View source for BitTorrent

{{Short description|Peer-to-peer file sharing protocol}}
{{About|the file sharing protocol}}
{{Use dmy dates|date=July 2016}}
{{Infobox software
| author              = [[Bram Cohen]]
| developer           = [[Rainberry, Inc.]]
| released            = {{Start date and age|2001|df=yes}}
| latest release date = {{Start date and age|2017|02|04|df=yes}}
| standard            = The BitTorrent Protocol Specification<ref name="Protocol1.0">{{cite web|url=https://www.bittorrent.org/beps/bep_0003.html|title=BitTorrent Protocol 1.0|last=Cohen|first=Bram|date=October 2002|publisher=BitTorrent.org|url-status=live|archive-url=https://web.archive.org/web/20140208002821/http://bittorrent.org/beps/bep_0003.html|archive-date=8 February 2014|access-date= 1 June 2020}}</ref>
| genre               = [[peer-to-peer file sharing]]
| repo                = {{URL|https://github.com/bittorrent/bittorrent.org}}
| website             = {{Official URL}}
}}
{{File sharing sidebar}}

'''BitTorrent''', also referred to as simply '''torrent''', is a [[Protocol (computing)|communication protocol]] for [[peer-to-peer file sharing]] (P2P), which enables users to distribute [[Data (computing)|data]] and [[electronic file]]s over the [[Internet]] in a [[Decentralised system|decentralized]] manner. The protocol is developed and maintained by [[Rainberry, Inc.]], and was first released in 2001.<ref name="BC">{{cite web |last=Cohen |first=Bram |date=2 July 2001 |title=BitTorrent&nbsp;– a new P2P app |url=http://finance.groups.yahoo.com/group/decentralization/message/3160 |url-status=dead |archive-url=https://web.archive.org/web/20080129085545/http://finance.groups.yahoo.com/group/decentralization/message/3160 |archive-date=29 January 2008 |access-date=15 April 2007 |publisher=Yahoo eGroups |df=dmy}}</ref> A 2004 study by Cachelogic found that one third of all internet traffic was BitTorrent traffic.

To send or receive files, users use a [[Comparison of BitTorrent clients|BitTorrent client]] on their Internet-connected [[computer]], which are available for a variety of [[computing platform]]s and [[Comparison of BitTorrent clients#Operating system support|operating systems]], including [[BitTorrent (software)|an official client]]. [[BitTorrent tracker]]s provide a list of files available for transfer and allow the client to find peer users, known as "seeds", who may transfer the files. BitTorrent downloading is considered to be faster than [[HTTP]] ("direct downloading") and [[File Transfer Protocol|FTP]] due to the lack of a central server that could limit bandwidth.<ref>{{Cite web |date=2023-04-19 |title=What Is BitTorrent and Is It Safe? |url=https://www.kaspersky.com/resource-center/definitions/bittorrent |access-date=2023-10-24 |website=www.kaspersky.com |language=en}}</ref><ref>{{Cite web |date=2009-06-13 |title=bittorrent vs HTTP |url=https://daniel.haxx.se/docs/bittorrent-vs-http.html |access-date=2023-10-24 |archive-url=https://web.archive.org/web/20090613010209/https://daniel.haxx.se/docs/bittorrent-vs-http.html |archive-date=13 June 2009 }}</ref>

BitTorrent is one of the most common protocols for transferring large files, such as [[digital video]] files containing [[TV show]]s and [[video clip]]s, or [[digital audio]] files containing [[song]]s. BitTorrent accounted for a third of all internet traffic in 2004, according to a study by Cachelogic.<ref>https://torrentfreak.com/bittorrent-the-one-third-of-all-internet-traffic-myth/</ref> As recently as 2019 BitTorrent remained a significant file sharing protocol according to [[Sandvine]], generating a substantial amount of Internet traffic, with 2.46% of [[Downstream (networking)|downstream]], and 27.58% of [[Upstream (networking)|upstream]] traffic,<ref name="BitTorrentSW">{{cite journal|last1=Marozzo|first1=Fabrizio|last2=Talia|first2=Domenico|last3=Trunfio|first3=Paolo|title=A Sleep-and-Wake technique for reducing energy consumption in BitTorrent networks|journal=Concurrency and Computation: Practice and Experience|volume=32|issue=14|year=2020|issn=1532-0634|doi=10.1002/cpe.5723|s2cid=215841734}}</ref> although this share has declined significantly since then.<ref>{{Cite web |title=BitTorrent is No Longer the 'King' of Upstream Internet Traffic * TorrentFreak |url=https://torrentfreak.com/bittorrent-is-no-longer-the-king-of-upstream-internet-traffic-240315/ |access-date=2024-03-19 |language=en}}</ref> 

== History ==
{{expand section|date=September 2021}}
[[File:BitTorrent network.svg|thumb|left|The middle computer is acting as a "seed" to provide a file to the other computers which act as peers.]]

Programmer [[Bram Cohen]], a [[University at Buffalo]] alumnus,<ref>{{cite web |url=https://twitter.com/UBengineering/status/357131210503356419 |title=UB Engineering Tweeter |publisher=University at Buffalo's School of Engineering and Applied Sciences |archive-url=https://web.archive.org/web/20131111070426/https://twitter.com/UBengineering/status/357131210503356419 |archive-date=11 November 2013 |url-status=live |access-date=4 January 2022 }}</ref> designed the protocol in April 2001, and released the first available version on 2 July 2001.<ref name="BC"/> Cohen and [[Ashwin Navin]] founded BitTorrent, Inc. (later renamed [[Rainberry, Inc.]]) to further develop the technology in 2004.

The first release of the BitTorrent client had no search engine and no peer exchange. Up until 2005, the only way to share files was by creating a small text file called a "[[Torrent file|torrent]]", that they would upload to a torrent index site. The first uploader acted as a ''[[Terminology of BitTorrent#Seed|seed]]'', and downloaders would initially connect as ''[[Terminology of BitTorrent#Peer|peers]]''. Those who wish to download the file would download the torrent, which their client would use to connect to a tracker which had a list of the IP addresses of other seeds and peers in the swarm. Once a peer completed a download of the complete file, it could in turn function as a seed. These files contain [[metadata]] about the files to be shared and the [[BitTorrent tracker|trackers]] which keep track of the other seeds and peers.

In 2005, first [[Vuze]] and then the BitTorrent client introduced distributed tracking using distributed hash tables which allowed clients to exchange data on swarms directly without the need for a torrent file.

In 2006, peer exchange functionality was added allowing clients to add peers based on the data found on connected nodes.

In 2017, BitTorrent, Inc. released the BitTorrent v2 protocol specification.<ref>{{cite web |date=15 May 2017 |title=Merge pull request #59 from the8472/new-hash-algos |url=https://github.com/bittorrent/bittorrent.org/commit/51fe877e6ed6f20fb7eea67fe234e7b266aaed84 |url-status=live |archive-url=https://web.archive.org/web/20211110162310/https://github.com/bittorrent/bittorrent.org/commit/51fe877e6ed6f20fb7eea67fe234e7b266aaed84 |archive-date=10 November 2021 |accessdate=13 September 2021 |publisher=BitTorrent}}</ref><ref>{{cite web |last1=Cohen |first1=Bram |title=The BitTorrent Protocol Specification v2 |url=https://www.bittorrent.org/beps/bep_0052.html |url-status=live |archive-url=https://web.archive.org/web/20201112030826/http://bittorrent.org/beps/bep_0052.html |archive-date=12 November 2020 |access-date=28 October 2020 |website=BitTorrent.org |publisher=BitTorrent}}</ref> BitTorrent v2 is intended to work seamlessly with previous versions of the BitTorrent protocol. The main reason for the update was that the old [[cryptographic hash function]], [[SHA-1]] is [[SHA-1#The SHAppening|no longer considered safe from malicious attacks]] by the developers, and as such, v2 uses [[SHA-256]]. To ensure backwards compatibility, the v2 .torrent file format supports a hybrid mode where the torrents are hashed through both the new method and the old method, with the intent that the files will be shared with peers on both v1 and v2 swarms. Another update to the specification is adding a [[Merkle tree|hash tree]] to speed up time from adding a torrent to downloading files, and to allow more granular checks for file corruption. In addition, each file is now hashed individually, enabling files in the swarm to be deduplicated, so that if multiple torrents include the same files, but seeders are only seeding the file from some, downloaders of the other torrents can still download the file. In addition, file hashes can be displayed on tracker, torrent indexing services, to search for swarms by searching for hashes of files contained in them. These hashes are different from the usual SHA-256 hash of files and can be obtained using tools.<ref>{{cite web |title=TMRR |url=https://github.com/kovalensky/tmrr |website=github.com |publisher=kovalensky}}</ref> Magnet links for v2 also support a hybrid mode to ensure support for legacy clients.<ref>{{cite web |title=Bittorrent-v2 |url=https://blog.libtorrent.org/2020/09/bittorrent-v2/ |website=libbittorrent.org |publisher=libbittorrent |access-date=28 October 2020 |archive-date=30 October 2020 |archive-url=https://web.archive.org/web/20201030011550/https://blog.libtorrent.org/2020/09/bittorrent-v2/ |url-status=live }}</ref>

== Design ==
[[File:Torrentcomp small.gif|thumb|Animation of [[Communications protocol|protocol]] use: The colored dots beneath each computer in the animation represent different parts of the [[file sharing|file being shared]]. By the time a copy to a destination computer of each of those parts completes, a copy to another destination computer of that part (or other parts) is already taking place between users.]]

The BitTorrent protocol can be used to reduce the server and network impact of distributing large files. Rather than downloading a file from a single source server, the BitTorrent protocol allows users to join a "swarm" of hosts to upload and download from each other simultaneously. The protocol is an alternative to the older single source, multiple mirror sources technique for distributing data, and can work effectively over networks with lower [[bandwidth (computing)|bandwidth]]. Using the BitTorrent protocol, several basic computers, such as home computers, can replace large servers while efficiently distributing files to many recipients. This lower bandwidth usage also helps prevent large spikes in [[internet traffic]] in a given area, keeping internet speeds higher for all users in general, regardless of whether or not they use the BitTorrent protocol.

The file being distributed is divided into segments called ''pieces''. As each peer receives a new piece of the file, it becomes a source (of that piece) for other peers, relieving the original seed from having to send that piece to every computer or user wishing a copy. With BitTorrent, the task of distributing the file is shared by those who want it; it is entirely possible for the seed to send only a single copy of the file itself and eventually distribute to an unlimited number of peers. Each piece is protected by a [[cryptographic hash]] contained in the torrent descriptor.<ref name="Protocol1.0"/> This ensures that any modification of the piece can be reliably detected, and thus prevents both accidental and malicious modifications of any of the pieces received at other nodes. If a node starts with an authentic copy of the torrent descriptor, it can verify the authenticity of the entire file it receives.

Pieces are typically downloaded non-sequentially, and are rearranged into the correct order by the BitTorrent client, which monitors which pieces it needs, and which pieces it has and can upload to other peers. Pieces are of the same size throughout a single download (for example, a 10&nbsp;MB file may be transmitted as ten 1&nbsp;MB pieces or as forty 256&nbsp;KB pieces).
Due to the nature of this approach, the download of any file can be halted at any time and be resumed at a later date, without the loss of previously downloaded information, which in turn makes BitTorrent particularly useful in the transfer of larger files. This also enables the client to seek out readily available pieces and download them immediately, rather than halting the download and waiting for the next (and possibly unavailable) piece in line, which typically reduces the overall time of the download.  This eventual transition from peers to seeders determines the overall "health" of the file (as determined by the number of times a file is available in its complete form).

