Section 2 of RFC 3326 says:
Proxies generating a CANCEL request upon reception of a CANCEL from
the previous hop that contains a Reason header field SHOULD copy it
into the new CANCEL request.
In normal SIP operation, a SIP status code in a response provides the
client with information about the request that triggered the
response, the session parameters, or the user. For example, a 405
(Method not allowed) response indicates that the request contained an
unsupported method. A 488 (Not Acceptable Here) indicates that the
session parameters are unacceptable and a 486 (Busy Here) provides
information about the status of the user.
So this means that Proxies SHOULD include the "Reason" header in its CANCEL if it receives a CANCEL containing it.
This would be very useful in big SIP environments involving various proxies.
Another different subject would be Kamailio adding by itself a "Reason" header to inform the cancelled branches about the reason of the cancellation (ie, "Call answered elsewhere"). But this is a different and independent wish/feature.
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