[SR-Users] To tags in 407 challenges

[Alex Balashov]

On Mon, Mar 26, 2018 at 10:27:35PM +0200, Henning Westerholt wrote:

> 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. "

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.

-- 
Alex Balashov | Principal | Evariste Systems LLC

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