Figure is an animation that shows a computer sending a file to a server using the FTP application, which utilizes TCP at the transport layer.  The data is segmented into six segments and to start the process three packets leave the computer and is sent to a router and then directed to the FTP. server on a remote server farm network.  The server receives the three packets and then it sends an acknowledgment packet back to the computer saying that it did receive three packets.  The computer now sends the next three packets to the server, but they become lost in the network for some unknown reason.  Since the server did not receive any packets it doesn’t send the acknowledgment.  After a period of time of not receiving an acknowledgment packet the sending computer determines that the final three packets were not received so it will resend those last three packets.  The server receives them this time and will send the acknowledgment packet.