| Protocol | nas | Network | 5g |
|---|---|---|---|
| Spec | 3GPP TS 24.501 | Spec Section | 8.3.8 |
| Direction | SMF to UE via AMF | Message Type | 5GSM signaling |
| Full message name | 5G NAS - PDU Session Modification 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 requested QoS, packet filter, DNN, LADN, service, or transaction context cannot be accepted by the network. |
| Main purpose | Stops the requested modification, returns the cause, and may provide retry or congestion guidance so the UE knows whether to wait, retry, or keep the session unchanged. |
| Main specification | 3GPP TS 24.501, 8.3.8 |
| Release added | Release 15 |
| Procedures where used | 5G PDU Session Modification, QoS Flow Establishment, LADN Modification, VoNR service adaptation |
| Related timers | T3581, T3396 |
| Related cause values | #26 insufficient resources, #31 request rejected, unspecified, #32 service option not supported, #35 PTI already in use, #43 invalid PDU session identity, #46 out of LADN service area, #59 unsupported 5QI value, #67 insufficient resources for specific slice and DNN, #69 insufficient resources for specific slice, #83 semantic error in the QoS operation, #84 syntactical error in the QoS operation, #95-111 protocol errors |
PDU Session Modification Reject is the 5GSM failure response the network sends when a UE-requested change to an active PDU session cannot be accepted.
Stops the requested modification, returns the cause, and may provide retry or congestion guidance so the UE knows whether to wait, retry, or keep the session unchanged.
Call flow position: Negative network response to a UE-initiated change for an already active PDU session.
Typical state: The UE already has a live PDU session and is asking the network to revise session behavior.
Preconditions:
Next likely message: PDU Session Modification Command, release handling, or no change after reject
Call flow position: Often appears when the modification request is tied to a specific service, slice, or local area restriction.
Typical state: The session exists, but the requested change is not allowed in the current context.
Preconditions:
Next likely message: PDU Session Modification Command or a retry after back-off
Previous message(s): PDU Session Modification Request, DL NAS Transport, Network evaluation of the requested modification
Next message(s): PDU Session Modification Command, PDU Session Release Command, Retry after back-off
Security context: Normally delivered while NAS security is already active, so engineers usually expect protected downlink NAS handling.
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.
PDU Session Modification Reject
Extended Protocol Discriminator: 5G Session Management
PDU Session ID: 10
PTI: 5
Message Type: PDU Session Modification Reject
5GSM Cause: Unsupported 5QI value
Back-off Timer Value: 5 minutes
5GSM Congestion Re-attempt Indicator: retry after congestion clears
Re-attempt Indicator: retry permitted
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.