  A Mobile Adhoc Network (MANET) is a cooperative engagement of a collection of
mobile nodes without any centralized access point or infrastructure to
coordinate among the peers. The underlying concept of coordination among nodes
in a cooperative MANET has induced in them a vulnerability to attacks due to
issues like lack of fixed infrastructure, dynamically changing network
topology, cooperative algorithms, lack of centralized monitoring and management
point, and lack of a clear line of defense. We propose a semi-distributed
approach towards Reputation Based Intrusion Detection System (IDS) that
combines with the DSR routing 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 observation results of the system are neither spread globally
nor restricted locally. The system assigns maximum weightage to self
observation by nodes for updating any reputation values, 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 test runs of the system. 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.
