Traffic has since seemed to drop off on the interface, which is likely unrelated. While communication is now fine, it was previously giving some delays, inconsistent pings, sometimes large pings. While using wireshark on the TUN device to try diagnosing some performance issues between sites, I noticed a semi-common occurrence of TCP retransmission ack and dup packets on the tun interface. 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.Hello, I've been working a bit with openvpn here, and was wondering a few things. Both situations are, unfortunately, entirely possible on the global Internet. If you're seeing a lot more duplicate ACK's followed by actual re-transmission then some amount of packet loss is taking place. So - if there are few random duplicate ACK's but no (or few) actual re-transmissions then it's likely packets arriving out of order. If this is the case, however, you're likely to see other problems on the link (.including other packets showing as dupes in Wireshark). A less common cause would be certain media problems where certain packets might end up being seen more than once. Upon seeing #6 arrive, though, it would stop sending the duplicate acknowledgements.

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 re-transmit. 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. 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.

The repeated acknowledgements at the last known value before the gap signal which packets the sender should re-transmit. There can be several things going on - the most common would be the use of TCP Fast Re-transmission 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. What would cause the duplicate ACKs seen in the FCIP statistics?īytes Sent: 2427483751544 Packets Sent: 1625906137 Round Trip Time 15 ms, HWM 15 ms, Variance 0, HWM 0 Send Window: 7448576 bytes, scale: 9 Slow Starts: 1 Slow Start Threshold: 16777216ĭuplicate ACKs 35838 <- Retransmits: 567, max: 1

Tcp dup ack wireshark