Proxy-based Flow Aggregation for Enhanced TCP over GPRS

Throughout the world, GSM cellular mobile networks are being upgraded to support the “alwayson” General Packet Radio Service (GPRS). Despite the apparent availability of levels of bandwidth not dissimilar to that provided by conventional fixed-wire telephone modems, the user experience using GPRS is considerably worse. In this paper we examine the performance of protocols such as TCP over GPRS, and show how certain network characteristics interact badly with TCP to yield problems such as: link under-utilization for short-lived flows, excess queueing for long-lived flows, ACK bunching, poor loss recovery, and gross unfairness between competing flows. We present the design and implementation of a transparent TCP proxy that mitigates many of these problems without requiring any changes to the TCP implementations in either mobile or fixed-wire end systems. The proxy splits TCP connections transparently into two halves, the wired and wireless sides. Connections destined for the same mobile host are treated as an aggregate due to their statistical dependence. We demonstrate packet scheduling and flow control algorithms that use information shared between the connections to maximise performance of the wireless link while inter-working with unmodified TCP peers. We also demonstrate how fairness between flows and response to loss is improved, and that queueing and hence network latency is reduced. We discuss how TCP enhancing proxies could be transparently deployed, and conclude that installing such a proxy into GPRS network would be of significant benefit to users. Index Terms Wireless, Transparent, Proxy, GPRS, TCP The authors are with the University of Cambridge Computer Laboratory, William Gates Building, Cambridge CB3 0FD, U.K.. Email: {FirstName.LastName}@cl.cam.ac.uk, e.g., [email protected]. Sachin Katti joins the Laboratory of Computer Science (LCS), Massachussets Institute of Technology (MIT), USA. He can also be reached via: [email protected]. IEEE JOURNAL OF SELECTED AREAS OF COMMUNICATIONS, VOL. X, NO. Y, JUNE 2003 (UNDER SUBMISSION) 2