13 Interworking with IMS

29.5613GPP5G SystemInterworking between 5G Network and external Data NetworksRelease 17Stage 3TS

13.1 General

Interworking with the IP Multimedia Core Network Subsystem (IMS) puts specific requirements on the SMF.

The SMF shall use the following mechanisms to support the interworking with the IMS:

– the P-CSCF discovery;

– N7 interface for the policy and charging control of QoS flows for IMS media flows; and

– the P-CSCF restoration.

These mechanisms are however not restricted only to the interworking with the IMS and may be used for other services that could benefit from these mechanisms.

If the PDU Session is used for IMS (identified by DNN), the SMF shall not modify the fields Type of Service (IPv4) and Traffic Class (IPv6).

NOTE: The P-CSCF can support paging policy differentiation for different traffic or service types over NG-RAN by marking the fields Type of Service (IPv4) and Traffic Class (IPv6) (see clause L.3.2.4 of 3GPP TS 24.229 [13]).

13.2 IMS interworking Model

13.2.1 Introduction

The signalling interface between the UE and the P-CSCF is a logical interface, i.e. it uses 5GC as a QoS flow. The Npcf_SMPolicyControl services, offered via N7 interface, are used for network communication between the SMF and the PCF. For a description of the IMS architecture, refer to 3GPP TS 23.228 [12].

13.2.2 IMS specific configuration in the SMF

The SMF shall have a list of preconfigured addresses of signalling servers (the P-CSCF servers). This list shall be provided to the UE at PDU session establishment. It shall be possible to preconfigure the list of preconfigured addresses of signalling servers per DNN.

The SMF/UPF may have the locally preconfigured packet filters, and/or the applicable PCC rules, to be applied on the QoS flow. The packet filters shall filter up-link and down-link packets, and shall only allow traffic to/from the signalling servers and to the DNS and the DHCP servers. It shall be possible to locally preconfigure the packet filters per DNN.

It shall be possible to enable/disable the use of the services offered via N7 interface per DNN.

The SMF shall support IPv4 and/or IPv6 addresses and protocol for IMS signalling and IMS QoS flows.

The methods for the UE to discover the P-CSCF address(es) may vary depending on the access technology that the UE is on. The details of the P-CSCF discovery mechanisms for various accesses are specified in 3GPP TS 23.228 [12] and 3GPP TS 24.229 [13]. The P-CSCF discovery mechanisms are:

– a 5GC procedure for the P-CSCF discovery;

– via DHCP servers i.e. the SMF shall provide the functionality of a DHCP relay agent; and

– if the UE has a P-CSCF FQDN locally configured and request the DNS IP address(es) from the SMF (via 5GC mechanism or DHCP procedures), the SMF shall be able to provide DNS IP address(es) to the UE.

The SMF shall have similar functional support depending on the P-CSCF discovery methods supported by the UE on the access technology. For example, for the UE in 3GPP 5G access network the SMF shall have DHCP server function towards the UE while acting as a DHCP client towards external DHCP server, if the SMF is configured to request DNS and/or P-CSCF IP addresses from the external DHCP servers.

The SMF shall be able to deliver DNS and/or P-CSCF addresses to the UE if requested by the UE via the 5G network or via DHCP procedures using the relevant DHCP options for IPv4/IPv6 DNS and SIP servers (see IETF RFC 2132 [14], IETF RFC 3361 [15], IETF RFC 3646 [16] and IETF RFC 3319 [17]).

On DNNs providing IMS services, the information advertised in Router Advertisements from the SMF to the UEs shall be configured in the same manner as for other DNNs providing IPv6 services except that the "O‑flag" shall be set.

The "O-flag" shall be set in IPv6 Router Advertisement messages sent by the SMF for DNNs used for IMS services. This will trigger a DHCP capable UE to start a DHCPv6 session to retrieve server addresses and other configuration parameters. The UE which doesn’t support DHCP shall ignore the "O-flag" and shall request the IMS specific configuration (e.g. the P-CSCF address) via other discovery methods supported in the UE (i.e. via locally configured P-CSCF address/FQDN in the UE or via 5G procedure, if applicable).

The SMF shall have configurable policy rules for controlling QoS flows used for signalling.

13.2.3 IMS specific procedures in the SMF

13.2.3.1 Provisioning of Signalling Server Address

At a PDU Session establishment procedure related to the IMS, the SMF shall support the capability to send the P-CSCF address(es) to the UE. The P-CSCF address information is sent by the visited SMF if LBO is used. For Home routed, the P-CSCF address information is sent by the SMF in the HPLMN. The P-CSCF address(es) shall be sent transparently through the AMF, and in case of Home Routed also through the SMF in the VPLMN.

NOTE 1: The SMF is located in the VPLMN if LBO is used.

NOTE 2: Other options to provide the P-CSCF address(es) to the UE as defined in 3GPP TS 23.228 [12] is not excluded.

NOTE 3: A PDU session for IMS is identified by "APN" or "DNN".

13.2.3.2 Failure of Signalling Server Address

If the SMF detects a failure:

– upon receiving the N4 session report from the UPF for the monitored P-CSCF address being used by the UE (as specified in 3GPP TS 23.380 [38], clause 5.8.3); or

– upon receiving a P-CSCF restoration indication from the UDM or the PCF,

then the SMF shall act as specified in 3GPP TS 23.380 [38], clause 5.8.