D.7.1 Network initiated QoS

When UE is accessing an overlay network via an underlay network as described in clause D.3, in order to ensure the underlay network to support the QoS required by the overlay network User Plane IPsec Child SA, the QoS differentiation mechanism based on network-initiated QoS modification as described in clause 5.30.2.7 and clause 5.30.2.8 can be used with the following considerations:

• An overlay network service can have specific QoS requirement that needs to be fulfilled by the underlay network, based on SLA between the two networks.
• The SLA covers selective services of the overlay network which require QoS support in underlay network. The rest of the overlay network traffic could be handled in best efforts basis by underlay network.
• The SLA between the overlay network and the underlay network includes a mapping between DSCP values of the User Plane IPsec Child SAs and the QoS requirement of the overlay network services. The QoS requirement includes the QoS parameters described in clause 5.7.2 that are necessary (e.g. 5QI, ARP, etc.) during the network-initiated QoS modification in underlay network. In order to facilitate the SLA, a guidance for details of the mapping between DSCP values of the User Plane IPSec Child SAs and QoS requirement of the overlay network services is described of TS 29.513 [133]. The SLA also includes the N3IWF IP address of the overlay network.
• The mapping agreed in SLA is configured at N3IWF of the overlay network and at the SMF/PCF of the underlay network. If a dedicated DNN/S-NSSAI is used in the underlay network for providing access to the N3IWF in the overlay network, the SMF/PCF in the underlay network can be configured to enable packet detection (based on N3IWF IP address and DSCP value) for PDU sessions associated with the dedicated DNN/S-NSSAI.
• When UE establishes a PDU Session in underlay network, the PCF in the underlay network determines PCC rules based on UE subscription information and local configuration which takes into account the SLA described above and installs the PCC rules on the SMF which generates and installs PDR/URR on UPF. The PCC rules indicate N3IWF IP address and the DSCP values of the User Plane IPsec Child SAs of the overlay network which require QoS differentiation by the underlay network. So, the UPF in the underlay network can detect packets of the User Plane IPsec Child SAs corresponding to the overlay network services which require QoS support by the underlay network.
• UE registers and establishes PDU Session in the overlay network via the User Plane connectivity established in the underlay network. When UE is accessing a specific service of overlay network, a QoS Flow can be created by the overlay network, then N3IWF creates dedicated User Plane IPsec Child SA for each overlay network QoS Flow that requires underlay network QoS support.
• N3IWF uses the QoS profile and the Session-AMBR it receives from SMF in overlay network along with the mapping agreed in the SLA to derive a specific DSCP value for the User Plane IPsec Child SA. N3IWF assigns a specific DSCP value only to one User Plane IPsec Child SA for a UE at the same time. UE (for UL) and N3IWF (for DL) will set the DSCP marking in the outer IP header of the User Plane IPsec Child SA accordingly.
• The overlay network traffic between UE and N3IWF using the specific DSCP marking will be detected by the UPF in the underlay network, based on previous installed PDR/URR. The SMF/PCF in underlay network will be informed when the overlay network traffic is detected. Then the PCF installs new PCC rules on the SMF including the QoS parameters for handling of packets corresponding to the specific User Plane IPsec Child SA based on the N3IWF IP address and the DSCP value of the User Plane IPsec Child SA and the SMF generates a QoS profile that triggers the PDU Session Modification procedure as described in clause 4.3.3 of TS 23.502 [3]. The QoS parameters are derived from the mapping agreed in SLA based on the detected DSCP value.