A BitTorrent tracker
is a server
which 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 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. 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.
A private tracker
is a tracker which restricts who can use it, often by requiring registration of a user account. A common method for controlling registration among private trackers is an invitation system, in which trusted users are given the ability to grant a new user permission to register at the site. Typically invitations, or the codes that are given to prospective users, are granted to users who have uploaded a pre-determined amount, meet specific ratio requirements, and have been registered. Private trackers usually register how much the users upload and download and may enforce a minimum upload-to-download ratio. As a result of the restricted access, their torrents usually offer better availability and speed compared to public trackers where leeching
is more common.
Many private trackers are now implementing Passkeys in the torrent file, which gives each user a specific address to contact in order to get a list of peers. This helps to prevent unauthorized distribution of torrent files from private trackers.
In most countries, it is typically illegal to distribute copyrighted material without the permission of the copyright holder. Because of their potential to be used in breach of copyright as well as in compliance, many torrent trackers have been the target of cease and desist lawsuits from copyright bodies; this despite the fact that a tracker does not actually host or distribute any copyrighted data 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.
Further complicating factors are that there are many circumstances under which it is legal to distribute copyrighted material - indeed there are currently some experiments at legally selling content that is distributed over BitTorrent using a "secure" tracker system. 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, 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.
Besides these complications with copyrighted files using various licenses, there is also the fact that many kinds of data are in the public domain and therefore not subject to copyright at all.
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 feature multiple trackers in the one torrent. This way, should one tracker fail, the others can continue supporting file transfer.
The original BitTorrent client
was the first to offer decentralized, distributed
tracking using a distributed hash table
(DHT), making torrents more independent from the tracker. Later, Vuze
adopted this feature. Azureus' "Distributed Database" feature uses its own form of DHT (Kademlia
) which is incompatible with the official BitTorrent client's implementation, although most of the other clients support the official DHT implementation.
One of the options for this [
] 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 (32bits).
though are 128bits 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, but this mechanism has not been adopted yet by the various BitTorrent clients.
The first IPv6 only worked with BitTorrent and is still run by NIIF/HUNGARNET
Recently SixXS released their IPv6 only BitTorrent Tracker This tracker has a permanent set of seeds available on high bandwidth links. The .torrents served by this tracker can be submitted to the catalog, the seeds will then automatically start distributing these .torrents amongst each other, ensuring that the seeds have the full file and providing for high speed downloads for clients. This service makes Open Source distributions readily available over IPv6 in a highspeed way.