2012/7/31 José Luis Millán jmillan@aliax.net:
Outbound provides the UAC a way to update a binding even if it reboots. For that, a unique and permanent value of instance-id is used, which in conjunction with the AoR and reg-id determines the binding to the UAC.
Having said this, I guess that the CSeq comparison between the one in the Register request and the one in the binding does not apply in this scenario since it is not guaranteed that a UAC saves the CSeq value of the registration among reboots.
This is correct. According to RFC 5626, when the REGISTER includes a Contact with +sip.instance and reg-id params the registrar MUST NOT check the call-id and cseq of the request, but just the +sip.instance and reg-id params (and the registering AoR in the To header of course).