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NAS5GUE to SMF via AMF3GPP TS 24.501
5G NAS - PDU Session Modification Command Reject
PDU Session Modification Command Reject is the 5GSM message the UE sends when it cannot accept the network's modification command for an active PDU session.
Message Fact Sheet
Protocol
nas
Network
5g
Spec
3GPP TS 24.501
Spec Section
8.3.11
Direction
UE to SMF via AMF
Message Type
5GSM signaling
Full message name
5G NAS - PDU Session Modification Command Reject
Protocol
NAS
Technology
5G
Direction
UE to SMF via AMF
Interface
N1
Signaling bearer / channel
NAS signaling / Usually carried inside UL NAS Transport on the access side
Typical trigger
The UE received a PDU Session Modification Command but could not accept the requested session update because of a QoS, packet filter, session identity, or syntax problem.
Main purpose
Tells the SMF that the UE rejected the command, gives the reason, and allows the network to abort or locally release the modification procedure correctly.
#26 insufficient resources, #43 invalid PDU session identity, #44 semantic error in packet filter(s), #45 syntactical error in packet filter(s), #83 semantic error in the QoS operation, #84 syntactical error in the QoS operation
What is PDU Session Modification Command Reject in simple terms?
PDU Session Modification Command Reject is the 5GSM message the UE sends when it cannot accept the network's modification command for an active PDU session.
Tells the SMF that the UE rejected the command, gives the reason, and allows the network to abort or locally release the modification procedure correctly.
Why this message matters
PDU Session Modification Command Reject means the UE could not apply the network's session update, so it tells the SMF why the modification was refused.
Where this message appears in the call flow
Network-requested PDU Session Modification
Call flow position: Negative UE response after the network sends the modification command for an active session.
Typical state: The UE has already received the command and determined that the requested update cannot be applied.
Preconditions:
The PDU session is active.
The UE received a valid PDU Session Modification Command.
Next likely message: The SMF stops T3591 and aborts the modification procedure, or the session continues unchanged
UE-requested PDU Session Modification
Call flow position: Negative response to the network-approved command that was returned after a UE-initiated modification request.
Typical state: The UE initiated the modification procedure, but the final command cannot be accepted.
Preconditions:
The UE previously sent PDU Session Modification Request.
The network responded with a PDU Session Modification Command.
Next likely message: The UE releases the PTI and the network stops T3581 if the command belonged to the UE-initiated transaction
Next message(s): Session remains active with original configuration, SMF aborts the modification procedure, Possible local release if 5GSM cause #43 is used
Message direction and transport
Sender and receiver: UE to SMF via AMF
Interface: N1
Domain: Core-side session management rejection 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 Command Reject
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 Reject - Example Dump
Start with PDU Session ID so you know which active session the UE rejected.
Check PTI to correlate the reject with the correct modification transaction.
Read the 5GSM cause next because it is the main root-cause field for the UE decision.
A small reject message is normal because the message usually carries only the identifiers and the rejection cause.
Important Information Elements
IE
Required
Description
PDU Session ID
Yes
Identifies the active PDU session for which the command was rejected.
PTI
Yes
Correlates the reject with the transaction and is especially important when the command belonged to a UE-requested modification.
5GSM cause
Yes
Explains why the UE could not accept the PDU Session Modification Command.
Detailed field explanation
PDU Session ID
Identifies the active PDU session for which the command was rejected.
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 reject with the transaction and is especially important when the command belonged to a UE-requested modification.
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
Explains why the UE could not accept the PDU Session Modification Command.
Presence: Required
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.
What to check in logs and traces
Confirm the reject follows a visible PDU Session Modification Command.
Verify the PDU session was already active before the command was sent.
Check `PDU Session ID` and `PTI` first to make sure the reject belongs to the same transaction as the command.
Inspect the `5GSM cause` and map it to the exact reject reason in the UE or protocol stack.
If the command came from a UE-requested modification, verify that PTI handling and T3581/T3591 behavior match the release rules.
Correlate the reject with SMF logs to see whether the procedure was aborted or the session was locally released.
Common Issues and Troubleshooting
The UE sends the reject immediately after the command.
Likely cause: The command carried a QoS, packet filter, or session identity that the UE could not apply.
What to inspect: Check the command contents, the 5GSM cause, and the UE capability or policy context.
Next step: Compare the approved parameters with what the UE implementation actually supports.
The network keeps waiting after the reject.
Likely cause: The reject was not correlated with the correct PTI or the SMF did not process the uplink NAS path correctly.
What to inspect: Check T3591, PTI correlation, and access-side NAS delivery.
Next step: Verify that the SMF received the reject on the expected session and transaction.
The session disappears after the reject.
Likely cause: The reject used 5GSM cause #43 invalid PDU session identity, which triggers local release handling on the network side.
What to inspect: Check the exact cause value and whether the SMF released the session locally without peer-to-peer signaling.
Next step: Confirm whether the session was intentionally torn down or whether the session identity was stale.
The command looked valid but the UE still rejected it.
Likely cause: The packet filter syntax, QoS operation, or capability support was not acceptable to the UE parser or implementation.
What to inspect: Check the command payload, release behavior, and any decoder or stack limitations.
Next step: Validate the message against the UE's supported feature set and the exact 3GPP semantics.
FAQ
What does PDU Session Modification Command Reject do?
It tells the network that the UE could not accept the PDU Session Modification Command and gives the rejection reason.
Who sends PDU Session Modification Command Reject?
The UE sends it to the SMF via the AMF.
When is PDU Session Modification Command Reject sent?
It is sent after the UE receives a PDU Session Modification Command and decides it cannot apply the requested update.
What are the important IEs in PDU Session Modification Command Reject?
Start with PDU Session ID and PTI, then inspect the 5GSM cause.
What happens if the UE rejects a modification command?
The SMF stops waiting for the completion path, aborts the modification procedure, and usually keeps the session unchanged unless cause #43 requires local release.
Does this message mean the session is new?
No. It only applies to an already active PDU session.
Is ASN.1 used for PDU Session Modification Command Reject?
No. It is a 5GSM NAS message defined by structured information elements in 3GPP TS 24.501.
Can this message be related to QoS or packet filter errors?
Yes. Those are among the most common reject reasons for this procedure.
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.