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. 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. 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. 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 length of the download. 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 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 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. Distributed downloading protocols in general provide redundancy against system problems, reduces dependence on the original distributor [ 9 ] 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.
A BitTorrent client is any program that implements the BitTorrent protocol. Each client is capable of preparing, requesting, and transmitting any type of computer file over a network, using the protocol.
A peer is any computer running an instance of a client. To share a file or group of files, a peer first creates a small file called a " torrent " e. This file contains metadata e. Peers that want to download the file must first obtain a torrent file for it and connect to the specified tracker, which tells them from which other peers to download the pieces of the file.
Though both ultimately transfer files over a network, a BitTorrent download differs from a classic download as is typical with an HTTP or FTP request, for example in several fundamental ways:. 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 " flash crowds " than regular 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 general, BitTorrent's non-contiguous download methods have prevented it from supporting progressive download or "streaming playback".
However, comments made by Bram Cohen in January [ 11 ] suggest that streaming torrent downloads will soon be commonplace and ad supported streaming [ 12 ] appears to be the result of those comments. In January Cohen demonstrated an early version of BitTorrent streaming, saying the feature was projected to be available by summer 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 kB and 16 MB each.
The peer creates a hash for each piece, using the SHA-1 hash function, and records it in the torrent file. Pieces with sizes greater than kB will reduce the size of a torrent file for a very large payload, but is claimed to reduce the efficiency of the protocol.
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. Torrent files have an "announce" section, which specifies the 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.
Torrent files are typically published on websites or elsewhere, and registered with at least one tracker. The tracker maintains lists of the clients currently participating in the torrent.
Azureus was the first [ citation needed ] BitTorrent client to implement such a system through the distributed hash table DHT method. 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, , but has not been accepted yet. Users find a torrent of interest, by browsing the web or by other means, download it, and open it with a BitTorrent client.
The client connects to the tracker s specified in the torrent file, from which it receives a list of peers currently transferring pieces of the file s specified in the torrent. 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; for example they download pieces in a random order to increase the opportunity to exchange data, which is only possible if two peers have different pieces of the file. 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 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 don't 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, so called preferred peers in hopes of discovering even better partners and to ensure that newcomers get a chance to join the swarm.
Although swarming scales well to tolerate flash crowds for popular content, it is less useful for unpopular 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.
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. However, there are ways to promote anonymity; for example, the OneSwarm project layers privacy-preserving sharing mechanisms on top of the original BitTorrent protocol. A growing number of individuals and organizations are using BitTorrent to distribute their own or licensed material.
Independent adopters report that without using BitTorrent technology, and its dramatically reduced demands on their private networking hardware and bandwidth, they could not afford to distribute their files. As of [update] BitTorrent has million users and a greater share of network bandwidth than Netflix and Hulu combined.
This refers to the number of active users at any instant and not to the total number of registered users. Routers that use network address translation NAT must maintain tables of source and destination IP addresses and ports.
Typical home routers are limited to about table entries [ citation needed ] while some more expensive routers have larger table capacities. This is a common cause of home routers locking up. 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 material without the authorization of copyright holders, rendering those sites especially vulnerable to lawsuits.
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 trackers 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. 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-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. However, recently some P2P , decentralized alternatives to Torrent search engines have emerged, see decentralized keyword search further down the page.
The BitTorrent protocol is still under development and may therefore still acquire new features and other enhancements such as improved efficiency. On May 2, , Azureus 2. The following month, BitTorrent, Inc. Both DHT implementations are based on Kademlia. 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. Peer exchange checks with known peers to see if they know of any other peers. With the 3. Web seeding was implemented in 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 through the use of existing, cheap, web hosting setups.
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. BitComet added support for web seeds in version 1. 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. In September , a new service named Burnbit was launched which generates a torrent from any URL using webseeding.
There are server-side solutions that provide initial seeding of the file from the webserver via standard BitTorrent protocol and when the number of external seeders reach a limit, they stop serving the file from the original source.
A technique called broadcatching combines RSS 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, The RSS feed will track the content, while BitTorrent ensures content integrity with cryptographic hashing of all data, so feed subscribers will receive uncorrupted content. One of the first and popular software clients free and open source for broadcatching is Miro. The BitTorrent web-service MoveDigital had the ability to make torrents available to any web application capable of parsing XML through its standard REST-based interface, [ 67 ] although this has since been discontinued.
Additionally, Torrenthut is developing a similar torrent API that will provide the same features, as well as further intuition to help bring the torrent community to Web 2. Since BitTorrent makes up a large proportion of total traffic, some ISPs have chosen to throttle slow down BitTorrent transfers to ensure network capacity remains available for other uses. For this reason, methods have been developed to disguise BitTorrent traffic in an attempt to thwart these efforts.
