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.