Explicit window-based transport control protocols in lossy environments

This paper addresses efficient packet loss recovery by retransmission in window-based congestion control protocols. It builds on explicit congestion control mechanisms to decouple the packet loss detection from the congestion feedback signals. Implicit algorithms alternatively infer congestion from losses (which yields to window size reduction), and therefore do not allow to evaluate the performance of window-based transmission algorithms in lossy environments. We first propose a simple modification of TCP that offers the possibility for explicit congestion control. Different retransmission strategies applicable to window-based congestion control protocols are then discussed in the framework of explicit congestion control. We introduce a new early retransmission timer that significantly improves the error resiliency when combined with explicit congestion control. Extensive simulations then compare the error recovery mechanisms generally used in recent TCP implementations, and the new loss monitoring and recovery strategies, combined with explicit congestion control protocols. Performances are analyzed in a simple network topology where a bottleneck link is shared by loss-free, and respectively lossy connections. Retransmissions triggered by the proposed accurate loss monitoring mechanism are shown to end up in a fair share of the bottleneck bandwidth between all connections, even for high loss ratios and bursty loss processes. The link utilization is in the same time close to optimal. Explicit congestion control, combined with efficient error control strategies, can therefore provide a valid solution to reliable and controlled connections over lossy network infrastructures.