6.3.6 N3IWF selection

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

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:

1) 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.

2) N3IWF identifier configuration: It contains the FQDN or IP address of the N3IWF in the HPLMN.

3) 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.

4) 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.

5) 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, the AMF:

– may, if the UE supports slice-based N3IWF selection, trigger the UE Policy Association Establishment or UE Policy Association Update procedure to provide the UE with updated N3IWF selection information as described in clause 6.15.2.1; when the AMF is informed that the update of N3IWF selection information is completed, the AMF may release UE Policy Association if it is not needed before proceeding to the Registration Reject;

– 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. 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.

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 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.

Editor’s note: The Prefixed N3IWF OI FQDN is assumed to take the form:

<Prefix>.tac-lb<TAC-low-byte>.tac-hb<TAC-high-byte>.tac.n3iwf.5gc.mnc<MNC>.mcc<MCC>.pub.3gppnetwork.org.

The Prefixed N3IWF TA FQDN is assumed to take the form <Prefix>.n3iwf.5gc.mnc<MNC>.mcc<MCC>.pub.3gppnetwork.org.

Once these FQDNs have been added to TS 23.003, this Editor’s note will be removed.

– 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:

a) 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].

b) 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.

6.3.6.2a SNPN N3IWF selection

This procedure applies when the UE is accessing the SNPN N3IWF via a PLMN or directly via untrusted non-3GPP access.

The UE shall first determine the country in which it is located. If the UE cannot determine the country in which the UE is located, the UE shall stop N3IWF selection and abort the attempt to access the SNPN via PLMN.

NOTE 1: It is up to UE implementation how to determine the country in which the UE is located.

If the UE determines that it is located in the country where the configured N3IWF is located, then the UE uses the configured N3IWF FQDN to select an N3IWF deployed in the SNPN.

If the UE determines that it is located in a country different from the country where the configured N3IWF is located (called the visited country), then:

– The UE shall construct an FQDN consisting of the SNPN ID of the SNPN subscription and the Visited Country FQDN and indicating the query is for SNPN, as specified in TS 23.003 [19] and perform a DNS query for the resulting FQDN.

– If the DNS response contains no records, then the UE determines that the visited country does not mandate the selection of an N3IWF in this country for the SNPN identified by the SNPN ID provided by the UE. In this case the UE uses the configured N3IWF FQDN to select an N3IWF deployed in the SNPN.

– If no DNS response is received, the UE shall stop the N3IWF selection.

NOTE 2: The DNS can be configured to return no records for the visited country regardless of the SNPN ID provided by the UE. This addresses the scenario that the visited country in general does not mandate selection of a local N3IWF.

– If the DNS response contains one or more records, then the UE determines that the visited country mandates the selection of an N3IWF in this country. Each record in the DNS response shall contain the identity of an N3IWF of the SNPN in the visited country which may be used for N3IWF selection. In this case:

– The UE shall select an N3IWF included in the DNS response based on its own implementation means.

– If the UE cannot select any N3IWF included in the DNS response, then the UE shall stop the N3IWF selection.

NOTE 3: Visited countries which mandate the selection of an N3IWF in the country are assumed to configure the DNS as follows:

(i) For SNPNs that do not have any dedicated N3IWFs in the country and which are not exempt from the requirement to select an N3IWF in the visited country, the DNS response contains a record that cannot be resolved to an IP address;

(ii) for SNPNs that have dedicated N3IWFs in the country, the DNS response contains the identities of the SNPN’s N3IWFs in the visited country;

(iii) for SNPNs that are exempt from the requirement to select an N3IWF in the visited country, the DNS response contains no records.

NOTE 4: Self-assigned NIDs are not supported, since a DNS cannot be properly configured for multiple SNPNs using the same self-assigned NID (i.e. in collision scenarios). If the visited country mandates the selection of an N3IWF in the same country, the NAPTR record(s) associated to the Visited Country FQDN of SNPNs that use a self-assigned NID can be provisioned with the replacement field containing an FQDN that cannot be resolved to an IP address.

NOTE 5: The identity of an SNPN’s N3IWF in the visited country can be any FQDN, i.e. is not required to include the SNPN ID.

NOTE 6: It is assumed that the AMF, SMF, UPF are located in the same country as the N3IWF.