The distributed nature of BitTorrent can lead to a [[Flooding algorithm|flood-like]] spreading of a file throughout many peer computer nodes. As more peers join the swarm, the likelihood of a successful download by any particular node increases. Relative to traditional Internet distribution schemes, this permits a significant reduction in the original distributor's hardware and bandwidth resource costs. Distributed downloading protocols in general provide [[wikt:redundancy|redundancy]] against system problems, reduce dependence on the original distributor,<ref>{{cite journal |arxiv=1004.0395|title=Estimating Self-Sustainability in Peer-to-Peer Swarming Systems |journal=Performance Evaluation |volume=67 |issue=11 |pages=1243–1258 |last1= Menasche |first1=Daniel S. |last2= Rocha |first2=Antonio A. A. |last3= de Souza e Silva |first3=Edmundo A. |last4= Leao |first4=Rosa M. |last5=Towsley |first5=Don |last6=Venkataramani |first6=Arun |year=2010 |doi=10.1016/j.peva.2010.08.013 |s2cid=9361889 }} by D. Menasche, A. Rocha, E. de Souza e Silva, R. M. Leao, D. Towsley, A. Venkataramani.</ref> and provide sources for the file which are generally [[Transient (computer programming)|transient]] and therefore there is no single point of failure as in one way server-client transfers.

Though both ultimately transfer files over a network, a BitTorrent download differs from a one way server-client download (as is typical with an [[Hypertext Transfer Protocol|HTTP]] or [[File Transfer Protocol|FTP]] request, for example) in several fundamental ways:
* BitTorrent makes many small data requests over different [[Internet Protocol|IP]] connections to different machines, while server-client downloading is typically made via a single [[Transmission Control Protocol|TCP]] connection to a single machine.
* BitTorrent downloads in a random or in a "rarest-first"<ref name="Rarest First and Choke Algorithms Are Enough">{{cite web | url=http://conferences.sigcomm.org/imc/2006/papers/p20-legout.pdf | title=Rarest First and Choke Algorithms Are Enough | date=December 2006 | last=Urvoy-Keller | publisher=SIGCOMM | access-date=9 March 2012 | archive-url=https://web.archive.org/web/20120523055500/http://conferences.sigcomm.org/imc/2006/papers/p20-legout.pdf | archive-date=23 May 2012 | url-status=live | df=dmy-all }}</ref> approach that ensures high availability, while classic downloads are sequential.

Taken together, these differences allow BitTorrent to achieve much lower cost to the content provider, much higher redundancy, and much greater resistance to abuse or to "[[Slashdot effect|flash crowds]]" than regular [[Server (computing)|server software]]. However, this protection, theoretically, comes at a cost: downloads can take time to rise to full speed because it may take time for enough peer connections to be established, and it may take time for a node to receive sufficient data to become an effective uploader. This contrasts with regular downloads (such as from an HTTP server, for example) that, while more vulnerable to overload and abuse, rise to full speed very quickly, and maintain this speed throughout. In the beginning, BitTorrent's non-contiguous download methods made it harder to support "streaming playback". In 2014, the client [[Popcorn Time]] allowed for streaming of BitTorrent video files. Since then, more and more clients are offering streaming options.

=== Searching ===

The BitTorrent protocol provides no way to index torrent files. As a result, a comparatively small number of websites have hosted a large majority of torrents, many linking to copyrighted works without the authorization of copyright holders, rendering those sites especially vulnerable to lawsuits.<ref>{{cite web| url=http://torrentfreak.com/publicbt-tracker-set-to-patch-bittorrents-achilles-heel-090712/| title=PublicBT Tracker Set To Patch BitTorrent' Achilles' Heel| date=12 July 2009| publisher=Torrentfreak |author=Ernesto |access-date=14 July 2009 | archive-url = https://web.archive.org/web/20140326093356/http://torrentfreak.com/publicbt-tracker-set-to-patch-bittorrents-achilles-heel-090712/ | archive-date = 26 March 2014| url-status=live}}</ref> A BitTorrent index is a "list of [[Torrent file|.torrent files]], which typically includes descriptions" and information about the torrent's content.<ref>Chwan-Hwa (John) Wu, J. David Irwin. ''Introduction to Computer Networks and Cybersecurity''. Chapter 5.4.: Partially Centralized Architectures. [[CRC Press]]. 4 February 2013. {{ISBN|9781466572133}}</ref> Several types of websites support the discovery and distribution of data on the BitTorrent network. Public torrent-hosting sites such as [[The Pirate Bay]] allow users to search and download from their collection of torrent files. Users can typically also upload torrent files for content they wish to distribute. Often, these sites also run [[BitTorrent tracker]]s for their hosted torrent files, but these two functions are not mutually dependent: a torrent file could be hosted on one site and tracked by another unrelated site. Private host/tracker sites operate like public ones except that they may restrict access to registered users and may also keep track of the amount of data each user uploads and downloads, in an attempt to reduce "[[leech (computing)|leeching]]".

[[Web search engine]]s allow the discovery of torrent files that are hosted and tracked on other sites; examples include The Pirate Bay and [[BTDigg]]. These sites allow the user to ask for content meeting specific criteria (such as containing a given word or phrase) and retrieve a list of links to torrent files matching those criteria. This list can often be sorted with respect to several criteria, relevance (seeders to leechers ratio) being one of the most popular and useful (due to the way the protocol behaves, the download bandwidth achievable is very sensitive to this value). [[Metasearch engine]]s allow one to search several BitTorrent indices and search engines at once.

The [[Tribler]] BitTorrent client was among the first to incorporate built-in search capabilities.  With Tribler, users can find .torrent files held by random peers and taste buddies.<ref>Zeilemaker, N., Capotă, M., Bakker, A., & Pouwelse, J. (2011). "Tribler P2P Media Search and Sharing." Proceedings of the 19th ACM International Conference on Multimedia - MM ’11.</ref> 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.<ref>{{cite web|url=https://www.tribler.org/DecentralizedRecommendation |title=DecentralizedRecommendation – |publisher=Tribler.org |access-date=9 July 2012 | archive-url = https://web.archive.org/web/20081202143338/http://www.tribler.org/DecentralizedRecommendation | archive-date = 2 December 2008| url-status=live}}</ref>

In May 2007, researchers at [[Cornell University]] published a paper proposing a new approach to searching a peer-to-peer network for inexact strings,<ref>
{{cite web|url=https://www.cs.cornell.edu/people/egs/papers/hyperspaces.pdf|title=Hyperspaces for Object Clustering and Approximate Matching in Peer-to-Peer Overlays|author=Wong, Bernard|author2=Vigfusson, Ymir|date=2 May 2007<!-- from PDF-->|publisher=Cornell University|url-status=live|archive-url=https://web.archive.org/web/20120617065142/http://www.cs.cornell.edu/People/egs/papers/hyperspaces.pdf|archive-date=17 June 2012|access-date=7 April 2013|author3=Gun Sirer, Emin|df=dmy-all}}
</ref> 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<ref>{{cite web
|url=http://www.cs.cornell.edu/~bwong/cubit/index.html
|author=Wong, Bernard
|title=Cubit: Approximate Matching for Peer-to-Peer Overlays
|year=2008
|access-date=26 May 2008
|publisher=Cornell University | archive-url = https://web.archive.org/web/20121231060445/http://www.cs.cornell.edu/~bwong/cubit/index.html |archive-date = 31 December 2012| url-status=live}}</ref> and published a follow-up paper reporting its success.<ref>
{{cite web
|url=http://www.cs.cornell.edu/~bwong/cubit/tr-cubit.pdf
|title=Approximate Matching for Peer-to-Peer Overlays with Cubit
|author=Wong, Bernard
|access-date=26 May 2008
|publisher=Cornell University
|archive-url=https://web.archive.org/web/20081029084030/http://www.cs.cornell.edu/~bwong/cubit/tr-cubit.pdf
|archive-date=29 October 2008
|url-status=live
|df=dmy-all
}}</ref>

A somewhat similar facility but with a slightly different approach is provided by the [[BitComet]] client through its "Torrent Exchange"<ref>{{cite web |url=http://wiki.bitcomet.com/Torrent_Exchange |title=Torrent Exchange | archive-url = https://web.archive.org/web/20131005065144/http://wiki.bitcomet.com/Torrent_Exchange | archive-date = 5 October 2013| url-status=live |quote=The torrent sharing feature of BitComet. Bitcomet.com. |access-date=31 January 2010}}</ref> feature. Whenever two peers using BitComet (with Torrent Exchange enabled) connect to each other they exchange lists of all the torrents (name and info-hash) they have in the Torrent Share storage (torrent files which were previously downloaded and for which the user chose to enable sharing by Torrent Exchange). Thus each client builds up a list of all the torrents shared by the peers it connected to in the current session (or it can even maintain the list between sessions if instructed).

At any time the user can search into that Torrent Collection list for a certain torrent and sort the list by categories. When the user chooses to download a torrent from that list, the .torrent file is automatically searched for (by info-hash value) in the [[Distributed hash table|DHT Network]] and when found it is downloaded by the querying client which can subsequently create and initiate a downloading task.

=== Downloading and sharing ===

Users find a torrent of interest on a torrent index site or by using a search engine built into the client, download it, and open it with a BitTorrent client. The client connects to the tracker(s) or seeds specified in the torrent file, from which it receives a list of seeds and peers currently transferring pieces of the file(s). The client connects to those peers to obtain the various pieces. If the swarm contains only the initial seeder, the client connects directly to it, and begins to request pieces. Clients incorporate mechanisms to optimize their download and upload rates.

