NAS 5G UE to AMF 3GPP TS 24.501

5G NAS - Identity Response

Identity Response is the NAS message the UE sends after Identity Request to provide the specific identity type the network asked for, such as SUCI, IMEI, IMEISV, or another requested identifier.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.2.11
Direction	UE to AMF	Message Type	5GMM signaling

Full message name	5G NAS - Identity Response
Protocol	NAS
Technology	5G
Direction	UE to AMF
Interface	N1
Signaling bearer / channel	NAS signaling / Dedicated NAS message, commonly transported via UL Information Transfer
Typical trigger	Sent immediately after the UE receives Identity Request and extracts the identity type the AMF wants.
Main purpose	Returns the requested UE identity so the AMF can continue registration, service handling, authentication, or another 5GMM procedure.
Main specification	3GPP TS 24.501, 8.2.11
Release added	Release 15
Procedures where used	5G Initial Registration, Service Request, Mobility Registration Update
Related timers	T3570
Related cause values	Identity type requested in the preceding Identity Request

What is Identity Response in simple terms?

Identity Response is the NAS message the UE sends after Identity Request to provide the specific identity type the network asked for, such as SUCI, IMEI, IMEISV, or another requested identifier.

Returns the requested UE identity so the AMF can continue registration, service handling, authentication, or another 5GMM procedure.

Why this message matters

Identity Response is the UE answering the network's identity question so the NAS procedure can continue.

Where this message appears in the call flow

5G Initial Registration

Call flow position: UE reply step after Identity Request when the network needs a more explicit or alternate identity before continuing.

Typical state: Registration is paused until the UE returns the requested identity.

Preconditions:

The AMF has already sent Identity Request.
The UE can provide the requested identity form.

Next likely message: Authentication Request or Registration Reject

Service Request

Call flow position: Identity clarification response when service recovery cannot continue on the basis of the identity already available.

Typical state: The network needs identity confirmation before restoring or continuing service handling.

Preconditions:

The network has requested a specific UE identity.

Next likely message: Service Accept, Authentication Request, or Service Reject

Call flow position

Previous message(s): Identity Request, Registration Request, Service Request

Next message(s): Authentication Request, Registration Reject, Service Accept

Message direction and transport

Sender and receiver: UE to AMF

Interface: N1

Domain: Core-side mobility management signaling with access-side NAS transport dependency

Signaling bearer: NAS signaling

Logical channel: Dedicated NAS message, commonly transported via UL Information Transfer

Transport / encapsulation: NAS 5GS message carried end-to-end between UE and AMF

Security context: Can appear before full NAS security is active because it often belongs to early registration identity exchange.

Message Structure Overview

Identity Response is a short NAS 5GMM reply whose main value is the actual identity value returned by the UE.
In trace analysis, engineers correlate this message directly with the preceding Identity Request and the next authentication or reject decision.

Structure Syntax

This message is not typically analyzed as ASN.1 on the wire. Engineers usually inspect it as a NAS or protocol field decode rather than an ASN.1 tree.

Identity Response follows NAS 24.501 encoding rather than ASN.1 syntax.

Decoded Message Dump

Identity Response
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Identity Response
  Mobile Identity
    Type of identity: SUCI
    SUPI format: IMSI
    MCC: 001
    MNC: 01
    Routing Indicator: 1234

How to read this dump

The important check is whether the UE returned the same identity type the network requested.
Engineers usually compare this response against the prior Identity Request and the next Authentication Request or Reject.

Important Information Elements

IE	Required	Description
`Mobile identity`	Yes	Carries the specific identity value requested by the network, such as SUCI, IMEI, or IMEISV.

Detailed field explanation

`Mobile identity`

Carries the specific identity value requested by the network, such as SUCI, IMEI, or IMEISV.

Presence: Required

What to check in logs and traces

Confirm which identity type the network requested in Identity Request.
Check that the UE returned the matching identity type in Identity Response.
Verify whether the response is plain or protected NAS and whether that fits the procedure stage.
Correlate the response with the next network decision such as Authentication Request or Registration Reject.

