6.3.6.2 Stand-alone N3IWF selection

The UE performs N3IWF selection based on the ePDG selection procedure as specified in clause 4.5.4 of TS 23.402 [43] except for the following differences:

• The Tracking/Location Area Identifier FQDN shall be constructed by the UE based only on the Tracking Area wherein the UE is located. The N3IWF Tracking/Location Area Identifier FQDN may use the 5GS TAI when the UE is registered to the 5GS, or the EPS TAI when the UE is registered to EPS. The Location Area is not applicable on the 3GPP access.
• The ePDG Operator Identifier (OI) FQDN format is substituted by with N3IWF OI FQDN format as specified in TS 23.003 [19].
• If the UE is configured with Slice-specific N3IWF prefix configuration, then the UE shall construct the Prefixed N3IWF OI FQDN or the Prefixed N3IWF TA FQDN as specified in clauses 28.3.2.2.8 and 28.3.2.2.9 of TS 23.003 [19] instead of the N3IWF OI FQDN and the N3IWF TA FQDN, respectively. To determine the prefix, the UE selects the Slice-specific N3IWF prefix configuration for the selected PLMN that contains S-NSSAIs that match all (or most, in case there is no full match) of the S-NSSAIs that the UE is going to include in the Requested NSSAI in the subsequent Registration procedure.
• The ePDG identifier configuration is substituted by the N3IWF identifier configuration and the Extended Home N3IWF identifier configuration. The Extended Home N3IWF identifier configuration takes precedence over the N3IWF identifier configuration. If the UE is located in the home country and the UE is configured with Extended Home N3IWF identifier configuration, then the UE uses the Extended Home N3IWF identifier configuration to select an N3IWF:
• The UE uses the FQDN or IP address from the Extended Home N3IWF identifier configuration that matches all (or most, if there is no full match) of the S-NSSAIs that the UE is going to request in the subsequent Registration.
• The ePDG selection information is substituted by the Non-3GPP access node selection information and slice-specific N3IWF prefix information. The UE shall give preference to the N3IWF in all PLMNs in the Non-3GPP access node selection information independent of the "Preference" parameter.
• If the UE determines to be located in a country other than its home country (called the visited country), then instead of clause 4.5.4.4, bullet 3 of TS 23.402 [43], the following applies:
• If the UE is registered via 3GPP access to a PLMN and this PLMN is included in the Non-3GPP access node selection information, then the UE shall select an N3IWF in this PLMN. If the UE fails to connect to an N3IWF in this PLMN, the UE shall select an N3IWF by performing the DNS procedure specified in clause 4.5.4.5 of TS 23.402 [43].
• In all other cases, (e.g. when the UE is not configured with the Non-3GPP access node selection information, or the UE is registered via 3GPP access to a PLMN but this PLMN is not included in the Non-3GPP access node selection information, or the UE is not registered via 3GPP access to any PLMN), the UE shall select an N3IWF by performing the DNS procedure specified in clause 4.5.4.5 of TS 23.402 [43] with the difference that the UE shall construct the Prefixed N3IWF OI FQDN if the UE is configured with Slice-specific N3IWF prefix configuration for the selected PLMN.

If the UE is accessing PLMN services via SNPN, the UE uses the procedure defined in this clause to select an N3IWF deployed in the PLMN. If the UE is accessing standalone non-public network service via a PLMN (see supported cases in clause 5.30.2.0), the UE uses the procedure defined in clause 6.3.6.2a.