PDU Session Resource Modify Confirm is the NGAP message sent by the AMF to the NG-RAN node in response to PDU Session Resource Modify Indication, confirming successful and failed per-session modification outcomes.
Message Fact Sheet
Protocol
ngap
Network
5g
Spec
3GPP TS 38.413
Spec Section
Clause 8.2.5, clause 9.2.1.8, and clause 9.2.1.9 (Release 18 baseline)
What is PDU Session Resource Modify Confirm in simple terms?
PDU Session Resource Modify Confirm is the NGAP message sent by the AMF to the NG-RAN node in response to PDU Session Resource Modify Indication, confirming successful and failed per-session modification outcomes.
Confirms the result of NG-RAN-initiated PDU session resource modification, carrying successful modified PDU session results, failed modified PDU session results, session-level transfer information, and optional Criticality Diagnostics.
Why this message matters
PDU Session Resource Modify Confirm is the AMF telling NG-RAN how the 5GC handled a RAN-initiated modify indication, session by session.
Where this message appears in the call flow
PDU Session Resource Modify Indication result
Confirm branch: NG-RAN initiates modification with Modify Indication, and AMF returns the per-session outcome with Modify Confirm.
Call flow position: AMF sends this successfulOutcome after NG-RAN has initiated PDU session modification using PDU Session Resource Modify Indication.
Typical state: The UE context remains active while NG-RAN learns which requested session modifications were accepted or failed by the 5GC.
Preconditions:
PDU Session Resource Modify Indication was received from NG-RAN.
AMF UE NGAP ID and RAN UE NGAP ID still identify the same UE-associated NGAP context.
AMF and SMF processed the indicated per-session modification details.
Next likely message: NG-RAN continuation for successful items and targeted handling for failed items
Mixed per-session confirm outcome
Per-session result branch: one PDU session can succeed while another fails, so confirm analysis must be done by PDU Session ID.
Call flow position: The confirm can carry both successfully modified PDU sessions and failed-to-modify PDU sessions in one message.
Typical state: One PDU session may continue with updated state while another requires follow-up handling.
Preconditions:
The earlier Modify Indication included one or more PDU session items.
AMF and SMF evaluated each indicated PDU Session ID.
Next likely message: Apply successful confirm transfers and troubleshoot failed unsuccessful transfers
Modify Confirm versus Modify Response
Branch distinction: Modify Confirm closes the NG-RAN-initiated indication branch; Modify Response closes the AMF-initiated request branch.
Call flow position: Modify Confirm belongs to the NG-RAN-initiated indication and confirm pair, while Modify Response belongs to the AMF-initiated request and response pair.
Typical state: Trace analysis must keep the two modification branches separate even though both can report per-session success and failure.
Preconditions:
The preceding message is PDU Session Resource Modify Indication for Modify Confirm.
The preceding message is PDU Session Resource Modify Request for Modify Response.
Next likely message: Continue the matching modify branch, not the other pair
Transport / encapsulation: NGAP over SCTP/IP between AMF and NG-RAN
Security context: The confirm assumes an existing UE-associated NGAP context and follows a RAN-initiated Modify Indication. It reports 5GC handling of existing session resources rather than initiating a new modification by itself.
Message Structure Overview
PDU Session Resource Modify Confirm is the AMF-to-NG-RAN successfulOutcome message for the NG-RAN-initiated Modify Indication procedure.
AMF UE NGAP ID and RAN UE NGAP ID anchor the confirm to the earlier indication and the live UE-associated NGAP context.
PDU Session Resource Modify List reports sessions successfully handled by the 5GC.
PDU Session Resource Failed to Modify List reports sessions that failed modification.
Each success or failure item is correlated with the earlier indication by PDU Session ID.
The Release 18 list bound is maxnoofPDUSessions = 256.
ASN.1 for 5G NGAP - PDU Session Resource Modify Confirm
PDUSessionResourceModifyConfirm ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {PDUSessionResourceModifyConfirm-IEs} },
...
}
PDUSessionResourceModifyConfirm-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-PDUSessionResourceModifyListModCfm CRITICALITY ignore TYPE PDUSessionResourceModifyListModCfm PRESENCE optional } |
{ ID id-PDUSessionResourceFailedToModifyListModCfm CRITICALITY ignore TYPE PDUSessionResourceFailedToModifyListModCfm PRESENCE optional } |
{ ID id-CriticalityDiagnostics CRITICALITY ignore TYPE CriticalityDiagnostics PRESENCE optional },
...
}
PDUSessionResourceModifyItemModCfm ::= SEQUENCE {
pduSessionID PDU Session ID,
pduSessionResourceModifyConfirmTransfer OCTET STRING,
...
}
PDUSessionResourceFailedToModifyItemModCfm ::= SEQUENCE {
pduSessionID PDU Session ID,
pduSessionResourceModifyUnsuccessfulTransfer OCTET STRING,
...
