The first release of the BitTorrent client had no search engine and no peer exchange. Up until 2005, the only way to share files was by creating a small text file called a "torrent", that they would upload to a torrent index site. The first uploader acted as a seed, and downloaders would initially connect as peers. Those who wish to download the file would download the torrent, which their client would use to connect to a tracker which had a list of the IP addresses of other seeds and peers in the swarm. Once a peer completed a download of the complete file, it could in turn function as a seed. These files contain metadata about the files to be shared and the trackers which keep track of the other seeds and peers.
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BitTorrent v2 is intended to work seamlessly with previous versions of the BitTorrent protocol. The main reason for the update was that the old cryptographic hash function, SHA-1 is no longer considered safe from malicious attacks by the developers, and as such, v2 uses SHA-256. To ensure backwards compatibility, the v2 .torrent file format supports a hybrid mode where the torrents are hashed through both the new method and the old method, with the intent that the files will be shared with peers on both v1 and v2 swarms. Another update to the specification is adding a hash tree to speed up time from adding a torrent to downloading files, and to allow more granular checks for file corruption. In addition, each file is now hashed individually, enabling files in the swarm to be deduplicated, so that if multiple torrents include the same files, but seeders are only seeding the file from some, downloaders of the other torrents can still download the file. Magnet links for v2 also support a hybrid mode to ensure support for legacy clients.[13]
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 the beginning, BitTorrent's non-contiguous download methods made it harder to support "streaming playback". In 2014, the client Popcorn Time allowed for streaming of BitTorrent video files. Since then, more and more clients are offering streaming options.
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 works without the authorization of copyright holders, rendering those sites especially vulnerable to lawsuits.[16] A BitTorrent index is a "list of .torrent files, which typically includes descriptions" and information about the torrent's content.[17] Several types of websites support the discovery and distribution of data on the BitTorrent network. 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. Private host/tracker sites operate like public ones except that they may restrict access to registered users and may also keep track of the amount of data each user uploads and downloads, in an attempt to reduce "leeching".
Web search engines allow the discovery of torrent files that are hosted and tracked on other sites; examples include The Pirate Bay and BTDigg. 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). Metasearch engines allow one to search several BitTorrent indices and search engines at once.
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 .torrent file is automatically searched for (by info-hash value) in the DHT Network and when found it is downloaded by the querying client which can after that create and initiate a downloading task.
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" exchange 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.[24]
Although "swarming" scales well to tolerate "flash crowds" for popular content, it is less useful for unpopular or niche market 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. This occurs on a huge scale; measurements have shown that 38% of all new torrents become unavailable within the first month.[25] A strategy adopted by many publishers which significantly increases availability of unpopular content consists of bundling multiple files in a single swarm.[26] More sophisticated solutions have also been proposed; generally, these use cross-torrent mechanisms through which multiple torrents can cooperate to better share content.[27]
BitTorrent does not, on its own, offer its users anonymity. One can usually see the IP addresses of all peers in a swarm in one's own client or firewall program. This may expose users with insecure systems to attacks.[24] In some countries, copyright organizations scrape lists of peers, and send takedown notices to the internet service provider of users participating in the swarms of files that are under copyright. In some jurisdictions, copyright holders may launch lawsuits against uploaders or downloaders for infringement, and police may arrest suspects in such cases.
Various means have been used to promote anonymity. For example, the BitTorrent client Tribler makes available a Tor-like onion network, optionally routing transfers through other peers to obscure which client has requested the data. The exit node would be visible to peers in a swarm, but the Tribler organization provides exit nodes. One advantage of Tribler is that clearnet torrents can be downloaded with only a small decrease in download speed from one "hop" of routing.
i2p provides a similar anonymity layer although in that case, one can only download torrents that have been uploaded to the i2p network.[34] The bittorrent client Vuze allows users who are not concerned about anonymity to take clearnet torrents, and make them available on the i2p network.[35]
On 2 May 2005, Azureus 2.3.0.0 (now known as Vuze) was released,[40] introducing support for "trackerless" torrents through a system called the "distributed database." This system is a Distributed hash table implementation which allows the client to use torrents that do not have a working BitTorrent tracker. Instead just bootstrapping server is used (router.bittorrent.com, dht.transmissionbt.com or router.utorrent.com[41][42]). The following month, BitTorrent, Inc. released version 4.2.0 of the Mainline BitTorrent client, which supported an alternative DHT implementation (popularly known as "Mainline DHT", outlined in a draft on their website) that is incompatible with that of Azureus. In 2014, measurement showed concurrent users of Mainline DHT to be from 10 million to 25 million, with a daily churn of at least 10 million.[43]
Web "seeding" was implemented in 2006 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 theory, this would make using BitTorrent almost as easy for a web publisher as creating a direct HTTP download. 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. This feature has two distinct specifications, both of which are supported by Libtorrent and the 26+ clients that use it. 2ff7e9595c
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