What is Bittorrent?

What it is
   Bittorrent is a cooperative distribution of content where each individual who is downloading (and those who have downloaded) the content use their upload capacity to give the content to others.

Traditional distribution of content relied upon a single server providing the content.  However, this creates a vicious circle where the distributor is punished as the content becomes more popular.  The more people that download the content, the higher the bandwidth cost to the distributor.  If the content becomes very popular and the server is flooded with requests for download, then the server crashes and no one receives the content.  Thus, only those with special equipment and lots of money were able to adequately distribute content.

Bittorrent allows individuals, and small enterprises, to distribute their content as effectively as traditional distribution, but at a small cost.  In bittorrent the popularity of a content creates a virtuous circle where the more popular the content is, the more efficient the distribution (with cost remaining minimal).

For an example, suppose a popular local band wishes to expose their music to new people and expand their fan base. The internet opens a large population to the band, but traditional distribution costs of bandwidth and server purchase or lease would prohibit them from a large scale distribution.  Bittorrent allows this group to distribute their content to a large group of people at a minimal cost as the distribution is split up among those obtaining the content.  Once the band has distributed the content to a number of individuals, then the band no longer needs to provide upload of the content as those individuals will now distribute the content.  The more individuals involved in the content, the faster the content will be distributed.

Prior to bittorrent, the upload capacity of an individual’s internet connection was barely used.  Bittorrent makes use of this capacity to ease the distribution costs of the original distributor.

How it works
   Bittorrent works by splitting content files into hundreds of smaller “bits” and sharing those bits across a “swarm” of linked users.  A “tracker” essentially serves as a communications hub for this swarm of individuals, helping them find each other.

First, a small file called a .torrent file is downloaded from a torrent search site.  This file contains information about the download and allows communication to and from the tracker regarding people involved in this content (the swarm).  

This correlation between the .torrent file and the content helps to keep bittorrent clean of infections as altered content will not be accepted by the .torrent.  This means that a user cannot add malicious content to an existing torrent and that torrents containing malicious content can be quickly discovered and eliminated.  

The .torrent file is opened with a bittorrent client which communicates to the tracker and then the tracker arranges connections between the users.  These users transfer the content among themselves.

Seeds and Leeches (Peers)
A “seed” is someone who has downloaded 100% of the content and is only uploading to peers in the swarm.  “Peers” are those who have less than 100% of the content and are downloading from seeds and other peers and uploading to other peers.

Originally (and still in some places), those in the swarm who had less than 100% of the content were referred to as “leeches” and the combined seeds/leeches were referred to as peers.  However, because of the negative connotations of the term “leeches”, those in the swarm with less than 100% of the content are now referred to as peers and “leeches” are those who download the content, but do not upload back at least a full copy of the content.

While most torrents still utilize a tracker, recent advances have made it so bittorrent now can support torrents when a tracker goes down or without a tracker by initiating contact directly between the bittorrent users.

Ratio
If my explanation above is any good, then you will understand that it is important for those who download the content to fully re-distribute the content.  This concept in bittorrent is referred to as ratio.  “Ratio” is the amount uploaded in bittorrent divided by the amount downloaded in bittorrent.  So, if one downloads a 100MB file and uploads 50MB of that file, their ratio would be 0.50 (50%).  

There is a lot of reference in bittorrent to maintaining at least a 1.00  (100%) ratio.  This means that you have given back as much as you have taken through bittorrent.  This helps keep torrents healthy and alive.

Conclusion
Bittorrent, which  was first available just over seven years ago in July of 2001, has grown to the point where it accounts for approximately 40% of all internet traffic.