Subject: [Sip] To-tag Corner Case

From: "Tom-PT Taylor"<[email-masked]>
Date: Wed, 24 Apr 2002 15:14:16 -0400

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.

(b) If the first response to an INVITE is a 1xx (greater than 100), then a
To-tag MUST be present in that response and in ALL succeeding responses,
even if the final response is a failure response.

Some unexpected tags in the call flows document stimulated this observation,
which I present for your confirmation.

Tom Taylor
[email-masked]
Ph. +1 613 736 0961 (ESN 396 1490)

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From: "Hussam Jarada" <[email-masked]>
Date: Wed, 24 Apr 2002 15:01:44 -0500

Hi,

The main issue I have with 2543bis-09 is that it's not clear with clear
examples that case were To tag is needed and not, cause in latest call flows
draft which been updated to reflect 2543bis-09 it may contain missing To tag
which I hope it just an typing error, I already I sent an email early for
clarification about this.

To my understanding
* To tag is added to any provisional response and final responses to all
requests, like it shown in section 11.1 line 1783, were no to tag in OPTIONS
request but in section 11.2 line 1818 there's To tag, and so 2543bis-09 is
missing To tag in F2 message, section 24.1

* I don't think (a) is true, cause I can not find any statement in the
2543bis-.09 that prove it, plus in section 17.1.1.3 which state that ACK
message for non-2xx differ from the To header field in the original request
by the addition of the tag parameter, it does not state that a provisional
response was sent early in order to have a To tag in the ACK message

Hope this helps.

Thanks.

Hussam Jarada
Ulticom Inc.

 -----Original Message-----
From: 	[email-masked] [mailto:[email-masked]]  On Behalf Of Tom-PT
Taylor
Sent:	Wednesday, April 24, 2002 2:14 PM
To:	[email-masked]'
Subject:	[Sip] To-tag Corner Case

Section 12.1 says:

Tom Taylor

---

From: Alan Johnston <[email-masked]>
Date: Thu, 25 Apr 2002 08:38:09 -0500

Hussam Jarada wrote: 

> To my understanding
> * To tag is added to any provisional response and final 
> responses to all requests, like it shown in section 11.1 line 
> 1783, were no to tag in OPTIONS request but in section 11.2 
> line 1818 there's To tag, and so 2543bis-09 is missing To tag 
> in F2 message, section 24.1
> 

The correct use of tags is shown in Section 11 with the OPTIONS.
Section 24.1 does contain an error in the lack of a To tag in the
response to the REGISTER.  This is the same error in the Call Flows.

The only change to tag usage in the Call Flows documents relates to
proxy behavior in forwarding non-2xx final responses.  It the past, the
call flows showed proxies replacing the existing tag in the non-2xx
response with its own tag.  Now, if possible, the proxy keeps the tag
when it proxies the response, which can help in troubleshooting -
figuring out which server sent the error.

Thanks,
Alan Johnston
WorldCom
sip:[email-masked]