SUP (Simple Update Protocol) is a simple and compact "ping feed" that web services can produce in order to alert the consumers of their feeds when a feed has been updated. This reduces update latency and improves efficiency by eliminating the need for frequent polling.

SUP is being developed by FriendFeed.

Benefits include:

• Simple to implement. Most sites can add support with only few lines of code if their database already stores timestamps.
• Works over HTTP, so it's very easy to publish and consume.
• Cacheable. A SUP feed can be generated by a cron job and served from a static text file or from memcached.
• Compact: updates can be about 21 bytes each. (8 bytes with gzip encoding)
• Does not expose usernames or secret feed urls (such as Google Reader Shared Items feeds)

Resources:

• Frequently Asked Questions about SUP
• Specification and example code
• SUP Validator (source)
• RSS/Atom Feed tester
• Public SUP feed (anyone can link to and contribute updates)
• Open source implementations of SUP are available as part of Laconica and Benjamin Golub's open-source blog software, as well as the example code provided by FriendFeed.
• See FriendFeed's SUP feed
• Discuss SUP in the SUP room on FriendFeed

The following services support SUP:

• BackType
• Brightkite
• Disqus
• FriendFeed
• Identi.ca and other Laconica powered microblogs
• Our Doings
• 12seconds.tv
• Blogs powered by Benjamin Golub's open-source blog software