Now of course if the application wants to be aware, it can
"subscribe to events" that let it know that it happened.
That sounds remarkably like a "presence" service.
No, what it is is a socket call you make that "subscribes" do address
events on the socket. Thus when a peer adds and deletes an
address you get a "notification" message up the socket buffer
telling you that an address was added and one was deleted.
Both are needed. That is, you need a way for the host IP stack to know
that a peer has moved to a different locator, and you need timely
notification that this has happened. This is a lot like a presence
service, but it needs to operate at layer 3, and it needs to act in
response to sending IP packets to the old locators (polling a presence
server is either too slow or - if you do it often enough - too
wasteful).
And yes, you probably also need a socket call that the app can use to
ask that it be notified of changes in peers' locations, because
location changes can also result in changes in the level of service
available to the app. though hopefully most apps won't need to use it.
Keith