5.37.3.4 Support of ECN marking for L4S in N3IWF and TNGF

ECN marking for L4S may be supported in N3IWF and TNGF, as specified in clauses 6.2.9 and 6.2.9A, respectively.

To enable ECN marking for L4S in N3IWF and TNGF, dedicated QoS Flow(s) and non-3GPP access resources (i.e. IPsec Child SAs) 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 N3IWF and TNGF for a L4S enabled QoS Flow(s) in UL and/or DL directions. In the absence of such PCC rule, the use of ECN marking for L4S in N3IWF and TNGF, on a QoS Flow is controlled by a coordinated configuration in N3IWF,TNGF and 5GC.

For DL, intermediate non-3GPP access nodes (i.e. N3IWF and TNGF) map the L4S-enabled QoS Flows to L4S enabled non-3GPP access resources.

NOTE: Any non-3GPP access node (i.e. N3IWF and TNGF) supporting L4S and acting as an IP-in-IP tunnel endpoint between the XR application client and server is assumed to implement encapsulation and decapsulation as specified in IETF RFC 6040 [198] and IETF RFC 9599 [199].

The criteria based on which N3IWF and TNGF decide to mark ECN bits for L4S is implementation specific.

ECN marking for L4S in W-AGF and 5G-RG is specified in clause 4.17.2 of TS 23.316 [84].