[SR-Users] To tags in 407 challenges

[Henning Westerholt]

Am Montag, 26. März 2018, 16:55:12 CEST schrieb Alex Balashov:
> This may be more a SIP question than a Kamailio question, but I am
> trying to figure out why 407 challenges from Kamailio contain To tags.
> 
> I suppose I've not run across it before, and I can't find an RFC-based
> rationale. A negative reply to an invite transaction without an
> intermediate early dialog-forming reply (e.g. 183 with To tag) does not
> form a dialog, nor takes place within a context in which a dialog was
> created. So what gives?

Hello Alex,

its indeed that common that it also included in the example RFC 3665, section 
3.3. and other.

There is a transaction relationship between the INVITE and the 407. RFC 5057 
mandates that the 407 only refers to the transaction, not to any dialog state. 
But you are right, if there is no provisional response, there is no (early) 
dialog at this state. 

But its also included in the 401 (for REGISTERs).

I would think this is normal UAC/UAS behavior for SIP request handling:

RFC 3261, sect. 8.2.6.2:
"However, if the To
header field in the request did not contain a tag, the URI in the To
header field in the response MUST equal the URI in the To header
field; additionally, the UAS MUST add a tag to the To header field in
the response (with the exception of the 100 (Trying) response, in
which a tag MAY be present). This serves to identify the UAS that is
responding, possibly resulting in a component of a dialog ID. "

Or I am wrong?

Henning