The fairness of TCP Vegas in networks with multiple congested gateways

We investigate the performance of TCP Vegas in networks with multiple congested gateways. Our work focuses on the fairness between multiple-hop connections and cross traffic connections at each gateway. We find that TCP Vegas tends to be biased against multiple-hop connections. By analysis and computer simulations, we show that this bias is rooted in Vegas's RTT-based congestion avoidance mechanism: it uses the variation of RTT (round trip time) as the tuning knob to adjust the congestion window. As the congestion status along the path cannot be correctly determined by measuring the RTT, the connection passing through a multiple-hop path receives degraded throughput as a result of the wrong response to congestion. We show that it is not easy for this unfairness problem to be solved solely by the end host; collaborative efforts from both end hosts and intermediate gateways should be considered.