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
Start typing to search message names, slugs, categories, directions, descriptions, and acronyms.
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.
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.
F1 Setup Failure
Rejects F1 setup with Cause, optional Time to Wait, and diagnostics.
F1 Reset
Requests full or partial reset of F1AP resources or selected UE-associated contexts.
F1 Reset Acknowledge
Confirms completion of F1 Reset handling and closes the reset procedure.
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.
gNB-DU Configuration Update
Updates DU-side configuration such as served cells or resource information.
Coming soongNB-DU Configuration Update Acknowledge
Confirms successful DU configuration update processing.
Coming soongNB-DU Configuration Update Failure
Rejects a DU configuration update with cause.
Coming soongNB-CU Configuration Update
Updates CU-side configuration over the F1 interface.
Coming soongNB-CU Configuration Update Acknowledge
Confirms successful CU configuration update processing.
Coming soongNB-CU Configuration Update Failure
Rejects a CU configuration update with cause.
Coming soonUE Context Management Messages
UE-associated procedures used by the gNB-CU and gNB-DU to create, modify, and release the DU-side UE context.
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 Setup Failure
Reports failed DU-side UE context setup.
Coming soonUE Context Modification Request
Requests modification of an existing DU-side UE context.
UE Context Modification Response
Reports successful UE context modification.
UE Context Modification Failure
Reports failed UE context modification.
UE Context Modification Required
Indicates that the DU requires UE context modification.
Coming soonUE Context Modification Confirm
Confirms handling of DU-initiated UE context modification.
Coming soonUE Context Release Command
Commands release of the DU-side UE context.
UE Context Release Complete
Confirms DU-side UE context release completion.
UE Context Release Request
Requests release of the UE context from the DU side.
Coming soonRRC Message Transfer Messages
F1AP messages used to transfer RRC messages between gNB-CU and gNB-DU for UE signalling.
Initial UL RRC Message Transfer
Carries the first uplink RRC message from DU to CU.
Coming soonDL RRC Message Transfer
Transfers downlink RRC messages from CU to DU.
Coming soonUL RRC Message Transfer
Transfers uplink RRC messages from DU to CU.
Coming soonPaging Messages
Messages used by the gNB-CU to trigger paging through the gNB-DU.
Paging
Requests paging transmission through the gNB-DU.
Coming soonSystem Information Messages
Messages used to deliver or request system information between gNB-CU and gNB-DU.
System Information Delivery Command
Commands system information delivery through the DU.
Coming soonSystem Information Delivery Confirm
Confirms system information delivery handling.
Coming soonWarning / PWS Messages
Public warning system messages used to deliver, replace, cancel, or report warning broadcast status over the F1 interface.
Write Replace Warning Request
Requests start or replacement of warning broadcast on DU cells.
Coming soonWrite Replace Warning Response
Reports write-replace warning handling result.
Coming soonPWS Cancel Request
Requests cancellation of public warning broadcast.
Coming soonPWS Cancel Response
Reports public warning cancellation result.
Coming soonPWS Restart Indication
Reports warning broadcast restart information.
Coming soonPWS Failure Indication
Reports warning broadcast failure information.
Coming soonTrace Messages
Messages used for trace activation, deactivation, and traffic trace reporting.
Trace Start
Requests trace activation for a UE or DU-side context.
Coming soonTrace Failure Indication
Reports trace failure on the DU side.
Coming soonDeactivate Trace
Requests deactivation of a trace session.
Coming soonCell Traffic Trace
Reports cell traffic trace information.
Coming soonPositioning / LPPa Messages
Messages used to transport positioning signalling between gNB-CU and gNB-DU.
DL Positioning Transport
Transports downlink positioning or LPPa signalling.
Coming soonUL Positioning Transport
Transports uplink positioning or LPPa signalling.
Coming soonIAB / Advanced Messages
Advanced F1AP messages for IAB, conditional mobility, multicast/broadcast, and release-specific features.
BAP Mapping Configuration
Configures BAP mapping for IAB-related operation.
Coming soonIAB TNL Address Allocation
Handles IAB transport network layer address allocation.
Coming soonIAB DU Resource Configuration
Configures DU resources for IAB operation.
Coming soonMost Used F1AP Messages
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)