BitTorrent is the name of a client application for the torrent peer-to-peer (P2P) file distribution protocool. created by programmer Bram Cohen. BitTorrent is designed to widely distribute large amounts of data without incurring the corresponding consumption in server and bandwidth resources.

The bittorrent protocol breaks the file(s) down into smaller fragments, typically a quarter of a megabyte (256KB) in size. Peers download missing fragments from each other and upload those that they already have to peers that request them. The protocol is ’smart’ enough to choose the peer with the best network connections for the fragments that it’s requesting. To increase the overall efficiency of the swarm (the ad-hoc P2P network temporarily created to distribute a particular file), the bittorrent clients request from their peers the fragments that are most rare; in other words, the fragments that are available on the least number of peers, making most fragments available widely across many machines and avoiding bottlenecks.

The file fragments are not usually downloaded in sequential order and need to be reassembled by the receiving machine. It is important to note that clients start uploading fragments to their peers before the entire file is downloaded. Sharing by each peer therefore begins when the first complete segment is downloaded and can begin to be uploaded if another peer requests it.

This scheme is particularly useful for trading large files such as videos and operating systems. This is contrasted with conventional file serving where high demand can lead to saturation of the host’s resources as the consumption of bandwidth to transfer the file to many requesting downloaders surges. With BitTorrent, high demand can actually increase throughput as more bandwidth and additional “seeds” of the file become available to the group. Cohen claims that for very popular files, BitTorrent can support about a thousand times as many downloads as HTTP.

Simple example of how BitTorrent works

• 10 people are looking for 100MB file. The original holder of the file would have to upload it 10 different times for a total upload of 1GB, making for a precious waste of time and bandwidth.
• With BitTorrent, the file is divided into tiny pieces. For arguments sake, well say 500 at 200KB each.
• As each user connects to the original seeder they download a different piece of the file from him. After doing this, BitTorrent searches for the 9 other users downloading the same file and finds out what pieces they have. It then begins downloading the pieces they have as well. Concurrently, it begins uploading his pieces to the other 9.

What are seeders, peers, and leechers?

Seeder
A seeder is someone who has the full file and is still sharing. Users are encouraged to do this whenever possible. Peers are those who are downloading a file but have not yet finished.

Leecher
Leechers are generally people who download lots of stuff but do not share (thus missing part of the point of file sharing). For BT, usually a leecher is someone who closes their client within minutes after the download is complete. This is frowned upon by most other users. Some trackers actively kick people who have bad enough upload to download ratios. Sometimes people use the word leecher when they mean peer.
In contrast to other file-transfer protocols, BitTorrent is designed to work better as the number of people interested in a certain file increases.

BitTorrent was started by Bram Cohen, who developed the foundations of BitTorrent whilst studying effective and efficient distribution of data.

Some Important BitTorrent Points

• Users do not have a shared folder.
• To download a file, or a group of files, you must find and download a torrent file.
• A torrent file tells the client what tracker to connect to and what to download.
• Torrent files are found at torrent websites.
• Whilst downloading, you upload parts of the file you have already downloaded.
• When finished downloading, it is important for the network that users continue to upload.

BitTorrent Terminology

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.

Swarm

Together, all users sharing a torrent are called a swarm. Six peers and two seeds make a swarm of eight.

Peer

A peer is one instance of a BitTorrent client running on a computer on the Internet that you connect to and transfer data. Usually a peer does not have the complete file, but only parts of it, however, ‘peer’ can be used to refer to any participant in the swarm (in this case, also known as a ‘client’).

Seed

A seed is a peer that has a complete copy of the torrent and still offers it for upload. The more seeds there are, the better the chances are for completion of the file.

Leech

A leech is usually a peer who has a negative effect on the swarm by having a very poor share ratios - in other words, downloading much more than they upload. Most leeches are users on asynchronous internet connections who do not leave their BitTorrent client open to seed the file after their download has completed. However, some leeches intentionally hurt the swarm to avoid uploading by using modified clients or excessively limiting their upload speed.
The term leech is also incorrectly used to refer to what should properly be called a peer, a member of the swarm who has not yet downloaded the complete file.

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 that they can connect to. The tracker is not directly involved in the data transfer and does not have a copy of the file.

Availability

(also distributed copies) The number of full copies of the file 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 (ie. a peer with 65.3% of the file downloaded increases the availability by 0.653).

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.

Choked

Describes an uploader to whom the client does not wish to upload. An uploading client ‘chokes’ another client in several situations:
The second client is a seed, in which case it does not want any pieces (ie. it is completely uninterested)
The uploading client is already uploading at its full capacity (ie. the value for max_uploads has been reached)

Snubbed

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