Cyberclinic: Can you explain what BitTorrent is, and how it works?

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If you suspect that BitTorrent is an internet-related tool used only for nefarious purposes, you're not far wide of the mark. But behind its success as a means of sharing copyrighted material among thousands of people who should probably know better lies a brilliantly devised piece of software which is now beginning to be used for legal purposes.

The programs used for torrenting (such as uTorrent for your PC, Bits On Wheels for the Mac, and Azureus for both) aren't illegal in themselves, any more than a meat cleaver isn't illegal until you start brandishing one on the high street. So feel free to go and download one – they come free of charge, after all.

But how do they work? Over to reader Tom Brogan: "If someone wants to share – or 'seed' – a video file of football highlights, they use a torrent program to create a small file relating to that video. When a copy of that file is opened by someone else, they are connected via a tracker to the original seeder of that video, and, more importantly, to everyone else who has opened it, too."

By using a clever system of breaking this file up into small chunks, the torrent then shares the load of distributing the goalmouth action between all its users; as soon as you've downloaded a chunk from the original seeder, other people are free to download that chunk from you. It's a cheap and remarkably efficient method of distribution.

The amount of data being shifted via torrents is astonishing; it already accounts for anything between 18 per cent and 35 per cent of all broadband traffic in North America. As a result, legal activity is becoming frenetic. Copyright holders have tried chasing those who run the aforementioned trackers, but as they store none of the copyrighted data, the buck tends to be passed to the torrent users, who are generally to be found whistling, hands in pockets, and saying: "What's the problem?"

Last year, the US broadcasting network HBO sent "cease and desist" letters to various internet service providers to get their customers to stop torrenting, but many just switched their ISP and carried on. And, as reader Sanjit Ghose points out, ISPs can't ban all BitTorrent traffic because there are legitimate uses: properly licensed movies and music are available at, and many software companies now choose to release software updates using a torrenting system.

But now that we've downloaded our torrent software, the chances of us using it legally and responsibly are, frankly, somewhere between slim and non-existent.

Diagnosis required

Next week's question comes from Steve Hart:

"For me, using webmail beats using my old email program in every way, with one exception: I can't get at it when I have no internet connection. Is there any way around this?"

Any comments, and new questions for the Cyberclinic, should be emailed to