The effectiveness of this data exchange depends largely on the policies that clients use to determine to whom to send data. Clients may prefer to send data to peers that send data back to them (a "[[tit for tat]]" exchange scheme), which encourages fair trading. But strict policies often result in suboptimal situations, such as when newly joined peers are unable to receive any data because they do not have any pieces yet to trade themselves or when two peers with a good connection between them do not exchange data simply because neither of them takes the initiative. To counter these effects, the official BitTorrent client program uses a mechanism called "optimistic unchoking", whereby the client reserves a portion of its available bandwidth for sending pieces to random peers (not necessarily known good partners, or "preferred peers") in hopes of discovering even better partners and to ensure that newcomers get a chance to join the swarm.<ref name = "Tamilmanistudy" >{{cite web| url=http://mnl.cs.stonybrook.edu/home/karthik/BitTorrent/Robustness_of_BT.doc |title=Studying and enhancing the BitTorrent protocol |first=Karthik |last=Tamilmani |publisher=Stony Brook University |date=25 October 2003 |access-date=6 May 2006 |format=DOC |archive-url=https://web.archive.org/web/20041119150847/http://mnl.cs.stonybrook.edu/home/karthik/BitTorrent/Robustness_of_BT.doc |archive-date=19 November 2004}}</ref><!-- mnl.cs.stonybrook.edu down, subst archive.org link -->

Although "swarming" scales well to tolerate "flash crowds" for popular content, it is less useful for unpopular or [[niche market]] content. Peers arriving after the initial rush might find the content unavailable and need to wait for the arrival of a "seed" in order to complete their downloads. The seed arrival, in turn, may take long to happen (this is termed the "seeder promotion problem"). Since maintaining seeds for unpopular content entails high bandwidth and administrative costs, this runs counter to the goals of publishers that value BitTorrent as a cheap alternative to a client-server approach. This occurs on a huge scale; measurements have shown that 38% of all new torrents become unavailable within the first month.<ref>{{cite arXiv |eprint=0912.0625 |title=Unraveling BitTorrent's File Unavailability: Measurements and Analysis |last=Kaune |first=Sebastian |display-authors=etal |class=cs.NI |year=2009 }}</ref> A strategy adopted by many publishers which significantly increases availability of unpopular content consists of bundling multiple files in a single swarm.<ref>{{cite book |url=http://conferences.sigcomm.org/co-next/2009/papers/Menasche.pdf |title=Content Availability and Bundling in Swarming Systems |author=D. Menasche |display-authors=etal |publisher=ACM via sigcomm.org |work=CoNEXT'09 |date=1–4 December 2009 |location=Rome, Italy |isbn=978-1-60558-636-6 |access-date=18 December 2009 |archive-url=https://web.archive.org/web/20110501082904/http://conferences.sigcomm.org/co-next/2009/papers/Menasche.pdf |archive-date=1 May 2011 |url-status=live |df=dmy-all }}</ref> More sophisticated solutions have also been proposed; generally, these use cross-torrent mechanisms through which multiple torrents can cooperate to better share content.<ref>{{cite web |url=http://www.eecs.qmul.ac.uk/~tysong/files/ICCCN09.pdf |title=The Seeder Promotion Problem: Measurements, Analysis and Solution Space |last=Kaune |first=Sebastian |display-authors=etal |publisher=Queen Mary's University London |access-date=20 July 2017 |archive-url=https://web.archive.org/web/20140809010501/http://www.eecs.qmul.ac.uk/~tysong/files/ICCCN09.pdf |archive-date=9 August 2014 |url-status=live |df=dmy-all }}</ref>

=== Creating and publishing ===
{{Update|date=January 2022|reason=Some extensions described in this section as experimental have been standardized. This section is factually incorrect about some aspects of v1 and v2.}}
The peer distributing a data file treats the file as a number of identically sized pieces, usually with byte sizes of a power of 2, and typically between 32&nbsp;kB and 16&nbsp;MB each. The peer creates a [[Hash Function|hash]] for each piece, using the [[SHA-1]] hash function, and records it in the torrent file. Pieces with sizes greater than 512&nbsp;kB will reduce the size of a torrent file for a very large payload, but is claimed to reduce the efficiency of the protocol.<ref>{{cite web |url=http://wiki.theory.org/index.php/BitTorrentSpecification |title=BitTorrent Specification |publisher=Wiki.theory.org |access-date=9 July 2012 | archive-url = https://web.archive.org/web/20130626195027/https://wiki.theory.org/index.php/BitTorrentSpecification | archive-date = 26 June 2013| url-status=live}}{{dubious|reason=Wikis are not citable sources – find a better source-Lexein|date=April 2013}}</ref> When another peer later receives a particular piece, the hash of the piece is compared to the recorded hash to test that the piece is error-free.<ref name = "Protocol1.0" /> Peers that provide a complete file are called seeders, and the peer providing the initial copy is called the initial seeder. The exact information contained in the torrent file depends on the version of the BitTorrent protocol.

By convention, the name of a torrent file has the suffix <code>.torrent</code>. Torrent files use the [[Bencode]] file format, and contain an "announce" section, which specifies the [[Uniform Resource Locator|URL]] of the tracker, and an "info" section, containing (suggested) names for the files, their lengths, the piece length used, and a [[SHA-1]] [[hash code]] for each piece, all of which are used by clients to verify the integrity of the data they receive. Though SHA-1 has shown signs of cryptographic weakness, Bram Cohen did not initially consider the risk big enough for a backward incompatible change to, for example, [[SHA-3]]. As of BitTorrent v2 the hash function has been updated to SHA-256.<ref>{{cite web|title=» BitTorrent v2|url=https://blog.libtorrent.org/2020/09/bittorrent-v2/|access-date=2020-09-27|language=en-US|archive-date=27 September 2020|archive-url=https://web.archive.org/web/20200927063545/https://blog.libtorrent.org/2020/09/bittorrent-v2/|url-status=live}}</ref>

In the early days, torrent files were typically published to torrent index websites, and registered with at least one tracker. The tracker maintained lists of the clients currently connected to the swarm.<ref name="Protocol1.0"/> Alternatively, in a ''trackerless system'' (decentralized tracking) every peer acts as a tracker. Azureus was the first<ref name="DHT-turns-10">{{cite web|title=BitTorrent's DHT Turns 10 Years Old|last=Jones|first=Ben|url=https://torrentfreak.com/bittorrents-dht-turns-10-years-old-150607/|date=7 June 2015|access-date=5 July 2015|website=[[TorrentFreak]]|archive-url=https://web.archive.org/web/20150611011335/http://torrentfreak.com/bittorrents-dht-turns-10-years-old-150607/|archive-date=11 June 2015|url-status=live|df=dmy-all}}</ref> BitTorrent client to implement such a system through the [[distributed hash table]] (DHT) method. An alternative and incompatible DHT system, known as [[Mainline DHT]], was released in the Mainline BitTorrent client three weeks later (though it had been in development since 2002)<ref name="DHT-turns-10"/> and subsequently adopted by the [[μTorrent]], [[Transmission (BitTorrent client)|Transmission]], [[rTorrent]], [[KTorrent]], [[BitComet]], and [[Deluge (BitTorrent client)|Deluge]] clients.

After the DHT was adopted, a "private" flag&nbsp;– analogous to the [[broadcast flag]]&nbsp;– was unofficially introduced, telling clients to restrict the use of decentralized tracking regardless of the user's desires.<ref>{{cite web|url=https://wiki.theory.org/index.php/BitTorrentSpecification#Info_Dictionary|title=Unofficial BitTorrent Protocol Specification v1.0|url-status=live|archive-url=https://web.archive.org/web/20061214094732/http://wiki.theory.org/BitTorrentSpecification|archive-date=14 December 2006|access-date=4 October 2009}}{{dubious|reason=Wikis are not citable sources – find a better source-Lexein|date=April 2013}}</ref> The flag is intentionally placed in the info section of the torrent so that it cannot be disabled or removed without changing the identity of the torrent. The purpose of the flag is to prevent torrents from being shared with clients that do not have access to the tracker. The flag was requested for inclusion in the official specification in August 2008, but has not been accepted yet.<ref>{{cite web|url=https://www.bittorrent.org/beps/bep_0027.html|title=Private Torrents|author=Harrison, David|date=3 August 2008|publisher=Bittorrent.org|url-status=live|archive-url=https://web.archive.org/web/20130324181003/http://www.bittorrent.org/beps/bep_0027.html|archive-date=24 March 2013|access-date=4 October 2009}}</ref> Clients that have ignored the private flag were banned by many trackers, discouraging the practice.<ref>{{cite web| url=http://www.slyck.com/news.php?story=1021| title=BitComet Banned From Growing Number of Private Trackers| access-date=4 October 2009 | archive-url = https://web.archive.org/web/20140326093846/http://www.slyck.com/news.php?story=1021 | archive-date = 26 March 2014| url-status=live}}</ref>

=== Anonymity ===

BitTorrent does not, on its own, offer its users anonymity.  One can usually see the [[IP address]]es of all peers in a swarm in one's own client or firewall program.  This may expose users with insecure systems to attacks.<ref name="Tamilmanistudy" /> In some countries, copyright organizations scrape lists of peers, and send takedown notices to the [[internet service provider]] of users participating in the swarms of files that are under copyright.  In some jurisdictions, copyright holders may launch lawsuits against uploaders or downloaders for infringement, and police may arrest suspects in such cases.

Various means have been used to promote anonymity.  For example, the BitTorrent client [[Tribler]] makes available a [[Tor (anonymity network)|Tor]]-like [[onion routing|onion network]], optionally routing transfers through other peers to obscure which client has requested the data.  The exit node would be visible to peers in a swarm, but the Tribler organization provides exit nodes.  One advantage of Tribler is that [[Clearnet (networking)|clearnet]] torrents can be downloaded with only a small decrease in download speed from one "hop" of routing.

[[i2p]] provides a similar anonymity layer although in that case, one can only download torrents that have been uploaded to the i2p network.<ref>{{cite web|url=https://geti2p.net/en/comparison/tor|title=I2P Compared to Tor - I2P|access-date=16 December 2015|archive-url=https://web.archive.org/web/20151222000747/https://geti2p.net/en/comparison/tor|archive-date=22 December 2015|url-status=live|df=dmy-all}}</ref>  The bittorrent client [[Vuze]] allows users who are not concerned about anonymity to take [[Clearnet (networking)|clearnet]] torrents, and make them available on the i2p network.<ref>{{cite web|url=https://wiki.vuze.com/w/I2PHelper_HowTo#Network_Mixing|title=I2PHelper HowTo - VuzeWiki|access-date=16 December 2015|archive-url=https://web.archive.org/web/20171020084042/https://wiki.vuze.com/w/I2PHelper_HowTo#Network_Mixing|archive-date=20 October 2017|url-status=live|df=dmy-all}}</ref>

Most BitTorrent clients are not designed to provide anonymity when used over Tor,<ref>{{cite web|url=https://blog.torproject.org/blog/bittorrent-over-tor-isnt-good-idea|title=Bittorrent over Tor isn't a good idea - The Tor Blog|access-date=2 October 2016|archive-url=https://web.archive.org/web/20161013120154/https://blog.torproject.org/blog/bittorrent-over-tor-isnt-good-idea|archive-date=13 October 2016|url-status=live|df=dmy-all}}</ref> and there is some debate as to whether torrenting over Tor acts as a drag on the network.<ref>{{cite web|url=https://www.torproject.org/docs/faq.html.en#FileSharing|title=Tor Project: FAQ|publisher=[[The Tor Project]]|access-date=2 October 2016|archive-url=https://web.archive.org/web/20161022004822/https://www.torproject.org/docs/faq.html.en#FileSharing|archive-date=22 October 2016|url-status=live|df=dmy-all}}</ref>
 
Private torrent trackers are usually invitation only, and require members to participate in uploading, but have the downside of a single centralized point of failure. [[Oink's Pink Palace]] and [[What.cd]] are examples of private trackers which have been shut down.

[[Seedbox]] services download the torrent files first to the company's servers, allowing the user to direct download the file from there.<ref>{{cite web|url=https://gizmodo.com/this-website-could-be-the-ultimate-all-in-one-torrent-m-1677265492|archive-url=https://web.archive.org/web/20160408154942/http://gizmodo.com/this-website-could-be-the-ultimate-all-in-one-torrent-m-1677265492|url-status=dead|archive-date=8 April 2016|title=This Website Could Be The Ultimate All-In-One Torrent Machine|date=8 April 2016}}</ref><ref>{{cite web|url=https://torrentfreak.com/torrent-from-the-cloud-with-seedr-160117/|title=Torrent From the Cloud With Seedr - TorrentFreak|date=17 January 2016|access-date=8 April 2016|archive-url=https://web.archive.org/web/20160419231351/https://torrentfreak.com/torrent-from-the-cloud-with-seedr-160117/|archive-date=19 April 2016|url-status=live|df=dmy-all}}</ref>  One's IP address would be visible to the Seedbox provider, but not to third parties.

