6.3.6.1 General

When the UE supports connectivity with N3IWF but does not support connectivity with ePDG, as specified in TS 23.402 [43], the UE shall perform the procedure in clause 6.3.6.2 for selecting an N3IWF.

When the UE supports connectivity with N3IWF, as well as with ePDG, as specified in TS 23.402 [43], the UE shall perform the procedure in clause 6.3.6.3 for selecting either an N3IWF or an ePDG, i.e. for selecting a non-3GPP access node.

In both cases above the UE can be configured by the HPLMN with the same information that includes:

• ePDG identifier configuration: It contains the FQDN or IP address of the ePDG in the HPLMN, as specified in clause 4.5.4.3 of TS 23.402 [43]. This is used only when the UE supports connectivity with ePDG and attempts to select an ePDG. It is ignored in all other cases.
• N3IWF identifier configuration: It contains the FQDN or IP address of the N3IWF in the HPLMN.
• Extended Home N3IWF identifier configuration: It contains one or multiple tuples of FQDN/IP address of the N3IWF in the HPLMN and the S-NSSAIs supported by this N3IWF.
• Non-3GPP access node selection information: It contains a prioritized list of PLMNs and for each PLMN it includes (i) a "Preference" parameter which indicates if ePDG or N3IWF is preferred in this PLMN and (ii) an FQDN parameter which indicates if the Tracking/Location Area Identity FQDN or the Operator Identifier FQDN (as specified in clause 4.5.4.4 of TS 23.402 [43]) should be used when discovering the address of an ePDG or N3IWF in this PLMN. The list of PLMNs shall include the HPLMN and shall include an "any PLMN" entry, which matches any PLMN the UE is connected to except the HPLMN.
• Slice-specific N3IWF prefix configuration: It contains one or multiple tuples consisting of:
• List of supported S-NSSAIs;
• Prefix for the Prefixed N3IWF OI or TA FQDNs.

NOTE 1: Extended Home N3IWF identifier configuration and Slice-specific N3IWF prefix configuration are assumed to be provided to the UE as part of ANDSP.

The ePDG identifier configuration, the N3IWF identifier configuration, the Extended Home N3IWF identifier configuration and the Slice-specific N3IWF Prefix Configuration are optional parameters, while the Non-3GPP access node selection information is required and shall include at least the HPLMN and the "any PLMN" entry.

If the ePDG identifier configuration is configured in the UE, then, when the UE decides to select an ePDG in the HPLMN (according to the procedure in clause 6.3.6.3), the UE shall use the ePDG identifier configuration to find the IP address of the ePDG in the HPLMN and shall ignore the FQDN parameter of the HPLMN in the Non-3GPP access node selection information.

If the N3IWF identifier configuration or the Extended Home N3IWF identifier configuration is configured in the UE, then, when the UE decides to select an N3IWF in the HPLMN (according to the procedure in clause 6.3.6.3 for combined N3IWF/ePDG selection and the procedure in clause 6.3.6.2 for Stand-alone N3IWF selection), the UE shall use the Extended Home N3IWF identifier configuration, if available and otherwise the N3IWF identifier configuration to find the IP address of the N3IWF in the HPLMN and shall ignore the FQDN parameter of the HPLMN in the Non-3GPP access node selection information.

The HPLMN's PCF takes the UE's subscribed S-NSSAIs into account when providing Extended Home N3IWF identifier configuration and/or Slice-specific N3IWF Prefix Configuration to the UE.

If a UE does not support the Extended Home N3IWF identifier configuration and the Slice-specific N3IWF Prefix Configuration, then the HPLMN provides to the UE the Non-3GPP access node selection information and the N3IWF identifier configuration by taking into account the UE's subscribed S-NSSAIs.

NOTE 2: If the HPLMN deploys multiple N3IWFs with different TAs which support different S-NSSAIs, then the HPLMN can configure a UE with N3IWF identifier configuration so that the UE selects an N3IWF that supports the UE's subscribed S-NSSAIs.

The UE can be configured by the VPLMN with the following information applicable for the V-PLMN:

Slice-specific N3IWF prefix configuration: It contains one or multiple tuples consisting of:

• List of supported S-NSSAIs;
• Prefix for the Prefixed N3IWF OI or TA FQDNs.

To enable the V-PCF to provide the UE with Slice-specific N3IWF prefix configuration, the AMF provides the V-PCF with the Configured NSSAI for the serving PLMN during the UE Policy Association Establishment/Modification procedure.

NOTE 3: In non-roaming cases, the UE PCF already receives the subscribed NSSAI from the UDR. Therefore, there is no need for the AMF to provide the Configured NSSAI to the PCF in the non-roaming case.

NOTE 4: PCF (V-PCF in the roaming case) is assumed to be locally configured with information about the slices supported by the different N3IWFs in the serving PLMN.

During the registration procedure the AMF may determine if the N3IWF selected by the UE is suitable for the S-NSSAI(s) requested by the UE considering the UE subscription. If the AMF determines that a different N3IWF should be selected (e.g. based on energy related local configuration for the N3IWF) as described in clause 4.12.2.2 of TS 23.502 [3], the AMF:

• may, if the UE supports slice-based N3IWF selection, triggers the UE Policy Association Establishment or UE Policy Association Update procedure to provide the UE with updated N3IWF selection information; when the AMF is informed by the PCF that the update of UE policy information on the UE is completed as described in clause 4.12.2.2.2 of TS 23.502 [3], the AMF releases UE Policy Association if the UE is not registered over 3GPP access before proceeding to the Registration Reject over untrusted non-3GPP access;
• shall send a Registration Reject message to the UE. The AMF may include target N3IWF information (FQDN and/or IP address) in the Registration Reject so that the UE can, if supported by the UE, use the target N3IWF information to select the N3IWF to register to 5GC if the UE wishes to send the same Requested NSSAI as during the previous Registration Request. The target N3IWF information only applies to the one N3IWF selection performed by the UE just after receiving the Registration Reject.

The AMF may determine the N3IWF based on the list of supported TAs and the corresponding list of supported slices for each TA obtained as defined in clause 5.15.8.

NOTE 5: The operator is assumed to ensure that UEs that do not support slice-based N3IWF selection always select an N3IWF that supports at least one slice requested by the UE. This is to avoid unnecessary and potentially repetitive rejections of those UEs. To ensure this, the operator is assumed to provide identifiers of N3IWFs that only support a subset of the slices configured in the network only to UEs that support slice-based N3IWF selection.