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The BitTorrent logo

BitTorrent is a peer-to-peer file sharing protocol used for distributing large amounts of data. BitTorrent is one of the most common protocols for transferring large files, and by some estimates it accounted for approximately 35% of all traffic on the entire Internet in 2004.^[1]

BitTorrent protocol allows users to receive large amounts of data without putting the level of strain on their computers that would be needed for standard Internet hosting. A standard host's servers can easily be brought to a halt if extreme levels of simultaneous data flow are reached. The protocol works as an alternative data distribution method that makes even small computers with low bandwidth capable of participating in large data transfers.

First, a user playing the role of file provider makes a file (or group of files) available to the network. This first user's file is called a seed and its availability on the network allows other users, called peers, to connect and begin to download the seed file. As new peers connect to the network and request the same file, their computer receives a different piece of the data from the seed. Once multiple peers have multiple pieces of the seed, BitTorrent allows each to become a source for that portion of the file. The effect of this is to take on a small part of the task and relieve the initial user, distributing the file download task among the seed and many peers. With BitTorrent, no one computer needs to supply data in quantities which could jeopardize the task by overwhelming all resources, yet the same final result—each peer eventually receiving the entire file—is still reached.

After the file is successfully and completely downloaded by a given peer, the peer is able to shift roles and become an additional seed, helping the remaining peers to receive the entire file. The community of BitTorrent users frowns upon the practice of disconnecting from the network immediately upon success of a file download, and encourages remaining as another seed for as long as practical, which may be days.

This distributed nature of BitTorrent leads to a viral spreading of a file throughout peers. As more peers join the swarm, the likelihood of a successful download increases. Relative to standard Internet hosting, this provides a significant reduction in the original distributor's hardware and bandwidth resource costs. It also provides redundancy against system problems, reduces dependence on the original distributor and provides a source for the file which is generally temporary and therefore harder to trace than when provided by the enduring availability of a host in standard file distribution techniques.

Programmer Bram Cohen designed the protocol in April 2001 and released a first implementation on July 2, 2001.^[2] It is now maintained by Cohen's company BitTorrent, Inc. There are numerous BitTorrent clients available for a variety of computing platforms. According to isoHunt, the total amount of shared content is currently more than 1.7 petabytes.^[3]

[edit] Operation

In this animation, the coloured bars beneath all of the 7 clients in the upper region above represent individual pieces of the file. After the initial pieces transfer from the seed (large system at the bottom), the pieces are individually transferred from client to client. The original seeder only needs to send out one copy of the file for all the clients to receive a copy.

A BitTorrent client is any program that implements the BitTorrent protocol. Each client is capable of preparing, requesting, and transmitting any type of computer file over a network, using the protocol. A peer is any computer running an instance of a client.

To share a file or group of files, a peer first creates a small file called a "torrent" (e.g. MyFile.torrent). This file contains metadata about the files to be shared and about the tracker, the computer that coordinates the file distribution. Peers that want to download the file must first obtain a torrent file for it, and connect to the specified tracker, which tells them from which other peers to download the pieces of the file.

Though both ultimately transfer files over a network, a BitTorrent download differs from a classic full-file d trackers suddenly become downloadable by non-members, as they can connect to a seed via an open tracker.

[edit] Decentralized keyword search

Even with distributed trackers, a third party is still required to find a specific torrent. This is usually done in the form of a hyperlink from the website of the content owner or through indexing websites like The Pirate Bay or Torrentz.

The Tribler BitTorrent client is the first to incorporate decentralized search capabilities. With Tribler, users can find .torrent files that are hosted among other peers, instead of on a centralized index sites. It adds such an ability to the BitTorrent protocol using a gossip protocol, somewhat similar to the eXeem network which was shut down in 2005. The software includes the ability to recommend content as well. After a dozen downloads the Tribler software can roughly estimate the download taste of the user and recommend additional content.^[34]

In May 2007 Cornell University published a paper proposing a new approach to searching a peer-to-peer network for inexact strings^[35] which could replace the functionality of a central indexing site. A year later, the same team implemented the system as a plugin for Vuze called Cubit^[36] and published a follow-up paper reporting its success^[37].

[edit] GIT-Torrent

The GitTorrent Protocol (GTP)^[38] is an, as of 2008, alpha-version of a protocol designed for collaborative git repository distribution across the Internet.

[edit] Implementations

Because the BitTorrent specification is free to use and many clients are open source, BitTorrent clients have been created for all common operating systems using a variety of programming languages. The official BitTorrent client, uTorrent, Vuze, and BitComet are some of the most popular clients.^{[citation needed]}

Some clients, like Torrentflux and TorrentVolve, can be run directly from a server, allowing hosting companies to offer speeds unavailable to most users. Services such as TorrentRelay, Instant Torrents, and ImageShack can download files on BitTorrent for the user, allowing them to download the entire file by HTTP once it is finished.

The Opera web browser supports BitTorrent^[39], as does Wyzo. BitLet allows users to download Torrents directly from their browser using a Java applet.

An increasing number of hardware devices are being made to support BitTorrent. These include routers and NAS devices that use BitTorrent-capable firmware like OpenWrt or Openslug.

