🚀 central-unit

A Golang implementation of a gNB Central Unit - Control Plane (CU-CP) for 5G Open RAN architecture

---

A Golang implementation of a gNB Central Unit - Control Plane (CU-CP) for 5G Open RAN architecture. This project implements an RRC core that orchestrates NGAP (AMF interface), F1AP (DU interface), and E1AP (CU-UP interface) protocols, mirroring the OAI (OpenAirInterface) CU-CP logic flow.

CU-CP is organized as:

• Central RRC task: Orchestrates procedures
• Protocol tasks: Handle SCTP and encode/decode
• Context storage: Maintains UE/DU/CU-UP state
• Event-driven: ITTI messages coordinate between tasks

The RRC task is the core, processing events from F1AP (DU), NGAP (AMF), and E1AP (CU-UP) to manage UE connections and radio resources.

Overview

The CU-CP is the control plane component of a 5G gNB that manages:

• UE Context: RRC state machine, security context, bearer management
• DU Management: F1AP interface for Distributed Unit connections
• AMF Communication: NGAP interface for Access and Mobility Management Function
• Protocol Handling: RRC, NGAP, F1AP message processing and routing

The codebase follows an event-driven architecture with a single-threaded logic loop processing events from transports, targeting scalability for high-throughput control-plane operations.

Requirements

Operating System Support

This project requires SCTP (Stream Control Transmission Protocol) support, which is available on:

• Linux (kernel 2.6+)
• FreeBSD
• macOS (with limitations)

Note: SCTP is not natively supported on Windows.

Ubuntu/Debian Package Installation

Install required SCTP development libraries:

sudo apt-get update
sudo apt-get install -y libsctp-dev lksctp-tools

Verify SCTP support:

# Check if SCTP module is loaded
lsmod | grep sctp

# If not loaded, load it manually
sudo modprobe sctp

Go Version

Requires Go 1.24.4 or later.

Configuration

The CU-CP is configured via a YAML file (config/config.yml). Here's the configuration structure:

Configuration File Structure

cucp:
  node_id: "0001"              # CU-CP node identifier
  node_name: "gNB-CU-CP"        # CU-CP node name
  plmn:
    mcc: "999"                  # Mobile Country Code
    mnc: "70"                   # Mobile Network Code
    mnc_length: 2               # MNC length (2 or 3 digits)
  slices:                       # Network Slice Support
    - sst: "01"                 # Slice/Service Type
      sd: "010203"              # Slice Differentiator
  tac: "000001"                 # Tracking Area Code

f1ap:                           # F1AP interface (CU-CP <-> DU)
  local_address: "192.168.1.10" # Local IP address for F1AP server
  local_port: 38472             # Local port for F1AP server
  sctp:
    in_streams: 2               # SCTP inbound streams
    out_streams: 2              # SCTP outbound streams
  timers:
    f1_setup_timer: "10s"       # F1 Setup timer duration

e1ap:                           # E1AP interface (CU-CP <-> CU-UP)
  local_address: "192.168.1.10" # Local IP address for E1AP server
  local_port: 38462             # Local port for E1AP server
  sctp:
    in_streams: 2               # SCTP inbound streams
    out_streams: 2              # SCTP outbound streams

ngap:                           # NGAP interface (CU-CP <-> AMF)
  gnb_id: "000001"              # gNB identifier
  amf_address: "192.168.1.15"   # AMF IP address
  amf_port: 38412               # AMF port
  local_address: "192.168.1.10"  # Local IP address for NGAP client
  local_port: 9487              # Local port for NGAP client
  sctp:
    in_streams: 2               # SCTP inbound streams
    out_streams: 2              # SCTP outbound streams

logging:
  level: "info"                 # Log level: debug, info, warn, error
  format: "json"                # Log format: json or text

Configuration Parameters

CU-CP Section

• node_id: Unique identifier for this CU-CP instance
• node_name: Human-readable name for the CU-CP
• plmn: Public Land Mobile Network configuration
  • mcc: Mobile Country Code (3 digits)
  • mnc: Mobile Network Code (2 or 3 digits)
  • mnc_length: Length of MNC (2 or 3)
• slices: List of supported network slices (S-NSSAI)
  • sst: Slice/Service Type (hex string)
  • sd: Slice Differentiator (hex string, optional)
• tac: Tracking Area Code (hex string)

F1AP Section

• local_address: IP address where CU-CP listens for DU connections
• local_port: Port number for F1AP server
• sctp: SCTP stream configuration
• timers: F1AP-specific timer configurations

NGAP Section

• gnb_id: gNB identifier (hex string)
• amf_address: AMF server IP address
• amf_port: AMF server port
• local_address: Local IP address for NGAP client connection
• local_port: Local port for NGAP client

Logging Section

• level: Logging verbosity level
• format: Output format (json recommended for production)

Running

Build and Run

# Clone the repository
git clone <repository-url>
cd central-unit

# Install dependencies
go mod download

# Run the application
go run ./cmd/main.go -config config/config.yml

Command Line Options

• -config: Path to configuration file (default: config/config.yml)

Example Run

# Using default config path
go run ./cmd/main.go

# Using custom config path
go run ./cmd/main.go -config /path/to/config.yml

The application will:

1. Load and validate the configuration
2. Initialize logging
3. Create CU-CP context
4. Start F1AP server (listening for DU connections)
5. Connect to AMF via NGAP
6. Process incoming messages

Press Ctrl+C to gracefully shutdown the application.

Features & Future Works

Planned Features

The following features are planned for future releases:

1. E1AP Message Handler

   • E1 Setup Request/Response procedures
   • Bearer Context Setup/Modification/Release
   • CU-UP connection management
   • E1AP message encoding/decoding

2. Multi-Connection Support

   • Multiple DU connections management
   • Multiple UE context handling
   • Multiple AMF associations support
   • Connection pooling and load balancing

3. Extended Procedures

   • Session Management: PDU Session Establishment/Modification/Release
   • Handover: Intra-DU, Inter-DU, and Inter-gNB handover procedures
   • Paging: Paging request handling and UE paging
   • UE Context Release: UE context release procedures
   • RRC Reestablishment: RRC connection reestablishment
   • RRC Resume: RRC connection resume procedures

4. Security Context Support

   • Complete 5G-AKA authentication flow
   • Security key derivation (KgNB, KUPenc, KUPint)
   • Integrity protection and verification
   • Ciphering/deciphering of RRC and NAS messages
   • Security mode command handling