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Sniffing http traffic to my (linux) server it seems like it always responds advertising a window size of 14. It isn't affected if I change the default initcwnd size (with ip route), and it doesn't increase if I download a big file, even if the speed grows. That doesn't seem to make a lot of sense. How would you explain what I see?

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can you provide a sample? If the value is 14, is this possibly "TCP window scale option"? –  benc Dec 8 '10 at 9:11
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I asume that you have a recently modern Linux kernel (> 2.6.8 IIRC) wich has TCP window autoscaling enabled? (sysctl net.ipv4.tcp_window_scaling should be 1)

You can specifically add tcp.window_size as an additional column (Edit->Preferences->User Interface->Columns-> Add custom (tcp.window_size) to wireshark while doing your measurements and post that here.

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