[RFC PATCH v2 net-next] tcp: remove 64 KByte limit for initial tp->rcv_wnd value

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

In 2018 commit a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin
to around 64KB") limited the initial value of tp->rcv_wnd to 65535, most
CDN team would not benefit from this change because they cannot have a
large window to receive a big packet, which will be slowed down especially
in long RTT.

According to RFC 7323, it says:
  "The maximum receive window, and therefore the scale factor, is
   determined by the maximum receive buffer space."

So we can get rid of this 64k limitation and let the window be tunable if
the user wants to do it within the control of buffer space. Then many
companies, I believe, can have the same behaviour as old days. Besides,
there are many papers conducting various interesting experiments which
have something to do with this window and show good outputs in some cases,
say, paper [1] in Yahoo! CDN.

To avoid future confusion, current change doesn't affect the initial
receive window on the wire in a SYN or SYN+ACK packet which are set within
65535 bytes according to RFC 7323 also due to the limit in
__tcp_transmit_skb():

    th->window      = htons(min(tp->rcv_wnd, 65535U));

In one word, __tcp_transmit_skb() already ensures that constraint is
respected, no matter how large tp->rcv_wnd is.

Let me provide one example if with or without the patch:
Before:
client   --- SYN: rwindow=65535 ---> server
client   <--- SYN+ACK: rwindow=65535 ----  server
client   --- ACK: rwindow=65536 ---> server
Note: for the last ACK, the calculation is 512 << 7.

After:
client   --- SYN: rwindow=65535 ---> server
client   <--- SYN+ACK: rwindow=65535 ----  server
client   --- ACK: rwindow=175232 ---> server
Note: I use the following command to make it work:
ip route change default via [ip] dev eth0 metric 100 initrwnd 120
For the last ACK, the calculation is 1369 << 7.

We can pay attention to the last ACK in 3-way shakehand and notice that
with the patch applied the window can reach more than 64 KByte.

[1]: https://conferences.sigcomm.org/imc/2011/docs/p569.pdf

Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
v2
Link: https://lore.kernel.org/all/20240517085031.18896-1-kerneljasonxing@gmail.com/
1. revise the title and body messages (Neal)
---
 net/ipv4/tcp_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 95caf8aaa8be..95618d0e78e4 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -232,7 +232,7 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
 	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
 		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
 	else
-		(*rcv_wnd) = min_t(u32, space, U16_MAX);
+		(*rcv_wnd) = space;
 
 	if (init_rcv_wnd)
 		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
-- 
2.37.3