I've been perusing this site among others to get an understanding of RSS and Atom, considering their use for an in-house data subscription feed. There's one thing that just seems glaringly missing to me, and I'm wondering whether I've just missed it, or if it just isn't there. In both RSS and Atom, it seems like the standards describe a document format that lists all of the content items/entries available on a feed. That allows a client to ask a feed provider "what all do you have?". So far so good. But it seems to me that any ongoing client of a feed provider would like to ask "what's new?" (or "what all do you have since datetime T?"). I keep expecting to see a description of a protocol to do that, but I haven't seen it anywhere. I'd expect something very simple, something like a HTTP GET URL parameter with a standard name like "since" and an IETF standard format datetime as its value, that you can tack on to any Atom URI.
I suppose such a thing is not absolutely needed, but it's very wanted. I could hit the Atom URI for a very active blog site every few minutes and sift through the entire 5-year archive that they send down, just to see whether there are one or two (or zero) new items posted. But that seems rather inefficient.
The closest I've seen to this is a fairly recent RFC about feed paging and history, which kinda sorta gets towards this, but not in a particularly client-friendly way.
It seems astonishing to me that anyone could conceive of a syndication "protocol" without the ability for a client to ask "what's new?". What am I missing?