Section: 5.37.3.2 Support of ECN marking for L4S in NG-RAN
Content: ECN marking for L4S may be supported in NG-RAN as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [27]. To enable ECN marking for L4S in NG-RAN, dedicated QoS Flow(s) are used for carrying L4S enabled IP traffic. The SMF may be instructed, based on either dynamic or predefined PCC rule, to provide an indication for ECN marking for L4S to NG-RAN for a corresponding QoS Flow(s) in UL and/or DL directions. In the absence of such PCC rule, the use of ECN marking for L4S in NG-RAN on a QoS Flow is controlled by a coordinated configuration in NG-RAN and 5GC. The criteria based on which NG-RAN decides to mark ECN bits for L4S is NG-RAN implementation specific. In the case of inter NG-RAN UE mobility, if the ECN marking for L4S has been enabled on source NG-RAN, but the target NG-RAN does not support ECN marking for L4S, then the SMF may, if supported, enable ECN marking for L4S in PSA UPF as defined in clause 5.37.3.3.