# CDAP — Custom Device API Protocol

> **Protocol Version**: 1.0  
> **Server Version**: BetterDesk v3.0.0  
> **Status**: Production-ready (Phases 0-7 implemented)

---

## Table of Contents

1. [Overview](#overview)
2. [Transport Layer](#transport-layer)
3. [Authentication](#authentication)
4. [Message Envelope](#message-envelope)
5. [Lifecycle](#lifecycle)
6. [Widgets](#widgets)
7. [Commands](#commands)
8. [Events](#events)
9. [Media Channel](#media-channel)
10. [Device Linking](#device-linking)
11. [RBAC](#rbac)
12. [Bridge SDK](#bridge-sdk)
13. [Error Codes](#error-codes)

---

## Overview

CDAP (Custom Device API Protocol) is a WebSocket-based protocol that connects non-RustDesk devices to the BetterDesk ecosystem. It provides:

- **Real-time state synchronization** — Devices push state updates; panel renders widgets
- **Bidirectional commands** — Operators send commands to devices; devices respond with results
- **Media relay** — Binary frame channel for remote desktop sessions (E2E encrypted)
- **Device management** — Enrollment, revocation, linking, grouping
- **Bridge ecosystem** — SDKs for Modbus TCP, SNMP, REST webhooks, and custom protocols

### When to Use CDAP

| Scenario | Use CDAP? | Alternative |
|----------|-----------|-------------|
| IoT sensor dashboard | ✅ Yes | — |
| SCADA/PLC monitoring | ✅ Yes | — |
| Network device management | ✅ Yes | — |
| Remote desktop (existing) | ❌ No | RustDesk client |
| Remote desktop (CDAP agent) | ✅ Yes | BetterDesk native agent |
| Custom automation agent | ✅ Yes | — |

---

## Transport Layer

### WebSocket Connection

```
ws://host:21122    (plain)
wss://host:21122   (TLS)
```

The CDAP gateway uses **dual-mode listening** — auto-detects TLS (first byte `0x16`) and plain connections on the same port. No separate TLS port needed.

### TLS Configuration

```bash
# Enable TLS on CDAP gateway
betterdesk-server --tls-cert /path/to/cert.pem --tls-key /path/to/key.pem

# CDAP auto-detects TLS when cert/key are provided
# Both plain and TLS connections accepted on port 21122
```

### Connection Parameters

| Parameter | Default | Description |
|-----------|---------|-------------|
| `CDAP_PORT` | 21122 | Gateway listen port |
| `CDAP_MAX_CONNS` | 10000 | Maximum concurrent connections |
| `CDAP_READ_LIMIT` | 1048576 | Maximum message size (1 MiB) |
| `CDAP_PING_INTERVAL` | 30s | WebSocket ping interval |
| `CDAP_PONG_TIMEOUT` | 10s | Pong response deadline |
| `CDAP_WRITE_TIMEOUT` | 10s | Write deadline per message |

### Reconnection

Clients should implement exponential backoff reconnection:

```
Attempt 1: wait 1s
Attempt 2: wait 2s
Attempt 3: wait 4s
Attempt N: wait min(2^(N-1), 60)s
```

On reconnect, clients must re-authenticate and re-register. The server preserves device state for 5 minutes after disconnection.

---

## Authentication

Authentication must be the **first message** after WebSocket upgrade. The server closes the connection after 10 seconds without auth.

### Method 1: API Key

```json
{
    "type": "auth",
    "id": "msg-001",
    "payload": {
        "method": "api_key",
        "key": "a1b2c3d4e5f6...",
        "device_id": "CDAP-SENSOR01"
    }
}
```

### Method 2: User/Password

```json
{
    "type": "auth",
    "id": "msg-002",
    "payload": {
        "method": "user_password",
        "username": "operator1",
        "password": "secure-password",
        "device_id": "CDAP-AGENT01"
    }
}
```

If the user has TOTP 2FA enabled, the server responds with `auth_2fa_required`:

```json
{
    "type": "auth_2fa_required",
    "id": "msg-002",
    "payload": {
        "message": "TOTP code required"
    }
}
```

Client must then send:

```json
{
    "type": "auth_2fa",
    "id": "msg-003",
    "payload": {
        "code": "123456"
    }
}
```

