Communication Networks Generalized Window Advertising for TCP Congestion Control MARIO GERLA Computer Science Department – Boelter Hall – UCLA, 405 Hilgard Ave., Los Angeles, CA, 90024, USA gerla@cs.ucla.edu RENATO LO CIGNO Dipartimento di Elettronica – Politecnico di Torino, Corso Duca Degli Abruzzi, 24 – 10129 Torino, Italy locigno@polito.it SAVERIO MASCOLO Dipartimento di Elettrotecnica ed Elettronica – Politecnico di Via Orabona, 4 – 70125 Bari, Italy mascolo@poliba.it WENJIE WENG Motorola, Inc., San Diego, California, USA W.Weng@motorola.com Abstract. Congestion in the Internet is a major cause of network performance degradation. The Generalized Window Advertising (GWA) scheme proposed in this paper is a new approach for enhancing the congestion control properties of TCP. GWA requires only minor modifications to the existing protocol stack and is completely backward compatible, allowing GWA-hosts tointeract with non-GWA hosts without modifications. GWA exploits the notion of end-host–network cooperation, with the congestion level notified from the network to end hosts. It is based on solid control theory results that guarantee performance and stable network operation. GWA is able to avoid window oscillations and the related fluctuations in offered load and network performance. This makes it more robust to sustained network overload due to a large number of connections competing for the same bottleneck, a situation where traditional TCP implementations fail to provide satisfactory performance. GWA-TCP is compared with traditional TCP, TCP with RED and also ECN using the ns-2 simulator. Results show that in most cases GWA-TCP outperforms the traditional schemes. In particular, when compared with ECN, it provides smoother network operation and increased fairness. 1 I NTRODUCTION AND RELATED WORK The boom of the Internet is stressing the entire TCP/IP protocol suite implementation, as the users grow in num- ber, along with the demand for enhanced service and per- formance. The majority of data services in the Internet are based on TCP (Transport Control Protocol), with ap- plication ranging from bulk data transmission (like FTP file transfers), to interactive, delay sensitive services (like Telnet remote terminal), to web browsing. TCP pro- vides a connection oriented, reliable communication chan- nel to upper layers. It also provides the functions of re- ceiver flow control and network congestion control. The This work was done when R. Lo Cigno was visiting the UCLA CS Dept. with CNR (Italian National Research Council) grant 203.15.8. The work was partially supported by NSF,DARPA and NASA through varius grants and by the Italian Ministry for University and Scientific Research (MURST) through the project MQOS. TCP congestion control mechanism in standard implemen- tations is based on a simple, wired network model, with very reliable nodes and links. Packet loss is mostly caused by buffer overflow and is thus taken as indication of con- gestion. Several studies have shown that this control ap- proach may fail if losses in the network are due to cases other than congestion, such as in the case of wireless links (see for instance[1, 2, 3]). Moreover, the control does not work well if the congestion is caused by an excessive num- ber of TCP connections allowed to compete for the same bottleneck [4]. Fortunately, routers of the next generation will be more intelligent than the ones used in traditional TCP implemen- tations. They will be able to predict impending congestion because of better environment awareness; however, a ma- jor problem consists in correctly elaborating and convey- ing congestion information from routers to TCP sources. 1