MQTT-SN
Overview
MQTT-SN is a variant of the well-known MQTT protocol - see MQTT.
In contrast to MQTT, MQTT-SN does not require a TCP transport, but is designed to be used over any message-based transport. Originally, it was mainly created with ZigBee in mind, but others like Bluetooth, UDP or even a UART can be used just as well.
Zephyr provides an MQTT-SN client library built on top of BSD sockets API. The
library can be enabled with CONFIG_MQTT_SN_LIB Kconfig option
and is configurable at a per-client basis, with support for MQTT-SN version
1.2. The Zephyr MQTT-SN implementation can be used with any message-based transport,
but support for UDP is already built-in.
MQTT-SN clients require an MQTT-SN gateway to connect to. These gateways translate between MQTT-SN and MQTT. The Eclipse Paho project offers an implementation of a MQTT-SN gateway, but others are available too. https://www.eclipse.org/paho/index.php?page=components/mqtt-sn-transparent-gateway/index.php
The MQTT-SN spec v1.2 can be found here: https://www.oasis-open.org/committees/download.php/66091/MQTT-SN_spec_v1.2.pdf
Sample usage
To create an MQTT-SN client, a client context structure and buffers need to be defined:
/* Buffers for MQTT client. */
static uint8_t rx_buffer[256];
static uint8_t tx_buffer[256];
/* MQTT-SN client context */
static struct mqtt_sn_client client;
Multiple MQTT-SN client instances can be created in the application and managed independently. Additionally, a structure for the transport is needed as well. The library already comes with an example implementation for UDP.
/* MQTT Broker address information. */
static struct mqtt_sn_transport tp;
The MQTT-SN library will inform clients about certain events using a callback.
static void evt_cb(struct mqtt_sn_client *client,
const struct mqtt_sn_evt *evt)
{
switch(evt->type) {
{
/* Handle events here. */
}
}
For a list of possible events, see API Reference.
The client context structure needs to be initialized and set up before it can be used. An example configuration for UDP transport is shown below:
struct mqtt_sn_data client_id = MQTT_SN_DATA_STRING_LITERAL("ZEPHYR");
struct sockaddr_in gateway = {0};
uint8_t tx_buf[256];
uint8_t rx_buf[256];
mqtt_sn_transport_udp_init(&tp, (struct sockaddr*)&gateway, sizeof((gateway)));
mqtt_sn_client_init(&client, &client_id, &tp.tp, evt_cb, tx_buf, sizeof(tx_buf), rx_buf, sizeof(rx_buf));
After the configuration is set up, the MQTT-SN client can connect to the gateway. While the MQTT-SN protocol offers functionality to discover gateways through an advertisement mechanism, this is not implemented yet in the library.
Call the mqtt_sn_connect function, which will send a CONNECT message.
The application should periodically call the mqtt_sn_input function to process
the response received. The application does not have to call mqtt_sn_input if it
knows that no data has been received (e.g. when using Bluetooth). Note that
mqtt_sn_input is a non-blocking function, if the transport struct contains a
poll compatible function pointer.
If the connection was successful, MQTT_SN_EVT_CONNECTED will be notified to the
application through the callback function.
err = mqtt_sn_connect(&client, false, true);
__ASSERT(err == 0, "mqtt_sn_connect() failed %d", err);
while (1) {
mqtt_sn_input(&client);
if (connected) {
mqtt_sn_publish(&client, MQTT_SN_QOS_0, &topic_p, false, &pubdata);
}
k_sleep(K_MSEC(500));
}
In the above code snippet, the event handler function should set the connected
flag upon a successful connection. If the connection fails at the MQTT level
or a timeout occurs, the connection will be aborted.
After the connection is established, an application needs to call mqtt_input
function periodically to process incoming data. Connection upkeep, on the other hand,
is done automatically using a k_work item.
If a MQTT message is received, an MQTT callback function will be called and an
appropriate event notified.
The connection can be closed by calling the mqtt_sn_disconnect function. This
has no effect on the transport, however. If you want to close the transport (e.g.
the socket), call mqtt_sn_client_deinit, which will deinit the transport as well.
Zephyr provides sample code utilizing the MQTT-SN client API. See MQTT-SN publisher for more information.
Deviations from the standard
Certain parts of the protocol are not yet supported in the library.
Pre-defined topic IDs
QoS -1 - it’s most useful with predefined topics
Gateway discovery using ADVERTISE, SEARCHGW and GWINFO messages.
Setting the will topic and message after the initial connect
Forwarder Encapsulation