|
BitTorrent is a communications protocol for the practice of peer-to-peer file sharing that is used to distribute large amounts of data over the Internet. BitTorrent is one of the most common protocols for transferring large files, and peer-to-peer networks have been estimated to collectively account for approximately 43% to 70% of all Internet traffic (depending on geographical location) . In November 2004, BitTorrent was responsible for 35% of all Internet traffic. As of February 2013, BitTorrent was responsible for 3.35% of all worldwide bandwidth, more than half of the 6% of total bandwidth dedicated to file sharing. To send or receive files the user must have a BitTorrent client; a computer program that implements the BitTorrent protocol. Some popular BitTorrent clients include μTorrent, Xunlei, Transmission, qBittorrent, Vuze, Deluge, and BitComet. BitTorrent trackers provide a list of files available for transfer, and assist the client in transferring and reconstructing the files. Programmer Bram Cohen, a former University at Buffalo student, designed the protocol in April 2001 and released the first available version on 2 July 2001, and the most recent version in 2013. BitTorrent clients are available for a variety of computing platforms and operating systems including an official client released by Bittorrent, Inc. , BitTorrent is utilized by 150 million active users (according to BitTorrent, Inc.). Based on this figure, the total number of monthly BitTorrent users can be estimated at more than a quarter of a billion. An extensive performance study of BitTorrent protocols has been done here. 〔Sharma P., Bhakuni A. & Kaushal R.( "Performance Analyis of BitTorrent Protocol ). National Conference on Communications, 2013 〕 ==Description== 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 to/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. 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. 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 (see diagram on the right). Segmented file transfer implementation: 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.〔 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 MB file may be transmitted as ten 1 MB pieces or as forty 256 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. Once a peer has downloaded a file completely, it becomes an additional seed. 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 flood-like spreading of a file throughout many peer computer nodes. As more peers join the swarm, the likelihood of a completely 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 redundancy against system problems, reduces dependence on the original distributor〔 by D. Menasche, A. Rocha, E. de Souza e Silva, R. M. Leao, D. Towsley, A. Venkataramani.〕 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). One such example of BitTorrent being used to reduce the distribution cost of file transmission is in the BOINC Client-Server system. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「BitTorrent」の詳細全文を読む スポンサード リンク
|