Am Dienstag, 27. März 2018, 15:30:01 CEST schrieb Emmanuel BUU:
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Honestly, in ten years of doing SIP, this is the first time I am confronting this question. I was accustomed to the notion that To tags are only added to a reply when it is desirable to form an early dialog (in the case of a non-100 1xx reply). But the passage you quote seems to settle it.
An additional question! Does second authenticated INVITE sent by UAC should also mention this to tag and the called UA should also accept it as part of the dialog id ?
Or does the second authenticated INVITE open a new dialog ?
Hello Emmanuel,
the first dialog is created at the second successful INVITE. The second INVITE is just a re-send of the first INVITE with the authentication credentials. To quote from RFC 3261, section 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. A dialog established by a non-final response to a request is in the "early" state and it is called an early dialog.
You will find a good example for a sessions establishment including authentication and two proxies in RFC 3665, section 3.2.
Best regards,
Henning