====== turn_auth ======
turn_auth - get TURN authentication credentials. In order to be able to use the built-in TURN server in IXNX you need in some configurations provide a username and password. Client needs to get those from server and use it in future TURN requests.

===== turn_auth request =====
**C → S {type:'turn_auth'}**

Client sends an empty turn_auth message to request fresh TURN credentials.

===== turn_auth answer =====
**S → C {type:'turn_auth', tauth:{tuname:..., tpwd:..., lifetime:...}}**

Server responds with turn_auth message containing username and password to use to get access to TURN server.

arguments:
  * tauth - TURN authentication credentials:
      * tuname - username to use to log in to TURN server
      * tpwd - password to use to log in to TURN server
      * lifetime - time (in seconds) username and password are valid
Client must send a new turn_auth request before end of lifetime to request a new fresh username and password.