UP                       CP             AAA
            |                        |  CoA Request  |
            |                       1|<--------------|
            | Session Update Request |               |
           2|<--------via Ci---------|               |
            |                        |               |
            | Session Update Response|               |
           3|---------via Ci-------->|               |
            |                        |  CoA Response |
            |                       4|-------------->|
            |                        |               |