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Content for  TS 23.402  Word version:  16.0.0

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4.5  High Level FunctionsWord‑p. 35

4.5.1  PDN GW Selection Function for Non-3GPP Accesses for S2a and S2b

PDN Gateway selection for non-3GPP accesses uses similar mechanisms as defined in TS 23.401, with the following modification:
  • The PDN Gateway selection function interacts with the 3GPP AAA Server or 3GPP AAA Proxy and uses subscriber information provided by the HSS to the 3GPP AAA Server. The HSS shall include the UE Usage Type in the UE's subscription information if any and, if included, the ePDG/TWAN shall select the PDN GW as described in TS 23.401, clause 4.3.25.1. To support separate PDN GW addresses at a PDN GW for different mobility protocols (PMIP, MIPv4 or GTP), the PDN GW Selection function takes mobility protocol type into account when deriving PDN GW address by using the Domain Name Service function.
During the initial authorization, PDN Gateway selection information for each of the subscribed PDNs is returned to the ePDG or the Trusted Non-3GPP Access Network. The PDN Gateway selection information includes:
  • The PDN GW identity, which is a logical name (FQDN) or IP address and an APN; or
  • an APN and an indication whether the allocation of a PDN GW from the visited PLMN is allowed or a PDN GW from the home PLMN shall be allocated.
This enables the entity requiring the IP address of the PDN Gateway to proceed with selection as per the procedures defined in TS 23.401, clauses 4.3.8.1 and 4.3.25.1. Once the selection has occurred, the PDN Gateway registers its association with a UE and the APN with the AAA/HSS by sending PDN GW identity, that is either its IP address (e.g. if it has a single IP address for all the mobility protocols it supports or if it only supports one mobility protocol) or its FQDN (e.g. if it has multiple IP addresses for the mobility protocols it supports), as well as information that identifies the PLMN in which the PDN GW is located, to the 3GPP AAA Server or AAA Proxy only when the Access Technology Type is non-3GPP. For 3GPP access types, the MME/S4-SGSN updates the HSS with the selected PDN GW identity, as well as information that identifies the PLMN in which the PDN GW is located, according to TS 23.401/TS 23.060. This permits the HSS and 3GPP AAA Server or Proxy to provide the association of the PDN Gateway identity and the related APN for the UE subsequently.
In the case that a UE already has assigned PDN Gateway(s), the PDN GW identity for each of the already allocated PDN Gateway(s), as well as information that identifies the PLMN in which the PDN GW is located, are returned by the 3GPP AAA Server or Proxy during the authorization step. This eliminates the need to repeat PDN Gateway selection for the PDNs the UE is already connected with. The information about the PLMN in which the PGW is located allows the receiving entity to determine an appropriate APN-OI. The ePDG may use this information to determine the S2b protocol type (PMIP or GTP). The TWAN may also use this information to determine the S2a protocol type (PMIP or GTP).
Upon mobility between 3GPP and non-3GPP accesses, PDN Gateway selection information for the subscribed PDNs the UE is not yet connected with is returned to the target access system as done during initial attachment. For the PDNs the UE is already connected with transfer of PDN GW information takes place as defined below:
  • If a UE attaches to a non-3GPP access and it already has assigned PDN Gateway(s) due to a previous attach in a 3GPP access, the HSS provides the PDN GW identity, as well as information that identifies the PLMN in which the PDN GW is located, for each of the already allocated PDN Gateway(s) with the corresponding PDN information to the 3GPP AAA server over the SWx reference point.
  • If a UE attaches to a 3GPP access and it already has an assigned PDN Gateway(s) due to a previous attach in a non-3GPP access, the HSS provides the PDN GW identity, as well as information that identifies the PLMN in which the PDN GW is located, for each of the already allocated PDN Gateway(s) with the corresponding PDN information to the MME over the S6a reference point and/or S4-SGSN over the S6d reference point.
The HSS receives the PDN GW identity for each of the selected PDN GWs and the corresponding PDN information for a given UE, from both the 3GPP AAA Server and also from the MME/S4-SGSN, depending on the currently in-use access. The HSS is responsible for the storage of the selected PDN GW identity as described in clause 12.
The ePDG may be configured with the S2b protocol variant(s) on a per HPLMN granularity, or may retrieve information regarding the S2b protocol variants supported by the PDN GW (PMIP or/and GTP) from the Domain Name Service function.
The TWAN may be configured with the S2a protocol variant(s) on a per HPLMN granularity, or may retrieve information regarding the S2a protocol variants supported by the PDN GW (PMIP or/and GTP) from the Domain Name Service function.
  • For PMIPv6 on S2a/b, the entity requesting the PDN Gateway is the entity acting as Mobile Access Gateway (MAG).
  • For GTP on S2b, the entity requesting the PDN Gateway is the ePDG.
  • For GTP on S2a, the TWAG, described in clause 16.1.2, is requesting the PDN Gateway.
  • For the PMIP-based S8-S2a/b chained cases, the PDN GW information is sent together with the selected Serving GW address from the 3GPP AAA proxy to the entity acting as MAG in the non-3GPP access network during access authentication and authorization. The PDN GW selection mechanism is the same as in the unchained case. The MAG function of the non-3GPP access network conveys the PDN GW address to the Serving GW as part of the PMIPv6 PBU message.
  • For MIPv4 FA mode on S2a, the entity requesting the PDN Gateway is the entity that plays the role of the FA.
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4.5.1a  PDN GW Selection Function for eHRPD with SIPTO support |R10|Word‑p. 36
In order to select the appropriate PDN GW for SIPTO in eHRPD access via HSGW, the PDN GW selection function needs to support DNS mechanism that allows selection of a PDN GW which is close to the HSGW for the UE. Details related to SIPTO support for eHRPD access is defined in 3GPP2 X.S0057 [51].