### Method 3: Device Token

For automated enrollment (one-time tokens generated by admin):

```json
{
    "type": "auth",
    "id": "msg-004",
    "payload": {
        "method": "device_token",
        "token": "enroll-abc123def456",
        "device_id": "CDAP-NEW01"
    }
}
```

### Auth Response

Success:

```json
{
    "type": "auth_ok",
    "id": "msg-001",
    "payload": {
        "session_id": "sess-xyz789",
        "expires_in": 86400,
        "role": "operator",
        "permissions": ["read", "control", "media"]
    }
}
```

Failure:

```json
{
    "type": "auth_error",
    "id": "msg-001",
    "payload": {
        "code": "AUTH_INVALID_KEY",
        "message": "Invalid API key"
    }
}
```

---

## Message Envelope

All CDAP messages use a JSON envelope:

```json
{
    "type": "message_type",
    "id": "unique-message-id",
    "payload": { ... },
    "ts": 1711834567890
}
```

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `type` | string | ✅ | Message type identifier |
| `id` | string | ✅ | Unique message ID (for request/response correlation) |
| `payload` | object | ✅ | Message-specific data |
| `ts` | number | ❌ | Unix timestamp in milliseconds |

### Message Types (Client → Server)

| Type | Description |
|------|-------------|
| `auth` | Authentication request |
| `auth_2fa` | TOTP 2FA code |
| `register` | Device registration with manifest |
| `state_update` | Widget state push |
| `command_response` | Response to server command |
| `heartbeat` | Keep-alive with optional metrics |
| `media_offer` | SDP offer for media channel |
| `media_answer` | SDP answer for media channel |
| `media_frame` | Binary frame (uses binary WebSocket frames) |
| `event` | Client-originated event |

### Message Types (Server → Client)

| Type | Description |
|------|-------------|
| `auth_ok` | Authentication success |
| `auth_error` | Authentication failure |
| `auth_2fa_required` | TOTP required |
| `registered` | Registration acknowledged |
| `command` | Command from operator |
| `state_request` | Server requests full state refresh |
| `media_offer` | SDP offer from peer |
| `media_answer` | SDP answer from peer |
| `config_update` | Server configuration change |
| `error` | Generic error |
| `ping` | Application-level ping |

---

## Lifecycle

### Connection Flow

```
Client                              Server
  │                                    │
  │──── WebSocket Upgrade ────────────>│
  │<─── 101 Switching Protocols ───────│
  │                                    │
  │──── auth {method, credentials} ───>│
  │<─── auth_ok {session, role} ───────│
  │                                    │
  │──── register {manifest} ──────────>│
  │<─── registered {ack} ─────────────│
  │                                    │
  │──── state_update {widgets} ───────>│  (periodic)
  │<─── command {action, params} ──────│  (on-demand)
  │──── command_response {result} ────>│
  │                                    │
  │──── heartbeat {metrics} ──────────>│  (every 30s)
  │<─── pong ─────────────────────────│
  │                                    │
  │──── close ────────────────────────>│
  │<─── close ─────────────────────────│
```

### Registration (Manifest)

The manifest describes the device's capabilities:

```json
{
    "type": "register",
    "id": "msg-010",
    "payload": {
        "manifest": {
            "name": "Factory Floor Controller",
            "device_type": "scada",
            "version": "2.1.0",
            "firmware": "PLC-FW-3.5.2",
            "capabilities": ["widgets", "commands", "media"],
            "categories": [
                {
                    "id": "temperature",
                    "label": "Temperature Sensors",
                    "icon": "thermostat"
                },
                {
                    "id": "actuators",
                    "label": "Actuators",
                    "icon": "settings"
                }
            ],
            "widgets": [
                {
                    "id": "temp_zone1",
                    "type": "gauge",
                    "label": "Zone 1 Temperature",
                    "category": "temperature",
                    "unit": "°C",
                    "min": -10,
                    "max": 100,
                    "thresholds": {
                        "warning": 60,
                        "danger": 85
                    },
                    "permissions": {
                        "read": "viewer"
                    }
                },
                {
                    "id": "motor_speed",
                    "type": "slider",
                    "label": "Motor Speed",
                    "category": "actuators",
                    "unit": "RPM",
                    "min": 0,
                    "max": 3000,
                    "step": 50,
                    "permissions": {
                        "read": "viewer",
                        "control": "operator"
                    }
                },
                {
                    "id": "emergency_stop",
                    "type": "button",
                    "label": "Emergency Stop",
                    "category": "actuators",
                    "confirm": true,
                    "confirmMessage": "Are you sure you want to trigger emergency stop?",
                    "dangerous": true,
                    "permissions": {
                        "read": "viewer",
                        "execute": "admin"
                    }
                }
            ],
            "commands": [
                {
                    "id": "reboot",
                    "label": "Reboot Device",
                    "confirm": true,
                    "permissions": {
                        "execute": "admin"
                    }
                },
                {
                    "id": "calibrate",
                    "label": "Calibrate Sensors",
                    "params": [
                        {
                            "id": "zone",
                            "type": "select",
                            "label": "Zone",
                            "options": ["zone1", "zone2", "zone3"]
                        }
                    ],
                    "permissions": {
                        "execute": "operator"
                    }
                }
            ]
        }
    }
}
```

