||This article has multiple issues. Please help improve it or discuss these issues on the talk page.
A BitTorrent tracker is a server that assists in the communication between peers using the BitTorrent protocol. It 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 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 indexers
A BitTorrent tracker, also simply referred to as a tracker, is different from a BitTorrent indexer, indexer for short, in that a tracker actually coordinates communication between peers attempting to download the payload of the torrents. By contrast, an indexer aggregates torrent data from one or more tracker sources into a central database, publicly or privately searchable, making available the metadata contained in the .torrent file such as the tracker URL. A tracker is not necessarily affiliated with an indexer.
However, some BitTorrent websites do operate and rely on their own tracker server. Sites such as these 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.
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 ones are OpenBitTorrent, PublicBitTorrent and Coppersurfer. The Pirate Bay used to run a tracker, but has since disabled it , although the website remains as a torrent index.
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. Invitations, typically sent via email or an invite code system, are normally granted to active users who have uploaded a pre-determined amount or meet specific upload-to-download ratio requirements. Trading invites for different sites is highly frowned upon in the private BitTorrent community as it allows anti-piracy groups to infiltrate private trackers more easily. Most private trackers monitor how much users upload or download, and in most situations, enforce a minimum upload-to-download ratio.
Some of the allure of private tracker versus a public one are: higher speeds, a tighter community, and safer downloads. Private trackers implement a strict set of rules, so generally files containing malware are extremely uncommon. Many private trackers keep in close contact with each other, so bad users (who trade invites or attempt to fake their ratio) can be quickly blacklisted. Almost all private trackers implement a passkey system, where each user is given a personalized announce URL so if there is unauthorized distribution, it can be pinpointed to the user responsible. Some private trackers have a higher level of security than others; many sites only allow their users to refer to their site as an abbreviation, and never as the site's full name or URL. Other trackers restrict invites to outstanding members, and many trackers, to increase security, have eliminated the invite system altogether. An example of a private tracker is Oink's Pink Palace, which was forcibly shut down in late 2007 by law enforcement officials.[examples needed]
The downside is that in a closed community it can be hard for all members to maintain the required ratio. Competitions may be offered, with prizes of improved ratios. Some trackers will exchange ratio credit for donations. Some trackers will also use "free leech" systems to improve the users ratio. When downloading a free leech torrent only the upload gets logged in, the download is ignored. Usually large torrents are offered as free leech. Seeding to a ratio of at least 1 is still recommended even for free leech torrents, for the sake of internet etiquette.
Perhaps the most complicated portion of this legal debate is the fact that although BitTorrent indexes and trackers provide means to receive and send copyrighted data between users, they do not host any copyrighted materials, and no copyrighted materials ever traverse these trackers or indexers. Many torrent trackers have been the target of cease and desist lawsuits from copyright bodies despite the fact that a tracker does not actually host or distribute any copyrighted data itself, due to the fact that files are transferred directly from each user's machine instead of distributed via the tracker itself.
A complicating factor is that torrent tracking websites operate in a variety of countries, including countries with copyright laws that differ from the country of origin of the copyrighted material, and countries in which different actions may be legal or illegal.
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 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 license family for free cultural works in text, audio, video or image format; or software licenses for Free Software / Open-source software like the BSD License and others. Actually, Wikipedia itself can be distributed via BitTorrent for the same reason.
- Public domain. Works that are in the public domain and therefore not (or no longer) subject to copyright law can also be legally distributed. For instance, Project Gutenberg regularly collects and publishes classical cultural works after their copyright has expired (which depends on the country the work was previously published).
- Fair use. Some countries also have fair use 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 at legally selling 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 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 220.127.116.11, Vuze (formerly Azureus) 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 (formerly Azureus) 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 18.104.22.168 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. There have been mentions of a "compact6" flag,[by whom?] but this mechanism has not been adopted yet by the various BitTorrent clients.
- See BitTorrent tracker software for a list of BitTorrent tracker software.
Atrack is a high performance open source tracker designed to run on Google App Engine. BitStorm is a small tracker written in PHP which does not require a database and runs on any PHP compatible web server. Anybody with even a little technical knowledge can use it to add a tracker fuctionalty to their webserver. BitStorm 2 uses a MySQL database instead of flat file to improve scalability.
Hefur is a standalone BitTorrent tracker written in C++, under the MIT license.
- BitTorrent (protocol)
- BitTorrent client
- Comparison of BitTorrent tracker software
- Bram Cohen
- Distributed hash table
- UDP tracker
- XBT Tracker – C++ BitTorrent tracker designed for performance (does not serve .torrent files or other web pages); requires MySQL
- "Worlds most resiliant tracking". November 17, 2009. Retrieved February 18, 2012. "... Now that the decentralized system for finding peers is so well developed, TPB has decided that there is no need to run a tracker anymore, so it will remain down! ..."
- "Trading BitTorrent Tracker Invites , Commodity or Curse?". TorrentFreak. 15 January 2008. Retrieved 23 October 2010.