How can I estimate the congestion window with the information shown in wireshark?

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The task is quite complex and usually you can't do this.

As current congestion window value is never transferred in packets, all you can do is to spot Bytes in Flight value (apply a filter to see only packets from sender you're interested in).

But in order to make any sense from Bytes in Flight value certain conditions are to be met:

• Capture must be taken on the sender side

• cwnd must be the limiting factor (not sending application)

• The path has to have certain parameters (be able to keep cwnd size on the flight; you wouldn't be able to correlate BIF and cwnd in local 1Gb segment)
• You have to keep in mind modern congestion control algorithms tend to move away from "cwnd-bound" way to "pacing-way" which means cwnd is not used so much to govern data transmission rate (BBR as an example).

So taking into consideration all these limitations, more optimal way is to spot cwnd by issuing commands on a sender OS.