### State Updates

Devices push widget state periodically or on change:

```json
{
    "type": "state_update",
    "id": "msg-020",
    "payload": {
        "widgets": {
            "temp_zone1": {
                "value": 42.5,
                "status": "normal"
            },
            "motor_speed": {
                "value": 1500
            },
            "emergency_stop": {
                "active": false
            }
        }
    }
}
```

### Heartbeat

Sent every 30s (configurable) with optional system metrics:

```json
{
    "type": "heartbeat",
    "id": "msg-030",
    "payload": {
        "uptime": 86400,
        "cpu": 23.5,
        "memory": 67.2,
        "disk": 45.0,
        "custom_metrics": {
            "queue_depth": 42,
            "error_rate": 0.01
        }
    }
}
```

---

## Widgets

### Toggle

Boolean on/off switch.

```json
{
    "id": "relay1",
    "type": "toggle",
    "label": "Main Relay",
    "category": "actuators"
}
```

State: `{ "value": true }`

Command: `{ "action": "set", "widget_id": "relay1", "value": false }`

### Gauge

Numeric value with thresholds and optional unit.

```json
{
    "id": "pressure",
    "type": "gauge",
    "label": "Pressure",
    "unit": "bar",
    "min": 0,
    "max": 10,
    "decimals": 2,
    "thresholds": {
        "warning": 7.5,
        "danger": 9.0
    }
}
```

State: `{ "value": 5.23 }`

### Button

Action trigger with optional confirmation dialog.

```json
{
    "id": "reset_counters",
    "type": "button",
    "label": "Reset Counters",
    "icon": "restart_alt",
    "confirm": true,
    "confirmMessage": "Reset all production counters to zero?",
    "cooldown": 5000
}
```

State: `{ "active": false, "lastTriggered": "2026-03-20T10:30:00Z" }`

Command: `{ "action": "execute", "widget_id": "reset_counters" }`

### LED

Status indicator with color states.

```json
{
    "id": "connection_status",
    "type": "led",
    "label": "PLC Connection",
    "states": {
        "green": "Connected",
        "yellow": "Reconnecting",
        "red": "Disconnected"
    }
}
```

State: `{ "color": "green", "label": "Connected" }`

### Text

Read-only text display.

```json
{
    "id": "firmware_version",
    "type": "text",
    "label": "Firmware"
}
```

State: `{ "value": "v3.5.2-stable" }`

### Slider

Numeric range input with step.

```json
{
    "id": "brightness",
    "type": "slider",
    "label": "LED Brightness",
    "unit": "%",
    "min": 0,
    "max": 100,
    "step": 5
}
```

State: `{ "value": 75 }`

Command: `{ "action": "set", "widget_id": "brightness", "value": 50 }`

### Select

Dropdown selection.

```json
{
    "id": "operating_mode",
    "type": "select",
    "label": "Operating Mode",
    "options": [
        { "value": "auto", "label": "Automatic" },
        { "value": "manual", "label": "Manual" },
        { "value": "maintenance", "label": "Maintenance" }
    ]
}
```

State: `{ "value": "auto" }`

Command: `{ "action": "set", "widget_id": "operating_mode", "value": "manual" }`

### Chart

Time-series or categorical chart data.

```json
{
    "id": "temp_history",
    "type": "chart",
    "label": "Temperature History",
    "chartType": "line",
    "maxPoints": 60,
    "unit": "°C"
}
```

