Bucking FreeRiders: Distributed Accounting and Settlement in Peer-toPeer Networks

The practice of free-riding-consuming service with-out providing equivalent service in return-is a well-documented problem in contemporary peer-to-peer (P2P) networks. For example, recent measurement studies of the Napster and Gnutella file-sharing sys-tems reveal that the majority of files are provided by only 7% of hosts, while the majority of requests are generated by the remaining 93% [8, 1]. This "tragedy of the commons" effect is driven by the combination of individual self-interest coupled with the absence of adequate incentives to encourage (or enforce) fair use. Unfortunately, concentrating service among a small number of hosts in this manner is not only unfair, but it also undermines the key advantages of service distribution, including scalability, availability and ro-bustness under attack. Existing approaches to the free-riding problem fall into two rough categories: reputation systems [4, 6, 7] and commerce systems [3]. The first approach relies on identifying greedy or misbehaving users after the fact (either automatically or manually) and then re-fusing service to hosts with "bad" reputations. In con-trast, the second approach is predicated on economic mechanisms that require each user to "purchase" ser-vice on demand, using a virtual currency that is ob-tained as payment for providing service in turn. While it is premature to reach strong conclusions about the limitations of each method, to a first approximation, reputation systems appear better suited to managing actively greedy users in a reactive fashion, while com-merce systems are a more natural approach for pro-actively implementing appropriate incentives among passively self-interested users. We believe this latter property represents a more pressing requirement for large-scale peer-to-peer systems and consequently it is the focus of this paper. There are two key components in any commerce-based resource management system: an accounting mechanism to securely store the currency held by in-dividual users and a settlement mechanism to fairly exchange currency for services. Since the integrity of