MQ Telemetry Transport format and protocol
IBM WebSphere MQ Telemetry uses version 3.1 of the MQ Telemetry Transport (MQTT) protocol. IBM publishes the protocol specification at https://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html.
If you have obtained an MQTT client from a source other then an installation of IBM WebSphere MQ, check the version of the MQTT protocol supported by the client.
Currently, clients from sources other than IBM WebSphere MQ Telemetry typically support a different level of the MQTT protocol and do not work correctly with the IBM WebSphere MQ Telemetry service. For these clients, a thin conversion layer is required that converts the clients to MQTT v3.1. Check with the source of your client if the conversion layer is available as an update to the client you intend to use.