Impact of Failures on Routing in Mobile Ad Hoc Networks Using DSR Xiaobing Hou and David Tipper Department of Information Science and Telecommunications University of Pittsburgh Pittsburgh, PA 15260 {xiaobing, dtipper}@mail.sis.pitt.edu Abstract - In this paper, we study the transient behavior of DSR protocol in case of path failures in mobile ad hoc networks. Transient analysis shows that after packets en route reach the failure point, they may block the interface queue and cause significant waste of wireless resources resulting in poor network performance. We present a scheme called Failure Record to prevent this situation and improve various performances of the network. Simulation results show that our scheme is very efficient. Keywords – Ad hoc networks, routing protocols, simulation, DSR, failure record, queuing, transient analysis. 1. Introduction In an ad hoc network, mobile nodes must cooperate to dynamically establish routes using multihop wireless links. There is no stationary infrastructure, and each node acts as a router. A packet may have to be forwarded by a sequence of nodes to reach its destination. With the mobile nature of ad hoc networks, routes are supposed to break very often and packet loss rate is supposed to be higher than wireline networks. So ad hoc networks require highly adaptive routing protocols and efficient failure recovery strategies to deal with the frequent topology changes. A mobile ad hoc networking (MANET) working group [1] has been formed within the Internet Engineering Task Force (IETF) to develop a routing framework for ad hoc networks. Many routing protocols have been proposed to solve the dynamic multihop routing problem in ad hoc networks. Traditional routing protocols, such as distance vector and link state, were designed for static infrastructured networks, and a dynamic topology was not considered in their design. Obviously, it will be unsuitable to use any non- adaptive protocol for a highly mobile network. Basically, there are two types of routing protocols for ad hoc networks, proactive routing and reactive routing. Proactive routing protocols attempt to maintain consistent, up-to-date routing information from each node to every other node in the network. Such protocols are termed proactive because they store route information even before it is needed. Proactive protocols suffer the disadvantage of additional control traffic that is needed to continually update stale routing entries. Some of the most popular proactive protocols are DSDV [2], WRP [3], OLSR [4] and FSR [5]. Reactive routing creates and maintains routes only when desired by the source node. Therefore, it's also known as on-demand, source-initiated, or demand-driven routing [6]. When a node requires a route to a destination, it initiates a route discovery process within the network, typically, by some form of flooding. This process is completed once a route is found or all possible route permutations have been examined. Once a route has been established, it is maintained by a route maintenance procedure until either the destination becomes inaccessible along every path from the source or until the route is no longer desired. Compared to proactive routing, reactive routing consumes far less bandwidth for maintaining the routing tables at each node when only a small subset of all available routes is in use at any time. Proposed reactive routing protocols include DSR [7][8][9], AODV [10][11], TORA [12][13], etc.. A review of ad hoc routing protocols is given in [6]. Our goal in this paper is to study the transient behavior of DSR protocol, one of the most popular ad hoc network routing protocols, when path failures occur. We present an improvement scheme termed the Failure Record to increase the packet delivery fraction and decrease the average end-to- end delay of data packets. The rest of the paper is organized as follows. Section 2 provides a brief overview of the DSR protocol and the manet simulation model used. In section 3, we study the transient behavior of the DSR and indicate the impact of failures on DSR. We present a scheme to improve the performance in section 4. Section 5 shows the simulation results. Conclusions and our future work are given in section 6. 2. DSR Overview and Simulation Model In the Dynamic Source Routing (DSR) [7][8][9] protocol, to send a packet to another node, the sender constructs a source route in the packet's header, giving the address of each node in the network through which the packet should be forwarded to reach the destination. The sender then transmits the packet to the first hop identified in the source route. When a node receives a packet, if this node is not the final destination of the packet, it simply transmits the packet to the next hop identified in the source route in the packet's header. DSR maintains a route cache in each node to contain the source routes the node is aware of. The protocol consists of two major phases: route discovery and route maintenance. When a node has a packet to send and has no valid route in the route cache, it initiates route discovery by broadcasting a route request packet. The intermediate nodes add their own address to the route record of the packet until the packet reaches the destination or an intermediate node that has a valid route to the destination in its route cache. Typically, the first copy of the route request packet reaching the destination contains the shortest route. A route reply is generated and sent back to the initiator via the reversed route in the route record if symmetric links are supported, or a new route discovered in a similar way. In route maintenance, acknowledgements are used to verify the correct operation of the route links and route error