Hi Jānis,

The ACK can have the different request URI if the 200 OK of INVITE contains an 
strict router address in Record-Route header.  The proxy modifies the 
Request-URI of ACK only if the next hope is a strict router.

Regards,
Alok Tiwari
Aricent
-----Original Message-----
From: [email protected] 
[mailto:[email protected]] On Behalf Of Janis 
Rukšans
Sent: Tuesday, February 12, 2013 6:58 AM
To: [email protected]
Subject: [Sip-implementors] Request-URI of ACK

Hi,

Is it OK for an ACK to have a different Request-URI than in the Contact header 
of the 200 OK being acknowledged?

RFC 3261 Section 13.2.2.4 says that "header fields of the ACK are constructed 
in the same way as for any request sent within a dialog", which AFAICT should 
always result in the remote target (on the UAC
side) being the Request-URI when the ACK is received by the UAS.
However, as only Call-ID and To and From tags are used to match requests to 
dialogs, as far as the UAS is concerned, the Request-URI can be whatever (true 
for other mid-dialog requests as well).

I'm facing a rather strange situation where a proxy is apparently modifying the 
Request-URI of ACK, causing problems on our side. Also, the other end (Server: 
Avaya CM/R016x.02.0.823.0
AVAYA-SM-6.2.3.0.623006) appears to be using a different route set depending on 
whether it is acting as an UAS or UAC (in the same dialog). That is not a real 
issue, though.


Thanks,
Jānis

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