Working up the stack from the bottom to top, we start with the switchboard (software below the IP layer which augments packet-level services). At this level, the control mechanisms used to perform traffic shaping, such as packet reordering and per-flow queuing extend the functionality and capability provided by the standard packet queue into the device. Packet tracking information available only at the network layer, such as post-reordering timing information, is relayed to network mechanisms at higher levels in the Backbay architecture. This interaction between layers is necessary for enabling service such as correlated packet-pair probes provided by Beacon. At the TCP layer, Backbay augments congestion control mechanisms to support aggregate congestion control for concurrent flows. Above the TCP layer is the scoreboard used to perform diagnosis and control at the level of a connection. In the scoreboard, diagnostic statistics about TCP connections, such as round-trip times and packet loss events are recorded. As described in earlier sections, these events are used to draw correlations between connections---the scoreboard facilitates that process. The scoreboard module will also use kernel-level message passing mechanisms and shared data structures to provide tight integration with lower level mechanisms in the switchboard. To perform packet-pair measurements, the switchboard must inform the scoreboard of accurate interdeparture times; likewise, the scoreboard must inform the switchboard of which packet pair measurements would be most beneficial to make.

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