MIT Department of Electrical Engineering & Computer Science

Three Challenges to Reliable Data Transport over Heterogeneous Wireless Networks

Hari Balakrishnan
University of California at Berkeley

Wednesday, April 15, 1998
4:00 PM (refreshments 3:45)
AI Playroom, Room NE43-800
EECS Special Seminar

Abstract

The Transmission Control Protocol (TCP) is the de-facto standard for reliable data transmission in the Internet today. While TCP has been tuned to work well over traditional wired networks, its performance over wireless networks is much worse. This performance degradation results from several factors: (i) the preponderance of packet losses due to wireless bit-errors and user mobility, (ii) asymmetric effects and latency variability, and (iii) low channel bandwidths.

While TCP adapts well to network congestion, it does not adequately handle the vagaries of wireless media. In this talk, I will discuss these challenges in detail and present solutions to them. These solutions incorporate local link-level techniques as well as modifications and enhancements to TCP at the sender and receiver. I will first describe the design and implementation of a novel protocol, called the Berkeley Snoop protocol, that deploys a soft-state agent at the base station to perform local retransmissions and shield the TCP sender from wireless errors.

I will also describe a mechanism called Explicit Loss Notification (ELN), by which we can successfully distinguish between congestion and corruption-induced packet losses to substantially enhance end-to-end performance. I will then present several techniques to overcome the challenges posed by latency variability and bandwidth asymmetry. Finally, I will present enhancements to TCP's loss recovery algorithm that reduce the frequency of sender timeouts and provide good performance when transmission windows are small--the common case for low-bandwidth connections and typical Web-like workloads. The talk will include experimental results from implementations on a variety of wireless networks, including Lucent Technologies' in-building WaveLAN and Metricom Inc.'s campus-area Ricochet network.