Keep alive: Keep alive timer is used to know if an MQTT client is on the network where the client sends regular PING request messages to the broker.The password is sent in plaintext if it is not encrypted underneath. MQTT allows sending username and password for authenticating a client and for authorization. When these fields are set, the credentials are expected in the payload. User Name and Password: Bit 7 and 6 of the connect flags respectively.If Will Retain is set to 0, the server must publish the Will message as a non-retained message and when it is set to 1, the Will message is published as a retained message. Will retain: Bit 5 of the connect flags.Denote the QoS level to be used while publishing the Will message. Will QoS: Bit 4 and 3 of the connect flags.When the Will flag is set to 1, the Will QoS and Will Retain fields in the Connect flags is used by the server. When the client disconnects, the broker sends this message on behalf of it. This is used while notifying other clients on disconnection. A Will message is an MQTT message with a Will Topic and a Will Message. This flag when set means, if the Connect request is accepted, a Will message must be stored on the server. Part of the last will and testament (LWT) feature of MQTT. Will flag: Bit 2 of the connect flags.The flag when set to true results in a clean session where the subscriptions are removed on disconnection and when set to false, a durable connection can be achieved where subscriptions remain and messages with high QoS are delivered on reconnection. This flag indicates the broker whether the client wants to establish a persistent connection or not. Clean session: Bit 1 of the connection flags.It denotes the presence or absence of fields in the payload. Connect Flags: The connect flag bytes contains parameters specifying the behavior of the MQTT connection.Header Flags: Holds information on the MQTT control packet type.The destination port is 1883, which is the default port for MQTT over TCP.
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