TCP Dup Ack Not Captured Packets

A Duplicate ACK (Dup ACK) is an acknowledgment (ACK) packet that a TCP receiver sends again with the same acknowledgment number as before, because it received data out of order.

TCP is a byte-stream protocol that expects in-order delivery.

If a packet is lost, the receiver will still get later packets but can’t deliver them to the application until the missing one arrives.

To tell the sender something is wrong, the receiver keeps ACKing the same last in-order byte it received.

This repeated ACK is what we call a Duplicate ACK.

Correlate with Retransmission

If dup ACKs appear → sender retransmits after 3 dup ACKs (fast retransmission).

If SACK blocks appear → sender retransmits exactly the missing segment(s), often faster and with fewer dup ACKs.