Skip to content

CoAP Protocol

CoAP (Constrained Application Protocol) is designed for constrained devices and low-power networks. Use CoAP when working with UDP-based networks or CoAP-specific infrastructure.

Overview

CoAP features: - UDP-based: Lower overhead than TCP - REST-like: Similar to HTTP methods - Lightweight: Smaller message size - Multicast: One-to-many communication - Observing: Subscribe to resource changes

CoAP Methods

Method HTTP Equivalent Use Case
GET GET Retrieve data
POST POST Create resource
PUT PUT Update resource
DELETE DELETE Remove resource

ESP32 CoAP Setup

Required Library

Install CoAP simple library via Arduino IDE Library Manager.

Basic Client

#include <WiFi.h>
#include <WiFiUdp.h>
#include <coap_simple.h>

const char* ssid = "YOUR_WIFI_SSID";
const char* password = "YOUR_WIFI_PASSWORD";

WiFiUDP udp;
Coap coap(udp);

void callback_response(CoapPacket &packet, IPAddress ip, int port) {
  Serial.println("[Coap Response got]");

  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = NULL;

  Serial.println(p);
}

void setup() {
  Serial.begin(115200);

  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("\nWiFi connected");

  coap.response(callback_response);
  coap.start();
}

void loop() {
  // Send sensor data every 10 seconds
  static unsigned long lastSend = 0;
  if (millis() - lastSend > 10000) {
    lastSend = millis();

    // TODO: Replace with actual sensor readings
    float temperature = 25.5;
    float humidity = 60.0;

    String payload = "temp=" + String(temperature) + "&hum=" + String(humidity);

    Serial.println("Sending CoAP message...");
    coap.put(IPAddress(192, 168, 1, 100), 5683, "sensors", payload.c_str());
  }

  coap.loop();
  delay(100);
}

CoAP Observing (Subscriptions)

// Subscribe to resource changes
void callback_observe(CoapPacket &packet, IPAddress ip, int port) {
  Serial.println("[Coap Observer got]");

  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = NULL;

  Serial.println(p);
}

void setup() {
  // ... WiFi setup

  coap.observe(IPAddress(192, 168, 1, 100), 5683, "sensors/temperature", callback_observe);
  coap.start();
}

CoAP Message Format

Request

CON [Message ID]
  GET
  Uri-Path: sensors/temperature

Response

ACK [Message ID]
  2.05 Content
  Payload: 25.5

CoAP Response Codes

Code Meaning Description
2.01 Created Resource created successfully
2.02 Deleted Resource deleted
2.03 Valid Response from cache
2.04 Changed Resource changed
2.05 Content Response to GET
4.00 Bad Request Invalid request
4.01 Unauthorized Authentication required
4.04 Not Found Resource not found
5.00 Internal Error Server error

CoAP vs HTTP

Feature CoAP HTTP
Transport UDP TCP
Overhead ~10 bytes ~800 bytes
Multicast Yes No
Battery use Lower Higher
Proxy support Yes Yes

When to Use CoAP

  • UDP networks: NAT traversal, no TCP overhead
  • Multicast: One-to-many communication
  • Low power: Battery-constrained devices
  • CoAP infrastructure: Existing CoAP brokers/proxies

CoAP Server Setup

For testing, set up a CoAP server:

Using Python (aiocoap)

pip install aiocoap

from aiocoap import Context, Message
from aiocoap.resource import Resource, Site
import asyncio

class SensorResource(Resource):
    async def render_get(self, request):
        payload = b'{"temperature": 25.5, "humidity": 60.0}'
        return Message(payload=payload, content_format=0)

    async def render_put(self, request):
        print(f"Received: {request.payload}")
        return Message(code=Code.CHANGED)

# Setup server
site = Site()
site.add_resource(['sensors'], SensorResource())

asyncio.run(Context.create_server_context(site))

Cloud Integration

Self-Hosted CoAP

CoAP requires a CoAP broker or server:

  • Eclipse Californium: Java CoAP framework
  • aiocoap: Python CoAP library
  • node-coap: Node.js CoAP library

AWS IoT Core

AWS IoT Core does not natively support CoAP. Use MQTT or HTTP for AWS.

Testing

CoAP CLI Tools

# Install libcoap tools
# Ubuntu/Debian: apt-get install libcoap2-bin

# GET request
coap-client -m get coap://192.168.1.100/sensors

# POST request
coap-client -m post -e "temp=25.5" coap://192.168.1.100/sensors

# Observe (subscribe)
coap-client -m get -s 60 coap://192.168.1.100/sensors/temperature

DTLS Security

CoAP supports Datagram TLS (DTLS) for encrypted UDP communication:

// DTLS requires additional setup with certificates
// Implementation depends on specific CoAP library

Troubleshooting

Issue Solution
No response Check UDP port 5683, verify server is running
Message lost Implement retry logic, check network quality
Observe not working Verify server supports observing
Payload too large CoAP max payload ~1024 bytes, fragment if needed

Comparison Summary

Protocol Best For Complexity
MQTT Real-time IoT, pub/sub Medium
HTTP Web APIs, REST Low
CoAP UDP networks, low-power Medium

Next Steps