Bittorent protocol information

BitTorrent is a collaborative file-sharing protocol that is ideal for quickly distributing large amounts of data. Unlike other file-sharing techniques, BitTorrent does not rely on a cross-file sharing network, but builds a separate distribution network for each file.

To participate in the distribution of the data of a torrent, the client usually uses a torrent file (filename extension .torrent or .tor). This contains the IP address (or host name) of the tracker as well as the file name, size and a list of checksums of segments of the data to be downloaded (one or more files). Torrent files can be created with many available bittorrent clients. In this case, the initial Seeder Peer (seeder) must establish the link to the file to be offered as a torrent and keep it available.

Torrent files are typically a few dozen kilobytes in size, and are made available to download from the vendor’s website or through index sites (for example, The Pirate Bay, Mininova, or isoHunt). Without tracker other methods must be used to locate remote sites (DHT, PEX, …), or it can only be exchanged with already known remote sites.

The client software receives from the tracker a list of remote sites that have or are interested in the data or parts thereof. As soon as a peer has received a chunk of the file and has verified the checksum, it reports this to the tracker and can now pass this file piece on to the other peers. The amount of peers interested in the same torrent is called a swarm. Peers in possession of the entire content of the torrent, and thus do not download anything from other clients, but only distribute data, is called Seeder (from seed to sow). As Leecher (leech leech: leech) are usually referred to the peers that do not have all the torrent content and download more segments. Peer generally refers to a client connected to a client.

In contrast to other known file-sharing systems, not any files are exchanged from the holdings of the participants. Rather, each swarm distributes only the files that the author of the torrent file has explicitly provided for downloading. The operator of the tracker also determines which downloads are to be managed by it. The individual trackers are not connected to each other, so there is no common network, but a separate for each Torrent. In this way, providers can more easily distance themselves from third-party, possibly illegal content.

What is DHT

BEP 5 describes the use of a distributed hash table (DHT) based on the Kademlia algorithm for “trackerless” operation. The tracker function is taken over by the client software. This avoids some of the previous problems (for example, the missing reliability of the tracker), even if tracker-based operation will continue to be possible. It also makes it easier to offer, since trackers are the most complex part in BitTorrent. Since the version 4.2.0 released in November 2005, the reference client supports the trackerless operation, relying on the Kademlia algorithm. The tracker can be decentralized, similar to the Kademlia network, as a distributed hash table on the clients themselves stored and managed.

Modern BitTorrent clients can therefore work without trackers and work decentrally (“trackerless”). However, they still use different protocols so that communication between different DHT-based clients is not always possible. Currently compatible are BitComet, Deluge, μTorrent, KTorrent, qBittorrent, rTorrent, Transmission. The developers are working harder to make compatibility.

You can download torrent files here.

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