Common Issues and Troubleshooting

The UE sends Identity Response but registration still does not progress.

Likely cause: The returned identity may not match what the AMF requested or the AMF may still reject the identity context.

What to inspect: Compare the requested identity type with the returned mobile identity contents and check the immediate next NAS message.

Next step: Move analysis to Authentication Request or Registration Reject to see how the network evaluated the returned identity.

Identity exchange repeats even after Identity Response.

Likely cause: The response may be malformed, incomplete, or not acceptable for the expected procedure context.

What to inspect: Check identity encoding, identity type, and whether the uplink NAS transport path delivered the message cleanly.

Next step: Compare with a known-good trace and verify that the UE can provide the identity format the network expects.

Detailed explanation

5G NAS - Identity Response Explained

Identity Response is the NAS message the UE sends after receiving Identity Request from the network. Its job is simple but important: provide the exact identity type the AMF asked for so the wider 5GMM procedure can continue.

For beginners, the simple meaning is: the UE is answering the network’s identity question.
For engineers, this message is where the identity exchange either clears the way for authentication or exposes an identity mismatch that later leads to reject or repeated querying behavior.

What is Identity Response in simple terms?

The network asks the UE for a specific identity. The UE sends Identity Response back with that identity.

Why Identity Response matters

This message matters because it closes the identity query step. If the UE returns the expected identity cleanly, the AMF can move forward. If it does not, later NAS behavior can stall, repeat the identity exchange, or branch into reject handling.

Identity Response is therefore a small message with a large impact on procedure continuity.

Where Identity Response appears in the call flow

UE                              gNB / AMF
|--- Registration Request ------>|
|<-- Identity Request -----------|
|--- Identity Response --------->|
|<-- Authentication Request -----|

It most commonly appears in registration, but it can also show up in other 5GMM procedures where the AMF needs a clearer or alternate UE identity.

Transport characteristics

Direction: UE to AMF
Interface: N1
Transport on access side: commonly via UL Information Transfer
Security expectation: often plain NAS during early procedure stages, depending on when identity clarification happens

What Identity Response means operationally

Operationally, Identity Response tells engineers that the UE has supplied the identity the network asked for and is waiting for the next decision from the AMF.

The key practical checks are:

did the UE return the correct identity type
was the identity encoded as expected
did the AMF accept that identity and continue into authentication or service handling

Important Information Elements

IE	Why it matters
`Mobile identity`	Carries the identity value requested by the network and determines whether the identity exchange succeeds.

Example message dump

Identity Response
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Identity Response
  Mobile Identity
    Type of identity: SUCI
    SUPI format: IMSI
    MCC: 001
    MNC: 01
    Routing Indicator: 1234

How to read this dump

Start by checking the identity type returned in Mobile Identity.
Then compare it with the identity type requested by the network in the earlier Identity Request.
Finally, look at the next network message to see whether the core accepted the returned identity and continued normally.

What to check in logs

correlate Identity Response with the preceding Identity Request
verify that the UE returned the identity type the network actually asked for
inspect whether the next message is Authentication Request, Registration Reject, or another unexpected branch
confirm that the uplink NAS transport path delivered the response without segmentation or integrity issues

FAQ

What does Identity Response do in 5G?

It returns the specific UE identity the network asked for in Identity Request so the NAS procedure can continue.

Is Identity Response usually protected?

It can appear before full NAS security is established because identity exchange often happens early in registration.

What should happen after Identity Response?

The network usually continues with Authentication Request, though a reject or another branch is also possible depending on the returned identity and core evaluation.

Summary

Identity Response is the NAS message the UE sends after Identity Request to provide the specific identity type the network asked for, such as SUCI, IMEI, IMEISV, or another requested identifier.

Advanced Engineer Notes

This message is most useful when read as part of an identity pair: what the AMF asked for, what the UE returned, and how the AMF reacted next.
Unexpected repeats or immediate reject handling usually point to identity-type mismatch, identity-format mismatch, or identity-policy constraints in the core.

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.