DCast : Sustaining Collaboration despite Rational Collusion Technical Report TRA 2 / 11 , School of Computing , National University of Singapore

A key and well-known problem in large-scale collaborative distributed systems is how to incentivize the rational/selfish users so that they will properly collaborate. Within such context, this paper focuses on designing incentive mechanisms for overlay multicast systems. A key limitation shared by existing proposals on the problem is that they are no longer able to provide proper incentives and thus will collapse when rational users collude or launch sybil attacks. This work explicitly aims to properly sustain collaboration despite collusion and sybil attacks by rational users. To this end, we propose a new decentralized DCast multicast protocol that uses a novel mechanism with debt-links and circulating debts. We formally prove that the protocol offers a novel concept of safety-net guarantee: A user running the protocol will always obtain a reasonably good utility despite the deviation of any number of rational users that potentially collude or launch sybil attacks. Our prototyping as well as simulation demonstrates the feasibility and safety-net guarantee of our design in prac-