Editing BitTorrent (section)

== History ==
{{expand section|date=September 2021}}
[[File:BitTorrent network.svg|thumb|left|The middle computer is acting as a "seed" to provide a file to the other computers which act as peers.]]

Programmer [[Bram Cohen]], a [[University at Buffalo]] alumnus,<ref>{{cite web |url=https://twitter.com/UBengineering/status/357131210503356419 |title=UB Engineering Tweeter |publisher=University at Buffalo's School of Engineering and Applied Sciences |archive-url=https://web.archive.org/web/20131111070426/https://twitter.com/UBengineering/status/357131210503356419 |archive-date=11 November 2013 |url-status=live |access-date=4 January 2022 }}</ref> designed the protocol in April 2001 with the idea of speeding up download time by enabling files to be downloaded from many sources. He then released the first available version on 2 July 2001.<ref name="BC"/> Cohen and [[Ashwin Navin]] founded BitTorrent, Inc. (later renamed [[Rainberry, Inc.]]) to further develop the technology in 2004.

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 file|torrent]]", that they would upload to a torrent index site. The first uploader acted as a ''[[Terminology of BitTorrent#Seed|seed]]'', and downloaders would initially connect as ''[[Terminology of BitTorrent#Peer|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 [[BitTorrent tracker|trackers]] which keep track of the other seeds and peers.

In 2005, first [[Vuze]] and then the BitTorrent client introduced distributed tracking using [[distributed hash table]]s which allowed clients to exchange data on swarms directly without the need for a torrent file.

In 2006, [[peer exchange]] functionality was added allowing clients to add peers based on the data found on connected nodes.

In 2017, BitTorrent, Inc. released the BitTorrent v2 protocol specification.<ref>{{cite web |date=15 May 2017 |title=Merge pull request #59 from the8472/new-hash-algos |url=https://github.com/bittorrent/bittorrent.org/commit/51fe877e6ed6f20fb7eea67fe234e7b266aaed84 |url-status=live |archive-url=https://web.archive.org/web/20211110162310/https://github.com/bittorrent/bittorrent.org/commit/51fe877e6ed6f20fb7eea67fe234e7b266aaed84 |archive-date=10 November 2021 |accessdate=13 September 2021 |publisher=BitTorrent}}</ref><ref>{{cite web |last1=Cohen |first1=Bram |title=The BitTorrent Protocol Specification v2 |url=https://www.bittorrent.org/beps/bep_0052.html |url-status=live |archive-url=https://web.archive.org/web/20201112030826/http://bittorrent.org/beps/bep_0052.html |archive-date=12 November 2020 |access-date=28 October 2020 |website=BitTorrent.org |publisher=BitTorrent}}</ref> 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 [[SHA-1#The SHAppening|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 [[Merkle tree|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. In addition, file hashes can be displayed on tracker, torrent indexing services, to search for swarms by searching for hashes of files contained in them. These hashes are different from the usual SHA-256 hash of files and can be obtained using tools.<ref>{{cite web |title=TMRR |url=https://github.com/kovalensky/tmrr |website=github.com |publisher=kovalensky}}</ref> Magnet links for v2 also support a hybrid mode to ensure support for legacy clients.<ref>{{cite web |title=Bittorrent-v2 |url=https://blog.libtorrent.org/2020/09/bittorrent-v2/ |website=libbittorrent.org |publisher=libbittorrent |access-date=28 October 2020 |archive-date=30 October 2020 |archive-url=https://web.archive.org/web/20201030011550/https://blog.libtorrent.org/2020/09/bittorrent-v2/ |url-status=live }}</ref>