Content for  TS 26.501  Word version:  19.0.0

Policy Template Provisioning. A Policy Template is provisioned with the requirement for L4S capability, indicated by setting the L4S enablement flag.

Service Access Information acquisition and Dynamic Policy activation. The Media Session Handler within the 5GMSd Client obtains Service Access Information and triggers a dynamic policy activation. A Policy Template Binding is present within the Service Access Information for each provisioned Policy Template. Policy Template Bindings suitable for L4S are indicated by an L4S enablement flag being set. The 5GMSd Client detects that an L4S-capable media transport stack is present and in use. The selected Policy Template is configured with the L4S enablement flag.

QoS request. The 5GMSd AF requests QoS handling using e.g. the Nnef_AfSessionWithQoS service or the Npcf_PolicyAuthorization service. If the L4S enablement flag is set in the selected Policy Template, this indicates that the new QoS flow is required to be L4S-enabled. The new QoS flow with the ECN marking for L4S indication setting propagates through the 5G System.

The Media Session Handler may inform the Media Player about the successful activation of the L4S-enabled Dynamic Policy via the client API at reference point M11d. Subject to availability of API access, the Media Player may use congestion notifications to perform early bit rate adaptation.

If the L4S enablement Dynamic Policy is successfully activated, the 5GMSd Client selects/enables the L4S capability of the used transport protocol.

The Media Player within the 5GMSd Client triggers the establishment of a TCP connection. The ECT(1) codepoint is set in the IP header, indicating an L4S-Capable Transport, and the SDAP entity ensures that the packet is forwarded via the matching QoS flow.

The 5GMSd AS responds to the TCP connection establishment request. The 5GMSd AS sets ECT(1) in the IP headers, indicating an L4S-Capable Transport.

The UPF finds the matching QoS Flow Identifier for the downlink packet and sends the packet via the corresponding QoS flow to the UE. TCP Connection setup continues, with one ECT bit set in all packets.

When the RAN detects an upcoming congestion event according to the congestion measurement (based on continuous congestion monitoring), the 5G System sets the CE (Congestion Experienced) codepoint in the IP header of the downlink packet.

The TCP protocol stack used by the Media Player in the 5GMSd Client reflects the Early Congestion Notification to the TCP sender by setting the ECN-Echo (ECE) flag in the TCP header of an uplink PDU of the same TCP connection. The TCP sender reacts to the ECN-Echo accordingly (i.e., by reducing its sending congestion window).

Based on the CE indication received in step 8, or by detecting a reduced bit rate in the downlink application flow, the Media Player in the 5GMSd Client reacts by, for example, changing the requested representation.

Policy Template Provisioning. A Policy Template is provisioned and shall include the QoS monitoring configuration. The QoS monitoring configuration includes the parameters to be monitored, the reporting frequency (event triggered, periodic), and optionally the notification via local UPF.

Service Access Information retrieval by 5GMSd AS. The 5GMSd AS retrieves Service Access Information from 5GMSd AF via reference point M3d. The 5GMSd AS subscribes to receive notifications from the 5GMSd AF about changes to the monitored QoS parameters for all relevant Dynamic Policies.

Service Access Information retrieval by Media Session Handler. The Media Session Handler retrieves Service Access Information from the 5GMSd AF via reference point M5d.

Dynamic Policy activation. The Media Session Handler within the 5GMSd Client obtains Service Access Information and triggers a dynamic policy activation. A Policy Template Binding is present within the Service Access Information for each provisioned Policy Template. Policy Template Bindings include a QoS monitoring availability flag indicating whether QoS monitoring may be enabled when instantiating the corresponding Policy Template. The Media Session Handler determines whether to enable QoS monitoring based on its own knowledge or based on input from the 5GMSd-Aware Application, and sets a QoS monitoring enabled flag accordingly in the Dynamic Policy activation. If successful, the Media Session Handler subscribes to receive notifications from the 5GMSd AF about changes to monitored QoS parameters for this Dynamic Policy.

QoS monitoring request. The 5GMSd AF invokes the Npcf_PolicyAuthorization service or the Nnef_AFsessionWithQoS service with the requested QoS monitoring configurations. In the case where the 5GMSd AS is deployed in the Edge DN, the 5GMSd AF may additionally enable the exposure of QoS monitoring results via the local UPF or local NEF in this step.

The PCF accepts the request and enables QoS monitoring within the 5G System, i.e., by configuring the RAN and/or the (local) UPF for monitoring and reporting of target QoS parameters for the downlink media streaming.

The PCF may expose QoS monitoring results to the 5GMSd AF periodically or by event triggers using the Npcf_PolicyAuthorization_Notify service operation directly at reference point N5, or else using the Nnef_EventExposure_Notify service operation via the NEF at reference point N33.

Alternatively, the QoS monitoring results may be exposed to the 5GMSd AF by the UPF directly using the Nupf_EventExposure_Notify service or via a locally deployed NEF using the Nnef_EventExposure_Notify service at reference point N33.

If QoS monitoring was requested by the Media Session Handler, the 5GMSd AF sends the notifications of the QoS monitoring results to the Media Session Handler via reference point M5d.

The Media Session Handler provides the QoS monitoring results to the Media Stream Handler at reference point M11d.

The Media Stream Handler may use the notified QoS monitoring results to modify its behaviour. For example, in the case of downlink media streaming, the Media Player may use the monitored packet latency to determine when to request the next media segment, and/or to change the bit rate of the next media segment based on the monitored congestion status.

The 5GMSd AF may provide the QoS monitoring results to the 5GMSd AS at reference point M3d.

The 5GMSd AS may use the notified QoS monitoring results to modify its behaviour.