State:
```json
{
    "points": [
        { "t": 1711834500, "v": 42.1 },
        { "t": 1711834560, "v": 42.3 },
        { "t": 1711834620, "v": 42.0 }
    ]
}
```

### Table (v3.0)

Dynamic sortable data table.

```json
{
    "id": "alarm_log",
    "type": "table",
    "label": "Active Alarms",
    "columns": [
        { "id": "time", "label": "Time", "type": "datetime" },
        { "id": "severity", "label": "Severity", "type": "badge" },
        { "id": "message", "label": "Message", "type": "text" },
        { "id": "ack", "label": "Acknowledge", "type": "action" }
    ],
    "sortable": true,
    "pagination": true,
    "pageSize": 20
}
```

State:
```json
{
    "rows": [
        {
            "id": "alarm-001",
            "time": "2026-03-20T10:30:00Z",
            "severity": { "value": "critical", "color": "red" },
            "message": "Zone 3 temperature exceeded 85°C",
            "ack": { "label": "Acknowledge", "action": "ack_alarm", "params": { "id": "alarm-001" } }
        }
    ],
    "total": 42
}
```

### Terminal (v3.0)

WebSocket shell relay for device management.

```json
{
    "id": "shell",
    "type": "terminal",
    "label": "Device Shell",
    "permissions": {
        "read": "operator",
        "control": "admin"
    }
}
```

Terminal widget uses a separate binary WebSocket channel for stdin/stdout/stderr.

---

## Commands

### Server → Client Command

```json
{
    "type": "command",
    "id": "cmd-001",
    "payload": {
        "action": "set",
        "widget_id": "motor_speed",
        "value": 2000,
        "operator": "admin@example.com",
        "timestamp": 1711834567890
    }
}
```

### Client → Server Response

```json
{
    "type": "command_response",
    "id": "cmd-001",
    "payload": {
        "status": "ok",
        "message": "Motor speed set to 2000 RPM",
        "applied_value": 2000
    }
}
```

### Command Error

```json
{
    "type": "command_response",
    "id": "cmd-001",
    "payload": {
        "status": "error",
        "code": "DEVICE_BUSY",
        "message": "Motor is in calibration mode, cannot change speed"
    }
}
```

---

## Events

Devices can emit events for audit logging and alerting:

```json
{
    "type": "event",
    "id": "evt-001",
    "payload": {
        "event_type": "alarm",
        "severity": "critical",
        "message": "Zone 3 temperature exceeded 85°C",
        "data": {
            "zone": 3,
            "temperature": 87.2,
            "threshold": 85
        }
    }
}
```

Event types:
- `alarm` — Threshold violation or abnormal condition
- `status_change` — Device state transition
- `maintenance` — Scheduled or manual maintenance event
- `security` — Security-related event (auth failure, tamper detection)
- `custom` — Application-specific event

---

## Media Channel

The media channel provides binary frame relay between CDAP devices and web clients for remote desktop sessions.

### Negotiation

1. Web client sends `media_offer` through CDAP gateway:

```json
{
    "type": "media_offer",
    "id": "media-001",
    "payload": {
        "target_device": "CDAP-AGENT01",
        "codecs": ["h264", "vp9"],
        "resolution": { "width": 1920, "height": 1080 },
        "fps": 30
    }
}
```

2. Target device responds with `media_answer`:

```json
{
    "type": "media_answer",
    "id": "media-001",
    "payload": {
        "accepted": true,
        "codec": "h264",
        "resolution": { "width": 1920, "height": 1080 },
        "fps": 30,
        "encryption": "nacl"
    }
}
```

3. Binary frames flow through the gateway:
   - Device sends video frames as binary WebSocket messages
   - Client sends input events (mouse, keyboard) as JSON commands
   - Gateway relays without decryption (E2E between client and device)

### Frame Encryption

Media frames use the same NaCl encryption as RustDesk:
- **Key exchange**: X25519 Diffie-Hellman
- **Encryption**: XSalsa20-Poly1305
- **Frame format**: `[24-byte nonce][encrypted payload]`

