IOT5: Setting up a CoAP service

The following scenario will show you how to create a CoAP server on the IoT end node device.
A CoAP service is an endpoint that provides information. It is UDP-based, and the protocol configuration (confirmable and non-confirmable messages) requires more than one handler to be implemented in complex and fully implemented scenarios.


To implement this scenario, it is necessary to get familiar with at least one of the following scenarios first:

and obligatory:

There are many implementations of the CoAP protocol, but we will use the following library:

lib_deps = 
    hirotakaster/CoAP simple library

Suggested Readings and Knowledge Resources

Hands-on Lab Scenario

Note—this scenario can be used in tandem with IOT6: Connecting to the CoAP service to build a client-server solution using two devices (CoAP server and CoAP client). You need to book two devices then and develop them in parallel. We suggest connecting them over the AP rather than setting up an access point on the ESP32 device.

Task to be implemented

Connect to the “internal IoT” WiFI access point as presented in the scenario IOT2: Reading IP address of the WiFi—present connection status on display. Then, set up a CoAP server, sending on its endpoint a “Hello World” message or any customized message of your choice; note to keep it short in case you visualise it on another node's display with limited capabilities. Another endpoint should present any data on the selected sensor (e.g., temperature and humidity, as in the EMBx scenarios) or control any actuator, as we present in this scenario. This example uses an RGB LED to switch it on and off (as presented in the scenario EMB9A: Use of RGB LEDs).

The following endpoints should be implemented:

  • endpoint 1
    • method GET - should return a “Hello World” or other string of our choice,
  • endpoint 2
    • method GET - should return the current status of the LED light (“ON” or “OFF”),
    • method PUT - should set the LED on and off, with the expected payload being “1” for on and “0” for off.


Check if you can clearly see a full display (of your choice) in your video stream. Book a device and create a dummy Arduino file with void setup()… and void loop()….
Implement a connection to the “internal IoT” network as a client. Refer to the supervisor or the technical documentation on credentials (SSID, passphrase). We do not provide the exact code on how to connect to the WiFi as it is a part of IOT2: Reading IP address of the WiFi scenario.

Step 1

Refer to the hardware documentation and ensure an understanding of the network infrastructure you're interfacing with.
Implement the code to display on the selected device.
Connect to the WiFi in the STA mode (as a client) and ensure the connection is OK and you got an IP from the DHCP server.
It is essential to note and present (using a display of your choice) the node's IP address, as you will later need to refer to it with a client to use your service.

Step 2

Include the WiFi UDP and CoAP implementation libraries headers in your code:

#include <WiFiUdp.h>
#include <coap-simple.h>

WiFi UDP is part of the Arduino for the ESP32 framework, so you do not need to add it explicitly to the platformio.ini file.

Step 3

Declare necessary constants, etc.:

bool LEDSTATE; //Keep LED's state.

Step 4

Declare communication objects:

WiFiUDP udp;      //UDP Communication class
Coap coap(udp);   //CoAP Communication class

Step 5

Declare function prototypes (not necessary if you implement them in the correct order):

void callback_response(CoapPacket &packet, IPAddress ip, int port); // CoAP client response callback
// CoAP server endpoint URL callback for GET and PUT methods
void callback_led(CoapPacket &packet, IPAddress ip, int port);      

Step 6

Implement them:

void callback_led(CoapPacket &packet, IPAddress ip, int port) {
  // send response
  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = NULL;
  String message(p);
  //for GET
  if (message.equals("0"))
    LEDSTATE = false;
  else if(message.equals("1"))
    LEDSTATE = true;
  //for PUT
  if (LEDSTATE) {
    coap.sendResponse(ip, port, packet.messageid, "1");
  } else { 
    digitalWrite(RGBLED_R_PIN, LOW) ; 
    coap.sendResponse(ip, port, packet.messageid, "0");
void callback_response(CoapPacket &packet, IPAddress ip, int port) { 
  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = NULL;
  //Do something with payload, e.g. print it to the display

Step 7

Setup CoAP services:

  coap.server(callback_led, "light");

Setup 8

Process CoAP services in the void loop():

If you plan to make frequent requests, decrease the time above (delay()).

Result validation

You should be able to connect to the WiFi and set up a CoAP service. Depending on whether you're fully remote or able to access our networks with an additional device, you need to implement a CoAP client on another laboratory node (as present in the scenario IOT6: Connecting to the CoAP service) or use some other client (such as NodeRED or even command line client): note, your client must be present in the same WiFi network as your CoAP service (laboratory node).

Sample command line client (Linux/MacOS) syntax to make a PUT request to the service ( is presented below (bash):
$ coap-client -e "0" -m put coap://
Sample command line client (Linux/MacOS) syntax to make a GET request to the service ( is presented below (bash):
$ coap-client -m get coap://


How do I implement a client to my CoAP service?: If you're fully remote, the only option is to implement a client on your own - use another laboratory node and scenario IOT6: Connecting to the CoAP service pointing to your service. For this reason, we suggest printing the IP address on the display, as you need to explicitly implement the client to connect to this IP (given by the DHCP server). If you're on a laptop or mobile within the WiFi network range to which the laboratory nodes are connected, you can connect to it with your laptop and use any CoAP client.

Project information

This Intellectual Output was implemented under the Erasmus+ KA2.
Project IOT-OPEN.EU Reloaded – Education-based strengthening of the European universities, companies and labour force in the global IoT market.
Project number: 2022-1-PL01-KA220-HED-000085090.

Erasmus+ Disclaimer
This project has been funded with support from the European Commission.
This publication reflects the views of only the author, and the Commission cannot be held responsible for any use that may be made of the information contained therein.

Copyright Notice
This content was created by the IOT-OPEN.EU Reloaded consortium, 2022,2024.
The content is Copyrighted and distributed under CC BY-NC Creative Commons Licence, free for Non-Commercial use.

en/iot-open/practical/hardware/sut/esp32/iot_5.txt · Last modified: 2024/05/01 13:28 by pczekalski
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