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5G QoS Flow Release Call Flow

call-flow 5G NR | 5GC | QoS | QFI | Session Modification

5G QoS Flow Release removes a dedicated QoS treatment from an active PDU session when that service-specific behavior is no longer needed.

The important distinction is that the session may survive even though one of its dedicated flows is withdrawn.

Introduction

This procedure is best understood as the cleanup counterpart to QoS Flow Establishment and QoS Flow Modification.

In practical traces, the engineer needs to prove three things: which QFI was removed, whether fallback traffic behavior is correct, and whether the gNB plus UPF truly deleted the old state.

What Is QoS Flow Release in Simple Terms?

What starts the procedure: The network decides a dedicated QoS flow should be removed.
What the UE and network want to achieve: Withdraw one flow cleanly while preserving the intended remaining session behavior.
What success looks like: The released QFI disappears and packets follow the updated policy correctly.
What failure means: Old flow state survives or post-release traffic behaves incorrectly.

Why this procedure matters

QoS flow cleanup is easy to underestimate. Many service issues after policy changes come from half-removed dedicated flows that still influence packet treatment somewhere in the path.

Quick Fact Sheet

Procedure name	5G QoS Flow Release
Domain	5G NR + 5GC QoS cleanup within an active PDU session
Main trigger	A dedicated QoS flow is no longer needed or is withdrawn by network policy
Start state	A PDU session has one or more active QoS flows beyond the default behavior
End state	The targeted QoS flow is removed while the parent PDU session may remain active
Main nodes	UE, gNB, AMF, SMF, UPF, PCF
Main protocols	NAS, NGAP, PFCP, policy control
Main success outcome	The flow disappears cleanly and packets stop using the released QFI
Main failure outcome	The flow appears removed in signaling but residual classification or enforcement state survives
Most important messages	PDU Session Modification Command/Complete, PFCP update, resource update
Main specs	TS 23.501, TS 23.502, TS 24.501, TS 29.244

Preconditions

A PDU session with an active dedicated QoS flow already exists.
The network has identified the exact QFI or rule set to remove.
The UE can accept an updated QoS rule set.
The gNB and UPF can delete flow-specific state without destabilizing the remaining session.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Removes the released QoS flow from its local rule set and stops mapping traffic to that QFI.
gNB	Updates radio-side treatment so the released flow no longer consumes dedicated handling.
AMF	Relays the session modification that removes the QoS flow.
SMF	Owns the decision to withdraw the QoS flow and coordinates cleanup.
UPF	Deletes the forwarding and enforcement state tied to the released flow.
PCF	May trigger withdrawal when policy conditions or service needs change.

Interfaces used

Interface	Path	Role
N1	UE <-> AMF	Carries the session modification that removes the flow.
N2	gNB <-> AMF	Aligns the access-side context with the reduced QoS model.
N11	AMF <-> SMF	Coordinates the removal decision.
N4	SMF <-> UPF	Deletes flow-specific enforcement state.
NR-Uu / N3	UE <-> gNB and gNB <-> UPF	Show whether traffic really stopped using the released flow.

End-to-End Call Flow

UE                gNB                AMF                SMF / PCF           UPF
|                  |                  |                    |                  |
|                  |                  |<-- flow withdrawal decision ----------|
|<-- Modification Command ------------|------------------->|-- delete rules -->|
|-- Modification Complete ----------->|                    |<-- PFCP ack ------|
|==== packets should stop using released QFI and follow new rules ==========>|

Major Phases

Phase	What happens
1. Flow withdrawal trigger	Policy or service logic decides the dedicated QoS flow should be removed.
2. Session modification signaling	The UE is told the flow or its rules are being withdrawn.
3. User-plane cleanup	UPF and gNB remove flow-specific treatment.
4. UE confirmation	The UE confirms the update and stops using the flow.
5. Post-release validation	Traffic proves the released flow is gone and the remaining session is stable.

Step-by-Step Breakdown

The network decides the QoS flow is no longer needed

Sender -> receiver: Service logic / PCF -> SMF

Message(s): Internal policy trigger or service-end condition

Purpose: Select the exact flow that should be removed while leaving the PDU session intact if appropriate.

State or context change: The session enters a targeted QoS cleanup path rather than full session release.

Note: It is important to separate dedicated-flow removal from full PDU session teardown.

SMF prepares the updated rule set

Sender -> receiver: SMF <-> UPF / AMF context

Message(s): QoS rule withdrawal and session modification preparation

Purpose: Define how the UE, gNB, and UPF should behave once the flow disappears.

