MQTT message format - Part I

MQTT contains a mandatory fixed-length header (2 bytes) and an optional message specific variable length header and message payload. 

Optional fields usually complicate the message processing. However, MQTT is optimized for bandwidth constrained unreliable networks (typically wireless networks), so optional fields are used to reduce data transmission as much as possible. 

MQTT uses network byte and bit ordering. 

References:

http://www.slideshare.net/PeterREgli/mq-telemetry-transport