. :::::::::::::: 2005-033 :::::::::::::: Title: Safe Compositional Specification of Networking Systems: A Compositional Analysis Approach Authors: Likai Liu, Assaf Kfoury, Azer Bestavros, Adam D. Bradley, Yarom Gabay, and Ibrahim Matta Date: December 28, 2005 Abstract: We present a type inference algorithm, in the style of compositional analysis, for the language TRAFFIC---a specification language for flow composition applications proposed in BUCS-TR-2005-014---and prove that this algorithm is correct: the typings it infers are principal typings, and the typings agree with syntax-directed type checking on closed flow specifications. This algorithm is capable of verifying partial flow specifications, which is a significant improvement over syntax-directed type checking algorithm presented in BUCS-TR-2005-015. We also show that this algorithm runs efficiently, i.e., in low-degree polynomial time. :::::::::::::: 2005-034 :::::::::::::: Title: Type Systems for a Network Specification Language With Multiple-Choice Let Authors: Yarom Gabay, Assaf J. Kfoury, Likai Liu, Azer Bestavros, Adam D. Bradley, and Ibrahim Matta Abstract: When analysing the behavior of complex networked systems, it is often the case that some components within that network are only known to the extent that they belong to one of a set of possible "implementations" -- e.g., versions of a specific protocol, class of schedulers, etc. In this report we augment the specification language considered in BUCS-TR-2004-021, BUCS-TR-2005-014, BUCS-TR-2005-015, and BUCS-TR-2005-033, to include a non-deterministic multiple-choice let-binding, which allows us to consider compositions of networking subsystems that allow for looser component specifications. :::::::::::::: 2005-035 :::::::::::::: Title: Inferring Intersection Typings that Are Equivalent to Call-by-Name and Call-by-Value Evaluations. Authors: Adam Bakewell, Sebastien Carlier, A.J. Kfoury, J.B. Wells Date: April 9, 2005 Abstract: We present a procedure to infer a typing for an arbitrary lambda-term M in an intersection-type system that translates into exactly the call-by-name (resp., call-by-value) evaluation of M. Our framework is the recently developed System E which augments intersection types with expansion variables. The inferred typing for M is obtained by setting up a unification problem involving both type variables and expansion variables, which we solve with a confluent rewrite system. The inference procedure is compositional in the sense that typings for different program components can be inferred in any order, and without knowledge of the definition of other program components. Using expansion variables lets us achieve a compositional inference procedure easily. Termination of the procedure is generally undecidable. The procedure terminates and returns a typing iff the input M is normalizing according to call-by-name (resp., call-by-value). The inferred typing is exact in the sense that the exact call-by-name (resp., call-by-value) behaviour of M can be obtained by a (polynomial) transformation of the typing. The inferred typing is also principal in the sense that any other typing that translates the call-by-name (resp., call-by-value) evaluation of M can be obtained from the inferred typing for M using a substitution-based transformation. :::::::::::::: 2006-001 :::::::::::::: Title: Computational Properties of SNAFU Authors: Yarom Gabay, Michael J. Ocean, Assaf J. Kfoury, and Likai Liu Date: February 6, 2006 Abstract: Sensor applications in Sensoria [BBKO:basenets05] are expressed using STEP "Sensorium Task Execution Plan". SNAFU (SensorNet Applications as Functional Units) serves as a high-level sensor-programming language, which is compiled into STEP. In SNAFU's current form, its differences with STEP are relatively minor, as they are limited to shorthands and macros not available in STEP. We show that, however restrictive it may seem, SNAFU has in fact universal power; technically, it is a Turing-complete language, i.e., any Turing program can be written in SNAFU (though not always conveniently). Although STEP may be allowed to have universal power, as a low-level language not directly available to Sensorium users, SNAFU programmers may use this power for malicious purposes or inadvertently introduce errors with destructive consequences. In future developments of SNAFU, we plan to introduce restrictions and high-level features with safety guards, such as those provided by a type system, which will make SNAFU programming safer. :::::::::::::: 2006-002 :::::::::::::: Title: On the Impact of Low-Rate Attacks Authors: Mina Guirguis, Azer Bestavros, and Ibrahim Matta Date: February 6, 2006 Abstract Recent research have exposed new breeds of attacks that are capable of denying service or inflicting significant damage to TCP flows, without sustaining the attack traffic. Such attacks are