The client who will receive messages subscribes to the topic, and the other client which issues messages publishes the topic to the server. The server reconstructs a message format to verify in the client by using the order flag set in the server and the SEQ of the received message. The client checks SEQ and order flag of the arrival message, and compares with a previous message SEQ. The client request a missed message if there is a missed message after comparing SEQ and the server resend the missed message to the client. Figure 6 shows the data flow in reliable message transmission system based on MQTT protocol.