TURN

TURN - Traversal Using Relay NAT - allows a device that is behind a firewall or NAT (Symmetric NAT or better known as Bi Directional NAT) device to receive incoming data leveraging TCP or UDP. TURN will most likely provide connectivity to the client but it does come at a high price to the provider. STUN is generally used first and TURN is used as a last resort.

The host sitting behind the NAT device is called a TURN client which connects the the TURN server on the public internet which acts as a relay. The TURN client communicates and arranges with the TURN server to have the server relay the packets to the desired peer. When the TURN client and peer want to communicate, the communication from the TURN client to the TURN Server is encapsulated within a TURN MESSAGE. The communication between the peer and the TURN server is not encapsulated.

TURN - RFC 5766