TCP over Wireless Multi-hop Protocols: Simulation and Experiments Mario Gerla, Rajive Bagrodia, Lixia Zhang, Ken Tang, Lan Wang {gerla, rajive, lixia, ktang, lanw}@cs.ucla.edu Wireless Adaptive Mobility Laboratory Computer Science Department University of California, Los Angeles Los Angeles, CA 90095 http://www.cs.ucla.edu/NRL/wireless Abstract In this study we investigate the interaction between TCP and MAC layer in a wireless multi-hop network. This type of network has traditionally found applications in the military (automated battlefield), law enforcement (search and rescue) and disaster recovery (flood, earthquake), where there is no fixed wired infrastructure. More recently, wireless "ad-hoc" multi-hop networks have been proposed for nomadic computing applications. Key requirements in all the above applications are reliable data transfer and congestion control, features that are generally supported by TCP. Unfortunately, TCP performs on wireless in a much less predictable way than on wired protocols. Using simulation, we provide new insight into two critical problems of TCP over wireless multi-hop. The first is the conflict between data packets and ACKs, which causes TCP performance to degrade for window sizes greater than 1 packet. The second is the interaction between MAC and TCP layer backoff timers which causes severe unfairness and capture conditions. In the paper, we identify these problems in several representative simulation runs on various topologies and traffic patterns and indicate possible remedies to improve TCP efficiency over a wireless multi-hop network. 1. Introduction The rapid advancement in portable computing platforms and wireless communication technology has led to significant interest in the design and development of protocols for instantly deployable, wireless networks often referred to as “Ad-Hoc Networks". Ad-hoc networks are required in situations where a fixed communication infrastructure, wired or wireless, does not exist or has been destroyed. The applications span several different sectors of society. In the civilian environment, they can be used to interconnect workgroups moving in an urban or rural area or a campus and engaged in collaborative operation such as distributed scientific experiments and search and rescue. In the law enforcement sector, applications such as crowd control and border patrol come to mind. In the military arena, the modern communications in a battlefield theater require a very sophisticated instant infrastructure with far more complex requirements and constraints than the civilian applications [8]. In a nutshell, the key characteristics which make the design and evaluation of ad-hoc networks unique and challenging include mobility, unpredictable wireless channel such as fading, interference and obstacles, broadcast medium shared by multiple users and very large number of heterogeneous nodes (e.g., thousands of sensors). To these challenging physical characteristics of the ad-hoc network, we must add the extremely demanding requirements posed on the network by the typical applications. These include multimedia support, multicast and multi-hop communications. Multimedia (voice, video and image) is a must when several individuals are collaborating in critical applications with real time constraints. Multicasting is a natural extension of the multimedia requirement. Multi- hopping is justified (among other things) by the limited power of the mobile devices, by obstacles and by the desire to reuse frequency and/or code. Two key requirements of the ad-hoc network environment are reliable data transfer and congestion control. These features are generally supported by TCP. An important question is how TCP (which has been designed and fine- tuned for wired networks) interacts with the wireless protocols, in particular the MAC layer. Both MAC and TCP layers strive to provide efficient transport in a shared environment, with some degree of efficiency and with protection from errors and interference. The MAC layer however has only a myopic view of the network, which is a critical limitation in multi-hop networks. In contrast, TCP provides a true end-to-end control on errors and congestion. In this paper, we study the TCP/MAC layer interaction via simulation. The simulation platform used is GloMoSim [18]. GloMoSim is a parallel simulation environment implemented in PARSEC, PARallel Simulation Environment for Complex Systems [1]. It includes several wireless protocols in its library (radio propagation, mobility, MAC, network, transport and applications). Most importantly, GloMoSim permits the detailed modeling of several layers and the study of their interaction, yet preserving very good runtime efficiency and yielding manageable execution time. The rest of the paper is organized as follows: Section 2 reports the configuration and parameters we used for our simulation. TCP over the MAC layer experiments are examined in section 3. Section 4 summarizes our grid topology simulation results. Finally, section 5 concludes the paper.