TCP Westwood: analytic model and performance evaluation

We present a performance model of TCP Westwood (TCPW), a new TCP protocol with a sender-side modification of the window congestion control scheme. TCP Westwood controls the window using end-to-end connection bandwidth share estimation. The key innovative idea is to continuously estimate, at the TCP sender, the packet rate of the connection by monitoring the ACK reception rate. The estimated connection rate is then used to compute congestion window and slow start threshold settings after a congestion episode. Resetting the window to match available bandwidth makes TCPW more robust to sporadic losses due to wireless channel problems. An analytic model using Markov Chain techniques is developed in this paper, and then used to assess the performance improvements obtained using TCPW. The model takes into account the estimation and filtering method used in TCPW, as well as the following system parameters, bottleneck link bandwidth, buffer space at the bottleneck router, end-to-end propagation time, and error rate. The model reveals substantial TCPW gains over Reno whenever losses due to link or other errors are taken into consideration. The analytic model accuracy is confirmed by comparing to simulation results. The results match very well for the range of parameter values of interest.