TCP DupACKS


           TCP DupACKS are found very frequently in Wireshark packet traces and you would be wondering why this is happening. Let us break down what this means and when this usually occurs.

When TCP DupACKS are sent?          

     
     Usually, DupACKs are sent by the receiver towards the sender side. TCP DupACKs are sent when there is a gap in the segments sequence numbers received at the receiver side. This happens during packet traverse some of the packets might have lost due to some reasons. This will trigger the receiver to send  DupACK.

This may also happen when the segments are received out of order due to some traffic congestion or slowness inside the network. Let's say packet one and two received but the next packet which was received four. So the receiver sends a DupACK.Now we will come to the point what is FAST RETRANSMISSION








TCP Fastretransmission

    Fast Retransmission is the enhancement in TCP protocol. If a receiver gets three DupACKs because the above things mentioned. After receiving three  DupACKs the sender will not wait to expire the retransmission timed out value since the sender is enough confident that packets are lost in transit.