Reflective QoS
A mechanism where the UE derives uplink QoS rules by observing the QoS markings of downlink packets, avoiding explicit signaling for uplink QoS flow-to-DRB mapping.
Reflective QoS is a neat trick for cutting signalling. Normally the network has to tell the UE how to map each uplink packet to a QoS flow. With reflective QoS it skips that: the UE watches the QFI marking on incoming downlink packets for a given flow and infers that its matching uplink packets — same five-tuple, reversed direction — should use the same QoS treatment. The UE builds the uplink rule itself from what it observed.
The network switches this on per packet using a Reflective QoS Indication (RQI) bit, and a timer governs how long a derived rule stays valid before the UE lets it lapse. The win is fewer explicit QoS rules to push down, which matters when an application opens lots of short-lived flows. The thing to keep straight is that it only applies to non-GBR flows and only covers the UE deriving uplink behaviour from downlink — it's not a general substitute for the SMF's explicitly signalled rules.
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