TURN stands for Traversal Using Relays around NAT.
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 TURN 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 TURN server (or pay for a hosted TURN service).
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