Not to be confused with point-to-point. This article is about peer-to-peer computing. For other uses see Peer-to-peer (disambiguation). This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2010) This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. The talk page may contain suggestions. (June 2009) A peer-to-peer system of nodes without central infrastructure. Centralized server-based service model.

Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged equipotent participants in the application. They are said to form a peer-to-peer network of nodes.

Peers make a portion of their resources such as processing power disk storage or network bandwidth directly available to other network participants without the need for central coordination by servers or stable hosts.1 Peers are both suppliers and consumers of resources in contrast to the traditional clientserver model where only servers supply and clients consume.

The peer-to-peer application structure was popularized by file sharing systems like Napster. The concept has inspired new structures and philosophies in many areas of human interaction. Peer-to-peer networking is not restricted to technology but covers also social processes with a peer-to-peer dynamic. In such context social peer-to-peer processes are currently emerging throughout society. Contents 1 Architecture of P2P systems 1.1 Structured systems 1.1.1 Distributed hash tables 1.2 Unstructured systems 1.3 Indexing and resource discovery 2 Peer-to-peer-like systems 3 Advantages and weaknesses 4 Social and economic impact 5 Applications 5.1 Content delivery 5.2 Peer-to-peer exchange of physical goods servicesor space 5.3 Networking 5.4 Science 5.5 Search 5.6 Communications networks 5.7 General 5.8 Miscellaneous 6 Historical perspective 7 Network neutrality controversy 8 See also 9 References 10 External links Architecture of P2P systems

Peer-to-peer systems often implement an abstract overlay network built at Application Layer on top of the native or physical network topology. Such overlays are used for indexing and peer discovery and make the P2P system independent from the physical network topology. Content is typically exchanged directly over the underlying Internet Protocol (IP) network. Anonymous peer-to-peer systems are an exception and implement extra routing layers to obscure the identity of the source or destination of queries.

In structured peer-to-peer networks peers (and sometimes resources) are organized following specific criteria and algorithms which lead to overlays with specific topologies and properties. They typically use distributed hash table-based (DHT) indexing such as in the Chord system (MIT).2

Unstructured peer-to-peer networks do not provide any algorithm for organization or optimization of network connections.citation needed. In particular three models of unstructured architecture can be distinguished: In pure peer-to-peer systems the entire network consists solely of equipotent peers. There is only one routing layer as there are no preferred nodes with any special infrastructure function. Hybrid peer-to-peer systems allow such infrastructure nodes to exist often called supernodes.3 In centralized peer-to-peer systems a central server is used for indexing functions and to bootstrap the entire system. Although this has similarities with a structured architecture the connections between peers are not determined by any algorithm.

The first prominent and popular peer-to-peer file sharing system Napster was an example of the centralized model.4 Freenet and early implementations of the gnutella protocol on the other hand are examples of the decentralized model. Modern gnutella implementations Gnutella2 as well as the now deprecated Kazaa network are examples of the hybrid model.

