Proceedings of 2 nd International Symposium on Communication and Information Technology (ISCIT’2002 ), 23-25 Oct. 2002, Central Hotels & Resorts, Pattaya, Chonburi, Thailand Proceedings 2002 International Symposium on Communication and Information Technology 105 Enhancement of TCP Performance over LEO Satellite Networks Ali M Abdelrahman, Mayada S Abdalla, Borhan M Ali, V Prakash, Pn. Ratna Faculty of Engineering & Technology Comp. & Commun. Sys. Engineering Multimedia University Universiti Putra Malaysia Jln Ayer Keroh, 75450 Melaka, Malaysia 43400 UPM Serdang, Selangor, Malaysia ali.abdelrahman@mmu.edu.my mmayda@hotmail.com, borhan@eng.upm.edu.my Abstract This paper studies the improvement of TCP performance over LEO satellite links by proposing an enhanced selective acknowledgements mechanism. The proposal takes into consideration the probability of satellite environment losses as well as the network congestion losses. The results obtained show that the proposed enhancements provide higher congestion windows than that of the standard selective acknowledgements mechanism, which result in improving the effective throughputs as well as reducing the total transmission times. 1. Introduction The transmission control protocol (TCP) is the reliable and most widely used transport protocol for popular Internet applications such as WWW and FTP. The TCP was designed and tuned to perform well in fixed networks, where the key functionality is to utilise the available bandwidth and avoid overloading the network. Basically, the TCP sender adapts its use of the bandwidth based on the feedback from the receiver. This adaptation can be obtained by implementing a number of TCP mechanisms including slow start/congestion avoidance, fast retransmit/fast recovery, and selective acknowledgements (SACK) [1]. Although these mechanisms work properly in wired networks they have negative impacts on the performance of TCP in satellite environment [2]. In this paper, an enhanced selective acknowledgements (ESACK) mechanism has been proposed to improve the performance of TCP over low earth orbit (LEO) satellite links. It considers losses in two consecutive windows by assuming that if losses occur due to the network congestion most probably they will appear in at least two successive windows. The ESACK mechanism has been compared with the standard TCP SACK mechanism. The results obtained indicate that the ESACK mechanism provides better performance in terms of average effective throughputs compared to the SACK mechanism as well as decreases the overall transmission times at the expense of slightly increases end to end packet delays. The rest of the paper is organized as follows: section 2 highlights the behaviour of TCP in LEO satellite environment. The ESACK has been introduced in section 3. In section 4 the simulation environment is described. The simulation results are discussed in section 5. Section 6 concludes the paper by some remarks. 2. TCP in LEO satellite environment LEO satellite links exhibit higher bit error rates than the typical terrestrial ones. The high mobility of the satellites leads to a rapidly and regularly changing of network topology which raises the needs for satellite-to-satellite handoffs [3]. On the other hand, the standard TCP mechanisms consider any loss as an indication of network congestion. Thus, for every window of data from which one or more packets are dropped, the TCP sender interprets this as network congestion and immediately reduces its congestion window by half [1]. Several studies reported that conventional TCP over LEO satellite networks have significantly degraded the throughput, the reliability and the efficiency of the TCP protocol [2]. The use of explicit congestion notification (ECN) bit has been proposed, where the sender can receive explicit messages from the routers when congestion occurs [4]. In [5] it has been argued that the ECN cannot completely eliminate packet losses as indications of congestion, and therefore would not allow the end nodes to interpret packet losses as indications of corruption instead of congestion. Recently, some researchers explored the use of SACK for lossy environment such as satellite links [1]. However, these studies have focused only on congestion issues and still the corruption needs to be addressed [5]. 3. Description of ESACK mechanism The ESACK mechanism takes actions against congestion in LEO satellite links by considering losses in two consecutive windows. It assumes that the losses in the first window are most probably due to transmission errors, delay variations, or handoffs. But when such losses appear in the second window it considers that this may due to network congestion. In a sense that if there is network