often referred to as ``low-rate'' attacks and they stand in sharp contrast against traditional Denial of Service (DoS) attacks that can completely shut off TCP flows by flooding an Internet link. In this paper, we study the impact of these new breeds of attacks and the extent to which defense mechanisms are capable of mitigating the attack's impact. Through adopting a simple discrete-time model with a single TCP flow and a non-oblivious adversary, we were able to expose new variants of these low-rate attacks that could potentially have high attack potency per attack burst. Our analysis is focused towards worst-case scenarios, thus our results should be regarded as upper bounds on the impact of low-rate attacks rather than a real assessment under a specific attack scenario. :::::::::::::: 2006-003 :::::::::::::: Title: Distributed Selfish Caching Authors: Nikolaos Laoutaris, Georgios Smaragdakis, Azer Bestavros, Ibrahim Matta, and Ioannis Stavrakakis Date: February 7, 2006 Abstract: Although cooperation generally increases the amount of resources available to a community of nodes, thus improving individual and collective performance, it also allows for the appearance of potential mistreatment problems through the exposition of one node's resources to others. We study such concerns by considering a group of independent, rational, self-aware nodes that cooperate using on-line caching algorithms, where the exposed resource is the storage at each node. Motivated by content networking applications -- including web caching, CDNs, and P2P -- this paper extends our previous work on the on-line version of the problem, which was conducted under a game-theoretic framework, and limited to object replication. We identify and investigate two causes of mistreatment: (1) cache state interactions (due to the cooperative servicing of requests) and (2) the adoption of a common scheme for cache management policies. Using analytic models, numerical solutions of these models, as well as simulation experiments, we show that on-line cooperation schemes using caching are fairly robust to mistreatment caused by state interactions. To appear in a substantial manner, the interaction through the exchange of miss-streams has to be very intense, making it feasible for the mistreated nodes to detect and react to exploitation. This robustness ceases to exist when nodes fetch and store objects in response to remote requests, i.e., when they operate as Level-2 caches (or proxies) for other nodes. Regarding mistreatment due to a common scheme, we show that this can easily take place when the "outlier" characteristics of some of the nodes get overlooked. This finding underscores the importance of allowing cooperative caching nodes the flexibility of choosing from a diverse set of schemes to fit the peculiarities of individual nodes. To that end, we outline an emulation-based framework for the development of mistreatment-resilient distributed selfish caching schemes. Our framework utilizes a simple control-theoretic approach to dynamically parameterize the cache management scheme. We show performance evaluation results that quantify the benefits from instantiating such a framework, which could be substantial under skewed demand profiles. :::::::::::::: 2006-004 :::::::::::::: Title: Authenticated Index Structures for Outsourced Database Systems Authors: Feifei Li, Marios Hadjieleftheriou, George Kollios, Leonid Reyzin Date: April 1, 2006 Abstract: In outsourced database (ODB) systems the database owner publishes its data through a number of remote servers, with the goal of enabling clients at the edge of the network to access and query the data more efficiently. As servers might be untrusted or can be compromised, query authentication becomes an essential component of ODB systems. Existing solutions for this problem concentrate mostly on static scenarios and are based on idealistic properties for certain cryptographic primitives, looking at the problem mostly from a theoretical perspective. In this work, first we define a variety of essential and practical cost metrics associated with ODB systems. Then we analytically evaluate a number of different approaches, in search for a solution that best leverages all metrics. Most importantly, we look at solutions that can handle dynamic scenarios, where owners periodically update the data residing at the servers. Finally, we discuss query freshness, a new dimension in data authentication that has not been explored before. A comprehensive experimental evaluation of the proposed and existing approaches is used to validate the analytical models and verify our claims. Our findings exhibit that the proposed solutions improve performance substantially over existing approaches, both for static and dynamic environments. :::::::::::::: 2006-005 :::::::::::::: Title: Amorphous Placement and Retrieval of Sensory Data