Section: 4.3.8.1 PDN GW selection function (3GPP accesses)
Content: The PDN GW selection function allocates a PDN GW that shall provide the PDN connectivity for the 3GPP access. The function uses subscriber information provided by the HSS and possibly additional criteria such as SIPTO/LIPA support per APN configured in the SGSN/MME, UE support for dual connectivity with NR, 15 EPS bearers support by the UE, CIoT EPS Optimisation(s) impacting PDN GW e.g. Non-IP support, Ethernet support, NB-IoT RAT support (for generation of accounting information), etc. NOTE 1: Selection of PDN GWs optimised for different RATs (e.g. NB-IoT) can be achieved by the allocation of different APNs to subscribers allowed to use specific RATs and/or using the UE Usage Type. The criteria for PDN GW selection may include load balancing between PDN GWs. When the PDN GW IP addresses returned from the DNS server include Weight Factors, the MME should use it if load balancing is required. The Weight Factor is typically set according to the capacity of a PDN GW node relative to other PDN GW nodes serving the same APN. For further details on the DNS procedure see TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61]. When the MME supports the GTP-C Load Control feature, it takes into account the Load Information received from the PDN GW in addition to the Weight Factors received from the DNS server to perform selection of an appropriate PDN GW. NOTE 2: How Weight Factors can be used in conjunction with Load Information received via GTP control plane signalling is left up to Stage 3. The PDN subscription contexts provided by the HSS contain: - the identity of a PDN GW and an APN (PDN subscription contexts with subscribed PDN GW address are not used when there is interoperation with pre Rel-8 2G/3G SGSN), or - an APN and an indication for this APN whether the allocation of a PDN GW from the visited PLMN is allowed or whether a PDN GW from the home PLMN shall be allocated. Optionally an identity of a PDN GW may be contained for handover with non-3GPP accesses. - optionally for an APN, an indication of whether SIPTO above RAN, or SIPTO at the Local Network, or both, is allowed or prohibited for this APN. - optionally for an APN, an indication of whether LIPA is conditional, prohibited, or only LIPA is supported for this APN. In the case of static address allocation, a static PDN GW is selected by either having the APN configured to map to a given PDN GW, or the PDN GW identity provided by the HSS indicates the static PDN GW. The HSS also indicates which of the PDN subscription contexts is the Default one for the UE. To establish connectivity with a PDN when the UE is already connected to one or more PDNs, the UE provides the requested APN for the PDN GW selection function. If one of the PDN subscription contexts provided by the HSS contains a wild card APN (see TS 23.003[ Numbering, addressing and identification ]  [9]), a PDN connection with dynamic address allocation may be established towards any APN requested by the UE. An indication that SIPTO (above RAN, at the local network, or both) is allowed or prohibited for the wild card APN allows or prohibits SIPTO for any APN that is not present in the subscription data. If the HSS provides the identity of a statically allocated PDN GW, or the HSS provides the identity of a dynamically allocated PDN GW and the Request Type indicates "Handover", no further PDN GW selection functionality is performed. If the HSS provides the identity of a dynamically allocated PDN GW, the HSS also provides information that identifies the PLMN in which the PDN GW is located. NOTE 3: The MME uses this information to determine an appropriate APN-OI and S8 protocol type (PMIP or GTP) when the MME and PDN GW are located in different PLMNs. If the HSS provides the identity of a dynamically allocated PDN GW and the Request Type indicates "initial Request", either the provided PDN GW is used or a new PDN GW is selected. When a PDN connection for an APN with SIPTO-allowed is requested, the PDN GW selection function shall ensure the selection of a PDN GW that is appropriate for the UE's location. The PDN GW identity refers to a specific PDN GW. If the PDN GW identity includes the IP address of the PDN GW, that IP address shall be used as the PDN GW IP address; otherwise the PDN GW identity includes an FQDN which is used to derive the PDN GW IP address by using Domain Name Service function, taking into account the protocol type on S5/S8 (PMIP or GTP). NOTE 4: Provision of a PDN GW identity of a PDN GW as part of the subscriber information allows also for a PDN GW allocation by HSS. If the HSS provides a PDN subscription context that allows for allocation of a PDN GW from the visited PLMN for this APN and, optionally, the MME is configured to know that the visited VPLMN has a suitable roaming agreement with the HPLMN of the UE, the PDN GW selection function derives a PDN GW identity from the visited PLMN. If a visited PDN GW identity cannot be derived, or if the subscription does not allow for allocation of a PDN GW from the visited PLMN, then the APN is used to derive a PDN GW identity from the HPLMN. The PDN GW identity is derived from the APN, subscription data and additional information by using the Domain Name Service function. If the PDN GW identity is a logical name instead of an IP address, the PDN GW address is derived from the PDN GW identity, protocol type on