TURN as in TURN server stands for Traversal Using Relays around NAT.

It is a standard method of NAT traversal used in WebRTC. It is defined in IETF RFC 5766.

TURN Server

TURN is used to relay media via a TURN server when the use of STUN isn’t possible.

The decision whether to use STUN or TURN is orchestrated by a protocol called ICE.

Since it relays all media through it, this can be a rather expensive endeavor (costing in bandwidth and CPU at a data center). This is why public TURN servers aren’t usually available and every service needs to install and maintain its own server (or pay for a hosted service).

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