NAS 5G Network to UE 3GPP TS 24.501

5G NAS - Relay Key Reject

Relay Key Reject is the NAS message the network sends when the relay key request cannot be accepted and the relay-related key procedure must stop or take a failure branch.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.2.36
Direction	Network to UE	Message Type	5GMM signaling

Full message name	5G NAS - Relay Key Reject
Protocol	NAS
Technology	5G
Direction	Network to UE
Interface	N1
Signaling bearer / channel	NAS signaling / Dedicated NAS message, transported over the existing NAS path
Typical trigger	Sent when the network cannot accept the relay key request or cannot provide valid relay-related key response information.
Main purpose	Rejects the relay key request and carries the cause information needed to explain why relay key handling could not continue.
Main specification	3GPP TS 24.501, 8.2.36
Release added	Release 17
Procedures where used	Relay key procedure, Relay security handling, ProSe relay context
Related timers	Procedure dependent
Related cause values	5GMM cause

What is Relay Key Reject in simple terms?

Relay Key Reject is the NAS message the network sends when the relay key request cannot be accepted and the relay-related key procedure must stop or take a failure branch.

Rejects the relay key request and carries the cause information needed to explain why relay key handling could not continue.

Why this message matters

Relay Key Reject means the network could not accept the relay key request and the relay key procedure failed.

Where this message appears in the call flow

Relay Key Procedure

Call flow position: Negative response branch after Relay Key Request.

Typical state: The relay-aware NAS context exists, but the network rejected the relay key request.

Preconditions:

Relay Key Request was received.
The network could not accept or complete the relay-key handling.

Next likely message: Procedure abort or retry

ProSe Relay Security Handling

Call flow position: Network reject step when relay-key response processing cannot continue.

Typical state: The relay security branch failed before successful relay-key response delivery.

Preconditions:

Relay-capable context exists.

Next likely message: Procedure retry or feature fallback

Call flow position

Previous message(s): Relay Key Request, Relay authentication context establishment

Next message(s): Relay procedure abort, Procedure retry with corrected context

Message direction and transport

Sender and receiver: Network to UE

Interface: N1

Domain: Core-side mobility management signaling with relay-specific security context

Signaling bearer: NAS signaling

Logical channel: Dedicated NAS message, transported over the existing NAS path

Transport / encapsulation: NAS 5GS message carried end-to-end between the network NAS entity and the UE-side requester

Security context: This message belongs to the relay-key handling failure branch, so it should be interpreted together with the earlier Relay Key Request, relay transaction identity, and the reject cause.

Message Structure Overview

Relay Key Reject is a specialized NAS relay-security failure message rather than a mainstream registration or service reject.
In trace analysis, engineers usually inspect the relay transaction identity first and then the reject cause that explains why the request failed.

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.

Relay Key Reject follows NAS 24.501 IE structure and is not an ASN.1 message.

Decoded Message Dump

Relay Key Reject
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Integrity protected and ciphered
  Message Type: Relay Key Reject
  ProSe Relay Transaction Identity: 7
  5GMM Cause: Semantically incorrect message

How to read this dump

The message is best read as a relay-security failure branch whose value comes from the relay transaction identity and reject cause.
The first useful correlation is always the earlier Relay Key Request.

Important Information Elements

IE	Required	Description
`ProSe relay transaction identity`	Yes	Identifies the relay-key transaction so the reject can be matched with the earlier request.
`5GMM cause`	Yes	Explains why the network rejected the relay key request.

Detailed field explanation

`ProSe relay transaction identity`

Identifies the relay-key transaction so the reject can be matched with the earlier request.

Presence: Required

`5GMM cause`

Explains why the network rejected the relay key request.

Presence: Required

What to check in logs and traces

Confirm the message directly follows the relevant Relay Key Request.
Inspect the ProSe relay transaction identity and match it to the request.
Check the 5GMM cause and correlate it with the relay scenario.
Look for procedure abort, retry, or later fallback behavior.

