EXPLICIT WINDOW-BASED CONTROL IN LOSSY PACKET NETWORKS
Christophe De Vleeschouwer, Pascal Frossard · 2006
AbstractThis paper addresses the problem of fair allocation of bandwidth resources on lossy channels and heterogeneous networks. Itdiscusses more particularly the ability of window-based congestion control to support non-congestion related losses. We investigatemethods for efficient packet loss recovery by retransmissio n, and builds on explicit congestion control mechanisms to decouplethe packet loss detection from the congestion feedback signals. For different retransmission strategies that respectively relyon conventional cumulative acknowledgments or accurate loss monitoring, we show how the principles underlying the TCPretransmission mechanisms have to be adapted in order to take advantage of an explicit congestion control framework. A novelretransmission timer is proposed to deal with multiple losses of data segments and, in consequence, to allow for aggressive resetof the connection recovery timer. It ensures significant ben efit from temporary inflation of the send-out window, and henc e thefair share of bottleneck bandwidth between loss-prone and lossy connections. Extensive simulations demonstrate the performanceof the new loss monitoring and recovery strategies, when used with two distinct explicit congestion control mechanisms. Thefirst one proposes a simple modification of TCP to support expl icit congestion control, based on a coarse binary congestionnotification from the routers. The second one, introduced in [1], relies on accurate feedback about congestion to compute finecongestion window adjustment. For both congestion control mechanisms, we observe that retransmissions triggered based on aprecise monitoring of losses allow for efficient utilizatio n of lossy links, and provide a fair share of the bottleneck bandwidthbetween heterogeneous connections, even for high loss ratios and bursty loss processes. Explicit congestion control, combined withappropriate error control strategies, can therefore provide a valid solution to reliable and controlled connections over lossy networkinfrastructures. In addition, our simulations also reveal that triggering retransmissions based on cumulative acknowledgments isonly efficient -in terms of bottleneck utilization and fairness- at high loss rates when used in conjunction with an accurate andfinely tuned congestion control. Therefore, we finally recom mend the implementation of accurate feedback mechanisms eitherin the routers (about the level of congestion) or at the receivers (about a packet arrival), in order to provide a fair bandwidthallocation in hybrid networks with explicit window-based control.