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.