Oh I'm well aware of that. :)
I assumed this whole discussion was theoretical.
In *practice* using sips is tough.  Some systems don't support it and will 
choke on the scheme, while some systems seem to ignore the extra "s".  And 
there are real problems with it even if you do everything by the book.  For 
example, it's not like Alice's UA will actually have a TLS cert to be able to 
be a TLS server/listen-socket, so you can't open a TLS connection to her UA 
regardless, ever.  And with TCP in general you have to treat her Registered 
Contact connection as an outbound-style flow (ie, like an alias'ed 
connection-reuse), even if the UAC doesn't indicate RFC 5626 nor 5923.  Once 
you do that, using "sip" instead of "sips" contact works, or has so far for us. 
 YMMV.

-hadriel


On Sep 15, 2011, at 12:03 PM, Iñaki Baz Castillo wrote:

> 2011/9/15 Hadriel Kaplan <[email protected]>:
>> No I mean if Bob wants to Refer Carol to Alice, or Alice to Carol (since 
>> that Refer can be sent out of dialog to Alice's contact).
> 
> Initial requests sent to a Contact address rather than being sent to
> an AoR are always problematic. The same occurs in attended trasfer
> when the REFER is sent within the dialog and contains a Refer-To with
> the endpoint Contact URI. Such URI could be no reachable if it's
> between some kind of NAT's (regardless the user used STUN).
> 
> -- 
> Iñaki Baz Castillo
> <[email protected]>

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