Free-Riding and Whitewashing in Peer-to-Peer Systems.pdfVIP

Free-Riding and Whitewashing in Peer-to-Peer Systems Michal Feldman Christos Papadimitriou
John Chuang Ion Stoica
School of Information Management and Systems, UC Berkeley
Computer Science Division, UC Berkeley 3rd Annual Workshop on Economics and Information Security (WEIS2004) May 6, 2004 Abstract – We devise a simple model to study the phenomenon of fr ee-riding and the effect of fr ee identities on user behavior in peer-to-peer systems. At the heart of our model is a strategic user of a certain type, an intrinsic and private parameter that reflects the user’s generosity. The user decides whether to contribute or free-ride based on how the current burden of contributing in the system compares to her type. We derive the emerging cooperation level in equilibrium and quantify the effect of providing free-riders with degraded service on the emerging cooperation. We find that this penalty mechanism is beneficial mostly when the “generosity level” of the society (i.e., the average type) is low. To quantify the social cost of free identities, we extend the model to account for dynamic scenarios with turnover (users joining and leaving) and with whitewashers: users who strategically leave the system and re-join with a new identity. We find that the imposition of penalty on all legitimate newcomers incurs a significant social loss only under high turnover rates in conjunction with intermediate societal generosity levels. 1 Introduction Why isfr ee-riding so widespread among users ofpeer -to-peer (P2P) systems? How costly is it in terms of theperfor - mance of the system? What mechanisms discouragefr ee-riding? What is the social cost of cheap pseudonyms? What is the tradeoff between tolerating whitewas