Section: 4.3.8.2 Serving GW selection function
Content: The Serving GW selection function selects an available Serving GW to serve a UE. The selection bases on network topology, i.e. the selected Serving GW serves the UE's location and for overlapping Serving GW service areas, the selection may prefer Serving GWs with service areas that reduce the probability of changing the Serving GW. When SIPTO is allowed then it is also considered as a criterion for Serving GW selection, e.g. when the first PDN connection is requested. Other criteria for Serving GW selection should include load balancing between Serving GWs, UE support for dual connectivity with NR, CIoT EPS Optimisation(s) impacting Serving GW e.g. Non-IP support, Ethernet support, NB-IoT RAT support (for generation of accounting information), etc. When the Serving 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 Serving GW node relative to other Serving GW nodes serving the same Tracking area. For further details on 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 Serving GW in addition to the Weight Factors received from the DNS server to perform selection of an appropriate Serving GW. NOTE 1: How Weight Factors can be used in conjunction with Load Information received via GTP control plane signalling is left up to Stage 3. If a subscriber of a GTP only network roams into a PMIP network, the PDN GWs selected for local breakout support the PMIP protocol, while PDN GWs for home routed traffic use GTP. This means the Serving GW selected for such subscribers may need to support both GTP and PMIP, so that it is possible to set up both local breakout and home routed sessions for these subscribers. For a Serving GW supporting both GTP and PMIP, the MME/SGSN should indicate the Serving GW which protocol should be used over S5/S8 interface. The MME/SGSN is configured with the S8 variant(s) on a per HPLMN granularity. If a subscriber of a GTP only network roams into a PMIP network, the PDN GWs selected for local breakout may support GTP or the subscriber may not be allowed to use PDN GWs of the visited network. In both cases a GTP only based Serving GW may be selected. These cases are considered as roaming between GTP based operators. If combined Serving and PDN GWs are configured in the network the Serving GW Selection Function may preferably derive a Serving GW that is also a PDN GW for the UE. In order to provide SIPTO at the local network service with stand-alone GW, the L-GW and Serving GW shall be co-located. The Serving GW selection function in the MME is used to ensure that the Serving GW is provided according to operator policy as described in clause 4.3.15a. When the L-GW is collocated with the (H)eNB, the Serving GW remains located in the mobile operator's core network. The Domain Name Service function may be used to resolve a DNS string into a list of possible Serving GW addresses which serve the UE's location. 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 Serving GW capabilities (e.g. whether the Serving GW supports PMIP-based or GTP-based S5/S8, or both). The details of the selection are implementation specific. For handover from non-3GPP accesses in roaming scenario, the Serving GW selection function for local anchoring is described in TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [2]. The Serving GW selection function in the MME is used to ensure that all Tracking Areas in the Tracking Area List belong to the same Serving GW service area. When DCNs with dedicated Serving GWs are used, the DNS procedure (TS 29.303[ Domain Name System Procedures; Stage 3 ]  [61]) for Serving GW selection may be used such that a Serving 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 Serving GW belonging to the DCN serving the particular UE. NOTE 2: Selection of Serving GWs optimised for different RATs (e.g. NB-IoT) can be achieved by using UE Usage Type and/or by using different TAIs for different RATs.