interworking(1/2) of INVITE and UPDATE

Hi all,

At this chance, I want to hear your opinion about the
interworking of INVITE and UPDATE.

This is related to chapter 4 in a offeranswer draft,

Regarding "5.1 Sending an UPDATE" of RFC3311, Paul said that
"this language itself is non-normative and is justified as
a corollary of 3261."

I think so, but these descriptions are also very messy.
About the timing when UA can send an UPDATE request,
at last I think as the following,

       A                               B
      *|                               |*
      *|ini/re-INVITE(offer)           |*
       |------------------------------>|
       |                1xx-rel(answer)|
       |<------------------------------|
       |PRACK                          |
       |------------------------------>|
       |                     200(PRACK)|
       |<------------------------------|
      *|                               |*<-- after this pont, UA may
      *|                               |*    send UPDATE with offer
      *|                               |*

       A                               B
      *|                               |*
      *|ini/re-INVITE(no offer)        |*
       |------------------------------>|
       |                 1xx-rel(offer)|
       |<------------------------------|
       |PRACK(answer)                  |
       |------------------------------>|
       |                     200(PRACK)|
       |<------------------------------|
      *|                               |*<-- after this pont, UA may
      *|                               |*    send UPDATE with offer
      *|                               |*

Do you have any questions or comments?

Regards,
Shinji
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