[OpenSIPS-Users] To-tag value in ACK

John Quick john.quick at smartvox.co.uk
Tue Feb 2 16:48:20 EST 2021

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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