BitTorrent tracker

A BitTorrent tracker is a special type of server, one that assists in the communication between peers using the . 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. 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. Since the creation of the distributed hash table (DHT) method for "Trackerless" torrents, BitTorrent trackers have largely become redundant, however, they are still often included with torrents to improve the speed of peer discovery.

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 operated one of the most popular public trackers until disabling it in 2009 amid legal trouble, and thereafter offered 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 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.

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. Additional extensions such as Peer exchange and DHT mitigate this effect by rapidly merging otherwise disjoint graphs of peers.

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Wikipedia