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This article may require cleanup to meet Wikipedia's quality standards.Please improve this article if you can. (March 2008)A peer-to-peer (or P2P) computer network uses diverse connectivity between participants in a network and the cumulative bandwidth of network participants rather than conventional centralized resources where a relatively low number of servers provide the core value to a service or application. P2P networks are typically used for connecting nodes via largely ad hoc connections. Such networks are useful for many purposes. Sharing content files (see file sharing) containing audio, video, data or anything in digital format is very common, and real time data, such as telephony traffic, is also passed using P2P technology.A pure P2P network does not have the notion of clients or servers but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client-server model where communication is usually to and from a central server. A typical example of a file transfer that is not P2P is an FTP server where the client and server programs are quite distinct: the clients initiate the download/uploads, and the servers react to and satisfy these requests.In contrast to the above discussed pure P2P network, an example of a distributed discussion system that also adopts a client-server model is the Usenet news server system, in which news servers communicate with one another to propagate Usenet news articles over the entire Usenet network. Particularly in the earlier days of Usenet, UUCP was used to extend even beyond the Internet. However, the news server system acted in a client-server form when individual users accessed a local news server to read and post articles. The same consideration applies to SMTP email in the sense that the core email relaying network of Mail transfer agents follows a P2P model while the periphery of e-mail clients and their direct connections is client-server. Tim Berners-Lee's vision for the World Wide Web, as evidenced by his WorldWideWeb editor/browser, was close to a P2P network in that it assumed each user of the web would be an active editor and contributor creating and linking content to form an interlinked "web" of links. This contrasts to the more broadcasting-like structure of the web as it has developed over the years.Some networks and channels such as Napster, OpenNAP and IRC server channels use a client-server structure for some tasks (e.g. searching) and a P2P structure for others. Networks such as Gnutella or Freenet use a P2P structure for all purposes, and are sometimes referred to as true P2P networks, although Gnutella is greatly facilitated by directory servers that inform peers of the network addresses of other peers.P2P architecture embodies one of the key technical concepts of the Internet, described in the first Internet Request for Comments, RFC 1, "Host Software" dated April 7, 1969. More recently, the concept has achieved recognition in the general public in the context of the absence of central indexing servers in architectures used for exchanging multimedia files.The concept of P2P is increasingly evolving to an expanded usage as the relational dynamic active in distributed networks, i.e. not just computer to computer, but human to human. Yochai Benkler has coined the term "commons-based peer production" to denote collaborative projects such as free software. Associated with peer production are the concept of peer governance (referring to the manner in which peer production projects are managed) and peer property (referring to the new type of licenses which recognize individual authorship but not exclusive property rights, such as the GNU General Public License and the Creative Commons licenses).// Classifications of P2P networksP2P networks can be classified by what they can be used for:file sharingtelephonymedia streaming (audio, video)discussion forumsOther classification of P2P networks is according to their degree of centralization.In 'pure' P2P networks:Peers act as equals, merging the roles of clients and serverThere is no central server managing the networkThere is no central routersome examples of pure P2P application layer networks designed for file sharing are Gnutella and Freenet.There also exist countless hybrid P2P systems:Has a central server that keeps information on peers and responds to requests for that information.Peers are responsible for hosting available resources (as the central server does not have them), for letting the central server know what resources they want to share, and for making its shareable resources available to peers that request it.Route terminals are used as addresses, which are referenced by a set of indices to obtain an absolute address.e.g.Centralized P2P network such as NapsterDecentralized P2P network such as KaZaAStructured P2P network such as CANUnstructured P2P network such as GnutellaHybrid P2P network (Centralized and Decentralized) such as JXTA (an open source P2P protocol specification) Advantages of P2P networksAn important goal in P2P networks is that all clients provide resources, including bandwidth, storage space, and computing power. Thus, as nodes arrive and demand on the system increases, the total capacity of the system also increases. This is not true of a client-server architecture with a fixed set of servers, in which adding more clients could mean slower data transfer for all users.The distributed nature of P2P networks also increases robustness in case of failures by replicating data over multiple peers, and -- in pure P2P systems -- by enabling peers to find the data without relying on a centralized index server. In the latter case, there is no single point of failure in the system. Unstructured and structured P2P networksThe P2P overlay network consists of all the participating peers as network nodes. There are links between any two nodes that know each other: i.e. if a participating peer knows the location of another peer in the P2P network, then there is a directed edge from the former node to the latter in the overlay network. Based on how the nodes in the overlay network are linked to each other, we can classify the P2P networks as unstructured or structured.An unstructured P2P network is formed when the overlay links are established arbitrarily. Such networks can be easily constructed as a new peer that wants to join the network can copy existing links of another node and then form its own links over time. In an unstructured P2P network, if a peer wants to find a desired piece of data in the network, the query has to be flooded through the network to find as many peers as possible that share the data. The main disadvantage with such networks is that the queries may not always be resolved. Popular content is likely to be available at several peers and any peer searching for it is likely to find the same thing. But if a peer is looking for rare data shared by only a few other peers, then it is highly unlikely that search will be successful. Since there is no correlation between a peer and the content managed by it, there is no guarantee that flooding will find a peer that has the desired data. Flooding also causes a high amount of signaling traffic in the network and hence such networks typically have very poor search efficiency. Most of the popular P2P networks such as Gnutella and FastTrack are unstructured.Structured P2P network employ a globally consistent protocol to ensure that any node can efficiently route a search to some peer that has the desired file, even if the file is extremely rare. Such a guarantee necessitates a more structured pattern of overlay links. By far the most common type of structured P2P network is the distributed hash table (DHT), in which a variant of consistent hashing is used to assign ownership of each file to a particular peer, in a way analogous to a traditional hash table's assignment of each key to a particular array slot. Some well known DHTs are Chord, Pastry, Tapestry, CAN, and Tulip. Not a DHT-approach but a structured P2P network is HyperCuP. The Chord protocolThe Chord protocol is one solution for connecting the peers of a P2P network together. Chord consistently maps a key onto a node. Both keys and nodes are assigned an m-bit identifier. This identifier is a hash of the node's IP address. A key's identifier is a hash of the key. There are many other algorithms in use by P2P, but this is a simple and common approach.A ring with positions numbered 0 to 2^(m-1) Key k is assigned to node successor(k), which is the node whose identifier is equal to or follows the identifier of k. If there are N nodes and K keys, then each node is responsible for roughly K / N keys.When the (N+1)st node joins or leaves the network, responsibility for O(K/N) keys changes hands. Each node knows the IP address of its successor. If each node knows the location of its successor, you can perform linear search over the network for a particular key. This is a naive method for searching the network.A faster searched will require each node to keep a "finger table" containing up to m entries. The i(th) entry of node n will contain the address of successor(n + 2i). The i(th) entry of node n will contain the address of successor(n + 2i)The number of node that must be contacted to find a successor in an N-node network is O(log n). Proof: Assume node n wants to resolve a query for key k. Let p be the node that contains k. We will analyze the number of steps to reach p. Let i be such that p is in . Node n will contact the smallest node in this interval; call this node f. Fact: f is closer to p than to n. Therefore, in one step, the distance to p decreases by at least half. US legal controversyIn Sony Corp. v. Universal Studios, 464 U.S. 417 (1984), the Supreme Court found that Sony's new product, the Betamax, did not subject Sony to secondary copyright liability
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