STUN (Simple Traversal of UDP over NATS) is a network protocol which allows the clients that are behind NAT to find their public IP and which kind of NAT they are behind, even the internet port. And those informations would be used to set up UDP connection between two systems which are both behind NAT.
A graphical illustration of the function of STUN:
The STUN algorithm:
**It might not pierce the Symmetric NAT
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