Section: 5.34 Support of deployments topologies with specific SMF Service Areas 5.34.1 General
Content: When the UE is outside of the SMF Service Area, or current SMF cannot serve the target DNAI for the traffic routing for local access to the DN, an I-SMF is inserted between the SMF and the AMF. The I-SMF has a N11 interface with the AMF and a N16a interface with the SMF and is responsible of controlling the UPF(s) that the SMF cannot directly control. The exchange of the SM context and forwarding of tunnel information if needed are done between two SMFs directly without involvement of AMF. Depending on scenario, a PDU Session in non-roaming case or local breakout is either served by a single SMF or served by an SMF and an I-SMF. When a PDU Session is served by both an SMF and an I-SMF, the SMF is the NF instance that has the interfaces towards the PCF and CHF. In this Release of the specification, deployments topologies with specific SMF Service Areas apply only for 3GPP access. The SMF shall release or reject the PDU Session if the DNN of the PDU Session corresponds to a LADN and the I-SMF is inserted to the PDU Session. NOTE 1: This implies that operators need to plan the LADN deployment in such a way that the LADN Service area needs to be within the SMF Service Area, but not across SMFs' Service Areas. NOTE 2: This is to cover the case where the UE is not in or moves out of SMF Service Area and an I-SMF is inserted to the PDU Session e.g. during PDU Session Establishment, Service Request. If the PDU Session is maintained with I-SMF, the SMF is not be able to enforce the LADN Service control, e.g. SMF is not notified in the case of Service Request. Independent of whether deployments topologies with specific SMF Service Areas apply, the SMF may trigger the PDU Session re-establishment to the same DN, if the PDU Session is associated with the SSC mode 2 or SSC mode 3. NOTE 3: SSC mode 2 or SSC mode 3 can be used to optimize SMF location for a PDU Session and/or, depending on deployment, ensure that the UE is always within the service area of the SMF controlling the PDU Session. In this case (when PDU Session continuity over the PLMN is not required) procedures described in this clause are not needed. In this Release, how TSC (as defined in clauses 5.27 and 5.28) is supported for PDU Sessions involving an I-SMF is not specified. In this Release, Redundant User Plane Paths as defined in clause 5.33.2.2 is not supported for PDU Sessions involving an I-SMF. Redundant PDU sessions support as defined in clause 5.33.2.1 is supported for PDU Sessions involving an I-SMF, when different S-NSSAIs are used for the redundant PDU sessions. Redundant User Plane Paths as defined in clause 5.33.2.3 is supported for PDU Sessions involving an I-SMF only if this PDU session is established for a S-NSSAI referring to network instances requiring redundant transmission at transport layer. QoS monitoring (as defined in clause 5.33.3) is supported as long as SMF and not I-SMF initiates the QoS monitoring function. Dynamic CN PDB provisioning (as defined in clause 5.7.3.4) is supported for PDU Sessions involving an I-SMF. In this Release, no dedicated functionality is specified for I-SMF and N16a in order to support NPN.