User attributes
Support user-defined attributes and transmission of additional custom information to expand more application scenarios.
Topic alias
Reduce the overhead of MQTT messages by shortening the topic name to a small integer.
Session expiration
Split the cleanup session flag into a new start flag and the session expiration interval can be modified when disconnected.
Flow control
Allow the client and server to specify the number of outstanding reliable messages (QoS> 0) respectively.
Message expiration
Allows messages to be published with an expiration interval.
Maximum packet length
Allow the client and server to individually specify the maximum message lengths they support.
Reason codes of all confirmation packets
All response packets contain reason codes so that the caller can determine whether the requested function was successful.
Optional server function availability
Inform the client the feature list supported by the server to avoid the client using unsupported features.
All confirmation message reason strings
Designed for problem location.
Enhanced authentication
Provide a mechanism to enable challenge/response style authentication including mutual authentication.
Server disconnection
The server is allowed to send a DISCONNECT message to indicate why the connection was closed.
Subscription options
Provide subscription options that are primarily used to define applications allowing message bridging.
Payload format and content type
Allows to specify payload format (binary, text) and MIME-style content types when publishing messages.
Will delay
Provide the ability to delay the sending of the specified will message after the connection is interrupted.
Request / response mode
Specify the MQTT request / response mode, provide the response topic and compare data attributes, and control the response message to be routed back to the request publisher.
Server keeps connected
Allow the server to specify the keep-alive values it wants the client to use.
Shared subscription
Implemented in v3.1.1, adding support for shared subscriptions to allow load balancing for multiple subscription consumers.
Assign client identifier
If the server has assigned a client identifier, this client identifier is returned to the client.
Subscription identifier
Allows to specify a digital subscription identifier in the subscription packet and returns this identifier when the message is distributed.
Server reference
Allows the server to specify a standby server.
