Several cooperation enforcement schemes based on rewarding mechanisms such as electronic cash or online credits have lately been proposed to prevent selfish behavior in ad-hoc networks. However, these schemes suffer from the lack of fairness guarantees or the reliance on costly mechanisms such as tamper- proof hardware or the requirement for trusted third parties (TTPs) that are not suitable for ad-hoc networks. In this paper, we present a new cooperation-enforcement scheme that is perfectly suitable for ad-hoc delay-tolerant networks. The protocol is based on a simple technique called hot- potato forwarding whereby in order to receive a packet, potential recipients must first deliver an advance reward to the sender prior to the transmission of the packet. Thanks to this technique cooperation among nodes becomes mandatory and poisoning attacks and cheating actions are inherently prevented. The second contribution in our scheme is an optimistic fair exchange protocol that solves the fairness problem that is inherent to peer rewarding schemes. The protocol achieves total fairness with the help of a TTP and is optimistic in that the TTP is only involved in case of conflict between peer nodes. Correct execution of the protocol does not require any access to the TTP, so fairness is achieved without any impact on well-behaving nodes. The fairness of the protocol is validated through the exhaustive analysis of all possible protocol traces.
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