BitTorrent is the name of a peer-to-peer (P2P) file distribution client application and also of its related file sharing protocol, both of which were created by programmer Bram Cohen. BitTorrent is designed to distribute large amounts of data widely without incurring the corresponding consumption in costly server and bandwidth resources. CacheLogic suggests that BitTorrent traffic accounts for ~35% of all traffic on the Internet, while other sources are skeptical.
BitTorrent is trademark of BitTorrent Inc and its use is subject to a trademark use policy.
The original BitTorrent application was written in Python. Its source code, as of version 4.0, has been released under the BitTorrent Open Source License (a modified version of the Jabber Open Source License). There are numerous compatible clients, written in a variety of languages and running on a variety of computing platforms.
BitTorrent clients are programs which implement the BitTorrent protocol. Each BitTorrent client is capable of preparing, requesting, and transmitting any type of computer file over a network using the BitTorrent protocol. This includes text, audio, video, encrypted content, and other types of digital information.
.torrent. Torrent files contain an “announce” section, which specifies the URL of the tracker, and an “info” section which contains a suggested name for the file, fragment size, a key length, file length, and a pass. A single torrent can contain information on one or more files. Clients who have finished downloading the file may also choose to act as seeders, providing a complete copy of the file. After the torrent file is created, a link to it is placed on a website, and it is registered with a tracker. BitTorrent trackers maintain lists of the clients currently downloading the file.
The computer with the initial copy of the file is referred to as the initial seeder.
Initially, there may be no other peers in the swarm, in which case the client connects directly to the initial seeder and begins to request fragments. The BitTorrent protocol breaks down files into a number of small fragments, typically a quarter of a megabyte (256 KB) in size. Larger file sizes typically have larger fragments. For example, a 4.37 GB file will often have a fragment size of up to 4 MB (4096 KB). File fragments are checked as they are received using a hash algorithm to ensure that they are error free.
As peers enter the swarm, they begin sharing fragments with one another. Because clients share fragments with one another, instead of directly from the seeder, BitTorrent networks easily scale to large numbers of clients. The protocol incorporates mechanisms so that clients choose peers with the best network connections for the fragment they are requesting. One major innovation that adds to the scalability of BitTorrent is the concept of “rare fragments.” The BitTorrent protocol specifies that clients should always request fragments that are the rarest, meaning they are held by the fewest number of clients in the swarm. By requesting the rarest fragments, the BitTorrent protocol ensures that one machine will not be swamped with requests, eliminating potential network bottlenecks.
BitTorrent also uses a mechanism called “optimistic unchoking” to create a group of “preferred peers.” Preferred peers are a subset of peers in a swarm who are known to offer the most bandwidth. Optimistic unchoking allows BitTorrent to determine which peers should be preferred by periodically testing peers. Peers outside the group are offered a file fragment. If the peer downloads the fragment faster than a peer in the preferred peered group, the new peer displaces the old. This ensures that clients are always downloading from a group of the fastest available peers.
Another drawback to the BitTorrent model is that file sharers have little incentive to become seeders after they finish downloading. The result of this is that seeds gradually die out as the swarm surrounding them dies out, meaning a lower possibility of obtaining older torrents. This has been addressed by certain tracking websites that record each user's download/upload ratio and provide access to older torrent files to people with better ratios.
BitTorrent excels in continuously connected, broadband environments. Dial-up users find it less efficient due to frequent disconnects and slow download rates.
max_uploads has been reached)
.torrent metadata file or all files described by it, depending on context. The torrent file contains metadata about all the files it makes downloadable, including their names and sizes and checksums of all pieces in the torrent. It also contains the address of a tracker that coordinates communication between the peers in the swarm.
The method used by BitTorrent to distribute files parallels the one used by the eDonkey2000 network, but nodes in eDonkey's file sharing network usually share and download a much larger number of files, making the bandwidth available to each transfer much smaller. Also eDonkey has queue-based system wherein there might be 200 people sharing the file but only one or two have queue free. Hence a user ends up getting files from only a few and rising up in rank in other users' queue list (while getting no download from them). BitTorrent transfers are typically very fast, because all nodes in a group concentrate on transferring a single file or collection of files. While the original eDonkey2000 client provided little "leech resistance", most new clients have some sort of system to encourage uploaders. eMule, for example, has a credits system whereby a client rewards other clients that upload to it by increasing their priority in its queue. However, the nature of the eDonkey2000 concept means download speeds tend to be much more variable, although the number of available files is far greater.
