Peer-to-peer

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 based network.

A server based network. (not peer-to-peer)

A peer-to-peer (or "P2P", or, rarely, "PtP") 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. Peer-to-peer 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 peer-to-peer 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 for a non peer-to-peer file transfer is an FTP server where the client and server programs are quite distinct, and the clients initiate the download/uploads and the servers react to and satisfy these requests.

The earliest peer-to-peer 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 peer-to-peer 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 peer-to-peer structure for others. Networks such as Gnutella or Freenet use a peer-to-peer structure for all purposes, and are sometimes referred to as true peer-to-peer networks, although Gnutella is greatly facilitated by directory servers that inform peers of the network addresses of other peers.

Peer-to-peer 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 peer to peer 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 License).

Classifications of peer-to-peer networks

Peer-to-peer networks can be classified by what they can be used for:

• file sharing
• telephony
• media streaming (radio, video)
• discussion forums

Other classification of peer-to-peer networks is according to their degree of centralization.

In 'pure' peer-to-peer 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

Some examples of pure peer-to-peer application layer networks designed for file sharing are Gnutella and Freenet.

There also exist countless hybrid peer-to-peer 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 addresses, which are referenced by a set of indices to obtain an absolute address.

e.g.

• 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, GreenTea and Shwup

Advantages of peer-to-peer networks

An important goal in peer-to-peer 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 peer-to-peer 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.

When the term peer-to-peer was used to describe the Napster network, it implied that the peer protocol was important, but, in reality, the great achievement of Napster was the empowerment of the peers (i.e., the fringes of the network) in association with a central index, which made it fast and efficient to locate available content. The peer protocol was just a common way to achieve this.

While the original Napster network was a P2P network, the newest version of Napster has no connection to P2P networking at all. The modern day version of Napster is a subscription based service which allows you to download music files legally.

Unstructured and structured P2P networks

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 signalling 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.

Legal controversy

Peer-to-peer technologies are rarely considered in and of themselves to be illegal.

However a frequent use of many peer-to-peer technologies is file sharing of copyright materials and this is very typically illegal (see File sharing and the law), unless a license exists that permits this (such as GPL or GFDL), or for materials that have entered the public domain.

Other uses of peer-to-peer such as telephony are not typically nearly so controversial, although provision of telephony is restricted in some legal jurisdictions around the world.

Public perception

According to a poll, 75% of young voters in Sweden (18-20) support filesharing, even if it is illegal when presented to the statement:

I think it is OK to download files from the Net, even if it is illegal

38% said they "adamantly agreed" while 39% said they "partly agreed".[1]

Computer science perspective

Technically, a completely pure peer-to-peer 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 peer-to-peer 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 peer-to-peer 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.

Peer-to-peer 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 Science

• Bioinformatics: Peer-to-peer 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 peer2peer 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 Academic: Due to the fast distribution and large storage space features, many organizations are trying to apply P2P network 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 topic on P2P network as part of its modern network war. In November, 2001, Colonel Robert Wardell from the Pentagon told a group of peer-to-peer 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 peer-to-peer 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 network. Due to security reasons, details are kept confidential.

• Business: P2P network has already been used in business areas, but it is still at the beginning line. 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-market place, Distributed Search engines, Groupware and Office Automation via P2P network. There are several reasons why companies prefer P2P sometimes such as: Real-time collaboration, a server cannot manage with increasing volume of contents, a process requires strong computing power, a process needs high-speed communications etc. At the same time, P2P is not fully used as it still confronts a lot of security issues.

• TV: One of the first applications of P2P in this area is Joost, which is expected to deliver (relay) near-TV resolution images.

• 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 built. It’s not very likely for companies to change all the cables. Many of them turn to use internet, more specifically, P2P network. For instance, Skype, one of the most widely used phone software is using P2P technology. Furthermore, many research organizations are trying to apply P2P network on cellular network.

Attacks on peer-to-peer networks

Many peer-to-peer networks are under constant attack by people with a variety of motives.

