coap_client

| Supported Targets | ESP32 | ESP32-C2 | ESP32-C3 | ESP32-C6 | ESP32-H2 | ESP32-S2 | ESP32-S3 |
| ----------------- | ----- | -------- | -------- | -------- | -------- | -------- | -------- |


# CoAP client example

(See the README.md file in the upper level esp-idf 'examples' directory for more information
about examples.)

This CoAP client example is very simplified adaptation of one of the
[libcoap](https://github.com/obgm/libcoap) examples.

CoAP client example will connect your ESP32 device to a CoAP server, send off a GET request and
fetch the response data from CoAP server.  The client can be extended to PUT / POST / DELETE requests,
as well as supporting the Observer extensions [RFC7641](https://tools.ietf.org/html/rfc7641).

If the URI is prefixed with coaps:// instead of coap://, then the CoAP client will attempt to use
the DTLS protocol using the defined Pre-Shared Keys(PSK) or Public Key Infrastructure (PKI) which the
CoAP server needs to know about.

If the URI is prefixed with coap+tcp://, then the CoAP will try to use TCP for the communication.

If the URI is prefixed with coap+ws://, then the CoAP will try to use WebSockets (over TCP) for
the communication, which assumes that CoAP WebSockets is enabled in the build.

The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with
constrained nodes and constrained networks in the Internet of Things.
The protocol is designed for machine-to-machine (M2M) applications such as smart energy and
building automation.

Please refer to [RFC7252](https://www.rfc-editor.org/rfc/pdfrfc/rfc7252.txt.pdf) for more details.

## How to use example

### Configure the project

```
idf.py menuconfig
```

Example Connection Configuration  --->
 * Set WiFi SSID
 * Set WiFi Password
Component config  --->
  CoAP Configuration  --->
    * Set encryption method definition, PSK (default) or PKI
    * Enable CoAP debugging if required
    * Disable CoAP using TCP if this is not required (TCP needed for TLS)
    * Disable CoAP server functionality to reduce code size
Example CoAP Client Configuration  --->
 * Set CoAP Target Uri
 * If PSK, Set CoAP Preshared Key to use in connection to the server
 * If PSK, Set CoAP PSK Client identity (username)

Note:
 * For enabled PKI, the certificates are stored in main/certs.
 * For enabled OSCORE, the OSCORE configuration is stored in main/oscore and will be used on
   every request.

### Build and Flash

Build the project and flash it to the board, then run monitor tool to view serial output:

```
idf.py build
idf.py -p PORT flash monitor
```

(To exit the serial monitor, type ``Ctrl-]``.)

See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.

## Example Output
Prerequisite: we startup a CoAP server on coap server example,
or use the default of coap://californium.eclipseprojects.io.

and you could receive data from CoAP server if succeed,
such as the following log:

```
...
I (332) wifi: mode : sta (30:ae:a4:04:1b:7c)
I (1672) wifi: n:11 0, o:1 0, ap:255 255, sta:11 0, prof:1
I (1672) wifi: state: init -> auth (b0)
I (1682) wifi: state: auth -> assoc (0)
I (1692) wifi: state: assoc -> run (10)
I (1692) wifi: connected with huawei_cw, channel 11
I (1692) wifi: pm start, type: 1

I (2582) event: sta ip: 192.168.3.89, mask: 255.255.255.0, gw: 192.168.3.1
I (2582) CoAP_client: Connected to AP
I (2582) CoAP_client: DNS lookup succeeded. IP=35.185.40.182
Received:
****************************************************************
CoAP RFC 7252                                  Cf 3.0.0-SNAPSHOT
****************************************************************
This server is using the Eclipse Californium (Cf) CoAP framework
published under EPL+EDL: http://www.eclipse.org/californium/

(c) 2014-2020 Institute for Pervasive Computing, ETH Zurich and others
****************************************************************
...
```

## libcoap Documentation
This can be found at [libcoap Documentation](https://libcoap.net/documentation.html).
The current API is 4.3.4.

## libcoap Specific Issues
These can be raised at [libcoap Issues](https://github.com/obgm/libcoap/issues).

## Troubleshooting
* Please make sure Target Url includes valid `host`, optional `port`,
optional `path`, and begins with `coap://`, `coaps://`, `coap+tcp://`, `coaps+tcp://`,
`coap+ws://` or `coaps+ws://`.
  * Not all hosts support TCP/TLS including coap+tcp://californium.eclipseprojects.io
  * Not all hosts support WebSockets, which needs to be enabled as an option

* CoAP logging can be enabled by running
'idf.py menuconfig -> Component config -> CoAP Configuration -> Enable CoAP debugging'
and setting appropriate log level.  If Mbed TLS logging is required, this needs to be
configured separately under mbedTLS Component Configuration.