MQTT Client Protocol (Message Queue Telemetry Transport)

Supported device types and versions 
Communication line configuration
Communication line parameters
Communication station configuration
I/O tag configuration
Literature
Document revisions

Supported device types and versions

The protocol is an implementation of the MQTT 3.1.1 standard (October 2014). MQTT protocol is a client/server protocol of subscribe/publish type. It is simple, has little overhead and is easy to implement. It is used for M2M communication (Machine to Machine) and in the IoT context (Internet of Things).
D2000 KOM implements the client part of the protocol. The protocol je implemented on a TCP/IP line. 
For transfer of LoRaWAN data encapsulated within the MQTT protocol, see LoRaWan protocol description.

Each PUBLISH message contains a topic (Topic), data (Payload) and level of confirmation (QoS). PUBLISH messages can be sent both by the client and the server. The clients at the beginning of the communication will use the SUBSCRIBE message to indicate what topics (parameter of Topic Filter protocol) they are interested in. 

• QoS_0 - PUBLISH message is not confirmed, it may be lost

• QoS_1 - PUBLISH message is confirmed by other side's PUBACK, it may be duplicate

• QoS_2 - PUBLISH message is confirmed by other side's PUBREC which is then confirmed back by the PUBREL message and that one by a final PUBCOMP message.

The level of confirmation of the messages sent by the D2000 KOM process is defined by the protocol parameter Publish QoS . D2000 KOM process considers the writing of the output tag to be successfully finished depending on the QoS:

• QoS_0 - after the data is successfully sent via TCP connection

• QoS_1 - after receiving PUBACK

• QoS_2 - after receiving PUBCOMP

The MQTT communication starts with the CONNECT message sent by client (D2000 KOM). Message contains User Name, Password and other parameters, from which only Clean Session Flag and Client ID can be modified (parameter Will Flag is not used, as  well as Will QoS and Will Retain, parameter Keep Alive is set to 0). Server replies with CONNACK message with a return code that contains information about the success of connect operation.

Then client sends SUBSCRIBE message with filter of topics (Topic Filter parameter), specifying which topics it is interest in, and with the required maximum level of confirmation (parameter Subscribe QoS).

The server responds with a return code that contains information about the success and maximum QoS that was assigned to the requested topics.

Then follows a phase of communication, during which both the client and the server send PUBLISH messages (the client with any topic, the server with topics relating to the filter of topics of the received SUBSCRIBE message) and confirm them according to the value of the QoS parameter of the received PUBLISH messages.

If the server does not send a message for longer than Ping Interval seconds, the client sends the PING request message, to which the server must respond with the PING response message (within the time specified by the Reply Timeout parameter).
If parameters change on the line, the connection is closed and re-created.

The communication has been tested with the MQTT server www.TheThings.network.

Communication line configuration

• Communication line category: TCP/IP-TCP.

• Host: IP address of MQTT server (or redundant addresses separated by a coma or semicolon).

• Port: default port number is 1883 or 8883 for the encrypted SSL/TLS variant.

• Line number: unused, set the value to 0.

Note: Default port for the MQTT protocol is 1883 or 8883 for SSL/TLS version. D2000 KOM does not contain implementation of the SSL/TLS protocol variant, but it is possible to configure it by using the stunnel utility http://www.stunnel.org working in a client mode (client = yes). Stunnel running on the same computer as the D2000 KOM should listen to the 1883 local port and after connecting of D2000 KOM process to the port should encrypt the communication using SLL/TLS and send to the target MQTT server (typically on port 8883).

Communication line parameters

Dialog link configuration - Protocol parameters tab.
They affect some optional protocol parameters. The following protocol line parameters can be entered:

Table 1