PDU Session Resource Notify is the NGAP message sent by the NG-RAN node to the AMF when the RAN needs to report PDU session resource status changes, including notified resources, released resources, and session-level transfer information.
Message Fact Sheet
Protocol
ngap
Network
5g
Spec
3GPP TS 38.413
Spec Section
PDU Session Management procedures and PDU Session Resource Notify message set (Release 18 baseline)
NG-RAN detects PDU session resource status or change information that must be reported to AMF, including notified session resources or resources released at the NG-RAN side.
Main purpose
Notifies AMF about PDU session resource status at the NG-RAN side, reports PDU sessions being notified, reports PDU sessions released at the NG-RAN side, carries per-session transfer containers, and helps AMF and SMF synchronize core state with RAN-side resource state.
Main specification
3GPP TS 38.413, PDU Session Management procedures and PDU Session Resource Notify message set (Release 18 baseline)
Release added
Release 15
Procedures where used
PDU Session Resource Notify, PDU Session Management, NG-RAN-initiated resource status notification, Released session resource reporting, RAN and core session-state synchronization
What is PDU Session Resource Notify in simple terms?
PDU Session Resource Notify is the NGAP message sent by the NG-RAN node to the AMF when the RAN needs to report PDU session resource status changes, including notified resources, released resources, and session-level transfer information.
Notifies AMF about PDU session resource status at the NG-RAN side, reports PDU sessions being notified, reports PDU sessions released at the NG-RAN side, carries per-session transfer containers, and helps AMF and SMF synchronize core state with RAN-side resource state.
Why this message matters
PDU Session Resource Notify is the NG-RAN telling the AMF that something about PDU session resources needs to be reflected in core state.
Notification branch: NG-RAN reports PDU session resource status or change information to AMF for core-side synchronization.
Call flow position: NG-RAN sends this initiatingMessage when it needs to notify AMF about PDU session resource status or change information for an existing UE context.
Typical state: The UE context exists, and one or more PDU session resources are active, changed, or recently released at the NG-RAN side.
Preconditions:
AMF UE NGAP ID and RAN UE NGAP ID identify a live UE-associated NGAP context.
NG-RAN has PDU session resource status information to report.
At least one notified or released session item may be available for reporting.
Next likely message: AMF and SMF session-state update, with follow-up signaling if needed
Notified and released session reporting
List branch: notified sessions and released sessions are reported in separate lists and decoded by PDU Session ID.
Call flow position: The message separates sessions being notified from sessions released at the NG-RAN side by using different lists.
Typical state: AMF and SMF need to interpret the Notify List and Released List separately before deciding whether follow-up handling is needed.
Preconditions:
Each reported item is identified by PDU Session ID.
The session-level reason and details are carried in the corresponding transfer container.
Next likely message: Core-side state synchronization and targeted follow-up for released resources
Resource-state synchronization
Procedure distinction: Notify is an NG-RAN-initiated status report, not a modify request and not a response to Release Command.
Call flow position: Notify is used to keep RAN and core session-resource state aligned when NG-RAN has status or release information to report.
Typical state: The message does not ask for modification like Modify Indication and does not answer a Release Command like Release Response.
Preconditions:
The UE-associated NGAP context remains identifiable.
NG-RAN has resource-state information that AMF or SMF must learn.
Next likely message: Policy, release, modification, or cleanup handling depending on the transfer details
Call flow position
Previous message(s): Existing UE context and active or recently changed PDU session resources, NG-RAN detects a session-resource status condition that must be reported
Transport / encapsulation: NGAP over SCTP/IP between NG-RAN and AMF
Security context: The message assumes an existing UE-associated NGAP context. It reports RAN-side session resource status or changes rather than initiating setup, release, or a paired response procedure by itself.
Message Structure Overview
PDU Session Resource Notify is an NG-RAN-to-AMF initiatingMessage in the NGAP PDU Session Management family.
It is a notification, not a direct response message.
AMF UE NGAP ID and RAN UE NGAP ID anchor the notification to the live UE-associated NGAP context.
