Free riders under control through service differentiation in peer-to-peer systems

Trust is required in a file sharing peer-to-peer system to achieve better cooperation among peers. In reputation-based peer-to-peer systems, reputation is used to build trust among peers. In these systems, highly reputable peers will usually be selected to upload requested files, decreasing significantly malicious uploads in the system. However, these peers need to be motivated to upload files by increasing the benefits that they receive from the system. In addition, it is necessary to motivate free riders to contribute to the system by sharing files. Malicious peers must be forced to contribute positively by uploading authentic files instead of malicious ones. In this paper, the contribution behavior of the peer is used as a guideline for service differentiation. The new concept of availability is introduced for partially-decentralized peer-to-peer systems. Both availability and involvement of the peer are used to assess its contribution behavior. Simulation results confirm the ability of the proposed scheme to effectively identify both free riders and malicious peers and reduce the level of service provided to them. Simulation results also confirm that based on rational behavior, peers are motivated to increase their contribution to receive services. Moreover, using our scheme, peers must continuously participate, reducing significantly the so-called milking phenomenon