Quality of Service
Quality of service in a 5G network is driven by QoS flows. A QoS flow represents the finest granularity of QoS differentiation in the protocol data unit (PDU) session. Each QoS flow is identified by a unique identifier called QoS Flow Identifier (QFI) and by QoS parameters that describe the characteristics of the packet flow. The session management function (SMF) whichcontrols the QoS and passes QoS characteristics to the user plane function (UPF) and the Access Gateway Function (AGF) when the PDU session is being established. The UPF enforces the QoS flows for a particular PDU session, and the AGF manages the aggregate of all the QoS flows going to the residential gateway (RG).
The 5G core (5GC) supports up to 64 QFIs for a single PDU session. The QoS flow parameters include the 5G QoS identifier (5QI). 5QI maps to the well-defined QoS characteristics, such as priority level, averaging window, maximum data burst volume, and so on. 3GPP Specification 23.501 defines how the standard 5QI values map to the QoS characteristics mappings.
The UPF identifies the QoS flows by sending a QFI, but the AGF does not use QFI to classify the packets. The AGF classifies the traffic by using the Differentiated Services code point (DSCP) in the GPRS tunneling protocol (GTP) header that the UPF sends over the N3 interface.. The SMF sends the Transport Level Marking (TLM) to the UPF. The UPF uses the TLM to mark up the GTP header in the outgoing N3 packets to the AGF. After classifying the packets (by using DSCP), the AGF uses the standard Junos class of service (CoS) classification configuration to classify and to shape the traffic.
The BBF standards specify that operators who use the per subscriber CoS parameters per household, should use Router Gateway Level Wireline Access Characteristics (RG-LWAC) to define the CoS limits and scope of service for the specified subscriber or a group of subscribers on a router. To configure the QoS characteristics of legacy access networks, the AGF uses the RG-LWAC information from the 5GC to set CoS queue shaping, firewall filters, and policers. The AGF leverages the existing shaping, policers, and firewall filters on Junos OS. The AGF uses the existing rewrite rules to set the PCP or DSCP value in the packet header sent to the access network.
For downstream traffic, the UPF sets the DSCP value based on the packet priority that was set by the 5GC and sends it to the AGF. The AGF uses hierarchical policers to shape downstream traffic. For this task, the AGF performs the following:
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Maps the outer DSCP value to a forwarding class.
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Assigns the forwarding class to an output queue.
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Assigns the forwarding class to the scheduler based on the forwarding class priority.
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Applies the RG-LWAC or the local configuration to classify the traffic and to rewrite the PCP or DSCP value when needed.
For more information on CoS, see CoS for Subscriber Access Overview.