[7] Programmer Bram Cohen, a University at Buffalo alumnus,[8] designed the protocol in April 2001, and released the first available version on 2 July 2001.

Relative to traditional Internet distribution schemes, this permits a significant reduction in the original distributor's hardware and bandwidth resource costs.

Though both ultimately transfer files over a network, a BitTorrent download differs from a one way server-client 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.

Public torrent-hosting sites such as The Pirate Bay allow users to search and download from their collection of torrent files.

Web search engines allow the discovery of torrent files that are hosted and tracked on other sites; examples include The Pirate Bay and BTDigg.

This list can often be sorted with respect to several criteria, relevance (seeders to 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).

[18] In May 2007, researchers at Cornell University published a paper proposing a new approach to searching a peer-to-peer network for inexact strings,[19] 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[20] and published a follow-up paper reporting its success.

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, or "preferred peers") in hopes of discovering even better partners and to ensure that newcomers get a chance to join the swarm.

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.

[24] A strategy adopted by many publishers which significantly increases availability of unpopular content consists of bundling multiple files in a single swarm.

Though SHA-1 has shown signs of cryptographic weakness, Bram Cohen did not initially consider the risk big enough for a backward incompatible change to, for example, SHA-3.

[33] The bittorrent client Vuze allows users who are not concerned about anonymity to take clearnet torrents, and make them available on the i2p network.

[36] Private torrent trackers are usually invitation only, and require members to participate in uploading, but have the downside of a single centralized point of failure.

[40] Current versions of the official BitTorrent client, μTorrent, BitComet, Transmission and BitSpirit all share compatibility with Mainline DHT.

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 BitTorrent web-service MoveDigital added the ability to make torrents available to any web application capable of parsing XML through its standard REST-based interface in 2006,[52] though this has since been discontinued.

As of November 2015, Vuze, BitComet, KTorrent, Transmission, Deluge, μTorrent, MooPolice, Halite, qBittorrent, rTorrent, and the latest official BitTorrent client (v6) support MSE/PE encryption.

Thus, even with MSE/PE, it may be possible for an ISP to recognize BitTorrent and also to determine that a system is no longer downloading but only uploading data, and terminate its connection by injecting TCP RST (reset flag) packets.

Another unofficial feature is an extension to the BitTorrent metadata format proposed by John Hoffman[58] and implemented by several indexing websites.

It is implemented in several clients, such as BitComet, BitTornado, BitTorrent, KTorrent, Transmission, Deluge, μTorrent, rtorrent, Vuze, and Frostwire.

Torrents with multiple trackers can decrease the time it takes to download a file, but also have a few consequences: As of December 2008,[update] BitTorrent, Inc. was working with Oversi on new Policy Discover Protocols that query the ISP for capabilities and network architecture information.

[66] Brave web browser ships with an extension which supports WebTorrent, a BitTorrent-like protocol based on WebRTC instead of UDP and TCP.

Independent adopters report that BitTorrent technology reduces demands on private networking hardware and bandwidth, an essential for non-profit groups with large amounts of internet traffic.

[70] Many major open source and free software projects encourage BitTorrent as well as conventional downloads of their products (via HTTP, FTP etc.)

[71] In addition, some video game installers, especially those whose large size makes them difficult to host due to bandwidth limits, extremely frequent downloads, and unpredictable changes in network traffic, will distribute instead a specialized, stripped down BitTorrent client with enough functionality to download the game from the other running clients and the primary server (which is maintained in case not enough peers are available).

[citation needed] Users may choose to run seedboxes or virtual private networks (VPNs) to circumvent these restrictions.

On 31 May 2006, The Pirate Bay's servers in Sweden were raided by Swedish police on allegations by the MPAA of copyright infringement;[109] however, the tracker was up and running again three days later.

In the study used to value NBC Universal in its merger with Comcast, Envisional examined the 10,000 torrent swarms managed by PublicBT which had the most active downloaders.

To achieve high bandwidths, the underlying protocol used is UDP, which allows spoofing of source addresses of internet traffic.