NAS 5G SMF to UE via AMF 3GPP TS 24.501

5G NAS - PDU Session Establishment Reject

PDU Session Establishment Reject is the 5GSM failure message the network sends when it cannot create the requested PDU session and needs to return the reason for rejection.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.3.3
Direction	SMF to UE via AMF	Message Type	5GSM signaling

Full message name	5G NAS - PDU Session Establishment 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	Sent when the requested DNN, slice, session type, subscription, operator policy, or session-capability assumptions do not allow the network to create the requested session.
Main purpose	Stops the current session-establishment attempt and returns the 5GSM cause plus any back-off, retry, or service-level context that explains why the session was not accepted.
Main specification	3GPP TS 24.501, 8.3.3
Release added	Release 15
Procedures where used	5G PDU Session Establishment, VoNR Registration, Enterprise or slice-specific service denial analysis
Related timers	T3580, Back-off timer value when provided
Related cause values	5GSM cause, Back-off timer value, Allowed SSC mode

What is PDU Session Establishment Reject in simple terms?

PDU Session Establishment Reject is the 5GSM failure message the network sends when it cannot create the requested PDU session and needs to return the reason for rejection.

Stops the current session-establishment attempt and returns the 5GSM cause plus any back-off, retry, or service-level context that explains why the session was not accepted.

Why this message matters

PDU Session Establishment Reject is the network telling the UE that the requested data session cannot be created, along with the reason why.

Where this message appears in the call flow

5G PDU Session Establishment

Call flow position: Negative 5GSM response when the requested session cannot be created.

Typical state: UE is registered but the requested session establishment path is being refused.

Preconditions:

The UE has already attempted session establishment.
The network evaluated the request and determined it cannot accept the requested session as sent.

Next likely message: Retry, wait, or remain without the requested session depending on cause and backoff

VoNR Registration

Call flow position: May appear when the IMS session request is rejected even though registration itself succeeded.

Typical state: UE is registered but cannot obtain the IMS or service-specific session it needs.

Preconditions:

Registration completed successfully.
The IMS-related DNN, slice, or policy path still failed during session creation.

Next likely message: Retry with corrected context or fail service bring-up

Call flow position

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

Next message(s): Backoff or retry behavior, Fresh PDU Session Establishment Request, Session remains unavailable

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 network to the UE through AMF mediation

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

Message Structure Overview

PDU Session Establishment Reject is a 5GSM failure response, so engineers read it as a TS 24.501 information-element structure rather than ASN.1.
Operationally, the most important field is the 5GSM cause, followed by any back-off timer or allowed SSC mode guidance.
In traces, the fastest root-cause path usually comes from correlating the reject with the original request's DNN, S-NSSAI, session type, and policy assumptions.

Structure Syntax

This message is not typically analyzed as ASN.1 on the wire. Engineers usually inspect it as a NAS or protocol field decode rather than an ASN.1 tree.

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

Decoded Message Dump

PDU Session Establishment Reject
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 1
  Message Type: PDU Session Establishment Reject
  5GSM Cause: Requested service option not subscribed
  Back-off Timer Value: 10 minutes
  Allowed SSC Mode: SSC mode 1
  Re-attempt Indicator: retry after back-off

How to read this dump

Start with PDU Session ID and PTI so you correlate the failure with the correct request.
Read 5GSM Cause next because it defines the troubleshooting path much more than the message name itself.
Back-off Timer Value is operationally important because a silent UE after reject may simply be respecting retry rules.
Allowed SSC Mode can explain why a request failed even when the DNN and slice looked valid.

Important Information Elements

IE	Required	Description
`PDU Session ID`	Yes	Identifies which requested session was rejected and is the first field used to correlate the failure with the original request.
`PTI`	Yes	Matches the reject to the original 5GSM transaction so engineers do not confuse it with another overlapping session attempt.
`5GSM cause`	Yes	Primary failure reason explaining why the session could not be created.
`Back-off timer value`	Optional	Tells the UE how long it should wait before retrying, which can dramatically change observed behavior after failure.
`Allowed SSC mode`	Optional	Shows which SSC mode would have been acceptable when the requested continuity model was not allowed.
`Re-attempt indicator`	Optional	Guides retry expectations and helps explain whether the UE should attempt the procedure again.
`Extended protocol configuration options`	Optional	May carry additional context relevant to the failed request and is useful in advanced service troubleshooting.

