PDU Session Resource Modify Indication is the NGAP message sent by the NG-RAN node to the AMF when the RAN side requests modification of one or more already established PDU session resources.
PDU Session Resource Modify Indication, PDU Session Management, NG-RAN-initiated session modification, DL transport and QoS flow mapping update, Security, ECN, congestion, and usage reporting
What is PDU Session Resource Modify Indication in simple terms?
PDU Session Resource Modify Indication is the NGAP message sent by the NG-RAN node to the AMF when the RAN side requests modification of one or more already established PDU session resources.
Requests AMF and SMF handling for NG-RAN-initiated modification of existing PDU session resources, carrying per-session modify indication transfers that can update downlink transport information, QoS flow mapping, security result, ECN or congestion reporting state, usage information, and optional UE location information.
Why this message matters
PDU Session Resource Modify Indication is the NG-RAN asking the core network to handle a change to an already established PDU session resource.
Where this message appears in the call flow
NG-RAN-initiated PDU session modification
NG-RAN-initiated branch: existing session resources are active, NG-RAN requests modification, and AMF later returns Modify Confirm.
Call flow position: NG-RAN sends this initiatingMessage when the UE context exists, one or more PDU session resources are already established, and the RAN side needs AMF and SMF to handle a requested modification.
Typical state: The session remains established while NG-RAN requests a change to its radio-side or transport-side realization.
Preconditions:
AMF UE NGAP ID and RAN UE NGAP ID already identify a live UE-associated NGAP context.
At least one PDU session resource is already established.
NG-RAN has per-session modification details ready in the Modify Indication Transfer.
Next likely message: PDU Session Resource Modify Confirm
Per-session modify indication handling
Per-session branch: each indicated PDU Session ID has its own Modify Indication Transfer carrying the actual modification details.
Call flow position: Each PDU session that NG-RAN wants to modify is represented by one item in the PDU Session Resource Modify Indication List.
Typical state: AMF and SMF process the requested change per PDU Session ID and later report the outcome in Modify Confirm.
Preconditions:
The indicated PDU Session IDs are valid for the active UE context.
Next likely message: PDU Session Resource Modify Confirm with per-session successful and failed results
DL transport and QoS flow mapping update
QoS and transport branch: the transfer can adjust DL QoS-flow-to-TNL mapping and report security, ECN, congestion, or usage state.
Call flow position: The indication can update downlink QoS-flow-to-TNL mapping, additional DL transport information, redundant DL transport information, security status, ECN or congestion reporting state, and related usage information.
Typical state: NG-RAN and 5GC must keep the session's transport bearer mapping aligned after the requested modification.
Preconditions:
The affected PDU session already has NG-U transport and QoS flow context.
The Modify Indication Transfer contains the relevant transport, QoS, security, or reporting details.
Next likely message: PDU Session Resource Modify Confirm after AMF and SMF process the requested change
Call flow position
Previous message(s): Existing UE context and established PDU session resources, NG-RAN detects that session-resource modification is needed
Transport / encapsulation: NGAP over SCTP/IP between NG-RAN and AMF
Security context: The message assumes an existing UE-associated NGAP context and already established PDU session resources. It requests modification of existing resources rather than setting up a fresh UE context.
Message Structure Overview
PDU Session Resource Modify Indication is an NG-RAN-to-AMF initiatingMessage for the NG-RAN-initiated PDU Session Resource Modify Indication procedure.
It is used for already established PDU session resources, not for initial setup.
AMF UE NGAP ID and RAN UE NGAP ID anchor the indication to the live UE-associated NGAP context.
PDU Session Resource Modify Indication List is the mandatory payload and contains one item per PDU session requested for modification.
Inside each item, PDU Session ID identifies the session and PDU Session Resource Modify Indication Transfer carries the actual session-level modification details.
The Release 18 list bound is maxnoofPDUSessions = 256.
ASN.1 for 5G NGAP - PDU Session Resource Modify Indication
PDUSessionResourceModifyIndication ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {PDUSessionResourceModifyIndication-IEs} },
...
}
PDUSessionResourceModifyIndication-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-PDUSessionResourceModifyListModInd CRITICALITY reject TYPE PDUSessionResourceModifyListModInd PRESENCE mandatory } |
{ ID id-UserLocationInformation CRITICALITY ignore TYPE UserLocationInformation PRESENCE optional },
...
}
PDUSessionResourceModifyItemModInd ::= SEQUENCE {
pduSessionID PDU Session ID,
pduSessionResourceModifyIndicationTransfer OCTET STRING,
...
}
How to read this ASN.1
Decode the UE identity pair first, then enumerate the PDU Session Resource Modify Indication List. The operational meaning is inside each per-session Modify Indication Transfer, so the top-level message only tells you the branch and direction.
Treat this as a teaching example based on the spec structure, not as a captured network trace.
The listed PDU Session IDs should already exist in the active UE context.
The per-session Modify Indication Transfer is where the real transport, QoS, security, ECN, congestion, or usage-reporting detail lives.
The expected response is PDU Session Resource Modify Confirm, not Modify Response.
Important Information Elements
IE
Required
Description
Message Type
Yes
Identifies the NGAP PDU as PDU SESSION RESOURCE MODIFY INDICATION.
