Why does SUP use SUP-IDs instead of URLs or URL-fingerprints?
For a variety of reasons, including the desire to guard private or competitive data, many feed publishers do not wish to publish a complete list of feed urls. SUP-IDs allow them to instead assign an arbitrary string of letters and numbers to identify each feed. The only way to discover a feed’s SUP-ID is by downloading the feed, therefore no additional information is revealed by a SUP-ID. SUP-IDs can also be much shorter than URLS (the SUP-IDs used by friendfeed.com are only 8 bytes), which is important on busy sites that have a high update rate. Keeping with the philosophy of making life simple for feed publishers, SUP-IDs are arbitrary, and feed publishers are free to decide how they should be assigned. One of the benefits of generating SUP-IDs based on something internal, such as user-ids, is that it feed publishers can assign the same SUP-ID to multiple urls. For example http://friendfeed.com/paul?format=atom, http://friendfeed.com/api/feed/user/paul?format=rss, and http://friendfe
