RE: To-tag in CANCEL response

> 1) All the 200 OK for the CANCEL responses do not
contain
> any To-tags. But according to RFC3261 section 16.10, a
> stateful proxy should act as a UAS, which then require
> the response to have a To-tag. Isn't it?

This is an error in RFC 3665. All final responses must have
to-tag.

> 2) Assume that CANCEL response should contain a To-tag,
then
> what tag should it use? The same one as in the 180
response,
> which then should match the 487 response for INVITE as
well?

The UAS should use the same to-tag for the 200-OK to the
CANCEL that it used in the 180, and that same to-tag should
be used in the 487 response for the INVITE. As it says in
section 9.2 of RFC 3261 :

   ....the UAS answers the CANCEL
   request itself with a 200 (OK) response.  This response
is
   constructed following the procedures described in Section
8.2.6
   noting that the To tag of the response to the CANCEL and
the To tag
   in the response to the original request SHOULD be the
same.

>3) If the proxy never received any provisional
response,
> except for 100 trying, meaning it has not received any
To-tag,
> then when it received the CANCEL request, should it
generate
> a local To-tag in the response?

Yes. The CANCEL is hop-by-hop and each proxy is responding
as a UAS, so it generates its own to-tag. If it previously
forwarded a 1xx response with a to-tag, it may use that
to-tag.

> 4) And since RFC3261 only stated that the To-tag in the
CANCEL
> response SHOULD,  not MUST, matches the one in the 487
response,
> I suppose it is OK to pass the 487 response from the
UAS (Bob)
> towards the UAC (Alice) without changing the To-tag to
match
> the one in the CANCEL response?

Yes. Proxies should not change the to-tag in a response it
is proxies back toward the UAC (see page 110 of RFC 3261).



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