5G NGAP - NAS Non Delivery Indication

Message Fact Sheet

Protocol	ngap	Network	5g
Spec	3GPP TS 38.413	Spec Section	8.6.4, 9.2.5.4 (Release 18 baseline)
Direction	NG-RAN -> AMF	Message Type	Failure indication

Full message name	5G NGAP - NAS Non Delivery Indication
Protocol	NGAP
Technology	5G
Direction	NG-RAN -> AMF
Interface	N2 / NG-C
Signaling bearer / channel	UE-associated NGAP signaling context / SCTP carried NGAP initiatingMessage
Typical trigger	The gNB receives a Downlink NAS Transport from the AMF but cannot deliver the NAS-PDU to the UE because of radio unreachability, context loss, or procedure interruption.
Main purpose	Reports N2 downlink NAS delivery failure so the AMF can stop waiting for UE response and move to recovery actions such as retry, paging, context release, or procedure abort.
Main specification	3GPP TS 38.413, 8.6.4, 9.2.5.4 (Release 18 baseline)
Release added	Release 15
Procedures where used	5G Initial Registration, 5G Authentication Procedure, 5G Security Mode Control, CM-IDLE reachability handling, Mobility interruption recovery

What is NAS Non Delivery Indication in simple terms?

NAS Non Delivery Indication is the NGAP message the NG-RAN sends to the AMF when a downlink NAS message cannot be delivered to the UE.

Reports N2 downlink NAS delivery failure so the AMF can stop waiting for UE response and move to recovery actions such as retry, paging, context release, or procedure abort.

Why this message matters

NAS Non Delivery Indication means the gNB could not deliver an AMF downlink NAS message to the UE and is informing the AMF of that failure.

Where this message appears in the call flow

5G Initial Registration failure branch

5G NGAP registration failure flow showing Downlink NAS Transport followed by NAS Non Delivery Indication when UE cannot receive NAS message — Registration failure branch: gNB reports delivery failure to AMF instead of receiving expected UE NAS response.

Call flow position: Failure indication from NG-RAN to AMF after an AMF downlink NAS step such as Authentication Request or Security Mode Command could not be delivered to the UE.

Typical state: AMF expects a UE NAS response, but the gNB detects that downlink delivery failed before the UE could process the NAS message.

Preconditions:

Downlink NAS Transport for the UE was already received from AMF.
AMF UE NGAP ID and RAN UE NGAP ID correlation is valid at detection time.
gNB determined NAS delivery failure on access side.

Next likely message: AMF recovery action such as retry, paging, or UE context release

CM-IDLE paging and reachability failure

5G NGAP paging failure flow showing unreachable UE after paging and gNB sending NAS Non Delivery Indication to AMF — Paging/reachability failure branch: pending downlink NAS cannot be delivered after reachability attempt, so gNB notifies AMF.

Call flow position: Failure indication when AMF downlink NAS cannot be delivered after paging attempts or reachability restoration steps.

Typical state: The UE is unavailable on the radio side within the expected delivery window, so gNB reports failed NAS delivery upward.

Preconditions:

A downlink NAS payload was pending delivery to a UE with weak or absent reachability.
gNB delivery attempt failed in the current paging or restoration branch.

Next likely message: AMF paging retry, timer expiry handling, or context cleanup

Mobility or handover interruption

5G NGAP mobility interruption flow showing context transition during handover and NAS Non Delivery Indication toward AMF — Mobility interruption branch: context transitions can block NAS delivery and trigger NAS Non Delivery Indication with cause details.

Call flow position: Failure indication when downlink NAS delivery is interrupted during mobility transitions, handover preparation, or context relocation windows.

Typical state: UE context may have changed cell, been released, or become temporarily inconsistent while AMF NAS delivery was in flight.

Preconditions:

Downlink NAS Transport arrived during a mobility-sensitive procedure window.
RAN could not deliver payload with active context conditions.

Next likely message: AMF decision to re-establish context, re-route, or release

Call flow position

Previous message(s): Downlink NAS Transport, Paging, Initial Context Setup Request

Next message(s): UE Context Release Command, Downlink NAS Transport, Paging, Initial Context Setup Request

Message direction and transport

Sender and receiver: NG-RAN -> AMF

Interface: N2 / NG-C

Domain: Access to core failure signaling for UE-specific NAS delivery

Signaling bearer: UE-associated NGAP signaling context

Logical channel: SCTP carried NGAP initiatingMessage

Transport / encapsulation: NGAP over SCTP/IP between NG-RAN and AMF

Security context: Protected by the N2 transport association. The embedded NAS-PDU is the failed downlink NAS payload that could not reach the UE.

Message Structure Overview

NAS Non Delivery Indication is an NGAP initiating message sent by NG-RAN to AMF for a UE-associated failure branch.
Four IEs form the full mandatory set: AMF UE NGAP ID, RAN UE NGAP ID, NAS-PDU, and Cause.
Operational value comes from correlating the failed NAS-PDU and Cause with the preceding Downlink NAS Transport step.

ASN.1 for 5G NGAP - NAS Non Delivery Indication

NASNonDeliveryIndication ::= SEQUENCE {
  protocolIEs ProtocolIE-Container { {NASNonDeliveryIndication-IEs} },
  ...
}

NASNonDeliveryIndication-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-NAS-PDU                     CRITICALITY reject TYPE NAS-PDU        PRESENCE mandatory } |
  { ID id-Cause                       CRITICALITY ignore TYPE Cause          PRESENCE mandatory },
  ...
}

How to read this ASN.1

Use this message as the negative outcome partner of Downlink NAS Transport. The NAS-PDU IE tells you exactly which downlink NAS message failed, and Cause tells you why the RAN could not deliver it.

