Subject: [Sipping] Explanation for "draft-gaoyang-sipping-session-state-criterion-01.txt"-2009/02/16-1

From: [email-masked]
Date: Mon, 16 Feb 2009 11:32:21 +0800

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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