PDU Session Resource Notify List reports sessions for which NG-RAN is sending session-resource status information.
PDU Session Resource Released List reports sessions released or no longer available at the NG-RAN side.
Each notified or released item is correlated by PDU Session ID and decoded through its transfer container.
The Release 18 list bound is maxnoofPDUSessions = 256.
ASN.1 for 5G NGAP - PDU Session Resource Notify
PDUSessionResourceNotify ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {PDUSessionResourceNotify-IEs} },
...
}
PDUSessionResourceNotify-IEs NGAP-PROTOCOL-IES ::= {
{ ID id-AMF-UE-NGAP-ID CRITICALITY reject TYPE AMF-UE-NGAP-ID PRESENCE mandatory } |
{ ID id-RAN-UE-NGAP-ID CRITICALITY reject TYPE RAN-UE-NGAP-ID PRESENCE mandatory } |
{ ID id-PDUSessionResourceNotifyList CRITICALITY ignore TYPE PDUSessionResourceNotifyList PRESENCE optional } |
{ ID id-PDUSessionResourceReleasedList CRITICALITY ignore TYPE PDUSessionResourceReleasedList PRESENCE optional } |
{ ID id-UserLocationInformation CRITICALITY ignore TYPE UserLocationInformation PRESENCE optional },
...
}
PDUSessionResourceNotifyItem ::= SEQUENCE {
pduSessionID PDU Session ID,
pduSessionResourceNotifyTransfer OCTET STRING,
...
}
PDUSessionResourceReleasedItem ::= SEQUENCE {
pduSessionID PDU Session ID,
pduSessionResourceNotifyReleasedTransfer OCTET STRING,
...
}
How to read this ASN.1
Decode the UE identity pair first, then inspect the Notify List and Released List separately. The top-level message says NG-RAN is notifying AMF; the engineering reason and state detail live inside each per-session transfer container.
5G NGAP - PDU Session Resource Notify - Example Dump
Treat this as a teaching example based on the spec structure, not as a captured network trace.
Notify List and Released List serve different purposes and should not be merged during analysis.
The message has no direct paired response; follow-up depends on AMF and SMF interpretation of the reported state.
Transfer-container decoding is required to understand the actual session-level notification reason.
Important Information Elements
IE
Required
Description
Message Type
Yes
Identifies the NGAP PDU as PDU SESSION RESOURCE NOTIFY.
AMF UE NGAP ID
Yes
Mandatory AMF-side UE identifier used to correlate the notification with the correct UE context.
RAN UE NGAP ID
Yes
Mandatory NG-RAN-side UE identifier used to bind the reported resource status to the correct radio-side UE context.
PDU Session Resource Notify List
Optional
Optional list of PDU sessions for which NG-RAN is reporting status. Each item includes PDU Session ID and PDU Session Resource Notify Transfer.
PDU Session Resource Released List
Optional
Optional list of PDU sessions released or no longer available at the NG-RAN side. Each item includes PDU Session ID and PDU Session Resource Notify Released Transfer.
User Location Information
Optional
Optional UE location information that can help AMF and SMF correlate session-resource changes with the serving area.
Detailed field explanation
Message Type
Identifies the NGAP PDU as PDU SESSION RESOURCE NOTIFY.
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.
AMF UE NGAP ID
Mandatory AMF-side UE identifier used to correlate the notification with the correct UE context.
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.
RAN UE NGAP ID
Mandatory NG-RAN-side UE identifier used to bind the reported resource status to the correct radio-side UE context.
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.
PDU Session Resource Notify List
Optional list of PDU sessions for which NG-RAN is reporting status. Each item includes PDU Session ID and PDU Session Resource Notify Transfer.
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.
PDU Session Resource Released List
Optional list of PDU sessions released or no longer available at the NG-RAN side. Each item includes PDU Session ID and PDU Session Resource Notify Released Transfer.
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.
User Location Information
Optional UE location information that can help AMF and SMF correlate session-resource changes with the serving area.
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.