[[Virtual private network]]s encrypt transfers, and substitute a different IP address for the user's, so that anyone monitoring a torrent swarm will only see that address.

== Associated technologies ==

=== Distributed trackers ===

On 2 May 2005, Azureus 2.3.0.0 (now known as [[Vuze]]) was released,<ref>{{cite web|url=http://azureus.sourceforge.net/changelog.php|title=Vuze Changelog|publisher=Azureus.sourceforge.net|url-status=live|archive-url=https://web.archive.org/web/20061201095553/http://azureus.sourceforge.net/changelog.php|archive-date=1 December 2006}}</ref> introducing support for "trackerless" torrents through a system called the "distributed database". This system is a [[distributed hash table]] implementation which allows the client to use torrents that do not have a working [[BitTorrent tracker]]. Instead just bootstrapping server is used (router.bittorrent.com, dht.transmissionbt.com or router.utorrent.com<ref>{{cite web|url=https://engineering.bittorrent.com/2013/12/19/dht-bootstrap-update/|title=DHT Bootstrap Update {{!}} The BitTorrent Engineering Blog|website=engineering.bittorrent.com|access-date=2019-11-21|archive-date=14 December 2019|archive-url=https://web.archive.org/web/20191214014200/https://engineering.bittorrent.com/2013/12/19/dht-bootstrap-update/|url-status=live}}</ref><ref>{{Citation|title=GitHub - bittorrent/bootstrap-dht: DHT bootstrap server|date=2019-11-11|url=https://github.com/bittorrent/bootstrap-dht|publisher=BitTorrent Inc.|access-date=2019-11-21|archive-date=14 January 2020|archive-url=https://web.archive.org/web/20200114211815/https://github.com/bittorrent/bootstrap-dht|url-status=live}}</ref>). The following month, BitTorrent, Inc. released version 4.2.0 of the Mainline BitTorrent client, which supported an alternative DHT implementation (popularly known as "[[Mainline DHT]]", outlined in a draft on their website) that is incompatible with that of Azureus. In 2014, measurement showed concurrent users of Mainline DHT to be from 10 million to 25&nbsp;million, with a daily churn of at least 10&nbsp;million.<ref>{{cite journal|last1=Wang|first1=Liang|last2=Kangasharju|first2=Jussi.|year=2013|title=Measuring Large-Scale Distributed Systems: Case of BitTorrent Mainline DHT|url=https://www.cs.helsinki.fi/u/lxwang/publications/P2P2013_13.pdf|url-status=live|journal=IEEE Peer-to-Peer|archive-url=https://web.archive.org/web/20140512225743/http://www.cs.helsinki.fi/u/lxwang/publications/P2P2013_13.pdf|archive-date=12 May 2014|access-date=15 May 2014|df=dmy-all}}</ref>

Current versions of the official BitTorrent client, [[μTorrent]], [[BitComet]], [[Transmission (BitTorrent client)|Transmission]] and BitSpirit all share compatibility with Mainline DHT. Both DHT implementations are based on [[Kademlia]].<ref>{{cite web|url=http://khashmir.sourceforge.net/ |title=Khashmir.SourceForge.net |publisher=Khashmir.SourceForge.net |access-date=9 July 2012 | archive-url = https://web.archive.org/web/20120702140624/http://khashmir.sourceforge.net/ | archive-date = 2 July 2012| url-status=live}}</ref> As of version 3.0.5.0, Azureus also supports Mainline DHT in addition to its own distributed database through use of an optional application plugin.<ref>{{cite web|url=http://azureus.sourceforge.net/plugin_details.php?plugin=mlDHT|title=plugins.vuze.com|publisher=plugins.vuze.com|url-status=live|archive-url=https://web.archive.org/web/20120801195122/http://azureus.sourceforge.net/plugin_details.php?plugin=mlDHT|archive-date=1 August 2012|access-date=9 July 2012}}</ref> This potentially allows the Azureus/Vuze client to reach a bigger swarm.

Another idea that has surfaced in Vuze is that of ''virtual torrents''. This idea is based on the distributed tracker approach and is used to describe some web resource. Currently, it is used for [[instant messaging]]. It is implemented using a special messaging protocol and requires an appropriate plugin. Anatomic P2P is another approach, which uses a decentralized network of nodes that route traffic to dynamic trackers. Most BitTorrent clients also use [[peer exchange]] (PEX) to gather peers in addition to [[BitTorrent tracker|trackers]] and [[Distributed hash table|DHT]]. Peer exchange checks with known peers to see if they know of any other peers. With the 3.0.5.0 release of Vuze, all major BitTorrent clients now have compatible peer exchange.

=== Web seeding ===
{{See|Torrent file#HTTP seeds}}
Web "seeding" was implemented in 2006 as the ability of BitTorrent clients to download torrent pieces from an HTTP source in addition to the "swarm". The advantage of this feature is that a website may distribute a torrent for a particular file or batch of files and make those files available for download from that same web server; this can simplify long-term seeding and [[Load balancing (computing)|load balancing]] through the use of existing, cheap, web hosting setups. In theory, this would make using BitTorrent almost as easy for a web publisher as creating a direct HTTP download. In addition, it would allow the "web seed" to be disabled if the swarm becomes too popular while still allowing the file to be readily available. This feature has two distinct specifications, both of which are supported by [[Libtorrent]] and the 26+ clients that use it.

The first was created by John "TheSHAD0W" Hoffman, who created BitTornado.<ref>{{cite web |url=http://bittornado.com/docs/webseed-spec.txt |title=HTTP-Based Seeding Specification |format=TXT |publisher=BitTornado.com |access-date=9 May 2006 |archive-url=https://web.archive.org/web/20040320142756/http://www.bittornado.com/docs/webseed-spec.txt |archive-date=20 March 2004 |url-status=dead|df=dmy }}</ref><ref>{{cite web|url=https://www.bittorrent.org/beps/bep_0017.html|title=''HTTP Seeding'' – BitTorrent Enhancement Proposal № 17|author=John Hoffman, DeHackEd|date=25 February 2008|url-status=live|archive-url=https://web.archive.org/web/20131213074432/http://www.bittorrent.org/beps/bep_0017.html|archive-date=13 December 2013|access-date=17 February 2012}}</ref> This first specification requires running a web service that serves content by info-hash and piece number, rather than filename.

The other specification is created by [[GetRight]] authors and can rely on a basic HTTP download space (using [[byte serving]]).<ref>{{cite web |url=http://www.getright.com/seedtorrent.html|title=HTTP/FTP Seeding for BitTorrent |publisher=GetRight.com|access-date=18 March 2010 |archive-url = https://web.archive.org/web/20091228072458/http://getright.com/seedtorrent.html |archive-date = 28 December 2009| url-status=live}}</ref><ref>{{cite web|url=https://www.bittorrent.org/beps/bep_0019.html|title=''WebSeed – HTTP/FTP Seeding (GetRight style)'' – BitTorrent Enhancement Proposal № 19|author=Michael Burford|date=25 February 2008|publisher=Bittorrent.org|url-status=live|archive-url=https://web.archive.org/web/20131213074337/http://www.bittorrent.org/beps/bep_0019.html|archive-date=13 December 2013|access-date=17 February 2012}}</ref>

In September 2010, a new service named Burnbit was launched which generates a torrent from any URL using webseeding.<ref name="tf-article">{{cite web|url=https://torrentfreak.com/burn-any-web-hosted-file-into-a-torrent-with-burnbit-100913/|title=Burn Any Web-Hosted File into a Torrent With Burnbit|date=13 September 2010|publisher=TorrentFreak|url-status=live|archive-url=https://web.archive.org/web/20110809053857/http://torrentfreak.com/burn-any-web-hosted-file-into-a-torrent-with-burnbit-100913/|archive-date=9 August 2011|access-date=9 July 2012}}</ref> There are server-side solutions that provide initial seeding of the file from the web server via standard BitTorrent protocol and when the number of external seeders reach a limit, they stop serving the file from the original source.<ref name="web-seeders">{{cite web|url=http://php-tracker.org/ |title=PHP based torrent file creator, tracker and seed server |publisher=PHPTracker |access-date=9 July 2012 | archive-url = https://web.archive.org/web/20131219084439/http://php-tracker.org/ | archive-date = 19 December 2013| url-status=live}}</ref>

=== RSS feeds ===
{{Main|Broadcatching}}

A technique called [[broadcatching]] combines [[RSS (file format)|RSS]] feeds with the BitTorrent protocol to create a content delivery system, further simplifying and automating content distribution. Steve Gillmor explained the concept in a column for [[Ziff-Davis]] in December 2003.<ref>Gillmor, Steve (13 December 2003). {{cite web |url=http://www.eweek.com/c/a/Messaging-and-Collaboration/BitTorrent-and-RSS-Create-Disruptive-Revolution/ |title=BitTorrent and RSS Create Disruptive Revolution |date=14 December 2003 |publisher=EWeek.com |access-date=22 April 2007 }}</ref> The discussion spread quickly among bloggers (Ernest Miller,<ref>Miller, Ernest (2 March 2004). {{cite web |url=http://importance.corante.com/archives/002223.html |title=BitTorrent + RSS = The New Broadcast | archive-url = https://web.archive.org/web/20131023060205/http://importance.corante.com/archives/002223.html | archive-date = 23 October 2013| url-status=live}}. ''The Importance of...'' Corante.com.</ref> [[Chris Pirillo]], etc.). In an article entitled ''Broadcatching with BitTorrent'', Scott Raymond explained:

{{quote|I want RSS feeds of BitTorrent files. A script would periodically check the feed for new items, and use them to start the download. Then, I could find a trusted publisher of an [[Alias (TV Show)|Alias]] RSS feed, and "subscribe" to all new episodes of the show, which would then start downloading automatically&nbsp;– like the "season pass" feature of the [[TiVo]].|Scott Raymond| scottraymond.net<ref>{{cite web|author=Raymond, Scott |url=http://scottraymond.net/archive/4745 |archive-url=https://web.archive.org/web/20040213093750/http://scottraymond.net/archive/4745 |archive-date=13 February 2004|title=Broadcatching with BitTorrent|publisher=scottraymond.net|date= 16 December 2003}}</ref>}}

The RSS feed will track the content, while BitTorrent ensures content integrity with [[cryptography|cryptographic]] [[Hash function|hashing]] of all data, so feed subscribers will receive uncorrupted content. One of the first and popular software clients ([[free software|free]] and [[open source software|open source]]) for ''broadcatching'' is [[Miro Media Player|Miro]]. Other free software clients such as PenguinTV and KatchTV are also now supporting broadcatching. The BitTorrent web-service MoveDigital added the ability to make torrents available to any web application capable of parsing [[XML]] through its standard [[Representational State Transfer|REST]]-based interface in 2006,<ref>{{cite web| url=http://www.movedigital.com/docs/index.php/MoveDigital_API| title=MoveDigital API REST functions| publisher=Move Digital|year=2006 |archive-url=https://web.archive.org/web/20060811154118/http://www.movedigital.com/docs/index.php/MoveDigital_API |archive-date=11 August 2006 |access-date=9 May 2006}} Documentation.</ref> though this has since been discontinued. Additionally, Torrenthut is developing a similar torrent [[API]] that will provide the same features, and help bring the torrent community to [[Web 2.0]] standards. Alongside this release is a first [[PHP]] application built using the API called PEP, which will parse any [[Really Simple Syndication]] (RSS 2.0) feed and automatically create and seed a torrent for each enclosure found in that feed.<ref>{{cite web|url=http://prodigem.com/code/pep/pep.txt|title=Prodigem Enclosure Puller(pep.txt)|publisher=Prodigem.com|format=TXT|access-date=9 May 2006|archive-url=https://web.archive.org/web/20060526130219/http://prodigem.com/code/pep/pep.txt|archive-date=26 May 2006}} via [[Internet Wayback Machine]].</ref><!-- – wi-fizzle unreachable 2007-4-15&nbsp;– found in Google cache&nbsp;– identical to archive.org copy. --Lexein -->

