Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

Preventing Replay Attacks for Secure Routing in Ad Hoc Networks 141the route passing through this tunnel will appear to be the shortest and thusgets selected. Consequently, attacker nodes at either ends of the tunnel can drop,delay or modify packets. We also identify a new type of replay attack that canoccur on the AODV protocol - the RREQ Flooding attack. In this type, attackerscan generate extra route discoveries by taking advantage of the “expanding ring”propagation of RREQ’s in that not all nodes have the knowledge that the RREQhas been processed. If performed massively, these packets will result in a numberof unnecessary resource-consuming route discoveries.Our technique is based on strengthening the neighbor authentication mechanismby a simple extension to the AODV protocol in order to determine if thesource nodes of RREQ packets are really in the neighborhood. By measuring theRound Trip Time (RTT) between two nodes and comparing the RTT value withan adaptive threshold, we can choose to discard or process the received RREQ.The threshold is calculated by requiring special Hello packets being replied immediatelyinstead of periodically. Analysis of the technique indicates that itachieves security with little overhead.The rest of the paper is organized as follows. Section 2 provides an overviewof attacks on the AODV protocol and describes the two types of replay attacks.Section 3 surveys related work in this area. Section 4 describes the proposedapproach. Section 5 gives a probabilistic analysis of the technique to provide anestimation of the overhead. Section 6 provides a discussion of the proposal andconcluding remarks.2 Attacks on the AODV Protocol2.1 Overview of AODVBriefly, the AODV routing protocol works as follows [2]. A node broadcasts aRoute Request (RREQ) if it wants to communicate with another node and novalid route is found in its routing table. The RREQ has the latest sequencenumber of the originator,an RREQ ID (or broadcast id) to mark that it hasnot been processed, and the latest sequence number of the destination that theoriginator has in its routing table. Each intermediate node increments the hopcount field in RREQ by one and broadcasts this RREQ until the RREQ reachesthe destination or a node that has a higher destination sequence number than theone in the packet. Multiple replies (Route Replies - RREP’s) may be generatedand transmitted along the reverse path. Each intermediate node increments thehop count in RREP and updates its routing table if the RREP has a highersequence number of the destination or a shorter hop count. This continues untilthe RREP arrives at the originator.2.2 Known Attacks on AODVA variety of known attacks on the AODV protocol have been identified [4,7,11,12,15,16].