State or context change: The network now has an updated session model without the released flow.

Note: This is the moment to verify which QFI is being removed and what traffic should fall back to afterward.

UE receives the release through session modification

Sender -> receiver: SMF -> AMF -> UE

Message(s): PDU Session Modification Command

Purpose: Deliver the new rule set with the targeted flow removed.

State or context change: The UE starts moving from the old flow layout to the reduced one.

Note: The command should make it clear whether the traffic now maps to a default flow or simply stops flowing.

UPF and gNB clean up the dedicated treatment

Sender -> receiver: SMF <-> UPF and AMF <-> gNB

Message(s): PFCP update and access-side resource adjustment

Purpose: Remove residual user-plane and radio-side state tied to the released flow.

State or context change: The dedicated QoS behavior should now disappear end to end.

Note: Residual enforcement is the most common reason these procedures look successful but behave badly later.

Traffic is validated after the release

Sender -> receiver: UE <-> gNB <-> UPF

Message(s): PDU Session Modification Complete and post-update packets

Purpose: Confirm that the released flow is gone and the remaining session behavior is correct.

State or context change: The session stabilizes with a reduced QoS configuration.

Note: Look for packets that still use the old QFI or appear with the wrong fallback treatment.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
PDU Session Modification Command	NAS	SMF -> UE via AMF	Carries the removal of the dedicated QoS flow.	Inspect which QFI or QoS rule was deleted.
PDU Session Modification Complete	NAS	UE -> SMF via AMF	Confirms the UE applied the updated rule set.	Check whether the UE acknowledged the intended flow removal.
PFCP session update	PFCP	SMF <-> UPF	Removes forwarding or enforcement state tied to the flow.	Best proof of data-plane cleanup.
PDU Session Resource Modify Request	NGAP	AMF -> gNB	Adjusts access-side resources after flow removal.	Shows whether the RAN was told to reduce dedicated handling.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Released QFI	Identifier of the QoS flow being removed.	Modification command and traces	Main anchor for correlating cleanup.	Wrong QFI interpretation breaks the whole analysis.
Updated QoS rules	Rule set after removal.	Modification command	Shows where traffic should go after the release.	Missing fallback understanding causes false alarms.
PDU Session ID	Parent session that remains active.	All related messages	Distinguishes flow release from full session release.	Easy to confuse when many sessions are active.
PFCP cleanup state	UPF-side removal of flow-specific logic.	N4 trace	Confirms the flow really disappeared in the user plane.	Residual state creates phantom QoS behavior.
Post-release traffic mapping	Where the application packets go after cleanup.	Live traffic traces	Validates the operational outcome of the release.	Traffic may fall back incorrectly or stop entirely.

Success Criteria

The correct QoS flow is selected for removal.
The UE applies the reduced rule set successfully.
gNB and UPF delete the dedicated treatment tied to the flow.
Live traffic no longer uses the released QFI and shows the intended fallback behavior.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
The released flow still appears in packet traces	The UE, gNB, or UPF kept stale state after the control-plane update.	Released QFI and PFCP cleanup.	Modification Command / Complete	N1, N4, N3	Residual QFI use is the clearest sign of incomplete release.
Traffic falls back to the wrong QoS treatment	Rule recalculation after release does not match the intended service behavior.	Updated rule set and packet classification.	Modification Command	N1, packet trace	This is often a design or filter problem rather than a signaling failure.
The whole PDU session becomes unstable after flow release	The removed dedicated flow was carrying more critical traffic than expected, or cleanup touched the wrong context.	PDU Session ID, filters, and live service behavior.	Modification flow	N1, N3	Verify the parent session was not damaged accidentally.
UE confirms release but gNB or UPF still enforces old behavior	The NAS path succeeded but access or user-plane cleanup lagged behind.	NGAP and PFCP updates after Complete.	Modification Complete	N2, N4	Treat this as incomplete convergence, not success.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is QoS Flow Release?

It is the targeted removal of a dedicated QoS flow while the parent PDU session may continue running.

How is it different from PDU Session Release?

QoS Flow Release removes one service treatment inside the session, while PDU Session Release tears down the entire session.

What proves success?

The released QFI disappears from rules and live traffic, and the remaining session behaves as expected.

What should I inspect first?

Start with the released QFI, updated QoS rules, and PFCP cleanup state.

Why is fallback behavior important?

Because traffic often moves to another flow after release, and that new behavior must still be correct.