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 tamperproof
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 hotpotato
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.