In this paper we address the issue of content availability in p2p file sharing systems. Content availability is a public good: the copying of a file by one peer does not prevent another peer also from copying it; but contributing files to the common pool is costly. The asymptotic analysis of certain public good models for p2p file sharing suggests that when the aim is to maximize social welfare, a fixed contribution scheme in terms of the number of files shared per unity of time can be asymptotically optimal as the number of participants n grows to infinity. However, the enforcement of such an incentive scheme is not straightforward in a realistic p2p system, where no trusted software or central entity accounting for peers' transactions can be assumed and peers are free to change their identity with no cost. We present a realistic version of the fixed contribution scheme, which does not require the use of system memory but relies only on the time peers are consuming resources to ensure that they contribute adequately. We describe the functionality that should be supported for enforcement and discuss the additional incentive issues that arise in this context, proposing some practical solutions to address them. We also formulate a suitable economic model to estimate the efficiency-loss of the proposed mechanism (compared to the one achieved using the theoretically optimal schemes under complete and incomplete information) and provide some insights for the correct tuning of its basic parameters. Our first results indicate that the proposed mechanism constitutes a good compromise between economic efficiency and implementability and should lead to some interesting and practical solutions for providing incentives for content availability in p2p systems.
ACM SIGecom Exchanges – Association for Computing Machinery
Published: Jul 1, 2005