Compound TCP - Principles of Operation

Principles of Operation

Like FAST TCP and TCP Vegas, Compound TCP uses estimates of queueing delay as a measure of congestion; if the queueing delay is small, it assumes that no links on its path are congested, and rapidly increases its rate. However, unlike FAST and Vegas, it does not seek to maintain a constant number of packets queued.

Compound TCP maintains two congestion windows: a regular AIMD window and a delay-based window. The size of the actual sliding window used is the sum of these two windows. The AIMD window is increased the same way that TCP Reno increases it. If the delay is small, the delay-based window increases rapidly to improve the utilisation of the network. Once queueing is experienced, the delay window gradually decreases to compensate for the increase in the AIMD window. The aim is to keep their sum approximately constant, at what the algorithm estimates is the path's bandwidth-delay product. In particular, when queueing is detected, the delay-based window is reduced by the estimated queue size to avoid the problem of "persistent congestion" reported for FAST and Vegas. Thus, unlike TCP-Illinois and its precursor TCP Africa, Compound TCP can reduce its window in response to delay. This increases its fairness to Reno.