6.3.6.3.3 Combined N3IWF/ePDG Selection

If the UE is attached to 5GC via an N3IWF that is located in the same country as the country in which the UE is currently located and the AMF has previously indicated support for emergency services over non-3GPP access as defined in clause 5.16.4.1, the UE reuses the existing N3IWF connection for emergency services. Otherwise, the UE terminates any existing N3IWF connection and performs PLMN and N3IWF or ePDG selection for emergency services.

If the UE is attached to EPC via an ePDG that has indicated support for the emergency services and is located in the same country as the country in which the UE is currently located, the UE reuses the existing ePDG connection for emergency services. Otherwise, the UE terminates the existing ePDG connection, if any and performs PLMN and N3IWF or ePDG selection for emergency services.

PLMN and N3IWF or ePDG selection for emergency services is performed as follows:

If the UE is equipped with a UICC:

• The UE determines whether it is located in the home country or a visited country;
• If the UE is located in the home country the UE selects the Home PLMN for emergency services and selects an N3IWF or ePDG as follows:
• If the Non-3GPP Access Node Selection Information for the HPLMN is available the UE selects first an N3IWF or ePDG based on the Non-3GPP Access Node type preference in the Non-3GPP Access Node Selection Information for the HPLMN. To select an N3IWF the UE uses the N3IWF identifier configuration (if available). If the N3IWF identifier configuration is not available, the UE constructs the FQDN format as indicated by the FQDN format in the Non-3GPP Access Node Selection Information for the HPLMN. To select an ePDG the UE selects the ePDG identified by the configured Emergency ePDG FQDN (if available). If the configured Emergency ePDG FQDN is not available, the UE constructs either the Tracking/Location Area Identity based Emergency ePDG FQDN or the Operator Identifier based Emergency ePDG FQDN as indicated by the FQDN format in the Non-3GPP Access Node Selection Information for the HPLMN.
• If the Non-3GPP Access Node Selection Information is not available, the UE shall first attempt to select an N3IWF following the procedure defined in clause 6.3.6.2 before attempting to select an ePDG. To select an ePDG the UE selects the ePDG identified by the configured Emergency ePDG FQDN (if available). If the configured Emergency ePDG FQDN is not available, the UE constructs the Operator Identifier based Emergency ePDG FQDN.
• If the UE is located in a visited country, the UE performs a DNS query using the Visited Country Emergency FQDN for N3IWF and using the Visited Country Emergency FQDN for ePDG, as specified in TS 23.003 [19] to determine which PLMNs in the visited country support emergency services in non-3GPP access.
• If the DNS responses contain one or more records, the UE selects a PLMN that supports emergency services in non-3GPP access for the UE. Each record in the DNS responses shall contain the identity of a PLMN in the visited country supporting emergency services in non-3GPP access via ePDG or N3IWF.
• The UE shall consider these PLMNs based on their priorities in the Non-3GPP Access Node Selection Information. If the UE cannot select a PLMN in the Non-3GPP Access Node Selection Information or if non-3GPP Access Node Selection Information is not available, the UE shall attempt to select any PLMN in the list of PLMNs returned in the DNS response.
• Once the UE has selected a PLMN the UE shall select an N3IWF or ePDG for the selected PLMN as follows:
• If the Non-3GPP Access Node Selection Information for the PLMN is available the UE selects first an N3IWF or ePDG based on the Non-3GPP Access Node type preference in the Non-3GPP Access Node Selection Information for the PLMN. To select an N3IWF the UE constructs the FQDN format as indicated by the FQDN format in the Non-3GPP Access Node Selection Information for the PLMN. To select an ePDG the UE constructs either the Tracking/Location Area Identity based Emergency ePDG FQDN or the Operator Identifier based Emergency ePDG FQDN as indicated by the FQDN format in the Non-3GPP Access Node Selection Information for the PLMN.
• If the Non-3GPP Access Node Selection Information is not available, the UE shall first attempt to select an N3IWF following the procedure defined in clause 6.3.6.2 before attempting to select an ePDG. To select an ePDG the UE constructs the Operator Identifier based Emergency ePDG FQDN.
• If the DNS response does not contain any record, or if the DNS response contains one or more records but the UE fails to select a PLMN that supports emergency services in non-3GPP access, or if the Emergency Registration procedure has failed for all PLMNs supporting emergency services in non-3GPP access, the UE notifies the user that emergency session cannot be established.

If the UE is not equipped with a UICC, the UE shall perform the emergency ePDG/N3IWF selection procedure above as if always in a visited country and without using the Non-3GPP Access Node Selection Information, i.e. the UE may construct the Operator Identifier FQDN for N3IWF or ePDG based on a PLMN ID obtained via implementation specific means.

When a N3IWF has been selected, the UE initiates an Emergency Registration. If the Emergency Registration fails, the UE shall attempt to select an ePDG before selecting another PLMN supporting emergency services in non-3GPP access. When an ePDG has been selected, the UE initiates an Emergency Registration. If the Emergency Registration fails, the UE shall attempt to select a N3IWF before selecting another PLMN supporting emergency services in non-3GPP access.