Thanks, I'm aware of Comet. Not exactly what I meant, in Comet the connection stays open after being initiated by the client. I'm looking for something where the interaction is first client-to-server, then client registers a "callback" address and port that it will listen to. This would be followed by a server-to-client call where the server will send updates/events to client as they happen, the client then decides what to do with those events. I want to avoid a persistent connection, and either the "heartbeat" client requests or other forms of polling by the client. The interaction that I'm looking for is more akin to a P2P sort of model.
I think firewall and NAT ubiquitousness (is that a word?) would make this unfeasible.
Yet another reason to speed the transition to IPv6.
IPv6 has nothing to do with firewalls and NAT issues. Firewalls in particular will remain in place and render this setup unfeasible.
You want a different and more complicated solution to an already solved problem?