=== Throttling and encryption ===
{{Main|BitTorrent protocol encryption}}

Since BitTorrent makes up a large proportion of total traffic, some [[ISP]]s have chosen to "throttle" (slow down) BitTorrent transfers. For this reason, methods have been developed to disguise BitTorrent traffic in an attempt to thwart these efforts.<ref>{{cite web|url=https://torrentfreak.com/encrypting-bittorrent-to-take-out-traffic-shapers/|title=Encrypting Bittorrent to take out traffic shapers|date=5 February 2006|publisher=Torrentfreak.com|url-status=live|archive-url=https://web.archive.org/web/20140326092903/http://torrentfreak.com/encrypting-bittorrent-to-take-out-traffic-shapers/|archive-date=26 March 2014|access-date=9 May 2006}}</ref> Protocol header encrypt (PHE) and [[BitTorrent protocol encryption|Message stream encryption/Protocol encryption (MSE/PE)]] are features of some BitTorrent clients that attempt to make BitTorrent hard to detect and throttle. As of November 2015, [[Vuze]], [[BitComet]], [[KTorrent]], [[Transmission (BitTorrent client)|Transmission]], [[Deluge (BitTorrent client)|Deluge]], [[μTorrent]], MooPolice, Halite, [[qBittorrent]], [[rTorrent]], and the latest official BitTorrent client (v6) support MSE/PE encryption.

In August 2007, [[Comcast]] was preventing BitTorrent seeding by monitoring and interfering with the communication between peers. Protection against these efforts is provided by [[proxy server|proxying]] the client-tracker traffic via an [[tunneling protocol|encrypted tunnel]] to a point outside of the Comcast network.<ref>{{cite web|url=https://torrentfreak.com/comcast-throttles-bittorrent-traffic-seeding-impossible/|title=Comcast Throttles BitTorrent Traffic, Seeding Impossible|url-status=live|archive-url=https://web.archive.org/web/20131011044156/http://torrentfreak.com/comcast-throttles-bittorrent-traffic-seeding-impossible/|archive-date=11 October 2013}}, ''TorrentFreak'', 17 August 2007.</ref> In 2008, Comcast called a "truce" with BitTorrent, Inc. with the intention of shaping traffic in a protocol-agnostic manner.<ref>{{cite web|url=https://www.cnet.com/news/comcast-and-bittorrent-agree-to-collaborate/|title=Comcast and BitTorrent Agree to Collaborate|last=Broache|first=Anne|date=27 March 2008|publisher=News.com|url-status=live|archive-url=https://web.archive.org/web/20080509150131/http://www.news.com/8301-10784_3-9904494-7.html|archive-date=9 May 2008|access-date=9 July 2012}}</ref> Questions about the ethics and legality of Comcast's behavior have led to renewed debate about [[net neutrality in the United States]].<ref>{{cite web|url=https://www.cnet.com/news/is-comcasts-bittorrent-filtering-violating-the-law/|title=Is Comcast's BitTorrent filtering violating the law?|last=Soghoian|first=Chris|date=4 September 2007|website=Cnet.com|url-status=live|archive-url=https://web.archive.org/web/20100715153001/http://www.cnet.com/8301-13739_1-9769645-46.html|archive-date=15 July 2010|access-date=9 July 2012}}</ref> In general, although encryption can make it difficult to determine ''what'' is being shared, BitTorrent is vulnerable to [[traffic analysis]]. Thus, even with MSE/PE, it may be possible for an ISP to recognize BitTorrent and also to determine that a system is no longer downloading but only uploading data, and terminate its connection by injecting [[Transmission Control Protocol|TCP]] RST (reset flag) packets.

=== Multitrackers ===

Another unofficial feature is an extension to the BitTorrent metadata format proposed by John Hoffman<ref>{{cite web|url=https://www.bittorrent.org/beps/bep_0012.html|title=BEP12: Multitracker Metadata Extension|publisher=BitTorrent Inc.|url-status=live|archive-url=https://web.archive.org/web/20121227233108/http://bittorrent.org/beps/bep_0012.html|archive-date=27 December 2012|access-date=28 March 2013}}</ref> and implemented by several indexing websites. It allows the use of multiple trackers per file, so if one tracker fails, others can continue to support file transfer. It is implemented in several clients, such as [[BitComet]], BitTornado, BitTorrent, [[KTorrent]], [[Transmission (BitTorrent client)|Transmission]], [[Deluge (software)|Deluge]], [[μTorrent]], [[rtorrent]], [[Vuze]], and [[Frostwire]]. Trackers are placed in groups, or tiers, with a tracker randomly chosen from the top tier and tried, moving to the next tier if all the trackers in the top tier fail.

Torrents with multiple trackers can decrease the time it takes to download a file, but also have a few consequences:
* Poorly implemented<ref>{{cite web| url=http://wiki.depthstrike.com/index.php/P2P:Protocol:Specifications:Multitracker#Bad_Implementations| title=P2P:Protocol:Specifications:Multitracker| access-date=13 November 2009| publisher=wiki.depthstrike.com| archive-url=https://web.archive.org/web/20140326095037/http://wiki.depthstrike.com/index.php/P2P%3AProtocol%3ASpecifications%3AMultitracker#Bad_Implementations| archive-date=26 March 2014| url-status=live| df=dmy-all}}{{dubious|reason=Wikis are not citable sources – find a better source-Lexein|date=April 2013}}</ref> clients may contact multiple trackers, leading to more overhead-traffic.
* Torrents from closed trackers suddenly become downloadable by non-members, as they can connect to a seed via an open tracker.

=== Peer selection ===
{{As of|2008|12|post=,}} BitTorrent, Inc. was 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.<ref>{{cite web|last=Johnston |first=Casey |url=https://arstechnica.com/news.ars/post/20081209-bittorrent-has-new-plan-to-shape-up-p2p-behavior.html |title=Arstechnica.com |publisher=Arstechnica.com |date=9 December 2008 |access-date=9 July 2012 | archive-url = https://web.archive.org/web/20081212061637/http://arstechnica.com/news.ars/post/20081209-bittorrent-has-new-plan-to-shape-up-p2p-behavior.html | archive-date = 12 December 2008| url-status=live}}</ref>

== Implementations ==
{{Main|Comparison of BitTorrent clients}}

The BitTorrent specification is free to use and many clients are [[Open-source software|open source]], so BitTorrent clients have been created for all common [[operating systems]] using a variety of [[programming languages]]. The official BitTorrent client, [[μTorrent]], [[qBittorrent]], [[Transmission (BitTorrent client)|Transmission]], [[Vuze]], and [[BitComet]] are some of the most popular clients.<ref>{{cite web|url=https://torrentfreak.com/thunder-blasts-utorrents-market-share-away-091204/|title=Thunder Blasts uTorrent's Market Share Away|last=Van Der Sar|first=Ernesto|date=4 December 2009|publisher=[[TorrentFreak]]|url-status=live|archive-url=https://web.archive.org/web/20091207062921/http://torrentfreak.com/thunder-blasts-utorrents-market-share-away-091204/|archive-date=7 December 2009|access-date=15 September 2011|df=dmy}}</ref><ref>{{cite web|title=uTorrent Dominates BitTorrent Client Market Share|url=https://torrentfreak.com/utorrent-dominates-bittorrent-client-market-share-090624/|publisher=[[TorrentFreak]]|date=24 June 2009|access-date=25 June 2013 | archive-url = https://web.archive.org/web/20140403051947/http://torrentfreak.com/utorrent-dominates-bittorrent-client-market-share-090624/ | archive-date = 3 April 2014| url-status=live}}</ref><ref>{{cite web|title=Windows Public File Sharing Market Share 2015|url=https://www.opswat.com/resources/reports/market-share-usage-analysis-file-sharing-antivirus-june-2015#public-file-sharing|publisher=opswat|access-date=1 April 2016|archive-url=https://web.archive.org/web/20160414045045/https://www.opswat.com/resources/reports/market-share-usage-analysis-file-sharing-antivirus-june-2015#public-file-sharing|archive-date=14 April 2016|url-status=live|df=dmy-all}}</ref><ref>{{cite web|last1=Henry|first1=Alan|title=Most Popular BitTorrent Client 2015|date=19 May 2015 |url=http://lifehacker.com/5813348/five-best-bittorrent-applications/1705622513|publisher=lifehacker|access-date=1 April 2016|archive-url=https://web.archive.org/web/20160409025410/http://lifehacker.com/5813348/five-best-bittorrent-applications/1705622513|archive-date=9 April 2016|url-status=live|df=dmy-all}}</ref>

Some BitTorrent implementations such as [[MLDonkey]] and [[Torrentflux]] are designed to run as servers. For example, this can be used to centralize file sharing on a single dedicated server which users share access to on the network.<ref>{{cite web |url=http://www.turnkeylinux.org/torrentserver |title=Torrent Server combines a file server with P2P file sharing |publisher=Turnkeylinux.org |access-date=9 July 2012 |archive-url=https://web.archive.org/web/20120707011646/http://www.turnkeylinux.org/torrentserver |archive-date=7 July 2012 |url-status=live |df=dmy-all }}</ref> Server-oriented BitTorrent implementations can also be hosted by [[Internet hosting service|hosting providers]] at [[colocation centre|co-located]] facilities with high bandwidth Internet connectivity (e.g., a datacenter) which can provide dramatic speed benefits over using BitTorrent from a regular home broadband connection. Services such as [[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)|Opera]] [[web browser]] supports BitTorrent natively.<ref>{{cite web| url=https://arstechnica.com/news.ars/post/20070201-8750.html| title=Does network neutrality mean an end to BitTorrent throttling?| first=Nate| last=Anderson| date=1 February 2007| publisher=Ars Technica, LLC| access-date=9 February 2007 | archive-url = https://web.archive.org/web/20081216140732/http://arstechnica.com/news.ars/post/20070201-8750.html | archive-date = 16 December 2008| url-status=live}}</ref> [[Brave (web browser)|Brave]] web browser ships with an [[Browser extension|extension]] which supports [[WebTorrent]], a BitTorrent-like protocol based on [[WebRTC]] instead of [[User Datagram Protocol|UDP]] and [[Transmission Control Protocol|TCP]].<ref>{{cite web| url=https://browserpulse.com/how-to-stream-movies-and-download-torrent-files-in-brave-browser/| title=How to Stream Movies and Download Torrent Files in Brave Browser| author=Mark| date=7 May 2020| publisher=Browser Pulse| access-date=6 October 2020| archive-date=9 October 2020| archive-url=https://web.archive.org/web/20201009082825/https://browserpulse.com/how-to-stream-movies-and-download-torrent-files-in-brave-browser/| url-status=live}}</ref><ref>{{cite web|title=What extensions are built into Brave?|url=https://support.brave.com/hc/en-us/articles/360035025231-What-extensions-are-built-into-Brave-|url-status=live|access-date=January 8, 2022|website=Brave Help Center|archive-date=8 January 2022|archive-url=https://web.archive.org/web/20220108170102/https://support.brave.com/hc/en-us/articles/360035025231-What-extensions-are-built-into-Brave-}}</ref> BitLet allowed users to download Torrents directly from their browser using a [[Java applet]] (until browsers removed support for Java applets).<ref>{{cite web|last=Bahgat|first=Alessandro|date=2008-10-10|title=BitLet - Bittorrent applet|url=https://www.abahgat.com/project/bitlet/|access-date=2022-01-08|website=Alessandro Bahgat|language=en-us|archive-date=21 November 2021|archive-url=https://web.archive.org/web/20211121144706/https://www.abahgat.com/project/bitlet/|url-status=live}}</ref> An increasing number of hardware devices are being made to support BitTorrent. These include routers and NAS devices containing BitTorrent-capable firmware like [[OpenWrt]]. Proprietary versions of the protocol which implement [[Digital rights management|DRM]], encryption, and authentication are found within managed clients such as [[Pando (application)|Pando]].

