Modifications of the Slow Start Algorithm to Improve TCP Performance Over Large Delay Satellite Channels

Abstract

The paper presents a study and performance analysis of a modified version of the TCP (Transmission Control Protocol) over a GEO (Geostationary Orbit) satellite link. The Round Trip Time (RTT) is above 500 ms. The high delay to receive acknowledgements decreases the performance of the TCP but, if the application field has a limited extension (as it is the test-bed emulated in this paper) the characteristics of the links and of the devices traversed are well known and the congestion aspects may be treated differently than in a large cable network, where the number of devices in the path is difficult to control. So, even if the performance of TCP is not satisfying, improvements can be obtained by properly tuning some parameters and modifying algorithms.

A new increase function of the slow start algorithm is introduced in the paper. It is called MTSI—Multi Threshold Smoothed Increase. The performance is measured by the throughput in bytes/s and by the overall transmission time. An ftp-like application designed for the aim has represented the reference application. The performance analysis is carried out by using both a real satellite test-bed and a satellite network emulator. The new proposal is compared with different strategies already in the literature, including the TCP New Reno, configurations with augmented buffer and initial window and transport layer solutions already adapted for the satellite environment. The analysis has included both the single and the multiple application case, where several connections share the satellite link at the same time. The file transfer dimension is varied along with the bandwidth available and the advantage of the new proposal depending on these two parameters is also evidenced. The performance has been measured both in clear sky condition (no packet loss) and during loss situations.

Indexing terms:

• Satellite communications
• Network protocols; TCP/IP
• TCP over satellite

Additional information

Notes on contributors

Mario Marchese

Mario Marchese was born in Genoa, Italy in 1967. He received the Laurea degree (cum laude) from the University of Genoa, Genoa, Italy, in 1992, his qualification as a Professional Engineer in 1992, and a PhD degree in telecommunications from the University of Genoa in 1996. From 1999 to 2004, he worked with the University of Genoa Research Unit, Italian National Consortium of Telecommunications (CNIT), where he was head of research. Since 2005, he has been an Associate Professor in the Department of Communication, Computer and Systems Science (DIST), University of Genoa. He is founder and still technically responsible for the CNIT/DIST Satellite Communication and Networking Laboratory (SCNL), University of Genoa, which contains high-value devices and tools, and implies the management of different units of specialized scientific and technical personnel. He is Vice-Chair of the IEEE Satellite and Space Communications Technical Committee. He is author and co-author of more than 80 scientific papers in international magazines, international conferences and book chapters. His main research interests include satellite networks, transport layer over satellite and wireless networks, quality of service over ATM, IP, and MPLS, and data transport over heterogeneous networks.