P2P networks are typically used for connecting nodes via largely ad hoc connections.citation needed Data including digital formats such as audio files and real time data such as telephony traffic is 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 clientserver model where communication is usually to and from a central server. A typical example of a file transfer that does not use the P2P model is the File Transfer Protocol (FTP) service in which the client and server programs are distinct: the clients initiate the transfer and the servers satisfy these requests. The 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. Structured systems Structured P2P networks 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. Distributed hash tables Main article: Distributed hash table Distributed hash tables Distributed hash tables (DHTs) are a class of decentralized distributed systems that provide a lookup service similar to a hash table: (key value) pairs are stored in the DHT and any participating node can efficiently retrieve the value associated with a given key. Responsibility for maintaining the mapping from keys to values is distributed among the nodes in such a way that a change in the set of participants causes a minimal amount of disruption. This allows DHTs to scale to extremely large numbers of nodes and to handle continual node arrivals departures and failures. DHTs form an infrastructure that can be used to build peer-to-peer networks. Notable distributed networks that use DHTs include BitTorrent's distributed tracker the Kad network the Storm botnet YaCy and the Coral Content Distribution Network. Some prominent research projects include the Chord project the PAST storage utility the P-Grid a self-organized and emerging overlay network and the CoopNet content distribution system (see below for external links related to these projects). DHT-based networks have been widely utilized for accomplishing efficient resource discovery56 for grid computing systems as it aids in resource management and scheduling of applications. Resource discovery activity involves searching for the appropriate resource types that match the users application requirements. Recent advances in the domain of decentralized resource discovery have been based on extending the existing DHTs with the capability of multi-dimensional data organization and query routing. Majority of the efforts have looked at embedding spatial database indices such as the Space Filling Curves (SFCs) including the Hilbert curves Z-curves k-d tree MX-CIF Quad tree and R*-tree for managing routing and indexing of complex Grid resource query objects over DHT networks. Spatial indices are well suited for handling the complexity of Grid resource queries. Although some spatial indices can have issues as regards to routing load-balance in case of a skewed data set all the spatial indices are more scalable in terms of the number of hops traversed and messages generated while searching and routing Grid resource queries. Unstructured systems 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. Many of the popular P2P networks are unstructured. In pure P2P networks: Peers act as equals merging the roles of clients and server. In such networks there is no central server managing the network neither is there a central router. Some examples of pure P2P Application Layer networks designed for peer-to-peer file sharing are gnutella (pre v0.4) and Freenet. There also exist hybrid P2P systems which distribute their clients into two groups: client nodes and overlay nodes. Typically each client is able to act according to the momentary need of the network and can become part of the respective overlay network used to coordinate the P2P structure. This division between normal and 'better' nodes is done in order to address the scaling problems on early pure P2P networks. As examples for such networks can be named modern implementations of gnutella (after v0.4) and Gnutella2. Another type of hybrid P2P network are networks using on the one hand central server(s) or bootstrapping mechanisms on the other hand P2P for their data transfers. These networks are in general called 'centralized networks' because of their lack of ability to work without their central server(s). An example for such a network is the eDonkey network (often also called eD2k). Indexing and resource discovery Older peer-to-peer networks duplicate resources across each node in the network configured to carry that type of information. This allows local searching but requires much traffic. Modern networks use central coordinating servers and directed search requests. Central servers are typically used for listing potential peers (Tor) coordinating their activities (Folding@home) and searching (Napster eMule). Decentralized searching was first done by flooding search requests out across peers. More efficient directed search strategies including supernodes and distributed hash tables are now used. Many P2P systems use stronger peers (super-peers super-nodes) as servers and client-peers are connected in a star-like fashion to a single super-peer. Peer-to-peer-like systems In modern definitions of peer-to-peer technology the term implies the general architectural concepts outlined in this article. However the basic concept of peer-to-peer computing was envisioned in earlier software systems and networking discussions reaching back to principles stated in the first Request for Comments RFC 1.7 A distributed messaging system that is often likened as an early peer-to-peer architecture is the USENET network news system that is in principle a clientserver model from the user or client perspective when they read or post news articles. However news servers communicate with one another as peers to propagate Usenet news articles over the entire group of network servers. The same consideration applies to SMTP email in the sense that the core email relaying network of Mail transfer agents has a peer-to-peer character while the periphery of e-mail clients and their direct connections is strictly a clientserver relationship. Tim Berners-Lee's vision for the World Wide Web as evidenced by his WorldWideWeb editor/browser was close to a peer-to-peer design 