A similar method to BitTorrent was the Participation Level introduced in Kazaa in 2002. A user's Participation Level would increase when they uploaded and decrease when they downloaded. Then when a user uploaded a file, the person with the highest Participation Level would get it first, then the next highest, and so on. This can be visualised as a pyramid, with the clients who have the most upload bandwidth available at the top and those with less bandwidth on progressively lower levels. This is the most efficient way to distribute a file to a large number of users: it is probable that even the people at the bottom of the pyramid will get the file faster than if the file was served by a non-P2P method. Unfortunately, the implementation adopted by Kazaa is considered by some to be flawed as it relies on the client accurately reporting their Participation Level and therefore it is easy to cheat using one of the many unofficial clients.
Bram Cohen, the creator of the protocol, was hired in 2004 by Valve Software to develop a means of distributing patches and other content for online video games.
In December 2004, the Finnish police raided a major BitTorrent site, Finreactor. Original investigation contained over 60 suspects. The case is being prepared by prosecution, and 33 people, mostly administrators and moderators will be filed charges. Originally there were 34 defendants under investigation, but one of them at the time was under 15 years old and charges against him were dropped. The case must enter court before December 2006, which is the timelimit for investigation period during which any charges must be filed or the case will be dismissed.
Suprnova.org, one of the most popular early BitTorrent sites, closed in December 2005, supposedly due to the pressure felt by Sloncek, the founder and administrator of the site. In December, 2005, Sloncek revealed that the Suprnova computer servers had in fact been confiscated by Slovenian authorities. LokiTorrent, arguably the biggest torrent source after the demise of Suprnova, closed down soon after Suprnova. Allegedly, after threats from the MPAA, Edward Webber (known as 'lowkee'), webmaster of the site, was ordered by the court to pay a fine and supply the MPAA with logs (the IP addresses of visitors).
Webber, in the weeks following his receipt of the subpoena, had begun a fundraising campaign to pay lawyers fees in a legal battle against the MPAA. Webber raised approximately US$45,000 through a PayPal-based donation system. It is unclear how much of that money went to the MPAA, but taking into account the amount of damages he most likely had to pay, probably much of it. Following the agreement, the MPAA changed the LokiTorrent website to display a message intended to intimidate filesharers. Webber did not comment on this change.
On May 25, 2005, the popular BitTorrent website EliteTorrents.org was shut down by the United States Federal Bureau of Investigation and Immigration and Customs Enforcement. At first it was thought that a malicious hacker had gained control of the website, but it was soon discovered that the website had been taken over by the US government. Ten search warrants relating to members of the website were executed.
On October 24, 2005, a 38-year-old Hong Kong BitTorrent user Chan Nai-ming (???, using the handle ???? Lit. The master of cunning, while the magistrate referred to him as Big Crook) allegedly distributed the three pirated movies Daredevil, Red Planet and Miss Congeniality, subsequently uploading the torrent file to a newsgroup (See HKSAR v Chan Nai Ming). He was convicted of breaching the copyright ordinance, Chapter 528 of Hong Kong law. The magistrate remarked that Chan's act caused significant damage to the interest of copyright holders. He was released on bail for *]5,000, awaiting a sentencing hearing, though the magistrate himself admitted the difficulty of determining how he should be sentenced due to the lack of precedent for such a case. On November 7, 2005, he was sentenced to jail for three months but was immediately granted parole pending an appeal to the High Court.
On November 23, 2005, the movie industry and Bram Cohen, the creator of BitTorrent signed a deal they hoped would reduce the number of pirated films shared on the downloading network. The deal covered films found via the bittorrent.com website run by BitTorrent, Inc. It meant BitTorrent.com had to remove any links to pirated films made by seven Hollywood movie studios. As it covered only the BitTorrent.com website, it is unclear what overall effect this has had on net piracy.BBC News
The Pirate Bay is another popular BitTorrent website which was formed out of a Swedish anti-copyright group. The site also contains torrents which point to copies of copyright-protected material. The Pirate Bay is notorious for its "legal" section in which letters and replies on the subject of alleged copyright infringements are publicly displayed. The replies are written in a humorous manner and a hard copy of one was even sold on eBay for USD $255. As of May 31, 2006, however, The Pirate Bay's servers, which are based in Sweden, have been raided by Swedish police and the site owners might be facing charges for copyright infringement or facilitating in it according to the accusations on the search warrant. No charges have been made so far. However, after securing new servers in The Netherlands and using a recent backup, The Pirate Bay was back online in less than 72 hours. Recently, The Pirate Bay has returned to Sweden. The return has been facilitated by the public and media backlash against the Swedish Government's actions. The Pirate Bay is now, supposedly, going to counter-sue the Swedish government for millions of Swedish kroner (SEK) lost from having their website shut down.
