Alice                                                Bob
           |                                                   |
           |        OPTIONS sip:b1@example.com SIP/2.0         |
           |-------------------------------------------------->|
           |                                                   |
           |                      200 OK                       |
           |    Contact: sip:b1@example.com;gr=hha9s8d-999a    |
           |            ;audio;video|;trickle-ice;...          |
           |<--------------------------------------------------|
           |                                                   |
           | INVITE sip:b1@example.com;gr=hha9s8d-999a SIP/2.0 |
           |             Supported: trickle-ice                |
           |                      (Offer)                      |
           |-------------------------------------------------->|
           |                                                   |
           |                  183 (Answer)                     |
           |<--------------------------------------------------|
           |                INFO/OK (Trickling)                |
           |<------------------------------------------------->|
           |                                                   |
           |                      ...                          |
           |                                                   |