Section: 5.37.3 Support of ECN marking for L4S to expose the congestion information 5.37.3.1 General
Content: L4S (Low Latency, Low Loss and Scalable Throughput) is described in IETF RFC 9330 [159], IETF RFC 9331 [160] and IETF RFC 9332 [161]. It exposes congestion information by marking ECN bits in the IP header of the user IP packets between the UE and the application server to trigger application layer rate adaptation. In 5G System, ECN marking for L4S may be supported. ECN marking for L4S is enabled on a per QoS Flow basis in the uplink and/or downlink direction and may be used for GBR and non-GBR QoS Flows. ECN marking for the L4S in the IP header is supported in either the NG-RAN (see clause 5.37.3.2 and TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [27]), or in the PSA UPF (see clause 5.37.3.3). NOTE 1: Based on operator's network configuration and policies, SMF decides whether NG-RAN or PSA UPF based ECN marking for L4S is used. In the case of ECN marking for L4S by PSA UPF, the NG-RAN is instructed to perform congestion information monitoring and report to the PSA UPF 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. NOTE 2: As for any QoS Flow, QoS rules in the UE and PDRs in the PSA UPF control which packets are bound to the L4S enabled QoS flow. The Packet Filter Set in the QoS rule or PDR can use packet filter(s) in clause 5.7.6.2 (e.g. match packets with ECT(1) or CE (See RFC 9331 [160]) together with IP 5 tuple) to steer traffic to an L4S enabled QoS Flow. NOTE 3: A QoS Flow may be enabled with ECN marking for L4S requirement e.g. statically when a PDU session is established based on configuration in SMF or PCF, or dynamically based on detection of the L4S traffic (e.g. match packets with ECT(1) or CE (See RFC 9331 [160]) together with IP 5 tuple) in the IP header whereby SMF or PCF triggers a setup of a QoS Flow enabled for L4S, or by requests by an AF. NOTE 4: To support this functionality, the UE needs to support L4S feedback as described in IETF RFC 9330 [159], which is not in the scope of 3GPP. When serving PSA UPF or NG-RAN is changed e.g. due to inter-NG-RAN handover or PSA UPF relocation, target NG-RAN and target PSA UPF, if supported, should continue to perform ECN marking for L4S for the QoS Flow. However, if not available (i.e. ECN marking for L4S is not supported in both, target NG-RAN and target PSA UPF), AF should be notified when ECN marking for L4S had been enabled for the QoS Flow based on AF request.