It is not clear, however, how long one should leave their client open after downloading has finished. Many Trackers/Sites asks their users to seed at least 72 hours and/or until a share ratio of 1.0 is reached. Members-only trackers and sites enforce this rule, thus files on these websites have a higher traffic than others, and the torrents on these websites remain active longer than other free torrent sites/trackers. Many clients report the byte traffic upstream as well as down, so the user can see how much they have contributed back to the network. Some clients also report the "share ratio", a number relating the amount of data uploaded to the amount downloaded. A share ratio of 1.0 means that a user has uploaded as much data as they have downloaded. A share ratio greater than 1 means that a user has uploaded more than they have downloaded. It is generally considered good form to at least share back the equivalent amount of traffic as the original file size.
Share ratios are more important on BitTorrent than they are on other peer-to-peer file sharing networks, because many BitTorrent trackers require users to maintain a minimum global share ratio. On some trackers that require users to register, the minimum global share ratio may start at around 0.5 and increase over time, so that the user has adequate time to upload and share their files. Users with a share ratio below the minimum may be put into a restricted "upload-only" mode, where they may not download until their share ratio reaches the minimum.
While it is highly unlikely that all users who download a given torrent will achieve a 1.0 ratio on it (because the net ratio of all users is 1.0, if any user uploads past 1.0 some other user will have to sustain a lower ratio), it is more of a guideline to encourage the average upstream of a given network. Some networks, for example, prevent access to new torrents for the first 24-48 hours that the torrent is active to people with overall ratios of less than 1.0 and a certain amount of data uploaded.
The amount of time the client is left open may be more important than the amount of traffic contributed, since new users attempting to download a file will first need to find peers hosting the file.
Many advanced trackers now track statistics such as how many seeders and downloaders were on a torrent at the time of a user's disconnect as many consider this information more important than just the user's ratio of data downloaded/uploaded.
In May 2005, Bram Cohen released a new beta version of BitTorrent that eliminated the need for web site hosting of centralized servers known as "trackers". It is now possible to have a torrent up in minutes, with a file, a website, and no understanding of how it works. In addition, Cohen launched a new search service on BitTorrent's website, similar to those found on other popular sites such as The Pirate Bay.
Cohen explained that the tracker removal feature is part of his ongoing effort to make publishing files online "painless and disruptively cheap". The move is only one of several designed to remove BitTorrent's dependence on centralized trackers.
This change is said to cause some trouble in the legal efforts to shut down illegal file sharing. However, Tarun Sawney, BSA Asia antipiracy director, said BitTorrent files could still be identified, since with or without the tracker sites, actual users still host the infringing files.
While potential illegal uses abound as is the case with any new distribution method, this idea lends itself to a great number of ideas that could turn traditional distribution models on their heads, giving smaller operations a new opportunity for content distribution. The system leans on the cost-saving benefit of BitTorrent, where expenses are virtually non-existent; each downloader of a file participates in a portion of the distribution. One early adoption of this concept is IPTV show mariposaHD, which uses BitTorrent to distribute large (1-2 GB) WMVHD files of high-definition video.
RSS feeds layered on top keep track of the content, and because BitTorrent does cryptographic hashing of all data, subscribers to the feed can be sure they're getting what they think they're getting, whether that winds up being the latest Sopranos episode, or the latest Sveasoft firmware upgrade. (Naturally, however, ensuring that the same data reaches all nodes neglects the possibility that the original, source file may be corrupted or incorrectly labeled.)
One of the first open source attempts to create a client specifically for this was Democracy Player. The idea is already gaining momentum however, with other Free Software clients such as PenguinTV and KatchTV also now supporting broadcatching.
Protocol header encrypt (PHE), Message stream encryption (MSE), or Protocol encryption (PE) are features of some BitTorrent clients that attempt to make BitTorrent hard to throttle. MSE and PE are two names for the same protocol. At the moment Azureus, Bitcomet and µTorrent, the three biggest BitTorrent clients, support PE/MSE encryption.
Some ISPs throttle BitTorrent traffic because it makes up a large proportion of total traffic and the ISPs don't want to spend money purchasing extra capacity. Instead, ISPs spend money purchasing hardware designed to look for and throttle BitTorrent traffic. Encryption makes BitTorrent traffic harder to detect and therefore harder to throttle. Recently, ISPs have announced possible future hardware upgrades in order to minimize BitTorrent traffic. Several universities have already taken these steps, including the University of Maryland, College Park, Brigham Young University, ASU, UTC, and WPI.
Peer exchange (PEX) is another method to gather peers for BitTorrent in addition to trackers and DHT. Peer exchange checks with known peers to see if they know of any other peers.
BitTorrent | Computer file formats | File sharing networks
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