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

148 J. Zhen and S. Srinivasthe time of finding a route with i hops being verified is Ti =3t∗i+t∗(Y −i).Theexpectation E i of the time consumed in finding a route will be ∑ i=Yi=0 P i ∗ T i .Forexample, with X = 10% and Y = 6 we can have following table:Table 1. Verification Probabilitiesi 0 1 2 3 4 5 6P i 0.531 0.354 0.010 0.015 0.001 0.000 0.000T i 6t 8t 10t 12t 14t 16t 18tFrom the table above, we can calculate E i =7.2t. Such that when neighborverification probability is 10%, the average extra time for finding a new routewould be (7.203t − 6t)/6t = 20%. The fact that this overhead will be sharedamong the processing of M RREQ’s makes it not significant.6 Discussion and ConclusionsAdding more mechanisms always needs more protection. The question is: willthis mechanism pose new security risks to the routing protocol? Can verificationrequests (VEF REQ) be replayed to some other area in the network? We arguethat this is not possible because each verification request needs the knowledge ofthe shared key between two nodes. The verification packets between two nodescannot be applied to other pairs.Can RTT REP packets be forged by illegitimate nodes to fool the node tocalculate wrong RTT threshold? This will not occur because if we have authentictrustable neighbors from initial stage of the network and each round of calculationof RTT threshold is based on the replies from trusted neighbors, the authenticationof RTT REPs will be guaranteed and the impact from a particularnode will be balanced by other nodes after RTT averaging.Can RTT REQs be replayed to some other area in the network? When receivinga RTT REQ replayed by an illegitimate node, the node may add theillegitimate node to its neighbor list and send back a RTT REP. Since the costof sending a RTT REP is just trivial this will not impact the sending node much.The replayed RTT REP will be discarded simply because the source node is notin the trusted neighbor list. The nodes in trusted neighbor list are added afterthe verification process.What if a neighbor moves away while the node still has not sent anotherRTT REQ since there is an interval? In this case, regular connectivity maintenancewill discover the leaving of the neighbor and purge it out of its neighborlist.In summary, we proposed a solution to prevent two important types of replayattacks, namely, the wormhole attack and the RREQ flooding attack, on theAODV routing protocol. Our technique is based on strengthening the neighborauthentication mechanism by a simple extension to the protocol. Analysis of the