What is PDU Session Modification Complete in simple terms?
PDU Session Modification Complete is the 5GSM message the UE sends after it applies the network-approved changes to an active PDU session.
Confirms that the UE accepted and applied the modification command so the SMF can finalize the updated session state.
Why this message matters
PDU Session Modification Complete means the UE successfully applied the network's session update and is telling the SMF that the modification is finished.
Where this message appears in the call flow
5G PDU Session Modification
Call flow position: UE confirmation step after the network-approved command has been applied.
Typical state: The UE has already received the command and is acknowledging that the active PDU session has been modified.
Preconditions:
The UE received a valid PDU Session Modification Command.
The UE accepted and applied the indicated changes.
Next likely message: The session returns to active state with the new parameters
Network-requested PDU Session Modification
Call flow position: UE acknowledgement after a network-initiated update instruction.
Typical state: The UE is confirming that the network-pushed update has been applied.
Preconditions:
The network sent a PDU Session Modification Command.
The UE accepted the command.
Next likely message: The session remains active with updated configuration
Domain: Core-side session management completion with access-side NAS delivery dependency
Signaling bearer: NAS signaling
Logical channel: Usually carried inside UL NAS Transport on the access side
Transport / encapsulation: 5GSM NAS message transported end-to-end from the UE to the SMF through AMF mediation
Security context: Normally delivered while NAS security is already active, so engineers usually expect protected uplink NAS handling.
ASN.1 Message Syntax for 5G NAS - PDU Session Modification Complete
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 Complete - Example Dump
PDU Session Modification Complete
Extended Protocol Discriminator: 5G Session Management
PDU Session ID: 10
PTI: 5
Message Type: PDU Session Modification Complete
Extended Protocol Configuration Options:
DNS IPv4 address: 10.1.1.1
Port Management Information Container:
Port status: updated
Port set: 443, 5004
How to read this dump
Start with PDU Session ID and PTI so you correlate the complete message with the correct active session and modification transaction.
If EPCO is present, check whether the UE is returning any configuration that is meaningful for the updated session state.
Port management information container is only relevant when the modification path included port-management information.
A minimal complete message is normal; many deployments only carry the identifiers and a small amount of optional context.
Important Information Elements
IE
Required
Description
PDU Session ID
Yes
Identifies the active PDU session whose modification is now being confirmed.
PTI
Yes
Correlates the complete message with the modification procedure when a UE-requested flow is in progress.
Extended protocol configuration options
Optional
May carry protocol configuration that the UE includes while acknowledging the modification.
Port management information container
Optional
Used when the modification includes port-management-related information that needs to be acknowledged back to the SMF.
Detailed field explanation
PDU Session ID
Identifies the active PDU session whose modification is now being confirmed.
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 complete message with the modification procedure when a UE-requested flow is in progress.
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.
Extended protocol configuration options
May carry protocol configuration that the UE includes while acknowledging the modification.
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.
Port management information container
Used when the modification includes port-management-related information that needs to be acknowledged back to the SMF.
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.
What to check in logs and traces
Confirm the complete message follows the expected PDU Session Modification Command.
Verify the PDU session was already active before the modification sequence began.
Check `PDU Session ID` and `PTI` first when correlating the request, command, and complete messages.
Correlate the complete with the final active session state in SMF logs.
If the command included reflective QoS or session-bitrate updates, verify the UE actually applied them after sending complete.
For network-requested modifications, make sure the downlink command and uplink complete match the same session context.
Common Issues and Troubleshooting
The UE never sends the complete message.
Likely cause: The UE did not accept the command, the command was not received, or the UE is still applying the update.
What to inspect: Check the command, the UE state, the uplink NAS path, and timer handling.
Next step: Determine whether the UE is waiting, rejecting, or missing the command entirely.
The complete message appears, but the session does not behave as expected.
Likely cause: The UE acknowledged the command, but the final QoS, AMBR, or port state does not match the intended service model.
What to inspect: Compare the command payload, the complete message, and user-plane behavior after the update.
Next step: Check whether the SMF and UE converged on the same session state.
The network still shows the modification procedure as pending.
Likely cause: The complete message was delayed, lost, or not correlated with the correct PTI.
What to inspect: Check T3591, PTI correlation, and access-side NAS delivery.
Next step: Validate uplink transport and ensure the network received the complete on time.
The complete message looks truncated or too small.
Likely cause: The message is usually minimal, but the decoder may also be missing optional IEs or be using stale parsing logic.
What to inspect: Check raw NAS bytes, optional IEs, and release context.
Next step: Confirm the raw capture before assuming a decode problem.
FAQ
What does PDU Session Modification Complete do?
It tells the network that the UE applied the session update and the modification procedure is complete.
Who sends PDU Session Modification Complete?
The UE sends it to the SMF via the AMF.
When is PDU Session Modification Complete sent?
It is sent after the UE receives and accepts a PDU Session Modification Command.
What are the important IEs in PDU Session Modification Complete?
Start with PDU Session ID and PTI, then inspect any Extended Protocol Configuration Options or Port Management Information Container fields.
Does this message mean the session is new?
No. It confirms that an already active PDU session has been modified.
What happens if PDU Session Modification Complete is missing?
The network may keep waiting on T3591, or the procedure can look stalled until the UE responds or the timer expires.
Is ASN.1 used for PDU Session Modification Complete?
No. It is a 5GSM NAS message defined by structured information elements in 3GPP TS 24.501.
Can this message carry configuration data?
Yes. It can include extended protocol configuration options and, in some cases, a port management information container.
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.