4.5.2  PDN GW Selection Function for S2c

For the S2c reference point, the UE needs to know the IP address of the PDN Gateway for the PDN the UE wants to connect to. This address is made known to the UE using one of the following methods:
  1. Via PCO at the attach procedure or UE requested PDN Connectivity procedure, for 3GPP access (as defined in TS 23.401) or trusted non-3GPP access (if supported).
  2. Via IKEv2 during tunnel setup to ePDG. For a UE's initial Attach, during the IKEv2 tunnel establishment procedure on the SWu interface (between UE and ePDG):
    • For non-roaming case, the 3GPP AAA Server selects the HA (PDN GW) which is close to the ePDG and sends the HA (PDN GW) FQDN or IP address to the ePDG;
    • For roaming with local breakout case, the 3GPP AAA Proxy selects the HA (PDN GW) which is close to the ePDG and sends the HA (PDN GW) FQDN or IP address to the ePDG;
      The HA (PDN GW) FQDN or IP address are then forwarded to the UE by the ePDG.
  3. If the IP address of the PDN GW is not received using options 1-2 above and if the UE knows that the HA is in the PDN where the UE is attached to then the UE shall request a PDN Gateway address via DHCP IETF RFC 6611 [41].
  4. If the IP address of the PDN GW is not delivered using options 1-3 above the UE can interact directly with the Domain Name Service function by composing a FQDN corresponding to the PDN.
For the S2c reference point, the network can force a reallocation of the PDN Gateway selected upon initial DSMIPv6 bootstrapping for the PDN the UE wants to connect to. This may happen if one of the following situations occurs:
  • The UE has done initial network attachment on an access system supporting network-based mobility, but the PDN Gateway discovered by the UE for the S2c reference point is different from the PDN Gateway allocated at initial network attachment. In this case, to enable IP address preservation based on DSMIPv6 upon inter-system mobility, the network must trigger a PDN Gateway reallocation for the S2c reference point, to re-direct the UE to the PDN Gateway that was selected upon initial network attachment.
  • The UE has done initial network attachment over S2c and, relying on DNS, has discovered a sub-optimal PDN Gateway. In this case, based on operator's policies, the network can optionally trigger a PDN Gateway reallocation to re-redirect the UE to a PDN Gateway that can provide better performance.
PDN Gateway reallocation for the S2c reference point is triggered by the AAA/HSS during DSMIPv6 bootstrapping. For a UE's initial Attach, if the UE has selected a initial PDN GW and initiated DSMIPv6 bootstrapping:
  • In non-roaming scenario, the PDN GW reports the UE Care of Address (allocated by the WLAN AN or ePDG) to the 3GPP AAA Server. According to the UE CoA and the pre-configuration, the 3GPP AAA Server finds there are other PDN GW(s) which are close to the UE, then it can initiate a PDN GW reallocation procedure (Clause 6.10 "PDN GW reallocation upon attach on S2c") to redirect the UE to the other PDN GW.
  • In roaming with local breakout scenario, the PDN GW reports the UE Care of Address (allocated by the WLAN AN or ePDG) to the 3GPP AAA Proxy. According to the UE CoA and the pre-configuration, the 3GPP AAA Proxy finds there are other PDN GW(s) which are close to the UE, then it can initiate a PDN GW reallocation procedure (clause 6.10 "PDN GW reallocation upon attach on S2c") to redirect the UE to the other PDN GW.
The HSS receives the values of identity(ies) of all allocated PDN GWs and the corresponding PDN information for a given UE from the 3GPP AAA. The HSS is responsible for the storage of PDN GW identity information.
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4.5.3  Serving GW Selection Function for Non-3GPP AccessesWord‑p. 37
The S GW selection function allocates an S GW that acts as a local anchor for non-3GPP access in the case of S8-S2a/b chained roaming. Whether S8-S2a/b chaining should be used is decided by 3GPP AAA Proxy based on per-HPLMN configuration.
The Serving GW selection function is located in 3GPP AAA Proxy. If an S GW is needed for non-3GPP access in the visited network, the 3GPP AAA proxy will select an S GW for the UE during initial attach or handover attach. The 3GPP AAA proxy shall send the selected S GW address to the MAG function of the Trusted non-3GPP IP access or ePDG in the chained S8-S2a/b scenarios.
There is no mechanism standardized for S GW address preservation for handover between 3GPP and non-3GPP in S2/S8 chained case within this Release of the specification.
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4.5.4  ePDG SelectionWord‑p. 38

