BitTorrent
Description
Bram Cohen created the BitTorrent protocol in April 2001, and released the first client on July 2 of that year, making it the original implementation of the technology it introduced. The client is named the same as the protocol itself, which sometimes causes confusion between the two. Developers often refer to the client as “Mainline” to differentiate it from dozens of third-party clients that also speak the same protocol.
Rather than downloading a complete file from a single server, BitTorrent divides a file into small chunks and allows the user downloading the file to gather the chunks from multiple sources simultaneously. Every user who already has a piece can provide it to others, so the more people download a file, the more upload capacity the swarm as a whole has. This design allows large files to spread out rapidly without taking all the bandwidth burden on a single origin server.
Since version 6.0, released in September 2007, the BitTorrent client is based on the same codebase as uTorrent and has Rainberry’s proprietary license. Earlier versions, up through 5.x, were open-source and were written in Python under licenses including the MIT License and later the GPL.
HOW IT WORKS
The user opens a .torrent file or pastes a magnet link into the client. The .torrent file contains metadata — file names, sizes, and a list of cryptographic hashes for each piece — as well as tracker addresses. The client contacts a tracker, which responds with a list of peers who are currently sharing the same torrent. The client then connects directly to those peers and begins exchanging pieces. It verifies each received piece against the hash in the metadata before writing it to disk, so any corrupted or tampered piece fails verification and the client resends a request for it from a different peer.
A Distributed Hash Table (DHT) implementation allows the client to find peers without having a central tracker, which allows a torrent to remain functional even if its original tracker goes offline. Peer Exchange (PEX) allows connected peers to exchange their own peer lists with one another, further increasing the pool of available sources.
VERSIONS AND VARIANTS
BitTorrent is released in four forms. BitTorrent Classic is a standalone desktop application for Windows that is targeted at users who prefer a traditional interface and bulk download management. BitTorrent Web is a browser-based application that runs within a browser window on Windows and macOS, and it adds the functionality to stream video content while the download is still running. The Android app is a scaled version of the Web interface. The Remote feature allows users to add new torrents to a running desktop session from a mobile device or any browser without being in front of the computer.
Classic still supports Windows XP through Windows 11 but dropped macOS support beyond Catalina 10.15. Users on newer Macs require the Web version instead.
DOWNLOAD and UPLOAD MANAGEMENT
The client manages multiple simultaneous downloads and allows the user to configure bandwidth limits for each individual torrent separately from the global limit. A priority system allows the user to tag some torrents or particular files within a multi-file torrent to download first, and the client will allocate available bandwidth to those items first. A scheduler manages when downloads and uploads occur, which is appropriate for users who do not wish to use their daytime bandwidth and prefer to allow the client to work at night. Swarm health — the ratio of seeds to peers — displays before and during a download so the user can judge whether a torrent has enough sources to complete quickly.
RSS AUTOMATION
An RSS feed reader is built into the client that monitors the configured feed URLs and begins new downloads automatically when matching items appear. Users aim it at feeds from indexing sites and specify filters based on keyword, category, or file size. This eliminates the manual part of checking sites and adding torrents one by one, which is suitable for anyone that follows a regular stream of releases.
SEEDING AND UPLOAD BEHAVIOR
Once a download is finished, the client continues to seed the finished files to other users in the swarm. The user can set a ratio target — for example, upload as much as downloaded — after which the client stops seeding that torrent automatically. Alternatively, the user can fix the seeding time or handle it manually. The client keeps track of the per-torrent upload and download totals in its statistics views.
SECURITY AND PAID TIERS
The free version has advertising. A paid Ad-Free tier is available for $4.95 per year, which removes the ads. The Pro tier at $19.95 per year includes integrated virus scanning that checks every downloaded torrent for malware before the files open on disk. The Pro + VPN tier at $69.95 per year combines the Pro features with a CyberGhost VPN subscription for those who want to conceal their IP address from other peers in the swarm.
SYSTEM Requirements and Availability
BitTorrent Classic is available for Windows XP to Windows 11 in a 32-bit or 64-bit build and is installed as a traditional desktop application. BitTorrent Web is available for Windows and macOS on any modern browser. The Android client is supported for Android 4.1 and above. The download for the free desktop version is a few megabytes. The client is distributed by BitTorrent, Inc. at bittorrent.com.
HISTORICAL NOTES
Versions 4.0 through 5.x added protocol extensions under the internal name “Allegro” that were intended to enhance download performance and provide ISPs with better tools for controlling the torrent traffic on their networks. Version 5.3 is the most recent open source release and can be found in the Internet Archive for those who wish to have the Python based original. The DNA (Delivery Network Accelerator) component, introduced around version 6.0, allowed content providers to incorporate BitTorrent delivery into their own distribution infrastructure as a commercial service independent of the end-user client.
