How to check current TCP Congestion Window on Linux?

UpTheCreek asked:

Does anyone know how to check the current value of initcwnd? (particularly on Debian [Squeeze])

I’ve searched, but can’t find a definitive answer to this. I haven’t changed it, but may want to depending on it’s current setting.

My answer:

Get the source code of your running kernel and inspect include/net/tcp.h. There you will find:

/* TCP initial congestion window */
#define TCP_INIT_CWND 10

This being the default value, and rarely changed even in userland, it isn’t otherwise exposed.

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