Peer-to-peer Systems Originally Invented for Music/data Sharing and Volunteer Computing, P2p Systems Now Enjoy Widespread Commercial and Non-commercial Use in Content Distribution, Iptv, and Ip Telephony

P EE R-TO-PEER (P2P) C OMPU TING has attracted significant interest in recent years, originally sparked by the release of three influential systems in 1999: the Napster music-sharing system, the Freenet anonymous data store, and the SETI@home volunteerbased scientific computing projects. Napster, for instance, allowed its users to download music directly from each other’s computers via the Internet. Because the bandwidth-intensive music downloads occurred directly between users’ computers, Napster avoided significant operating costs and was able to offer its service to millions of users for free. Though unresolved legal issues ultimately sealed Napster’s fate, the idea of cooperative resource sharing among peers found its way into many other applications. More than a decade later, P2P technology has gone far beyond music sharing, anonymous data storage, or scientific computing; it now enjoys significant research attention and increasingly widespread use in open software communities and industry alike. Scientists, companies, and open-software organizations use BitTorrent to distribute bulk data such as software updates, data sets, and media files to many nodes;5 commercial P2P software allows enterprises to distribute news and events to their employees and customers;29 millions of people use Skype to make video and phone calls;1 and hundreds of TV channels are available using live streaming applications such as PPLive,17 CoolStreaming,38 and the BBC’s iPlayer.4 The term P2P has been defined in different ways, so we should clarify what exactly we mean by a P2P system. For the purposes of this article, a P2P system is a distributed system with the following properties: High degree of decentralization. The peers implement both client and server functionality and most of the system’s state and tasks are dynamically allocated among the peers. There are few if any dedicated nodes with centralized state. As a result, the bulk of the computation, bandwidth, and storage needed to operate the system are contributed by participating nodes. Self-organization. Once a node is introduced into the system (typically by providing it with the IP address of a participating node and any necessary Peer-to-Peer Systems DOI:10.1145/1831407.1831427