5G PDU Session Release Procedure Explained

Introduction

The PDU Session Release procedure in 5G networks is used to terminate an existing PDU session between the User Equipment (UE) and the Data Network (DN).

When a PDU session is released:

• the IP connectivity is removed
• user plane tunnels are deleted
• QoS flows are released
• network resources are freed

This procedure is defined by the 3rd Generation Partnership Project in:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol

When PDU Session Release Happens

A PDU session may be released for several reasons.

UE Initiated

The UE releases the session when:

• an application disconnects
• the device disables data
• the UE powers off

Network Initiated

The network may release the session due to:

• policy enforcement
• network congestion
• subscription limitations
• mobility events

Network Functions Involved

The procedure involves several 5G Core Network Functions.

UE (User Equipment)

Initiates or responds to the release procedure.

gNB

Handles radio signaling and forwards control-plane messages.

AMF (Access and Mobility Management Function)

Acts as the signaling anchor between UE and SMF.

SMF (Session Management Function)

Controls the lifecycle of the PDU session and coordinates session release.

UPF (User Plane Function)

Handles the removal of user plane forwarding rules.

Interfaces Used

PDU Session Release Call Flow

Below is the simplified signaling sequence.

UE           gNB           AMF            SMF            UPF
 |            |             |              |              |
 |--PDU Session Release Request---------->|
 |            |----N2-------------------->|
 |            |             |----N11----->|
 |            |             |              |----N4----->|
 |            |             |              |<---N4-----|
 |            |             |<---N11------|
 |<--PDU Session Release Command---------|
 |--PDU Session Release Complete-------->|

After this procedure finishes, the PDU session context is removed from the network.

Step-by-Step Explanation

Step 1: PDU Session Release Request

The UE sends a PDU Session Release Request NAS message to the network.

The message contains:

• PDU Session ID
• release cause

Important parameters to check

Engineers should verify:

• PDU Session ID
• release cause
• UE state

Step 2: Session Management Processing

The AMF forwards the request to the SMF.

The SMF:

• identifies the session context
• prepares the session release
• instructs the UPF to delete forwarding rules

Important parameters to check

Check:

• SMF session context
• policy rules
• subscriber data

Step 3: UPF Resource Release

The SMF updates the UPF via the N4 interface.

The UPF removes:

• packet detection rules (PDR)
• forwarding action rules (FAR)
• QoS enforcement rules (QER)

This effectively stops the user plane traffic.

Important parameters to check

Verify:

• UPF rule deletion
• tunnel removal
• resource cleanup

Step 4: PDU Session Release Command

The network sends a PDU Session Release Command to the UE.

This message informs the UE that the session is being terminated.

Important parameters to check

Look for:

• session ID
• release cause
• NAS security protection

Step 5: Release Complete

The UE confirms the procedure by sending PDU Session Release Complete.

After this step:

• the session is fully removed
• the UE no longer has IP connectivity for that session

Common Release Causes

Troubleshooting PDU Session Release

Session Release Fails

Possible causes:

• incorrect session ID
• SMF context mismatch
• NAS signaling errors

User Plane Not Released

Possible reasons:

• UPF rule deletion failure
• N4 signaling problem
• tunnel cleanup issue

Session Reappears Automatically

Possible causes:

• application reconnect
• UE auto-connect feature
• policy-based session re-establishment

Key NAS Messages

Relevant 3GPP Specifications

The procedure is defined in the specifications from the 3rd Generation Partnership Project.

Important documents include:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol
• 3GPP TS 38.300 - NR Overall Description

Summary

The PDU Session Release procedure terminates the data session between the UE and the network.

It ensures that:

• user plane tunnels are removed
• QoS flows are released
• network resources are freed

This procedure plays an important role in efficient resource management in 5G networks.