[SIPForum-techwg] Route header in ACK to a negative response

[Brett Tate]

> What should be the expected behavior of the SIP client, 
> Alice, in such a scenario? Should the ACK contain the 
> Route header or should it be left out.

As specified within RFC 3261, the Route MUST be added.


> What is the reasoning behind either course of action? 

The call flows of RFC 3665 do not normatively update the MUST and MUST
NOT requirements of RFC 3261.

The reason to include the Route within the ACK are presented within RFC
3261 section 17.1.1.3.  I'll refrain from commenting upon the potential
uselessness of following the RFC 3261 recommendation concerning adding
such a Route to the INVITE.  However if you add a Route header to the
INVITE, you must add it to the non-2xx's ACK (and the INVITE's CANCEL).