Adding incentives to file-sharing systems

Modern peer-to-peer file sharing systems rely heavily on the willingness of users to distribute files to others. A selfish user can choose to download a file and consume resources without uploading in return. This form of free-riding plagues all currently deployed peer-to-peer systems. We present a novel protocol for a BitTorrent-like system (i.e., one in which only one file is being shared) that strongly discourages users from downloading a file without sharing it. Our protocol requires very little computation, and can easily be implemented in today's peer-to-peer systems. It is resistant to all forms of manipulation, including the use of multiple free identities, and does not require any coordination among seeds. We analyze our protocol and show that if downloading peers are rational, a new system equilibrium is reached in which all peers upload at least some percentage of the file they are given.