BitTorrent is an ad hoc, P2P (peer-to-peer) protocol.
Cohen created it to address the problem of transferring popular, large files over the Internet.
Instead of using the client-server model of traditional Internet file distribution systems, such as FTP or the Webs HTTP or older P2P systems such as Kazaa, wherein files are shared directly among individual users, BitTorrent took a new approach.
BitTorrent works by using a central server, or tracker, to coordinate all the peers sharing a particular file.
The tracker, however, may or may not have a master copy of the file.
Its job is to simply coordinate the connections—nothing more, nothing less.
Instead, systems with a complete copy of the file, also known as seeders, start sharing it with systems that request it.
Then, unlike other P2P systems, those systems that have only part of the file begin sharing it with other users requesting the file.
The end result is that with multiple users both sending and receiving the file, download speeds tend to be fast without requiring high-bandwidth connections or multiple servers by the files owner.
Technically, BitTorrent works by connecting over TCP ports 6881-6999.
A smaller subset of ports can be used. Ten, one per each transfer session, such as 6881-6891, is a typical configuration.
However, a BitTorrent client must have outbound access to port 6969 to connect with most trackers.
It all sounds more complicated then it really is, as far as end-users are concerned.
For a user, all thats required is that the systems firewall or NAT (Network Address Translation) be set to allow BitTorrent access to its TCP ports.
Some anti-worm programs, like the one in Norton SystemWorks 2005, must also be set to allow BitTorrent inbound connections.
With those factors taken care of, all you need to do is to click on a BitTorrent link, and the download will begin.
With BitTorrent, data transfer rates can be achieved that make the downloading of hundreds of megabytes of data, or even gigabytes of data, practical over even slow broadband connections.
So, for the end user, BitTorrent is easy. Its another story for network administrators.
British P2P analysis firm CacheLogic claims that BitTorrent protocol traffic accounts for an amazing 35 percent of all Internet traffic.
According to CacheLogics studies, BitTorrent traffic accounts for more Internet traffic than any other single protocol, such as http, or e-mails POP (Post Office Protocol) and SMTP.
That same popularity makes BitTorrent a major problem for network administrators.
Even though an individual BitTorrent client doesnt use much bandwidth, dozens, hundreds, or even thousands of them are enough to bring strong LANs to their knees.
Another problem with BitTorrent is, like other P2P services before it, its often used to copy and share copyrighted materials.
Unlike Napster of old, though, which only transferred songs of a few megabytes, BitTorrent is often used to move the hundreds of megabytes of movies.
Together, these problems are bad enough that many institutions, like the University of Florida, ban the use of BitTorrent and other P2P clients.