Correct me if I'm wrong. STUN/TURN is the one that is doing the UDP punching. And in the event that fails, all the data are being passed through the STUN/TURN server, which makes it just like client/server setup where the server became the bottle neck again.
Stun is a protocol to query the network about the topology. Turn is a protocol to route through a third party when p2p fails. Both of these are part of the ICE methodologies.
STUN is basically a way to know your external IP address to share to others and have both peers try to connect to one another through them. It's lightweight.
If that fails, then there's TURN which acts like a proxy between the peers.
