Performance analysis of heterogeneous interacting TCP sources

In this paper we develop a fluid model for TCP sources that share a bottleneck router. The model allows to specify for each source individually the maximum window size, the packet size, and the (stochastic) roundtrip time. First we consider the situation in which one source uses the link. In this setting we study the impact of buffer size, link rate, and source parameters on the link utilization. We observe that conservative source behavior is optimal with respect to utilization. Then we focus on two sources sharing the link. By computing the throughput for each source separately we obtain insight in fairness issues. We find analytic support for TCP's bias against sources with: (1) longer roundtrip times; (2) smaller maximum congestion windows; (3) smaller packet sizes. Another interesting point is that when buffering delay forms a substantial part of the roundtrip time, on which the feedback loop depends, a large buffer size is detrimental to link utilization.