Common Issues and Troubleshooting

Relay Key Reject appears immediately after Relay Key Request.

Likely cause: The relay request parameters or relay context were not acceptable to the network.

What to inspect: Check the relay transaction identity, request parameters in Relay Key Request, and the 5GMM cause returned in the reject.

Next step: Compare the relay request against a known-good trace and verify relay feature support and transaction correctness.

Relay-specific functionality fails even though ordinary NAS signaling is fine.

Likely cause: The failure belongs to the relay-specific key branch rather than to the general NAS path.

What to inspect: Focus on relay-context correlation and the reject cause rather than on mainstream registration signaling.

Next step: Treat the relay-key failure as a specialized feature problem and trace the feature-specific branch.

Detailed explanation

5G NAS - Relay Key Reject Explained

Relay Key Reject is the negative network response to Relay Key Request. It means the relay-key handling request could not be accepted and the relay security procedure has entered a failure branch.

For beginners, the simple meaning is: the network could not approve the relay key request.
For engineers, the key point is that this is a specialized relay-security failure message, not a mainstream registration reject.

What is Relay Key Reject in simple terms?

The UE asked for relay-related key handling with Relay Key Request. The network could not accept that request, so it answered with Relay Key Reject.

Why Relay Key Reject matters

This message matters because it marks the clear failure point of the relay-key branch. If engineers see this message, they should stop looking at general registration logic and switch to relay-specific analysis.

The most useful practical checks are:

whether the relay transaction identity matches the earlier request
what 5GMM cause the network returned
whether the procedure retries or falls back afterward

Where Relay Key Reject appears in the call flow

UE                              Network
|--- Relay Key Request --------->|
|<-- Relay Key Reject -----------|
|<-- Abort / Retry / Fallback ---|

It is best interpreted as part of a relay key procedure rather than as part of ordinary registration or service signaling.

Transport characteristics

Direction: Network to UE
Interface: N1
Transport on access side: carried over the existing NAS path
Security expectation: normally tied to an already established protected relay-capable NAS context

What Relay Key Reject means operationally

Operationally, Relay Key Reject tells engineers that the relay-key request failed and that the network is not returning usable relay-key response information.

That means the first practical questions are:

which relay transaction this reject belongs to
what cause the network gave
whether the failure is recoverable with a retry or corrected context

Important Information Elements

IE	Why it matters
`ProSe relay transaction identity`	Ties the reject to a specific earlier relay-key request.
`5GMM cause`	Explains why the network rejected the relay-key request.

Example message dump

Relay Key Reject
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Integrity protected and ciphered
  Message Type: Relay Key Reject
  ProSe Relay Transaction Identity: 7
  5GMM Cause: Semantically incorrect message

How to read this dump

Start with the relay transaction identity.
Then inspect the 5GMM Cause.
After that, move back to the earlier Relay Key Request and compare what was requested against what the network rejected.

What to check in logs

verify that the message follows the correct Relay Key Request
inspect the relay transaction identity
check the 5GMM cause
correlate the reject with later abort, retry, or fallback behavior

FAQ

What does Relay Key Reject do in 5G?

It tells the UE that the relay key request failed and provides the cause for that failure.

Is Relay Key Reject part of ordinary registration?

No. It belongs to a specialized relay security procedure rather than the standard registration path.

What should engineers inspect first?

Start with the relay transaction identity and the 5GMM cause.

Summary

Relay Key Reject is the NAS message the network sends when the relay key request cannot be accepted and the relay-related key procedure must stop or take a failure branch.

Advanced Engineer Notes

This is a niche Release 18-era failure message, so most engineering value comes from correct relay-context correlation rather than from the outer NAS wrapper alone.
If this message appears, the root cause usually belongs to relay-specific capability, parameter, or transaction handling.

Related message pages

Related Procedures

Relay Key Procedure

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.