The nRF Connect SDK provides the CoAP library, which is Zephyr’s implementation of the CoAP standard. The library supports both the server and client roles and is highly configurable.

The library is implemented using plain buffers, meaning the user must create the sockets for communication and then pass these to the library. This is similar to what we did in Lesson 3 Exercise 1.

Enabling and configuring library

Enable the CoAP library in your application by enabling the following Kconfig in the prj.conf file.

CONFIG_COAP=y

Include the header file for the CoAP library in your source code.

#include <zephyr/net/coap.h>

Create a CoAP message

Create a CoAP packet message of type struct coap_packet, using coap_packet_init() which has the following signature:

• cpkt – Packet to be initialized, of type struct coap_packet
• data – Data can contain CoAP packet information to be used as input data
• max_len – length of data
• ver – CoAP header version, currently the value 1.
• type – CoAP header type, specified in enum coap_msgtype.
• token_len – length of token
• token – CoAP header token
• code – CoAP header code, specified in enum coap_method.
• id – CoAP header message ID

Add options

You can add options to the CoAP packet using coap_packet_append_option(), which has the following signature

• cpkt – Packet to be updated
• code – Code for the packet option, see enum coap_option_num.
• value – Pointer to the value of the option
• len – Size of data in value

Parse response from CoAP server

Parse a received CoAP packet using coap_packet_parse(), which has the following signature

• cpkt – Packet to be initialized, of type struct coap_packet
• data – Pointer to the CoAP packet to be parsed
• len – Length of the data
• options – Parse options of type struct coap_option.
• opt_num – Pass the number of options passed.

Helper functions

When initiating a packet, we can use the function coap_next_id(), which has the following signature

Extract the payload from a CoAP packet using coap_packet_get_payload(), which has the following signature

Extract the token from a CoAP packet using coap_header_get_token(), which has the following signature

Extract the code from the CoAP packet using coap_header_get_code(), which has the following signature

We have now covered all the CoAP API functions we will be using in the exercise for this lesson. See the CoAP library documentation for the full API reference of this library.

Using CoAP with DTLS

As opposed to the MQTT Library covered in Lesson 4, the CoAP library does not natively support implementing a DTLS connection. Therefore, we will do this using the Modem key management library and the secure sockets implementation in Zephyr’s native BSD sockets API.