Home / Call Flows / 5G / PDU Session Release

5G PDU Session Release Procedure Call Flow

call-flow 5G NR | 5GC | NAS | SMF | UPF | Session Teardown

5G PDU Session Release is the controlled teardown of an active PDU session so the UE and network can remove data connectivity and free related resources.

The key engineering question is not only whether a release command appeared, but whether the right session disappeared completely across UE, SMF, and UPF.

Introduction

This page explains the end-to-end teardown path for an existing 5G data session, including both UE-initiated and network-initiated release behavior.

For practical troubleshooting, the strongest checks are the target session ID, the release cause, the PFCP cleanup on the UPF side, and the absence of traffic after release.

What Is PDU Session Release in Simple Terms?

What starts the procedure: The UE or network decides an active session should be removed.
What the UE and network want to achieve: Delete the correct session cleanly without leaving stale context behind.
What success looks like: The selected session is removed and related data-plane resources disappear.
What failure means: The wrong session is touched, cleanup is partial, or residual traffic remains.

Why this procedure matters

Release problems cause subtle outages because the session may appear gone in one layer but still exist in another. A structured teardown analysis avoids chasing ghost sessions.

Quick Fact Sheet

Procedure name	5G PDU Session Release Procedure
Domain	5G NR + 5GC session teardown and resource cleanup
Main trigger	UE or network decides an active PDU session should be removed
Start state	UE has an active PDU session with working user-plane state
End state	The selected session is deleted and related resources are released
Main nodes	UE, gNB, AMF, SMF, UPF
Main protocols	NAS, NGAP, PFCP
Main success outcome	The targeted session is removed cleanly and no residual user-plane state remains
Main failure outcome	The release is partial, stale context remains, or the wrong session is removed
Most important messages	PDU Session Release Request, PDU Session Release Command, PDU Session Release Complete
Main specs	TS 23.502, TS 24.501, TS 23.501, TS 29.244

5G PDU Session Release procedure flow across UE, gNB, AMF, SMF, and UPF

Preconditions

A valid active PDU session exists before release begins.
The network can correlate the release action to the intended session context.
SMF and UPF can remove the session state successfully.
The UE can process release signaling and remove the session locally.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Requests release or confirms network-initiated release for the selected PDU session.
gNB	Relays control signaling and tears down access-side handling tied to the session.
AMF	Provides the signaling anchor between UE and SMF during session removal.
SMF	Owns the decision to release the session and coordinates removal of session context.
UPF	Deletes forwarding and enforcement rules associated with the session.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> gNB	Carries the release-related signaling while the access path still exists.
N1	UE <-> AMF	Carries release request, command, and completion confirmation.
N2	gNB <-> AMF	Keeps access-side state aligned with session removal.
N11	AMF <-> SMF	Coordinates teardown of the selected PDU session context.
N4 / N3	SMF <-> UPF and gNB <-> UPF	Remove forwarding state and stop user-plane transport for the session.

End-to-End Call Flow

UE                gNB                AMF                SMF                UPF
|                  |                  |                  |                  |
|-- Release Request ----------------->|----------------->|----------------->|
|                  |                  |                  |-- delete rules -->|
|                  |                  |                  |<-- cleanup done --|
|<-- Release Command -----------------|<-----------------|                  |
|-- Release Complete ---------------->|----------------->|----------------->|
|==== no user-plane traffic should remain on released session ==========>|

Major Phases

Phase	What happens
1. Release need appears	The UE or network determines the session is no longer required or no longer allowed.
2. Release signaling	The selected session is identified and a release exchange begins.
3. Core teardown	The SMF and UPF remove session, forwarding, and QoS state.
4. UE confirmation	The UE confirms the session has been removed from its local context.
5. Post-release validation	Logs and traffic confirm the session is actually gone and resources are cleanly freed.

Step-by-Step Breakdown

The session selected for removal is identified

Sender -> receiver: UE or network policy logic

Message(s): Release trigger and target session identification

Purpose: Make sure the correct active session is being removed.

State or context change: The network correlates release intent with a specific PDU Session ID.

Note: Always prove which session is being released before interpreting the rest of the trace.

Release signaling starts

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

Message(s): PDU Session Release Request or PDU Session Release Command

Purpose: Initiate controlled removal of the selected session.

State or context change: The session moves from active service toward explicit teardown.