in Sparse Mobile Ad-Hoc Networks Authors: Hany Morcos, Azer Bestavros, and Ibrahim Matta Date: April 4, 2006 Abstract: Personal communication devices are increasingly being equipped with sensors that are able to passively collect information from their surroundings -- information that could be stored in fairly small local caches. We envision a system in which users of such devices use their collective sensing, storage, and communication resources to query the state of (possibly remote) neighborhoods. The goal of such a system is to achieve the highest query success ratio using the least communication overhead (power). We show that the use of Data Centric Storage (DCS), or directed placement, is a viable approach for achieving this goal, but only when the underlying network is well connected. Alternatively, we propose, amorphous placement, in which sensory samples are cached locally and informed exchanges of cached samples is used to diffuse the sensory data throughout the whole network. In handling queries, the local cache is searched first for potential answers. If unsuccessful, the query is forwarded to one or more direct neighbors for answers. This technique leverages node mobility and caching capabilities to avoid the multi-hop communication overhead of directed placement. Using a simplified mobility model, we provide analytical lower and upper bounds on the ability of amorphous placement to achieve uniform field coverage in one and two dimensions. We show that combining informed shuffling of cached samples upon an encounter between two nodes, with the querying of direct neighbors could lead to significant performance improvements. For instance, under realistic mobility models, our simulation experiments show that amorphous placement achieves 10% to 40% better query answering ratio at a 25% to 35% savings in consumed power over directed placement. :::::::::::::: 2006-006 :::::::::::::: Title: A customizable camera-based human computer interaction system allowing people with disabilities autonomous hands free navigation of multiple computing tasks Authors: Wajeeha Akram, Laura R. Tiberii and Margrit Betke Date: May 11, 2006 Abstract Many people suffer from conditions that lead to deterioration of motor control and makes access to the computer using traditional input devices difficult. In particular, they may loose control of hand movement to the extent that the standard mouse cannot be used as a pointing device. Most current alternatives use markers or specialized hardware to track and translate a user's movement to pointer movement. These approaches may be perceived as intrusive, for example, wearable devices. Camera-based assistive systems that use visual tracking of features on the user's body often require cumbersome manual adjustment. This paper introduces an enhanced computer vision based strategy where features, for example on a user's face, viewed through an inexpensive USB camera, are tracked and translated to pointer movement. The main contributions of this paper are (1) enhancing a video based interface with a mechanism for mapping feature movement to pointer movement, which allows users to navigate to all areas of the screen even with very limited physical movement, and (2) providing a customizable, hierarchical navigation framework for human computer interaction (HCI). This framework provides effective use of the vision-based interface system for accessing multiple applications in an autonomous setting. Experiments with several users show the effectiveness of the mapping strategy and its usage within the application framework as a practical tool for desktop users with disabilities. :::::::::::::: 2006-007 :::::::::::::: Title: Web Mediators for Accessible Browsing Authors: Benjamin N. Waber, John J. Magee, and Margrit Betke Date: May 11, 2006 Abstract We present a highly accurate method for classifying web pages based on link percentage, which is the percentage of text characters that are parts of links normalized by the number of all text characters on a web page. K-means clustering is used to create unique thresholds to differentiate index pages and article pages on individual web sites. Index pages contain mostly links to articles and other indices, while article pages contain mostly text. We also present a novel link grouping algorithm using agglomerative hierarchical clustering that groups links in the same spatial neighborhood together while preserving link structure. Grouping allows users with severe disabilities to use a scan-based mechanism to tab through a web page and select items. In experiments, we saw up to a 40-fold reduction in the number of commands needed to click on a link with a scan-based interface, which shows that we can vastly improve the rate of communication for users with disabilities. We used web page classification and link grouping to alter web page display on an