1. Introduction

Mobile computing is characterized by many constraints: small, slow, battery-powered portable devices, variable and low-bandwidth communication links. Together, they complicate the design of mobile information systems and require the rethinking of traditional approaches to information access and application design. Finding approaches to reduce power consumption and to improve application performance is a vital and interesting challenge. Many ideas have been developed to address this problem, ranging from hardware to software level approaches.

Designing applications that adapt to the challenges posed by the wireless environment is a hot research area. One group of approaches concentrates on mobile applications that adapt to the scarce and varying wireless link bandwidth by filtering and compressing the data stream between a client application on a portable device and a server executing on a stationary host. Some [BOL 98] enhance the server to generate a data stream that is suited to the currently available bandwidth. Others [ANG 98, FOX 98] extend the client-server structure to a client-proxy-server structure, where a proxy executes in the wireless access network, close to the portable unit. This proxy transparently filters and compresses the data stream originating from the server to suit the current wireless bandwidth.

A second set of approaches provides general solutions that do not change the data stream, focusing on improving TCP throughput [BAL 95]. They usually treat IP packets as opaque, i.e., they neither require knowledge of, nor do...