---

## Device Linking

CDAP devices can be linked to RustDesk peers for unified management:

```json
{
    "type": "register",
    "id": "msg-050",
    "payload": {
        "manifest": {
            "name": "Server Room Agent",
            "device_type": "os_agent",
            "linked_peer_id": "1340238749",
            "version": "1.0.0",
            "widgets": [ ... ]
        }
    }
}
```

Linked devices appear in the same device detail page. The panel shows:
- RustDesk tab: Remote desktop, file transfer
- CDAP tab: Widgets, commands, metrics
- Combined connection status

---

## RBAC

### Per-Widget Permissions

Each widget can specify minimum role requirements:

```json
{
    "permissions": {
        "read": "viewer",
        "control": "operator",
        "execute": "admin"
    }
}
```

Permission types:
- `read` — View widget state (default: `viewer`)
- `control` — Change widget value (sliders, toggles, selects)
- `execute` — Trigger actions (buttons, commands)

### Dangerous Widgets

Widgets marked `"dangerous": true` are hidden from non-admin users and require explicit confirmation:

```json
{
    "id": "factory_reset",
    "type": "button",
    "label": "Factory Reset",
    "dangerous": true,
    "confirm": true,
    "permissions": {
        "execute": "admin"
    }
}
```

---

## Bridge SDK

### Python SDK

```bash
pip install betterdesk-cdap
```

```python
from betterdesk_cdap import CDAPBridge, Widget, WidgetType

bridge = CDAPBridge(
    server="ws://betterdesk.example.com:21122",
    api_key="your-api-key",
    device_id="BRIDGE-MODBUS01"
)

bridge.register(
    name="Modbus Gateway",
    device_type="scada",
    widgets=[
        Widget("coil_0", WidgetType.TOGGLE, "Output Coil 0"),
        Widget("register_0", WidgetType.GAUGE, "Holding Register 0",
               unit="mA", min=0, max=20, thresholds={"warning": 16, "danger": 19}),
    ]
)

@bridge.on_command("set", "coil_0")
async def handle_coil(value: bool):
    # Write to Modbus device
    await modbus_client.write_coil(0, value)
    return {"status": "ok"}

bridge.run()  # Starts event loop
```

### Reference Bridges

| Bridge | Protocol | Status |
|--------|----------|--------|
| `betterdesk-bridge-modbus` | Modbus TCP/RTU | v3.0 |
| `betterdesk-bridge-snmp` | SNMP v2c/v3 | v3.0 |
| `betterdesk-bridge-rest` | REST webhook | v3.0 |
| `betterdesk-bridge-mqtt` | MQTT 3.1.1/5.0 | Planned |
| `betterdesk-bridge-opcua` | OPC UA | Planned |

---

## Error Codes

| Code | HTTP Equiv | Description |
|------|-----------|-------------|
| `AUTH_INVALID_KEY` | 401 | Invalid API key |
| `AUTH_INVALID_CREDENTIALS` | 401 | Wrong username/password |
| `AUTH_2FA_REQUIRED` | 401 | TOTP code needed |
| `AUTH_2FA_INVALID` | 401 | Wrong TOTP code |
| `AUTH_TOKEN_EXPIRED` | 401 | Device token expired |
| `AUTH_TOKEN_REVOKED` | 401 | Device token revoked |
| `AUTH_RATE_LIMITED` | 429 | Too many auth attempts |
| `DEVICE_ALREADY_REGISTERED` | 409 | Device ID already connected |
| `DEVICE_BANNED` | 403 | Device is banned |
| `DEVICE_REVOKED` | 403 | Device has been revoked |
| `DEVICE_SOFT_DELETED` | 403 | Device has been soft-deleted |
| `PERMISSION_DENIED` | 403 | Insufficient role for action |
| `INVALID_MANIFEST` | 400 | Manifest validation failed |
| `INVALID_COMMAND` | 400 | Unknown command or missing params |
| `DEVICE_NOT_FOUND` | 404 | Target device not connected |
| `DEVICE_BUSY` | 503 | Device cannot process command |
| `MEDIA_REJECTED` | 403 | Media channel rejected by device |
| `MEDIA_CODEC_UNSUPPORTED` | 406 | No common codec found |
| `INTERNAL_ERROR` | 500 | Server internal error |
| `GATEWAY_DISABLED` | 503 | CDAP gateway not enabled |

---

*Last updated: March 2026 — BetterDesk v3.0.0*