I can manipulate such value with:
ip route change ... initcwnd 10
and then have a feedback with:
ip route show
But what about the default value before any modification? Is there a way to query that value from the system?
Alternatively, can you provide a valid reference that shows the default hardcoded value for each kernel version?
I don't really know for sure, but this seems like a legit reference
Hunch:
$ grep -A 2 initcwnd `find /usr/src/linux/include -type f -iname '*h'`
out:
/usr/src/linux/include/net/tcp.h:
/* TCP initial congestion window as per draft-hkchu-tcpm-initcwnd-01 */
#define TCP_INIT_CWND 10
Well, I can't say I'm 100 % sure this should be the answer, buuut, as it often comes, ss is the good choice to get some info revealed, for e. g.:
ss -nli|fgrep cwnd
westwood rto:1000 mss:536 cwnd:10
westwood rto:1000 mss:536 cwnd:10
westwood rto:1000 mss:536 cwnd:10
-n is typical to get rid of annoying DNS resolving, -l is we stick to listening sockets only and -i (the key) is "Show internal TCP information". As it can be seen, both congestion algorithm and default cwnd are shown.
If I understood you correctly, you're looking for the initial value of the snd_cwnd parameter set when a TCP socket is initialized.
It looks like starting with linux kernel 2.6.39, a macro TCP_INIT_CWND has been introduced in linux/include/net/tcp.h which populates the value of snd_cwnd when initializing a TCP socket.
I know where this code is in the kernel for IPv4, and unfortunately it does not seem to use any macro to populate the value for kernels older than 2.6.39
/* net/ipv4/tcp_ipv4.c from 2.6.37 kernel */
static int tcp_v4_init_sock(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
....
....
....
/* So many TCP implementations out there (incorrectly) count the
* initial SYN frame in their delayed-ACK and congestion control
* algorithms that we must have the following bandaid to talk
* efficiently to them. -DaveM
*/
tp->snd_cwnd = 2;
....
....
....
}
A similar init code exists for IPv6 as well inside tcp_v6_init_sock() function in net/ipv6/tcp_ipv6.c
