A BitTorrent tracker is a special type of

server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...

that assists in the communication between peers using the BitTorrent protocol. In

peer-to-peer file sharing Peer-to-peer file sharing is the distribution and sharing of digital media using peer-to-peer (P2P) networking technology. P2P file sharing allows users to access media files such as books, music, movies, and games using a P2P software program t ...

, a software client on an end-user PC requests a file, and portions of the requested file residing on peer machines are sent to the client, and then reassembled into a full copy of the requested file. The "tracker" server keeps track of where file copies reside on peer machines, which ones are available at time of the client request, and helps coordinate efficient transmission and reassembly of the copied file. Clients that have already begun downloading a file communicate with the tracker periodically to negotiate faster file transfer with new peers, and provide network performance statistics; however, after the initial peer-to-peer file download is started, peer-to-peer communication can continue without the connection to a tracker. Modern BitTorrent clients may implement a

distributed hash table A distributed hash table (DHT) is a distributed system that provides a lookup service similar to a hash table: key–value pairs are stored in a DHT, and any participating node can efficiently retrieve the value associated with a given key. The ...

and the

peer exchange Peer exchange or PEX is a communications protocol that augments the BitTorrent file sharing protocol. It allows a group of users (or peers) that are collaborating to share a given file to do so more swiftly and efficiently. In the original design ...

protocol to discover peers without trackers; however, trackers are still often included with torrents to improve the speed of peer discovery.

Public vs private trackers

Public trackers

Public or open trackers can be used by anyone by adding the tracker address to an existing torrent, or they can be used by any newly created torrent, like OpenBitTorrent.

The Pirate Bay The Pirate Bay (sometimes abbreviated as TPB) is an online index of digital content of entertainment media and software. Founded in 2003 by Swedish think tank Piratbyrån, The Pirate Bay allows visitors to search, download, and contribute ...

operated one of the most popular public trackers until disabling it in 2009 due to legal trouble, and thereafter offered only

magnet link Magnet is a URI scheme that defines the format of magnet links, a de facto standard for identifying files ( URN) by their content, via cryptographic hash value rather than by their location. Although magnet links can be used in a number of co ...

Private trackers

A ''private tracker'' is a BitTorrent tracker that restricts use by requiring users to register with the site. The method for controlling registration used among many private trackers is an invitation system, in which active and contributing members are given the ability to grant a new user permission to register at the site, or a new user goes through an interview process.

Legal issues

Legal uses

There are several circumstances under which it is legal to distribute copyrighted material or parts thereof. * Free distribution. Copyright holders may choose to allow free distribution of their works. Dedicated copyright licenses—usable by anyone who wants to upload their own material—are available for that purpose. Such licenses are often used in situations with large numbers of copyright holders, like in online communities. For example, the

Creative Commons Creative Commons (CC) is an American non-profit organization and international network devoted to educational access and expanding the range of creative works available for others to build upon legally and to share. The organization has release ...

license family for

free cultural works The Definition of Free Cultural Works is a definition of free content from 2006. The project evaluates and recommends compatible free content licenses. History The Open Content Project by David A. Wiley in 1998 was a predecessor project which ...

in text, audio, video or image format; or software licenses for

Free Software Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, n ...

Open-source software Open-source software (OSS) is computer software that is released under a license in which the copyright holder grants users the rights to use, study, change, and distribute the software and its source code to anyone and for any purpose. ...

like the

BSD License BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft licenses, which have share-alike requirements. The original BSD lice ...

and others.

Wikipedia Wikipedia is a multilingual free online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and using a wiki-based editing system. Wikipedia is the largest and most-read refer ...

itself can be distributed via BitTorrent for the same reason. * Public domain. Works that are in the

public domain The public domain (PD) consists of all the creative work to which no exclusive intellectual property rights apply. Those rights may have expired, been forfeited, expressly waived, or may be inapplicable. Because those rights have expired ...

and therefore not (or no longer) subject to copyright law can also be legally distributed. For instance,

Project Gutenberg Project Gutenberg (PG) is a volunteer effort to digitize and archive cultural works, as well as to "encourage the creation and distribution of eBooks." It was founded in 1971 by American writer Michael S. Hart and is the oldest digital libr ...

regularly collects and publishes classical cultural works after their copyright has expired (which depends on the country in which the work was previously published). * Fair use. Some countries also have

fair use Fair use is a doctrine in United States law that permits limited use of copyrighted material without having to first acquire permission from the copyright holder. Fair use is one of the limitations to copyright intended to balance the interests ...

provisions in copyright law, which allow people the right to access and use certain classes of copyrighted material without breach of the law. There are also experiments that legally sell content that is distributed over BitTorrent using a "secure" tracker system.

