LTE - SecurityModeComplete | LTE-RRC Message

Message Fact Sheet

Protocol	lte-rrc	Network	lte
Spec	3GPP TS 36.331	Spec Section	5.3.10, 6.2.2
Direction	UE -> eNodeB	Message Type	Security Control

Full message name	LTE - SecurityModeComplete
Protocol	LTE-RRC
Technology	LTE
Direction	UE -> eNodeB
Interface	Uu
Signaling bearer / channel	SRB1 / UL-DCCH
Typical trigger	The UE has accepted SecurityModeCommand and is ready to confirm completion of LTE AS security activation.
Main purpose	Confirms successful LTE AS security activation so later protected dedicated signaling can continue.
Main specification	3GPP TS 36.331, 5.3.10, 6.2.2
Release added	Release 8
Procedures where used	RRC Connection Establishment, Initial Access, AS Security Activation
Related timers	Correlate with the surrounding access and security sequence rather than a message-specific timer
Related cause values	There is no reject cause here because this message is the successful completion path

What is SecurityModeComplete in simple terms?

Uplink LTE RRC message sent by the UE to confirm that SecurityModeCommand was accepted and LTE AS security activation succeeded.

Confirms successful LTE AS security activation so later protected dedicated signaling can continue.

Why this message matters

SecurityModeComplete is the UE telling the eNodeB that LTE radio-layer security was activated successfully.

Where this message appears in the call flow

LTE initial access

Call flow position: UE confirmation message sent after SecurityModeCommand during early AS security activation.

Typical state: UE is in connected mode and is crossing from early setup into protected dedicated signaling.

Preconditions:

SecurityModeCommand has been received.
The UE has accepted the selected security configuration.

Next likely message: Protected dedicated control such as reconfiguration or bearer-related continuation

Connected-mode continuation

LTE connected-mode security flow showing SecurityModeComplete followed by protected reconfiguration and bearer control — In connected-mode continuation, SecurityModeComplete confirms that later dedicated LTE RRC signaling can proceed under the protected context.

Call flow position: Security confirmation step before later dedicated LTE RRC signaling continues under the protected context.

Typical state: UE is already in connected mode and confirming that protected control-plane operation can proceed.

Preconditions:

SRB1 is active.
SecurityModeCommand was handled successfully.

Next likely message: Later protected LTE RRC signaling

Call flow position

Previous message(s): SecurityModeCommand

Next message(s): Protected dedicated LTE RRC signaling, RRCConnectionReconfiguration

Message direction and transport

Sender and receiver: UE -> eNodeB

Interface: Uu

Domain: Access-side radio control

Signaling bearer: SRB1

Logical channel: UL-DCCH

Transport / encapsulation: RRC over DCCH on SRB1 after the UE applies the selected AS security configuration

Security context: This message confirms the security activation step and marks the point after which later dedicated LTE RRC signaling should be treated as protected.

Message Structure Overview

SecurityModeComplete is a compact confirmation message.
The practical reading path is to match it to the preceding SecurityModeCommand and then verify that later dedicated signaling continues under the protected context.
If this message is missing, later control-plane behavior should not be assumed healthy.

ASN.1 for LTE - SecurityModeComplete

SecurityModeComplete ::= SEQUENCE {
  rrc-TransactionIdentifier RRC-TransactionIdentifier,
  criticalExtensions CHOICE {
    c1 CHOICE {
      securityModeComplete-r8 SecurityModeComplete-r8-IEs,
      spare3 NULL,
      spare2 NULL,
      spare1 NULL
    },
    criticalExtensionsFuture SEQUENCE {}
  }
}

SecurityModeComplete-r8-IEs ::= SEQUENCE {
  nonCriticalExtension SecurityModeComplete-v8a0-IEs OPTIONAL
}

How to read this ASN.1

The main practical check is transaction correlation with SecurityModeCommand and the presence of later protected LTE RRC signaling.

Decoded Message Dump

UL-DCCH-Message
  message c1 : securityModeComplete : {
    rrc-TransactionIdentifier 2,
    criticalExtensions c1 : securityModeComplete-r8 : {
    }
  }

How to read this dump

The transaction identifier should match the earlier SecurityModeCommand.
This message is structurally simple, so the main value is its timing and successful presence.
After this point, later dedicated LTE RRC signaling should be treated as protected.

Important Information Elements

IE	Required	Description
`rrc-TransactionIdentifier`	Yes	Transaction identifier matching the preceding SecurityModeCommand.
`criticalExtensions`	Yes	Versioned wrapper carrying the SecurityModeComplete payload.
`nonCriticalExtension`	Optional	Release-extension branch used for later additions.

Detailed field explanation

`rrc-TransactionIdentifier`

Transaction identifier matching the preceding SecurityModeCommand.

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.

`criticalExtensions`

Versioned wrapper carrying the SecurityModeComplete payload.

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.

`nonCriticalExtension`

Release-extension branch used for later additions.

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 SecurityModeCommand without an intervening failure.
Verify the transaction identifier matches the security command.
Check that later dedicated LTE RRC signaling continues after this point.
If later control-plane messages are missing, confirm whether the completion step really arrived.

Common Issues and Troubleshooting

SecurityModeCommand is present but SecurityModeComplete never appears.

Likely cause: The UE may have rejected or failed to apply the selected security configuration.

What to inspect: Check the command, UE capability alignment, and whether SecurityModeFailure appears instead.

Next step: Walk backward into SecurityModeCommand and the earlier security preparation path.

Later dedicated LTE RRC signaling is inconsistent after security activation.

Likely cause: The security path may not have completed cleanly even if earlier setup looked healthy.

What to inspect: Confirm SecurityModeComplete is present and correctly matched to the command.

Next step: Validate the protected signaling boundary before debugging only later messages.

SecurityModeComplete is present but the procedure still fails.

Likely cause: The problem may be after security activation rather than in the security confirmation itself.

What to inspect: Use this message as the boundary marker, then continue into later reconfiguration or bearer-related signaling.

Next step: Move forward into the next dedicated LTE RRC message.

LTE / 5G / Variant Comparison

Compared with SecurityModeCommand

SecurityModeCommand selects and activates LTE AS security. SecurityModeComplete is the UE confirmation that the activation succeeded.

Compared with SecurityModeFailure

SecurityModeComplete is the success path. SecurityModeFailure is the branch to inspect when the UE cannot continue with the selected security configuration.

FAQ

What is SecurityModeComplete in LTE?

It is the uplink LTE RRC message the UE sends to confirm successful completion of SecurityModeCommand.

What comes after SecurityModeComplete?

Later protected dedicated LTE RRC signaling continues, often including reconfiguration or bearer-related control.

What should I inspect first in SecurityModeComplete?

Start with the transaction identifier and its correlation to the preceding SecurityModeCommand.

Why is SecurityModeComplete important?

It marks the successful completion of LTE AS security activation and the start of later protected dedicated signaling.

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.