Message format

The message format that is common to all packet types,

The message header for each MQTT command message contains a fixed header. Some messages also require a variable header and a payload. The format for each part of the message header is described in the following sections:

Fixed header

The message header for each MQTT command message contains a fixed header.

Message type

Variable header

Protocol name

Protocol version

Connect flags

Clean session flag

Will flag

Will QoS

Will Retain flag

Username and password flags

Keep alive timer

Connect return code

Topic name

Payload

Message identifiers

MQTT and UTF-8

Unused bits