Improving torrent reliability

Trackers are the primary reason for a damaged BitTorrent "swarm". (Other reasons are mostly related to damaged or hacked clients uploading corrupt data.) The reliability of trackers has been improved through two main innovations in the BitTorrent protocol.

Multi-tracker torrents

Multi-tracker torrents contain multiple trackers in a single torrent file. This provides redundancy in the case that one tracker fails, the other trackers can continue to maintain the swarm for the torrent. One disadvantage to this is that it becomes possible to have multiple unconnected swarms for a single torrent where some users can connect to one specific tracker while being unable to connect to another. This can create a disjoint set which can impede the efficiency of a torrent to transfer the files it describes. Additional extensions such as

Peer exchange Peer exchange or PEX is a communications protocol that augments the BitTorrent file sharing protocol. It allows a group of users (or peers) that are collaborating to share a given file to do so more swiftly and efficiently. In the original design ...

and DHT mitigate this effect by rapidly merging otherwise disjoint graphs of peers.

Trackerless torrents

Vuze Vuze (previously Azureus) is a BitTorrent client used to transfer files via the BitTorrent protocol. Vuze is written in Java, and uses the Azureus Engine. In addition to downloading data linked to .torrent files, Azureus allows users to view, p ...

(formerly Azureus) was the first BitTorrent client to implement such a system through the

(DHT) method. An alternative and incompatible DHT system, known as Mainline DHT, was developed simultaneously and later adopted by the BitTorrent (Mainline), μTorrent, Transmission, rTorrent, KTorrent, BitComet, and Deluge clients. Current versions of the official BitTorrent client,

μTorrent μTorrent, or uTorrent (see pronunciation) is a proprietary adware BitTorrent client owned and developed by Rainberry, Inc. with over 150 million users. It is the most widely used BitTorrent client outside China; globally only behind Xunlei. ...

BitComet BitComet (originally named SimpleBT client from versions 0.11 to 0.37) is a cross-protocol BitTorrent, HTTP and FTP client written in C++ for Microsoft Windows and available in 52 different languages. Its first public release was version 0.28. ...

Transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...

and BitSpirit all share compatibility with

Mainline DHT Mainline DHT is the name given to the Kademlia-based distributed hash table (DHT) used by BitTorrent clients to find peers via the BitTorrent protocol. The idea of using a DHT for distributed tracking in BitTorrent was first implemented in Azure ...

. Both DHT implementations are based on

Kademlia Kademlia is a distributed hash table for decentralized peer-to-peer computer networks designed by Petar Maymounkov and David Mazières in 2002. It specifies the structure of the network and the exchange of information through node lookups. Kademl ...

. As of version 3.0.5.0, Vuze also supports Mainline DHT in addition to its own distributed database through use of an optional application plugin MainlineDHT Plugin. This potentially allows the Vuze client to reach a bigger swarm. Most BitTorrent

client Client(s) or The Client may refer to: * Client (business) * Client (computing), hardware or software that accesses a remote service on another computer * Customer or client, a recipient of goods or services in return for monetary or other valuabl ...

s also use

(PeX) to gather peers in addition to trackers and DHT. Peer exchange checks with known peers to see if they know of any other peers. With the 3.0.5.0 release of Vuze, all major BitTorrent clients now have compatible peer exchange.

IPv6 support

One of the options for this

HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

based tracker protocol is the "compact" flag. This flag, as defined in BEP 23, specifies that the tracker can compact the response by encoding

IPv4 Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...

addresses as a set of 4 bytes (32 bits).

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...

though are 128 bits long, and as such, the "compact" would break IPv6 support. To handle that situation clients and trackers must either avoid using compact announces over IPv6 or implement BEP 07

Software

opentracker from Dirk Engling powered one of the biggest BitTorrent trackers,

tracker.

qBittorrent qBittorrent is a cross-platform free and open-source BitTorrent client written in native C++. It relies on Boost, Qt 6 toolkit and the libtorrent-rasterbar library (for the torrent back-end), with an optional search engine written in Python. ...

is an open source BitTorrent client with a built-in tracker support. BitTorious is an open source, commercially supported tracker with integrated web-based management portal.

References

{{DEFAULTSORT:BitTorrent Tracker Application layer protocols BitTorrent Servers (computing)