== Adoption ==

A growing number of individuals and organizations are using BitTorrent to distribute their own or licensed works (e.g. [[independent music|indie]] bands distributing digital files of their new songs). Independent adopters report that BitTorrent technology reduces demands on private networking hardware and bandwidth, an essential for non-profit groups with large amounts of internet traffic.<ref>See for example {{cite web |url=https://www.makeuseof.com/tag/8-legal-uses-for-bittorrent-youd-be-surprised/#the-internet-archive |title=8 Legal Uses for BitTorrent |date=17 August 2013 | archive-url = https://web.archive.org/web/20210505161456/https://www.makeuseof.com/tag/8-legal-uses-for-bittorrent-youd-be-surprised/ | archive-date = 5 May 2021| url-status=live}} The Internet Archive</ref>

Many major [[Open-source software|open source]] and [[free software]] projects encourage BitTorrent as well as conventional downloads of their products (via [[HTTP]], [[FTP]] etc.) to increase availability and to reduce load on their own servers, especially when dealing with larger files.<ref>{{cite web|url=https://ubuntu.com/download/alternative-downloads|title=Complete Download Options List – BitTorrent|publisher=Ubuntu.com|url-status=live|archive-url=https://web.archive.org/web/20100424013939/http://www.ubuntu.com/getubuntu/downloadmirrors|archive-date=24 April 2010|access-date=7 May 2009}}</ref> In addition, some video game installers, especially those whose large size makes them difficult to host due to bandwidth limits, extremely frequent downloads, and unpredictable changes in network traffic, will distribute instead a specialized, stripped down BitTorrent client with enough functionality to download the game from the other running clients and the primary server (which is maintained in case not enough peers are available).

Some uses of BitTorrent for file sharing may violate laws in some jurisdictions (see [[#Legislation|legislation]] section).

=== Popularity and traffic statistics ===
{{As of|2012|1}}, BitTorrent is utilized by 150 million active users. Based on this figure, the total number of monthly users may be estimated to more than a quarter of a billion (≈ 250 million).<ref>{{cite web |date=9 January 2012 |title=BitTorrent and μTorrent Software Surpass 150 Million User Milestone |url=http://www.bittorrent.com/intl/es/company/about/ces_2012_150m_users |url-status=dead |archive-url=https://web.archive.org/web/20140326102305/http://www.bittorrent.com/intl/es/company/about/ces_2012_150m_users |archive-date=26 March 2014 |access-date=9 July 2012 |publisher=Bittorrent.com |df=dmy-all}}</ref> {{As of|2013|02|post=,}} BitTorrent was responsible for 3.35% of all worldwide [[Bandwidth (computing)|bandwidth]]—more than half of the 6% of total bandwidth dedicated to file sharing.<ref>{{cite web |title=Application Usage & Threat Report |url=http://researchcenter.paloaltonetworks.com/app-usage-risk-report-visualization/ |url-status=live |archive-url=https://web.archive.org/web/20131031153132/http://researchcenter.paloaltonetworks.com/app-usage-risk-report-visualization/ |archive-date=31 October 2013 |access-date=7 April 2013 |publisher=Palo Alto Networks. 2013}}</ref> {{As of|2013}}, BitTorrent had 15–27&nbsp;million concurrent users at any time.<ref>{{cite book |last1=Wang |first1=Liang |title=IEEE P2P 2013 Proceedings |last2=Kangasharju |first2=J. |date=1 September 2013 |isbn=978-1-4799-0515-7 |pages=1–10 |chapter=Measuring large-scale distributed systems: Case of BitTorrent Mainline DHT |doi=10.1109/P2P.2013.6688697 |access-date=7 January 2016 |chapter-url=https://www.cl.cam.ac.uk/~lw525/MLDHT/ |archive-url=https://web.archive.org/web/20151118101022/http://www.cl.cam.ac.uk/~lw525/MLDHT/ |archive-date=18 November 2015 |url-status=live |df=dmy-all |s2cid=5659252}}</ref> 

=== Film, video, and music ===

* [[BitTorrent (company)|BitTorrent Inc.]] has obtained a number of licenses from Hollywood studios for distributing popular content from their websites.{{citation needed|date=January 2020}}
* [[Sub Pop]] Records releases tracks and videos via BitTorrent Inc.<ref>{{cite web| url=http://www.bittorrent.com/users/subpoprecords/| title=Sub Pop page on BitTorrent.com| access-date=13 December 2006| archive-url=https://web.archive.org/web/20070114140652/http://www.bittorrent.com/users/subpoprecords/| archive-date=14 January 2007|url-status=dead| df=dmy-all}}</ref> to distribute its 1000+ albums. [[Babyshambles]] and [[The Libertines]] (both bands associated with [[Pete Doherty]]) have extensively used torrents to distribute hundreds of demos and live videos. US [[industrial rock]] band [[Nine Inch Nails]] frequently distributes albums via BitTorrent.
* [[Podcast]]ing software is starting to integrate BitTorrent to help podcasters deal with the download demands of their MP3 "radio" programs. Specifically, [[Juice (podcasting)|Juice]] and [[Miro Media Player|Miro]] (formerly known as Democracy Player) support automatic processing of .torrent files from [[RSS]] feeds. Similarly, some BitTorrent clients, such as [[μTorrent]], are able to process [[web feeds]] and automatically download content found within them.
* [[Discipline Global Mobile|DGM Live]] purchases are provided via BitTorrent.<ref>{{cite web |url=http://www.dgmlive.com/help.htm#whatisbittorrent |title=DGMlive.com |publisher=DGMlive.com |access-date=9 July 2012 |archive-url=https://web.archive.org/web/20131111075532/https://www.dgmlive.com/help.htm#whatisbittorrent |archive-date=11 November 2013 |url-status=dead|df=dmy-all }}</ref>
* [[VODO]], a service which distributes "free-to-share" movies and TV shows via BitTorrent.<ref>[http://vo.do/about "VODO – About..."]. Retrieved 15 April 2012. ([https://web.archive.org/web/20101003111942/http://vo.do/about WebCite]).</ref><ref>Cory Doctorow (15 October 2009). [https://boingboing.net/2009/10/15/vodo-a-filesharing-s.html "Vodo: a filesharing service for film-makers"]. [[Boing Boing]]. Happy Mutants LLC. Retrieved 15 April 2012. ([https://web.archive.org/web/20091018045456/http://www.boingboing.net/2009/10/15/vodo-a-filesharing-s.html WebCite])</ref><ref>Ernesto. [https://torrentfreak.com/pioneer-one-the-bittorrent-exclusive-tv-series-continues-101215/ "Pioneer One, The BitTorrent Exclusive TV-Series Continues"]. TorrentFreak. Retrieved 15 April 2012. ([https://web.archive.org/web/20101218191831/http://torrentfreak.com/pioneer-one-the-bittorrent-exclusive-tv-series-continues-101215/ WebCite])</ref>

=== Broadcasters ===

* In 2008, the [[Canadian Broadcasting Corporation|CBC]] became the first public broadcaster in North America to make a full show (''[[Canada's Next Great Prime Minister]]'') available for download using BitTorrent.<ref>{{cite news
| title = CBC to BitTorrent Canada's Next Great Prime Minister
| date = 19 March 2008
| url = http://www.cbc.ca/nextprimeminister/blog/2008/03/canadas_next_great_prime_minis.html
| archive-url = https://web.archive.org/web/20100614220807/http://www.cbc.ca/nextprimeminister/blog/2008/03/canadas_next_great_prime_minis.html
| archive-date = 14 June 2010
| access-date=19 March 2008
| work=CBC News}}</ref>
* The [[Norwegian Broadcasting Corporation]] (NRK) has since March 2008 experimented with bittorrent distribution, available online.<ref>{{cite web|url=http://nrkbeta.no/bittorrent/ |title=Bittorrent |publisher=Nrkbeta.no |year=2008 |access-date=7 April 2013 |language=no | archive-url = https://web.archive.org/web/20131024144950/http://nrkbeta.no/bittorrent/ | archive-date = 24 October 2013| url-status=live}}</ref> Only selected works in which NRK owns all royalties are published. Responses have been very positive, and NRK is planning to offer more content.
* The Dutch [[VPRO]] broadcasting organization released four documentaries in 2009 and 2010 under a [[Creative Commons]] license using the content distribution feature of the [[Mininova]] tracker.<ref>{{cite web|url=http://www.mininova.org/user/EeuwvandeStad|title=Torrents uploaded by EeuwvandeStad|publisher=MiniNova|year=2009|access-date=7 April 2013|archive-url=https://web.archive.org/web/20131104210531/http://www.mininova.org/user/EeuwvandeStad|archive-date=4 November 2013|url-status=dead|df=dmy-all}}</ref><ref>{{cite web|url=http://tegenlicht.vpro.nl/nieuws/2010/november/creative-commons.html |title=Tegenlicht – Download California Dreaming |date=11 August 2010 |author=Denters, M. |publisher=VPRO.nl |access-date=7 April 2013 | archive-url = https://web.archive.org/web/20140326102335/http://tegenlicht.vpro.nl/nieuws/2010/november/creative-commons.html | archive-date = 26 March 2014| url-status=live}}</ref><ref>{{cite web|url=http://tegenlicht.vpro.nl/nieuws/2009/oktober/vpro-gemeengoed.html |date=1 October 2009 |title=Tegenlicht – VPRO gemeengoed |author=Bol, M. |publisher=VPRO.nl |language=nl|access-date=7 April 2013 | archive-url = https://web.archive.org/web/20140326094708/http://tegenlicht.vpro.nl/nieuws/2009/oktober/vpro-gemeengoed.html | archive-date = 26 March 2014| url-status=live}}</ref>

=== Cloud Service Providers ===

* The Amazon AWS's [[Amazon S3|Simple Storage Service]] (S3), until April 29, 2021, had supported sharing of bucket objects with BitTorrent protocols. As of June 13, 2020, the feature is only available in service regions launched after May 30, 2016.<ref>{{cite web|title=Announcements about S3 BitTorrent discontinuation shouldn't be buried in the commit history · Issue #27 · awsdocs/amazon-s3-userguide|url=https://github.com/awsdocs/amazon-s3-userguide/issues/27|access-date=2021-09-30|website=GitHub|language=en|archive-date=30 September 2021|archive-url=https://web.archive.org/web/20210930005409/https://github.com/awsdocs/amazon-s3-userguide/issues/27|url-status=live}}</ref><ref>{{cite web|title=Periodic update - 2021-05-13 · awsdocs/amazon-s3-userguide@0d17598|url=https://github.com/awsdocs/amazon-s3-userguide/commit/0d1759880ccb1818ab0f14129ba1321c519d2ac1|access-date=2021-09-30|website=GitHub|language=en|archive-date=30 September 2021|archive-url=https://web.archive.org/web/20210930005410/https://github.com/awsdocs/amazon-s3-userguide/commit/0d1759880ccb1818ab0f14129ba1321c519d2ac1|url-status=live}}</ref><ref>{{cite web|date=2020-06-13|title=Using BitTorrent with Amazon S3 - Amazon Simple Storage Service|url=https://docs.aws.amazon.com/AmazonS3/latest/dev/S3Torrent.html|access-date=2021-09-30|archive-url=https://web.archive.org/web/20200613221201/https://docs.aws.amazon.com/AmazonS3/latest/dev/S3Torrent.html|archive-date=13 June 2020}}</ref> The feature for the existing customers will be extended for an additional 12 months following the deprecation. After April 29, 2022, BitTorrent clients will no longer connect to Amazon S3.

