- For other uses of the term see Peer-to-peer (disambiguation)
- For peer-to-peer networks used for file sharing see File sharing
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 realtime 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.
The earliest P2P network in widespread use was the Usenet news server system, in which peers communicated 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 also 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 is a P2P network while the periphery of Mail user agents and their direct connections is client server.
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 7 April 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 networksEdit
P2P networks can be classified by what they can be used for:
- file sharing
- media streaming (audio, video)
- discussion forums
Other 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 server
- There is no central server managing the network
- There is no central router
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.
- Centralized P2P network such as Napster
- Decentralized P2P network such as KaZaA
- Structured P2P network such as CAN
- Unstructured P2P network such as Gnutella
- Hybrid P2P network (Centralized and Decentralized) such as JXTA (an open source P2P protocol specification)
Advantages of P2P networksEdit
An 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 networksEdit
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.
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.
US legal controversyEdit
In 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 because it was capable of substantial non-infringing uses. Decades later, this case became the jumping-off point for all peer-to-peer copyright infringement litigation.
The first peer-to-peer case was A&M Records v. Napster, 239 F.3d 1004 (9th Cir. 2001). In the Napster case, the 9th Circuit considered whether Napster was liable as a secondary infringer. First, the court considered whether Napster was contributorily liable for copyright infringement. To be found contributorily liable, Napster must have engaged in "personal conduct that encourages or assists the infringement."  The court found that Napster was contributorily liable for the copyright infringement of its end-users because it "knowingly encourages and assists the infringement of plaintiffs' copyrights." The court goes on to analyze whether Napster was vicariously liable for copyright infringement. The standard applied by the court is whether Napster "has the right and ability to supervise the infringing activity and also has a direct financial interest in such activities." The court found that Napster did receive a financial benefit, and had the right and ability to supervise the activity, meaning that the plaintiffs demonstrated a likelihood of success on the merits of their claim of vicarious infringement. The court denied all of Napster's defenses, including its claim of fair use.
The next major peer-to-peer case was MGM v. Grokster, 514 U.S. 913 (2005). In this case, the Supreme Court found that even if Grokster was capable of substantial non-infringing uses, which the Sony Court found was enough to relieve one of secondary copyright liability, Grokster was still secondarily liable because it induced its users to infringe. 
Around the world in 2006, an estimated five billion songs, equating to 38,000 years in music were swapped on peer-to-peer websites, while 509 million were purchased online .
Computer science perspectiveEdit
Technically, a completely pure P2P application must implement only peering protocols that do not recognize the concepts of "server" and "client". Such pure peer applications and networks are rare. Most networks and applications described as P2P actually contain or rely on some non-peer elements, such as DNS. Also, real world applications often use multiple protocols and act as client, server, and peer simultaneously, or over time. Completely decentralized networks of peers have been in use for many years: two examples are Usenet (1979) and FidoNet (1984).
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.
Sun added classes to the Java technology to speed the development of P2P applications quickly in the late 1990s so that developers could build decentralized real time chat applets and applications before Instant Messaging networks were popular. This effort is now being continued with the JXTA project.
P2P systems and applications have attracted a great deal of attention from computer science research; 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).
Application of P2P network outside computer scienceEdit
- Bioinformatics: P2P networks have also begun to attract attention from scientists in other disciplines, especially those that deal with large datasets such as bioinformatics. P2P networks can be used to run large programs designed to carry out tests to identify drug candidates. The first such program was begun in 2001 the Centre for Computational Drug Discovery at Oxford University in cooperation with the National Foundation for Cancer Research. There are now several similar programs running under the auspices of the United Devices Cancer Research Project. On a smaller scale, a self-administered program for computational biologists to run and compare various bioinformatics software is available from Chinook. Tranche is an open-source set of software tools for setting up and administrating a decentralized network. It was developed to solve the bioinformatics data sharing problem in a secure and scalable fashion.
- Academic Search engine: The sciencenet P2P search engine provides a free and open search engine for scientific knowledge. sciencenet is based on yacy technology. Universities / research institutes can download the free java software and contribute with their own peer(s) to the global network. Liebel-Lab @ Karlsruhe institute of technology KIT.
- Education and Academia: Due to the fast distribution and large storage space features, many organizations are trying to apply P2P networks for educational and academic purposes. For instance, Pennsylvania State University, MIT and Simon Fraser University are carrying on a project called LionShare designed for facilitating file sharing among educational institutions globally.
- Military: The U.S. Department of Defense has already started research on P2P networks as part of its modern network warfare strategy. In November, 2001, Colonel Robert Wardell from the Pentagon told a group of P2P software engineers at a tech conference in Washington, DC: "You have to empower the fringes if you are going to... be able to make decisions faster than the bad guy". Wardell indicated he was looking for P2P experts to join his engineering effort. In May, 2003 Dr. Tether. Director of Defense Advanced Research Project Agency testified that U.S. Military is using P2P networks. Due to security reasons, details are kept classified.