4.5.4.1  General |R13|

The UE performs ePDG selection based on a set of information configured by the HPLMN in the UE, and based on the UE's knowledge of the PLMN it is attached to.
A UE connected to one or multiple PDN GWs uses a single ePDG.

4.5.4.2  ePDG FQDNs Construction |R13|

When the UE attempts to construct an FQDN for selecting an ePDG in a certain PLMN-x (either a VPLMN or the HPLMN), then the UE shall construct one of the following FQDN formats:
  • Operator Identifier FQDN: The UE constructs the FQDN by using the PLMN-x ID as the Operator Identifier.
  • Tracking/Location Area Identity FQDN: The UE constructs the FQDN by using the identity of the Tracking Area/Location Area it is located in (i.e. based on PLMN-x ID and TAC/LAC). The Tracking/Location Area Identity FQDN is used to support location-specific ePDG selection within a PLMN.
The ePDG FQDN formats are specified in TS 23.003.
The UE selects one of the above FQDN formats as follows:
  1. If the UE attempts to select an ePDG in the registered PLMN and the UE is configured to use for this PLMN the Tracking/Location Area Identity FQDN as defined in point 2) of clause 4.5.4.3; and
  2. the UE knows the TAI/LAI of the area the UE it is located in (e.g. the TAI/LAI from the most recent Attach or TAU/LAU),
then the UE constructs a Tracking/Location Area Identity FQDN. Otherwise the UE constructs the Operator Identifier FQDN.
Also, the UE constructs the Operator Identifier FQDN as a fallback in the case of failure of DNS resolution of a Tracking/Location Area Identity based FQDN.
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4.5.4.3  UE Configuration By HPLMN |R13|

The UE may be configured (e.g. via H-ANDSF, USIM, etc.) by the HPLMN with the following configuration, whose usage is defined in clause 4.5.4.4:
  1. ePDG identifier configuration: It contains the FQDN or IP address of an ePDG in the HPLMN.
  2. ePDG selection information: It contains a prioritized list of PLMNs which are preferred for ePDG selection. It also indicates if selection of an ePDG in a PLMN should be based on Tracking/Location Area Identity FQDN or on Operator Identifier FQDN, as specified in clause 4.5.4.4. The list of PLMNs may include the HPLMN.
The PLMNs included in the ePDG selection information are PLMNs that have roaming agreements with HPLMN for interworking with untrusted WLANs.
The ePDG selection information may include an "any PLMN" entry, which matches any PLMN the UE is attached to except the HPLMN. If the ePDG selection information contains both the "any PLMN" and the PLMN the UE is attached to, the UE shall give precedence to the latter.
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4.5.4.4  UE ePDG Selection Procedure |R13|

