+----------+                      +-----------+
      |  Alice   |                      |    Bob    |
      |          |                      |           |
      +----+-----+                      +-----+-----+
           |                                  |
           |                                  |
           | SIP INVITE 1                     |
           |--------------------------------->|
           |                                  |
           |                                  |
           |                         200 0K 1 |
           |<---------------------------------|
           |                                  |
           |                                  |
           | SIP ACK 1                        |
           |--------------------------------->|
           |                                  |
           |                                  |
           |                                  |
           |<########### MEDIA 1 ############>|
           |   1 video A->B, 1 video B->A     |
           |<################################>|
           |                                  |
           |                                  |
           |                                  |
           |                                  |
           v                                  v