Traversal Using Relay NAT (TURN), a standard-based effort started as early as in November 2001, was meant to be the complementary method for the (old) STUN to complete the solution. The original idea was the host to use STUN to detect the NAT type, and when it has found that the NAT type is symmetric it would use TURN to relay the traffic. But as stated above, this approach was deemed to be unreliable, and now the prefered way to use TURN is to combine it with ICE.