accessible web browser that we developed to make a usable browsing interface for users with disabilities. Our classification method consistently outperformed a baseline classifier even when using minimal data to generate article and index clusters, and achieved classification accuracy of 94.0% on web sites with well-formed or slightly malformed HTML, compared with 80.1% accuracy for the baseline classifier. :::::::::::::: 2006-008 :::::::::::::: Title: An Adaptive Management Approach to Resolving Policy Conflicts Authors: Selma Yilmaz and Ibrahim Matta Date: May 25, 2006 Abstract: The Border Gateway Protocol (BGP) is the current inter-domain routing protocol used to exchange reachability information between Autonomous Systems (ASes) in the Internet. BGP supports policy-based routing which allows each AS to independently define a set of local policies on which routes it accepts and advertises from/to other networks, as well as on which route it prefers when more than one route becomes available. However, independently chosen local policies may cause global conflicts, which result in protocol divergence. In this paper, we propose a new algorithm, called Adaptive Policy Management Scheme (APMS), to resolve policy conflicts in a distributed manner. Akin to distributed feedback control systems, each AS independently classifies the state of the network as either conflict-free or potentially conflicting by observing its local history only (namely, route flaps). Based on the degree of measured conflicts, each AS dynamically adjusts its own path preferences---increasing its preference for observably stable paths over flapping paths. APMS also includes a mechanism to distinguish route flaps due to topology changes, so as not to confuse them with those due to policy conflicts. A correctness and convergence analysis of APMS based on the sub-stability property of chosen paths is presented. Implementation in the SSF network simulator is performed, and simulation results for different performance metrics are presented. The metrics capture the dynamic performance (in terms of instantaneous throughput, delay, etc.) of APMS and other competing solutions, thus exposing the often neglected aspects of performance. :::::::::::::: 2006-009 :::::::::::::: Title: On the Interaction between TCP and the Wireless Channel in CDMA2000 Networks Authors: Karim Mattar, Ashwin Sridharan, Hui Zang, Ibrahim Matta and Azer Bestavros Date: June 6, 2006 Abstract: In this work, we conducted extensive active measurements on a large nationwide CDMA2000 1xRTT network in order to characterize the impact of both the Radio Link Protocol and more importantly, the wireless scheduler, on TCP. Our measurements include standard TCP/UDP logs, as well as detailed RF layer statistics that allow observability into RF dynamics. With the help of a robust correlation measure, normalized mutual information, we were able to quantify the impact of these two RF factors on TCP performance metrics such as the round trip time, packet loss rate, instantaneous throughput etc. We show that the variable channel rate has the larger impact on TCP behavior when compared to the Radio Link Protocol. Furthermore, we expose and rank the factors that influence the assigned channel rate itself and in particular, demonstrate the sensitivity of the wireless scheduler to the data sending rate. Thus, TCP is adapting its rate to match the available network capacity, while the rate allocated by the wireless scheduler is influenced by the sender's behavior. Such a system is best described as a closed loop system with two feedback controllers, the TCP controller and the wireless scheduler, each one affecting the other's decisions. In this work, we take the first steps in characterizing such a system in a realistic environment. :::::::::::::: 2006-010 :::::::::::::: Author: Vassilis Athitsos Title: Learning Embeddings for Indexing, Retrieval, and Classification, with Applications to Object and Shape Recognition in Image Databases Date: June 14, 2006 Abstract: Nearest neighbor retrieval is the task of identifying, given a database of objects and a query object, the objects in the database that are the most similar to the query. Retrieving nearest neighbors is a necessary component of many practical applications, in fields as diverse as computer vision, pattern recognition, multimedia databases, bioinformatics, and computer networks. At the same time, finding nearest neighbors accurately and efficiently can be challenging, especially when the database contains a large number of objects, and when the underlying distance measure is computationally expensive. This thesis proposes new methods for improving the efficiency and accuracy of nearest neighbor retrieval and classification in spaces with computationally expensive distance measures. The proposed methods are domain-independent, and can be applied in