[Sipping] : Re: Summary of Closing the offer/answer rollback issue? //Explanation

[gao.yang2 at zte.com.cn]

I arranged a Q/A list. And the questions
are people usually asked. And it is the process of my thought, so I once
asked myself too.

[Q1] Why we need rollback?
[A1] That is the original
concept of RFC3261. And it is the intuitionistic requirement for rejecting
modification from user's view.

[Q2] And I think other SIP
transaction(such as UPDATE) is independent transaction from Re-INVITE.
[A2] Yes.

[Q3] If I think that Offer/Answer
in UPDATE/200OK will be committed alone, and if the UPDATE/200OK pair is
during the Re-INVITE, what will happen. Committment/Rollback? It seems
antinomy. 
[A3] Yes. It seems that.

But the 200OK of Re-INVITE
is the "Late Commitment" of the modification triggered by Re-INVITE.
The commitment of other "new modification" with other transaction
is the duty of that transaction 's 200OK.

[Q5] It puzzled me more. How
can we treat a new modification during Re-INVITE?
[A5]If UA can not accept
it at once, it should be rejected by 504. If UA accepts it, it will be
committed at once. And it will have nothing to do with the Re-INVITE's
failure or success.

[Q4] But there are really
some special Offer/Answer. How can we treat it?
[A4] Such as precondition,
it has the semantics of suspending/resuming of modification. So, it can
be treated as part of the original modification. Then it will rollback
with Re-INVITE's failure.
Before user is hinted, there
can be more than one Offer/Answer pairs. But when he/she rejected the modification,
he rejected all as atomic.

[Q6] Your mean differentiate
the UPDATE during Re-INVITE?
[A6] Yes. Because some are
really parts of the original modification and some are really new modifications.

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