Introduction to QtCoAP

QtCoAP, M2M, Qt

It is a client-side implementation of the Constrained Application Protocol (CoAP) for the Internet of Things. The library provides a quick and easy way to use the CoAP protocol in your cross-platform Qt applications. As previously announced, QtCoAP will be available in the new Qt 5.13 release as part of Qt for Automation along with other IoT protocol implementations such as MQTT, KNX and OPC UA.

What is CoAP?

CoAP was designed as a lightweight machine-to-machine (M2M) communication protocol that can run on devices with limited memory and computing resources. It is based on the concept of RESTful APIs and is very similar to HTTP.

CoAP is a client-server architecture and uses GET, POST, PUT, and DELETE requests to interact with data. But unlike HTTP, it uses lightweight UDP for transport instead of TCP. In addition, it supports some interesting features such as multicast requests, resource discovery, and monitoring.

Due to its low cost and simplicity, CoAP has become one of the popular IoT protocols for use on embedded devices. It acts like a kind of HTTP for the built-in world.

QtCoAP implementation overview

QtCoAP supports the following features:

• Resource monitoring.
• Resource discovery.
• Group communication (multicast).
• Block translations.
• Security.

The library is really easy to use. You just need to instantiate the QCoapClient object and connect its signals:

QCoapClient client;
connect(&client, &QCoapClient::finished, this, &CoapHandler::onFinished);
connect(&client, &QCoapClient::error, this, &CoapHandler::onError);

You are now ready to send requests and receive responses:

// GET requests
client.get(QUrl("coap://127.0.0.1:5683/resource"));
// or simply
client.get(QUrl("127.0.0.1/resource"));
 
// PUT/POST requests
QFile file("data.json");
// ...
client.post(QUrl("127.0.0.1/resource"), file.readAll());
client.put(QUrl("127.0.0.1/resource"), file.readAll());
 
// DELETE requests
client.deleteResource(QUrl("127.0.0.1/resource"));

Using the QCoapRequest class, you can pass parameters and customize your requests. For example:

QCoapRequest request;
request.setUrl(QUrl("127.0.0.1"));
request.setPayload(file.readAll());
request.addOption(QCoapOption::UriPath, "resource");
client.put(request);

CoAP also provides a publish-subscribe mechanism, which is achieved through "observe" requests:

QCoapReply *observeReply = client.observe(QUrl("127.0.0.1/temperature"));
connect(observeReply, &QCoapReply::notified, this, &CoapHandler::onNotified);

Now your application will be notified whenever the “/temperature” resource (“/temperature”) changes.

What makes CoAP even more interesting is the ability to find and discover CoAP resources. You can find resources on a given host:

QCoapResourceDiscoveryReply
*discoverReply = client.discover(QUrl("127.0.0.1"));

Or across the network:

QCoapResourceDiscoveryReply
*discoverReply = client.discover();

This will send an IPv4 CoAP multicast group discovery request. You can also run discovery for IPv6 hosts:

discoverReply = client.discover(QtCoap::MulticastGroup::AllCoapNodesIPv6LinkLocal);
// or
discoverReply = client.discover(QtCoap::MulticastGroup::AllCoapNodesIPv6SiteLocal);
...
connect(discoverReply, &QCoapResourceDiscoveryReply::discovered, this, &CoapHandler::onDiscovered);

You will receive multiple discovery responses from each CoAP device in your network. For example:

Host 1:

RES: 2.05 Content
;rt="temperature-c";if="sensor";obs,
;rt="firmware";sz=262144

Host 2:

RES: 2.05 Content
;rt="temperature-c";if="sensor";obs,

This will mean that there are 2 devices on your network running CoAP servers: one of them is connected to temperature and light sensors, and the other is only connected to a temperature sensor.

Security

Last but not least is safety. The Qt CoAP library supports the following security modes:

• Authentication using pre-shared keys (shared keys).
• Use X.509 certificates (X.509 certificates).

To secure a CoAP connection, you need to go through one of these modes when creating a client and configure it accordingly. For example:

QCoapClient secureClient(QtCoap::SecurityMode::PreSharedKey);
QCoapSecurityConfiguration config;
config.setIdentity("Client_identity");
config.setPreSharedKey("secretPSK");
secureClient.setSecurityConfiguration(config);