Dealing with Cheaters in Anonymous Peer-to-Peer Networks

As anonymous peer-to-peer file sharing networks transition from intellectual curiosity to societal reality, their long-term viability is seriously threatened by cheaters. A cheater either consumes resources without producing them (a freeloader), or advertises valuable content, but ultimately delivers that which is useless (a spoofer). In both cases, the cheater realizes some benefit from his actions without having to pay a commensurate cost. Because these networks are anonymous, the traditional accountability mechanisms developed for classic distributed systems do not apply. In this paper we present a protocol that dramatically reduces and in many cases eliminates the benefit gained by cheaters in anonymous peer-to-peer file sharing networks. Our protocol is based on the notion of exchange: instead of allowing users to unidirectionally download content, in order to acquire a file, a user must simultaneously provide a file to somebody else. We organize users into “exchange groups,” in which each user provides one file in order to acquire one file, with the aggregate exchange satisfying all participants. Through exposition we show that our composite protocol works well in theory, eliminating the incentive to freeload and forcing spoofers to spend resources commensurate with the damage they cause. Through tracedriven simulation, we show that it works well in practice, resulting in a system in which users can acquire the content they want with reasonable delay.