What to check in logs and traces
Confirm the message direction is NG-RAN node to AMF.
Verify AMF UE NGAP ID and RAN UE NGAP ID match the active UE context.
List every PDU Session ID in PDU Session Resource Notify List.
List every PDU Session ID in PDU Session Resource Released List.
Decode PDU Session Resource Notify Transfer for notified items.
Decode PDU Session Resource Notify Released Transfer for released items.
Use User Location Information when the status change is location-sensitive.
Confirm AMF and SMF follow-up behavior matches the reported state.
Common Issues and Troubleshooting
The message is treated as a response to another procedure.
Likely cause: PDU Session Resource Notify is being confused with Release Response or another successfulOutcome message.
What to inspect: Check that the NGAP PDU is an initiatingMessage sent from NG-RAN to AMF and that there is no direct paired response expected.
Next step: Analyze Notify as an NG-RAN-initiated resource-state report.
A released session is missed during trace analysis.
Likely cause: The PDU Session Resource Released List may have been ignored or merged with the Notify List.
What to inspect: Decode Notify List and Released List separately and enumerate PDU Session IDs in both lists.
Next step: Handle released items as session-level state changes that require core-side synchronization.
The reason for the notification is unclear.
Likely cause: The per-session transfer containers may not have been decoded.
What to inspect: Open PDU Session Resource Notify Transfer or PDU Session Resource Notify Released Transfer for the affected PDU Session ID.
Next step: Use transfer details, not the top-level message name, to determine follow-up handling.
AMF or SMF updates the wrong session.
Likely cause: Correlation may have used only the UE identity pair without matching PDU Session ID.
What to inspect: Compare AMF UE NGAP ID, RAN UE NGAP ID, and each PDU Session ID against the active session inventory.
Next step: Fix PDU Session ID correlation before diagnosing policy or release behavior.
Notify is confused with Modify Indication.
Likely cause: Both messages are NG-RAN-to-AMF initiating messages, but their purposes differ.
What to inspect: Check whether the message requests modification or reports resource status/change information.
Next step: Use Modify Indication analysis only when NG-RAN is requesting modification; use Notify analysis for resource-state reporting.
LTE / 5G / Variant Comparison
Compared with PDU Session Resource Modify Indication
Modify Indication asks AMF and SMF to modify existing PDU session resources. Notify reports PDU session resource status or change information.
Compared with PDU Session Resource Release Response
Release Response answers an AMF-triggered Release Command. Notify is NG-RAN initiated and can report released resources without being a direct release-command response.
Compared with PDU Session Resource Modify Confirm
Modify Confirm is a paired response to Modify Indication. Notify has no direct paired response; follow-up depends on AMF and SMF handling.
FAQ
What is PDU Session Resource Notify in NGAP?
It is the NG-RAN-to-AMF message used to report PDU session resource status or change information for an existing UE context.
Who sends PDU Session Resource Notify?
The NG-RAN node sends PDU Session Resource Notify to the AMF.
When is this message used?
It is used when NG-RAN needs to notify AMF about PDU session resource status changes, including notified resources or resources released at the RAN side.
Is PDU Session Resource Notify a response message?
No. It is an NG-RAN-initiated initiatingMessage and has no direct paired response message.
What is PDU Session Resource Notify List?
It is the optional list of PDU sessions for which NG-RAN is reporting status, with one item per PDU Session ID.
What is PDU Session Resource Released List?
It is the optional list of PDU sessions released or no longer available at the NG-RAN side.
What does PDU Session Resource Notify Transfer contain?
It carries the session-level notification details for a notified PDU session item.
How is Notify different from Modify Indication?
Modify Indication requests modification of established PDU session resources, while Notify reports resource status or change information.
How is Notify different from Release Response?
Release Response answers an AMF-triggered Release Command, while Notify is NG-RAN initiated and can report released resources without being a direct response.
How do you troubleshoot PDU Session Resource Notify?
Check direction, match UE IDs, enumerate PDU Session IDs in Notify and Released lists separately, and decode each transfer container.
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.
