Duplicate delivery of a PUBLISH Control Packet
flags to ubyte
Quality Of Service for a message
PUBLISH Retain flag
Represented as a 4-bit unsigned value
The Remaining Length is the number of bytes remaining within the current packet, including data in the variable header and the payload. The Remaining Length does not include the bytes used to encode the Remaining Length.
Each MQTT Control Packet contains a fixed header.
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Figure_2.2_-