5G NAS - PDU Session Release Reject

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.3.13
Direction	SMF to UE via AMF	Message Type	5GSM signaling

Full message name	5G NAS - PDU Session Release Reject
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 UE sent a PDU Session Release Request, but the SMF did not accept the request to release the active session.
Main purpose	Tells the UE that the release request was not accepted, returns the rejection cause, and keeps the session alive unless another procedure later changes it.
Main specification	3GPP TS 24.501, 8.3.13
Release added	Release 15
Procedures where used	UE-requested PDU Session Release, Release refusal analysis, Session cleanup troubleshooting, Timer-driven release retry analysis
Related timers	T3582
Related cause values	#34 service option temporarily out of order, #35 PTI already in use, #43 invalid PDU session identity

What is PDU Session Release Reject in simple terms?

PDU Session Release Reject is the 5GSM failure response the network sends when it does not accept the UE's request to release an active PDU session.

Tells the UE that the release request was not accepted, returns the rejection cause, and keeps the session alive unless another procedure later changes it.

Why this message matters

PDU Session Release Reject means the network did not accept the UE's request to end the session, so the session stays active.

Where this message appears in the call flow

UE-requested PDU Session Release

Call flow position: Negative network response to a UE-originated attempt to release an active PDU session.

Typical state: The UE has already sent a release request and is waiting for the network's decision.

Preconditions:

A PDU session already exists and is active.
The UE has sent PDU Session Release Request.

Next likely message: The UE keeps the session active and stops T3582, or retries later if appropriate

Release cleanup or collision handling

Call flow position: Often appears when the release transaction collides with another session-management decision or when the network sees a bad session identity.

Typical state: The session is still active in the network and the UE is waiting for a clear response.

Preconditions:

The UE release procedure is pending.
The network determined that the release request cannot be accepted.

Next likely message: The session remains active unless a later release command or another procedure changes it

Call flow position

Previous message(s): PDU Session Release Request, DL NAS Transport, SMF evaluation of the release request

Next message(s): Session remains active, UE retries after timer expiry, PDU Session Release Command if the network later decides to release the session

Message direction and transport

Sender and receiver: SMF to UE 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 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 Release 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 Release Reject - Example Dump

PDU Session Release Reject
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 6
  Message Type: PDU Session Release Reject
  5GSM Cause: PTI already in use

How to read this dump

Start with PDU Session ID to confirm which session release was rejected.
Check PTI so you correlate the reject with the exact release request.
Read the 5GSM cause next because it usually tells you whether the issue is a transaction collision, bad identity, or a temporary service problem.
A short reject message is normal and usually carries only the identifiers and the reason for rejection.

Important Information Elements

IE	Required	Description
`PDU Session ID`	Yes	Identifies the active PDU session whose release was rejected.
`PTI`	Yes	Correlates the reject with the UE-requested release transaction.
`5GSM cause`	Yes	Explains why the SMF did not accept the release request.

Detailed field explanation

`PDU Session ID`

Identifies the active PDU session whose release 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 UE-requested release transaction.

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 SMF did not accept the release request.

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 Release Request.
Verify `PDU Session ID` and `PTI` first so the reject is matched to the right release attempt.
Inspect the `5GSM cause` and map it to the exact reason the network did not accept the release.
Check whether the UE should stop T3582 and keep the session active.
If `#43 invalid PDU session identity` is used, verify whether the SMF treated the session identity as stale or unknown.
Correlate the reject with SMF logs to confirm whether the session remains active.

Common Issues and Troubleshooting

The UE wants to release the session but the network rejects the request.

Likely cause: The session identity, transaction identity, or release context does not match what the SMF expects.

What to inspect: Check the `5GSM cause`, `PDU Session ID`, PTI, and the SMF session state.

Next step: Correct the request or confirm whether the session should remain active.

The reject says PTI already in use.

Likely cause: The UE reused a PTI that is already active in another procedure.

What to inspect: Check overlapping NAS SM transactions and the UE PTI allocation logic.

Next step: Ensure the UE allocates a free PTI before starting a new release request.

The reject says invalid PDU session identity.

Likely cause: The release request referenced a session ID that the SMF does not consider active.

What to inspect: Check the active session list in the SMF and compare it with the UE's session map.

Next step: Fix the session correlation or clear stale UE state.

The UE keeps retrying after the reject.

Likely cause: T3582 handling or retry logic may not be aligned with the rejection cause.

What to inspect: Check T3582 expiry behavior and UE retry policy.

Next step: Make sure the UE stops or retries only as the spec and cause allow.

FAQ

What does PDU Session Release Reject do?

It tells the UE that the network did not accept the request to release the PDU session.

Who sends PDU Session Release Reject?

The SMF sends it to the UE via the AMF.

When is PDU Session Release Reject sent?

It is sent after the UE requests a PDU session release and the network does not accept that release.

What are the important IEs in PDU Session Release Reject?

Start with PDU Session ID and PTI, then inspect the 5GSM cause.

What happens after PDU Session Release Reject?

The UE stops T3582, releases the PTI, and keeps the session active unless another procedure later changes it.

Does this message mean the session is gone?

No. It means the release was rejected, so the session remains active.

Is ASN.1 used for PDU Session Release Reject?

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

Which cause values are typical here?

Common causes are #34 service option temporarily out of order, #35 PTI already in use, and #43 invalid PDU session identity.

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.