An Analysis of Incentives to Contribute in Collaborative File Sharing Environments

Projects developing infrastructure for the pooling of distributed resources (data, storage, or computation) [1, 2] often assume that resource owners have committed their resources and that the chief task is to integrate and use them efficiently. Such projects frequently ignore the question of whether individual resource owners are willing to share their personal resources for the overall good of the community. However, experiences [3-6] with peer-to-peer (P2P) file sharing systems like Gnutella, Napster, and Kazaa suggest that users are not altruistic. In Gnutella, for example, 70% of all users do not share files, and 50% of all requests are satisfied by the top 1% sharing hosts. Thus, incentive mechanisms that motivate users to contribute resources may be critical to eventual success of such systems. Various approaches to incentives have been proposed, including pricing or micro-currency schemes [7] and so-called “soft incentive” or nonpricing schemes [8]. However, the effectiveness of these different schemes is not well understood. In this paper we take a step towards understanding the performance of incentive schemes by defining and applying an analytic model based on Schelling’s Multi-Person Prisoner’s Dilemma (MPD) [9]. We use both this framework and simulations to study the effectiveness of different schemes for encouraging sharing in distributed file sharing systems. We consider three such schemes: the soft-incentive, reputation-based Peer-Approved and Service-Quality, and the Token-Exchange pricing scheme. After introducing the MPD model (Section 2), we use it to explain the rational behavior of users in a P2P file sharing community without incentives (Section 3). We then analyze user behavior when Peer-Approved is used as the incentive mechanism and find that it is effective in incentivizing rational users to share more files (Section 4). We then shift to the use of simulations so as to look beyond the assumptions of the MPD model. We measure the effectiveness of the reputation-based soft-incentive Peer-Approved scheme and compare it with that of the Token-Exchange pricing scheme (in Section 5). We find that even simple soft-incentive schemes can motivate users of P2P file-sharing systems to increase contributions in a way that benefits all users, including themselves.