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Top1. Introduction
A Mobile Ad Hoc Network (MANET) is an autonomous system of mobile nodes connected by wireless links to form a communication network. The challenge in the design of protocol architectures for MANETs is to provide a certain level of Quality of Service (QoS) in information transfer using the limited network resources. This design goal is further constrained by the unreliable physical channel, the mobility of the nodes, and the lack of infrastructure for network coordination. To meet the QoS requirements for real-time data communication, some coordination of the channel is needed, but centralized coordination is not feasible in MANETs. Furthermore, since a MANET is a dynamic and distributed entity, the optimal control of MANET should also be dynamic and adaptive.
In a MANET, there are no basic network devices, such as routers or access points. Data transfer among nodes is realized by means of multiple hops. Every node in MANET is not only serving as a single terminal but also acts as a router to establish a route. When a source node intends to transfer data to a destination node, packets are transferred through the intermediate nodes. Thus, searching for and quickly establishing a route from a source to a destination node is an important issue for MANETs. The currently available routing protocols are mainly categorized into proactive routing protocols and reactive routing protocols.
In a proactive routing protocol, every node proactively searches for routes to other nodes. The nodes, periodically, exchange routing messages, in order to ensure the correctness and freshness of the information in the routing table. DSDV (Destination Sequence Distance Vector) and OLSR (Optimized Link State Routing Protocol) are two popular proactive routing protocols for MANETs. Each node in a MANET is limited to a certain power and bandwidth, thus, continuous transmission of routing messages would lead to congestion of the network.
In a reactive routing protocol, a route is searched and established only when two nodes intend to transfer data; and therefore, it is also called an on-demand routing protocol, such as Ad hoc On-Demand Distance Vector (AODV) (Perkins et al., 2003) or Dynamic Source Routing(DSR) (Johnson et al., 2004). A source node generally broadcasts a route request message to the entire network by means of flooding, in order to search for and establish a route to the destination node. The multi-path extension to the well-defined single path routing protocol AODV is referred as Ad hoc On-demand Multi-path Distance Vector (AOMDV). This protocol computes multiple loop-free and link-disjoint paths. Loop freedom is guaranteed by using a notion of “advertised hop-count” in AOMDV.
Most of these routing protocols rely on cooperation between nodes due to the lack of a centralized administration and assume that all nodes are trustworthy and well-behaved. However, in a hostile environment, a malicious node can launch routing attacks to disrupt routing operations. Thus, MANETs often suffer from security attacks because of their specification such as open medium, dynamic topology, lack of central monitoring and management, cooperative algorithms and no clear defense mechanism.
One such active Interruption attack in MANET on-demand routing protocol is the black hole attack, in which, the malicious node waits for the neighbors to initiate a Route Request (RREQ) packet. As the node receives the RREQ packet, it will immediately send a false Route Reply (RREP) packet with a modified higher sequence number. So, the source node assumes that the malicious node is having the fresh route towards the destination. The source node ignores the RREP packet received from other nodes and begins to send the data packets through the malicious node. Thus, the malicious node takes all the routes towards itself. It does not allow forwarding any packet anywhere. This attack is called a black hole as it swallows all data packets (Dokurer, 2006).