Abstract: Consider a resource sharing system in peer-to-peer (P2P) networks where peers act as both suppliers and customers of resources. Each participant obtains the utility by exchanging its resources with its neighbors according to the preset rules. A series of recent work considered a market equilibrium mechanism and studied the robustness of such a protocol against the Sybil attack strategy, which is a kind of grave threat in P2P system. The concept of incentive ratio is applied to measure how much a participant could gain from the Sybil attack by splitting its identity and reconstructing its communication connections with others. Although Chen et al. (Incentive ratios of a proportional sharing mechanism in resource sharing. In:23rd Annual International Computing and Combinatorics Conference, 2017) proved the incentive ratio on cycle networks is bounded by 2 and 4, an open problem is left that is how to narrow the gap furthermore. In this paper, we improve the upper bound of incentive ratio on cycle networks to 3. This improvement comes from a better understanding of the market equilibrium mechanism and a novel analysis technique for the improvement in utility.

Key words: Game theory, Resource sharing, Market equilibrium mechanism, Incentive ratio, Sybil attack