Mobile ad hoc networks inherently have very different properties from conventional networks. A mobile ad hoc network (MANET) is a collection of mobile nodes that are self configuring (network can be run solely by the operation of the end-users), capable of communicating with each other, establishing and maintaining connections as needed. Nodes in MANET are both routers and terminals. These networks are dynamic in the sense that each node is free to join and leave the network in a nondeterministic way. These networks do not have a clearly defined physical boundary, and therefore, have no specific entry or exit point. Although MANET is a very promising technology, challenges are slowing its development and deployment. Nodes in ad hoc networks are in general limited in battery power, CPU and capacity. Hence, the transmission ranges of these devices are also limited and nodes have to rely on the neighboring nodes in the network to route the packet to its destination node. Ad hoc networks are sometimes referred to as multi-hop networks, where a hop is a direct link between two nodes. MANET has many important applications, including battlefield operations, emergency rescues, mobile conferencing, home and community networking, sensor dust and so forth.

A Mobile Adhoc Network (MANET) is a cooperative engagement of a collection of mobile nodes without any centralized access point. The underlying concept of coordination among nodes in a cooperative MANET has induced in them a vulnerability to attacks due to issues like dynamically changing network topology, cooperative algorithms and lack of centralized monitoring point. We propose a semi-distributed approach towards a reputation-based Intrusion Detection System (IDS) that combines with the Dynamic Source Routing (DSR) protocol for strengthening the defense of a MANET. Our system inherits the features of reputation from human behavior, hence making the IDS socially inspired. It has a semi-distributed architecture as the critical observations of the system are neither spread globally nor restricted locally. The system assigns maximum priority to self observation by nodes for updating any reputation parameters, thus avoiding the need of a trust relationship between nodes. Our system is also unique in the sense that it features the concepts of Redemption and Fading with a robust Path Manager and Monitor system. Simulation studies show that DSR fortified with our system outperforms normal DSR in terms of the packet delivery ratio and routing overhead even when up to half of nodes in the network behave as malicious. Various parameters introduced such as timing window size, reputation update values, congestion parameter and other thresholds have been optimized over several simulation runs. By combining the semi-distributed architecture and other design essentials like path manager, monitor module, redemption and fading concepts, our system proves to be robust enough to counter most common attacks in MANETs.

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