Examples include:

• poisoning attacks (e.g. providing files whose contents are different from the description)
• polluting attacks (e.g. inserting "bad" chunks/packets into an otherwise valid file on the network)
• freeloaders (Sometimes known as 'Leechers') (users or software that make use of the network without contributing resources to it)
• insertion of viruses to carried data (e.g. downloaded or carried files may be infected with viruses or other malware)
• malware in the peer-to-peer network software itself (e.g. distributed software may contain spyware)
• denial of service attacks (attacks that may make the network run very slowly or break completely)
• filtering (network operators may attempt to prevent peer-to-peer network data from being carried)
• identity attacks (e.g. tracking down the users of the network and harassing or legally attacking them)
• spamming (e.g. sending unsolicited information across the network- not necessarily as a denial of service attack)

Most attacks can be defeated or controlled by careful design of the peer-to-peer network and through the use of encryption. P2P network defense is in fact closely related to the "Byzantine Generals Problem". However, almost any network will fail when the majority of the peers are trying to damage it, and many protocols may be rendered impotent by far fewer numbers.

Security

Anonymity

Main article: Anonymous P2P

Some peer-to-peer protocols (such as Freenet) attempt to hide the identity of network users by passing all traffic through intermediate nodes.

Encryption

Some peer-to-peer 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 applications

Other types of peer-to-peer applications

• 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 protocols

Network or Protocol	Use	Applications
Ares	File sharing	Ares Galaxy, Warez P2P, Filecroc
BitTorrent	File sharing/Software distribution/Media distribution	ABC, AllPeers, Azureus, BitComet, BitLord, BitSpirit, BitTornado, BitTorrent, Burst!, Deluge FlashGet, G3 Torrent, Halite, KTorrent, Limewire, MLDonkey, Opera, QTorrent, rTorrent, Shareaza, TorrentFlux, Transmission, Tribler, µTorrent,Thunder
Buzm	Shared HTML wiki	a peer-to-peer wiki platform
CSpace	File sharing, text chat, remote desktop	a peer-to-peer based communications system
Direct Connect	File sharing	DC++, NeoModus Direct Connect, SababaDC, BCDC++, ApexDC++, StrongDC++
Domain Name System	Internet information retrieval	See Comparison of DNS server software
eDonkey	File sharing	aMule, eDonkey2000 (discontinued), eMule, eMule Plus, Hydranode, Jubster, lMule, Lphant, MLDonkey, Morpheus, Pruna, Shareaza, xMule, iMesh
FastTrack	File sharing	giFT, Grokster, iMesh (and its variants stripped of adware including iMesh Light), Kazaa (and its variants stripped of adware such as Kazaa Lite), KCeasy, Mammoth, MLDonkey, Poisoned
Freenet	Distributed data store	Entropy (on its own network), Freenet
GNUnet	File sharing, chat	GNUnet, (GNUnet-gtk)
Gnutella	File sharing	Acquisition, BearShare, Cabos, Gnucleus, Grokster, iMesh, gtk-gnutella, Kiwi Alpha, LimeWire, FrostWire, MLDonkey, Morpheus, Poisoned, Swapper, Shareaza, XoloX
Gnutella2	File sharing	Adagio, Caribou, Gnucleus, iMesh, Kiwi Alpha, MLDonkey, Morpheus, Shareaza, TrustyFiles
Kad Network	File sharing	aMule, eMule, MLDonkey
JXTA	Peer applications	WiredReach Platform, Collanos Workplace (Teamwork software)
Krawler	Social network	Krawler[x]
MANOLITO/MP2P	File sharing	Blubster, Piolet
MFPnet	File sharing	amiciPhone (no longer available)
Napster	File sharing	Napigator, Napster
NeoEdge	File sharing, peer applications	MostFun Game Player, NeoARM game delivery
OpenNap	File sharing	WinMX, Utatane, XNap, Lopster, WinLop, Napster
P2PTV	Video stream or file sharing	TVUPlayer, Joost, CoolStreaming, Cybersky-TV, TVants, PPLive, Kontiki, LiveStation
PDTP	Streaming media or file sharing	DistribuStream
Peercasting	Multicasting streams	PeerCast, IceShare, FreeCast, PeerStream, Rawflow
Retroshare		Retroshare serverless Filesharing with Chat Messenger
Tranche
Usenet	Distributed discussion	expressLoad. See list of news clients
Windows Peer-to-Peer		Advanced Networking Pack for Windows XP,Windows XP SP2, Windows Vista (This is a Windows component that provides a 'meta' peer-to-peer network that applications can piggyback)
WPNP	File sharing	WinMX

