What does the D/C bit mean in SDAP?

The D/C field is 1 bit. A value of 1 means Data PDU and a value of 0 means Control PDU.

Are RQI and RDI used in uplink the same way as in downlink?

Their main behavioral significance is tied to downlink SDAP header processing, where they are used for reflective QoS related handling at the UE.

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5G SDAP Header Fields: QFI, RQI, RDI, and Bit Layout

Q: How long is the QFI field in the SDAP header?

The QFI field is 6 bits long and identifies the QoS flow to which the SDAP PDU belongs.

The SDAP header is a compact field set used in header-bearing SDAP data PDUs for downlink, uplink, and unicast sidelink. The key fields are the D/C bit, QFI, the reserved R bit, RQI, RDI, and the sidelink PQFI context.

Quick facts

Technology	5G NR
Area / Protocol	SDAP header fields and bit layout
Primary baseline	3GPP TS 37.324
Main focus	D/C, QFI, reserved bit, RQI, RDI, PQFI context, and DL or UL or SL header-bearing formats
Main distinction	Header-bearing SDAP data PDUs vs headerless SDAP data PDUs and control PDUs
Behavior link	RDI and RQI connect the SDAP header to reflective QoS behavior in downlink processing

What This Page Covers

This reference page covers the SDAP header parameter set, including field length, field meaning, placement in header-bearing SDAP PDUs, and how the same fields are interpreted in downlink, uplink, and unicast sidelink contexts.

SDAP Header Overview

The SDAP header appears only in the SDAP data PDU formats where header presence is configured. These are the DL SDAP data PDU with header, the UL SDAP data PDU with header, and the SL SDAP data PDU with header for unicast NR sidelink communication.

Headerless SDAP data PDUs do not carry any SDAP header fields. For groupcast and broadcast sidelink communication, only the headerless SDAP data PDU is supported.

Generic SDAP Header Bit Layout

The exact field order depends on the PDU format, but the SDAP header family is built from a compact parameter set: D/C, QFI or PQFI context, reserved bit R, RQI, and RDI.

DL SDAP header

UL SDAP header

SL unicast SDAP header

D/C Bit

The D/C field is 1 bit long and indicates whether the SDAP PDU is a data PDU or a control PDU.

D/C value	Meaning
0	Control PDU
1	Data PDU

QFI Field

The QFI field is 6 bits long and identifies the QoS flow to which the SDAP PDU belongs. This is the central SDAP header field for normal Uu QoS flow identification.

In reflective handling, the UE processes the QFI in the downlink SDAP header to determine which QoS flow is associated with the received packet and whether mapping state should be refreshed.

Reserved Bit

The SDAP header includes a reserved field identified as R. It is a structural part of the header and should be shown explicitly in bit-layout diagrams instead of being silently omitted.

Because this is a reserved field, it is best treated here as a defined header position rather than as an active trigger for standalone procedure behavior.

RQI Field

The RQI field carries the Reflective QoS Indication. In practical SDAP reading, this is the header-level signal that tells the receiving UE that reflective QoS information is relevant for the received flow.

In downlink processing, a received SDAP data PDU with RQI = 1 causes the SDAP entity to inform the NAS layer of the corresponding RQI and QFI.

RDI Field

The RDI field carries the Reflective QoS Flow to DRB Mapping Indication. This is the header-level trigger for reflective mapping actions at the UE.

In downlink processing, a received SDAP data PDU with RDI = 1 causes the SDAP entity to process the QFI field, determine the corresponding QoS flow, and update the uplink mapping rule based on the received downlink DRB context.

PQFI in Sidelink Context

For NR sidelink, the SDAP field family extends to the PQFI, or PC5 QoS Flow Identifier. This is the sidelink counterpart to QFI and belongs to the unicast sidelink SDAP header context rather than the normal Uu header context.

When reading sidelink SDAP traces, it is helpful to think of PQFI as the flow identifier that ties the SDAP PDU to PC5 QoS treatment in the same way QFI ties Uu SDAP PDUs to QoS flow treatment.

DL vs UL vs SL Header Comparison

Header context	Main flow identifier	Reflective fields relevant
DL SDAP header	QFI	RQI and RDI are behaviorally important
UL SDAP header	QFI	Mainly identifies the uplink QoS flow on the transmitted PDU
SL unicast SDAP header	PQFI context	Sidelink-specific header use

How Header Fields Connect to SDAP Behavior

QFI identifies the QoS flow carried by the SDAP PDU.
RDI triggers reflective QoS flow to DRB mapping update behavior.
RQI triggers SDAP-to-NAS notification of reflective QoS information.
D/C distinguishes normal SDAP data PDUs from control PDUs such as End-Marker.

Reference Examples

Example 1: DL reflective packet

A DL SDAP data PDU arrives with header present, carrying QFI 9 and RDI = 1. The UE processes the QFI, identifies the QoS flow, and updates the reflective uplink QoS flow to DRB mapping state.

Example 2: DL RQI notification case

A DL SDAP data PDU arrives with RQI = 1. The SDAP entity informs the NAS layer of the RQI and QFI associated with the packet.

Example 3: Control PDU discrimination

A received SDAP PDU with D/C = 0 is interpreted as a control PDU rather than as a normal SDAP data PDU.

Key Specification References

3GPP TS 37.324 - primary SDAP header field, parameter, and format specification
3GPP TS 37.324 clause 5.3.2 - reflective QoS flow to DRB mapping behavior using RDI and QFI
3GPP TS 37.324 clause 5.4 - RQI handling behavior and NAS notification