TCP throughput and timeout - steady state and time-varying dynamics

While many models of TCP´s dynamics have been developed, few focus on the effects of timeout and high loss probability. Active queue management (AQM) is an important application of these dynamic models. However, recent work has shown that AQM provides little performance benefit over drop-tail queueing for HTTP traffic, except, possibly, at high utilizations. It is at these utilizations that the dynamic models of TCP are the least accurate. The paper presents a dynamic model of TCP that accurately models timeout. This model is also applicable to the static case. The paper also presents a model of the variance and the distribution of the congestion window. It is shown that, while the dynamics of the mean value of the congestion window are rather mild, the dynamics of timeout display large oscillations that take several seconds to decay. These oscillations cause the average bit-rate also to oscillate wildly. The paper includes results from several million simulations providing a detailed view of the dynamics of timeout.