5G F1AP Messages – Complete CU-DU Message List & Procedures

F1AP, or F1 Application Protocol, is the control-plane protocol used on the F1-C interface between the gNB-CU and gNB-DU in a split 5G RAN architecture. Defined in 3GPP TS 38.473, F1AP supports F1 interface setup, gNB-DU configuration, UE context setup, modification and release, RRC message transfer, paging, system information delivery, warning messages, trace, positioning, and other CU-DU coordination procedures.

Use this page to search or browse 5G F1AP messages by procedure category. Each message page explains the message direction, trigger, call-flow role, key Information Elements, and practical troubleshooting notes.

Based on 3GPP TS 38.473

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F1 Interface Management Messages

Non-UE-associated procedures used to establish, reset, update, and manage the F1-C interface between gNB-CU and gNB-DU.

6 messages

gNB-DU -> gNB-CU

F1 Setup Request

Starts F1-C setup and advertises DU identity, served cells, and configuration.

gNB-CU -> gNB-DU

F1 Setup Response

Accepts F1 setup and returns CU-side configuration and activation context.

gNB-CU -> gNB-DU

F1 Setup Failure

Rejects F1 setup with Cause, optional Time to Wait, and diagnostics.

gNB-CU <-> gNB-DU

F1 Reset

Requests full or partial reset of F1AP resources or selected UE-associated contexts.

gNB-CU <-> gNB-DU

F1 Reset Acknowledge

Confirms completion of F1 Reset handling and closes the reset procedure.

gNB-CU <-> gNB-DU

Error Indication

Reports protocol errors, missing context, invalid messages, or unsupported F1AP procedure handling.

gNB-DU Configuration Messages

Messages used to update or acknowledge gNB-DU configuration and gNB-CU configuration on the F1 interface.

6 messages

gNB-DU -> gNB-CU

gNB-DU Configuration Update

Updates DU-side configuration, served cells, cell status, resource information, and TNL associations.

gNB-CU -> gNB-DU

gNB-DU Configuration Update Acknowledge

Accepts a DU configuration update and reports any cells that failed activation.

gNB-CU -> gNB-DU

gNB-DU Configuration Update Failure

Rejects a DU configuration update with Cause, optional Time to Wait, and diagnostics.

gNB-CU -> gNB-DU

gNB-CU Configuration Update

Updates CU-side configuration, transport endpoints, and TNL associations after F1 setup.

gNB-DU -> gNB-CU

gNB-CU Configuration Update Acknowledge

Accepts a CU configuration update and confirms DU-side synchronization of CU transport or configuration changes.

gNB-DU -> gNB-CU

gNB-CU Configuration Update Failure

Rejects a CU configuration update with Cause, optional Time to Wait, and diagnostics.

UE Context Management Messages

UE-associated procedures used by the gNB-CU and gNB-DU to create, modify, and release the DU-side UE context.

11 messages

gNB-CU -> gNB-DU

UE Context Setup Request

Requests DU-side setup of UE context and radio resources.

gNB-DU -> gNB-CU

UE Context Setup Response

Reports successful DU-side UE context setup.

gNB-DU -> gNB-CU

UE Context Setup Failure

Reports failed DU-side UE context setup.

gNB-CU -> gNB-DU

UE Context Modification Request

Requests modification of an existing DU-side UE context.

gNB-DU -> gNB-CU

UE Context Modification Response

Reports successful UE context modification.

gNB-DU -> gNB-CU

UE Context Modification Failure

Reports failed UE context modification.

gNB-DU -> gNB-CU

UE Context Modification Required

Indicates that the DU requires UE context modification.

gNB-CU -> gNB-DU

UE Context Modification Confirm

Confirms handling of DU-initiated UE context modification.

gNB-CU -> gNB-DU

UE Context Release Command

Commands release of the DU-side UE context.

gNB-DU -> gNB-CU

UE Context Release Complete

Confirms DU-side UE context release completion.

gNB-DU -> gNB-CU

UE Context Release Request

Requests release of the UE context from the DU side.

RRC Message Transfer Messages

F1AP messages used to transfer RRC messages between gNB-CU and gNB-DU for UE signalling.

3 messages

gNB-DU -> gNB-CU

Initial UL RRC Message Transfer

Carries the first uplink RRC message from DU to CU.

gNB-CU -> gNB-DU

DL RRC Message Transfer

Transfers downlink RRC messages from CU to DU.

gNB-DU -> gNB-CU

UL RRC Message Transfer

Transfers uplink RRC messages from DU to CU.

Paging Messages

Messages used by the gNB-CU to trigger paging through the gNB-DU.

1 messages

gNB-CU -> gNB-DU

Paging

Requests paging transmission through the gNB-DU.

Coming soon

System Information Messages

Messages used to deliver or request system information between gNB-CU and gNB-DU.

2 messages

gNB-CU -> gNB-DU

System Information Delivery Command

Commands system information delivery through the DU.

Coming soon

gNB-DU -> gNB-CU

System Information Delivery Confirm

Confirms system information delivery handling.

Coming soon

Warning / PWS Messages

Public warning system messages used to deliver, replace, cancel, or report warning broadcast status over the F1 interface.

