Section: 4.23.11.4 Xn based handover with removal of intermediate SMF
Content: This procedure is used to hand over a UE from a Source NG-RAN to a Target NG-RAN using Xn interface(in this case the AMF is unchanged) and the AMF decides that removal of intermediate I-SMF is needed. This procedure is used for non-roaming or local breakout roaming scenario. The call flow is shown in figure 4.23.11.4-1. Figure 4.23.11.4-1: Xn based inter NG-RAN handover with removal of intermediate SMF 1. Step 1 is the same as described in clause 4.9.1.2.2. 2. For each PDU Session Rejected in the list of PDU Sessions received in the N2 Path Switch Request, the AMF sends Nsmf_PDUSession_UpdateSMContext Request to source I-SMF and then the source I-SMF sends the Nsmf_PDUSession_Update Request to SMF forwarding the failure cause. The SMF decides whether to release the PDU Session. The rest of this procedure applies for each PDU Session To Be Switched. 3a. The AMF performs I-SMF selection as described in clause 5.34.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and the AMF decides to remove I-SMF in this case. 3b. The AMF sends Nsmf_PDUSession_CreateSMContext Request (SUPI, PDU Session ID, AMF ID, PDU Session To Be Switched with N2 SM Information (Secondary RAT usage data), UE Location Information, UE presence in LADN service area, Target NG-RAN Tunnel Info) to the SMF. 4a. [Conditional] SMF to I-UPF: N4 Session Establishment Request (Target NG-RAN Tunnel Info, UL CN Tunnel Info of the UPF (PSA)). The SMF may select an I-UPF based on UPF Selection Criteria according to clause 6.3.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If an I-UPF is selected, an N4 Session Establishment Request message is sent to the I-UPF. The target NG-RAN Tunnel Info is included in the N4 Session Establishment Request message. 4b. I-UPF to SMF: N4 Session Establishment Response. The I-UPF sends an N4 Session Establishment Response message to the I-SMF. The UL and DL CN Tunnel Info of I-UPF is sent to the I-SMF. 5a. SMF to UPF (PSA): N4 Session Modification Request (DL CN Tunnel Info of the I-UPF if I-UPF is selected, or Target NG-RAN Tunnel Info if I-UPF is not selected). The SMF provides the DL CN Tunnel Info of the I-UPF to the UPF (PSA) if I-UPF is selected. If an I-UPF is not selected, the SMF provides the target NG-RAN Tunnel Info to the UPF (PSA). If different CN Tunnel Info need to be used by PSA UPF, i.e. the CN Tunnel Info for N3 and N9 are different, the SMF retrieves the new CN Tunnel Info from UPF. 5b. UPF (PSA) to SMF: N4 Session Modification Response. If different CN Tunnel Info needs to be used by PSA UPF, i.e. the CN Tunnel Info for N3 and N9 are different, the CN Tunnel Info is allocated by the UPF and provided to the SMF. The PDU Session Anchor responds with the N4 Session Modification Response message after requested PDU Sessions are switched. PDU Session Anchor sends one or more "end marker" packets for each N3/N9 tunnel on the old path immediately after switching the path, the source NG-RAN shall forward the "end marker" packets to the target NG-RAN. At this point, PDU Session Anchor starts sending downlink packets to the Target NG-RAN via I-UPF. 6a. [Conditional] SMF to I-UPF: N4 Session Modification Request (UL CN Tunnel Info of the UPF (PSA)). If the UL CN Tunnel Info of the UPF (PSA) has been changed, the SMF provides the UL CN Tunnel Info of the UPF (PSA) to I-UPF. 6b. I-UPF to SMF: N4 Session Modification Response. The I-UPF responds with the N4 Session Modification Response message. 7. In order to assist the reordering function in the Target NG-RAN, the PDU Session Anchor sends one or more "end marker" packets for each N3/N9 tunnel on the old path immediately after switching the path, the source NG-RAN shall forward the "end marker" packets to the target NG-RAN. 8. SMF to AMF: Nsmf_PDUSession_CreateSMContext Response (UL CN Tunnel Info of the I-UPF if I-UPF is selected, or CN Tunnel Info (on N3) of UPF (PSA) if I-UPF is not selected, updated CN PDB in the QoS parameters for accepted QoS Flows). The SMF may update the CN PDB in the response or using a separate PDU Session Modification procedure, based on local configuration. The SMF sends an Nsmf_PDUSession_CreateSMContext response to the AMF. 9. Step 9 is same as step 7 defined in clause 4.9.1.2.2. 10a. AMF to source I-SMF: Nsmf_PDUSession_ReleaseSMContext request (I-SMF only Indication). The AMF sends Nsmf_PDUSession_ReleaseSMContext request to source I-SMF. The I-SMF only indication is included in this message to avoid invoking resource release in SMF. 10b. Source I-SMF to source I-UPF: N4 Session Release Request/Response. The source I-SMF sends N4 Session Release Request to source I-UPF in order to release resources for the PDU Session. 10c. Source I-SMF to AMF: Nsmf_PDUSession_ReleaseSMContext Response. The source I-SMF responds to AMF with Nsmf_PDUSession_ReleaseSMContext response. 11-12. Steps 11-12 are same as steps 8-9 defined in clause 4.9.1.2.2.