Note: UE-initiated and network-initiated releases share most of the teardown logic after the trigger.

The SMF removes core session state

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

Message(s): Session release handling and PFCP deletion

Purpose: Delete the user-plane and policy context for the session.

State or context change: The session stops being a valid transport path in the core.

Note: Stale UPF rules after a clean NAS release are a common operational trap.

The UE confirms removal

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

Message(s): PDU Session Release Complete

Purpose: Tell the network that the UE removed the session locally.

State or context change: The release becomes complete from both UE and network viewpoints.

Note: Without Complete, you may have a half-finished release that leaves confusing traces later.

Residual traffic and state are checked

Sender -> receiver: UE, gNB, UPF, service monitoring

Message(s): Post-release traffic and context observation

Purpose: Prove the session is fully gone and not silently lingering.

State or context change: The network returns to a clean post-release state.

Note: Residual user-plane traffic usually means incomplete cleanup, not a mysterious new failure.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
PDU Session Release Request	NAS	UE -> SMF via AMF	Starts a UE-initiated release of the selected session.	Inspect the session ID and release reason first.
PDU Session Release Command	NAS	SMF -> UE via AMF	Instructs the UE to remove the session.	Useful for network-initiated release and cleanup validation.
PDU Session Release Complete	NAS	UE -> SMF via AMF	Confirms the UE removed the session locally.	Use it to confirm the teardown reached the UE side fully.
PFCP session deletion	PFCP	SMF <-> UPF	Deletes forwarding and QoS enforcement state in the UPF.	Critical for catching stale N3/N4 context after release.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
PDU Session ID	Identifier of the session being removed.	Request, Command, and Complete	Keeps the teardown tied to the correct active session.	Wrong ID creates severe correlation errors.
Release cause	Reason for session teardown.	Request or network decision path	Helps explain whether the release is user-driven, policy-driven, or failure-driven.	Missing cause hides the real story.
PFCP rule deletion state	UPF cleanup related to the released session.	N4 teardown	Proves whether data forwarding was actually removed.	Residual rules cause phantom traffic.
UE local session state	Whether the UE still believes the session exists.	After Command and Complete	Important when the network looks clean but the UE does not.	Creates confusing retry or reuse behavior.
Traffic after release	Any packets still crossing the old path.	Post-release monitoring	Best final proof that the session is really gone.	Residual packets mean incomplete cleanup.

Success Criteria

The correct PDU session is selected for release.
The UE and network both remove session state cleanly.
PFCP deletion removes forwarding and enforcement state in the UPF.
No residual traffic or stale session behavior remains afterward.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Wrong session is released	The release was correlated to the wrong PDU Session ID.	Session ID in Request, Command, and all related context.	Release Request or Command	N1, N11	Always start here when the service impact looks larger than expected.
Release signaling completes but UPF state remains	NAS teardown succeeded but user-plane cleanup was partial.	PFCP deletion state and residual traffic.	Release Complete	N4, N3	This is a classic stale-context problem.
UE never confirms release	The command reached the UE poorly or local cleanup failed.	Command delivery, NAS integrity, and UE behavior.	Release Command, Release Complete	N1, radio	Separate command-delivery issues from core cleanup issues.
Application still sends traffic after release	Another session exists, or cleanup is incomplete, or the application is mis-correlated.	PDU Session ID, post-release traffic path, and application binding.	Post-release traffic	N3, service layer	Do not assume all post-release traffic belongs to the released session.

What to Check in Logs and Traces

Validate the PDU Session ID before anything else.
Use the release cause to understand why the teardown happened.
Inspect PFCP deletion to confirm UPF cleanup really occurred.
Check for residual N3 traffic after release to catch stale context.
If the UE still behaves as if the session exists, compare Release Command and Release Complete handling closely.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

The release is only finished when residual state is gone. Message completion without UPF cleanup is still an operational failure.

FAQ

What is 5G PDU Session Release?

It is the procedure that removes an active PDU session and frees the related network resources.

Can it be initiated by both UE and network?

Yes. The UE may request it, or the network may command it.

What proves success?

The correct session is removed, PFCP state is deleted, and no residual traffic remains on that path.

What is the main troubleshooting trap?

Assuming NAS success means the user plane is clean. You still need to validate UPF cleanup.

What should I inspect first?

Start with the PDU Session ID, the trigger for release, and PFCP deletion state.