}
How to read this ASN.1
Decode the UE identity pair first, then inspect the successful and failed-to-modify lists separately. The top-level successfulOutcome only confirms that the response message completed; it does not mean every indicated PDU session modification succeeded.
Treat this as a teaching example based on the spec structure, not as a captured network trace.
One confirm can contain both successful and failed PDU session items.
The presence of successfulOutcome does not mean every session in the earlier indication succeeded.
Failure analysis depends on decoding PDU Session Resource Modify Unsuccessful Transfer for each failed item.
Important Information Elements
IE
Required
Description
Message Type
Yes
Identifies the NGAP PDU as PDU SESSION RESOURCE MODIFY CONFIRM.
AMF UE NGAP ID
Yes
Mandatory AMF-side UE identifier used to correlate the confirm with the earlier Modify Indication and the correct UE context.
RAN UE NGAP ID
Yes
Mandatory NG-RAN-side UE identifier used to bind the confirm result to the correct radio-side UE context.
PDU Session Resource Modify List
Optional
Optional list of sessions successfully handled by the 5GC. Each item includes PDU Session ID and PDU Session Resource Modify Confirm Transfer.
PDU Session Resource Failed to Modify List
Optional
Optional list of sessions that failed modification. Each item includes PDU Session ID and PDU Session Resource Modify Unsuccessful Transfer.
Criticality Diagnostics
Optional
Optional diagnostics that can explain IE processing, decode, or criticality-handling issues.
Detailed field explanation
Message Type
Identifies the NGAP PDU as PDU SESSION RESOURCE MODIFY CONFIRM.
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 confirm with the earlier Modify Indication and 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 confirm result 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 Modify List
Optional list of sessions successfully handled by the 5GC. Each item includes PDU Session ID and PDU Session Resource Modify Confirm 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 Failed to Modify List
Optional list of sessions that failed modification. Each item includes PDU Session ID and PDU Session Resource Modify Unsuccessful 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.
Criticality Diagnostics
Optional diagnostics that can explain IE processing, decode, or criticality-handling issues.
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 follows a matching PDU Session Resource Modify Indication.
Verify AMF UE NGAP ID and RAN UE NGAP ID match the earlier indication and active UE context.
Compare PDU Session IDs in the indication list with success and failed-to-modify lists in the confirm.
Decode PDU Session Resource Modify Confirm Transfer for successful items.
Decode PDU Session Resource Modify Unsuccessful Transfer for failed items.
Check Criticality Diagnostics when present.
Keep Modify Confirm separate from AMF-initiated Modify Response.
Common Issues and Troubleshooting
The confirm is present, but one indicated PDU session did not update.
Likely cause: The confirm may contain a failed-to-modify item even though the NGAP successfulOutcome was delivered.
What to inspect: Read PDU Session Resource Modify List and PDU Session Resource Failed to Modify List separately, then correlate each item by PDU Session ID.
Next step: Troubleshoot the failed PDU Session ID and its unsuccessful transfer rather than treating the whole confirm as successful.
Trace analysis labels the message as Modify Response.
Likely cause: The AMF-initiated request/response branch is being confused with the NG-RAN-initiated indication/confirm branch.
What to inspect: Check whether the preceding message is Modify Indication or Modify Request and verify message direction.
Next step: Analyze this message as Modify Confirm only when it responds to Modify Indication.
NG-RAN applies the result to the wrong session.
Likely cause: Correlation may have stopped at the UE identity pair and missed per-session PDU Session ID matching.
What to inspect: Compare AMF UE NGAP ID, RAN UE NGAP ID, and every PDU Session ID across the indication and confirm.
Next step: Fix per-session correlation before diagnosing the transfer-container contents.
A failed modification has no clear top-level cause.
Likely cause: The failure cause is carried in PDU Session Resource Modify Unsuccessful Transfer, not in the top-level confirm name.
What to inspect: Decode the unsuccessful transfer for the failed PDU Session ID.
Next step: Use the session-level failure details to decide whether to retry, adjust policy, or clean up resources.
LTE / 5G / Variant Comparison
Compared with PDU Session Resource Modify Indication
Modify Indication is the NG-RAN request. Modify Confirm is the AMF-to-NG-RAN result message for that request.
Compared with PDU Session Resource Modify Response
Modify Response belongs to the AMF-initiated request/response pair. Modify Confirm belongs to the NG-RAN-initiated indication/confirm pair.
Compared with PDU Session Resource Modify Request
Modify Request starts an AMF-initiated modification. Modify Confirm closes a RAN-initiated modification.
FAQ
What is PDU Session Resource Modify Confirm in NGAP?
It is the AMF-to-NG-RAN message used to confirm the result of a RAN-initiated PDU Session Resource Modify Indication.
Who sends PDU Session Resource Modify Confirm?
The AMF sends PDU Session Resource Modify Confirm to the NG-RAN node.
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.