=== Software ===

* [[Blizzard Entertainment]] uses BitTorrent (via its Blizzard [[Battle.net]] distribution platform, formerly the "Blizzard Downloader") to distribute content and patches for ''[[Diablo III]]'', ''[[StarCraft II]]'' and ''[[World of Warcraft]]'', including the games themselves.<ref>{{cite web|url=https://wow.gamepedia.com/Blizzard_Downloader|title=Blizzard Downloader|date=4 November 2010|publisher=Curse Inc.|url-status=live|archive-url=https://web.archive.org/web/20140326100329/http://wowpedia.org/Blizzard_Downloader|archive-date=26 March 2014|access-date=4 November 2010}}</ref>
* [[Wargaming (company)|Wargaming]] uses BitTorrent in their popular titles ''[[World of Tanks]]'', ''[[World of Warships]]'' and ''[[World of Warplanes]]'' to distribute game updates.<ref>{{cite web|url=https://worldoftanks.eu/en/content/guide/general/frequently_asked_questions/|title=World of Tanks FAQ|date=15 December 2014|publisher=Wargaming|url-status=live|archive-url=https://web.archive.org/web/20141218111040/http://worldoftanks.eu/en/content/guide/general/frequently_asked_questions/|archive-date=18 December 2014|access-date=15 December 2014|df=dmy-all}}</ref>
* [[CCP Games]], maker of the space simulation MMORPG ''[[Eve Online]]'', has announced that a new launcher will be released that is based on BitTorrent.<ref>{{cite web |author=MJ Guthrie |date=11 March 2013 |url=http://massively.joystiq.com/2013/03/11/eve-online-reconfiguring-launcher-to-use-bittorrent/ |title=EVE Online reconfiguring launcher to use BitTorrent |publisher=Massively.joystiq.com |access-date=7 April 2013 |archive-url=https://web.archive.org/web/20140213023755/http://massively.joystiq.com/2013/03/11/eve-online-reconfiguring-launcher-to-use-bittorrent/ |archive-date=13 February 2014 |url-status=dead}}</ref><ref>{{cite web|author=CCP Games |url=http://community.eveonline.com/devblog.asp?a=blog&nbid=74573 |title=All quiet on the EVE Launcher front? – EVE Community |publisher=Community.eveonline.com |date=20 July 2010 |access-date=7 April 2013 | archive-url = https://web.archive.org/web/20130313171657/http://community.eveonline.com/devblog.asp?a=blog&nbid=74573 | archive-date = 13 March 2013| url-status=live}}</ref>
* [[Resilio Sync]] is a BitTorrent-based<ref>{{cite web |url=http://labs.bittorrent.com/experiments/sync/technology.html |title=BitTorrent Sync technology |accessdate=29 August 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20130830055935/http://labs.bittorrent.com/experiments/sync/technology.html |archivedate=30 August 2013 |df=dmy-all }}</ref> folder-syncing tool which can act as an alternative to server-based synchronisation services such as [[Dropbox]].

=== Government ===

* The British government used BitTorrent to distribute [[Combined Online Information System|details about how the tax money of British citizens was spent]].<ref>{{cite web|url=https://data.gov.uk/dataset/coins|title=Combined Online Information System|author=HM Government|date=4 September 2012|work=Data.Gov.Uk Beta|publisher=Controller of Her Majesty's Stationery Office|url-status=live|archive-url=https://web.archive.org/web/20140326093701/http://data.gov.uk/dataset/coins|archive-date=26 March 2014|access-date=7 September 2012}}</ref><ref>{{cite web|url=https://torrentfreak.com/uk-government-uses-bittorrent-to-share-public-spending-data-100604/|title=UK Government Uses BitTorrent to Share Public Spending Data|author=Ernesto|date=4 June 2010|publisher=TorrentFreak|url-status=live|archive-url=https://web.archive.org/web/20131027201557/http://torrentfreak.com/uk-government-uses-bittorrent-to-share-public-spending-data-100604/|archive-date=27 October 2013|access-date=7 September 2012}}</ref>

=== Education ===

* [[Florida State University]] uses BitTorrent to distribute large scientific data sets to its researchers.<ref>{{cite web|url=http://www.hpc.fsu.edu/index.php?option=com_wrapper&view=wrapper&Itemid=80|title=HPC Data Repository|publisher=Florida State University|access-date=7 April 2013|archive-url=https://web.archive.org/web/20130402200554/https://www.hpc.fsu.edu/index.php?option=com_wrapper&view=wrapper&Itemid=80|archive-date=2 April 2013|url-status=live|df=dmy-all}}</ref>
* Many universities that have [[BOINC]] distributed computing projects have used the BitTorrent functionality of the client-server system to reduce the bandwidth costs of distributing the client-side applications used to process the scientific data.  If a BOINC [[distributed computing]] application needs to be updated (or merely sent to a user), it can do so with little impact on the BOINC server.<ref>{{cite book |author=Costa, Fernando |author2=Silva, Luis |author3=Fedak, Gilles |author4=Kelley, Ian |title=2008 IEEE International Symposium on Parallel and Distributed Processing |page=1 |publisher=IEEE |year=2008 |doi=10.1109/IPDPS.2008.4536446 |chapter=Optimizing the data distribution layer of BOINC with Bit ''Torrent'' |isbn=978-1-4244-1693-6 |s2cid=13265537 |df=dmy-all |chapter-url=https://zenodo.org/record/3432525 }}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}{{registration required}}</ref>
* The developing [[Human Connectome Project]] uses BitTorrent to share their [http://www.developingconnectome.org/project/data-release-user-guide/ open dataset].<ref>{{cite web|url=https://torrentfreak.com/torrents-help-researchers-worldwide-to-study-babies-brains-170603/|title=Torrents Help Researchers Worldwide to Study Babies' Brains|publisher=Torrent Freak|date=3 June 2017|access-date=4 January 2018|archive-url=https://web.archive.org/web/20180105011741/https://torrentfreak.com/torrents-help-researchers-worldwide-to-study-babies-brains-170603/|archive-date=5 January 2018|url-status=live|df=dmy-all}}</ref>
* [[Academic Torrents]] is a BitTorrent tracker for use by researchers in fields that need to share large datasets<ref name="website">{{cite web |title=Academic Torrents Website |url=https://academictorrents.com/ |access-date=4 May 2020 |archive-date=7 May 2020 |archive-url=https://web.archive.org/web/20200507000702/https://academictorrents.com/ |url-status=live }}</ref><ref name="MyScienceWork 2014">{{cite web |last1=Miccoli |first1=Fräntz |title=Academic Torrents: Bringing P2P Technology to the Academic World |url=https://www.mysciencework.com/omniscience/academic-torrents-bringing-p2p-technology-to-the-academic-world |website=MyScienceWork |access-date=6 May 2020 |date=2014 |archive-date=26 July 2020 |archive-url=https://web.archive.org/web/20200726233620/https://www.mysciencework.com/omniscience/academic-torrents-bringing-p2p-technology-to-the-academic-world |url-status=live }}</ref>

=== Others ===
* Facebook uses BitTorrent to distribute updates to Facebook servers.<ref>{{cite web|url=https://torrentfreak.com/facebook-uses-bittorrent-and-they-love-it-100625/|title=Facebook Uses BitTorrent, and They Love It|author=Ernesto|date=25 June 2010|work=Torrent Freak|url-status=live|archive-url=https://web.archive.org/web/20140419233159/https://torrentfreak.com/facebook-uses-bittorrent-and-they-love-it-100625/|archive-date=19 April 2014|access-date=7 September 2012}}</ref>
* Twitter uses BitTorrent to distribute updates to Twitter servers.<ref>{{cite web|title=Twitter Uses BitTorrent For Server Deployment|url=http://torrentfreak.com/twitter-uses-bittorrent-for-server-deployment-100210/|work=Torrent Freak|access-date=7 September 2012 |author=Ernesto|date=10 February 2010 | archive-url = https://web.archive.org/web/20140326094120/http://torrentfreak.com/twitter-uses-bittorrent-for-server-deployment-100210/ | archive-date = 26 March 2014| url-status=live}}</ref><ref>{{cite web|title=BitTorrent Makes Twitter's Server Deployment 75x Faster|url=http://torrentfreak.com/bittorrent-makes-twitters-server-deployment-75-faster-100716/|work=Torrent Freak|access-date=7 September 2012 |author=Ernesto|date=16 July 2010 | archive-url = https://web.archive.org/web/20140326093158/http://torrentfreak.com/bittorrent-makes-twitters-server-deployment-75-faster-100716/ | archive-date = 26 March 2014| url-status=live}}</ref>
* The [[Internet Archive]] added BitTorrent to its file download options for over 1.3&nbsp;million existing files, and all newly uploaded files, in August 2012.<ref name="TFBT">{{cite web |url=https://torrentfreak.com/internet-archive-starts-seeding-1398635-torrents-120807/ |title=Internet Archive Starts Seeding 1,398,875 Torrents |author=Ernesto |date=7 August 2012 |publisher=TorrentFreak |access-date=7 August 2012 |archive-url=https://web.archive.org/web/20120808212731/https://torrentfreak.com/internet-archive-starts-seeding-1398635-torrents-120807/ |archive-date=8 August 2012 |url-status=live |df=dmy }}</ref><ref>{{cite web |url=http://bt1.archive.org/hotlist.php |title=Hot List for bt1.us.archive.org (Updated August 7, 2012, 7:31&nbsp;pm PDT) |access-date=8 August 2012 |archive-url=https://web.archive.org/web/20120803014019/http://bt1.archive.org/hotlist.php |archive-date=3 August 2012 |url-status=live |df=dmy-all }}. Archive.org.</ref> This method is the fastest means of downloading media from the Archive.<ref name=TFBT/><ref>{{cite web |url=https://archive.org/details/bittorrent |title=Welcome to Archive torrents |access-date=22 December 2015 |archive-url=https://web.archive.org/web/20160119064508/https://archive.org/details/bittorrent |archive-date=19 January 2016 |url-status=live |df=dmy-all }}. Archive.org. 2012.</ref>

