  Empirical data shows that in the absence of incentives, a peer participating
in a Peer-to-Peer (P2P) network wishes to free-riding. Most solutions for
providing incentives in P2P networks are based on direct reciprocity, which are
not appropriate for most P2P multimedia sharing networks due to the unique
features exhibited by such networks: large populations of anonymous agents
interacting infrequently, asymmetric interests of peers, network errors, and
multiple concurrent transactions. In this paper, we design and rigorously
analyze a new family of incentive protocols that utilizes indirect reciprocity
which is based on the design of efficient social norms. In the proposed P2P
protocols, the social norms consist of a social strategy, which represents the
rule prescribing to the peers when they should or should not provide content to
other peers, and a reputation scheme, which rewards or punishes peers depending
on whether they comply or not with the social strategy. We first define the
concept of a sustainable social norm, under which no peer has an incentive to
deviate. We then formulate the problem of designing optimal social norms, which
selects the social norm that maximizes the network performance among all
sustainable social norms. Hence, we prove that it becomes in the self-interest
of peers to contribute their content to the network rather than to free-ride.
We also investigate the impact of various punishment schemes on the social
welfare as well as how should the optimal social norms be designed if
altruistic and malicious peers are active in the network. Our results show that
optimal social norms are capable of providing significant improvements in the
sharing efficiency of multimedia P2P networks.
