Département Réseau, Sécurité et Multimédia Rapport d'Activités 2008

Managing a Peer-to-Peer Storage System in a Selfish SocietyResearch Staff : Patrick Maillé, Laszlo TokaKeywords : Peer-to-peer, pricing, incentives, game theoryApplications : Network pricing and optimizationPartners & Funding : Institut TELECOM project DisPairSeIntroductionThe possibility of storing data online appearsas a value-adding service in the context of thesoaring “society of information”. Indeed,having access to the Internet becomes moreand more easy, with higher and higheravailable transmission rates in accessnetworks, which renders transfer timesreasonable, even for large files. Moreover,online storage systems are able to cope withdocument versioning, and to protect data notonly against user device failures but alsoagainst disk failures, through the use of datareplicates stored on different disks.For those reasons, many companies nowpropose a service of online data storage.However, while creating such a storage serviceimplies owning huge memory capacities andaffording the associated energy andwarehouse costs, one can imagine using thesmaller but numerous possible storage spacesof the service users themselves, as is done inpeer-to-peer file sharing systems. In such apeer-to-peer storage system, the participantsare at the same time the providers and theusers of the service. To work properly, a peerto-peerstorage network needs thatparticipants offer a sufficient part of their diskspace to the system, and remain online oftenenough. However, both of those requirementsimply costs (or at least constraints) forparticipants, who may be reluctant to devotesome of their storage capacity to the systeminstead of using it for their own needs.Note that the economic models developed forpeer-to-peer file sharing systems do not applyto peer-to-peer storage services: in file sharingsystems, the data offered by a peer canbenefit to all other peers; in that sense theresource offered to the system is a publicgood. On the contrary, in a peer-to-peerstorage system the memory space offered by apeer is a private good: it can be shared amongdifferent users but each part is then devotedto only one user. Therefore the economicimplications of those systems are necessarilydifferent.Eliciting participants to contributeWe consider two possible incentive schemes,by suggesting that either each peer's use ofthe service should be limited to hercontribution level (symmetric schemes), or thatstorage space be bought from and sold topeers by a system operator that seeks tomaximize profit.The figure presented next illustrates thequantity C i * that is exchanged by user i withdemand function d i and supply function s i inthe case of a symmetric scheme, and thequantity of storing capacity C i o (resp. C i s ) thathe offers (resp. buys) if an operator buys fromand sells storing capacity to participants withunit price p o (resp. p s ).RealizationIn this work, we consider that users behaveselfishly, i.e. are only sensitive to the quality ofservice they experience, regardless of theeffects of their actions on the other users. Theframework of noncooperative Game Theory [1]is therefore particularly well-suited to study theinteractions among peers.We therefore focus on the incentives that canbe introduced to make participants contributeto the system, i.e. the changes that can bebrought to the game to modify its outcomes.Which scheme to prefer?The main question addressed in this work iswhether it is socially better to impose asymmetric scheme or to let a profit-maximizing60 Pracom’s Annual Report 2008