ADHOCTCP: Improving TCP Performance in Ad Hoc Networks

Abstract

A mobile ad-hoc network (MANET) is a special type of wireless networks. It consists of a collection of mobile nodes that are capable of communicating with each other without help from a fixed infrastructure. The interconnections between nodes are capable of changing on a continual and arbitrary basis. Nodes within each other's radio range communicate directly via wireless links, while those that are far apart use other nodes as relays in a multi-hop routing fashion. The typical applications of MANETs include conferences or meetings, emergency operations such as disaster rescue, and battlefield communications. Transmission Control Protocol (TCP) [1] is a reliable, connection-oriented, full-duplex, transport protocol widely used in wired networks. TCP’s flow and congestion control mechanisms are based upon the assumption that packet loss is an indication of congestion. While this assumption holds in wired networks, it does not hold in the case of mobile wireless networks. In addition to congestion, a transport protocol in an ad hoc network must handle mobilityinduced disconnection and reconnection, route change-induced packet out-of-order delivery for mobile hosts, and error/contention prone wireless transmissions. Reaction to these events might require transport control actions different from congestion control. It might be better to periodically probe the network during disconnection than to back off exponentially [2], and it makes more sense simply to re-transmit a packet lost to random channel error than to multiplicatively decrease the current congestion window [3]. Even if the correct action is executed in response to each type of network event, it is not immediately obvious how to construct an engine that will accurately detect and classify events. Packet loss alone cannot detect and differentiate all these new network events [4]. In this paper, we first describe the necessary network states in an ad hoc network to be identified by TCP and use an end-to-end approach for identification of congestion state in ad hoc network then examine metrics that can be measured end-to-end. Two metrics are devised to detect congestion, IDD (Inter Delay Difference) and STT (Short Term Throughput).The approach we propose in this paper utilizes network layer feedback (from intermediate hops) for identification of disconnection state to put TCP sender into persist mode. Therefore we use from advantage of both end to end measurements and network layer feedback. The remainder of the chapter is organized as follows: It starts with describing TCP’s challenges in MANETs environment in Section 2. Section 3 provides an overview of related