TCP westwood with agile probing: dealing with dynamic, large, leaky pipes

TCP westwood (TCPW) has been shown to provide significant performance improvement over high-speed heterogeneous networks. The key idea of TCPW is to use eligible rate estimation (ERE) methods, to set the congestion window (cwnd) and slow start threshold (ssthresh) after a packet loss. ERE is defined as the transmission rate a sender ought to use to achieve high utilization and remain friendly to other TCP variants. This paper presents TCP westwood with agile probing (TCPW-A), a sender-side only enhancement of TCPW. TCPW-A perform well when faced with highly dynamic bandwidth, large propagation time/bandwidth, and random loss in the current and future heterogeneous Internet. TCPW-A achieves its goal by incorporating the following, two mechanisms: 1) when a connection initially begins or re-starts after a timeout, instead of exponentially expanding cwnd to an arbitrary preset ssthresh and then going into linear increase. TCPW-A uses agile probing, a mechanism that repeatedly resets ssthresh based on ERE and forces cwnd into an exponential climb each time. The result is fast convergence to a more appropriate ssthresh value. 2) In congestion avoidance, TCPW-A invokes agile probing upon indication of unused extra bandwidth via a scheme we call load gauge (LG). Experimental results, both in Ns-2, and in measurements using FreeBSD implementation, show that TCPW-A can significantly improve link utilization over a wide range of bandwidth, propagation delay and dynamic network loading.