By early 2015, [[AT&T]] estimated that BitTorrent accounted for 20% of all broadband traffic.<!-- this reference should be replaced by the original paper by Terry Shaw and Jim Martin, which I couldn't locate --><ref name="Ellis">{{cite web |url=http://thestack.com/atandt-patents-system-speed-up-bittorrent-traffic-190215 |date=8 May 2006 |title=AT&T Patents System to 'Fast-Lane' BitTorrent Traffic |website=TheStack.com |access-date=5 March 2015 |archive-url=https://web.archive.org/web/20150223032802/http://thestack.com/atandt-patents-system-speed-up-bittorrent-traffic-190215 |archive-date=23 February 2015 |url-status=live |df=dmy-all }}</ref>

Routers that use [[network address translation]] (NAT) must maintain tables of source and destination IP addresses and ports. Because BitTorrent frequently contacts 20–30 servers per second, the NAT tables of some consumer-grade routers are rapidly filled. This is a known cause of some home routers ceasing to work correctly.<ref>{{cite web|title=FAQ:Modems/routers that are known to have problems with P2P apps |website=uTorrent.com|url=http://www.utorrent.com/faq.php#Modems_routers_that_are_known_to_have_problems_with_P2P |archive-url=https://web.archive.org/web/20080913085527/http://www.utorrent.com/faq.php#Modems_routers_that_are_known_to_have_problems_with_P2P |archive-date=13 September 2008|access-date=7 April 2013}}</ref><ref>{{cite book|title=UDP NAT and Firewall Puncturing in the Wild|author=Halkes, Gertjan |author2=Pouwelse, Johan|editor=Jordi Domingo-Pascual |display-editors=etal |page=7|work=NETWORKING 2011:10th International IFIP TC 6 Networking Conference, Valencia, Spain, May 9–13, 2011, Proceedings|publisher=Springer|year=2011|url=https://books.google.com/books?id=j6HPyxvPFcsC&pg=PA7|access-date=7 April 2013|isbn=9783642207976|archive-url=https://web.archive.org/web/20130509032124/http://books.google.com/books?id=j6HPyxvPFcsC&pg=PA7#v=onepage&f=false|archive-date=9 May 2013|url-status=live|df=dmy-all}}</ref>

== Legislation ==
{{Main|Legal issues with BitTorrent}}

Although the protocol itself is legal,<ref>{{cite web|url=https://www.comparitech.com/blog/vpn-privacy/is-torrenting-safe-illegal-will-you-be-caught/|title=Is torrenting safe? Is it illegal? Are you likely to be caught?|date=29 November 2018|access-date=5 October 2018|archive-url=https://archive.today/20180406235039/https://www.comparitech.com/blog/vpn-privacy/is-torrenting-safe-illegal-will-you-be-caught/|archive-date=6 April 2018|url-status=live|df=dmy-all}}</ref> problems stem from using the protocol to traffic copyright infringing works, since BitTorrent is often used to download otherwise paid content, such as movies and video games. There has been much controversy over the use of BitTorrent trackers. BitTorrent metafiles themselves do not store file contents. Whether the publishers of BitTorrent metafiles violate copyrights by linking to copyrighted works without the authorization of copyright holders is controversial. Various jurisdictions have pursued legal action against websites that host BitTorrent trackers.

As a result the use of BitTorrent may sometimes be limited by [[Internet service provider|Internet Service Providers]] (ISPs) due to legal or copyright grounds.{{Citation needed|date=January 2022}} Users may choose to run [[Seedbox|seedboxes]] or [[Virtual private network|virtual private networks]] (VPNs) to circumvent these restrictions.

High-profile examples include the closing of [[Suprnova.org]], [[TorrentSpy]], [[LokiTorrent]], [[BTJunkie]], [[Mininova]], [[Oink's Pink Palace]] and [[What.cd]]. BitTorrent search engine [[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;<ref>{{cite web| url=http://torrentfreak.com/the-piratebay-is-down-raided-by-the-swedish-police/| date=31 May 2006| publisher=TorrentFreak| title=The Piratebay is Down: Raided by the Swedish Police| access-date=20 May 2007 | archive-url = https://web.archive.org/web/20140416060632/https://torrentfreak.com/the-piratebay-is-down-raided-by-the-swedish-police/ | archive-date = 16 April 2014| url-status=live}}</ref> however, the tracker was up and running again three days later. In the study used to value NBC Universal in its merger with Comcast, Envisional examined the 10,000 torrent swarms managed by PublicBT which had the most active downloaders. After excluding pornographic and unidentifiable content, it was found that only one swarm offered legitimate content.<ref>{{cite web |url=http://documents.envisional.com/docs/Envisional-Internet_Usage-Jan2011.pdf |date=1 January 2011 |title=Technical report: An Estimate of Infringing Use of the Internet |publisher=Envisional |access-date=6 May 2012 |archive-url=https://web.archive.org/web/20120425151935/http://documents.envisional.com/docs/Envisional-Internet_Usage-Jan2011.pdf |archive-date=25 April 2012 |url-status=live |df=dmy-all }}</ref>

In the United States, more than 200,000 lawsuits have been filed for copyright infringement on BitTorrent since 2010.<ref>{{cite news|url=https://www.theguardian.com/technology/pda/2011/aug/09/bittorrent-piracy |title=BitTorrent: Copyright Lawyers' Favourite Target Reaches 200,000 Lawsuits |newspaper=The Guardian |date=9 August 2011 |access-date=10 January 2014 |archive-url=https://web.archive.org/web/20131204002125/http://www.theguardian.com/technology/pda/2011/aug/09/bittorrent-piracy |archive-date=4 December 2013 }}</ref>  In the United Kingdom, on 30 April 2012, the [[High Court of Justice]] ordered five ISPs to [[List of websites blocked in the United Kingdom|block The Pirate Bay]].<ref>{{cite magazine |url=https://www.pcmag.com/news/297264/u-k-high-court-orders-isps-to-block-the-pirate-bay |title=U.K. High Court Orders ISPs to Block The Pirate Bay |last=Albanesius |first=Chloe |date=30 April 2012 |magazine=PC Magazine |url-status=live |archive-url=https://web.archive.org/web/20130525155105/http://www.pcmag.com/article2/0%2C2817%2C2403749%2C00.asp |archive-date=25 May 2013 |access-date=6 May 2012 |df=dmy-all }}</ref>

== Security ==

One concern is the [[UDP flood attack]]. BitTorrent implementations often use [[μTP]] for their communication. To achieve high bandwidths, the underlying protocol used is [[User Datagram Protocol|UDP]], which allows spoofing of source addresses of internet traffic. It has been possible to carry out [[denial-of-service attack]]s in a P2P lab environment, where users running BitTorrent clients act as amplifiers for an attack at another service.<ref>{{cite web|last=Adamsky|first=Florian|year=2015|url=https://www.usenix.org/conference/woot15/workshop-program/presentation/p2p-file-sharing-hell-exploiting-bittorrent|title=P2P File-Sharing in Hell: Exploiting BitTorrent Vulnerabilities to Launch Distributed Reflective DoS Attacks|access-date=21 August 2015|archive-url=https://web.archive.org/web/20151001113522/https://www.usenix.org/conference/woot15/workshop-program/presentation/p2p-file-sharing-hell-exploiting-bittorrent|archive-date=1 October 2015|url-status=live|df=dmy-all}}</ref> However this is not always an effective attack because ISPs can check if the source address is correct.

Several studies on BitTorrent found files available for download containing [[malware]]. In particular, one small sample indicated that 18% of all executable programs available for download contained malware.<ref>{{cite web |url=http://www.techrepublic.com/whitepapers/searching-for-malware-in-bittorrent/1681115 | title=Searching for Malware in Bit Torrent |first1=Andrew D. |last1=Berns |first2=Eunjin (EJ) |last2=Jung |date=24 April 2008 |publisher=[[University of Iowa]], via [[TechRepublic]] |access-date=7 April 2013 | archive-url = https://web.archive.org/web/20130501051349/http://www.techrepublic.com/whitepapers/searching-for-malware-in-bittorrent/1681115 | archive-date = 1 May 2013|url-status=dead}}{{registration required}}</ref> Another study claims that as much as 14.5% of BitTorrent downloads contain [[Zero-day attack#Viruses|zero-day malware]], and that BitTorrent was used as the distribution mechanism for 47% of all zero-day malware they have found.<ref>{{cite conference|last1=Vegge|first1=Håvard|last2=Halvorsen|first2=Finn Michael|last3=Nergård|first3=Rune Walsø|conference=2009 Fourth International Conference on Internet Monitoring and Protection|pages=66|publisher=IEEE Computer Society|year=2009|url=http://www.rookconsulting.com/Downloads/NTNU-zeroday-project-2008.pdf|archive-url=https://web.archive.org/web/20130617143905/http://www.rookconsulting.com/Downloads/NTNU-zeroday-project-2008.pdf|archive-date=2013-06-17|format=PDF ''(orig. work + pub. paper)''|doi=10.1109/ICIMP.2009.19|title=Where Only Fools Dare to Tread: An Empirical Study on the Prevalence of Zero-Day Malware|isbn=978-1-4244-3839-6|s2cid=15567480}}</ref>

== See also ==

* [[Anonymous P2P]]
* [[Anti-Counterfeiting Trade Agreement]]
* [[Bencode]]
* [[Cache Discovery Protocol]]
* [[Comparison of BitTorrent clients]]
* [[Comparison of BitTorrent sites]]
* [[Comparison of BitTorrent tracker software]]
* [[Glossary of BitTorrent terms]]
* [[Magnet URI scheme]]
* [[Simple file verification]]
* [[Super-seeding]]
* [[Torrent poisoning]]

== References ==

{{reflist|colwidth=30em}}

== Further reading ==
* {{cite book |chapter= The Bittorrent P2P File-Sharing System: Measurements and Analysis |title=Peer-to-Peer Systems IV |volume=3640 |last=Pouwelse |first=Johan |year=2005 |publisher=Springer |location=Berlin |isbn=978-3-540-29068-1 |doi=10.1007/11558989_19 |pages=205–216|chapter-url=https://books.google.com/books?id=Dnw7E8xzQUMC&q=The%20Bittorrent%20P2P%20File-Sharing%20System%3A%20Measurements%20and%20Analysis%20Johan%20Pouwelse%20%2C%20Pawel%20Garbacki%2C%20Dick%20Epema%2C&pg=PA205|access-date=4 September 2011|display-authors=etal|series=Lecture Notes in Computer Science }}
* {{cite thesis |first=Michal |last=Czerniawski |ssrn=1540913 |title=Responsibility of Bittorrent Search Engines for Copyright Infringements |publisher=[[SSRN]] |date=20 December 2009 |doi=10.2139/ssrn.1540913 }}
* {{cite web |url=https://web.stanford.edu/class/ee380/Abstracts/050216.html |title=Under the hood of BitTorrent |first=Bram |last=Cohen |publisher=[[Stanford University]] |date=16 February 2005 |work=Computer Systems Colloquium (EE380) }}

== External links ==
{{Commons category}}

* {{Official website}}
* [https://www.bittorrent.org/beps/bep_0003.html Specification]
* {{curlie|Computers/Internet/File_Sharing/BitTorrent|BitTorrent}}

{{Cloud computing}}
{{BitTorrent}}
{{File sharing}}

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[[Category:Application layer protocols]]
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