BitTorrent is a peer-to-peer file sharing (P2P) communications protocol. BitTorrent is a method of distributing large amounts of data widely without the original distributor incurring the entire costs of hardware, hosting, and bandwidth resources. Instead, when data is distributed using the BitTorrent protocol, each recipient supplies pieces of the data to newer recipients, reducing the cost and burden on any given individual source, providing redundancy against system problems, and reducing dependence on the original distributor.

The protocol is the brainchild of programmer Bram Cohen, who designed it in April 2001 and released a first implementation on 2 July 2001.[1] It is now maintained by Cohen's company BitTorrent, Inc.

Usage of the protocol accounts for significant traffic on the Internet, but the precise amount has proven difficult to measure.

There are numerous compatible BitTorrent clients, written in a variety of programming languages, and running on a variety of computing platforms.

Operation[edit | edit source]

Main article: Operation of the BitTorrent protocol

Adoption[edit | edit source]

Main article: Adoption of the BitTorrent protocol

Network impact[edit | edit source]

CableLabs, the research organization of the North American cable industry, estimates that BitTorrent represents 18% of all broadband traffic.[2] In 2004, CacheLogic put that number at roughly 35% of all traffic on the Internet.[3] The discrepancies in these numbers are caused by differences in the methodology used to measure P2P traffic on the Internet.[4]

Routers that use NAT, Network Address Translation, must maintain tables of source and destination IP addresses and ports. Typical home routers are limited to about 2000 table entries while some more expensive routers have larger table capacities. BitTorrent frequently contacts 300-500 servers per second rapidly filling the NAT tables. This is a common cause of home routers locking up.[5]

Indexing[edit | edit source]

The BitTorrent protocol provides no way to index torrent files. As a result, a comparatively small number of websites have hosted the large majority of torrents linking to (possibly) copyrighted material, rendering those sites especially vulnerable to lawsuits. Several types of websites support the discovery and distribution of data on the BitTorrent network.

Public tracker sites such as The Pirate Bay allow users to search in and download from their collection of .torrent files; they also run BitTorrent trackers for those files. Users can typically also upload .torrent files for content they wish to distribute.

Private tracker sites such as Demonoid operate like public ones except that they restrict access to registered users and keep track of the amount of data each user uploads and downloads, in an attempt to reduce leeching.

There are specialized tracker sites such as FlixFlux for films, bitme for educational content, for metal music, PureTnA for pornographic content, and tv torrents for television series. Often these will also be private.

Search engines allow the discovery of .torrent files that are hosted and tracked on other sites; examples include Mininova, Monova, Btjunkie, Torrentz and isoHunt. 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 is often sorted with respect to relevance or number of seeders. Bram Cohen launched a BitTorrent search engine on that commingles licensed content with search results.[6] Metasearch engines allow to search several BitTorrent indices and search engines at once.

Legal issues[edit | edit source]

Main article: Legal issues with BitTorrent

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 copy copyrighted material that contravenes laws in some locations.

Various jurisdictions have pursued legal action against websites that host BitTorrent trackers. High-profile examples include the closing of,, LokiTorrent, Demonoid (now back online), and The Pirate Bay torrent website, formed by a Swedish anti-copyright group, is notorious for the "legal" section of its website in which letters and replies on the subject of alleged copyright infringements are publicly displayed. On May 31, 2006, The Pirate Bay's servers in Sweden were raided by Swedish police on allegations by the MPAA of copyright infringement;[7] 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.[8] 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.[9]

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

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.

Limitations and security vulnerabilities[edit | edit source]

Anonymity (or Lack Thereof)[edit | edit source]

BitTorrent does not offer its users anonymity. It is possible to obtain the IP addresses of all current, and possibly previous, participants in a swarm from the tracker. This may expose users with insecure systems to attacks.[10]

Dialup versus Broadband[edit | edit source]

BitTorrent is best suited to continuously connected broadband environments, since dial-up users find it less efficient due to frequent disconnects and slow download rates.

The Leech Problem[edit | edit source]

Another drawback is that BitTorrent file sharers, compared to users of client/server technology, often have little incentive to become seeders after they finish downloading. The result of this is that torrent swarms gradually die out, meaning a lower possibility of obtaining older torrents. Some BitTorrent websites have attempted to address this by recording each user's download and upload ratio for all or just the user to see, as well as the provision of access to newer torrent files to people with better ratios. Also, users who have low upload ratios may see slower download speeds until they upload more. This prevents (statistical) leeching, since after a while they become unable to download much faster than 1-10 kB/s on a high-speed connection. Some trackers exempt dial-up users from this policy, because they cannot upload faster than 1-3 kB/s.

The Leech Compensation Problem[edit | edit source]

Some seeders deliberately withhold one specific piece of the data. This results in clients downloading all but the final piece from the seed and from each other, thus leaving a large number of potential seeders once they receive the withheld piece of data.

The reason this is done is to combat the leeching problem described above. With clients each awaiting that one final piece, the seeder ensures that there will be many more seeds once the final piece is released.

The Cheater Problem[edit | edit source]

There are "cheating" clients like BitThief which claim to be able to download without uploading, and because of this can sometimes download faster than regular clients. Such exploitation negatively affects the cooperative nature of the BitTorrent protocol.

Technologies built on BitTorrent[edit | edit source]

Main article: Technologies built upon BitTorrent

Implementations[edit | edit source]

Main article: Comparison of BitTorrent software

Because of the open nature of the protocol, many clients have been developed that support numerous platforms and written using various programming languages. The official client is also named BitTorrent.

Some clients, like Torrentflux, can be run straight from a server, allowing hosting companies to offer speeds unavailable to most users. Sites such as Torrent2FTP offer services to download torrents and then make them available to the customer on a FTP server.

Opera Software now incorporates BitTorrent downloads through its popular browser software[11], as does Wyzo.

An increasing number of hardware devices are being made to support BitTorrent. These include routers and NAS devices, as well as anything capable of running OpenWrt (routers) or Openslug (NAS).

Development[edit | edit source]

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.[12] Andersen believes that this technique could be immediately used by developers with the BitTorrent file sharing system.[13]

See also[edit | edit source]

Notes[edit | edit source]


References[edit | edit source]



External links[edit | edit source]

This page uses Creative Commons Licensed content from Wikipedia (view authors). Smallwikipedialogo.png

From BitTorrent Wiki, a Wikia wiki.
Community content is available under CC-BY-SA unless otherwise noted.