Get cwnd of my TCP connection from a program

Question

I am creating a TCP connection from my linux program with boost.asio. I wonder how do I get the value of its congestion window (cwnd) from the program? The only way I know of is to parse /proc/net/tcp, but this does not feel right. I'd rather use a dedicated syscall to get this info.

A solution to a similar question (How to monitor cwnd and ssthresh values for a TCP connection?) suggests using TCP Probe, but it feels even less appealing.

So what is the best way to get the value of cwnd?

Answer 1

It turned out getsockopt() is able to return the same tcp_info when called with TCP_INFO option:

  tcp_info tcpi = {};
  socklen_t len = sizeof(tcp_info);
  getsockopt(tcp_socket, SOL_TCP, TCP_INFO, &tcpi, &len);
  tcpi.tcpi_snd_cwnd;  // <-- CWND

Answer 2

I did this with netlink and INET_DIAG-sockets based on this helpful example: https://github.com/kristrev/inet-diag-example