Content for  TS 26.501  Word version:  19.2.0

Policy Template Provisioning. A Policy Template is provisioned with the L4S enablement preference flag set, indicating a preference for enabling ECN marking for L4S functionality.

Service Access Information acquisition and Dynamic Policy activation. The Media Session Handler within the 5GMSu 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 preference flag being set. When instantiating the Dynamic Policy, the Media Session Handler only sets the L4S required flag in the Dynamic Policy request when the Media Player supports an L4S-capable media transport stack.

QoS request. The 5GMSu AF requests QoS handling using e.g. the Nnef_AfSessionWithQoS service or the Npcf_PolicyAuthorization service. If the L4S required flag is set in the Dynamic Policy request from the Media Session Handler, 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 Streamer about the successful activation of the L4S-enabled Dynamic Policy via the client API at reference point M11u. Subject to availability of API access, the Media Player may use congestion notifications to perform early adaptation.

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

The Media Streamer within the 5GMSu 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 5GMSu AS responds to the TCP connection establishment request. The 5GMSu 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 according to the congestion measurement (based on continuous congestion monitoring), the 5G System the CE (Congestion Experienced) codepoint in the IP header of the uplink packet.

The TCP protocol stack used by the 5GMSu AS reflects the Early Congestion Notification to the TCP sender by setting the ECN-Echo (ECE) flag in the TCP header of a downlink 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 uplink application flow, the Media Streamer in the 5GMSu Client may react by, for example, changing the bit rate of the representation.

Policy Template Provisioning. A Policy Template is provisioned with a QoS monitoring configuration, indicating a preference for enabling QoS monitoring functionality. The QoS monitoring configuration includes the parameters to be monitored, the reporting frequency (event triggered, periodic), and optionally the notification via UPF.

Service Access Information retrieval by 5GMSu AS. The 5GMSu AS retrieves Service Access Information from 5GMSu AF via reference point M3u. The 5GMSu AS subscribes to receive notifications from the 5GMSu 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 5GMSu AF via reference point M5u. A Policy Template Binding is present within the Service Access Information for each provisioned Policy Template. Policy Template Bindings include a QoS monitoring enablement preference flag indicating a preference that QoS monitoring is be enabled when instantiating the corresponding Policy Template.

Dynamic Policy activation. The Media Session Handler within the 5GMSu Client obtains Service Access Information and triggers a dynamic policy activation. If the Media Session Handler determines that the Media Access Function is capable of consuming QoS monitoring results (based on interrogating the capabilities of the Media Access Function) it shall set a QoS monitoring enabled flag accordingly in the Dynamic Policy activation. If successful, the Media Session Handler subscribes to receive notifications from the 5GMSu AF about QoS monitoring results for this Dynamic Policy.

QoS monitoring request. The 5GMSu AF invokes the Npcf_PolicyAuthorization service or the Nnef_AFsessionWithQoS service with the requested QoS monitoring configurations if the QoS monitoring enabled flag is set to true in the Dynamic Policy request from the Media Session Handler. In the case where the 5GMSu AS is deployed in the Edge DN, the 5GMSu 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 uplink media streaming. Following the QoS monitoring request(s):

The PCF may expose the QoS monitoring results to the 5GMSu 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 5GMSu 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 5GMSu AF sends the notifications of the QoS monitoring results to the Media Session Handler via reference point M5u.

The Media Session Handler provides QoS monitoring results to the Media Stream Handler at reference point M11u.

The Media Stream Handler may use the notified QoS monitoring results to modify its behaviour. For example, in the case of uplink media streaming, the Media Player may use the monitored packet latency, congestion status, etc. to determine the bit rate of the uplink streaming.

The 5GMSu AF may provide the QoS monitoring results to the 5GMSu AS at reference point M3u.

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