DSCP Marking for Downlink and Uplink Packets

Transport-level marking is the process of marking traffic at the UPF with a Differentiated Services Code Point (DSCP) value. The transport-level marking, executed on per-QoS flow, is based on the mapping from the 5QI and optional Allocation and Retention Policy (ARP) configuration from the SMF.

The SMF controls the transport-level marking by providing the DSCP in the ToS (IPv4) or Traffic Class (IPv6) within the "Transport Level Marking" IE in the FAR, that is associated to the PDR matching the traffic to be marked. The UPF performs the transport level marking for the detected traffic and sends the marked packet to the peer entity. The SMF can change the transport-level marking by changing the "Transport Level Marking" IE in the related FAR.

The UPF also supports the inner packet marking in which it marks the tunnel packets. As the 3GPP specification does not determine any specific IE, the UPF uses a private IE named "Inner Packet Marking".

In addition, there is also a provision to copy the DSCP of inner packet to the outer IP header. As the 3GPP specification does not determine any specific IE, the UPF uses a private IE named "Transport Level Marking Options".