• Other networks: Abacast, Alliance, 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, Skype, Solipsis, soribada, Soulseek, SPIN, Swarmcast, WASTE, Winny

An earlier generation of peer-to-peer systems were called "metacomputing" or were classed as "middleware". These include: Legion, Globus

Multi-network applications

Main article: Comparison of file sharing applications

Applications	Network or Protocol	Operating systems	License
AMIGIFT	FastTrack, Ares, OpenFT, Gnutella, BitTorrent	AmigaOS	GPL / PD
aMule	eDonkey network, Kad network	Cross-platform	GPL
eMule	eDonkey network, Kad network	Windows	GPL
FileScope	eDonkey network, Gnutella, Gnutella2, OpenNAP	Cross-platform	GPL
giFT	eDonkey network, FastTrack, Gnutella	Cross-platform	GPL
Gnucleus	Gnutella, Gnutella2	Windows	GPL
iMesh	FastTrack, eDonkey network, Gnutella, Gnutella2	Windows
KCeasy	Ares, FastTrack, Gnutella, OpenFT	Windows	GPL
Kiwi Alpha	Gnutella, Gnutella2	Windows
MLDonkey	BitTorrent, Direct Connect, eDonkey network, FastTrack, Gnutella, Gnutella2, Kad Network, OpenNap, SoulSeek, HTTP/FTP	Cross-platform	GPL
Morpheus	NEO Network, Gnutella, Gnutella2, BitTorrent	Windows
Napshare	Key network, MUTE network	Linux, Windows	GPL
Shareaza	BitTorrent, eDonkey, Gnutella, Gnutella2	Windows	GPL
Vagaa	BitTorrent, eDonkey, Kad	Windows	Closed source
Zultrax	Gnutella, ZEPP	Windows
DC++	BCDC++	Windows

History

• 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

See also

• Ambient network
• Mobile VoIP
• Anonymous P2P
• Byzantine Fault Tolerance
• Client-server
• Comparison of P2P applications
• Compulsory licensing
• Computer cluster
• Distributed hash table
• File sharing
• Friend-to-friend (or F2F)
• Friend-to-friend with third party storage
• Grid computing
• Overlay network
• Private P2P
• Servent
• Swarm intelligence
• ad hoc network
• Peer-to-server-peer (or P2sp)

References

• 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):335–371, December 2004. doi:10.1145/1041680.1041681.

• 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, 18 November 2002.

• Antony Rowstron and Peter Druschel, Pastry: Scalable, Decentralized Object Location, and Routing for Large-Scale Peer-to-Peer Systems. In proceedings Middleware 2001 : IFIP/ACM International Conference on Distributed Systems Platforms. Heidelberg, Germany, November 12-16, 2001. Lecture Notes in Computer Science, Volume 2218, Jan 2001, Page 329.

• Andy Oram et al., Peer-to-Peer:Harnessing the Power of Disruptive Technologies, Oreilly 2001

• 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.

• I. Stoica, R. Morris, D. Karger, M. F. Kaashoek, and H. Balakrishnan. Chord: A scalable peer-to-peer lookup service for internet applications. In Proceedings of SIGCOMM 2001, August 2001.

• Ralf Steinmetz, Klaus Wehrle (Eds). Peer-to-Peer Systems and Applications. ISBN 3-540-29192-X, Lecture Notes in Computer Science, Volume 3485, Sep 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.