BitTorrent is a particularly elegant solution to the problem of how to distribute a popular file without running up astronomical
bandwidth costs or falling prey to the
Slashdot effect. Most average users do a lot of downloading, but hardly use their
upstream bandwidth for anything other than
chatting or sending
email. Since they aren't using it anyway, why not harness all that unused bandwidth to help other people get these popular files? And so that is precisely what BitTorrent does. Just as
distributed computing applications such as
Seti @Home distribute a problem over many peoples' computer CPUs, BitTorrent distributes serving a file over many peoples'
Internet bandwidth. It also appeals to the
community ethic that still has some strength among
geeks—particularly those in the
Free Software or
Open Source communities.
BitTorrent is not so much a peer-to-peer application as it is a specialized form of world-wide-web download manager. Activated by clicking a link on a website, the BitTorrent client connects to a type of of file server called a tracker, and begins downloading the file from whichever seeds are available. As soon as a small portion of the file has been downloaded, the downloading BitTorrent client turns right around and starts uploading that portion to somebody else. In this way, the bandwidth of the first seed computer is magnified many times. It becomes feasible for even people whose upstream bandwidth is capped at 128 kbps to nontrivially contribute to the distribution of a file: in the time it would take for them to upload it once or twice to a single person, BitTorrent's trickle-down effect can have distributed it to dozens of people.
BitTorrent seems to be finding its greatest niche among distributors of large files such as captured TV shows and movies (which were formerly distributed solely by the slow and unwieldy IRC bot method), some more licit than others. www.animesuki.com serves as a clearing house for anime digisub fansubs, and bt.base0.net/tv-n hosts the tokusatsu fansubs of fansubbing group TV-Nihon. www.torrentse.cx formerly hosted user-contributed torrents of all kinds, but closed down after threats from the RIAA. www.suprnova.com has taken its place in terms of illicit content, but charges a $19-25 subscription fee.
On the more legitimate side, f.scarywater.net distributes CDROM ISO images and other files that are mentioned on Slashdot and thus subject to the website-killing Slashdot effect. Thanks to BitTorrent, British fans of the popular Fox TV series 24 were able to see episodes before they even aired on American TV, and American and British fans of Farscape (which sometimes aired first in America or in Britain) were able to see the newest episode of the show at the same time.
BitTorrent is not without its problems. Some users (particularly those using a certain less-advanced operating system) may find that their uploads clobber their downloads, or vice versa. The solution to this is to apply traffic shaping so that certain types of packet have priority over the BitTorrent traffic (or else use a more recent BitTorrent client which allows bandwidth throttling).
However, another problem, less easily solved, is the tendency of the average user to close his BitTorrent client soon after his download of the file completes—meaning that his bandwidth is no longer helping other would-be downloaders receive their own copy of the file. If enough people do this, particularly after the original seeder has stopped seeding in order to move on to another file, the torrent can be left with no seeds while there are still people trying to download—and thus, none of the downloads would be able to complete until someone else who had the file started seeding. These file leeches are a common problem, which different sites deal with in different ways; some purport to monitor length of connection after download and threaten to ban offenders' IP addresses; others threaten to make no new content available unless they see an improvement in their BitTorrent stats. Regardless, it is often a part of human nature to care less about other people than oneself, and so this behavior will continue to be a problem.