Although the use-case is a bit different from what is being discussed by Andy Roberts, at least one form of “delta” document for Atom/RSS has already become an important part of the syndication infrastructure. Servers and clients that support “RFC3229+feed” as described on my blog ( [link] [link] ) save a great deal of bandwidth and computing resource by working with Delta Feeds.
bob wyman
via the MS RSS Team Blog:
Windows RSS Platform Download Engine [link]
Delta Encoding
In addition to standard conditional GETs the RSS Platform download engine supports Delta Encoding (for details, see Bob Wyman’s post "Using RFC 3229 with Feeds") which allows the server to respond with only the feed items that are new or have been updated, thereby possibly reducing the response significantly.
also see: Default and Minimum Intervals, TTL, Compression (gzip encoding), Automatic unsubscribing, Features to help manage client network usage