Klaus Darilion writes:
UA1 behaves wrong - per RFC 3261 the dialog will be established by the 200 OK message (not by 180), therefore the to-tag from the 200 OK must be used.
it may be that to tag from 200 ok must be used, but you are wrong about the dialog part. as per rfc 3261 dialog is established already by 180 ringing.
-- juha
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12.1 Creation of a Dialog
Dialogs are created through the generation of non-failure responses to requests with specific methods. Within this specification, only 2xx and 101-199 responses with a To tag, where the request was INVITE, will establish a dialog.
12.1.2 UAC Behavior
When a UAC receives a response that establishes a dialog, it constructs the state of the dialog. This state MUST be maintained for the duration of the dialog.
The local tag component of the dialog ID MUST be set to the tag in the
From field in the request, and the remote tag component of the dialog ID
MUST be set to the tag in the To field of the response.