Detailed field explanation

`PDU Session ID`

Identifies which requested session was rejected and is the first field used to correlate the failure with the original request.

Presence: Required

`PTI`

Matches the reject to the original 5GSM transaction so engineers do not confuse it with another overlapping session attempt.

Presence: Required

`5GSM cause`

Primary failure reason explaining why the session could not be created.

Presence: Required

`Back-off timer value`

Tells the UE how long it should wait before retrying, which can dramatically change observed behavior after failure.

Presence: Optional

`Allowed SSC mode`

Shows which SSC mode would have been acceptable when the requested continuity model was not allowed.

Presence: Optional

`Re-attempt indicator`

Guides retry expectations and helps explain whether the UE should attempt the procedure again.

Presence: Optional

`Extended protocol configuration options`

May carry additional context relevant to the failed request and is useful in advanced service troubleshooting.

Presence: Optional

What to check in logs and traces

Confirm the reject matches the expected request transaction using PDU Session ID and PTI.
Inspect the 5GSM cause before analyzing later service symptoms.
Check whether the requested DNN, S-NSSAI, PDU session type, and SSC mode were actually allowed for the subscriber and area.
Verify whether a Back-off Timer Value or retry-related indicator is present.
Correlate the reject with registration-time Allowed NSSAI and any slice-selection behavior.
If the session was for IMS or enterprise service, compare the failure against the intended service policy and DNN mapping.

Common Issues and Troubleshooting

The session request is rejected immediately.

Likely cause: The requested DNN, slice, subscription, or session type does not match what the network allows.

What to inspect: Check 5GSM cause, DNN, S-NSSAI, PDU session type, and request type in the original request.

Next step: Troubleshoot by the returned cause rather than retrying blindly.

The UE does not retry after the reject.

Likely cause: A back-off timer or retry restriction is present and the UE is correctly waiting.

What to inspect: Check Back-off Timer Value and any retry or re-attempt guidance.

Next step: Validate timer-driven behavior before treating it as a separate access failure.

Registration succeeds but IMS or enterprise service still cannot start.

Likely cause: The session-establishment path, not the registration path, is failing due to DNN, slice, or policy mismatch.

What to inspect: Compare this reject against the original request and registration-time Allowed NSSAI or service context.

Next step: Shift troubleshooting from 5GMM success to 5GSM denial cause.

Detailed explanation

5G NAS - PDU Session Establishment Reject Explained

PDU Session Establishment Reject is the 5GSM failure message the network sends when it cannot create the session requested by the UE. In simple terms, it means the UE asked for a data session, but the network refused it and returned the reason why.

For beginners, the simple meaning is: the network is saying “this requested data session is not allowed or cannot be created.”
For experienced engineers, this is one of the most important 5GSM troubleshooting messages because it reveals the session-side denial cause after registration may already have succeeded.

What is PDU Session Establishment Reject in simple terms?

The UE asked the network to create a new data session. The network checked the request and decided it could not accept it.

This message tells the UE:

the requested session was not created
why it was rejected
whether the UE should wait before retrying
whether a different SSC mode would have been acceptable

So this message does not just say “no.” It gives the reason and sometimes the retry guidance.

Why this message matters

This message matters because many service failures happen after successful registration, not during registration.

Engineers inspect this message to understand:

whether the requested DNN is allowed
whether the requested slice is permitted
whether the PDU Session Type is acceptable
whether the UE asked for an unsupported SSC Mode
whether the network is forcing a retry pause through Back-off Timer Value

That makes this message the key starting point for many “registered but no data session” cases.