6.3.6.3 Combined N3IWF/ePDG Selection

When the UE wants to select a non-3GPP access node (either an N3IWF or an ePDG), the UE shall perform the following procedure:

1) The UE shall attempt to determine the country it is located in. This is determined by implementation-specific methods not defined in this specification. If the UE cannot determine the country it is located in, the UE shall stop the non-3GPP access node selection.

2) If the UE determines to be located in its home country, then:

a) The UE shall select the HPLMN. If the UE fails to connect to an ePDG/N3IWF in the HPLMN, then the UE shall stop the non-3GPP access node selection.

3) If the UE determines to be located in a country other than its home country (called the visited country), then:

a) 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 this PLMN. If the UE fails to connect to an ePDG/N3IWF in this PLMN, the UE shall select another PLMN by performing the DNS procedure specified in bullet 3c) below.

c) The UE shall select a PLMN as follows:

i) The UE shall determine if the non-3GPP access node selection is required for an IMS service or for a non-IMS service. The means of that determination are implementation specific.

ii) If the UE determines that the non-3GPP access node selection is required for a non-IMS service, the UE shall select a PLMN as specified in clause 6.3.6.2. As defined below, if the UE fails to connect to an N3IWF in any PLMN, the UE may attempt to select an ePDG according to the procedure specified in clause 4.5.4.5 of TS 23.402 [43].

iii) If the UE determines that the non-3GPP access node selection is required for an IMS service, the UE shall select a PLMN as follows:

– First, the UE shall perform a DNS query using the Visited Country FQDN for N3IWF, as specified in TS 23.003 [19] to determine if the visited country mandates the selection of N3IWF in this country. The DNS response received by the UE may be empty or may contain the identities of one or more PLMNs in the visited country, which may be used for N3IWF selection, if the UE decides to select an N3IWF, as specified below. For example, the DNS response may contain the identity of PLMN-1 and the identity of PLMN-2.

– Then, the UE shall perform a DNS query using the Visited Country FQDN for ePDG, as specified in TS 23.003 [19] to determine if the visited country mandates the selection of ePDG in this country. The DNS response received by the UE may be empty or may contain the identities of one or more PLMNs in the visited country, which may be used for ePDG selection, if the UE decides to select an ePDG, as specified below. For example, the DNS response may contain the identity of PLMN-1 and the identity of PLMN-3.

– If the UE does not receive a DNS response in none of the above two DNS queries, then the UE shall stop the non-3GPP access node selection. Otherwise, the next steps are executed.

– The UE shall consolidate the PLMN identities received in the above two DNS responses and shall construct a candidate list of PLMNs. For example, the candidate list of PLMNs may contain the identities of PLMN-1, PLMN-2, PLMN-3.

– If the candidate list of PLMNs is empty, then:

– If the Non-3GPP access node selection information contains one or more PLMNs in the visited country, the UE shall select one of these PLMNs based on their priorities in the Non-3GPP access node selection information. If the UE fails to connect to a non-3GPP access node in any of these PLMNs, the UE shall select the HPLMN.

– Otherwise, the UE shall select the HPLMN.

– If the candidate list of PLMNs is not empty, then:

– If the UE is registered via 3GPP access to a PLMN which is included in the candidate list of PLMNs, then the UE shall select this PLMN. If the UE fails to connect to a non-3GPP access node in this PLMN, then the UE shall select a different PLMN included in the candidate list of PLMNs as specified in the next bullet.

– If the UE is registered via 3GPP access to a PLMN which is not included in the candidate list of PLMNs, or the UE is not registered via 3GPP access to any PLMN, or the UE fails to connect to a non-3GPP access node according to the previous bullet, then the UE shall select one of the PLMNs included in the candidate list of PLMNs based on the prioritized list of PLMNs in the Non-3GPP access node selection information (i.e. the UE shall select first the highest priority PLMN in the Non-3GPP access node selection information that is contained in the candidate list of PLMNs). If the Non-3GPP access node selection information does not contain any of the PLMNs in the candidate list of PLMNs, or the UE is not configured with the Non-3GPP access node selection information, or the UE was not able to connect to a non-3GPP access node in any of the PLMNs included in the Non-3GPP access node selection information and in the candidate list of PLMN, then the UE shall select a PLMN included in the candidate list of PLMNs based on its own implementation means.

– If the UE cannot select a non-3GPP access node in any of the PLMNs included in the candidate list of PLMNs, then the UE shall stop the non-3GPP access node selection.

