Split TCP for mobile ad hoc networks

The fairness and throughput of TCP suffer when it is used in mobile ad hoc networks. This is a direct consequence of TCP wrongly attributing packet losses due to link failures (a consequence of mobility) to congestion. While this problem causes an overall degradation of throughput, it especially affects connections with a large number of hops, where link failures are more likely. Thus, short connections enjoy an unfair advantage over long connections. Furthermore, if the MAC protocol defined in the IEEE 802.11 standard is used, the problems exacerbate due to the capture effect induced by this protocol, leading to a larger degree of unfairness and a further degradation of throughput. In this paper we develop a scheme which we call Split TCP. This scheme separates the functionalities of TCP congestion control and reliable packet delivery. For any TCP connection, certain nodes along the route take up the role of being proxies for that connection. The proxies buffer packets upon receipt and administer rate control. The buffering enables dropped packets to be recovered from the most recent proxy. The rate control helps in controlling congestion on inter-proxy segments. Thus, by introducing proxies we emulate shorter TCP connections and can thereby achieve better parallelism in the network. As shown by our simulations, the use of proxies abates the problems described i.e., a) it improves the total throughput by as much as 30% in typical scenarios and b) it reduces unfairness significantly. In terms of an unfairness metric that we introduce, the unfairness decreases from 0.8 to 0.2 (1.0 being the maximum unfairness). We conclude that incorporating TCP proxies is beneficial in terms of improving TCP performance in ad hoc networks.