First I want to define my question: 1, I am talking about computer program doing the IM, not mobile app. The program should be working in every home, or office environment.
2, Right now all (or most) home computers are behind a router, protected by ISP's security policy. It is not like 20 years ago, when a computer connects to an adapter, it gets a universal IP, as good as Microsoft.com and whitehouse.gov. 10 years ago, you can advise user to do port forwarding, or use UPnP to stealthily perform the port forwarding task. But now in the name of security, ISP is taking port forwarding out from the ISP-managed router (xfinity is doing that, as far as I know).
I know how to do IM in LAN setting, or the universal IP setting, that you get the IP of the friend, establish connection using TCP or UDP, then transfer information. Now when all computers are behind the routers, what is the feasible way / popular way / right way to do IM again? Using a central server to transfer information, by establishing connections of UserA-Server and UserB-Server? I know it would work, but not very "instant", in my humble opinion.
Please advise, thanks.
Upnp and NAT-PMP can still work, it will map a external port like punchhole. The problem is that our external IP may not be sure. So if you want to send IM, there are two choices, DHT or Central Controller.
I suggest you to use DHT and a central server using ICE structure, if DHT can work then it is ok, or we can use the central server to do the UDP/TCP traversal. In some circumstances like symmetric NAT, the server need to do relay, but in others, it just start the connection and then it is a p2p connection without server.