Study and performance evaluation of TCP modifications and tuning over satellite links

The paper presents a study and an analysis of the performance offered by TCP over a GEO (geostationary orbit) satellite link. The characteristics of satellite channels (notably the large round trip delay) heavily influence the TCP flow control, which is essentially based on the acknowledgments. The effect is a delay in the acknowledgement reception and, consequently, in the delivery of messages, with respect to cabled networks. The drawbacks of using TCP in a satellite environment may be mitigated by a proper tuning of some TCP parameters. The behavior of the protocol as a consequence of variations in the buffer length of both the transmitter and receiver and in the initial congestion window is investigated in the paper and a proper configuration that drastically improves performance (measured by the throughput in bytes/s and by the overall transmission time) is proposed. Two test environments have been used to evaluate the proposed modifications: a real testbed, composed of two remote hosts connected through a satellite channel, and a satellite network emulator, composed of three PCs (two of them representing two hosts, the third one reproducing the behavior of the satellite link). In both environments, a single ftp-like application designed for the aim has represented the reference application; three different file sizes have been used and the different effect of the tuning depending on the transfer length has been evidenced.