esp-idf/examples/protocols/coap_client/README.md

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# 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
* If PSK, Set CoAP Preshared Key to use in connection to the server
* If PSK, Set CoAP PSK Client identity (username)
Component config --->
CoAP Configuration --->
* Set encryption method definition, PSK (default) or PKI
* Enable CoAP debugging if required
### 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 'idf.py menuconfig -> Component config -> CoAP Configuration' and setting appropriate log level