TURN                                 TURN          Peer         Peer
client                               server         A            B
  |-- Allocate request --------------->|            |            |
  |   (invalid or missing credentials) |            |            |     
  |                                    |            |            |
  |<--------------- Allocate failure --|            |            |
  |              (401 Unauthenticated) |            |            |
  |                                    |            |            |
  |-- Allocate request --------------->|            |            |
  |               (valid credentials)  |            |            |
  |                                    |            |            |
  |<---------- Allocate success resp --|            |            |
  |            (192.0.2.15:50000)      |            |            |
  //                                   //           //           //
  |                                    |            |            |
  |-- Refresh request ---------------->|            |            |
  |                                    |            |            |
  |<----------- Refresh success resp --|            |            |
  |                                    |            |            |