Section: 5.37.3.3 Support of ECN marking for L4S in PSA UPF
Content: To enable ECN marking for L4S by a PSA UPF, a QoS Flow level ECN marking for L4S indicator may be sent by SMF to PSA UPF over N4. SMF also indicates to NG-RAN to report the congestion information (i.e. a percentage of packets that UPF uses for ECN marking for L4S) of the QoS Flow on UL and/or DL directions via GTP-U header extension to PSA UPF. If there is no UL packet when report for DL and/or UL needs to be provided, NG-RAN may generate an UL Dummy GTP-U Packet for such a reporting. The SMF may be instructed, based on either dynamic or predefined PCC rule, to provide an indication for ECN marking for L4S to PSA UPF for a corresponding QoS Flow(s) in UL and/or DL directions. Upon successful activation of congestion information reporting for UL and/or DL directions, PSA UPF uses information sent by NG-RAN in GTP-U header extension (see TS 38.415[ NG-RAN; PDU session user plane protocol ]  [116] and TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [27]) to perform ECN bits marking for L4S for the corresponding direction. NOTE: How the congestion information is converted to ECN markings is UPF implementation specific. The criteria based on which NG-RAN decides to provide the congestion information is up to NG-RAN implementation. In the case of PSA UPF relocation, if the ECN marking for L4S has been enabled on source PSA UPF, SMF should select a target PSA UPF supporting ECN marking for L4S. If the target PSA UPF does not support ECN marking for L4S, then SMF may, if supported, switch to ECN marking for L4S in target NG-RAN by following clause 5.37.3.2. In such case, the target NG-RAN stops sending congestion information to the target PSA UPF. In the case of inter NG-RAN UE mobility, if the congestion information reporting has been enabled on source NG-RAN while the target NG-RAN does not support congestion information reporting, then the SMF shall inform PSA UPF to stop ECN marking for L4S. If ECN marking for L4S is supported by the target NG-RAN, the SMF may instruct the target NG-RAN to perform ECN marking for L4S in NG-RAN by following clause 5.37.3.2. For a given QoS Flow, if the target NG-RAN supports congestion information reporting, the target NG-RAN shall report congestion information to UPF once it is available.