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1 Enhancing TCP Congestion Control Via Connection Bandwidth Estimation , A Performance Study

We present performance studies of TCP Westwood (TCPW), a sender-side modification of the congestion window control scheme in TCP. TCP Westwood relies on end-to-end rate estimation. The key innovative idea is to continuously measure at the TCP sender the packet rate of the connection by monitoring the rate of returning ACKs. The estimate is then used to compute congestion window and slow start threshold after a congestion episode, that is, after three duplicate acknowledgments or a timeout. The rationale of this strategy is simple: in contrast with TCP Reno, which “blindly” halves the congestion window after three duplicate ACKs, TCP Westwood attempts to select a slow start threshold and a congestion window which are consistent with the effective connection rate at the time congestion is experienced. The proposed faster recovery mechanism is particularly effective over wireless links where sporadic losses due to radio link problems are often misinterpreted as a symptom of congestion by current TCP schemes, and thus lead to unnecessary window reduction. Experimental studies reveal improvements in throughput performance, as well as in fairness. In addition, friendliness with TCP Reno was confirmed in a set of experiments. TCP Reno connections are not starved by TCPW connections; on the contrary, the TCP Reno connections continue to make satisfactory progress. TCPW is shown here to be extremely effective in mixed wired and wireless networks and in high speed networks. Throughput improvements of up to 615 % are observed. Internet measurements using a Linux TCPW implementation are also reported in this paper, providing further evidence of the gains achievable via TCPW.