arbitrary spaces, including non-Euclidean and non-metric spaces. In this thesis particular emphasis is given to computer vision applications related to object and shape recognition, where expensive non-Euclidean distance measures are often needed to achieve high accuracy. The first contribution of this thesis is the BoostMap algorithm for embedding arbitrary spaces into a vector space with a computationally efficient distance measure. Using this approach, an approximate set of nearest neighbors can be retrieved efficiently - often orders of magnitude faster than retrieval using the exact distance measure in the original space. The BoostMap algorithm has two key distinguishing features with respect to existing embedding methods. First, embedding construction explicitly maximizes the amount of nearest neighbor information preserved by the embedding. Second, embedding construction is treated as a machine learning problem, in contrast to existing methods that are based on geometric considerations. The second contribution is a method for constructing query-sensitive distance measures for the purposes of nearest neighbor retrieval and classification. In high-dimensional spaces, query-sensitive distance measures allow for automatic selection of the dimensions that are the most informative for each specific query object. It is shown theoretically and experimentally that query-sensitivity increases the modeling power of embeddings, allowing embeddings to capture a larger amount of the nearest neighbor structure of the original space. The third contribution is a method for speeding up nearest neighbor classification by combining multiple embedding-based nearest neighbor classifiers in a cascade. In a cascade, computationally efficient classifiers are used to quickly classify easy cases, and classifiers that are more computationally expensive and also more accurate are only applied to objects that are harder to classify. An interesting property of the proposed cascade method is that, under certain conditions, classification time actually decreases as the size of the database increases, a behavior that is in stark contrast to the behavior of typical nearest neighbor classification systems. The proposed methods are evaluated experimentally in several different applications: hand shape recognition, off-line character recognition, online character recognition, and efficient retrieval of time series. In all datasets, the proposed methods lead to significant improvements in accuracy and efficiency compared to existing state-of-the-art methods. In some datasets, the general-purpose methods introduced in this thesis even outperform domain-specific methods that have been custom-designed for such datasets. :::::::::::::: 2006-011 :::::::::::::: Title: Authenticated Index Sturctures for Aggregation Queries in Outsourced Databases Authors: Feifei Li, Marios Hadjieleftheriou, George Kollios, and Leonid Reyzin. Date: July 10, 2006 Abstract: In an outsourced database system the data owner publishes information through a number of remote, untrusted servers with the goal of enabling clients to access and query the data more efficiently. As clients cannot trust servers, query authentication is an essential component in any outsourced database system. Clients should be given the capability to verify that the answers provided by the servers are correct with respect to the actual data published by the owner. While existing work provides authentication techniques for selection and projection queries, there is a lack of techniques for authenticating aggregation queries. This article introduces the rst known authenticated index structures for aggregation queries. First, we design an index that features good performance characteristics for static environments, where few or no updates occur to the data. Then, we extend these ideas and propose more involved structures for the dynamic case, where the database owner is allowed to update the data arbitrarily. Our structures feature excellent average case performance for authenticating queries with multiple aggregate attributes and multiple selection predicates. We also implement working prototypes of the proposed techniques and experimentally validate the correctness of our ideas. :::::::::::::: 2006-012 :::::::::::::: Title: Extending snBench to Support Hierarchical and Configurable Scheduling Authors: Gabriel Parmer, Georgios Zervas, Angshuman Bagchi Date: July 14, 2006 Abstract: It is useful in systems that must support multiple applications with various temporal requirements to allow application-specific policies to manage resources accordingly. However, there is a tension between this goal and the desire to control and police possibly malicious programs. The Java-based Sensor Execution Environment (SXE) in snBench presents a situation where such considerations add value to the system. Multiple applications can be run by multiple users with varied temporal requirements, some Real-Time and others best effort.