TCP Dup ACK and TCP Previous segment

asked 2019-02-06 17:42:43 +0000

updated 2019-02-06 17:55:53 +0000

flag of United Kingdom of Great Britain and Northern Ireland

23790 ●4 ●950 ●227 https://www.wireshark.org

Hello, I have a problem with the database on the server (19.168.1.100). There are cases where the same records are duplicated to the database.

No.  Time      Source          Destination    Protocol Length Info
19   0.160086  195.69.209.165  192.168.1.100  TCP      60     [TCP Dup ACK 18#1] 443 → 56386 [ACK] Seq=156 Ack=551 Win=4930 Len=0

and 
No.  Time       Source          Destination    Protocol Length Info
812  12.160016  195.69.209.165  192.168.1.100  TLSv1    91     [TCP Previous segment not captured] , Application Data

What could be the reason for these errors? Switcher or network card in the server?

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answered 2019-02-07 12:25:36 +0000

Jasper
24103 ●5 ●111 ●292 Düsseldorf, Germany

updated 2019-02-07 12:26:15 +0000

It seems highly unlikely that the TCP symptoms you pasted have anything to do with duplicate records in the database. A duplicate ACK is just a message telling the other side that a packet hasn't been received when it should have, and "Previous segment not captured" means that a packet is missing. TCP connections transport data reliably (or fail completely, after retrying without success), but it doesn't duplicate anything when it comes to the actual application payload.

Having programmed database applications myself I am reasonably sure that the problem is somewhere in the database/application logic, not the network That would mean that switches and network cards are not to blame for this.

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