CoAP Protocol

CoAP (Constrained Application Protocol) is a lightweight RESTful protocol designed for constrained devices and low-power networks. It uses UDP and follows a request/response model similar to HTTP, making it familiar for web developers working with IoT.

When to Use CoAP

Use CoAP when your device:

• Uses NB-IoT or other low-power wide-area networks (LPWAN)
• Needs a simple request/response model (like HTTP)
• Has limited memory and cannot maintain persistent connections
• Sends data infrequently (e.g., once per hour)

For devices that need continuous data streaming or real-time commands, standard MQTT over TLS is recommended.

Connection Details

Parameter	Value
Broker Host	mqtt.siliconwit.io
Port	5683 (UDP)
Authentication	Device ID (username) + Access Token (password)
Method	POST to publish telemetry

How It Works

CoAP messages are translated by the gateway into MQTT messages, which then flow through the standard SiliconWit telemetry pipeline:

Device (CoAP/UDP) --> Gateway (port 5683) --> MQTT Broker --> SiliconWit Platform

Key characteristics:

• RESTful: Uses familiar GET, POST, PUT, DELETE methods
• UDP-based: Low overhead, no TCP handshake
• Observe: Subscribe to resource changes (similar to MQTT subscribe)
• Block transfer: Send large payloads in smaller chunks
• Content negotiation: Supports JSON, CBOR, and other formats

Publishing Telemetry

To send data, make a CoAP POST request to the telemetry resource:

POST coap://mqtt.siliconwit.io:5683/mqtt/d/{device_id}/t
Content-Format: application/json
Payload: {"temperature": 25.5, "humidity": 60}

Authentication

CoAP uses the same credentials as standard MQTT:

• Username: Your Device ID (from the SiliconWit dashboard)
• Password: Your Access Token (from the SiliconWit dashboard)

Credentials are passed in the CoAP request. The exact method depends on your CoAP library.

Example: Python with CoAP

import asyncio
import json
from aiocoap import Context, Message, POST

BROKER = "mqtt.siliconwit.io"
PORT = 5683
DEVICE_ID = "YOUR_DEVICE_ID"
ACCESS_TOKEN = "YOUR_ACCESS_TOKEN"

async def publish_telemetry():
    context = await Context.create_client_context()

    payload = json.dumps({
        "temperature": 25.5,
        "humidity": 60,
        "battery": 3.7
    }).encode()

    uri = f"coap://{BROKER}:{PORT}/mqtt/d/{DEVICE_ID}/t"

    request = Message(
        code=POST,
        uri=uri,
        payload=payload
    )
    request.opt.content_format = 50  # application/json

    response = await context.request(request).response
    print(f"Response: {response.code}")

asyncio.run(publish_telemetry())

Note

Python CoAP libraries include aiocoap and CoAPthon3. For embedded C devices, consider libcoap or your platform’s built-in CoAP stack.

Observing Commands

To receive commands from the SiliconWit platform, use CoAP’s Observe option:

GET coap://mqtt.siliconwit.io:5683/mqtt/d/{device_id}/c
Observe: 0

The gateway will push notifications whenever a command is published to your device’s command topic.

Payload Format

CoAP payloads follow the same JSON format as standard MQTT:

{
    "temperature": 25.5,
    "humidity": 60,
    "battery": 3.7
}

See Topics & Payloads for detailed formatting guidelines.

Limitations

• No encryption: CoAP currently runs over plain UDP without DTLS. For security-sensitive deployments, use standard MQTT with TLS on port 8883
• Gateway translation: Some CoAP features (like block-wise transfer) may not map directly to MQTT concepts
• Payload size: Keep payloads under 1 KB for reliable UDP delivery

Protocol Comparison

Feature	MQTT (TLS)	MQTT-SN	CoAP
Transport	TCP + TLS	UDP	UDP
Port	8883	1884	5683
Encryption	TLS	None	None
Model	Pub/Sub	Pub/Sub	Request/Response
Best for	WiFi, Ethernet	2G/GPRS, NB-IoT	NB-IoT, LPWAN
Persistent connection	Yes	Optional	No

Next Steps

• MQTT-SN Protocol - UDP-based publish/subscribe alternative
• MQTT Basics - Standard MQTT guide
• Topics & Payloads - Message formatting
• Authentication - Credential management