The BitTorrent Guide : Part I

BitTorrent is currently the world’s most popular Peer-to-Peer file-sharing protocol. BitTorrent is similar to other P2P networks, but with some differences that makes it a winner - every person who downloads a file, must upload it simultaneously, which means that more the number of people sharing a file, the faster it transfers, in contrast to a client-server model like FTP or HTTP. Another difference is that with BitTorrent, files are broken into smaller fragments, typically a quarter of a megabyte each. As the fragments are distributed to the peers in a random order, they can be reassembled on a requesting machine. Each peer takes advantage of the best connections to the missing pieces while providing an upload connection to the pieces it already has. For this reaso, BitTorrent generally deals in large files. BitTorrent was written by programmer Bram Cohen and debuted at CodeCon 2002. The name “BitTorrent” refers to the distribution protocol, the original client application, and the .torrent file type.

Using BitTorrent

To share a file using BitTorrent, a user creates a .torrent file (the process for which will be explained in the next part of the series), which contains the following :

• The file name, size, and the hash of each block in the file (which allows users to make sure they are downloading the real thing)
• The address of a “tracker” server
• And some other data (like client instructions).

Now, the user who wants to download the software that has been put up must first download the .torrent file which basically serves as co-ordinates of the real thing that the seeder is hosting. Then he must use a BitTorrent client to use the torrent file to download the software he wants.

BitTorrent Terminology

• Torrent : A torrent can mean either a .torrent meta data file or all files described by it, depending on context. The .torrent file contains meta data about shared files. It contains names of files, their sizes and checksums of all blocks in the torrent. It also holds the address of a tracker that hosts the torrent.
  peer
• 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.
  seed
• 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 a peer that does not have a complete copy of the torrent yet. When downloading is complete, it may stay around and seed the file as a seed so that others can complete their download. The term leech is also used for peers that have very poor upload/download ratios or leave the swarm immediately after their downloads are complete. (See Etiquette.) The leeches usually contribute a majority of the bandwidth in a swarm.
• Swarm : Together, all peers sharing a torrent are called a swarm. Six leeches and two seeds makes a swarm of eight.
• Tracker : A BitTorrent tracker is a server which directs the uploading and downloading of packets between clients using the BitTorrent protocol. It is also, without any extension to the existing protocol, the only major single point of failure, as peers are required to communicate with the server at regular intervals to receive up-to-date information.