FriendFeed's Simple Update Protocol

FriendFeed is working on Simple Update Protocol (SUP), a means of
discovering when feeds on a particular service have been updated
without polling the individual feeds:

http://code.google.com/p/simpleupdateprotocol/

Feeds indicate their updates can be tracked with SUP by using a new
channel-link tag, as in this example from an Atom feed:

<link rel="http://api.friendfeed.com/2008/03#sup"
href="http://friendfeed.com/api/sup.json#53924729"
type="application/json" />

The rel attribute identifies an ID for the feed, called its SUP-ID.
The href attribute contains a URL that uses JSON data to identify
updated feeds by their SUP-IDs.

My first take on the protocol is that defining a relationship with a
URI is too different than standard link relationships in HTML:

http://www.w3.org/TR/REC-html40/types.html#type-links

Also, neither RSS 1.0 nor RSS 2.0 allows multiple channel-link tags,
so this would only be valid in an Atom feed.
 
Both of these concerns could be addressed by identifying the SUP
provider with a new namespace like this:

<rss xmlns:sup="http://friendfeed.com/api/sup/">
<channel>
<sup:provider href="http://friendfeed.com/api/sup.json#53924729"
type="application/json" />
...

If the definition of the channel-cloud element wasn't tied so closely
to remote procedure calls, it could have been used to identify a sup
provider.


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