I've been working on creating my first feed for a forum and the only stumbling block I've come across is the ID in the top level element. As I understand it it has to be valid URL and unique which I've seen how to create. The ID for each entry being unique I understand but I'm not so sure about the ID in the top level when the contents of the feed change.

For example I have 20 entries listed, someone posts a new topic so the feed is now going to be updated with a new entry. Does the ID in the top level need to change at this point?