Published in IET Communications Received on 23rd January 2009 Revised on 2nd December 2009 doi: 10.1049/iet-com.2009.0394 ISSN 1751-8628 Optimal design of forward error correction for fairness maximisation among transmission control protocol flavours over wireless networks T. Mahmoodi 1 V. Friderikos 2 O. Holland 2 A. Hamid Aghvami 2 1 Intelligent Systems and Networks Group, Department of Electrical and Electronic Engineering, Imperial College, London SW7 2BT, UK 2 Centre for Telecommunications Research, King’s College London, Strand, London WC2R 2LS, UK E-mail: t.mahmoodi@imperial.ac.uk Abstract: A plethora of modifications to transmission control protocol (TCP) have recently been proposed, a major aim being to improve its performance over wireless links. Two schools of thought have emerged: the first investigates changes to the transport-layer protocol, whereas the second explores the potential to enhance the characteristics of lower layers to improve the end-to-end performance of TCP.This study focuses on the latter, and, in contrast to most research in this area, which thus-far has concentrated on a single TCP flavour, examines the case where different TCP flavours are competing over a wireless link. To this end, the authors present and assess a cross-layer solution to adapt the coding rate at the link-layer based on the detected TCP flavour, to maximise fairness among TCP flows. Through both analysis and simulation, the authors show that the proposed scheme considerably improves the fairness among different TCP flavours that compete over a wireless link. Furthermore, the proposed approach has minimal detrimental effect on the aggregate throughput of TCP flows. 1 Introduction Among the tapestry of solutions that have made the development and success of the Internet possible, the transmission control protocol (TCP), and numerous variations thereof, have become the norm for the provision of a reliable end-to-end unicast transport layer. As the take-up of wireless technologies further proliferates, it is expected, for compatibility reasons, that TCP will be used much more often over wireless networks. A significant amount of research has therefore evolved over the past few years, aimed at improving the performance of TCP over wireless networks [1]. Many such enhancements fit into one of two categories, the first of which proposes changes to the end-to-end protocol, and the second of which explores the potential to enhance lower layers in order to optimise end-to-end performance. The mass of prior art on designs to improve TCP over wireless links has thus far assumed that all competing flows are of the same flavour. However, with the spiralling number of TCP flavours in existence over the Internet [2], the end-to- end performance characteristics of TCP, particularly in cases of random (wireless) packet loss, have become increasingly diverse. In this paper, we study a more generalised framework, whereby the different competing TCP flows are heterogeneous in nature, that is, they can be based on different TCP flavours (such as for example Reno, New Reno, Westwood, or using the Selective ACKnowledgement (SACK) option). In this context, because TCP flavours react differently to random packet losses, a significant degree of unfairness among flows can surface in terms of achieved end- to-end throughput. The performances seen by the different TCP flows therefore depend not only on the packet loss rates over the wireless link, but also on the exact combination of currently utilised TCP flavours over that wireless link. Hence, designs to improve end-to-end TCP performance should also take into consideration the mix of TCP flavours operating over the wireless link. 1196 IET Commun., 2010, Vol. 4, Iss. 10, pp. 1196–1206 & The Institution of Engineering and Technology 2010 doi: 10.1049/iet-com.2009.0394 www.ietdl.org