Where this message appears in the call flow

A simplified failure path is:

UE sends PDU Session Establishment Request
Network evaluates DNN, slice, policy, and subscription
Network sends PDU Session Establishment Reject
UE waits, retries, or remains without the requested session

Mini sequence:

UE                              gNB / AMF / SMF
|                                   |
|--- UL NAS Transport ------------->|
|    PDU Session Establishment Req  |
|<-- DL NAS Transport --------------|
|    PDU Session Establishment Rej  |

In practice, this is the message that turns a generic session failure into a cause-driven troubleshooting path.

Transport characteristics

sender: network / SMF side
receiver: UE
interface: N1
access-side transport: usually inside DL NAS Transport
signaling class: 5GSM
security expectation: normally sent over an already protected NAS path

This is a NAS session-management failure response, not an RRC problem by itself.

What PDU Session Establishment Reject means operationally

Operationally, PDU Session Establishment Reject means the network understood the request but could not grant the requested session under the current conditions.

This is where engineers verify:

which PDU Session ID was denied
whether the PTI matches the original request
what 5GSM Cause the network returned
whether a Back-off Timer Value changes retry behavior
whether a different SSC Mode would have been acceptable

That is why this message is usually more useful than repeated retry attempts when the service still cannot come up.

Important Information Elements

IE	Why it matters
`PDU Session ID`	Identifies which requested session was rejected.
`PTI`	Matches the reject to the original 5GSM transaction.
`5GSM Cause`	Main reason the session could not be created.
`Back-off Timer Value`	Controls how long the UE should wait before retrying.
`Allowed SSC Mode`	Shows what continuity mode would have been acceptable when relevant.
`Re-attempt Indicator`	Helps explain retry expectations.

Example message dump

PDU Session Establishment Reject
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 1
  Message Type: PDU Session Establishment Reject
  5GSM Cause: Requested service option not subscribed
  Back-off Timer Value: 10 minutes
  Allowed SSC Mode: SSC mode 1
  Re-attempt Indicator: retry after back-off

How to read this dump

Start with PDU Session ID and PTI to make sure you are looking at the correct failed transaction.
Read 5GSM Cause next because it drives the whole troubleshooting path.
Then inspect Back-off Timer Value and Allowed SSC Mode because they explain whether the UE should retry and whether a different request profile might have succeeded.
Finally compare the reject against the original request, especially DNN, S-NSSAI, PDU Session Type, and SSC Mode.

What to check in logs

confirm the reject matches the correct request transaction
inspect the returned 5GSM Cause
check DNN, S-NSSAI, PDU Session Type, and SSC Mode in the original request
verify whether a Back-off Timer Value is present
compare the reject with registration-time slice context and subscriber policy
correlate IMS or enterprise-service failures with this session-management denial before blaming radio behavior

FAQ

What is PDU Session Establishment Reject in 5G?

It is the 5GSM failure response that tells the UE the requested session could not be created and provides the reason.

What should engineers inspect first?

Start with PDU Session ID, PTI, and especially the 5GSM cause, then inspect any back-off or SSC-mode guidance.

Does this always mean the radio path failed?

No. It usually points to session-management, DNN, slice, subscription, or policy issues rather than radio decode failure.

Why is the back-off timer important?

Because it changes retry behavior and can make the UE appear idle when it is actually following network retry instructions.

What comes after PDU Session Establishment Reject?

That depends on the cause and any retry guidance. The UE may wait, retry with changed context, or remain without the requested service.

Summary

PDU Session Establishment Reject is the 5GSM failure message the network sends when it cannot create the requested PDU session and needs to return the reason for rejection.

Advanced Engineer Notes

Many field issues blamed on user plane or radio actually start with a 5GSM reject that was overlooked in the NAS trace.
The combination of 5GSM cause, back-off timer, and requested DNN or slice usually gives the fastest root cause.
For IMS and slice-aware services, session rejection after successful registration is a strong clue that mobility context succeeded but service authorization did not.

LTE / 5G / Variant Comparison

Compared with PDU Session Establishment Accept

Establishment Reject explains why the requested session could not be created, while Establishment Accept returns the final address, QoS, and service context for a successful session.

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.