RG            UP              CP         AAA
           | User Access |  User Access   |   AAA    |
           |   Request   |    Request     |  Req/Rep |
          1|<----------->|<----via Si---->|<-------->|
           |             |                |          |
           |             | Create Session |          |
           |             |    Request     |          |
           |            2|<----via Ci---->|          |
           |             |                |          |
           |             | Create Session |          |
           |             |    Response    |          |
           |            3|----via Ci----->|          |
           |             |                |          |
           |  Multicast  |                |          |
           | negotiation |                |          |
          4|<----------->|                |          |
           |             |                |          |