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ISBN: 978-972-8939-25-0 © 2010 IADISLet v( A,B)be the total value of the resources donated from peer A to peer B over the past history of thesystem, peer A calculates rA (B), the reputation of peer B , using the following Equation:rΑ{ 0,v( B,A) − v( A,B) log( v( Β,Α))}(Β) = max+(1)Using (1), a peer A calculates the reputation of peer B with the value of favors that A has receivedfrom B , decreased by the value of favors that B has received from A . Using a non-negative reputationfunction makes it possible to avoid prioritizing malicious ID-changing peers over collaborating peers whohave consumed more resources than they have contributed. The sub linear term log( v ( B,A)) has beenintroduced in the equation to distinguish between a malicious ID-changing peer who never donates anyresources and a collaborating peer B that has donated resources to A in the past, but consumed at least thesame amount of resources from A . Also, it is possible to identify a collaborator even if the collaborator hasconsumed more resources than it has donated, provided that it has donated enough in the past.3. IMPROVING TRUST IN PEER-TO-MANETWe adopt a variant of the Network of Favors to compute reputation scores, based on interaction among peersto identify and discourage rogue peers activities in a P2MAN system. We combine the NoF score mechanismwith a new blacklist feature, and the sending node selection mechanism of P2MAN to improve robustness,systematically avoiding that rogue peers receive content pieces from their neighbors. As previous works, weassume that if the system has some mechanism by which it can identify collaborators with sufficientaccuracy, and known collaborators get priority in resources, it pays to be a collaborator. As an expectedconsequence, nodes will change their strategy to collaborating and the system evolves to a state where thereare no rogue peers. Our approach works as follows.3.1 Adapting Network of Favors to P2MANSimilarly to the NoF, a P2MAN node assigns a score to each neighbor node it interacts with, and storeslocally the information, according to the favors it receives and gives. In P2MAN, transmitting a content pieceto a node that requests it is a favor. However, a P2MAN owner node must decide if it will reply contentrequests or not. This decision is based on its health. Health is a P2MAN metaphor which represents theintuitive notion of the availability of the combined node resources. For instance, as a wireless mobile node isconstrained by a limited power source, it must save energy, and so the transmissions are costly. A node witha full energy source is more suitable to transmit contents than a node with an exhausted power source.Let ρ ∈R: 0 ≤ ρ ≤1be the probability of which a P2MAN owner node A will share contents when requested,θ A be the health threshold of node A and rA(max)the maximum node reputation recorded in A . Wedefine ρ in the Equation (2), as follows.rA(B)ρ = min( 1,( θA+ θA.)) :0≤θ ≤ 1rA(max)(2)Hence, when a P2MAN node A is healthy, θ A will be 1. Otherwise, when A has exhausted all of itsresources, θ A will be 0. The second term in the Equation is to compare the reputation of a requesting nodewith the best reputation recorded in the node A . The aim of this term is to improve the probability of a goodcollaborator receiving pieces. Hence, when a newcomer (i.e., with reputation zero) requests a content for thenode A , the probability of node A replying depends only from the health threshold of A (i.e., θ A ). If at somepoint node A is fully healthy, it will share contents, even to newcomers. However, when the best collaborator(i.e., r A(max)) requests a content for the node A , the probability of the node A replying is twice of a222