Establishing a direct connection between two clients under the control of the server

Question

Communication protocols

There is a server with a white (real) IP, there are several clients scattered over the Internet (dynamic addresses, nat ...) It is necessary to organize direct data exchange between clients.
The questions are:
1) What are the best protocols to use to exchange data and establish a connection?
2) What can I read on this topic (I tried to google, but it seems I can’t ask the question correctly)?
3) If someone did something similar, what libraries were used?

Answer 1

vpn make it and do not invent bicycles.

Answer 2

What bittorrent did not please? The protocol is open, even if you sculpt your own special clients, no one interferes.

Answer 3

Regarding NAT bypass, you can read about STUN

Answer 4

Hamachi will make a virtual (5.*.*.*) network to work through P2P VPN using UDP traffic.
Passes through NAT.