Im interested in any communication that can be used to create a real-time
based multiplayer game.
You can use the Multiuser xtra (as opposed to the server - and even if
you use the server you will still need to use the xtra to communicate
with it).
Alternatively you could have a web server in between both machines and
repeatedly hit them with GET or POST to see whether there are any new
messages, but using the xtra is a better solution.
I think you can also use the Flash Multimedia Server to make voice/video chat
connections, but that requires the server.