5G NAS - PDU Session Release Request

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.3.12
Direction	UE to SMF via AMF	Message Type	5GSM signaling

Full message name	5G NAS - PDU Session Release Request
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 wants to terminate an active PDU session, or it needs to release the session because of QoS, packet filter, or local service conditions.
Main purpose	Starts the UE-requested release procedure so the SMF can release the session cleanly and know whether the UE has a specific reason for the release.
Main specification	3GPP TS 24.501, 8.3.12
Release added	Release 15
Procedures where used	UE-requested PDU Session Release, Service deactivation, QoS or packet-filter cleanup, VoNR teardown
Related timers	T3582
Related cause values	#36 regular deactivation, #41 semantic error in the TFT operation, #42 syntactical error in the TFT operation, #44 semantic errors in packet filter(s), #45 syntactical error in packet filter(s)

What is PDU Session Release Request in simple terms?

PDU Session Release Request is the 5GSM message the UE sends when it wants to end an already active PDU session, or when it needs to release the session because of a local protocol or service condition.

Starts the UE-requested release procedure so the SMF can release the session cleanly and know whether the UE has a specific reason for the release.

Why this message matters

PDU Session Release Request is the UE's way of saying it wants to end an already active PDU session.

Where this message appears in the call flow

UE-requested PDU Session Release

Call flow position: UE-originated request that begins the release procedure for an already active PDU session.

Typical state: The UE has a live PDU session and is asking the network to release it or is releasing it due to local conditions.

Preconditions:

A PDU session already exists and is active.
The UE has a reason to end the session or clean up session-specific state.

Next likely message: PDU Session Release Command or PDU Session Release Reject

QoS or packet-filter cleanup

Call flow position: Often appears when the UE cannot keep the current traffic filters or QoS state and decides to release the session.

Typical state: The session is still active, but the UE's local session state has become inconsistent or no longer needed.

Preconditions:

The session is active.
The UE is cleaning up TFT, QoS, or application-driven session state.

Next likely message: PDU Session Release Command or a local retry if the network rejects the request

Call flow position

Previous message(s): PDU Session Establishment Accept, PDU Session Modification Complete, UL NAS Transport, Active PDU session state

Next message(s): PDU Session Release Command, PDU Session Release Reject, Session becomes inactive after release

Message direction and transport

Sender and receiver: UE to SMF via AMF

Interface: N1

Domain: Core-side session management request 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 Release Request

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 Request - Example Dump

PDU Session Release Request
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 6
  Message Type: PDU Session Release Request
  5GSM Cause: Regular deactivation
  Extended Protocol Configuration Options:
    DNS IPv4 address: not present

How to read this dump

Start with PDU Session ID so you know which active session the UE wants to release.
Check PTI to correlate the release request with the correct UE-initiated transaction.
Read the 5GSM cause if present, because it can explain whether the release is normal or triggered by a local error condition.
A small message is normal, and many captures will show only the identifiers and perhaps the release reason.

Important Information Elements

IE	Required	Description
`PDU Session ID`	Yes	Identifies the active PDU session that the UE wants to release.
`PTI`	Yes	Correlates the release request with the correct UE-initiated transaction.
`5GSM cause`	Optional	May indicate the reason for the release, such as regular deactivation or a QoS or packet-filter error condition.
`Extended protocol configuration options`	Optional	Can carry additional protocol or configuration context that the UE wants to return during release.

Detailed field explanation

`PDU Session ID`

Identifies the active PDU session that the UE wants to release.

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 release request with the correct UE-initiated 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`

May indicate the reason for the release, such as regular deactivation or a QoS or packet-filter error condition.

Presence: Optional

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.

`Extended protocol configuration options`

Can carry additional protocol or configuration context that the UE wants to return during release.

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 request follows an established active PDU session.
Verify `PDU Session ID` and `PTI` first so you correlate the release request with the correct session.
Check whether the UE included a `5GSM cause` and whether that cause matches the release reason seen in the application or UE logs.
Inspect T3582 handling on the UE side.
Correlate the request with the eventual `PDU Session Release Command` or `PDU Session Release Reject`.
Check whether the release was driven by service shutdown, QoS cleanup, or a packet-filter/TFT error.

Common Issues and Troubleshooting

The UE wants to release the session but the network does not respond.

Likely cause: The request may not have reached the SMF, or the uplink NAS path was interrupted.

What to inspect: Check UL NAS Transport, AMF delivery, PTI correlation, and T3582 behavior.

Next step: Verify whether the request is visible at the SMF before assuming the network ignored it.

The release request is rejected.

Likely cause: The SMF did not accept the UE's request to release the session.

What to inspect: Check the returned `PDU Session Release Reject`, its cause, and the session state in the core network.

Next step: Determine whether the session should stay up or whether a local cleanup is needed on the UE.

The session remains active longer than expected after the request.

Likely cause: The release procedure may still be pending, or the network may have initiated a release command path that has not yet completed.

What to inspect: Check for `PDU Session Release Command`, `PDU Session Release Reject`, and timer expiry.

Next step: Trace the full release procedure rather than treating the request as the final step.

The release request carries a cause value that looks unexpected.

Likely cause: The UE may be signaling a local QoS, packet-filter, or TFT problem rather than a normal user-driven disconnect.

What to inspect: Check the exact `5GSM cause`, the local application state, and the feature or policy context.

Next step: Map the cause to the UE behavior before assuming the session was intentionally closed.

FAQ

What does PDU Session Release Request do?

It starts the UE-requested release procedure for an active PDU session.

Who sends PDU Session Release Request?

The UE sends it to the SMF via the AMF.

When is PDU Session Release Request sent?

It is sent when the UE wants to end an active PDU session or needs to release it because of a local session condition.

What are the important IEs in PDU Session Release Request?

Start with PDU Session ID and PTI, then inspect any 5GSM cause or Extended Protocol Configuration Options fields.

Does this message always include a cause value?

No. The 5GSM cause is optional, but it can be helpful when the UE is releasing the session for a specific reason.

What happens after PDU Session Release Request?

The SMF can accept the request and proceed with release handling, or it can reject the request if the release is not accepted.

Is ASN.1 used for PDU Session Release Request?

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

What timer is related to this message?

T3582 is the main UE-side timer after the release request is sent.

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.