S5/S8 (PMIP or GTP) by using the Domain Name Service function. The S8 protocol type (PMIP or GTP) is configured per HPLMN in MME/SGSN. In order to select the appropriate PDN GW for SIPTO above RAN service, the PDN GW selection function uses the TAI (Tracking Area Identity), the serving eNodeB identifier, or TAI together with serving eNodeB identifier depending on the operator's deployment during the DNS interrogation as specified in TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61] to find the PDN GW identity. In roaming scenario PDN GW selection for SIPTO is only possible when a PDN GW in the visited PLMN is selected. Therefore in a roaming scenario with home routed traffic, PDN GW selection for SIPTO is not performed. In order to select the appropriate GW for SIPTO at the local network service with a stand-alone GW (with S-GW and L-GW collocated), the PDN GW selection function uses the APN and the Local Home Network ID during the DNS interrogation as specified in TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61] to find the PDN GW identity. The Local Home Network ID is provided to the MME by the (H)eNB in every INITIAL UE MESSAGE and every UPLINK NAS TRANSPORT control message as specified in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]. The MME uses the Local Home Network ID to determine if the UE has left its current local network and if S-GW relocation is needed. For SIPTO at the Local Network with L-GW function collocated with the (H)eNB the PDN GW selection function uses the L-GW address proposed by the (H)eNB in the S1-AP message, instead of DNS interrogation. In order to select the appropriate L-GW for LIPA service, if permitted by the CSG subscription data and if the UE is roaming, the VPLMN LIPA is allowed, the PDN GW selection function uses the L-GW address proposed by HeNB in the S1-AP message, instead of DNS interrogation. If no L-GW address is proposed by the HeNB and the UE requested an APN with LIPA permissions set to "LIPA-only", the request shall be rejected. If no L-GW address is proposed by the HeNB and the UE requested an APN with LIPA permissions set to "LIPA-conditional", the MME uses DNS interrogation for PDN GW selection to establish a non-LIPA PDN connection. The PDN subscription context for an APN with LIPA permissions set to "LIPA-only" shall not contain a statically configured PDN address or a statically allocated PDN GW. A static PDN address or a static PDN GW address, if configured by HSS for an APN with LIPA permissions set to "LIPA-conditional", is ignored by MME when the APN is established as a LIPA PDN connection. When establishing a PDN connection for a LIPA APN, the VPLMN Address Allowed flag is not considered. The PDN GW domain name shall be constructed and resolved by the method described in TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61], which takes into account any value received in the APN-OI Replacement field for non-roaming or home routed traffic. Otherwise, or when the resolution of the above PDN GW domain name fails, the PDN GW domain name shall be constructed by the serving node using the method specified in Annex A of TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [7] and clause 9 of TS 23.003[ Numbering, addressing and identification ]  [9]. If the Domain Name Service function provides a list of PDN GW addresses, one PDN GW address is selected from this list. If the selected PDN GW cannot be used, e.g. due to an error, then another PDN GW is selected from the list. The specific interaction between the MME/SGSN and the Domain Name Service function may include functionality to allow for the retrieval or provision of additional information regarding the PDN GW capabilities (e.g. whether the PDN GW supports PMIP-based or GTP-based S5/S8, or both). NOTE 5: The APN as constructed by the MME/SGSN for PDN GW resolution takes into account the APN-OI Replacement field. This differs from the APN that is provided in charging data to another SGSN and MME over the S3, S10 and S16 interfaces as well as to Serving GW and PDN GW over the S11, S4 and S5/S8 interfaces, in that the APN-OI Replacement field is not applied. See clause 5.7.2 of the present document for more details. If the UE provides an APN for a PDN, this APN is then used to derive the PDN GW identity as specified for the case of HSS provided APN if one of the subscription contexts allows for this APN. If there is an existing PDN connection to the same APN used to derive the PDN GW address, the same PDN GW shall be selected. As part of PDN GW selection, an IP address of the assigned PDN GW may be provided to the UE for use with host based mobility as defined in TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [2], if the PDN GW supports host-based mobility for inter-access mobility towards accesses where host-based mobility can be used. If a UE explicitly requests the address of the PDN GW and the PDN GW supports host based mobility then the PDN GW address shall be returned to the UE. When DCNs with dedicated PDN GWs are used, the DNS procedure (TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61]) for PDN GW selection may be used such that a PDN GW belonging to a DCN serving a particular category of UEs, e.g. identified by UE Usage Type, is selected. When UEs with the same UE Usage type are served by multiple DCNs, it shall also be possible to select the PDN GW belonging to the DCN serving the particular UE.