Systemic risk and user-level performance in private P2P communities

Many peer-to-peer communities, including private BitTorrent Communities that serve hundreds of thousands of users, utilizecredit-based or sharing ratio enforcement schemes to incentivize their members to contribute. In this paper, we analyze the performanceof such communities from both the system-level and the user-level perspectives. We show that both credit-based and sharing ratioenforcement policies can lead to system-wide “crunches” or “crashes” where the system seizes completely due to too little or to toomuch credit, respectively. We explore the conditions that lead to these system pathologies and present a theoretical model that predictsif a community will eventually crunch or crash. We apply this analysis to design an adaptive credit system that automatically adjustscredit policies to maintain sustainability. Given private communities that are sustainable, it has been demonstrated that they are greatlyoversupplied in terms of excessively high seeder-to-leecher ratios. We further analyze the user-level performance by studying theeffects of oversupply. We show that although achieving an increase in the average downloading speed, the phenomenon of oversupplyhas three undesired effects: long seeding times, low upload capacity utilizations, and an unfair playing field for late entrants into swarms.To alleviate these problems, we propose four different strategies, which have been inspired by ideas in social sciences and economics.We evaluate these strategies through simulations and demonstrate their positive effects.Accepted Author Manuscript Published online 5-12-2012Distributed System