pping, the UE maps that packet to the default DRB of the PDU session. Within each PDU session, it is up to NG-RAN to decide how to map multiple QoS flows to a DRB. The NG-RAN may map a GBR flow and a non-GBR flow, or more than one GBR flow to the same DRB. The time when a non-default DRB between NG-RAN and UE is established for QoS flow can be different from the time when the PDU session is established. It is up to NG-RAN to decide when non-default DRBs are established. In dual connectivity scenarios, the QoS flows belonging to the same PDU ses- sion can be mapped to different bearer types and, consequently, there can be two different SDAP entities configured for the same PDU session, that is, one for MCG and another one for SCG [16]. 3-Drive and insert 2-N3 marking in 1-RQI marking UE packet filters packet filter encapsulation header downlink packet for applying uplink QoS RQI 1 IP packet (III) QoS flows Radio bearers NG-RAN Server 4-Uplink packet for the same QoS flow Figure 1.87 Illustration of the reflective QoS concept (example) [82]. 5G Network Architecture 177 As we mentioned earlier, the 5G QoS model is based on QoS flows. The 5G QoS model supports both QoS flows that require GBFR QoS flows and QoS flows that do not require guaranteed flow bit rate (non-GBFR QoS flows). The 5G QoS model also supports reflec- tive QoS. The QoS flow is the finest granularity to differentiate QoS classes in the PDU ses- sion. A QFI is used to identify a QoS flow in the 5G system. User-plane traffic with the same QFI within a PDU session receives the same traffic forwarding treatment (e.g., sched- uling and admission control). The QFI is carried in an encapsulated header format on N3 (and N9), that is, without any changes to the end-to-end packet header. The QFI is used for all PDU session types and is unique within a PDU session. The QFI may be dynamically assigned or may be equal to the 5QI. Monitoring of user-plane traffic (e.g., MFBR enforce- ment) is not considered as QoS differentiation and is done by UPFs on an SDF-lev