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A BitTorrent tracker is a server that assists in the communication between peers using the BitTorrent protocol. In peer-to-peer file sharing 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. The BitTorrent tracker is also, in the absence of extensions to the original protocol, the only major critical point, as clients are required to communicate with the tracker to initiate downloads. 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.
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. The most popular are OpenBitTorrent, PublicBitTorrent and istole.it. The Pirate Bay operated one of the most popular public trackers until disabling it in 2009 amid legal trouble, opting to offer only magnet links.
A private tracker is a BitTorrent tracker that restricts use by requiring users to register with the site. The method for controlling registration used amongst 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.
There are several circumstances under which it is legal to distribute copyrighted material or parts thereof.
There are also experiments at legally selling content that is distributed over BitTorrent using a "secure" tracker system.
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 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.
Vuze (formerly Azureus) was the first BitTorrent client to implement such a system through the distributed hash table (DHT) method. An alternative and incompatible DHT system, known as Mainline DHT, was later developed and adopted by the BitTorrent (Mainline), µTorrent, Transmission, rTorrent, KTorrent, BitComet, and Deluge clients.
Current versions of the official BitTorrent client, µTorrent, BitComet, Transmission and BitSpirit all share compatibility with Mainline DHT. Both DHT implementations are based on Kademlia. As of version 126.96.36.199, 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 clients also use Peer exchange (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 188.8.131.52 release of Vuze, all major BitTorrent clients now have compatible peer exchange.
One of the options for this HTTP based tracker protocol is the "compact" flag. This flag specifies that the tracker can compact the response by encoding IPv4 addresses as a set of 4 bytes (32 bits). IPv6 though are 128 bits long, and as such, the "compact" flag breaks IPv6 support. Trackers which support IPv6 clients thus currently ignore the compact flag. No current alternative is available for IPv6.
BitStorm is a small tracker written in PHP which does not require a database and runs on any PHP compatible web server.
Hefur is a standalone BitTorrent tracker written in C++, under the MIT license.