5G NAS - PDU Session Modification Command

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.3.9
Direction	SMF to UE via AMF	Message Type	5GSM signaling

Full message name	5G NAS - PDU Session Modification Command
Protocol	NAS
Technology	5G
Direction	SMF to UE via AMF
Interface	N1
Signaling bearer / channel	NAS signaling / Usually carried inside DL NAS Transport on the access side
Typical trigger	The network has accepted a PDU session modification request, or the SMF needs to update an active PDU session based on policy, QoS, or session-management requirements.
Main purpose	Carries the network-approved session updates so the UE can apply the new QoS, timer, AMBR, or session behavior without creating a new PDU session.
Main specification	3GPP TS 24.501, 8.3.9
Release added	Release 15
Procedures where used	5G PDU Session Modification, Network-requested PDU Session Modification, QoS Flow Establishment, VoNR service adaptation
Related timers	T3591
Related cause values	#39 reactivation requested

What is PDU Session Modification Command in simple terms?

PDU Session Modification Command is the 5GSM message the network sends to tell the UE how an active PDU session should be updated after the modification request is accepted or when the network itself is changing the session.

Carries the network-approved session updates so the UE can apply the new QoS, timer, AMBR, or session behavior without creating a new PDU session.

Why this message matters

PDU Session Modification Command tells the UE how to change an already active data session. It is the network's approved update instruction, not a new session setup.

Where this message appears in the call flow

5G PDU Session Modification

Call flow position: Positive network response that tells the UE what has been modified in the active PDU session.

Typical state: The UE already has an active PDU session and is waiting to apply the network-approved change.

Preconditions:

A PDU session already exists and is active.
The network has accepted the requested modification or decided to update the session itself.

Next likely message: PDU Session Modification Complete

Network-requested PDU Session Modification

Call flow position: Network-initiated update instruction for a live PDU session.

Typical state: The SMF is pushing a change that the UE must apply and acknowledge.

Preconditions:

The session is active.
The SMF has already evaluated the new session parameters.

Next likely message: PDU Session Modification Complete

Call flow position

Previous message(s): PDU Session Modification Request, DL NAS Transport, SMF policy and subscription evaluation

Next message(s): PDU Session Modification Complete, PDU Session Release Command, UE applies updated session parameters

Message direction and transport

Sender and receiver: SMF to UE via AMF

Interface: N1

Domain: Core-side session management update with access-side NAS delivery dependency

Signaling bearer: NAS signaling

Logical channel: Usually carried inside DL NAS Transport on the access side

Transport / encapsulation: 5GSM NAS message transported end-to-end from the SMF to the UE through AMF mediation

Security context: Normally delivered while NAS security is already active, so engineers usually expect protected downlink NAS handling.

ASN.1 Message Syntax for 5G NAS - PDU Session Modification Command

This message is not typically analyzed as ASN.1 on the wire. It is usually read as a NAS or protocol field structure instead.

This is a 5GSM NAS message defined by ordered information elements in 3GPP TS 24.501 rather than ASN.1 syntax.

5G NAS - PDU Session Modification Command - Example Dump

PDU Session Modification Command
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 5
  Message Type: PDU Session Modification Command
  5GSM Cause: Reactivation requested
  Session-AMBR: 100 Mbps
  RQ timer value: 12 seconds
  Always-on PDU session indication: set
  Authorized QoS rules:
    Rule 1:
      QFI: 9
      Packet filter: permit out ip from any to any
      Precedence: 255
  Authorized QoS flow descriptions:
    5QI: 1
    GFBR UL/DL: 128 kbps / 128 kbps
  Extended Protocol Configuration Options:
    DNS IPv4 address: 10.1.1.1

How to read this dump

Start with PDU Session ID and PTI so you correlate the command with the correct active session and request transaction.
Inspect Session-AMBR and the authorized QoS fields because they tell you what the network actually approved.
RQ timer value matters when reflective QoS is in use, especially in voice and service-continuity scenarios.
If the 5GSM cause is #39, the command may be driving a reactivation or anchor-relocation style behavior rather than a simple QoS tweak.
If EPCO is present, verify whether the returned configuration should be applied immediately or only after the UE completes the modification.

Important Information Elements

IE	Required	Description
`PDU Session ID`	Yes	Identifies which active PDU session is being updated and is the first field to verify in traces.
`PTI`	Yes	Correlates the command to the transaction or identifies that no procedure transaction identity is assigned for a network-initiated update.
`5GSM cause`	Optional	Can carry a session-management reason or trigger context, such as reactivation requested, when the network needs the UE to act on the update.
`Session-AMBR`	Optional	Conveys the aggregate bitrate the UE should apply for the session after the modification.
`RQ timer value`	Optional	Tells the UE how long to apply reflective QoS behavior when that feature is in use.
`Always-on PDU session indication`	Optional	Marks that the session should remain always on when the subscription and service model support it.
`Authorized QoS rules`	Optional	Carries the network-approved QoS rules that the UE must use for the active session.
`Mapped EPS bearer contexts`	Optional	Relevant in EPS interworking scenarios where the network updates how QoS flows map to EPS bearer context.
`Authorized QoS flow descriptions`	Optional	Provides the final QoS flow parameters that the UE should apply for user traffic.
`Extended protocol configuration options`	Optional	May carry DNS or other protocol configuration that should be applied along with the session update.