Proprietary versions of the protocol which implement DRM, encryption, and authentication are found within managed clients such as Pando.

[edit] Development

An as-yet (2 February 2008) unimplemented unofficial feature is Similarity Enhanced Transfer (SET), a technique for improving the speed at which peer-to-peer file sharing and content distribution systems can share data. SET, proposed by researchers Pucha, Andersen, and Kaminsky, works by spotting chunks of identical data in files that are an exact or near match to the one needed and transferring these data to the client if the 'exact' data are not present. Their experiments suggested that SET will help greatly with less popular files, but not as much for popular data, where many peers are already downloading it.^[40] Andersen believes that this technique could be immediately used by developers with the BitTorrent file sharing system.^[41]

The BitTorrent client μTorrent implemented the UDP Torrent Protocol, beginning with version 1.8.2. This protocol is believed to be more resilient against traffic analysis while offering more control over the connection between peers.

As of December 2008, BitTorrent, Inc. is working with Oversi on new Policy Discover Protocols that query the ISP for capabilities and network architecture information. Oversi's ISP hosted NetEnhancer box is designed to "improve peer selection" by helping peers find local nodes, improving download speeds while reducing the loads into and out of the ISP's network. ^[42]

[edit] Legal issues

There has been much controversy over the use of BitTorrent trackers. BitTorrent metafiles themselves do not store copyrighted data, hence BitTorrent itself is not illegal — it is the use of it to make unauthorized copies of copyrighted material that contravenes laws in many jurisdictions.

Various jurisdictions have pursued legal action against websites that host BitTorrent trackers. High-profile examples include the closing of Suprnova.org, Torrentspy, LokiTorrent, Demonoid, OiNK.cd and EliteTorrents.org. The Pirate Bay torrent website, formed by a Swedish group, is noted for the "legal" section of its website in which letters and replies on the subject of alleged copyright infringements are publicly displayed. On 31 May 2006, The Pirate Bay's servers in Sweden were raided by Swedish police on allegations by the MPAA of copyright infringement;^[43] however, the tracker was up and running again three days later.

HBO, in an effort to combat the distribution of its programming on BitTorrent networks, has sent cease and desist letters to the Internet Service Providers of BitTorrent users. Many users have reported receiving letters from their ISPs that threatened to cut off their Internet service if the alleged infringement continues.^[44] HBO, unlike the RIAA, has not been reported to have filed suit against anyone for sharing files as of April 2007. In 2005 HBO began "poisoning" torrents of its show Rome, by providing bad chunks of data to clients.^[45]

On 23 November 2005, the movie industry and BitTorrent Inc. CEO Bram Cohen, signed a deal they hoped would reduce the number of unlicensed copies available through bittorrent.com's search engine, run by BitTorrent, Inc. It meant BitTorrent.com had to remove any links to unlicensed copies of films made by seven of Hollywood's major movie studios.

More recently, the BitTorrent network has been subject to scrutiny by the British Phonographic Industry (BPI). There are suggestions that they are using the network to obtain the IP addresses of those currently connected to the tracker. The information is then used to contact the ISP of each downloader so that notifications can be made (this was given sizeable coverage in the UK press with regard to Virgin Media sending letters out to customers suspected of using P2P networks).

There are two major differences between BitTorrent and many other peer-to-peer file-trading systems, which advocates suggest make it less useful to those sharing copyrighted material without authorization. First, BitTorrent itself does not offer a search facility to find files by name. A user must find the initial torrent file by other means, such as a web search. Second, BitTorrent makes no attempt to conceal the host ultimately responsible for facilitating the sharing: a person who wishes to make a file available must run a tracker on a specific host or hosts and distribute the tracker address(es) in the .torrent file. Because it is possible to operate a tracker on a server that is located in a jurisdiction where the copyright holder cannot take legal action, the protocol does offer some vulnerability that other protocols lack. It is far easier to request that the server's ISP shut down the site than it is to find and identify every user sharing a file on a peer-to-peer network. However, with the use of a distributed hash table (DHT), trackers are no longer required, though often used for client software that does not support DHT to connect to the stream.

[edit] See also

Free software portal

• Anti-Counterfeiting Trade Agreement
• BitTorrent tracker software
• Bencode
• Comparison of BitTorrent clients
• Magnet URI scheme
• Segmented downloading
• Similarity Enhanced Transfer
• .sfv files in .torrents
• Super-seeding
• Terminology of BitTorrent
• UDP Torrent Protocol

[edit] References

[edit] Further reading

• Pouwelse, Johan; et al. (2005). "The Bittorrent P2P File-Sharing System: Measurements and Analysis". Peer-to-Peer Systems IV. Berlin: Springer. pp. 205–216. doi:10.1007/11558989_19. ISBN 9783540290681. 

[edit] External links

Wikimedia Commons has media related to: BitTorrent

• Official BitTorrent Website
• Official BitTorrent Specification
• BitTorrent at the Open Directory Project
• Interview with chief executive Ashwin Navin
• Unofficial BitTorrent Protocol Specification v1.0 at wiki.theory.org
• Unofficial BitTorrent Location-aware Protocol 1.0 Specification at wiki.theory.org
• Interview with Bram Cohen about Bittorrent and The Pirate Bay