} This paper outlines and documents an implementation of a hierarchical and configurable scheduling system with which different applications can be executed using application-specific scheduling policies. Concurrently the system administrator can define fairness policies between applications that are imposed upon the system. Additionally, to ensure forward progress of system execution in the face of malicious or malformed user programs, an infrastructure for execution using multiple threads is described. :::::::::::::: 2006-013 :::::::::::::: Title: Extending snBench to Provide Concurrency Support in the Sensorium Execution Environment (SXE) Authors: Jorge Londono, Sowmya Manjanatha, and Zhinan Han. Date: July 14, 2006 Abstract: The SNBENCH is a general-purpose programming environment and run-time system targeted towards a variety of Sensor applications such as environmental sensing, location sensing, video sensing, etc. In its current structure, the run-time engine of the SNBENCH namely, the Sensorium Execution Environment (SXE) processes the entities of execution in a single thread of operation. In order to effectively support applications that are time-sensitive and need priority, it is imperative to process the tasks discretely so that specific policies can be applied at a much granular level. The goal of this project was to modify the SXE to enable efficient use of system resources by way of multi-tasking the individual components. Additionally, the transformed SXE offers the ability to classify and employ different schemes of processing to the individual tasks. :::::::::::::: 2006-014 :::::::::::::: Title: Extending snBench to Support a Graphical Programming Interface for a Sensor Network Tasking Language (STEP) Authors: Ching Chang, Raymond Sweha, Panagiotis Papapetrou Date: July 14, 2006 Abstract: We report on our development and implementation of a graphical "programming" interface for a sensor network tasking language called STEP. The graphical interface allows the user to specify a program execution graphically from an extensible pallet of functionalities and save the results as a properly formatted STEP file. Moreover, the software is able to load a file in STEP format and convert it into the corresponding graphical representation. During both phases a type-checker is running on the background to ensure that both the graphical representation and the STEP file are syntactically correct. This project has been motivated by the Sensorium project at Boston University. In this technical report we present the basic features of the software, the process that has been followed during the design and implementation. Finally, we describe the approach used to test and validate our software. :::::::::::::: 2006-015 :::::::::::::: Title: Extending snBench to Support a Video-Based Intrusion Detection and Alerting System with a Centralized Hash Table Author: Dustin Burke, Dave Cecere, and Ben Freiberg Date: July 14, 2006 Abstract: In this project we design and implement a centralized hashing table in the snBench sensor network environment. We discuss the feasibility of this approach and compare and contrast with the distributed hashing architecture, with particular discussion regarding the conditions under which a centralized architecture makes sense. There are numerous computational tasks that require persistence of data in a sensor network environment. To help motivate the need for data storage in snBench we demonstrate a practical application of the technology whereby a video camera can monitor a room to detect the presence of a person and send an alert to the appropriate authorities. :::::::::::::: 2006-016 :::::::::::::: Title: Integrating Sensor-Network Research and Development into a Software Engineering Curriculum Author: Michael J Ocean, Assaf J. Kfoury, and Azer Bestavros Date: July 14, 2006 Abstract: The emergence of a sensor-networked world produces a clear and urgent need for well-planned, safe and secure software engineering. It is the role of universities to prepare graduates with the knowledge and experience to enter the work-force with a clear understanding of software design and its application to the future safety of computing. The snBench (Sensor Network WorkBench) project aims to provide support to the programming and deployment of Sensor Network Applications, enabling shared sensor embedded spaces to be easily tasked with various sensory applications by different users for simultaneous execution. In this report we discus our experience using the snBench research project as the foundation for semester-long project in a graduate level software engineering class at Boston University (CS511). :::::::::::::: 2006-017 :::::::::::::: Title: The Cache Inference Problem and its Application to