COMPUTER SCIENCE SEMINAR
Wednesday, March 4, 4-5pm, Jorgenson 74, Caltech.
Three Challenges to Reliable Data Transport over Heterogeneous Wireless
Department of Electrical Engineering and Computer Science,
Berkeley, CA 94720-1776.
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 effects:
(1) the preponderance of packet losses due to wireless bit-errors and
(2) asymmetric effects caused by the adverse interaction between
media-access protocols and TCP, bandwidth asymmetry, etc.;
(3) 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 media-access
protocols 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
Power is me sneaking into your home through a wire.
-- Todd Rungren