The UE shall perform ePDG selection by executing the steps below. Unless otherwise specified, when the UE attempts to select an ePDG, the UE shall construct an FQDN for this ePDG as specified in clause 4.5.4.2 and shall use the DNS server function to obtain the IP address(es) of this ePDG.:
  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 ePDG selection.
  2. If the UE determines to be located in its home country, then:
    1. The UE shall select an ePDG in the HPLMN. If the ePDG selection information contains the HPLMN, the UE shall construct an FQDN as specified in clause 4.5.4.2. If the ePDG selection information does not contain the HPLMN and the UE is configured with the ePDG identifier defined in bullet 1) of clause 4.5.4.3, then the UE shall either use the configured FQDN and use the DNS server function to obtain the IP address(es) of the ePDG(s) in the HPLMN, or the UE shall use the configured IP address. Otherwise, the UE shall construct an Operator Identifier FQDN and shall use the DNS server function to obtain the IP address(es) of the ePDG(s) in the HPLMN.
    2. If the UE cannot select an ePDG in the HPLMN, then the UE shall stop the ePDG selection.
  3. If the UE determines to be located in a country other than its home country (called the visited country), then:
    1. If the UE is registered via 3GPP access to a PLMN and this PLMN matches an entry in the ePDG selection information, then the UE shall select an ePDG in this PLMN. If the UE fails to connect to an ePDG in this PLMN, the UE shall select an ePDG by performing the DNS procedure specified in clause 4.5.4.5.
    2. In all other cases, (e.g. when the UE is not configured with the ePDG selection information, or the UE is registered via 3GPP access to a PLMN but this PLMN does not match an entry in the ePDG selection information, or the UE is not registered via 3GPP access to any PLMN), the UE shall select an ePDG by performing the DNS procedure specified in clause 4.5.4.5.
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4.5.4.5  ePDG Selection with DNS-based Discovery of Regulatory Requirements |R13|Word‑p. 39
The UE shall perform ePDG selection according to the following procedure when the UE determines to be located in a country other than its home country (called the visited country) and when the conditions defined in clause 4.5.4.4 apply.
The UE shall perform a DNS query using Visited Country FQDN, as specified in TS 23.003 to determine if the visited country mandates the selection of ePDG in this country as specified below.
  1. If the DNS response contains no records, then the UE determines that the visited country does not mandate the selection of ePDG in this country. In this case:
    1. If the ePDG selection information contains one or more PLMNs in the visited country, the UE shall select an ePDG in one of these PLMNs. The UE shall consider these PLMNs based on their priorities in the ePDG selection information. If the UE fails to connect to an ePDG in one or more of these PLMNs, the UE shall select an ePDG in the HPLMN according to bullet 1b below.
    2. Otherwise, including the case when the UE fails to connect to an ePDG according to bullet 1a above, the UE shall select an ePDG in the HPLMN. If the UE is configured with the ePDG identifier defined in bullet 1) of clause 4.5.4.3, then the UE shall either use the configured FQDN and use the DNS server function to obtain the IP address(es) of the ePDG(s) in the HPLMN, or the UE shall use the configured IP address. Otherwise, the UE shall construct an Operator Identifier FQDN as specified in clause 4.5.4.2 and shall use the DNS server function to obtain the IP address(es) of the ePDG(s) in the HPLMN.
  2. If the DNS response contains one or more records, then the UE determines that the visited country mandates the selection of ePDG in this country. Each record in the DNS response shall contain the identity of a PLMN in the visited country which may be used for ePDG selection. In this case:
    1. If the UE is registered via 3GPP access to a PLMN which is included in the DNS response, then the UE shall select an ePDG in this PLMN. If the UE fails to connect to an ePDG in this PLMN, then the UE shall select an ePDG in one of the other PLMNs included in the DNS response as specified in bullet 2b below.
    2. If the UE is registered via 3GPP access to a PLMN which is not included in the DNS response or the UE is not registered via 3GPP access to any PLMN or the UE fails to connect to an ePDG according to bullet 2a above, then the UE shall select an ePDG in one of the PLMNs included in the DNS response as follows:
      The UE shall select one of the PLMNs included in the DNS response based on the prioritized list of PLMNs in the ePDG selection information (i.e. the UE shall select first the highest priority PLMN in the ePDG selection information that is contained in the DNS response). If the ePDG selection information does not contain any of the PLMNs in the DNS response or the UE is not configured with the ePDG selection information, or the UE was not able to connect to an ePDG in the PLMNs included in the ePDG selection information and in the DNS response, then the UE shall select a PLMN included in the DNS response based on its own implementation means.
    3. If the UE cannot select an ePDG in any of the PLMNs included in the DNS response, then the UE shall stop the ePDG selection.
  3. If the UE does not receive a DNS response, then the UE shall stop the ePDG selection.
After the UE selects a PLMN for ePDG selection as specified above, UE shall construct an Operator Identifier FQDN for the selected PLMN and shall use the DNS server function to obtain the IP address(es) of the ePDG(s) in this PLMN.
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4.5.4a  ePDG Selection for Emergency Services |R13|Word‑p. 40