Content and Request Routing Authors: Nikolaos Laoutaris, Georgos Zervas, Azer Bestavros, and George Kollios Date: July 14, 2006 In many networked applications, independent caching agents cooperate by servicing each other's miss streams, without revealing the operational details of the caching mechanisms they employ. Inference of such details could be instrumental for many other processes. For example, it could be used for optimized forwarding (or routing) of one's own miss stream (or content) to available proxy caches, or for making cache-aware resource management decisions. In this paper, we introduce the ``Cache Inference Problem'' (CIP) as that of inferring the characteristics of a caching agent, given the miss stream of that agent. While CIP is insolvable in its most general form, there are special cases of practical importance in which it is, including when the request stream follows an Independent Reference Model (IRM) with generalized power-law (GPL) demand distribution. To that end, we design two basic ``litmus'' tests that are able to detect LFU and LRU replacement policies, the effective size of the cache and of the object universe, and the skewness of the GPL demand for objects. Using extensive experiments under synthetic as well as real traces, we show that our methods infer such characteristics accurately and quite efficiently, and that they remain robust even when the IRM/GPL assumptions do not hold, and even when the underlying replacement policies are not ``pure'' LFU or LRU. We exemplify the value of our inference framework by considering example applications. :::::::::::::: 2006-018 :::::::::::::: Title: Distributed Placement of Service Facilities in Large-Scale Networks Authors: Nikolaos Laoutaris, Georgios Smaragdakis, Konstantinos Oikonomou, Ioannis Stavrakakis, and Azer Bestavros Date: July 14, 2006 Abstract: The effectiveness of service provisioning in large-scale networks is highly dependent on the number and location of service facilities deployed at various hosts. The classical, centralized approach to determining the latter would amount to formulating and solving the ``uncapacitated k-median'' (UKM) problem (if the requested number of facilities is fixed), or the ``uncapacitated facility location'' (UFL) problem (if the number of facilities is also to be optimized). Clearly, such centralized approaches require knowledge of global topological and demand information, and thus do not scale and are not practical for large networks. The key question posed and answered in this paper is the following: ``How can we determine in a distributed and scalable manner the number and location of service facilities?'' We propose an innovative approach in which topology and demand information is limited to neighborhoods, or ``balls'' small radius around selected facilities, whereas demand information is captured implicitly for the remaining (remote) clients outside these neighborhoods, by mapping them to clients on the edge of the neighborhood; the ball radius regulates the trade-off between scalability and performance. We develop a scalable, distributed approach that answers our key question through an iterative re-optimization of the location and the number of facilities within such balls. We show that even for small values of the radius (1 or 2), our distributed approach achieves performance under various synthetic and real Internet topologies that is comparable to that of optimal, centralized approaches requiring full topology and demand information. :::::::::::::: 2006-019 :::::::::::::: Title: Implications of Selfish Neighbor Selection in Overlay Networks Authors: Nikolaos Laoutaris, Georgios Smaragdakis, Azer Bestavros, and John Byers Date: July 14, 2006 Abstract: In a typical overlay network for routing or content sharing, each node must select a fixed number of immediate overlay neighbors for routing traffic or content queries. A selfish node entering such a network would select neighbors so as to minimize the weighted sum of expected access costs to all its destinations. Previous work on selfish neighbor selection has built intuition with simple models where edges are undirected, access costs are modeled by hop-counts, and nodes have potentially unbounded degrees. However, in practice, important constraints not captured by these models lead to richer games with substantively and fundamentally different outcomes. Our work models neighbor selection as a game involving directed links, constraints on the number of allowed neighbors, and costs reflecting both network latency and node preference. We express a node's ``best response'' wiring strategy as a $k$-median problem on asymmetric distance, and use this formulation to obtain pure Nash equilibria. We experimentally examine the properties of such stable wirings on synthetic topologies, as well as on real topologies and maps constructed from