Fault-Tolerant Routing in Mobile Ad Hoc Networks

Mobile Ad Hoc Networks (MANETs) are characterized by the cooperative engagement of mobile nodes that constitute networks possessing continuously-changing infrastructures, the absence of centralized network managers, access points, fixed base stations, a backbone network for controlling the network management functions, and the absence of designated routers for making routing decisions.All the nodes in MANETs participate in the routing process by acting as routers for one another.However, for the transmission of data from one node to another, such networks normally require several hops because of the limited wireless transmission range associated with the operation of the mobile nodes [2,7,9].The above-mentioned characteristics of MANETs, particularly those arising due to the mobility of the nodes, and the continuously-changing network infrastructure, pose several challenges.Due to the continuously changing infrastructure, the routes that were once considered to be the "best" may no longer remain as the "best" at a later time instant.Therefore, one needs to continuously re-compute the routes, implying that in such networks, there is no permanent convergence to a fixed set of routes.Thus, any routing protocol that needs to operate in MANET network environments should take these issues into consideration [2].Designing routing protocols poses further challenges when one needs to design routing schemes in the presence of adversarial environments in MANET networks.This is the primary focus of this chapter.More specifically, we discuss fault-tolerant routing schemes when the network contains malfunctioning nodes.To motivate this, we observe that most existing MANET protocols were postulated considering scenarios in which all the mobile nodes in the ad hoc network function properly and in an idealistic manner.However, adversarial environments are common in MANET environments, and misbehaving nodes degrade the performance of these routing protocols [11].The need for fault-tolerant routing protocols was identified to address routing in adversarial environments in the presence of faulty nodes by exploring redundancies in the networks [10,11].Despite the challenges that we mention above, it is worthwhile to note a few applications of MANETs which have made them popular.One of the popular application domains of www.intechopen.comMobile Ad-Hoc Networks: Protocol Design 324 MANETs is communications in moving battlefields [7].Other applications may be found in rural regions where building up fixed wired or wireless infrastructures can be costly and/or difficult.Although our primary discussion centers around fault-tolerant routing in MANETs, since this chapter is intended to be of a survey nature, we shall first briefly include an overview of the field and the corresponding routing protocols. Routing protocols for MANETsRouting in MANETs is currently a challenging and interesting problem studied by the community primarily due to the dynamic nature of the infrastructure present in MANETs, e.g., due to nodes joining and leaving the network.For routing, the transmission of data from one node to another is direct, if the source and destination nodes are neighbors, i.e., if they are within the wireless range of each other.On the other hand, the transmission is indirect, if the source and destination nodes are not within their range of operation [7].In such a case, routing is achieved through a series of multiple hops, with intermediate nodes between the source and the destination nodes serving the purpose of routers for relaying the information in between.The dynamic nature of the topology of MANETs due to the constant migration of nodes renders routing considerations difficult.The following characteristics of MANETs make their routing further challenging [7]: 1.The terrain in which the mobile nodes operate in MANETs may pose to be hostile with hazardous conditions that can lead to the frequent failure of the nodes and their mutual links.2. The medium of transmission of information in MANETs is wireless.Wireless media are relatively unreliable, insecure, and quite susceptible to different kinds of errors and unwanted noise.3. MANETs operate with battery-powered nodes, which are normally low powered, and resource constrained.If the region of operation of the nodes is in a hostile terrain, the frequent recharging of the nodes may not always be feasible.Consequently, all routing algorithms should be energy-efficient, of low complexity, and should be capable of operating under limited bandwidth.The different types of errors that can occur in MANETs are the following [7]: 1. Transmission errors 2. Node failures 3. Link failures 4. Route breakages 5. Packet loss due to congested nodes/links.The currently available MANET routing protocols can be classified into two categories [7]: (i) Unipath routing protocols, and (ii) Multipath routing protocols, explained below. Unipath routing protocolsIn unipath routing protocols, the transmission of messages between a source-destination pair of nodes takes places through a unique path.All the unipath routing protocols may be classified to be either table-based or on-demand [7].Table-based protocols are characterized by their ability to maintain routing tables that store information about routes from one node in the network to the others.Obviously, this requires that the nodes in the network maintain the table up-to-date by exchanging routing information between the participating nodes.Although, in general, table-based protocols may be easy to implement, the major limitation www.intechopen.comFault-Tolerant Routing in Mobile Ad Hoc Networks 325 associated with these protocols is that due to the highly-mobile and dynamic nature of ad hoc networks, maintaining the routing information in these tables is a very challenging task [7].On-demand routing protocols, on the other hand, alleviate the above problems, and make routing more scalable to highly dynamic and large networks.As the name suggests, ondemand routing protocols are characterized by the computation of routes on an "asrequired" basis.In on-demand routing protocols, there is initially a route discovery phase in which a route is found between two nodes.The route discovery phase is normally followed by a route maintenance phase in which a broken link in a route is repaired, or a new route is found [7,9].Various unipath routing protocols have been proposed in the literature (e.g., [5,9]).Of these, the Ad Hoc On-Demand Distance Vector (AODV) routing protocol [9], and the Dynamic Source Routing (DSR) protocol [5] are the most popular ones.In the interest of completeness, we briefly discuss these protocols below, with sufficient details so as to introduce the context for the fault-tolerant routing problem discussed later in this chapter. 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