The present annex defines IP-CAN specific requirements for a call control protocol for use in the IP Multimedia (IM) Core Network (CN) subsystem based on the Session Initiation Protocol (SIP), and the associated Session Description Protocol (SDP), where the IP-CAN is a DOCSIS cable access network.
DOCSIS (Data Over Cable Service Interface Specification) is a term referring to the ITU-T Recommendation J112  Annex B standard for cable modem systems.
A UE accessing the IM CN subsystem, and the IM CN subsystem itself, utilise the services provided by the DOCSIS cable access network to provide packet-mode communication between the UE and the IM CN subsystem.
From the perspective of the UE, the necessary IP-CAN bearer for signalling is transparently available to the UE.
The UE is not directly involved in requests for IP-CAN bearer(s) for media flow(s). The IM CN interacts with the PCRF in the DOCSIS IP-CAN to establish IP-CAN bearer(s) for media flow(s), on behalf of the UE.
Prior to communication with the IM CN subsystem, the UE shall perform a Network Attachment procedure as defined in the CableLabs PacketCable specifications PKT-TR-ARCH-FRM . When using DOCSIS, both IPv4 and IPv6 UEs may access the IM CN subsystem. The procedures for P-CSCF discovery defined in subclause 9.2.1 of this document apply.
A UE supporting the P-CSCF restoration procedure uses the keep-alive procedures described in RFC 6223.
If the P-CSCF fails to respond to the keep-alive request the UE shall acquire a different P-CSCF address using any of the methods described in the subclause H.2.2.1 and perform an initial registration as specified in subclause 5.1.
If the access-type field in the P-Access-Network-Info header field indicated DOCSIS access the P-CSCF shall assume that the initial request for a dialog or standalone transaction or an unknown method destined for a PSAP is initiated in the same country.