Reports in August indicated that Comcast was preventing BitTorrent seeding by monitoring and interfering with the communication between peers. Protection against these efforts is provided by proxying the client-tracker traffic via an encrypted tunnel to a point outside of the Comcast network. In general, although encryption can make it difficult to determine what is being shared, BitTorrent is vulnerable to traffic analysis. Another unofficial feature is an extension to the BitTorrent metadata format proposed by John Hoffman [ 74 ] 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. 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 [ 75 ] can decrease the time it takes to download a file, but also have a few consequences:. Even with distributed trackers, a third party is still required to find a specific torrent. This is usually done in the form of a hyperlink from the website of the content owner or through indexing websites like isoHunt , Torrentz , BTDigg or The Pirate Bay. The Tribler BitTorrent client is the first to incorporate decentralized search capabilities.
With Tribler, users can find. It adds such an ability to the BitTorrent protocol using a gossip protocol, somewhat similar to the eXeem network which was shut down in 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.
In May Cornell University published a paper proposing a new approach to searching a peer-to-peer network for inexact strings, [ 78 ] 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 [ 79 ] and published a follow-up paper reporting its success. A somewhat similar facility but with a slightly different approach is provided by the BitComet client through its "Torrent Exchange" [ 81 ] 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. 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. For example, this can be used to centralize file sharing on a single dedicated server which users share access to on the network.
The Opera web browser supports BitTorrent, [ 84 ] as does Wyzo. BitLet allows users to download Torrents directly from their browser using a Java applet.
An increasing number of hardware devices are being made to support BitTorrent. Proprietary versions of the protocol which implement DRM , encryption, and authentication are found within managed clients such as Pando. An unimplemented as of February [update] 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. As of December [update] , BitTorrent, Inc. 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.
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 controversial. Various jurisdictions have pursued legal action against websites that host BitTorrent trackers. High-profile examples include the closing of Suprnova.
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. The Internet Archive is a non-profit organization that preserves content and makes it downloadable on the Internet. It's known for its Wayback Machine , which stores copies of websites and allows you to go back in time and relive the past.
The organization also offers a huge archive of public domain media -- recordings of live concerts, eBooks, old movies and TV shows, and other audio recordings. The Internet Archive recommends people use BitTorrent to download its content, as it's the fastest method and allows the non-profit organization to save on bandwidth costs.
In , the UK government released several large data sets showing how public money was being spent. To make these available, they offered them via BitTorrent. This allowed the government to save on bandwidth costs. And, let's face it -- BitTorrent is also the fastest way to make such documents available to the largest number of people possible. BitTorrent, Inc. BitTorrent Sync works differently from standard BitTorrent clients.
It's entirely private: you install the client, choose one or more folders to share, and then link it up with other computers. Files anyone places in their copy of the shared folder are all automatically synced with all other copies of the shared folders. In this way, BitTorrent Sync is a lot like Dropbox.
Unlike Dropbox, it doesn't store your files in a centralized server online -- it just syncs them between computers you own or computers your friends own. This means that it offers easy file sharing over the Internet and, unlike Dropbox, you can sync an unlimited number of files as long as you have the space on your computers for them. BitTorrent Sync could be used to share pirated content, but that would be silly when pirated content is available in so many public BitTorrent streams.
It's a great way to roll your own Dropbox-like service and share files across the Internet without trusting them to a central server or being limited by the size of your cloud storage account.
If you're familiar with BitTorrent, you'll know that BitTorrent users always say they're downloading "Linux ISOs" as a joke when they're actually downloading pirated content. Whether you're downloading the latest release of Ubuntu, Fedora, Debian, or any of the other best Linux distributions , there's a good chance you're getting it via BitTorrent.
These distributions offer themselves for free to everyone and they're often 1 GB or larger. BitTorrent can help them save on bandwidth costs and speed up downloads. If you want to make media availableperhaps you've produced a documentary and want to release it for free or you're a band that wants to release free music as a promotionBitTorrent is one of the best ways to do it. If you hosted the files yourself, you'd have to pay for a lot of bandwidth.
If you make the files available via BitTorrent, you'd save a lot of bandwidth by letting your fans contribute their bandwidth as they downloaded your content. You'd also receive press just for making your files available via BitTorrent. The official BitTorrent website has a list of "bundles" of music and videos artists make available to hook fans, just as radio was used to offer free music to large number of people in hopes that they'll attend live shows and buy albums.
BitTorrent is a great way to distribute any large chunk of data as fast as possible, saving money on bandwidth. In addition to all the uses above, BitTorrent has been used to share large scientific data sets with anyone interested. Any large chunk of data that's free for anyone to access can be distributed publically with BitTorrent.
If we look at the examples above, we can see that BitTorrent is very useful in several situations:. BitTorrent is a tool, and a particularly useful one -- that's why it's so widely used for piracy. BitTorrent also allows the Internet to be more participatory, enabling average people to share their files without paying for massive amounts of bandwidth and contribute their own bandwidth to share other people's files.
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