 header extension of the GTP-U (GPRS Tunnel Protocol - User Plane) protocol. The gNB base station associates each QFI flow of a PDU session with the corresponding DRB radio bearer. In the downlink direction, to apply the reflective QoS at the mobile level, the RQI (Reflective QoS Indication) is set to 1 (see Figure 8.3). The RQI identifier is transmitted from the UPF to the gNB entity in the header extension of the GTP-U protocol. The SDAP header of the frame transmitted by the base station contains the QFI identifier. The SDAP frame is transmitted, for each bearer, to the PDCP sublayer corresponding to the bearer. In the uplink direction, the base station applies the routing rules to transmit the IP packet towards the UPF. 8.1.2. The protocol structure The structure of the protocol concerns the transmission and reception function. Both are implemented at the gNB and at the UE. The SDAP entity transmits the flows to the lower layers. Depending on the NAS configuration, the entity SDAP adds or not a header (see Figure 8.4). The SDAP entity receives the frames from the PDCP layer. Figure 8.4 on the left describes the SDAP header added by the base station for the transmission of traffic towards the UE and the reception of traffic from the UE. 5G-NR Radio Interface - Data Link Layer Figure 8.4 on the right describes the SDAP header added by the UE for transmission towards the gNB and reception from the gNB. The SDAP entity is not compelled to add an SDAP header. Octet 1 Octet 1 Octet 2 Octet 2 Octet n Octet n Figure 8.4. SDAP frame structure RDI (Reflective QoS flow to DRB mapping Indication): when this bit is coded to 1, the mobile updates the matching rule. RQI (Reflective QoS Indication): when this bit is coded to 1, the mobile informs the NAS layer to update the matching rules. D/C (Data/Control): this bit indicates whether the frame contains SDAP frames (bit at 1) or control messages (bit at 0). 8.2. PDCP PDCP (Packet Data Convergence Protocol) is used for RRC (Radio Resource Control) signaling messages, relativ