BitTorrent vocabulary

This list explains terms used when discussing BitTorrent clients, and in particular the BitTorrent protocol used by these clients.

Common BitTorrent terms

Availability

(Also known as distributed copies.) The number of full copies of a file (or set of files and directories) directly available to the client. Each seed adds 1.0 to this number, as they have one complete copy of the file. A connected peer with a fraction of the file available adds that fraction to the availability, if no other peer has this part of the file.

Example: a peer with 65.3% of the file downloaded increases the availability by 0.653. However, if two peers both have the same portion of the file downloaded - say 50% - and there is only one seeder, the availability is 1.5.

Sometimes "distributed copies" is considered to be "availability minus 1". So if the availability is 1.6 , the distributed copies will be 0.6 because it is only counting the "copies" of the file.

Choked

Describes a peer to whom the client refuses to send file pieces. A client chokes another client in several situations:

The second client is a seed, in which case it does not want any pieces (i.e., it is completely uninterested)
The client is already uploading at its full capacity (it has reached the value of max_uploads)
The second client has been blacklisted for being abusive or is using a blacklisted BitTorrent client.

Client

The program that enables p2p file sharing via the BitTorrent protocol. Examples of clients include µTorrent and Vuze.

Downloader

A downloader is any peer that does not have the entire file and is downloading the file. This term, used in Bram Cohen's Python implementation, lacks the negative connotation attributed to leech. Bram prefers downloader to leech because BitTorrent's tit-for-tat ensures downloaders also upload and thus do not unfairly qualify as leeches.

End Game

Also known as Endgame Mode on many clients. BitTorrent has several download strategies for initializing a download, downloading normally among the middle of the torrent, and downloading the last few pieces (see below) of a torrent.

In typical client operation the last download pieces arrive more slowly than the others. This is because the faster and more easily accessible pieces should have already been obtained. In order to prevent the last pieces becoming unobtainable BitTorrent clients attempt to get the last missing pieces from all of its peers. Upon receiving the last pieces a cancel request command is sent to other peers.

Fake

A fake torrent is a torrent that does not contain what is specified in its name or description (e.g. a torrent is said to contain a video, but it contains only a snapshot of a moment in the video, or in some cases a virus).

Hash

The hash is a string of alphanumeric characters (typically hexadecimal) in the .torrent file that the client uses to verify the data that is being transferred. Hash is the shorter form of the word Hashsum.

Torrent files contain information like the file list, sizes, pieces, etc. Every piece received is first checked against the hash. If it fails verification, the data is discarded and requested again. The Hash Fails field in the torrent's General tab shows the number of these hash fails.

Hash checks greatly reduce the chance that invalid data is incorrectly identified as valid by the BitTorrent client, but it is still possible for invalid data to have the same hash value as the valid data and be treated as such. This is known as a hash collision. Torrent and p2p files typically use 160 bit hashes that are reasonably free from hash collision problems, so the probability of bad data being received and passed on is very rare.

Health

Health is shown in a bar or in % usually next to the torrent's name and size, on the site where the .torrent file is hosted. It shows if all pieces of the torrent are available to download (i.e. 50% means that only half of the torrent is available).

Index

An index is a list of .torrent files (usually including descriptions and other information) managed by a website and available for searches. An index website can also be a tracker.

Interested

Describes a downloader who wishes to obtain pieces of a file the client has. For example, the uploading client would flag a downloading client as 'interested' if that client did not possess a piece that it did, and wished to obtain it.

Leech

A leech is a term with two meanings. Primarily leech (or leeches) refer to a peer (or peers) who has a negative effect on the swarm by having a very poor share ratio (downloading much more than they upload, creating a ratio less than 1.0).

Most leeches are users on asymmetric internet connections and do not leave their BitTorrent client open to seed the file after their download has completed. However, some leeches intentionally avoid uploading by using modified clients or excessively limiting their upload speed.

The often used second meaning of leech is synonymous with downloader (see above): used simply to describe a peer or any client that does not have 100% of the data. This alternative meaning was mainly introduced by most BitTorrent tracker sites.

Lurker

A lurker is a user that only downloads files from the group but does not add new content. It does not necessarily mean that the lurker will not seed. Not to be confused with a leecher.

p2p

Stands for "peer to peer", which is the technology used for file sharing among computer users over the internet. In a p2p network, each node (or computer on the network) acts as both a client and a server. In other words, each computer is capable of both sending and receiving data.

Peer

A peer is one instance of a BitTorrent client running on a computer on the Internet to which other clients connect and transfer data. Usually a peer does not have the complete file, but only parts of it. However, in the colloquial definition, "peer" can be used to refer to any participant in the swarm (in this case, it's synonymous with "client").

Piece

This refers to the torrented files being divided up into equal specific sized pieces (e.g. 64kb, 128kb, 512Kb, 1Mb, 2Mb, 4Mb). The pieces are distributed in a random fashion among peers in order to optimize trading efficiency.

Ratio credit

A ratio credit, also known as upload credit or ratio economy, is a currency system used on a number of private trackers to provide an incentive for higher upload/download ratios among member file-sharers. In such a system, those users who have greater amounts of bandwidth, hard drive space (particularly seedboxes) or idle computer uptime are at a greater advantage to accumulate ratio credits versus those who are lacking in any one or more of the same resources.

Scrape

This is when a client sends a request to the tracking server for information about the statistics of the torrent, such as with whom to share the file and how well those other users are sharing.

Seed

A Seed is used to refer to a peer who has 100% of the data. When a leech obtains 100% of the data, that peer automatically becomes a Seed.

Seeding refers to leaving a peer's connection available for other peers, i.e. leeches, to download from. Normally, a peer should seed more data than leech. However, whether to seed or not, or how much to seed, is dependent on the availability of leeches and the choice of the peer at the seeding end.

Share ratio

A user's share ratio for any individual torrent is a number determined by dividing the amount of data that user has uploaded by the amount of data they have downloaded. Final share ratios over 1 carry a positive connotation in the BitTorrent community, because they indicate that the user has sent more data to other users than they received. Likewise, share ratios under 1 have negative connotation.

Snubbed

An uploading client is flagged as snubbed if the downloading client has not received any data from it in over 60 seconds.

Super-seeding

When a file is new, much time can be wasted because the seeding client might send the same file piece to many different peers, while other pieces have not yet been downloaded at all. Some clients, like ABC, Vuze, BitTornado, TorrentStorm, and µTorrent have a "super-seed" mode, where they try to only send out pieces that have never been sent out before, theoretically making the initial propagation of the file much faster. However the super-seeding becomes less effective and may even reduce performance compared to the normal "rarest first" model in cases where some peers have poor or limited connectivity. This mode is generally used only for a new torrent, or one which must be re-seeded because no other seeds are available.

Swarm

Main article: segmented downloading

Together, all peers (including seeders) sharing a torrent are called a swarm. For example, six ordinary peers and two seeders make a swarm of eight.

Torrent

A torrent can mean either a .torrent metadata file or all files described by it, depending on context. The torrent file contains metadata about all the files it makes downloadable, including their names and sizes and checksums of all pieces in the torrent. It also contains the address of a tracker that coordinates communication between the peers in the swarm.

Tracker

A tracker is a server that keeps track of which seeds and peers are in the swarm. Clients report information to the tracker periodically and in exchange, receive information about other clients to which they can connect. The tracker is not directly involved in the data transfer and does not have a copy of the file.