Explicit Window-based Control in Lossy Packet Networks

This paper addresses the problem of fair allocation of bandwidth resources on lossy channels and heterogeneous networks. It discusses more particularly the ability of window-based congestion control to support non-congestion related losses. We investigate methods for efficient packet loss recovery by retransmission, and builds on explicit congestion control mechanisms to decouple the packet loss detection from the congestion feedback signals. For different retransmission strategies that respectively rely on conventional cumulative acknowledgments or accurate loss monitoring, we show how the principles underlying the TCP retransmission mechanisms have to be adapted in order to take advantage of an explicit congestion control framework. A novel retransmission timer is proposed to deal with multiple losses of data segments and, in consequence, to allow for aggressive reset of the connection recovery timer. It ensures significant benefit from temporary inflation of the send-out window, and hence the fair share of bottleneck bandwidth between loss-prone and lossy connections. Extensive simulations demonstrate the performance of the new loss monitoring and recovery strategies, when used with two distinct explicit congestion control mechanisms. The first one proposes a simple modification of TCP to support explicit congestion control, based on a coarse binary congestion notification from the routers. The second one, introduced in [1], relies on accurate feedback about congestion to compute fine congestion window adjustment. For both congestion control mechanisms, we observe that retransmissions triggered based on a precise monitoring of losses allow for efficient utilization of lossy links, and provide a fair share of the bottleneck bandwidth between heterogeneous connections, even for high loss ratios and bursty loss processes. Explicit congestion control, combined with appropriate error control strategies, can therefore provide a valid solution to reliable and controlled connections over lossy network infrastructures. In addition, our simulations also reveal that triggering retransmissions based on cumulative acknowledgments is only efficient -in terms of bottleneck utilization and fairnessat high loss rates when used in conjunction with an accurate and finely tuned congestion control. Therefore, we finally recommend the implementation of accurate feedback mechanisms either in the routers (about the level of congestion) or at the receivers (about a packet arrival), in order to provide a fair bandwidth allocation in hybrid networks with explicit window-based control.