? After receiving an out of order segment, the receiver acknowledges the last packet. If the missing segment is not received, the receiver retransmits the last acknowledgement and this is first duplicate ACK. If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost.
 ?  TCP has a timer for each segment when transmitted known as retransmission timeout period (RTO). Sender is expected to receive ACK for the packet within RTO. However, if the desired ACK is not received, sender waits. If TCP timer expires , it is assumed that the segmment is lost and the Timeout occurs.