Section: 4.3.7 CN-initiated selective deactivation of UP connection of an existing PDU Session
Content: The following procedure is used to deactivate UP connection (i.e. data radio bearer and N3 tunnel) for an established PDU Session of a UE in CM-CONNECTED state. For an always-on PDU Session, the SMF should not configure the UPF to report inactivity. Figure 4.3.7-1: CN-initiated deactivation of UP connection for an established PDU Session 1. The SMF determines that the UP connection of the PDU Session can be deactivated in following cases: - During handover procedure, if all the QoS Flows of a PDU Session are rejected by the target NG-RAN (as described in clause 4.9.1), or if a PDU Session is failed to setup indicated by the AMF (see step 7 of clause 4.9.1.3.3). SMF proceeds with step 2 and step 3, the steps 5 to 9 are skipped; - The UPF detects that the PDU Session has no data transfer for a specified Inactivity period as described in clause 4.4.2.2; - For PDU sessions belonging to a network slice that is in the Partially Allowed NSSAI (see clause 5.15.17 in TS 23.501[ System architecture for the 5G System (5GS) ]  [2]), or with Network Slice Area of Service not matching deployed Tracking Areas (see clause 5.15.18 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]) the AMF notifies to the SMF that it has detected that the UE moved out of the network slice area of support or availability; - For a LADN PDU Session, the AMF notifies to the SMF that the UE moved out of the LADN service area; or - The AMF notifies to the SMF that the UE moved out of the Allowed Area. The SMF may decide to release the UPF of N3 terminating point. In that case the SMF proceeds with step 2 and step 3. Otherwise, if the SMF decides to keep the UPF of N3 terminating points, the SMF proceeds with step 4. To assist SMF in this decision the SMF may make use of UE presence pattern in LADN service area based on UE mobility analytics from the NWDAF as described in clause 6.7.2 of TS 23.288[ Architecture enhancements for 5G System (5GS) to support network data analytics services ]  [50]. The SMF may make use of UE Communication analytics provided by NWDAF, as described in clause 6.7.3 of TS 23.288[ Architecture enhancements for 5G System (5GS) to support network data analytics services ]  [50], to determine the value of an Inactivity Timer for a PDU Session provided to the UPF. 2. The SMF may initiate an N4 Session Release procedure to release the intermediate UPF of N3 terminating point. If there are multiple intermediate UPFs, this step can be performed for each UPFs to be released. The SMF needs to initiate N4 Session Modification procedure to the UPF (i.e. N9 terminating point or PDU Session Anchor) connecting to the released UPF in step 3. 3. If the intermediate UPF(s) of N3 terminating point is released in step 2, the SMF initiates an N4 Session Modification procedure towards the UPF (PDU Session Anchor or another intermediate UPF) connecting to the released UPF, indicating the need to remove CN Tunnel Info for N9 tunnel of the corresponding PDU Session. In this case, the UPF connecting to the released UPF buffers the DL packets for this PDU Session or drops the DL packets for this PDU session or forwards the DL packets for this PDU session to the SMF, based on buffering instruction provided by the SMF as described in clause 5.8.3.2 or clause 5.8.3.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If the PDU Session corresponds to a LADN and the UE moved out of the LADN service area, the SMF may notify the UPF connecting to the released UPF to discard downlink data for the PDU Sessions and/or to not provide further Data Notification messages. Otherwise, N4 Session Modification procedure occurs toward N3 terminating point. 4. If the UPF of N3 terminating point is not released in step 2, the SMF initiates an N4 Session Modification procedure indicating the need to remove AN Tunnel Info for N3 tunnel of the corresponding PDU Session. In this case, the UPF buffers the DL packets for this PDU Session or drops the DL packets for this PDU session or forwards the DL packets for this PDU session to the SMF, based on buffering instruction provided by the SMF as described in clause 5.8.3.2 or clause 5.8.3.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If the PDU Session corresponds to a LADN and the UE moved out of the LADN service area, the SMF may notify the UPF to discard downlink data for the PDU Sessions and/or to not provide further Data Notification messages. 5. The SMF invokes the Namf_Communication_N1N2MessageTransfer service operation (PDU Session ID, N2 SM Information (N2 Resource Release Request (PDU Session ID))) to release the NG-RAN resources associated with the PDU Session. 6. The AMF sends the N2 PDU Session Resource Release Command including N2 SM information (N2 Resource Release Request (PDU Session ID)) received from the SMF via N2 to the NG-RAN. 7. The NG-RAN may issue NG-RAN specific signalling exchange (e.g. RRC Connection Reconfiguration) with the UE to release the NG-RAN resources related to the PDU Session received from the AMF in step 5. When a User Plane connection for a PDU Session is released, the AS layer in the UE indicates it to the NAS layer. If the UE is in RRC_INACTIVE state, this step is skipped. When the UE becomes RRC_CONNECTED state from RRC_INACTIVE state, the NG-RAN and UE synchronize the released radio resources for the deactivated PDU Session as described in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [16] and TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [12]. 8. The NG-RAN acknowledges the N2 PDU Session Resource Release Command to the AMF including N2 SM Resource Release Ack (User Location Information, Secondary RAT Usage Data). 9. The AMF invokes the Nsmf_PDUSession_UpdateSMContext service operation (N2 SM Information(Secondary RAT Usage Data)) to acknowledge the Namf service received in step 5.