1 LN-MAC: a Cross-layer Explicit Loss Notification Solution for TCP over IEEE 802.11 Ayyappan Ravichandran 1 , Marco Tacca 1 , Michael Welzl 2 , and Andrea Fumagalli 1 1 OpNear Lab, Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75083, USA Email: [axr065100|mtacca|andreaf]@utdallas.edu 2 Institute of Computer Science, University of Innsbruck, A-6020 Innsbruck, Austria Email: [michael.welzl]@uibk.ac.at Abstract— WiFi, i.e., IEEE 802.11 is one of the most widely used technologies to implement internet access in today’s net- works. Typical frame error rates in IEEE 802.11 are much higher than in wired links. This negatively impacts the behavior of TCP, which assumes that packet loss is due to congestion. This paper presents LN-MAC, a cross-layer solution based on explicit loss notification (ELN), where the IEEE 802.11 MAC is enhanced to provide loss notifications to the TCP layer. Extensive simulation results demonstrate the benefits of LN-MAC. Numerical results demonstrate that under any condition TCP throughput is always higher when using LN-MAC than when using conventional IEEE 802.11. I. I NTRODUCTION With wireless data transfers becoming more and more popular, the expectations of these networks increase everyday. The IEEE 802.11 [1] family of standards, referred to as WiFi, is the most commonly used in establishing wireless access. However, wireless channels are subject to heavier packet loss due to corruption and other synchronization issues when compared to wired channels. Additionally, most household wireless networks are subject to interferences from home appliances, such as microwaves, mobile phones, and other electronic devices which produce interfering signals. As a result, typical wireless link error rates are in the range 10 -3 to 10 -1 , while typical wired link error rates are in the range of 10 -6 to 10 -8 [2]. This hugely affects the transport layer (TCP) performance [3] which was originally designed to operate over networks with wired links. Unless otherwise specified, TCP interprets the occurrence of a packet loss as an indicator of network congestion, which is resolved by promptly decreasing the congestion window, i.e., the TCP sender’s transmission rate. One possible approach to contain the TCP performance loss due to packet corruption losses is based on performance enhancing proxies (PEP) [4], Berkeley SNOOP protocol [5] or TULIP [6]. Another solution, i.e., Split-TCP [7] and indirect TCP [8] tries to enhance performance by splitting the TCP connection between the wireless links and the wired links. This paper presents a study of the performance of LN- MAC, a cross-layer explicit loss notification (ELN) [9] based method in TCP/IP over IEEE 802.11. The study is based This research is supported in part by NSF grants No. ECS-0225528 and CNS-0435429 on the use of two TCP options introduced by one of the authors, i.e., the Corruption Detection Option (CDO) and Corruption Notification Option (CNO) [10]. In a real scenario, the CNO/CDO option technique alone improves marginally the TCP performance, as demonstrated in [11]. The CNO/CDO technique is augmented with a cross-layer solution, where the IEEE 802.11 MAC and TCP layers cooperate to achieve better performance in the presence of errors due to the wireless channel. Performance gain is obtained by taking advantage of the knowledge of the link status at the MAC layer, i.e., the MAC obtains information about the sequence number of the TCP segments. Upon a MAC frame loss, the MAC can directly inform the TCP layer. This effectively decreases the round trip time in the presence of errors and the TCP reaction time to losses. Simulation results demonstrate that the cross-layer LN- MAC protocol allows to always gain in terms of throughput under any wireless link conditions. Furthermore, the protocol design is backward compatible with existing implementations of TCP. II. SYSTEM DESCRIPTION This section briefly describes how CNO/CDO options are used in conjunction with TCP and how the LN-MAC cross- layer solution takes advantage of the ELN technique. Notice that IEEE 802.11 uses a CRC that covers both MAC headers and MAC payload. IEEE 802.11 cards discard frames where the CRC detects an error. The CNO/CDO solution requires a modification to the behavior of IEEE 802.11: when a frame has error(s), it should not be discarded, it should be passed to the higher layers. Higher layers will handle the packets as described. A. TCP Using CNO/CDO Fig. 1. CDO as specified in [10]. Corruption detection is performed using a TCP header option in the data segment. CDO is a six byte option (Fig. 1).