UL RRC Message Transfer is the F1AP UE-associated message the gNB-DU sends to the gNB-CU to deliver an uplink RRC message received from the UE over the selected SRB.
The gNB-DU receives an uplink RRC message from an already identified UE over NR radio and needs to forward the transparent RRC payload to the gNB-CU for centralized RRC processing.
Main purpose
Transfers uplink RRC signaling from DU to CU, enables centralized CU-side RRC processing, transports transparent RRC payloads, supports mobility, bearer, security, capability, and configuration handling, and separates DU radio reception from CU control-plane logic.
Main specification
3GPP TS 38.473, Sections 8.4.3 and 9.2.3.3
Release added
Release 15
Procedures where used
RRC setup completion, Security activation confirmation, Mobility measurement reporting, Bearer reconfiguration confirmation, UE capability handling, NAS transport through RRC, Handover-related RRC signaling
What is UL RRC Message Transfer in simple terms?
UL RRC Message Transfer is the F1AP UE-associated message the gNB-DU sends to the gNB-CU to deliver an uplink RRC message received from the UE over the selected SRB.
Transfers uplink RRC signaling from DU to CU, enables centralized CU-side RRC processing, transports transparent RRC payloads, supports mobility, bearer, security, capability, and configuration handling, and separates DU radio reception from CU control-plane logic.
Why this message matters
UL RRC Message Transfer is the DU's way to hand an uplink RRC message from the UE to the CU after UE context handling exists. Match both UE IDs, check SRB ID, decode the RRC container, and then follow the CU's next procedure.
Where this message appears in the call flow
Uplink RRC response transfer
Delivery branch: the DU receives uplink RRC over NR radio and forwards the transparent RRC container to the CU over F1-C.
Call flow position: The UE has sent an uplink RRC message over NR radio. The DU receives it and forwards it to the CU using UL RRC Message Transfer.
Typical state: A UE-associated logical F1 connection exists and both CU and DU UE identifiers should correlate to the active UE context.
Preconditions:
The DU has a UE context with both F1AP UE IDs.
The uplink RRC payload is encoded in RRC-Container.
SRB ID identifies the signaling bearer used by the UE.
Next likely message: DL RRC Message Transfer, UE Context Modification Request, or NGAP procedure handling
SRB and RRC payload validation
Decode branch: match UE IDs, confirm SRB ID, decode the RRC payload, and check optional PLMN or new DU UE ID fields.
Call flow position: SRB ID tells the CU which signaling bearer carried the uplink RRC payload, while RRC-Container carries the transparent UL-DCCH payload or CG-SDT-related UL-CCCH/UL-CCCH1 payload.
Typical state: F1AP transports the RRC bytes; the CU-side RRC layer interprets the message semantics.
Preconditions:
gNB-CU UE F1AP ID and gNB-DU UE F1AP ID are present.
RRC-Container is present and decodable.
Next likely message: CU-side RRC, bearer, mobility, security, or NAS handling
UL RRC versus Initial UL and NGAP
Scope branch: separate ongoing F1 uplink RRC transport from initial access and N2 NAS transport.
Call flow position: UL RRC Message Transfer is the later DU-to-CU F1 uplink path. Initial UL RRC Message Transfer is used for first access, and NGAP Uplink NAS Transport is a different N2 procedure.
Typical state: Trace analysis should separate F1 RRC payload transport from NGAP/NAS transport even when NAS is carried inside RRC.
Preconditions:
The UE-associated F1 context exists or is already being maintained.
Next likely message: DL RRC response, NGAP Uplink NAS Transport, or UE context update
Transport / encapsulation: F1AP over SCTP/IP between gNB-DU and gNB-CU
Security context: UL RRC Message Transfer does not establish NAS security by itself. It transports an uplink RRC payload, which may contain NAS information depending on the RRC procedure stage.
Message Structure Overview
UL RRC Message Transfer is sent by the gNB-DU to the gNB-CU after the DU receives an uplink RRC message from the UE.
The procedure uses UE-associated F1AP signaling and normally assumes that both CU-side and DU-side UE F1AP IDs already exist.
The mandatory core is Message Type, both UE F1AP IDs, SRB ID, and RRC-Container.
RRC-Container carries a transparent uplink RRC payload. F1AP transports it; RRC semantics are decoded by the CU RRC layer.
Selected PLMN ID and New gNB-DU UE F1AP ID are optional and should be checked when PLMN selection, mobility, or re-establishment context changes are involved.
This message is different from Initial UL RRC Message Transfer, which is used for first access before normal UE-associated handling is fully established.
This page follows the Release 18 message table in TS 38.473. The normal decode path is both UE IDs, SRB ID, RRC-Container, then optional Selected PLMN ID or New gNB-DU UE F1AP ID.
Treat this as a teaching example based on the expected message structure, not as a captured network trace.
Match both UE F1AP IDs before following the RRC payload.
Decode SRB ID before the RRC container because SRB1 and SRB2 point to different RRC delivery contexts.
Decode RRC-Container as the transparent RRC payload; do not expect F1AP itself to interpret RRC procedure semantics.
If Selected PLMN ID or New gNB-DU UE F1AP ID is present, check PLMN selection and UE ID continuity carefully.
Important Information Elements
IE
Presence
Description
Message Type
Mandatory
Identifies the F1AP PDU as UL RRC MESSAGE TRANSFER.
gNB-CU UE F1AP ID
Mandatory
Mandatory CU-side UE identifier for the UE-associated logical F1 connection.
gNB-DU UE F1AP ID
Mandatory
Mandatory DU-side UE identifier used to correlate the uplink RRC message with the DU UE context.
