Duplicate ACK

TCP supporta fast retransmit algorithm to detect and repair loss pakcets. The Receiver send a duplicate ACK (which is idintical to the ACK segment before) to inform the sender that a segment was recieved out-of-order. After the arrival of 3 duplicate ACKs (4 identical ACKs without the arrival of any other intervening packet), TCP performs a retransmission of what appears to be the missing segment, without waiting for the retransmission timer to expire.

We take a simple example of congested network

As you can see at the chart segment number 13 was retransmitted roughly around 0.640 second (The chart displays the segments that were sent from the client)

The ACK chart is a useful way to see what happen with the control packets. Zoom in to ACK segment 12. You can see that the 4th ACK (3 duplicate ACK) was received exactly at that time causing the sender to retransmitted segment 13