The Effects of Asymmetry on TCP Performance 1

Hari's research interests are in the areas of computer networks and protocol architectures, wireless networks and mobile systems, adaptive network applications, and large-scale communication systems. He has received awards for papers at the ACM/IEEE Mobi-com Conference and the Usenix Technical Conference. He is also a recipient of the C. V. Ramamoorthy award for distinguished graduate research at Berkeley and the winner of a Segasoft research grant from the Okawa Foundation. He is a member of the ACM and the IEEE. His email address is hari@lcs.mit.edu and his WWW URL is Venkat's research interests are in the areas of Computer Networks, Mobile Computing, and Operating Systems. The focus of his Ph.D. dissertation was on developing network protocols for efficient Web access and effective data transport over asymmetric networks. His work on persistent-connection HTTP has been adopted by the HTTP/1.1 standard. Among the awards he has received are the National Talent Scholarship in India, the Charles Fish fellowship at Berkeley, and the best student paper award at the Usenix '95 conference. Venkat may be reached via e-mail at padmanab@microsoft.com and on the Web at http:// and is a principal investigator in the Bay Area Research Wireless Access Network (BARWAN) project. He has taught at Berkeley since 1983, with the exception of 1993 and 1994 when he was a program manager and deputy director of the Computing Systems Technology Office at the Defense Department's Advanced Research Projects Agency. He has written over 130 technical publications on CAD, database management, multiprocessor architectures, high performance storage systems, video server architectures, and computer networks. 19 Our methodology has involved three major steps. First, we measured the various asymmetric networks, including the Hybrid wireless cable network and the Ricochet packet radio network, to identify performance bottlenecks. Then, we modeled these networks in the ns simulator and experimented with several techniques to improve performance. Finally, we implemented the promising techniques in our experimental testbed and confirmed the performance trends observed in the simulations. • SLIP header compression [13] alleviates some of the problems due to bandwidth asymmetry, but does not completely eliminate all problems, especially those that arise in the presence of bidirectional traffic. • Connections traversing packet radio networks suffer from large variations in round-trip times caused by the half-duplex nature of the radios and asymmetries in the media-access protocol, which lead to variable latencies. This adversely affects TCP's loss recovery mechanism and results in degraded performance. …