Subject: [Sip] Bis-8: To tag question

From: Christer Holmberg <[email-masked]>
Date: Fri, 22 Feb 2002 12:11:45 +0200

Hi,

Chapter 8.2.6.2 says that a server MUST use the same To tag in all
responses, both provisional and final. So, if a UAS eg sends 18x with a
To tag, and then 4xx, my understanding is that the To tag must be in
BOTH of those responses.

However, chapter 17.2.3 says:

"The usage of the tag in the To header field helps disambiguate ACK for
2xx from
 ACK for other responses at a proxy, which may have forwarded both
responses (which can occur in unusual conditions)."

Is this really true? IF the non-2xx responses had a To tag, shouldn't
the ACK for that response use the same To tag? In that case a server
can't use the To tag in the ACK to check if the ACK was for a 2xx-, or
non-2xx-,response.

Is there a bug, or have I missunderstood something?

Regards,

Christer Holmberg
Ericsson Finland

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From: "Jo Hornsby" <[email-masked]>
Date: Tue, 26 Feb 2002 02:24:44 -0000

> Is there a bug, or have I missunderstood something?

I think the parenthesized "which can occur in unusual
conditions" is the important thing to note here.

Basically, this case arises when a proxy thinks it has
exhausted all branches, and returns some non-2xx final
response.  Then, seemingly out of the blue, a UA returns
a 2xx.  The proxy is forced to forward this 2xx;
however, the tag in the 2xx will be different from the
tag in the non-2xx, so any ACKs can be disambiguated.

 - Jo.

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From: Christer Holmberg <[email-masked]>
Date: Tue, 26 Feb 2002 10:50:42 +0200

Hi,

> >2xx from ACK for other responses at a proxy, which may have forwarded both
>

> conditions" is the important thing to note here.

True. However, I think the text should still be clear - no matter how unusual
the case described is :)

> tag in the non-2xx, so any ACKs can be disambiguated.

Yes. The tag will be different for each final response, but there will still
be a tag in each final response (2xx or non-2xx), so that's why I
wondered how the server can decide if an ACK is for a 2xx just by looking if
there is a tag or not (that's at least how I understood the text, so I
appologise if I missunderstood)...

Of course, using the tag in the ACK it will help finding the corresponding
INVITE transaction, but that's the case no matter what the final response
(2xx or non-2xx) was, so...

Thanks for your reply!

Regards,

Christer Holmberg
Ericsson Finland

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From: "Jo Hornsby" <[email-masked]>
Date: Tue, 26 Feb 2002 18:23:49 -0000

[Regarding 17.2.3]
> appologise if I missunderstood)...

It's not so much that the proxy can just look at the
tag and somehow magically determine that it was in
a 2xx response.  No, rather it's because the proxy
knows the tag it used in the non-2xx it sent; therefore,
if the tag in an ACK is not the same, it must be for
some 2xx.

 - Jo.

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From: Christer Holmberg <[email-masked]>
Date: Tue, 26 Feb 2002 21:40:40 +0200

Hi,

> [Regarding 17.2.3]
> >Yes. The tag will be different for each final response, but there
> >will still be a tag in each final response (2xx or non-2xx), so that's
> why I
> >wondered how the server can decide if an ACK is for a 2xx just by
> >looking if there is a tag or not (that's at least how I understood the
> text, so I

True. I just thought the text was a little unclear, giving the impression
that the tag as such is an indication that the ACK is for a 2xx response...

Thanks!

Regards,

Christer Holmberg
Ericsson Finland

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 27 Feb 2002 03:30:54 -0500

I added a bit of clarifying text here.

-Jonathan R.

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