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5GS – UE‑Triggered Service Request – 09 of 36
Signalling flow for TS 23.502 UE-Triggered Service Request: SMF to UPF (PSA): N4 Session Modification Request

Step 7a, verbatim
[Conditional] SMF to UPF (PSA): N4 Session Modification Request
If the SMF selects a new intermediate UPF for the PDU Session or removed the I-UPF, the SMF sends N4 Session Modification Request message to PDU Session Anchor UPF, providing the Data forwarding indication and DL tunnel information from new intermediate UPF.
If the new intermediate UPF was added for the PDU Session, the UPF (PSA) begins to send the DL data to the new I-UPF as indicated in the DL tunnel information.
If the Service Request is triggered by the network, and the SMF removes the old I-UPF but does not replace it with a new I-UPF, the SMF includes the Data Forwarding indication to the request. The Data Forwarding Indication indicates to the UPF (PSA) that a second tunnel endpoint needs to be reserved for buffered DL data from the old I-UPF. In this case, the UPF (PSA) begins to buffer the DL data it may receive at the same time from the N6 interface.