- Business: P2P networks have already been used in business areas, but it is still in the beginning stages. Currently, 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, e-marketplace, Distributed Search engines, Groupware and Office Automation via P2P networks. There are several reasons why companies prefer P2P sometimes, such as: Real-time collaboration--a server cannot scale well with increasing volume of content; a process which requires strong computing power; a process which needs high-speed communications, etc. At the same time, P2P is not fully used as it still faces a lot of security issues.
- TV: Quite a few applications available to delivery TV content over a P2P network (P2PTV)
- Telecommunication: Nowadays, people are not just satisfied with “can hear a person from another side of the earth”, instead, the demands of clearer voice in real-time are increasing globally. Just like the TV network, there are already cables in place, and it's not very likely for companies to change all the cables. Many of them turn to use the internet, more specifically P2P networks. For instance, Skype, one of the most widely used internet phone applications is using P2P technology. Furthermore, many research organizations are trying to apply P2P networks to cellular networks.
- Main article: Anonymous P2P
Some P2P protocols (such as Freenet) attempt to hide the identity of network users by passing all traffic through intermediate nodes.
Some P2P networks encrypt the traffic flows between peers.
This may help to:
- make it harder for an ISP to detect that peer-to-peer technology is being used (as some artificially limit bandwidth)
- hide the contents of the file from eavesdroppers
- impede efforts towards law enforcement or censorship of certain kinds of material
- authenticate users and prevent 'man in the middle' attacks on protocols
- aid in maintaining anonymity
Networks, protocols and applicationsEdit
Other types of peer-to-peer applicationsEdit
- File sharing (using application layer protocols as BitTorrent)
- VoIP (using application layer protocols as SIP)
- Streaming media
- Instant messaging
- Software publication and distribution
- Media publication and distribution (radio, video)
Networks and protocolsEdit
- Other networks: ANts P2P, Applejuice, Audiogalaxy, Avalanche, CAKE, Chord, The Circle, Coral, Dijjer, FileTopia, Groove, Hamachi, iFolder, konspire2b, Madster/Aimster, MUTE, OpenFT, P-Grid, IRC, JXTA, MojoNation, Mnet, Octoshape, Omemo, Overnet, Peersites, Perfect Dark, Scour, SharingZone, Skype, Solipsis, soribada, Soulseek, SPIN, Swarmcast, WASTE, Winny
- Main article: Comparison of file sharing applications
|Applications||Network or Protocol||Operating systems||License|
|aMule||eDonkey network, Kad network||Cross-platform||GPL|
|eMule||eDonkey network, Kad network||Windows||GPL|
|giFT||eDonkey network, FastTrack, Gnutella||Cross-platform||GPL|
|iMesh||FastTrack, eDonkey network, Gnutella, Gnutella2||Windows||Proprietary|
|KCeasy||Ares, FastTrack, Gnutella, OpenFT||Windows||GPL|
|Kiwi Alpha||Gnutella, Gnutella2||Windows||Proprietary|
|MLDonkey||BitTorrent, Direct Connect, eDonkey network, FastTrack, Gnutella, Gnutella2, Kad Network, OpenNap, SoulSeek, HTTP/FTP||Cross-platform||GPL|
|Morpheus||Gnutella, Gnutella2, BitTorrent||Windows||Proprietary|
|Shareaza||Gnutella, Gnutella2, eDonkey, BitTorrent, HTTP/FTP||Windows||GPL|
|Vagaa||BitTorrent, eDonkey, Kad||Windows||Proprietary|
- July, 1999: publication of Freenet protocol
- September, 1999: creation of Napster
- November, 1999: first release of Direct Connect client
- March 14, 2000: first release of Gnutella
- September 6, 2000: first release of eDonkey2000
- March, 2001: introduction of the FastTrack protocol
- April, 2001: design of the BitTorrent protocol
- May, 2001: first release of WinMX Peer Network Protocol
- July, 2001: shutdown of Napster
- November 6, 2001: first release of GNUnet
- March, 2002: publication of the Kademlia DHT
- November, 2002: start of the Gnutella2 project
- Ad hoc network
- Ambient network
- Anonymous P2P
- Byzantine Fault Tolerance
- Comparison of P2P applications
- Compulsory license
- Computer cluster
- Decentralized computing
- Distributed hash table
- File sharing
- Friend-to-friend (or F2F)
- Friend-to-friend with third party storage
- Grid computing
- Mobile VoIP
- Overlay network
- Private P2P
- Swarm intelligence
- 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
- Carsten Rossenhövel, Peer-to-Peer Filters: The Big Report. Internet Evolution, March 2008
- Stephanos Androutsellis-Theotokis and Diomidis Spinellis. A survey of peer-to-peer content distribution technologies. ACM Computing Surveys, 36(4):335–371, 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.
- 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.af:Eweknienetwerk
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