PlanetLab and the AS-level Internet measurements. Our results indicate that selfish nodes can reap substantial performance benefits when connecting to overlay networks composed of non-selfish nodes. On the other hand, in overlays that are dominated by selfish nodes, the resulting stable wirings are optimized to such great extent that even non-selfish newcomers can extract near-optimal performance through naive wiring strategies. :::::::::::::: 2006-020 :::::::::::::: Title: Scalable Overlay Multicast Tree Construction for QoS-Constrained Media Streaming Authors: Gabriel Parmer, Richard West, and Gerald Fry Date: July 14, 2006 Abstract: Overlay networks have become popular in recent times for content distribution and end-system multicasting of media streams. In the latter case, the motivation is based on the lack of widespread deployment of IP multicast and the ability to perform end-host processing. However, constructing routes between various end-hosts, so that data can be streamed from content publishers to many thousands of subscribers, each having their own QoS constraints, is still a challenging problem. First, any routes between end-hosts using trees built on top of overlay networks can increase stress on the underlying physical network, due to multiple instances of the same data traversing a given physical link. Second, because overlay routes between end-hosts may traverse physical network links more than once, they increase the end-to-end latency compared to IP-level routing. Third, algorithms for constructing efficient, large-scale trees that reduce link stress and latency are typically more complex. This paper therefore compares various methods to construct multicast trees between end-systems, that vary in terms of implementation costs and their ability to support per-subscriber QoS constraints. We describe several algorithms that make trade-offs between algorithmic complexity, physical link stress and latency. While no algorithm is best in all three cases we show how it is possible to efficiently build trees for several thousand subscribers with latencies within a factor of two of the optimal, and link stresses comparable to, or better than, existing technologies. :::::::::::::: 2006-021 :::::::::::::: Mina's Technical Note :::::::::::::: 2006-022 :::::::::::::: Title: An Independent-Connection Model for Traffic Matrices Authors: Vijay Erramilli, Mark Crovella Dept. of Computer Science, Boston Univ. , Nina Taft, Intel Research, Berkeley Date: 09/06/2006 Abstract: The `gravity' model has been used both for traffic matrix (TM) estimation and for generating synthetic TMs. It is based on the assumption that a packet's network egress is independent of its ingress. We argue that in real IP networks, this assumption should not and does not hold. The fact that most traffic consists of two-way exchanges of packets means that traffic streams flowing in opposite directions at any point in the network are {\em not\/} independent. In this paper we propose a model for traffic matrices based on independence of {\em connections\/} rather than packets. We argue that the independent-connection (IC) model is simpler, more intuitive, and has a more direct connection to underlying network phenomena than the gravity model. Using publicly available TMs, we show that the IC model fits real data better than the gravity model. We then characterize the parameters involved in the IC model based on our datasets; these results can be used to construct synthetic TMs. Finally, we turn to the well-studied problem of choosing a prior for TM estimation. Assuming that certain parameters of model can be measured in advance and remain constant in time, we show that the IC model yields a better prior for TM estimation than the gravity model. :::::::::::::: 2006-023 :::::::::::::: Title: Notes on the Effect of Different Access Patterns on the Intensity of Mistreatment in Distributed Caching Groups Author: Georgios Smaragdakis Date: September 18, 2006 Abstract: In this report, we extend our study of the intensity of mistreatment in distributed caching groups due to state interaction. In our earlier work (published as BUCS-TR-2006-003), we analytically showed how this type of mistreatment may appear under homogeneous demand distributions. We provided a simple setting where mistreatment due to state interaction may occur. According to this setting, one or more ``overactive'' nodes generate disproportionately more requests than the other nodes. In this report, we extend our experimental evaluation of the intensity of mistreatment to which non-overactive nodes are subjected, when the demand distributions are not homogeneous. :::::::::::::: 2006-024 :::::::::::::: Title: Spatiotemporal Gesture Segmentation Authors: Jonathan Alon Date: September 18, 2006 Abstract: Spotting patterns of interest in an input signal is a very useful task in many different