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 broadcasting-like structure of the web as it has developed over the years. Advantages and weaknesses In P2P networks clients provide resources which may include bandwidth storage space and computing power. As nodes arrive and demand on the system increases the total capacity of the system also increases. In contrast in a typical clientserver architecture clients share only their demands with the system but not their resources. In this case as more clients join the system fewer resources are available to serve each client. The decentralized nature of P2P networks also increases robustness because it removes the single point of failure that can be inherent in a client-server based system.8 As with most network systems unsecure and unsigned codes may allow remote access to files on a victim's computer or even compromise the entire network.citation needed In the past this has happened for example to the FastTrack network when anti P2P companies managed to introduce faked chunks into downloads and downloaded files (mostly MP3 files) were unusable afterwards or even contained malicious code.citation needed Consequently the P2P networks of today have seen an enormous increase of their security and file verification mechanisms. Modern hashing chunk verification and different encryption methods have made most networks resistant to almost any type of attack even when major parts of the respective network have been replaced by faked or nonfunctional hosts. Internet service providers (ISPs) have been known to throttle P2P file-sharing traffic due to the high-bandwidth usage.9 Compared to Web browsing e-mail or many other uses of the internet where data is only transferred in short intervals and relative small quantities P2P file-sharing often consists of relatively heavy bandwidth usage due to ongoing file transfers and swarm/network coordination packets. As a reaction to this bandwidth throttling several P2P applications started implementing protocol obfuscation such as the BitTorrent protocol encryption. Techniques for achieving "protocol obfuscation" involves removing otherwise easily identifiable properties of protocols such as deterministic byte sequences and packet sizes by making the data look as if it were random.10 A possible solution to this is called P2P caching where a ISP stores the part of files most accessed by P2P clients in order to save access to the Internet. Social and economic impact Main article: Peer-to-peer (meme) 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 and open source software and Wikipedia. Associated with peer production are the concepts of: peer governance (referring to the manner in which peer production projects are managed) 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) peer distribution (or the manner in which products particularly peer-produced products are distributed) Some researchers have explored the benefits of enabling virtual communities to self-organize and introduce incentives for resource sharing and cooperation arguing that the social aspect missing from today's peer-to-peer systems should be seen both as a goal and a means for self-organized virtual communities to be built and fostered.11 Ongoing research efforts for designing effective incentive mechanisms in P2P systems based on principles from game theory are beginning to take on a more psychological and information-processing direction. Applications There are numerous applications of peer-to-peer networks. The most commonly known is for content distribution Content delivery Many file sharing networks such as gnutella G2 and the eDonkey network popularized peer-to-peer technologies. From 2004 on such networks form the largest contributor of network traffic on the Internet. Peer-to-peer content delivery networks (P2P-CDN) (Giraffic Kontiki Ignite RedSwoosh). Peer-to-peer content services e.g. caches for improved performance such as Correli Caches12 Software publication and distribution (Linux several games); via file sharing networks. Streaming media. P2PTV and PDTP. Applications include TVUPlayer Joost CoolStreaming Cybersky-TV PPLive LiveStation and Didiom. Spotify uses a peer-to-peer network along with streaming servers to stream music to its desktop music player. Peercasting for multicasting streams. See PeerCast IceShare FreeCast Rawflow Pennsylvania State University MIT and Simon Fraser University are carrying on a project called LionShare designed for facilitating file sharing among educational institutions globally. Osiris (Serverless Portal System) allows its users to create anonymous and autonomous web portals distributed via P2P network. Peer-to-peer exchange of physical goods servicesor space Peer-to-peer renting/sharing web platforms (ex: http://www.rentalic.com ) enable people to find and reserve goods services or space on the virtual platform but carry out the actual P2P transaction in the physical world. Networking Domain Name System for Internet information retrieval. See Comparison of DNS server software cloud computing Dalesa a peer-to-peer web cache for LANs (based on IP multicasting). Science In bioinformatics drug candidate identification. The first such program was begun in 2001 the Centre for Computational Drug Discovery at the University of Oxford in cooperation with the National Foundation for Cancer Research. There are now several similar programs running under the United Devices Cancer Research Project. The sciencenet P2P search engine. BOINC Search YaCy a free distributed search engine built on principles of peer-to-peer networks. Communications networks Skype one of the most widely used internet phone applications is using P2P technology. VoIP (using application layer protocols such as SIP) Instant messaging and online chat Completely decentralized networks of peers: Usenet (1979) and WWIVnet (1987). General Research like the Chord project the PAST storage utility the P-Grid and the CoopNet content distribution system. JXTA for Peer applications. See Collanos Workplace (Teamwork software) Sixearch Miscellaneous The U.S. Department of Defense has started research on P2P networks as part of its modern network warfare strategy.13 In May 2003 Dr. Tether. Director of Defense Advanced Research