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Enforcing fairness in P2P storage systems using asymmetric reciprocal exchanges

In P2P storage systems peers need to contribute some local storage resources in order to obtain a certain online and reliable storage capacity. To guarantee that the storage service works, P2P storage systems have to meet two main requirements. First, the storage system needs to maintain fairness among peers by ensuring that peers consuming more online storage capacity contribute more local storage resources. And second, to reduce redundancy costs and improve reliability, the storage system must incentivize low-available peers to improve their online availability. Traditionally, P2P storage systems achieved these two requirements by (i) using symmetric reciprocal exchanges between peers, and by (ii) allowing peers to selfishly select their set of storage partners. However, in this paper we show that these two mechanisms are suboptimal in terms of the overall storage resources contributed by all peers. To minimize this amount of contributed resources, we design a novel incentive mechanism based on asymmetric reciprocal exchanges between peers. Our mechanism incentivizes peers to select storage partners uniformly at random, and to establish asymmetric exchange relationships with them. These asymmetric exchange relationships allow low-available peers to compensate the increase of redundancy of high-available peers by giving them more storage capacity. We show that our solution reduces the overall amount of contributed resources as well as the resources contributed by each peer individually. Using real P2P availability traces, we show that our incentive mechanism can reduce the overall savings up to 60%, and individual savings from 2% up to 75%, depending on peers' availabilities.