Subject: Re: [Sip] To-tag Corner Case

From: Jiri Kuthan <[email-masked]>
Date: Thu, 25 Apr 2002 07:43:54 +0200

At 09:14 PM 4/24/2002, Tom-PT Taylor wrote:
>Section 8.2.6.2 of 2543bis says a To tag MUST be added to any provisional
>response (except perhaps 100 Trying).  It goes on to say:
>
>   The same tag MUST be used for all responses to that request, both 
>   final and provisional (again excepting the 100 (Trying)). 
>
>Section 12.1 says:
>
>   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 to INVITE establish a dialog.
>
>and of course, creation of a dialog involves the UAS adding a To-tag.
>
>What I get out of this is:
>
>(a) If the first response to an INVITE is a failure response, a To-tag MUST
>NOT be added.

I don't see how that is implied. Actually I think doing so would be badly 
restrictive (for example, To-tags are one way how to make an upstream UAC 
mirror piece of state in ACK).

-Jiri

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 25 Apr 2002 02:13:01 -0400

> mirror piece of state in ACK).

Jiri is right - this is not implied at all. In fact, bis is quite
explicit about adding tags to all responses. Section 8.2.6.2 reads:

If a request contained a To tag in the request, the To header field in
the response MUST equal that of 1358
the request. However, if the To header field in the request did not
contain a tag, the URI in the To header 1359
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 1361
present). This serves to identify the UAS that is responding, possibly
resulting in a component of a dialog 1362
ID. The same tag MUST be used for all responses to that request, both
final and provisional (again excepting 1363
the 100 (Trying)). Procedures for generation of tags are defined in
Section 19.3. 1364

The second sentence above is explicit that the UAS MUST add a tag to the
To header in the response, and the only exception is 100 trying. This
section is not specific to any type of response, and presents general
guidelines for constructing a response. Thus, it applies to all
responses except the 100 trying.

-Jonathan R.

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