- Connect Flags: to specify the behavior of the connection.
- Keep Alive: maximum time interval, in seconds, that can elapse between client transmission of control packets.
- Clean Session: controls the lifetime of the session state (0 to resume state, 1 to discard previous state).
- Will Flag: indicates that a will message is to be sent upon dirty client disconnection.
- Will QoS: indicates QoS level for the will message.
- Will Retain: indicates the retain policy for the will message.
- Password: indicates whether (1) or not (0) a password must be present in the payload.
- User Name: indicates whether (1) or not (0) a user name must be present in the payload.
Length prefixed fields whose presence is determined according to the value of flags in the variable header. The fields are, in order: client identifier, will topic, will message, user name, password.