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NAS5GSMF to UE via AMF3GPP TS 24.501
5G NAS - PDU Session Release Command
PDU Session Release Command is the 5GSM message the network sends to tell the UE that an active PDU session is being released and to provide any release-related cause or retry guidance.
Message Fact Sheet
Protocol
nas
Network
5g
Spec
3GPP TS 24.501
Spec Section
8.3.14
Direction
SMF to UE via AMF
Message Type
5GSM signaling
Full message name
5G NAS - PDU Session Release 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 SMF wants to release an active PDU session due to policy, resource, authorization, inter-system relocation, or completion of a UE-requested release procedure.
Main purpose
Starts the network-requested release procedure, informs the UE that the session is released, and can carry cause or retry information for reactivation or congestion handling.
#26 insufficient resources, #29 user authentication or authorization failed, #36 regular deactivation, #39 reactivation requested, #67 insufficient resources for specific slice and DNN, #69 insufficient resources for specific slice
What is PDU Session Release Command in simple terms?
PDU Session Release Command is the 5GSM message the network sends to tell the UE that an active PDU session is being released and to provide any release-related cause or retry guidance.
Starts the network-requested release procedure, informs the UE that the session is released, and can carry cause or retry information for reactivation or congestion handling.
Why this message matters
PDU Session Release Command is the network telling the UE that the data session is being released and giving any extra guidance it needs.
Where this message appears in the call flow
Network-requested PDU Session Release
Call flow position: Network instruction that ends the active PDU session and starts the UE release confirmation step.
Typical state: The session is active in the UE and the network is now telling the UE to consider it released.
Preconditions:
A PDU session already exists and is active.
The SMF has decided to release the session or has accepted a UE-requested release.
Next likely message: PDU Session Release Complete
UE-requested PDU Session Release
Call flow position: Network acceptance or rejection follow-up when the UE asked to release the session.
Typical state: The UE already sent PDU Session Release Request and the network is now resolving the transaction.
Preconditions:
The UE previously sent PDU Session Release Request.
The SMF has decided how the release should proceed.
Next likely message: PDU Session Release Complete if accepted, or release remains active if another branch occurs
Domain: Core-side session management release 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 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 Release Command - Example Dump
PDU Session Release Command
Extended Protocol Discriminator: 5G Session Management
PDU Session ID: 10
PTI: 6
Message Type: PDU Session Release Command
SM Cause: Reactivation requested
Back-off Timer Value: not present
Extended Protocol Configuration Options:
No additional data
How to read this dump
Start with PDU Session ID so you know which session the network is releasing.
Check PTI to decide whether the message is tied to a UE-requested release transaction.
Read the SM cause next because it explains why the release is happening and whether reactivation is expected.
Back-off timer value is optional and usually appears when resource-related causes need retry pacing.
A short command is normal even though the message can carry important release behavior.
Important Information Elements
IE
Required
Description
PDU Session ID
Yes
Identifies the active PDU session that the network is releasing.
PTI
Yes
Correlates the command with the UE-requested release transaction when one exists.
SM cause
Yes
Tells the UE why the network is releasing the session and may influence reactivation behavior.
Back-off timer value
Optional
Tells the UE how long to wait before retrying when the release is driven by a resource-related cause.
EAP message
Optional
Used when external DN re-authentication and re-authorization fails and a protocol payload needs to be returned.
Extended protocol configuration options
Optional
Carries optional protocol data such as configuration parameters, error codes, or service messages.
Detailed field explanation
PDU Session ID
Identifies the active PDU session that the network is releasing.
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 with the UE-requested release transaction when one exists.
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.
SM cause
Tells the UE why the network is releasing the session and may influence reactivation behavior.
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.
Back-off timer value
Tells the UE how long to wait before retrying when the release is driven by a resource-related cause.
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.
EAP message
Used when external DN re-authentication and re-authorization fails and a protocol payload needs to be returned.
Presence: Optional
In practice: If present, it reflects the external DN authentication result. It is worth checking when the session succeeds on NAS but service access still depends on an external authentication flow.
Extended protocol configuration options
Carries optional protocol data such as configuration parameters, error codes, or service messages.
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 follows a visible release request or a known network release trigger.
Verify `PDU Session ID` and `PTI` first so you correlate the release command with the right session and transaction.
Inspect the `SM cause` and map it to the intended release reason.
If `#39 reactivation requested` is present, verify that the UE restarts establishment behavior after release as required.
Check whether `T3592` started on the SMF side and whether `T3582` should be stopped on the UE side when PTI matches a UE-requested release transaction.
If a back-off timer is present, confirm the UE obeys the retry delay before attempting a new session.
Common Issues and Troubleshooting
The UE receives the command but does not treat the session as released.
Likely cause: The UE did not parse the release command correctly or the NAS delivery path was not fully processed.
What to inspect: Check the command payload, PTI correlation, and access-side NAS handling.
Next step: Confirm the UE state transition to released and whether `PDU Session Release Complete` followed.
The network expects release completion but never gets it.
Likely cause: The UE may not have received the command, or the UE may still be handling the release and retry logic.
What to inspect: Check T3592 on the network side, the downlink NAS path, and UE release state.
Next step: Determine whether the issue is transport, sequencing, or UE parser behavior.
The session is released, and a new session starts immediately after.
Likely cause: The release command included `#39 reactivation requested`, which tells the UE to re-initiate establishment.
What to inspect: Check the SM cause and any follow-up PDU session establishment request.
Next step: Correlate the reactivation behavior with DNN, SSC mode, and S-NSSAI.
The release is blocked from retrying too soon.
Likely cause: A back-off timer was included for a resource-related release.
What to inspect: Check the back-off timer value and the cause value that triggered it.
Next step: Verify the UE respects the retry delay before sending a new establishment request.
FAQ
What does PDU Session Release Command do?
It tells the UE that the PDU session is being released and can provide cause or retry guidance.
Who sends PDU Session Release Command?
The SMF sends it to the UE via the AMF.
When is PDU Session Release Command sent?
It is sent when the SMF decides to release the session, or after it accepts a UE-requested release.
What are the important IEs in PDU Session Release Command?
Start with PDU Session ID and PTI, then inspect the SM cause and any back-off timer value.
What happens after PDU Session Release Command?
The UE considers the PDU session released and sends PDU Session Release Complete.
What does cause #39 reactivation requested mean here?
It means the UE should re-initiate PDU session establishment after the release completes.
Is ASN.1 used for PDU Session Release Command?
No. It is a 5GSM NAS message defined by structured information elements in 3GPP TS 24.501.
Can this message carry a back-off timer?
Yes. The network may include a back-off timer value for resource-related release causes.
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.