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Why retransmission occured before duplicated ACK ?


Hi everyone,

I'm struggling with TCP analysis from my home lan network. I tried to understand which TCP version is implemented in my OS (Windows 8.1 in two computers) and how it's working. I generated 7 TCP streams to get retransmissions. I think that my TCP version is TCP SACK.

First of all, I got first retransmission because of the timeout elapsed (probably). Picture Next, I got duplicated ACK(1) and in this segment I got information about SLE = 1595113 and SRE = 1596573. SLE is a seq number of first TCP retransmission. Did I get information that retransmission was correct ?

This duplicated ACK(1) occured because there was one or more segments lost. After that I got another retransmission but not connected with that lost segment. Why I did not get retransmission connected with that loose ?

Finally, I got another duplicated ACK and there was not any reaction for that.

Could anybody tell me if I am thinking on the right way ?

Cheers !

asked 19 Jun '15, 04:37

kub4ss's gravatar image

accept rate: 0%

Your questions aren't clear, probably due to the lack of a capture file.

Can you share a capture in a publicly accessible spot, e.g. CloudShark, Google Drive, Dropbox?

Also your picture link doesn't seem to show anything useful and then traps my browser in some AliExpress hell.

(19 Jun '15, 05:17) grahamb ♦

Here is a capture file: link

(21 Jun '15, 14:14) kub4ss

@kub4ss: I converted your answer to a comment, as that's how this Q&A site works. Please read the FAQ!

(21 Jun '15, 14:15) Kurt Knochner ♦

One Answer:


Do I understnad you correct, that you are wondering about the DUP ACK of FRAME 10824? It is D-SACK for Frame 10456. But also it has the same SEQ und ACK number as the ACK 10638, but with the additional D-SACK options, so Wireshark interpreted it as a DUP ACK. (Maybe this could irritated you.)

There has been a quite similar thread here: TCP SACK in capture shows range of previous segments instead of subsequent

answered 22 Jun '15, 12:36

Christian_R's gravatar image

accept rate: 16%