5G Reflective QoS, RQI, RDI, and RQ Timer

Quick facts

What Reflective QoS Means in 5G

Reflective QoS is a 5GS QoS behavior where the UE can derive uplink QoS flow treatment from downlink information instead of relying only on explicit configuration. In SDAP terms, this appears as reflective QoS flow to DRB mapping at the UE.

The 5GS architecture specification defines Reflective QoS as part of the QoS framework, and related service models expose a ReflectiveQosAttribute with values such as RQOS and NO_RQOS.

Where Reflective Behavior Appears in SDAP

In SDAP, reflective QoS flow to DRB mapping is a UE-side function tied to downlink SDAP processing on DRBs that are configured with SDAP header presence.

That means reflective behavior is not a standalone control-plane feature inside SDAP. It is triggered during downlink packet processing when the UE receives SDAP data PDUs with the relevant header information.

RQI and RDI in SDAP

RQI

RQI is the Reflective QoS Indication carried in SDAP-related handling. It tells the UE that reflective QoS behavior is relevant for the received flow.

RDI

RDI is the Reflective QoS Flow to DRB Mapping Indication. It is used together with SDAP header processing to indicate that the UE should apply or update reflective QoS flow to DRB mapping state.

When the UE Creates Reflective QoS Flow to DRB Mapping

The UE creates or refreshes reflective QoS flow to DRB mapping when it receives downlink SDAP data on a DRB with SDAP header present and the downlink indication set implies reflective behavior for that QoS flow.

In practical terms, the UE uses the downlink QFI context together with the receiving DRB and reflective indications to derive the uplink mapping that should later be used for packets belonging to the same flow.

When Reflective State Is Updated or Restarted

Reflective state is not just created once. It can be updated when later downlink SDAP data indicates the same QoS flow on a DRB with reflective handling, and the UE refreshes the associated validity timer.

This is why the RQ timer matters: reflective mapping is meant to remain valid only while recent downlink behavior continues to justify that UE-side derived rule.

RQ Timer and T3583

The RQ timer controls how long the UE keeps reflective QoS information active. In NAS behavior, this is tied to the timer model around T3583, which is part of 5GS QoS rule handling.

The timer ensures that UE-derived reflective mapping does not remain valid forever after a single downlink indication. Once the timer expires, the UE removes or stops using the reflective rule unless newer downlink information has refreshed it.

Relationship Between NAS QoS Rules and SDAP Reflective Mapping

Reflective QoS is not purely an SDAP-local invention. The 5GS QoS framework defines whether traffic may be subject to Reflective QoS, while SDAP is the user-plane layer that turns downlink packet indications into actual UE-side flow-to-bearer behavior.

A good reference reading order is: 5GS QoS architecture first, then NAS QoS rule or timer behavior, and finally SDAP header-driven mapping behavior.

Reflective QoS Limits and Non-Applicable Cases

• reflective QoS flow to MRB mapping is not supported
• reflective QoS is not supported over the PC5 interface for sidelink
• without suitable downlink SDAP header processing, reflective mapping cannot be derived in SDAP

Practical Reflective QoS Flow

1. the network marks a QoS flow as eligible for Reflective QoS at the 5GS level
2. the UE receives downlink SDAP data on a DRB configured with SDAP header presence
3. the DL SDAP header carries QFI and reflective indications such as RQI and RDI
4. the UE derives or updates reflective QoS flow to DRB mapping for that flow
5. the UE starts or refreshes the RQ timer associated with the reflective state
6. later uplink packets for that QoS flow can use the UE-derived mapping while the timer remains valid

Reference Examples

Example 1: Reflective mapping creation

A UE receives DL SDAP data with SDAP header present on DRB 3 for QFI 7 together with reflective indications. The UE stores a reflective mapping so later UL traffic for QFI 7 can use DRB 3 while the RQ timer is valid.

Example 2: Reflective mapping refresh

More DL packets for the same QFI arrive later on the same DRB with reflective handling. The UE refreshes the reflective state and restarts or extends the relevant timer context.

Example 3: Timer expiry

No qualifying DL packets arrive before the timer expires. The UE stops using the stale reflective rule and falls back to other valid mapping behavior.

Key Reflective QoS Concepts

Key Specification References

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