Modelling Real P2P Networks: The Effect of Altruism

We develop a model of the interaction of rational peers in an incentive-free peer-to-peer (P2P) network and use game theoretic analysis to derive results about peer and network behavior. We calculate and discuss Nash equilibria and predict peer behavior in terms of individual contribution. At the heart of our model is altruism, an intrinsic parameter reflecting peers inherent willingness to contribute. Two different approaches for modelling altruistic behavior and its attendant benefit are introduced and discussed. We consider the cases of P2P networks of peers that (i) have homogeneous altruism levels or (ii) have heterogeneous altruism levels, but with known probability distributions. We find that, under the effects of altruism, a substantial fraction of peers will contribute when altruism levels are within certain intervals, even though no incentive mechanism is used. Our results corroborate empirical evidence of large P2P networks surviving or even flourishing without or with barely functioning incentive mechanisms.