UAC UAS
| session established |
|<===================>|
| |
| F1 re-INVITE (SDP) |
|-------------------->|
| F2 1xx-rel (SDP) |
|<--------------------|
| F3 UPDATE |
|<--------------------| Target refreshing
| F4 2xx UPT |
|-------------------->|
| |
| F5/6 Cancel/200OK |
|-------------------->|
| F7 487 INV |
|<--------------------|
| F7 ACK |
|-------------------->|
| F8 re-INVITE (SDP) |
|-------------------->| Using which dialog state
F3/F4 is about target refreshing.
And if UAC sends Cancel， UAS sends 487. The first Re-INVITE is discarded.
But if UAC sends another Re-INVITE, which dialog state should it use?
I think is the one refreshed by F3. That's the merit of regarding UPDATE/200OK's atomicity.
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