Abstract.  Cooperative peer-to-peer (p2p) applications are designed to share the resources of participating computers for the common good of all users. However, users do not necessarily have an incentive to donate resources to the system if they can use the system's services for free. In this paper, we describe Scrivener, a fully decentralized system that ensures fair sharing of bandwidth in cooperative content distribution networks. We show how participating nodes, tracking only first-hand observed behavior of their peers, can detect when their peers are behaving selfishly and refuse to provide them service. Simulation results show that our mechanisms effectively limit the quality of service recevied by a user to a level that is proportional to the amount of resources contributed by that user, while incurring modest overhead.