What is test1, test2 and test3 in case of STUN?

Question

I didn't understood importance of three tests in case of STUN ? STUN server is used for knowing external port and ip address so why three requests?

Thanks

Answer 1

The three tests are used by a sip client to determine in which mode the STUN-Gateway is operating

• Open Internet
• Full Cone NAT
• Restricted Cone
• Restricted Port
• UDP Blocked
• Symmetric UDP Firewall
• Symmetric NAT

If the three tests algorithm ends up in one of the later three states no UDP communication is possible.

Check this nice wikimedia graphic for a flowchart of the NAT characterization algorithm