1. Recommendation of signal flow
For usage of SIP/SDP, UA is better to avoid triggering a new modification of session while the previous one is in pending state.
For examples, when UAC want to triggering a new modification, it should Cancel the current Re-INVITE, and then trigger a new Re-INVITE(F11):
UAC UAS
| session established |
|<===================>|
| |
| F1 re-INVITE (SDP) |
|-------------------->|
| F2 1xx-rel (SDP) |
|<--------------------|
| F3 UPDATE | <- UPDATE request include state
|-------------------->| notification
| F4 2xx UPT |
|<--------------------|
| |
| F5 180 ringing |
|<--------------------|
| |
| F6 Cancel |
|-------------------->| <- UAC request termination of Re-INVITE
| F7 2xx |
|<--------------------|
| F8 487 INV |
|<--------------------| <- final response of Re-INVITE
| F9 ACK |
|-------------------->|
| |
| F11 re-INVITE (SDP)|
|-------------------->| <- Re-INVITE request inclue a new
modification
And when UAS want to triggering a new modification, it can just send 4xx to current Re-INVITE, and then trigger a new Re-INVITE.
2. Why UPDATE during INVITE has no such ambiguous state of session as Re-INVITE?
Unsuccessful INVITE will terminate session and dialog, so it is meaningless to talk about session state after unsuccessful INVITE.
But unsuccessful Re-INVITE will not terminate session and dialog, so it is meaningful to talk about session state after unsuccessful Re-INVITE.
3. Triggering a new modification while the previous one is in pending state
The behavior like this is unfriendly(or immoderate), but legal. But while the new modification is committed, the previous one in pending state MUST be discarded.
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