[OpenSIPS-Users] To-tag value in ACK

[Donat Zenichev]

Good day John,
it looks like your own deduction is absolutely right!

If we talk about the SIP protocol in terms of RFC 3261, then I guess it's
clear that the acknowledgement which has a To tag,
which is different from the previously defined one (in 200 OK) of the same
dialog, should be essentially considered as out of the dialog request.
Which will not be correlated with the dialog is being established (the one
in the early stage you are talking about).

So that, it's known - "Call-ID" + "To tag" + "From tag" gives us a full
uniqueness of certain dialog/call branch (in your case it looks like it's
only one branch though).
If you change at least a To tag in the subsequent request (in-dialog
request) for the same dialog/call branch,
then the remote side will likely fail to match it to a needed dialog
(transaction check).

Please see this section of RFC 3261 to understand the concept:
https://tools.ietf.org/html/rfc3261#section-19.3

I hope I was clear enough and you find my response useful.
Have a nice day!

On Tue, Feb 2, 2021 at 6:50 PM John Quick <john.quick at smartvox.co.uk> wrote:

> Johan,
> Thanks for your comment, but in this instance the problem is something
> very subtle.
> OpenSIPS is acting as a Proxy, not as an endpoint. So the Contact header
> in the ACK contains the address of the UAC.
> OpenSIPS identified itself earlier in the dialog using the correct FQDN in
> the topmost Record-Route header of the INVITE request and using TLS with a
> certificate whose subject name matches the FQDN.
>
> Like I said, I have been able to put this ACK side-by-side with the ACK in
> a similar case where the call works correctly.
> Doing an A-B comparison, the only obvious difference I could identify was
> in the order of the headers.
> For example, the Route header is before the Via headers in one case and
> after in the other. I don't believe this is important.
>
> So then I looked at values in the 200 OK to see if they were the same in
> the ACK:
> The R-URI in the ACK is the same as the Contact in the 200 OK, including
> parameters.
> The From tag is identical in both
> The To tag in the ACK is not the same as the To tag in the 200 OK, but in
> my sip trace for a call that worked okay, the To tag did not get changed.
>
> My question is about the To tag. Should it be the same in the ACK as it
> was in the 200 OK?
>
> John Quick
> Smartvox Limited
> Tel:  +44-1727-221221
>
>
> > From: Johan De Clercq <Johan at democon.be>
> > Sent: 02 February 2021 14:42
> > To: John Q <john.quick at smartvox.co.uk>; OpenSIPS users mailling list <
> users at lists.opensips.org>
> > Subject: Re: [OpenSIPS-Users] To-tag value in ACK
> >
> > is contact an fqdn ?
> > If not, look no further.
>
>
> _______________________________________________
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> Users at lists.opensips.org
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>


-- 

Best regards,
Donat Zenichev
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