TCP congestion window size ..?

Question

I am interested in getting congestion window size of a connection. The connection is created by another program. I am hoping that we can get this congestion window size using some file in proc, or, there is a call to get this info from kernel...

So I need more leads on any of those approaches ...

Answer 1

If you are on Linux, you can use getsockopt() on the socket using the IPPROTO_TCP socket level and TCP_INFO socket option.

The struct tcp_info structure has the member tcpi_snd_cwnd. A fairly extensive write up can be found here.

FreeBSD also has a similar feature.

Windows provides congestion window information with the GetPerTcpConnectionEStats() call using TcpConnectionEstatsSndCong as the stats type.

Answer 2

My solution was to use tcpdump to capture packet retrans , sack, ecn-cwr. Those indicate that window size will be collapsed, each of those has different collapse magnitudes. But its now just a matter of calculating those magnitudes and pluging them in the initcwnd size.

I think this is easier than what jxh suggested.