Section: 4.23.11.2 Xn based handover with insertion 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 insertion of a new intermediate I-SMF is needed. This procedure is used for non-roaming or local breakout roaming scenario. The call flow is shown in figure 4.23.11.2-1. Figure 4.23.11.2-1: Xn based inter NG-RAN handover with insertion 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 perform same step as step 2 in clause 4.9.1.2.2. The rest of this procedure applies for each PDU Session To Be Switched. 3a. The AMF checks if an I-SMF needs to be selected as described in clause 5.34.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 3b. If a new I-SMF is selected the AMF sends Nsmf_PDUSession_CreateSMContext Request (SUPI, AMF ID, SMF ID, SM Context ID, PDU Session To Be Switched with N2 SM Information (Secondary RAT usage data), UE Location Information, UE presence in LADN service area) to the new selected I-SMF. 4. The new I-SMF sends Nsmf_PDUSession_Context Request (SM context type, SM Context ID) to SMF to retrieve the SM Context. The new I-SMF uses SM Context ID received from AMF for this service operation. SM context type indicates that the requested information is all SM context, i.e. PDN Connection Context and 5G SM context. The SM Context ID is used by the recipient of Nsmf_PDUSession_Context Request in order to determine the targeted PDU Session. 5a. I-SMF to I-UPF: N4 Session Establishment Request (Target NG-RAN Tunnel Info). The I-SMF then selects a 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]. 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. 5b. I-UPF to I-SMF: N4 Session Establishment Response. The I-UPF sends an N4 Session Establishment Response message to the I-SMF. The UL CN Tunnel Info and DL CN Tunnel Info of I-UPF are sent to the I-SMF. 6. I-SMF to SMF: Nsmf_PDUSession_Create Request to the SMF (SUPI, PDU Session ID, Secondary RAT usage data, UE Location Information, UE presence in LADN service area, DL CN Tunnel Info of the I-UPF, DNAI list supported by the I-SMF). The I-SMF provides the DNAI list it supports to the SMF as defined in Figure 4.23.9.1-1 step 1. Secondary RAT usage data is extracted from N2 SM Information within PDU Session To Be Switched received from NG RAN. 7a. SMF to UPF (PSA): N4 Session Modification Request (DL CN Tunnel Info of the I-UPF). The SMF provides the DL CN Tunnel Info of the I-UPF to the UPF(PSA). If old I-UPF controlled by SMF does not exist and if different CN Tunnel Info need to be used by PSA UPF, i.e. the CN Tunnel Info at the PSA for N3 and N9 are different, the CN Tunnel Info at the PSA for N9 needs to be allocated. The CN Tunnel Info is provided from UPF to SMF in the response. 7b. UPF (PSA) to SMF: N4 Session Modification Response. The PDU Session Anchor responds with the N4 Session Modification Response message after requested PDU Sessions are switched. At this point, PDU Session Anchor starts sending downlink packets to the Target NG-RAN via I-UPF. 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. 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. 9. SMF to I-SMF: Nsmf_PDUSession_Create Response(Information for local traffic steering location, CN Tunnel Info at the PSA for N9, 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. 9a. If the CN Tunnel Info at PSA for N9 is allocated, it is included in the response and the I-SMF provides the CN Tunnel Info at the PSA for N9 to I-UPF via N4 Session Modification Request. In the case of I-SMF insertion and the PDU session corresponds to a LADN, the SMF shall release the PDU session after the handover procedure is completed. 10. I-SMF to AMF: Nsmf_PDUSession_CreateSMContext Response (UL CN Tunnel Info of the I-UPF, updated CN PDB in the QoS parameters for accepted QoS Flows). The SMF sends an Nsmf_PDUSession_CreateSMContext response to the AMF. 11-13. Steps 11-13 are same as steps 7-9 defined in clause 4.9.1.2.2 with the following addition: If a Source I-UPF controlled by SMF was serving the PDU Session, the SMF initiates Source I-UPF Release procedure by sending an N4 Session Release Request toward the Source I-UPF.