The Orchard Algorithm: P2P Multicasting without Free-Riding

The main purpose of many current peer-to-peer (P2P) networks is off-line file sharing. However, a potentially very promising use of such networks is to share video streams (e.g., TV programs) in real time. In order to do so, the peers in a P2P network who are interested in the same video stream may employ application level multicasting (ALM). In existing P2P networks, peers may exhibit behaviour which is problematic for ALM: peers tend not to donate any resources (free-riding), and they arrive and depart at a high rate (churn). In this paper we propose the orchard algorithm for ALM of video streams in P2P systems, which deal with both these problems. By employing a technique called multiple description coding, we split a video stream into several substreams. Orchard creates a dynamic spanning tree for each of these substreams in such a way that in the re suiting forest, no peer has to forward more substreams than it receives. Our experiments show that orchard is capable of providing a good quality of service to every peer, even when peers join and leave the forest at a high rate