Project Agency testified that U.S. Military is using P2P networks. Kato et al.s studies indicate over 200 companies with approximately $400 million USD are investing in P2P network. Besides File Sharing companies are also interested in Distributing Computing Content Distribution. Wireless community network Netsukuku An earlier generation of peer-to-peer systems were called "metacomputing" or were classed as "middleware". These include: Legion Globus Bitcoin is a peer-to-peer based digital currency. Historical perspective Tim Berners-Lee's vision for the World Wide Web 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.citation needed This contrasts to the current broadcasting-like structure of the web.citation needed Some networks and channels such as Napster OpenNAP and IRC serving channels use a clientserver structure for some tasks (e.g. searching) and a P2P structure for others. Networks such as gnutella or Freenet use a P2P structure for nearly all tasks with the exception of finding peers to connect to when first setting up. 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. Network neutrality controversy Peer-to-peer applications present one of the core issues in the network neutrality controversy. In October 2007 Comcast one of the largest broadband Internet providers in the USA started blocking P2P applications such as BitTorrent. Their rationale was that P2P is mostly used to share illegal content and their infrastructure is not designed for continuous high-bandwidth traffic. Critics point out that P2P networking has legitimate uses and that this is another way that large providers are trying to control use and content on the Internet and direct people towards a client-server-based application architecture. The client-server model provides financial barriers-to-entry to small publishers and individuals and is quite inefficient for sharing large files.citation needed See also Clientserver model Decentralized computing Friend-to-friend Peercasting Segmented downloading Semantic P2P networks Wireless ad hoc network Dead Drop (USB) References Rdiger Schollmeier A Definition of Peer-to-Peer Networking for the Classification of Peer-to-Peer Architectures and Applications Proceedings of the First International Conference on Peer-to-Peer Computing IEEE (2002). Kelaskar M.; Matossian V.; Mehra P.; Paul D.; Parashar M. (2002) A Study of Discovery Mechanisms for Peer-to-Peer Application http://portal.acm.org/citation.cfmid873218  Beverly Yang and Hector Garcia-Molina Designing a super-peer network Proceedings of the 19th International Conference on Data Engineering (2003). Napster - the first prominent example of a centralized P2P system Ranjan Rajiv; Harwood Aaron; Buyya Rajkumar (1 December 2006) A Study on Peer-to-Peer Based Discovery of Grid Resource Information http://www.cs.mu.oz.au/%7Erranjan/pgrid.pdf  Ranjan Rajiv; Chan Lipo; Harwood Aaron; Karunasekera Shanika; Buyya Rajkumar. "Decentralised Resource Discovery Service for Large Scale Federated Grids" (PDF). http://gridbus.org/papers/DecentralisedDiscoveryGridFed-eScience2007.pdf.  RFC 1 Host Software S. Crocker IETF Working Group (April 7 1969) Lua Eng Keong; Crowcroft Jon; Pias Marcelo; Sharma Ravi; Lim Steven (2005). "A survey and comparison of peer-to-peer overlay network schemes". http://academic.research.microsoft.com/Publication/2633870/a-survey-and-comparison-of-peer-to-peer-overlay-network-schemes.  Janko Roettgers 5 Ways to Test Whether your ISP throttles P2P http://newteevee.com/2008/04/02/5-ways-to-test-if-your-isp-throttles-p2p/ Hjelmvik Erik; John Wolfgang (2010-07-27). "Breaking and Improving Protocol Obfuscation". http://www.iis.se/docs/hjelmvikbreaking.pdf.  Antoniadis P. & Le Grand B. (2007). Incentives for resource sharing in self-organized communities: From economics to social psychology. Digital Information Management 2007. ICDIM '07 Gareth Tyson Andreas Mauthe Sebastian Kaune Mu Mu and Thomas Plagemann. Corelli: A Dynamic Replication Service for Supporting Latency-Dependent Content in Community Networks. In Proc. 16th ACM/SPIE Multimedia Computing and Networking Conference (MMCN) San Jose CA (2009).1 "Walker Leslie. Uncle Sam Wants Napster! The Washington Post November 8 2001". 2001-11-08. http://www.washingtonpost.com/ac2/wp-dynpagenamearticle&nodewashtech/techthursday/columns/dotcom&contentIdA59099-2001Nov7. Retrieved 2010-05-22.  External links Wikimedia Commons has media related to: P2P Glossary of P2P terminology Foundation of Peer-to-Peer Computing Special Issue Elsevier Journal of Computer Communication (Ed) Javed I. Khan and Adam Wierzbicki Volume 31 Issue 2 February 2008 Ross J. Anderson. The eternity service. In Pragocrypt 1996 1996. Marling Engle & J. I. Khan. Vulnerabilities of P2P systems and a critical look at their solutions May 2006 Stephanos Androutsellis-Theotokis and Diomidis Spinellis. A survey of peer-to-peer content distribution technologies. ACM Computing Surveys 36(4):335371 December 2004. Biddle Peter Paul England Marcus Peinado and Bryan Willman The Darknet and the Future of Content Distribution. In 2002 ACM Workshop on Digital Rights Management November 2002. John F. Buford Heather Yu Eng Keong Lua P2P Networking and Applications. ISBN 30-12374-214-5 Morgan Kaufmann December 2008 Djamal-Eddine Meddour Mubashar Mushtaq and Toufik Ahmed Open Issues in P2P Multimedia Streaming in the proceedings of the 1st Multimedia Communications Workshop MULTICOMM 2006 held in conjunction with IEEE ICC 2006 pp 4348 June 2006 Istanbul Turkey. Detlef Schoder and Kai Fischbach Core Concepts in Peer-to-Peer (P2P) Networking. In: Subramanian R.; Goodman B. (eds.): P2P Computing: The Evolution of a Disruptive Technology Idea Group Inc Hershey. 2005 Ralf Steinmetz Klaus Wehrle (Eds). Peer-to-Peer Systems and Applications. ISBN 3-540-29192-X Lecture Notes in Computer Science Volume 3485 September 2005. Ramesh Subramanian and Brian Goodman (eds) Peer-to-Peer Computing: Evolution of a Disruptive Technology ISBN 1-59140-429-0 Idea Group Inc. Hershey PA USA 2005. Shuman Ghosemajumder. Advanced Peer-Based Technology Business Models. MIT Sloan School of Management 2002. Silverthorne Sean. Music Downloads: Pirates- or Customers. Harvard Business School Working Knowledge 2004.