When the QoS level is set to one, the protocol ensures that
a message arrives at the server "at least once". A published
message is stored in the publisher internal buffer until it
receives the ACK packet. Once the acknowledgement is
received, the message is discarded from the buffer, and the
delivery is complete. If a TCP/IP session is broken, only a few
messages can be stored in the buffer until the time when the
session is again restored and acknowledgement messages again
are delivered.