TCP DUP ACK – What Does TCP DUP ACK Mean?

There can be several things going on - the most common would be the use of TCP Fast Retransmission which is a mechanism by which a receiver can indicate that it has seen a gap in the received sequence numbers that implies the loss of one or more packets in transit. The repeated acknowledgements at the last known value before the gap signal which packets the sender should retransmit. This can occur without waiting for the acknowledgement timeout for the lost packet to hit on the transmitter - which, as the name implies, means recovering a lot faster.

It's also possible that the same symptom of gaps in sequence numbers might be seen in a situation where packets are being delivered out of order. As above, if the receiver sees (for example) a segment with sequence #5 followed by another with #7 before seeing sequence #6 then it might try to begin to trigger a fast retransmit. Upon seeing #6 arrive, though, it would stop sending the duplicate acknowledgements.

A less common cause would be certain media problems where certain packets might end up being seen more than once. If this is the case, however, you're likely to see other problems on the link (...including other packets showing as dupes in Wireshark).

So - if you're seeing a few random duplicate ACK's but no (or few) actual retransmissions then it's likely packets arriving out of order. If you're seeing a lot more duplicate ACK's followed by actual retransmission then some amount of packet loss is taking place. Both situations are, unfortunately, entirely possible on the global Internet. If you're seeing other kinds of duplicate packets as CRC issues and generally slow performance then it might make sense to look at link issues on your own network.