> can B side respond to the timer offer with
> a new offer "before" answering the first one?

Yes.

> 200OK for INV is the last to be sent so if it
> supersedes the UPD/200OK transaction what's
> the point of putting session timer in UPDATE
> anyway ...

Among other reasons, it allows the proxies to know that the caller's
device is still aware of the early dialog.  The Timer C mechanism does not
provide this knowledge unless 100rel is being used.


> but if you omit then is it correct from a
> protocol perspective?

Yes; however if the sender does not indicate timer supported, they are
indicating that they don't support RFC 4028.

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