4.5.4a.1  General

UE initiates the ePDG selection for emergency services when it detects a user request for emergency session and determines that WLAN shall be used for the emergency access.
Unless the UE is attached to an ePDG that has indicated support for the emergency services and is located in the same country where the UE is currently located, the UE terminates the exisitng ePDG connection, if any, and performs the emergency ePDG selection procedure described in clause 4.5.4a.2. Otherwise, the UE should reuse the existing ePDG connection.
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4.5.4a.2  Emergency ePDG Selection Procedure

The ePDG selection for emergency services shall use the ePDG selection procedure for non-emergency services specified in clause 4.5.4, with the following modifications:
  1. Separately configured ePDG Emergency Identifier shall be used instead of the ePDG Identifier specified in clause 4.5.4.3;
  2. For a UE equipped with a UICC, the Operator Identifier Emergency FQDN and the Tracking/Location Area Identity Emergency FQDN (specified in TS 23.003) shall be constructed based on the rules specified in clause 4.5.4.2 and shall be used instead of the Operator Identifier FQDN and the Tracking/Location Area Identity FQDN respectively;
  3. The DNS-based discovery of the regulatory requirements described in clause 4.5.4.5 in the context of emergency ePDG selection shall be based on a Visited Country Emergency FQDN (specified in TS 23.003), instead of the Visited Country FQDN;
  4. If the UE is not equipped with a UICC, the UE shall perform the emergency ePDG selection procedure without using the ePDG selection configuration data (the ePDG Emergency Identifier and the ePDG selection information), i.e., the UE shall consider those data as not configured and the UE may construct the Operator Identifier FQDN format based on a PLMN ID obtained via implementation specific means.
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4.5.5  PCRF Selection

In addition to the PDN-GW and AF being served by one or more PCRF nodes in a HPLMN and, where applicable , in VPLMN as in TS 23.401, the following nodes in this specification also are served by PCRF:
  • Serving GW;
  • Elements in trusted non-3gpp access;
  • ePDG.
Selection of a PCRF by nodes served by PCRF in this specification, is the same as that in specified in TS 23.203.

4.5.6  DSMIPv6 Home Link Detection FunctionWord‑p. 41
The DSMIPv6 Home Link Detection Function is used by the UE to detect if, for a specific PDN, an access interface is the Home Link from a DSMIPv6 perspective.
It is up to the UE configuration to decide when to trigger the home link detection function for a specific PDN connection, except that homelink detection for an access interface shall be performed before sending any DSMIPv6 Binding Update via that access interface.
The UE detects the home link comparing the IPv6 prefix associated with a specific access system of the UE , and the Home Network Prefix (HNP) associated with the PDN connection. If there is a match, the UE detects it is in the home link for this specific PDN over the access interface. Otherwise, the UE detects it is not in the home link for this specific PDN over the access interface.
Home Network Prefix (HNP) may be assigned in a 3GPP access via PCO during 3GPP attach, if supported by the UE, or via IKEv2.
The UE knows the HNP associated with a specific PDN from the IPsec security association bootstrap (see clause 6.3, step 4) or from PCO received in 3GPP attach.
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