5G SDAP End-Marker Control PDU and Flow Remapping

Quick facts

What This Page Covers

This reference page covers the SDAP End-Marker Control PDU and the procedures that cause it to be transmitted during mapping changes. That includes QoS flow to DRB reconfiguration, reflective mapping replacement, and PC5 QoS flow to SL-DRB remapping for unicast sidelink.

What the SDAP End-Marker Control PDU Is

In SDAP, the End-Marker Control PDU is used by the UE SDAP entity to indicate that it stops mapping the SDAP SDU of a given flow to the bearer on which the End-Marker is transmitted.

For Uu operation the flow is identified by QFI. For sidelink context the flow is identified by QFI or PQFI depending on the applicable SDAP context.

Why SDAP Needs End-Marker During Remapping

When a flow moves from one bearer to another, the UE may still have an old mapping state associated with the previous bearer. The End-Marker is the SDAP mechanism used to close that old mapping path cleanly before the new mapping becomes active.

This is why End-Marker belongs with remapping logic, not only with control-PDU format description.

End-Marker in Configured QoS Flow to DRB Remapping

When RRC configures a QoS flow to DRB mapping rule, the SDAP entity may need to send an End-Marker before storing the new rule.

• If no stored mapping rule exists and a default DRB is configured, the UE constructs an End-Marker for the QoS flow, maps it to the default DRB, and submits it to lower layers.
• If the stored uplink QoS flow to DRB mapping rule differs from the configured rule, the UE constructs an End-Marker for the QoS flow, maps it to the DRB according to the old stored rule, and submits it to lower layers.
• After that, the UE stores the configured uplink QoS flow to DRB mapping rule.

End-Marker in Reflective QoS Mapping Updates

Reflective QoS can also trigger End-Marker transmission. For each received downlink SDAP data PDU with RDI = 1, the SDAP entity processes the QFI and may replace the UE-side uplink mapping rule based on the downlink bearer.

• If no stored mapping rule exists and a default DRB is configured, the UE constructs an End-Marker for the QoS flow, maps it to the default DRB, and submits it to lower layers.
• If the stored uplink mapping rule differs from the downlink QoS-flow-to-DRB mapping and the old DRB has uplink SDAP header configured, the UE constructs an End-Marker, maps it to the old DRB, and submits it to lower layers.
• The UE then stores the downlink QoS-flow-to-DRB mapping as the new uplink rule.

End-Marker in PC5 QoS Flow to SL-DRB Remapping

For unicast NR sidelink, End-Marker is also used when the PC5 QoS flow to SL-DRB mapping changes.

• If no stored SL-DRB mapping rule exists and a default SL-DRB is configured, the UE constructs an End-Marker for the PC5 QoS flow, maps it to the default SL-DRB, and submits it to lower layers.
• If the stored PC5 QoS flow to SL-DRB rule differs from the configured rule and the old SL-DRB has SL SDAP header configured, the UE constructs an End-Marker, maps it to the old SL-DRB, and submits it to lower layers.
• The UE then stores the configured PC5 QoS flow to SL-DRB rule.

End-Marker Transmission Logic Summary

1. A new mapping rule is about to replace an old or default mapping path.
2. The UE constructs an End-Marker for the affected QoS flow or PC5 QoS flow.
3. The End-Marker is mapped to the old bearer path that is being stopped.
4. The UE submits the End-Marker to lower layers.
5. The UE stores the new mapping rule.

Uu vs Sidelink End-Marker Comparison

End-Marker Format View

At format level, the End-Marker is a control PDU carrying the flow identity needed to indicate termination of mapping on the bearer where it is sent.

Relationship to DRB and SL-DRB Release

End-Marker is about remapping away from an old bearer path. DRB or SL-DRB release is a separate cleanup action where SDAP removes all mappings associated with a released bearer.

These are related but different concepts: End-Marker signals termination of mapping use, while bearer release removes stored mapping state.

Reference Examples

Example 1: Explicit DRB remap

QFI 5 was previously mapped to DRB 2. RRC reconfigures it to DRB 4. The UE sends an End-Marker on DRB 2, then stores the new DRB 4 mapping rule.

Example 2: Reflective remap from downlink

A DL SDAP data PDU with RDI = 1 indicates that QFI 9 now arrives on a different DRB than the stored uplink rule. The UE sends an End-Marker on the old bearer path, then updates the uplink mapping to match the new downlink bearer mapping.

Example 3: Unicast sidelink remap

A PC5 QoS flow is moved from one SL-DRB to another. The UE sends an End-Marker on the old SL-DRB, then stores the new SL-DRB mapping rule.

Key Specification References

• 3GPP TS 37.324 clause 6.1.2 / 6.2.3 - End-Marker Control PDU definition and format
• 3GPP TS 37.324 clause 5.3.1 - configured QoS flow to DRB remapping behavior using End-Marker
• 3GPP TS 37.324 clause 5.3.2 - reflective QoS remapping behavior using End-Marker
• 3GPP TS 37.324 clause 5.5.1 - PC5 QoS flow to SL-DRB remapping behavior using End-Marker

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