In the selected PLMN the UE shall attempt to select a non-3GPP access node as follows:

1. The UE shall determine if the non-3GPP access node selection is required for an IMS service or for a non-IMS service. The means of that determination are implementation-specific.

2. When the selection is required for an IMS service, the UE shall choose a non-3GPP access node type (i.e. ePDG or N3IWF) based on the "Preference" parameter specified in clause 6.3.6.1, unless the UE has its 5GS capability disabled in which case it shall choose an ePDG independent of the "Preference" parameter setting.

If the "Preference" parameter for the selected PLMN indicates that ePDG is preferred, the UE shall attempt to select an ePDG. If the "Preference" parameter for the selected PLMN indicates that N3IWF is preferred, the UE shall attempt to select an N3IWF.

If the selection fails, including the case when, during the registration performed over either 3GPP or non-3GPP access, the UE receives the IMS Voice over PS session Not Supported over Non-3GPP Access indication (specified in clause 5.16.3.2a), the UE shall attempt selecting the other non-3GPP access node type in the selected PLMN, if any. If that selection fails too, or it is not possible, then the UE shall select another PLMN, according to the procedure specified bullet 3c) above.

3. When the selection is required for a non-IMS service, the UE shall perform the selection by giving preference to the N3IWF independent of the "Preference" parameter setting. If the N3IWF selection fails, or it is not possible, the UE should select another PLMN based on the procedure specified in clause 4.5.4.4 of TS 23.402 [43], and shall attempt to select an N3IWF in this PLMN. If the UE fails to select an N3IWF in any PLMN, the UE may attempt to select an ePDG according to the procedure specified in clause 4.5.4.5 of TS 23.402 [43].

In the above procedure, when the UE attempts to construct a Tracking/Location Area Identifier FQDN either for ePDG selection or for N3IWF selection, the UE shall use the Tracking Area wherein the UE is located and shall construct either:

– an ePDG or N3IWF TAI FQDN based on the 5GS TAI, when the UE is registered to the 5GS; or

– an ePDG or N3IWF TAI FQDN based on the EPS TAI, when the UE is registered to EPS.

NOTE: A UE performing both a selection for an IMS service and a selection for a non-IMS service could get simultaneously attached to a N3IWF and to an ePDG in the same PLMN or in different PLMNs.

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 TS 23.003 [19] instead of the N3IWF OI FQDN and the N3IWF TA FQDN, respectively. Further details on constructing the Prefixed N3IWF OI and TA FQDN are described in clause 6.3.6.2.

6.3.6.4 PLMN and non-3GPP access node Selection for emergency services

6.3.6.4.1 General

UE initiates PLMN and non-3GPP access node selection for emergency services when it detects a user request for emergency session and determines that untrusted non-3GPP access shall be used for the emergency access.

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 6.3.6.4.2 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 6.3.6.4.3 for selecting either an N3IWF or an ePDG, i.e. for selecting a non-3GPP access node.

6.3.6.4.2 Stand-alone N3IWF 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 continues 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, then the UE selects the Home PLMN for emergency services and selects an N3IWF based on the procedure defined in clause 6.3.6.2.

– If the UE is located in a visited country, the UE performs a DNS query using the Visited Country Emergency N3IWF FQDN, as specified in TS 23.003 [19] to determine which PLMNs in the visited country support emergency services in non-3GPP access via N3IWF; and:

– If the DNS response contains one or more records, the UE selects a PLMN that supports emergency services in non-3GPP access via N3IWF. Each record in the DNS response shall contain the identity of a PLMN in the visited country supporting emergency services in non-3GPP access via N3IWF.

– The UE shall consider these PLMNs based on their priorities in the Non-3GPP Access Node Selection Information (if available). 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 for the selected PLMN as follows:

– If non-3GPP Access Node Selection Information is available for the selected PLMN the UE constructs the Tracking Area Identity based N3IWF FQDN or the Operator Identifier based N3IWF FQDN as indicated in the non-3GPP Access Node Selection Information for the selected PLMN.

– If non-3GPP Access Node Selection Information is not available for the selected PLMN the UE constructs the Operator Identifier based N3IWF FQDN for the selected PLMN.

– 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 an emergency session cannot be established.

If the UE is not equipped with a UICC, the UE shall perform the emergency 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 based N3IWF FQDN format based on a PLMN ID obtained via implementation specific means.

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

6.3.6.3.3 Combined N3IWF/ePDG Selection

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 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.