The default distribution mechanism for Amazon S3 data is via client/server download. In client/server distribution, the entire object is transferred point-to-point from Amazon S3 to every authorized user who requests that object. The costs of client/server distribution increase linearly as the number of users downloading objects increases. This can make it expensive to distribute popular objects.
BitTorrent addresses this problem by recruiting the very clients that are downloading the object as distributors themselves: Each client downloads some pieces of the object from Amazon S3 and some from other clients, while simultaneously uploading pieces of the same object to other interested “peers.” The benefit for publishers is that for large, popular files the amount of data actually supplied by Amazon S3 can be substantially lower than what it would have been serving the same clients via client/server download. Less data transferred means lower costs for the publisher of the object.
Every anonymously readable object stored in Amazon S3 is automatically available for download using BitTorrent. Retrieving a .torrent file for any publicly available object is easy. Simply add a “?torrent” query string parameter at the end of the REST GET request for the object. No authentication is required.