5.37.3.1 General

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. In the case of 3GPP access, 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 [27]), or in the PSA UPF (see clause 5.37.3.3). In the case of untrusted/trusted non-3GPP access, ECN marking for L4S in the IP header is supported in the N3IWF/TNGF (see clause 5.37.3.4, clause 6.2.9 and clause 6.2.9A).

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 and accordingly, the PSA UPF may mark the UL and/or DL direction packets of the QoS Flow.

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 or by requests by an AF.

NOTE 4: For enabling ECN marking for L4S based on dynamic detection of L4S traffic, according to local configuration, SMF instructs UPF (as described in clause 5.4.11 of TS 29.244 [65] for Application Reporting) to detect L4S traffic (e.g. match packets with ECT(1) or CE (See RFC 9331 [160]) in the IP header) on a QoS Flow. Upon receiving report from UPF, the SMF establishes another QoS Flow enabled for L4S, executing the actions defined in clause 5.7.1.5 (as if a PCC rule enabling L4S is processed and an appropriate QoS profile is derived e.g. identical to the QoS Flow where the L4S traffic was detected) such that the detected L4S traffic is now mapped to the newly established QoS Flow. It is recommended that other traffic processing rules are aligned to those of the PDR of the QoS Flow where L4S traffic was detected (different configuration for specific traffic processing rules can exist).

NOTE 5: 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 that ECN marking for L4S can no longer be performed if ECN marking for L4S had been enabled for the QoS Flow based on AF request. When ECN marking for L4S is supported again either in target NG-RAN or in target PSA UPF, AF should be notified that ECN marking for L4S can be performed again if ECN marking for L4S had been enabled for the QoS Flow based on AF request.