A Cross layer Analysis of TCP Instability in Multihop Ad hoc Networks

It is well-known that due to the nature of some ad hoc network applications (e.g. emergency operation, battlefield communication), TCP instability can have a devastating impact on the Quality of Service requirements. As we will show in this paper, TCP instability is truly a cross layer problem which needs to be addressed by considering the interaction of multiple layers with each other. We first divide the TCP instability problem into intra-flow and inter-flow instability and then propose a set of simple but effective solutions and show through extensive simulations the improvements achieved using the proposed algorithms. I) INTRODUCTION Multihop ad hoc networks are autonomous systems of mobile devices connected by wireless links without the use of any pre-existing network infrastructure or centralized administration. During recent years ad hoc networks have attracted considerable research interest thanks to their easy deployment, maintenance and application variety. To enable seamless integration of ad hoc networks with the Internet (for instance in ubiquitous computing applications), TCP seems to be the natural choice for users of ad hoc networks that want to communicate reliably with each other and with the Internet. On the other hand, one of the prominent random access protocols known for ad-hoc networks is the IEEE 802.11 MAC standard [1] which has been widely used and adopted. However, neither TCP nor 802.11 were primarily designed and optimized to work in multihop ad hoc networks. Not surprisingly, as shown in [2,3], the network exhibits serious performance issues when TCP runs over 802.11 in multihop ad hoc networks. In particular interest to us in this paper is the network instability issue as we believe due to the nature of some of the ad hoc network applications (e.g. emergency operation, battlefield communication) the disconnectivity or starvation of one or more connections for even a short period of time is not acceptable and can have a devastating impact on QoS. More specifically, the ad-hoc network users are more willing to receive a continuous and stable flow of data rather than sending/receiving large bulk of data instantly. This argument holds also true for jitter sensitive applications such as audio or video streaming. As we will show later in the paper, instability is truly a cross layer problem which needs to be addressed by considering the interaction of multiple layers with each other. During recent years, there has been a number of valuable works that have investigated the TCP instability by investigating the interaction of routing protocols and TCP in multihop ad hoc networks [4-6]. In this paper, we take a different approach to address TCP instability by carefully tracking the chain of events occurring between link layer and TCP. In particular, we divide the TCP instability problem into intra-flow and interflow instability where the former instability is caused by the interaction of nodes belonging to the same TCP connection while the latter happens when nodes belonging to different connections interact. Based on the findings, we then propose a set of simple but effective solutions and show through extensive simulations the improvements achieved using the proposed algorithms. The rest of the paper is organized as follows. In section 2, we will give an overview of the IEEE 802.11 MAC and TCP with special emphasis on backoff algorithm and TCP congestion control mechanism, respectively. In section 3, the main causes of inter-flow and intra-flow instability are discussed and explained in fine details. Then, based on the drawn facts, we propose two different schemes in section 4 that are applied to TCP and 802.11. This is followed by the simulation model and the key results obtained by simulating the proposed model against the default TCP and 802.11 MAC protocol in section 5. Finally, in section 6, we conclude the paper with some outlines towards future work. II) PROTOCOLS OVERVIEW