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I admit that I only have rudimentary understanding, but: my understanding was that TURN uses a modified STUN format that returns the address/port on the peer facing side of the TURN server, a la address of a hotel room or PO box, of querying user. My point is that STUN/TURN(especially STUN) are not encapsulation protocols for WebRTC, but just means to determine(or get assigned, so TURN sort of is encapsulating, by being a transparent proxy) client's own public IP/port because there is no way to do so than by asking someone with a global IP.