5G NGAP - NAS Non Delivery Indication - Example Dump

NGAP-PDU
  initiatingMessage
    procedureCode: id-NASNonDeliveryIndication
    criticality: ignore
    value: NASNonDeliveryIndication
      protocolIEs:
        - id: id-AMF-UE-NGAP-ID
          value: 0x10A1
        - id: id-RAN-UE-NGAP-ID
          value: 0x0041
        - id: id-NAS-PDU
          value:
            Security Mode Command
              Extended Protocol Discriminator: 5GMM
              Security Header Type: Plain NAS
              Message Type: Security Mode Command
        - id: id-Cause
          value:
            radioNetwork: ue-in-rrc-inactive-state

How to read this dump

Start by decoding NAS-PDU to identify the exact AMF downlink NAS step that failed.
Then read Cause to understand whether failure is reachability, context, or transport related.
Finally correlate with preceding Downlink NAS Transport and expected UE response path that never occurred.

Important Information Elements

IE	Required	Description
`AMF UE NGAP ID`	Yes	AMF-side UE identifier used to correlate the failure indication to the exact UE context in the AMF.
`RAN UE NGAP ID`	Yes	NG-RAN-side UE identifier used to identify the affected RAN UE context where NAS delivery failed.
`NAS-PDU`	Yes	Carries the specific downlink NAS payload that failed to be delivered, allowing AMF to identify exactly which NAS message was impacted.
`Cause`	Yes	Explains why delivery failed using NGAP cause categories and values, typically radio network or transport related for this procedure.

Detailed field explanation

`AMF UE NGAP ID`

AMF-side UE identifier used to correlate the failure indication to the exact UE context in the AMF.

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`

NG-RAN-side UE identifier used to identify the affected RAN UE context where NAS delivery failed.

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.

`NAS-PDU`

Carries the specific downlink NAS payload that failed to be delivered, allowing AMF to identify exactly which NAS message was impacted.

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.

`Cause`

Explains why delivery failed using NGAP cause categories and values, typically radio network or transport related for this procedure.

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.

What to check in logs and traces

Find the immediately preceding Downlink NAS Transport and verify it targets the same AMF/RAN UE NGAP ID pair.
Decode the failed NAS-PDU to understand which procedure branch was interrupted.
Inspect Cause and map it to radio reachability, UE context, or signaling transport conditions.
Check whether UE was in expected RRC state and whether paging or resume steps were ongoing.
Confirm AMF recovery behavior after indication: retry, paging, release, or procedure abort.

Common Issues and Troubleshooting

AMF keeps waiting for UE response after Downlink NAS Transport but receives NAS Non Delivery Indication instead.

Likely cause: The UE did not receive the downlink NAS payload because of RRC unreachability, context loss, or interruption in access-side delivery path.

What to inspect: Review the failed NAS-PDU, NGAP Cause, and UE RRC/paging state around the same timestamp.

Next step: Treat this as a delivery-path failure first, then decide whether AMF should retry delivery or release context.

Repeated NAS Non Delivery Indications for the same UE.

Likely cause: Persistent reachability failure, stale UE context mapping, or unresolved mobility transition may prevent successful NAS delivery retries.

What to inspect: Compare AMF/RAN UE NGAP ID continuity, mobility events, and repeated Cause values across attempts.

Next step: Escalate to context repair path: re-page, re-establish context, or perform explicit UE context release and fresh access.

Registration or security procedure aborts unexpectedly after NAS delivery failure indication.

Likely cause: The failed NAS message was critical to continuing registration/security handshake, so AMF terminated the branch after non-delivery report.

What to inspect: Identify procedure stage from failed NAS-PDU and verify AMF decision logic immediately after indication.

Next step: Align AMF recovery policy with observed Cause profile to avoid repeated abort loops for transient radio failures.

LTE / 5G / Variant Comparison

Compared with Downlink NAS Transport

Downlink NAS Transport carries the AMF NAS message toward UE. NAS Non Delivery Indication reports that the exact payload could not be delivered by NG-RAN.

Compared with Uplink NAS Transport

Uplink NAS Transport carries successful UE NAS responses upward. NAS Non Delivery Indication appears instead when the UE never receives or never returns a response due to delivery failure.

Compared with Error Indication

NAS Non Delivery Indication is a procedure-specific delivery failure report that includes failed NAS-PDU. Error Indication is a broader NGAP protocol error signaling mechanism.

FAQ

What is NAS Non Delivery Indication in 5G NGAP?

It is the NGAP message sent by NG-RAN to AMF when a downlink NAS message cannot be delivered to the UE.

Which fields are mandatory in NAS Non Delivery Indication?

AMF UE NGAP ID, RAN UE NGAP ID, NAS-PDU, and Cause are mandatory.

Why does this message include NAS-PDU?

So AMF can identify exactly which downlink NAS message failed and choose procedure-specific recovery instead of generic retry.

What should I check first when this appears in trace?

Start with the immediately preceding Downlink NAS Transport, then decode Cause and verify UE reachability/RRC state.

Is NAS Non Delivery Indication the same as Error Indication?

No. NAS Non Delivery Indication is a dedicated downlink NAS delivery-failure report with failed NAS payload context.

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.