BitTorrent tracker

A BitTorrent tracker is a server which assists in the communication between peers using the BitTorrent protocol. It is also, without any extension to the original protocol, the only major single point of failure, as clients are required to communicate with the tracker to initiate downloads. (Clients that have already begun downloading also communicate with the tracker periodically to negotiate with newer peers and provide statistics, however, after the initial reception of peer data, peer communication can continue without a tracker.)

Trackers and indexes

A tracker should be differentiated from a BitTorrent index by the fact that it does not necessarily list files that are being tracked. A BitTorrent index is a list of .torrent files (usually including descriptions and other information). Trackers merely coordinate communication between peers attempting to download the payload of the torrents.

Many BitTorrent websites act as both tracker and index, for example The Pirate Bay. Sites such as these publicize the tracker's URL and allow users to upload torrents to the index with the tracker's URL embedded in them, providing all the features necessary to initiate a download.

Legal issues

Many torrent trackers have been the target of cease and desist lawsuits from copyright bodies such as the MPAA and RIAA, despite the fact that they do not actually host any of the copyrighted data themselves. Some torrent tracking websites are operated in countries with copyright laws that differ from the country of origin of the copyrighted material. For example, The Pirate Bay (located in Sweden) is famous for publishing and making fun of legal threats it receives. Most of these threats originate from companies in the United States, but US copyright law does not, of course, extend to Sweden.

In many countries (including the United States), it is illegal to distribute copyrighted material without the permission of the copyright holder. Of course there are many circumstances under which it is legal to distribute copyrighted material. For instance, copyright holders may choose to allow their content to be distributed for free, using licenses such as the Creative Commons family, or the GPL In addition, there are currently some experiments at legally selling content that is distributed over BitTorrent using a "secure" tracker system.

Improving tracker 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 feature multiple trackers in the one torrent. This way, should one tracker fail, the others can continue supporting file transfer.

'Trackerless' torrents

There are two incompatible 'trackerless' BitTorrent transfer (aka. decentralized tracking) methods: DHT-based implementations, and Azureus's 'Distributed Database'.

The term 'trackerless' is something of a misnomer, as decentralized tracking essentially treats every peer in the swarm as a tracker. BitComet was the first client to offer decentralized tracking through its DHT method. Later, Azureus and Bram Cohen's original BitTorrent client adopted this feature, although Azureus's method of implementation is incompatible with the DHT offered by all other supporting clients.

Prominent sites running trackers

• The Pirate Bay

BitTorrent Tracking Software

• BNBT - C++ port of the original BitTorrent tracker written in Python by Bram Cohen
  • CBTT, XBNBT and EasyTracker - popular modifications of BNBT
• TBDev.net Updated and modification supported fork of the older TorrentBits code.
• TorrentTrader - Free PHP and MySQL tracker
• OpenTracker - PHP and MySQL tracker

See also

• BitTorrent
• Bram Cohen
• BitTorrent client
• Distributed hash tables
• Share ratio
• UDP tracker

External Links

• AnimeSuki Japanese anime tracker.
• LinuxTracker - Directory of Linux distributions.
• The Pirate Bay – Swedish BitTorrent tracker and search engine.