6 messages

gNB-CU -> gNB-DU

Write Replace Warning Request

Requests start or replacement of warning broadcast on DU cells.

Coming soon

gNB-DU -> gNB-CU

Write Replace Warning Response

Reports write-replace warning handling result.

Coming soon

gNB-CU -> gNB-DU

PWS Cancel Request

Requests cancellation of public warning broadcast.

Coming soon

gNB-DU -> gNB-CU

PWS Cancel Response

Reports public warning cancellation result.

Coming soon

gNB-DU -> gNB-CU

PWS Restart Indication

Reports warning broadcast restart information.

Coming soon

gNB-DU -> gNB-CU

PWS Failure Indication

Reports warning broadcast failure information.

Coming soon

Trace Messages

Messages used for trace activation, deactivation, and traffic trace reporting.

4 messages

gNB-CU -> gNB-DU

Trace Start

Requests trace activation for a UE or DU-side context.

Coming soon

gNB-DU -> gNB-CU

Trace Failure Indication

Reports trace failure on the DU side.

Coming soon

gNB-CU -> gNB-DU

Deactivate Trace

Requests deactivation of a trace session.

Coming soon

gNB-DU -> gNB-CU

Cell Traffic Trace

Reports cell traffic trace information.

Coming soon

Positioning / LPPa Messages

Messages used to transport positioning signalling between gNB-CU and gNB-DU.

2 messages

gNB-CU -> gNB-DU

DL Positioning Transport

Transports downlink positioning or LPPa signalling.

Coming soon

gNB-DU -> gNB-CU

UL Positioning Transport

Transports uplink positioning or LPPa signalling.

Coming soon

IAB / Advanced Messages

Advanced F1AP messages for IAB, conditional mobility, multicast/broadcast, and release-specific features.

3 messages

gNB-CU -> gNB-DU

BAP Mapping Configuration

Configures BAP mapping for IAB-related operation.

Coming soon

gNB-CU <-> gNB-DU

IAB TNL Address Allocation

Handles IAB transport network layer address allocation.

Coming soon

gNB-CU -> gNB-DU

IAB DU Resource Configuration

Configures DU resources for IAB operation.

Coming soon

Most Used F1AP Messages

F1 Setup Request Starts F1-C setup and advertises DU identity, served cells, and configuration. F1 Setup Response Accepts F1 setup and returns CU-side configuration and activation context. gNB-DU Configuration Update Updates DU-side configuration, served cells, cell status, resource information, and TNL associations. UE Context Setup Request Requests DU-side setup of UE context and radio resources. UE Context Setup Response Reports successful DU-side UE context setup. UE Context Modification Request Requests modification of an existing DU-side UE context. UE Context Modification Response Reports successful UE context modification. UE Context Release Command Commands release of the DU-side UE context. UE Context Release Complete Confirms DU-side UE context release completion. DL RRC Message Transfer Transfers downlink RRC messages from CU to DU. UL RRC Message Transfer Transfers uplink RRC messages from DU to CU. Initial UL RRC Message Transfer Carries the first uplink RRC message from DU to CU.

Common F1AP Procedure Flows

F1 Setup Flow

F1 Setup Request -> F1 Setup Response / F1 Setup Failure

gNB-DU Configuration Update Flow

gNB-DU Configuration Update -> Acknowledge / Failure

UE Context Setup Flow

UE Context Setup Request -> UE Context Setup Response / UE Context Setup Failure

UE Context Modification Flow

UE Context Modification Request -> UE Context Modification Response / Failure

UE Context Release Flow

UE Context Release Command -> UE Context Release Complete

RRC Transfer Flow

Initial UL RRC Message Transfer; DL RRC Message Transfer <-> UL RRC Message Transfer

FAQ

What is F1AP in 5G?

F1AP is the F1 Application Protocol used on the F1-C interface between gNB-CU and gNB-DU.

Which interface uses F1AP?

F1AP is used on the F1-C control-plane interface between the gNB-CU and gNB-DU.

Which 3GPP specification defines F1AP?

F1AP is defined in 3GPP TS 38.473.

What is the difference between F1AP and NGAP?

NGAP runs between NG-RAN and AMF over N2, while F1AP runs inside the RAN split between gNB-CU and gNB-DU.

What is the difference between gNB-CU and gNB-DU?

The gNB-CU centralizes higher-layer control and user-plane functions, while the gNB-DU handles lower-layer radio functions closer to the cell site.

What are the most important F1AP messages?

Commonly analyzed F1AP messages include F1 Setup Request, F1 Setup Response, UE Context Setup Request, UE Context Modification Request, UE Context Release Command, and RRC Message Transfer messages.

How does F1AP carry RRC messages?

F1AP coordinates CU-DU RRC transfer using Initial UL RRC Message Transfer, DL RRC Message Transfer, and UL RRC Message Transfer messages.

References

3GPP TS 38.473 – NG-RAN; F1 Application Protocol (F1AP)
3GPP TS 38.401 – NG-RAN architecture description
3GPP TS 38.331 – NR Radio Resource Control (RRC)