SRB ID
Mandatory
Mandatory signaling radio bearer identifier that carried the uplink RRC message.
RRC-Container
Mandatory
Mandatory transparent container carrying the uplink RRC UL-DCCH message encapsulated in a PDCP PDU. In CG-SDT cases, it may include UL-CCCH or UL-CCCH1 content.
Selected PLMN ID
Optional
Optional PLMN identity selected by the UE.
New gNB-DU UE F1AP ID
Optional
Optional replacement DU-side UE identifier used in specific mobility, re-establishment, or context-change handling.
Detailed field explanation
Message Type
Identifies the F1AP PDU as UL RRC MESSAGE TRANSFER.
Presence: Mandatory
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.
gNB-CU UE F1AP ID
Mandatory CU-side UE identifier for the UE-associated logical F1 connection.
Presence: Mandatory
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.
gNB-DU UE F1AP ID
Mandatory DU-side UE identifier used to correlate the uplink RRC message with the DU UE context.
Presence: Mandatory
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.
SRB ID
Mandatory signaling radio bearer identifier that carried the uplink RRC message.
Presence: Mandatory
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.
RRC-Container
Mandatory transparent container carrying the uplink RRC UL-DCCH message encapsulated in a PDCP PDU. In CG-SDT cases, it may include UL-CCCH or UL-CCCH1 content.
Presence: Mandatory
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.
Selected PLMN ID
Optional PLMN identity selected by the UE.
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.
New gNB-DU UE F1AP ID
Optional replacement DU-side UE identifier used in specific mobility, re-establishment, or context-change handling.
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 message direction is gNB-DU to gNB-CU.
Verify gNB-CU UE F1AP ID and gNB-DU UE F1AP ID match an active UE-associated F1 connection.
Check SRB ID and confirm it matches the expected RRC message channel.
Confirm RRC-Container is present and decodable as the expected uplink RRC message.
Validate whether the payload is RRCSetupComplete, RRCReconfigurationComplete, MeasurementReport, SecurityModeComplete, or ULInformationTransfer.
Decode Selected PLMN ID if present and compare it with access and core-network selection expectations.
Check New gNB-DU UE F1AP ID if present and follow subsequent UE ID correlation.
Look for the expected CU follow-up: DL RRC Message Transfer, UE Context Modification, NGAP signaling, mobility handling, or release.
Common Issues and Troubleshooting
CU takes no action after UL RRC Message Transfer.
Likely cause: The RRC payload may be malformed, the SRB ID may be wrong, or the CU may not correlate the UE IDs to an active context.
What to inspect: Check both UE F1AP IDs, SRB ID, RRC-Container decode, and CU RRC logs.
Next step: Fix UE ID correlation or RRC payload decoding before expecting a valid CU follow-up.
RRC payload does not decode as expected.
Likely cause: The selected SRB context may be wrong, the container may be truncated, or the trace decoder may be interpreting UL-DCCH/UL-CCCH incorrectly.
What to inspect: Compare SRB ID, PDCP/RLC logs, RRC message type, and whether the case involves CG-SDT.
Next step: Decode F1AP first, then the RRC container with the correct RRC channel context.
Trace analysis confuses NAS and RRC.
Likely cause: A NAS PDU embedded inside an RRC ULInformationTransfer is being treated as if F1AP directly carries NAS.
What to inspect: Decode F1AP, then RRC-Container, then any NAS payload inside the RRC message.
Next step: Separate F1, RRC, and NGAP/NAS layers in the timeline.
LTE / 5G / Variant Comparison
Compared with DL RRC Message Transfer
UL RRC Message Transfer carries UE-originated uplink RRC from DU to CU. DL RRC Message Transfer carries CU-originated downlink RRC from CU to DU toward the UE.
Compared with Initial UL RRC Message Transfer
Initial UL RRC Message Transfer is used for the first uplink RRC message before or during early UE-associated setup. UL RRC Message Transfer is used later after UE context handling exists.
Compared with NGAP Uplink NAS Transport
NGAP Uplink NAS Transport carries NAS from gNB/CU to AMF over N2. UL RRC Message Transfer carries an RRC container from DU to CU over F1-C; NAS may be embedded inside RRC.
FAQ
What is UL RRC Message Transfer in F1AP?
It is the F1AP message the gNB-DU sends to the gNB-CU to transfer an uplink RRC message received from the UE.
Who sends UL RRC Message Transfer?
The gNB-DU sends UL RRC Message Transfer to the gNB-CU over F1-C.
What RRC messages can it carry?
It commonly carries RRCSetupComplete, RRCReconfigurationComplete, MeasurementReport, SecurityModeComplete, ULInformationTransfer, or other uplink RRC payloads.
What is the purpose of the RRC Container?
RRC-Container carries the transparent uplink RRC payload. F1AP transports it, while the CU RRC layer interprets the RRC semantics.
What is SRB ID used for?
SRB ID tells the CU which signaling radio bearer carried the uplink RRC message.
How is this different from Initial UL RRC Message Transfer?
Initial UL RRC Message Transfer is used for the first access-related uplink RRC message. UL RRC Message Transfer is used later after UE context handling exists.
How is this different from NGAP Uplink NAS Transport?
UL RRC Message Transfer is F1-C RRC transport between DU and CU. NGAP Uplink NAS Transport is N2 NAS transport between gNB/CU and AMF.
How do you troubleshoot UL RRC Message Transfer issues?
Check both UE F1AP IDs, SRB ID, RRC-Container decode, Selected PLMN ID, New gNB-DU UE F1AP ID if present, and the expected CU follow-up action.
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.
