The practice of free-riding -- consuming service without providing equivalent service in return -- is a well-documented problem in contemporary peer-to-peer (P2P) networks that undermines the key advantages of service distribution, including scalability, availability and robust ness under attack. Existing approaches to the free-riding problem fall into two rough categories: reputation systems and commerce systems. To a first approximation, reputation systems appear better suited to managing actively greedy users in a reactive fashion, while commerce systems are more natural for pro-actively implementing appropriate incentives among passively self-interested users. We believe this l atter property represents a more pressing requirement for large-scale peer-to-peer systems and consequently focus on the commerce system approach. We propose in this work a design for the technical infrastructure necessary to implement a commerce-based resource sharing policy. There are two key components to any such infrastructure: an accounting mechanism to securely store the currency held by individual users and a settlement mechanism to fairly exchange currency for services. Traditionally, such systems have relied on the use of a single trusted third-party. Such a centralized solution, however, is at odds with the goals of P2P networks, which are by their nature highly distributed. Instead, we offer an alternative design that hinges on an accounting scheme based on the use of a distributed set of peers as accountants providing accounting functionality for each user in the system. We also propose several settlement protocols that leverage this accounting facility. Finally, we show that our design distributes accounting and settlement functions widely, providing powerful incentives without depending on a centralized trusted third-party to mediate each transaction.
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