Colin's Journal

Colin's Journal: A place for thoughts about politics, software, and daily life.

April 1st, 2003

BitTorrent and distributing large files

I’ve heard about BitTorrent before, but it was only today that I saw a great example of how it can change the nature of distribution of large files on the Internet. Red Hat released ISOs of version 9 of their Linux distribution to paying subscribers, and someone (legally) made them available through BitTorrent and announced their availability on slashdot. The result was that people could get hold of the ISOs through the peer-to-peer swarm more quickly than they could through the overloaded FTP site.

The reason the peer-to-peer network was faster is down to the way BitTorrent works, which is that each downloading client also becomes a provider of the file. A major strength of BitTorrent is that the downloading client doesn’t have to complete the download before it can offer uploads, whatever portions have already been downloaded are made available for upload to others in the swarm that might need them.

The architecture consists of three main components. The .torrent file contains a description of the file (or directory) that is to be downloaded, including name, file size, and a secure hash of each chunk of the file. It additionally contains the URL of a BitTorrent tracker.

The tracker maintains a list of peers currently involved in transferring a particular file (or directory), as well as some stats around what each peer is up to. The client, after parsing the .torrent file, connects to the tracker and gets the list of peers in the swarm. The client then contacts peers from this list directly, offering up portions of the file that the client already has, and asking for portions that it requires.

There is some load balancing to ensure that clients are uploading their fair share, you get faster downloads the more bandwidth you can provide on upload, and multiple downloads are performed at once (so enabling modem users to make a real contribution of bandwidth even to those on broadband connections).

It’s an excellent way of distributing large files without having to foot a huge bandwidth bill.

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