TCP Congestion Window Validation

TCP’s congestion window controls the number of packets a TCP flow may have in the network at any time. However, long periods when the sender is idle or application-limited can lead to the invalidation of the congestion window, in that the congestion window no longer reflects current information about the state of the network. In this paper we propose a simple modification to TCP’s congestion control algorithms to decay the congestion window cwnd after the transition from a sufficiently-long applicationlimited period, while using the slow-start threshold ssthresh to save information about the previous value of the congestion window. An invalid congestion window also results when the congestion window is increased (i.e., in TCP’s slow-start or congestion avoidance phases) during application-limited periods, when the previous value of the congestion window might never have been fully utilized. We propose that the TCP sender should not increase the congestion window when the TCP sender has been application-limited (and therefore has not fully used the current congestion window). This note illustrates these algorithms both with simulations and with experiments from an implementation in FreeBSD.