[Conditional] SMF to old UPF (intermediate): N4 Session Modification Request (New UPF address, New UPF DL Tunnel ID)
If the service request is triggered by the network, and the SMF removes the old (intermediate) UPF, the SMF sends the N4 Session Modification Request message to the old (intermediate) UPF, providing the DL tunnel information for the buffered DL data. If the SMF allocated new I-UPF, the DL tunnel information is from the new (intermediate) UPF acting as N3 terminating point. If the SMF did not allocate a new I-UPF, the DL tunnel information is from the new UPF (PSA) acting as N3 terminating point. The SMF starts a timer to monitor the forwarding tunnel.
If the SMF find the PDU Session is activated when receiving the Nsmf_PDUSession_UpdateSMContext Request in step 4 with Cause set to indicate "establishment of user plane resources" for the PDU Session(s), it deletes the RAN tunnel info and initiates an N4 Session Modification procedure to remove Tunnel Info of AN in the UPF.