Content for  TS 23.402  Word version:  18.3.0

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 clause 4.3.25.1 of TS 23.401. 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 clauses 4.3.8.1 and 4.3.25.1 of TS 23.401. 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.

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].

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 RFC 6611.
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.

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.

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.

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.