Man page improvements - document MQTT support.
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Roger A. Light
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@ -91,6 +91,231 @@
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</para>
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</refsect1>
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<refsect1>
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<title>MQTT Support</title>
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<para>Mosquitto supports MQTT v5.0, v3.1.1, and v3.1.</para>
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<refsect2>
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<title>MQTT v5.0</title>
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<para>
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Mosquitto provides full MQTT v5.0 support, but some features
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are not used directly. The following sections describe the new
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features and explain where Mosquitto does not make use of a feature.
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</para>
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<refsect3>
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<title>Features</title>
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<variablelist>
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<varlistentry>
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<term><option>Enhanced authentication</option></term>
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<listitem><para>
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Basic MQTT authentication uses username/password
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checks. Enhanced authentication allows different
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authentication schemes to be integrated into MQTT,
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and even those schemes with multiple step processes.
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Clients request a particular type of authentication
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and if the broker is configured for that scheme the
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authentication continues. Mosquitto supports
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enhanced authentication through plugins.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>Error handling</option></term>
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<listitem><para>
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Most MQTT packets now have the concept of a
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<option>reason code</option>
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which indicates success or failure, and what the failure
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was. Mosquitto provides full support for reason codes,
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but does not make use of the
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<option>reason string</option>
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feature which can be used to provide a human readable
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error string to explain the reason code.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>Flow control</option></term>
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<listitem><para>
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The number of "in flight" messages for QoS 1 and QoS
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2 can be controlled by both the client and the
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broker.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>Request / response</option></term>
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<listitem><para>
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MQTT v5.0 adds a request/response pattern that
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allows a client to publish a message and instruct
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the subscribers of that message where to publish a
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response.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>Server redirection</option></term>
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<listitem><para>
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Server redirection is the concept of telling a
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client to connect to a different MQTT broker, either
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on CONNECT or with a broker initiated DISCONNECT.
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Mosquitto does not currently make use of this
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feature.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>Shared subscriptions</option></term>
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<listitem><para>
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When multiple clients subscribe to the same shared
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subscription, only one client out of the group will
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receive each message which allows for distributing
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work loads.
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</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect3>
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<refsect3>
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<title>Packet properties</title>
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<para>
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MQTT v5.0 allows properties to be added to packets to
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control certain behaviour. Unless noted, Mosquitto
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support the properties listed below.
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</para>
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<variablelist>
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<varlistentry>
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<term><option>CONNECT</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Authentication data</para></listitem>
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<listitem><para>Authentication method</para></listitem>
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<listitem><para>Maximum packet size</para></listitem>
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<listitem><para>Receive maximum</para></listitem>
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<listitem><para>Request problem information - supported but not used</para></listitem>
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<listitem><para>Request response information - supported but not used</para></listitem>
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<listitem><para>Session expiry interval</para></listitem>
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<listitem><para>Topic alias maximum</para></listitem>
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<listitem><para>User property</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>Last will and testament</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Content type</para></listitem>
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<listitem><para>Correlation data</para></listitem>
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<listitem><para>Message expiry interval</para></listitem>
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<listitem><para>Payload format indicator</para></listitem>
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<listitem><para>Response topic</para></listitem>
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<listitem><para>User property</para></listitem>
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<listitem><para>Will delay interval</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>CONNACK</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Assigned client identifier</para></listitem>
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<listitem><para>Authentication data</para></listitem>
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<listitem><para>Authentication method</para></listitem>
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<listitem><para>Maximum packet size</para></listitem>
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<listitem><para>Maximum qos</para></listitem>
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<listitem><para>Reason string - supported but not used</para></listitem>
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<listitem><para>Receive maximum</para></listitem>
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<listitem><para>Response information - supported but not used</para></listitem>
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<listitem><para>Retain available</para></listitem>
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<listitem><para>Server keep alive</para></listitem>
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<listitem><para>Server reference - supported but not used</para></listitem>
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<listitem><para>Session expiry interval</para></listitem>
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<listitem><para>Shared subscription available</para></listitem>
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<listitem><para>Subscription identifiers available</para></listitem>
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<listitem><para>Topic alias maximum</para></listitem>
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<listitem><para>User property</para></listitem>
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<listitem><para>Wildcard subscription available</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>PUBLISH</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Content type</para></listitem>
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<listitem><para>Correlation data</para></listitem>
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<listitem><para>Message expiry interval</para></listitem>
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<listitem><para>Payload format indicator</para></listitem>
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<listitem><para>Response topic</para></listitem>
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<listitem><para>Subscription identifier</para></listitem>
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<listitem><para>Topic alias</para></listitem>
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<listitem><para>User property</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>PUBACK / PUBREC / PUBREL / PUBCOMP / SUBACK / SUBSCRIBE / SUBACK</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Reason string - supported but not used</para></listitem>
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<listitem><para>User property</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>SUBSCRIBE</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Subscription identifier</para></listitem>
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<listitem><para>User property</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>DISCONNECT</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Reason string - supported but not used</para></listitem>
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<listitem><para>Server reference - supported but not used</para></listitem>
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<listitem><para>Session expiry interval</para></listitem>
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<listitem><para>User property</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>AUTH</option></term>
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<listitem>
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<itemizedlist>
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<listitem><para>Authentication method</para></listitem>
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<listitem><para>Authentication data</para></listitem>
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<listitem><para>Reason string - supported but not used</para></listitem>
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<listitem><para>User property</para></listitem>
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</itemizedlist>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect3>
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</refsect2>
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<refsect2>
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<title>MQTT v3.1.1</title>
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<para>Mosquitto provides full MQTT v3.1.1 support.</para>
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</refsect2>
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<refsect2>
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<title>MQTT v3.1</title>
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<para>Mosquitto provides full MQTT v3.1 support.</para>
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</refsect2>
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<refsect2>
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<title>MQTT v3</title>
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<para>
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MQTT v3 is an obsolete version of the protocol that does not
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support username/password authentication and used the
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<option>clean start</option> flag in the CONNECT packet which
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applied only to the start of a session. An MQTT v3 client
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will be able to successfully connect to a Mosquitto instance
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that does not require authentication.
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</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Broker Status</title>
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<para>Clients can find information about the broker by subscribing to
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