Торрент-трекер
BitTorrent трекер — сервер, осуществляющий соединение пиров в протоколе BitTorrent. Клиентам пользователей необходимо соединяться с трекером для того, что бы начать закачку (клиенты, которые уже начали закачку также соединяются с трекером для того, что бы получить информацию о новых пирах и сообщить статистику, однако после начала соединения сообщение между пирами может происходить и без участия трекера).
Трекеры необходимо отличать от хранилищ торрентов, так как они не обязательно хранят список файлов, которые через них скачиваются. Хранилище торентов содержит список .torrent файлов и описаний к ним. Трекер координирует соединение между пирами, которые пытаются скачать информацию, описанную торентом.
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.
[edit] Private Trackers
A private tracker is a tracker which restricts who can use it, often by requiring registration of a user account. Private trackers usually register how much the users upload and download and may enforce a minimum upload-to-download ratio. As a result their torrents usually offer better availability and speed compared to public trackers where leeching is more common. The concept was pioneered by the (now defunct) site TorrentBits on whose code many private trackers are now based.
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 unauthorised distribution of torrent files from private trackers.
[edit] Legal Issues
In most countries, it is 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.
[edit] 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:
[edit] 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.
[edit] 'Trackerless' torrents
There are two incompatible 'trackerless' BitTorrent transfer (aka. decentralized tracking) methods: DHT-based implementations, and Azureus' 'Distributed Database'.
The term 'trackerless' is something of a misnomer, as decentralized or distributed tracking essentially treats every peer in the swarm as a tracker. The original BitTorrent client was the first to offer decentralized tracking through its DHT method. Later, Azureus, µTorrent, BitComet and KTorrent adopted this feature, although Azureus's method of implementation is incompatible with the DHT offered by all other supporting clients; with the possible exception of BitComet.
[edit] IPv6 support
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 (32bits). IPv6 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 BitTorrent tracker was and still is 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.
Програмные реализации
Реализации бывают трех видов: