samples: net: mqtt_publisher: Enable MQTT 5.0 support

Enable MQTT 5.0 support in the mqtt_publisher sample and make use
of the topic aliasing feature.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

samples/net/mqtt_publisher/README.rst

@@ -11,11 +11,15 @@ Overview
 publish/subscribe messaging protocol optimized for small sensors and
 mobile devices.
 
-The Zephyr MQTT Publisher sample application is a MQTT v3.1.1
-client that sends MQTT PUBLISH messages to a MQTT broker.
-See the `MQTT V3.1.1 spec`_ for more information.
+The Zephyr MQTT Publisher sample application is a MQTT client that sends
+MQTT PUBLISH messages to a MQTT broker. The sample supports MQTT client in
+version v3.1.1 (default) and v5.0.
 
-.. _MQTT V3.1.1 spec: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html
+See the `MQTT v3.1.1 spec`_ and `MQTT v5.0 spec`_ for more information about
+MQTT v3.1.1 and v5.0, respectively.
+
+.. _MQTT v3.1.1 spec: https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/mqtt-v3.1.1.html
+.. _MQTT v5.0 spec: https://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-v5.0.html
 
 The source code of this sample application can be found at:
 :zephyr_file:`samples/net/mqtt_publisher`.
@@ -136,6 +140,23 @@ Open another terminal window and type:
 
 	$ mosquitto_sub -t sensors
 
+MQTT v5.0 support
+=================
+
+The sample can be configured to use MQTT v5.0 instead of MQTT v3.1.1. To enable
+MQTT v5.0 in the sample, build it with ``-DEXTRA_CONF_FILE=overlay-mqtt-5.conf``
+parameter. The sample should work with any broker supporting MQTT v5.0, however
+it was specifically tested with mosquitto version 2.0.21. Server side
+configuration in this particular case is the same as for MQTT v3.1.1.
+
+When the sample is configured in the MQTT v5.0 mode, it makes use of the topic
+aliasing feature. i.e. if the broker reports it supports topic aliases, the
+client will register a topic alias for the default ``sensors`` topic, and use it
+for consecutive MQTT Publish messages. It can be observed (for example using
+Wireshark) how the actual topic is only present in the first Publish message, and
+all subsequent Publish messages are smaller, as they include topic alias
+property only.
+
 Connecting securely using TLS
 =============================
 

samples/net/mqtt_publisher/overlay-mqtt-5.conf

@@ -0,0 +1 @@
+CONFIG_MQTT_VERSION_5_0=y

samples/net/mqtt_publisher/src/main.c

@@ -53,9 +53,14 @@ static APP_BMEM struct sockaddr socks5_proxy;
 
 static APP_BMEM struct pollfd fds[1];
 static APP_BMEM int nfds;
-
 static APP_BMEM bool connected;
 
+/* Whether to include full topic in the publish message, or alias only (MQTT 5). */
+static APP_BMEM bool include_topic;
+static APP_BMEM bool aliases_enabled;
+
+#define APP_TOPIC_ALIAS 1
+
 #if defined(CONFIG_MQTT_LIB_TLS)
 
 #include "test_certs.h"
@@ -155,6 +160,18 @@ void mqtt_evt_handler(struct mqtt_client *const client,
 		connected = true;
 		LOG_INF("MQTT client connected!");
 
+#if defined(CONFIG_MQTT_VERSION_5_0)
+		if (evt->param.connack.prop.rx.has_topic_alias_maximum &&
+		    evt->param.connack.prop.topic_alias_maximum > 0) {
+			LOG_INF("Topic aliases allowed by the broker, max %u.",
+				evt->param.connack.prop.topic_alias_maximum);
+
+			aliases_enabled = true;
+		} else {
+			LOG_INF("Topic aliases disallowed by the broker.");
+		}
+#endif
+
 		break;
 
 	case MQTT_EVT_DISCONNECT:
@@ -242,12 +259,18 @@ static char *get_mqtt_topic(void)
 
 static int publish(struct mqtt_client *client, enum mqtt_qos qos)
 {
-	struct mqtt_publish_param param;
+	struct mqtt_publish_param param = { 0 };
 
-	param.message.topic.qos = qos;
+	/* Always true for MQTT 3.1.1.
+	 * True only on first publish message for MQTT 5.0 if broker allows aliases.
+	 */
+	if (include_topic) {
 		param.message.topic.topic.utf8 = (uint8_t *)get_mqtt_topic();
 		param.message.topic.topic.size =
 			strlen(param.message.topic.topic.utf8);
+	}
+
+	param.message.topic.qos = qos;
 	param.message.payload.data = get_mqtt_payload(qos);
 	param.message.payload.len =
 			strlen(param.message.payload.data);
@@ -255,6 +278,13 @@ static int publish(struct mqtt_client *client, enum mqtt_qos qos)
 	param.dup_flag = 0U;
 	param.retain_flag = 0U;
 
+#if defined(CONFIG_MQTT_VERSION_5_0)
+	if (aliases_enabled) {
+		param.prop.topic_alias = APP_TOPIC_ALIAS;
+		include_topic = false;
+	}
+#endif
+
 	return mqtt_publish(client, &param);
 }
 
@@ -308,7 +338,11 @@ static void client_init(struct mqtt_client *client)
 	client->client_id.size = strlen(MQTT_CLIENTID);
 	client->password = NULL;
 	client->user_name = NULL;
+#if defined(CONFIG_MQTT_VERSION_5_0)
+	client->protocol_version = MQTT_VERSION_5_0;
+#else
 	client->protocol_version = MQTT_VERSION_3_1_1;
+#endif
 
 	/* MQTT buffers configuration */
 	client->rx_buf = rx_buffer;
@@ -435,6 +469,9 @@ static int publisher(void)
 {
 	int i, rc, r = 0;
 
+	include_topic = true;
+	aliases_enabled = false;
+
 	LOG_INF("attempting to connect: ");
 	rc = try_to_connect(&client_ctx);
 	PRINT_RESULT("try_to_connect", rc);