[Conditional] SMF to new UPF (intermediate): N4 Session Establishment Request
If the SMF selects a new UPF to act as intermediate UPF for the PDU Session, or if the SMF selects to insert an intermediate UPF for a PDU Session which did not have an intermediate UPF, an N4 Session Establishment Request message is sent to the new UPF, providing Packet detection, Data forwarding, enforcement and reporting rules to be installed on the intermediate UPF. The PDU Session Anchor addressing information (on N9) for this PDU Session is also provided to the intermediate UPF.
If a new UPF is selected by the SMF to replace the old (intermediate) UPF, SMF includes the Data forwarding indication. The Data Forwarding Indication indicates to the UPF that a second tunnel endpoint needs to be reserved for buffered DL data from the old I-UPF.