Detailed field explanation

`PDU Session ID`

Identifies which active PDU session is being updated and is the first field to verify in traces.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`PTI`

Correlates the command to the transaction or identifies that no procedure transaction identity is assigned for a network-initiated update.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`5GSM cause`

Can carry a session-management reason or trigger context, such as reactivation requested, when the network needs the UE to act on the update.

Presence: Optional

In practice: When this appears in an accept, it often means the network normalized a requested value rather than failing the session outright. Check it together with the selected session type, not in isolation.

`Session-AMBR`

Conveys the aggregate bitrate the UE should apply for the session after the modification.

Presence: Optional

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`RQ timer value`

Tells the UE how long to apply reflective QoS behavior when that feature is in use.

Presence: Optional

In practice: Reflective QoS timing only matters when the feature is actually in use. If the session uses reflective QoS, this timer helps explain how long the UE should keep applying the reflected behavior.

`Always-on PDU session indication`

Marks that the session should remain always on when the subscription and service model support it.

Presence: Optional

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`Authorized QoS rules`

Carries the network-approved QoS rules that the UE must use for the active session.

Presence: Optional

In practice: QoS rules are the real service profile of the session. Inspect the QFI mapping, packet filters, and precedence because those values explain how user traffic will actually be classified and forwarded.

`Mapped EPS bearer contexts`

Relevant in EPS interworking scenarios where the network updates how QoS flows map to EPS bearer context.

Presence: Optional

In practice: When EPS interworking is enabled, this field explains how QoS flows map back to EPS bearers. It matters most when the same service has to behave consistently across 5G and LTE anchor handling.

`Authorized QoS flow descriptions`

Provides the final QoS flow parameters that the UE should apply for user traffic.

Presence: Optional

In practice: QoS rules are the real service profile of the session. Inspect the QFI mapping, packet filters, and precedence because those values explain how user traffic will actually be classified and forwarded.

`Extended protocol configuration options`

May carry DNS or other protocol configuration that should be applied along with the session update.

Presence: Optional

In practice: This is where DNS and other operational configuration can hide. When the session is accepted but applications still fail, EPCO is often one of the first optional fields worth validating.

What to check in logs and traces

Confirm the command matches the expected modification request using PDU Session ID and PTI.
Verify the active session already existed before this command appeared.
Inspect Session-AMBR, RQ timer value, and authorized QoS rules or flow descriptions.
Check whether the UE should apply the change immediately or only after the follow-up complete message.
Correlate the command with `PDU Session Modification Complete` to confirm the UE accepted the update.
If the command was network-initiated, verify that policy and session-state logs agree with the downlink NAS content.
If the session is for VoNR or IMS, confirm that the updated QoS parameters match the expected voice profile.

Common Issues and Troubleshooting

The UE receives the command but the session behavior does not change.

Likely cause: The UE did not apply the authorized QoS, AMBR, or timer update correctly.

What to inspect: Check the command payload, the follow-up complete message, and user-plane traces after the update.

Next step: Compare the approved network state with what the UE actually enforced.

The command is missing even though the request was accepted.

Likely cause: The downlink NAS message was not delivered, was lost in transport, or the trace missed the AMF-to-UE leg.

What to inspect: Check DL NAS Transport, AMF logs, and access-side delivery.

Next step: Trace the full N1 transport path before assuming the SMF skipped the command.

QoS changes look wrong after the command.

Likely cause: The final authorized QoS rules or flow descriptions do not match the requested service model.

What to inspect: Compare the command against the original request, policy result, and session AMBR.

Next step: Validate whether policy normalization narrowed the requested settings.

Reflective QoS behavior persists longer or shorter than expected.

Likely cause: The RQ timer value was set differently than the UE or service expected.

What to inspect: Check the RQ timer value and the reflective QoS support declared earlier in the session.

Next step: Adjust the timer interpretation against the active release and UE capability.

FAQ

What does PDU Session Modification Command do?

It tells the UE how to update an already active PDU session using the network-approved parameters.

Who sends PDU Session Modification Command?

The SMF sends it to the UE via the AMF.

When is PDU Session Modification Command sent?

It is sent after the network accepts a modification request or when the SMF needs to update an active session.

What are the important IEs in PDU Session Modification Command?

Start with PDU Session ID and PTI, then inspect Session-AMBR, RQ timer value, and the authorized QoS fields.

What happens after PDU Session Modification Command?

The UE applies the indicated changes and then sends PDU Session Modification Complete.

Does this message mean the session is new?

No. It updates an already active PDU session rather than creating a new one.

Is ASN.1 used for PDU Session Modification Command?

No. It is a 5GSM NAS message defined by structured information elements in 3GPP TS 24.501.

Why is the RQ timer value important?

Because it controls how long reflective QoS behavior should be applied, which can change observed service behavior.

Related message pages

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.