BitTorrent

= BitTorrent = toc
 * BitTorrent** is a peer-to-peer file sharing protocol used for distributing large amounts of data over the Internet. BitTorrent is one of the most common protocols for transferring large files, and it has been estimated that it accounted for roughly 43% to 70% of all Internet traffic (depending on geographical location) as of February 2009 . Programmer Bram Cohen designed the protocol in April 2001 and released a first implementation on July 2, 2001. It is now maintained by Cohen's company BitTorrent, Inc. There are numerous BitTorrent clients available for a variety of computing platforms . As of January 2011, BitTorrent has 100 million users and a greater share of network bandwidth than Netflix and Hulu combined. At any given instant of time BitTorrent has, on average, more active users than YouTube and Facebook combined. (This refers to the number of active users at any instant and not to the total number of unique users.)

How does it work?
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 download and upload from each other simultaneously. The protocol is an alternative to the older single source, multiple mirror sources technique for distributing data, and can work over networks with lower bandwidth so many small computers, like mobile phones, are able to efficiently distribute files to many recipients. A user who wants to upload a file first creates a small //torrent// descriptor file that they distribute by conventional means (web, email, etc.). They then make the file itself available through a BitTorrent node acting as a **// seed //**. Those with the torrent descriptor file can give it to their own BitTorrent nodes which, acting as **// peers //** or **// leechers //**, download it by connecting to the seed and/or other peers.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 to other peers, relieving the original seed from having to send a copy to every computer or user wishing for 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. 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. When a peer completely downloads a file, it becomes an additional seed. This eventual shift 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 leads to a flood like spreading of a file throughout many peer computer nodes. As more peers join the swarm, the likelihood of a complete 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. It also provides redundancy against system problems, reduces dependence on the original distributor and provides sources for the file which are generally transient and therefore harder to trace by those who would block distribution compared to the situation provided by limiting availability of the file to a fixed host machine (or even several).

BitTorrent Clients
The BitTorrent specification is free to use and many clients are open source, so BitTorrent clients have been created for all common operating systems using a variety of programming languages. The official BitTorrent client, µTorrent, Xunlei, Vuze and BitComet are some of the most popular clients. 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. Server-oriented BitTorrent implementations can also be hosted by hosting providers at 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 supports BitTorrent, as does Wyzo. BitLet allows users to download Torrents directly from their browser using a Java applet. Sites such as xFiles and DuShare allow to transfer big files directly using bittorrent inside adobe Flash. 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 DRM, encryption, and authentication are found within managed clients such as Pando.

Is it Legal?
The BitTorrent protocol's wide use for copyright infringement has led to legal issues with BitTorrent. The technology itself is perfectly legal, but it has been debated if its implementation in connection with copyrighted material or otherwise illegal material makes the issuer of the BitTorrent file, as opposed to the copyrighted material itself, liable as an accomplice or an infringing party. 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 material without the authorization of copyright holders is still being debated. A BitTorrent file can be seen as a hyperlink or very specific instruction of how to obtain something on the internet, including illegal or copyrighted content. The degree of illegality varies, but, in general, court decisions in various nations deem it illegal. Due to the nature of the internet, it is possible to host the BitTorrent file in areas where it is not illegal - making it more an issue if ISPs can provide access there and if anyone can access that file (or the illegal content), which again would vary by jurisdictions. Various jurisdictions have pursued legal action against websites that host BitTorrent trackers. High-profile examples include the closing of Suprnova.org, Torrentspy, LokiTorrent, Mininova and OiNK.cd. 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 May 31, 2006, The Pirate Bay's servers in Sweden were raided by Swedish police on allegations by the MPAA of copyright infringement; however, the tracker was up and running again three days later.

**References**
**[|BitTorrent]**
 * [|Swedish Police Close Pirate Bay]**
 * [|HBO attacks BitTorrent]**