TCP Westwood: End-to-End Bandwidth Estimation for Enhanced Transport over Wireless Links
TCP Westwood: End-to-End Bandwidth Estimation for Enhanced Transport over Wireless Links
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发表时间：
2002
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通讯作者：
C. Casetti;M. Gerla;S. Mascolo;M. Sanadidi;Ren Wang
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作者：
C. Casetti;M. Gerla;S. Mascolo;M. Sanadidi;Ren Wang
TCP Westwood（TCPW）是TCP拥塞窗口算法的发送端修改，它改进了TCP里诺在有线和无线网络中的性能。这种改进在具有有损链路的无线网络中最为显著。事实上，TCPW的性能对随机错误并不十分敏感，而TCP里诺对随机丢失和拥塞丢失同样敏感，无法区分它们。因此，TCP里诺倾向于对错误反应过度。TCP Westwood相对于以前的无线TCP“扩展”的一个重要区别特征是，它不需要在中间（代理）节点处检查和/或拦截TCP分组。相反，TCPW完全符合端到端TCP设计原则。关键的创新思想是在TCP发送端通过监控ACK的返回率来连续测量连接所使用的带宽。然后，该估计用于计算拥塞事件之后（即，在三次重复拥塞之后或在超时之后）的拥塞窗口和慢启动阈值。该策略的基本原理很简单：与TCP里诺在三个重复ACK之后“盲目地”将拥塞窗口减半相比，TCP韦斯特伍德尝试选择与经历拥塞时使用的有效带宽一致的慢启动阈值和拥塞窗口。我们称之为快速恢复机制。所提出的机制是特别有效的无线链路中，由于无线电信道问题的零星损失往往被误解为拥塞的症状，由当前的TCP方案，从而导致不必要的窗口减少。实验研究表明，在吞吐量每千分之一，以及在公平性的改善。此外，在一组实验中观察到与TCP里诺的友好性，表明TCP里诺连接不会被TCPW连接所饥饿。最重要的是，TCPW在有线和无线混合网络中非常有效，其中吞吐量提高了550%。最后，TCPW的性能几乎与流行的Snoop方案等本地化链路层方法一样好，而不会产生专门的链路层协议的开销。
TCP Westwood (TCPW) is a sender-side modification of the TCP congestion window algorithm that improves upon the perfor- mance of TCP Reno in wired as well as wireless networks. The improvement is most significant in wireless networks with lossy links. In fact, TCPW performance is not very sensitive to random errors, while TCP Reno is equally sensitivity to random loss and congestion loss and cannot discriminate between them. Hence, the tendency of TCP Reno to overreact to errors. An important distinguishing feature of TCP Westwood with respect to previous wireless TCP "extensions" is that it does not require inspection and/or interception of TCP packets at intermediate (proxy) nodes. Rather, TCPW fully complies with the end-to-end TCP design principle. The key innovative idea is to continu- ously measure at the TCP sender side the bandwidth used by the connection via 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 after 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 bandwidth used at the time congestion is experienced. We call this mechanism faster recovery. The proposed mechanism is particularly effective over wireless links where sporadic losses due to radio channel problems are often misinterpreted as a symptom of congestion by current TCP schemes and thus lead to an unnecessary window reduction. Experimental studies reveal improvements in throughput per- formance, as well as in fairness. In addition, friendliness with TCP Reno was observed in a set of experiments showing that TCP Reno connections are not starved by TCPW connections. Most importantly, TCPW is extremely effective in mixed wired and wireless networks where throughput improvements of up to 550% are observed. Finally, TCPW performs almost as well as localized link layer approaches such as the popular Snoop scheme, without incurring the overhead of a specialized link layer protocol.