based on MLDonkey via Don Park

BitTorrent is smart software to distribute large downloads.

It divides a large download into small pieces.

Each piece is tagged and a checksum is computed for each piece. These are used to track each piece of the download and to ensure that the data received matches the data sent.

The software nominates a tracker – a Web resource that acts as a broker and introduces peers/clients to each other.

Peer machines then contact each other, learn what pieces each has available and request the rarest (least commonly seen) pieces first.

Fairly quickly, clients can start to put together the original file by grabbing different pieces of the download from various different peers.

Meanwhile each client in turn supports other peers as they too ask for pieces of the download.

What this means in practice is that the original “source” (say, the BBC) doesn’t get “hit” too badly, because most of the download traffic is peer-to-peer rather than server-client.