AMF UE NGAP ID
Yes
Mandatory AMF-side UE identifier used to correlate the indication with the correct existing UE context.
RAN UE NGAP ID
Yes
Mandatory NG-RAN-side UE identifier used to bind the requested modification to the correct radio-side UE context.
PDU Session Resource Modify Indication List
Yes
Mandatory list of PDU sessions requested for modification. Each item includes PDU Session ID and PDU Session Resource Modify Indication Transfer.
User Location Information
Optional
Optional UE location information reported by NG-RAN when supported.
Detailed field explanation
Message Type
Identifies the NGAP PDU as PDU SESSION RESOURCE MODIFY INDICATION.
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 indication with the correct existing 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 requested modification 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 Indication List
Mandatory list of PDU sessions requested for modification. Each item includes PDU Session ID and PDU Session Resource Modify Indication Transfer.
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.
User Location Information
Optional UE location information reported by NG-RAN when supported.
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-associated NGAP context.
List every PDU Session ID in PDU Session Resource Modify Indication List.
Decode each PDU Session Resource Modify Indication Transfer.
Check DL QoS Flow per TNL Information and any additional or redundant DL transport information.
Validate Security Result, ECN Marking, Congestion Information Reporting Status, and Secondary RAT Usage Information when present.
Confirm the follow-up message is PDU Session Resource Modify Confirm.
Common Issues and Troubleshooting
The trace is interpreted as an AMF-initiated modification even though NG-RAN sent the message.
Likely cause: PDU Session Resource Modify Indication is being confused with PDU Session Resource Modify Request.
What to inspect: Check the NGAP PDU type, direction, procedure name, and whether the follow-up is Modify Confirm rather than Modify Response.
Next step: Analyze the branch as NG-RAN-initiated indication/confirm, not AMF-initiated request/response.
AMF or SMF updates the wrong PDU session.
Likely cause: Correlation may be using only the UE identity pair instead of the PDU Session ID inside each indication item.
What to inspect: Compare AMF UE NGAP ID, RAN UE NGAP ID, and every PDU Session ID against the active session inventory.
Next step: Fix per-session correlation before diagnosing the Modify Indication Transfer.
Downlink user-plane behavior changes unexpectedly after the indication.
Likely cause: DL QoS Flow per TNL Information, additional DL transport information, or redundant transport information may have changed the QoS-flow-to-bearer mapping.
What to inspect: Decode the transfer for the affected PDU Session ID and compare QoS flow mapping before and after the indication.
Next step: Repair the TNL mapping or bearer allocation for the affected session item.
An NG-U bearer appears to be removed after the modification.
Likely cause: After the modification, no QoS flow may remain associated with that existing NG-U bearer, making its NG-U UP TNL information available again.
What to inspect: Check whether any QoS flow remains mapped to the bearer after processing the Modify Indication Transfer.
Next step: Treat the removal as a mapping result unless the transfer content contradicts it.
Security or congestion behavior looks stale after modification.
Likely cause: Security Result, ECN Marking, or Congestion Information Reporting Status may have been ignored or misread.
What to inspect: Decode those fields when present and confirm SMF state reflects the new session-level status.
Next step: Update the session state handling for the affected PDU Session ID.
LTE / 5G / Variant Comparison
Compared with PDU Session Resource Modify Confirm
Modify Indication carries the NG-RAN request. Modify Confirm is the AMF-to-NG-RAN outcome message for that indication.
Compared with PDU Session Resource Modify Request
Modify Request is AMF-initiated and travels AMF to NG-RAN. Modify Indication is NG-RAN-initiated and travels NG-RAN to AMF.
Compared with PDU Session Resource Modify Response
Modify Response reports NG-RAN processing of an AMF request. Modify Indication asks AMF and SMF to process a RAN-initiated change.
FAQ
What is PDU Session Resource Modify Indication in NGAP?
It is the NG-RAN-to-AMF initiating message used when NG-RAN requests modification of already established PDU session resources.
Who sends PDU Session Resource Modify Indication?
The NG-RAN node sends PDU Session Resource Modify Indication to the AMF.
When is this message used?
It is used when the RAN side detects that one or more established PDU session resources need modification.
What is PDU Session Resource Modify Indication List?
It is the mandatory list of PDU sessions that NG-RAN requests to modify, with one item per PDU Session ID.
What does PDU Session Resource Modify Indication Transfer contain?
It carries per-session modification details such as DL QoS Flow per TNL Information, additional or redundant DL transport information, Security Result, ECN or congestion reporting status, Secondary RAT Usage Information, and related session-level data.
What is the response to this message?
The response is PDU Session Resource Modify Confirm.
What is the difference between Modify Indication and Modify Request?
Modify Indication is NG-RAN-initiated and goes from NG-RAN to AMF. Modify Request is AMF-initiated and goes from AMF to NG-RAN.
Can this message update DL transport-layer information?
Yes. The Modify Indication Transfer can include DL QoS Flow per TNL Information, additional DL transport information, and redundant DL transport information.
Can this message include UE location information?
Yes. User Location Information is optional and may be included when supported.
How do you troubleshoot a failed RAN-initiated PDU session modification?
Correlate by AMF UE NGAP ID, RAN UE NGAP ID, and PDU Session ID; decode each Modify Indication Transfer; then inspect the later Modify Confirm for failed sessions or failed QoS flows.
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.
