Determining the peer resource contributions in a P2P contract

In this paper we study a scheme called P2P contract which explicitly specifies the resource contributions that are required from the peers. In particular, we consider a P2P file sharing system in which when a peer downloads the file it is required to serve the file to up to N other peers within a maximum period of time T. We study the behavior of this contribution scheme in both centralized and decentralized P2P networks. In a centralized architecture, new requests are forwarded to a central server which hands out the contract along with a list of peers from where the file can be downloaded. We show that a simple fixed contract (i.e., fixed values of N and T) is sufficient to create the required server capacity which adapts to the load. Furthermore, we show that T, the time part of the contract is a more important control parameter than N. In the case of a decentralized P2P architecture, each new request is broadcast to a certain neighborhood determined by the time-to-live (TTL) parameter. Each server receiving the request independently doles out a contract and the requesting peer chooses the one which is least constraining. If there are no servers in the neighborhood, the request fails. To achieve a good request success ratio, we propose an adaptive scheme to set the contracts without requiring global information. Through both analysis and simulation, we show that the proposed scheme adapts to the load and achieves low request failure rate with high server efficiency.