has been shown to provide significant performance improvement over high-speed heterogeneous networks. The key idea of TCPW is use eligible rate estimation (ERE) methods to intelligently set the congestion window (cwnd) and slow-start threshold (ssthresh) after a packet loss. ERE is defined as the efficient transmission rate eligible for a sender to achieve high utilization and be friendly to other TCP variants. This paper presents TCP Westwood with agile probing (TCPW-A), a sender-side only enhancement of TCPW, that deals well with highly dynamic bandwidth, large propagation time/bandwidth, and random loss in the current and future heterogeneous Internet. TCPW-A achieves this goal by adding the following two mechanisms to TCPW. 1) When a connection initially begins or restarts after a timeout, instead of exponentially expanding cwnd to an arbitrary preset sthresh 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 detection of persistent extra bandwidth via a scheme we call persistent noncongestion detection (PNCD). While in congestion avoidance, agile probing is actually invoked under the following conditions: a) a large amount of bandwidth that suddenly becomes available due to change in network conditions; b) random loss during slow-start that causes the connection to prematurely exit the slow-start phase. Experimental results, both in ns-2 simulation and lab measurements using actual protocols implementation, show that TCPW-A can significantly improve link utilization over a wide range of bandwidth, propagation delay, and dynamic network loading. Index Terms—Bandwidth estimation, congestion control, highspeed networks, random errors, simulation and measurement. I.

One of the most promising solution for transport protocol over very high bandwidth-delay product networks is High-Speed TCP. However, little is known about its performance as well as its interaction with other elements of the network (such as the RED queue management). In this paper, a comprehensive control-theoretic analysis of HighSpeed TCP is provided. We develop a fluid-flow model of the HighSpeed TCP/RED network and use it to study its performance. The main contributions of this paper are the following. Firstly, we provide a fluid-flow model for High-Speed TCP/RED networks. Secondly, a comprehensive and systematic implementation methodology is described in detail. We also provide a Simulink-based framework for analyzing fluid-based models. Thirdly, we give stability conditions for HighSpeed TCP/RED networks. Finally, the results are validated by simulations using Ns-2 [11].