# CoAP client example

(See the README.md file in the upper level '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.

NOTE: coaps+tcp:// is not currently supported, even though both libcoap and MbedTLS support it.

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 under Example Configuration
 * Set WiFi Password under Example Configuration
Example CoAP Client Configuration  --->
 * Set CoAP Target Uri
 * Set encryption method definitions (None, PSK or PKI)
  * If PSK Set CoAP Preshared Key to use in connection to the server
  * If PSK Set CoAP PSK Client identity (username)
 Enable CoAP debugging if required
Component config  --->
 mbedTLS  --->
  [*]   Enable mbedtls certificate expiry check
       TLS Key Exchange Methods  --->
        [*] Enable pre-shared-key ciphersuites
        [*]   Enable PSK based ciphersuite modes
  [*] Support DTLS protocol (all versions)

### 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.eclipse.org.  

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=104.196.15.150
Received:
************************************************************
CoAP RFC 7252                              Cf 2.0.0-SNAPSHOT
************************************************************
This server is using the Eclipse Californium (Cf) CoAP framework
published under EPL+EDL: http://www.eclipse.org/californium/

(c) 2014, 2015, 2016 Institute for Pervasive Computing, ETH Zurich and others
************************************************************
...
```

## libcoap Documentation
This can be found at https://libcoap.net/doc/reference/4.2.0/

## Troubleshooting
* Please make sure Target Url includes valid `host`, optional `port`,
optional `path`, and begins with `coap://`, `coaps://` or `coap+tcp://`
for a coap server that supports TCP
(not all do including coap+tcp://californium.eclipse.org).

* CoAP logging can be enabled by running 'make menuconfig' and enable debugging

* Encryption (MbedTLS) can be enabled by running 'make menuconfig' and enable debugging