Mathematical analysis of Westwood plus TCP congestion control

TCP congestion control is based on an additive-increase/multiplicative-decrease (AIMD) probing paradigm aimed at adapting the sending rate of TCP data sources to match the Internet time-varying available bandwidth. Westwood+ TCP has been recently proposed to improve the tracking of available bandwidth of classic TCP. It is based on an end-to-end estimate of the available bandwidth, which is obtained by properly counting and filtering the stream of acknowledgement packets. The estimate is used to adaptively decrease the congestion window and slow start threshold after congestion so that it can be said that Westwood+ TCP substitutes the classic multiplicative decrease with an adaptive decrease paradigm. The authors propose a mathematical analysis of the additive-increase/adaptive-decrease (AIADD) paradigm to analyse the steady-state throughput provided by Westwood+ TCP and investigate the intra-protocol fairness of the AIADD paradigm and the inter-protocol friendliness between AIADD and AIMD algorithms. It is shown that (i) both classic and Westwood+ TCP provide a throughput that is proportional to 1/rootp, where p is the segment drop probability, that is they are friendly to each other; and (ii) the throughput of Westwood+ TCP is proportional to 1/rootRTT, where RTT is the round trip time, whereas the throughput of Reno TCP is proportional to 1/RTT, i.e. Westwood+ TCP improves the intra-protocol fairness. Finally, Ns-2 simulations are reported in order to validate the mathematical model in the presence of a wide range of network loads, loss probabilities and round trip times.