Basics of BitTorrent

BitTorrent is a collaborative file-sharing protocol that is particularly suitable for the rapid distribution of large amounts of data and was designed and first published in 2001. In contrast to other file-sharing techniques, BitTorrent does not rely on an overarching file-sharing network, but builds a decentralized distribution network for each file.

Technically, the protocol is assigned to OSI layer 7, i.e. the application layer, and is based on the TCP/IP reference model. The reference implementation (BitTorrent client) by the inventor Bram Cohen was originally done in the Python programming language. There are now a number of alternative programs available that implement the BitTorrent protocol.

In contrast to downloading a file via HTTP or FTP, BitTorrent technology uses the (otherwise unused) upload capacities of the downloaders, even if they have not yet downloaded the file completely. Files are not only distributed from a server, but also passed from user to user (peer-to-peer or P2P). This approach avoids bottlenecks due to the exhausted capacities of a single provider.

In order to participate in the distribution of the data of a torrent, the client usually uses a torrent file (filename extension .torrent or .tor). It contains the IP address (or hostname) 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 generated using many available Bittorrent clients. The initial seeder (= seeder) must establish the link to the file to be offered as a torrent and keep it available.

To find other peers (interested in a particular file), there is, among other things, a system in which special (web) servers – the trackers mediate contacts. Normally, the tracker only keeps a unique ID of the torrent locally, to which the IP addresses of the peers who keep the file available are assigned. Peers connect to the tracker quite frequently in order to be able to react quickly to changes in the available peers.

Torrent files are usually a few tens of kilobytes in size and are made available on the provider’s website or via index sites. Without a tracker, other methods have to be used to find remote stations (DHT, PEX, …), or it can only be exchanged with already known remote stations.

The client software receives a list of remote sites from the tracker that have the data or parts of it or are interested. Once a peer has received a chunk of the file and verified the checksum, it reports this to the tracker and can now pass this piece of file to the other peers. The set of all peers interested in the same torrent is called a swarm. Peers who are in possession of the complete content of the torrent, and thus do not download anything from other clients, but only distribute data, are called seeders. Leechers are those peers who do not yet have all the torrent content and download additional segments. Mainly in other contexts, peers are sometimes pejoratively referred to as “leechers” who only download without redistributing segments that have already been downloaded, thus violating the P2P principle. Peer generally refers to another client connected to a client.

In contrast to other file sharing systems, arbitrary files from the participants’ holdings are not exchanged. Rather, each swarm only distributes the files that the author of the torrent file has explicitly intended for distribution. The operator of the tracker also decides which downloads are to be managed by the tracker. The individual trackers are not connected to each other, so there is no common network, but a separate one for each individual torrent. This also makes it easier for providers to distance themselves from third-party, possibly illegal content.