5.8.2.19.2 Buffering at UPF

When the SMF decided to activate buffering in UPF, the SMF shall inform the UPF to start buffering packets for this PDU Session.

The SMF provides instructions to the UPF for at least the following behaviour:

• buffer downlink packets with the following additional options:
• reporting the arrival of first downlink packet (for a QoS Flow or a service data flow) and/or
• reporting the first discarded downlink packet (for a service data flow), or
• drop downlink packets with the following additional options:
• reporting the first discarded downlink packet (for a service data flow).
• buffer uplink packets.

When the SMF instructs the UPF for a service data flow to buffer downlink packets and to report the first discarded downlink packet, the SMF shall also instruct the UPF to report the arrival of the first downlink packet for this service data flow to enable the SMF check if this is also the first report for the QoS Flow (as described below). Based on local policy and/or an indication from the AMF, e.g. the RAT type of REDCAP or the CN based MT handling indication, the SMF may also instruct the UPF to report the DL data size in case of arrival of the first downlink packet.

Buffering in the UPF may be configured based on timers or the amount of downlink data to be buffered. The SMF decides whether buffering timers or amount of downlink data are handled by the UPF or SMF.

After starting buffering, when the first downlink packet (of a QoS Flow or a service data flow) arrives, UPF shall inform the SMF if it is setup to report. UPF sends a Downlink Data Report to the SMF via N4 unless specified otherwise and indicates the PDR by which the downlink packet was received. If the SMF receives a Downlink Data Report for a service data flow, the SMF shall also check if this is the first report for the QoS Flow corresponding to the PDR. If so, the SMF shall also proceed as described in clause 5.4.3.1.

After starting buffering, when the first downlink packet (of a service data flow) in a configured period of time that has been buffered is discarded by the UPF because the configured buffering time or amount of downlink data to be buffered is exceeded, the UPF shall inform the SMF if it is setup to report. UPF sends a Downlink Data Report to the SMF via N4 and indicates the PDR by which the discarded downlink packet was received.

A new report is sent if the SMF terminates and subsequently re-activates the buffering action at the UPF and the UPF again receives downlink packets.

NOTE: For the notification about the downlink data delivery status "buffered" or "discarded" related to packets from a particular AF as part of the Nsmf_EventExposure service, it is expected that a PDR with a traffic filter identifying that AF as source and a Forwarding Action rule with action "buffer" is installed.

When the UP connection of the PDU Session is activated, the SMF updates the UPF of the change in buffering state. The buffered downlink packets, if any, are then forwarded to the (R)AN by the UPF.

If the UP connection of the PDU Session has been deactivated for a long time, the SMF may indicate the UPF to stop buffering for this PDU Session.

The SMF may indicate to the UPF to start or stop the buffering of uplink packets of an application associated to the PCC rule as described in clause 6.3.5 of TS 23.548 [130]. When the buffering of uplink packets is stopped the UPF shall forward all buffered uplink packets before it forwards any new uplink packets.