[OpenSIPS-Users] To-tag value in ACK

John Quick john.quick at smartvox.co.uk
Fri Feb 5 15:45:16 EST 2021

Thank you Donat. You have confirmed my suspicions.
The reference to the section in the RFC is very useful.

John Quick
Smartvox Limited
Tel:   +44-1727-221221

> From: Donat Zenichev <donat.zenichev at gmail.com> 
> Sent: 03 February 2021 09:57
> To: john.quick at smartvox.co.uk; OpenSIPS users mailling list <users at lists.opensips.org>
> Cc: Johan De Clercq <Johan at democon.be>
> Subject: Re: [OpenSIPS-Users] To-tag value in ACK
> 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!

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