5G PDU Session Modification Procedure Explained

Introduction

The PDU Session Modification procedure in 5G networks is used to update an existing PDU session after it has already been established.

This procedure allows the network or UE to modify session parameters such as:

• QoS flows
• user plane routing
• session policies
• network slice parameters

Unlike PDU Session Establishment, this procedure does not create a new session, but instead modifies the configuration of an existing one.

The 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 Modification Happens

PDU Session Modification may be triggered by either the UE or the network.

Typical triggers include:

• new QoS flow request
• policy changes from the core network
• application bandwidth changes
• network slicing adjustments
• mobility-related session updates

Network Functions Involved

Several core network components participate in this procedure.

UE (User Equipment)

The UE may request session modification when applications require new QoS resources.

gNB

The gNB handles radio configuration updates and forwards control-plane signaling.

AMF (Access and Mobility Management Function)

The AMF forwards signaling messages between the UE and session management entities.

SMF (Session Management Function)

The SMF controls session parameters including:

• QoS flow creation
• policy enforcement
• routing updates

UPF (User Plane Function)

The UPF updates forwarding rules and traffic handling based on the new session configuration.

Interfaces Used

PDU Session Modification Call Flow

Below is the simplified signaling sequence.

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

This procedure updates the existing PDU session configuration.

Step-by-Step Explanation

Step 1: PDU Session Modification Request

The UE sends a PDU Session Modification Request message.

This may request:

• new QoS flow
• removal of QoS flow
• modification of session parameters

Important parameters to check

Engineers should verify:

• PDU Session ID
• requested QoS flow parameters
• DNN (Data Network Name)
• slice information (S-NSSAI)

Step 2: Session Management Processing

The AMF forwards the request to the SMF.

The SMF:

• validates the request
• checks policy rules
• determines if the modification is allowed

Important parameters to check

Check:

• SMF session context
• policy control rules
• QoS authorization

Step 3: UPF Update

The SMF updates the UPF using the N4 interface.

This may include:

• creating new forwarding rules
• updating QoS flows
• modifying packet filtering rules

Important parameters to check

Verify:

• forwarding rules
• QoS Flow Identifier (QFI)
• packet detection rules (PDR)

Step 4: PDU Session Modification Command

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

This message instructs the UE to update session parameters.

The message may contain:

• new QoS flow identifiers
• updated session policies
• traffic rules

Important parameters to check

Look for:

• QFI values
• QoS rule updates
• traffic filter parameters

Step 5: Modification Complete

The UE sends PDU Session Modification Complete.

After this step:

• the new session parameters become active
• the updated QoS flows are applied

Common PDU Session Modification Scenarios

Troubleshooting PDU Session Modification

Modification Request Rejected

Possible causes:

• invalid QoS request
• policy restrictions
• subscription limitations

QoS Flow Not Activated

Possible reasons:

• SMF configuration error
• UPF rule mismatch
• incorrect QFI mapping

Data Path Not Updated

Possible causes:

• N4 signaling failure
• UPF forwarding rule error
• radio bearer mismatch

Key NAS Messages

Relevant 3GPP Specifications

This procedure is defined in:

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

Published by the 3rd Generation Partnership Project.

Summary

The PDU Session Modification procedure allows the 5G Core Network to update session parameters dynamically during an active data session.

This enables:

• dynamic QoS adjustments
• policy enforcement
• optimized data routing

The procedure is essential for adaptive network performance and application-aware traffic management in 5G networks.