Recently, we had some servers upgraded to the latest kernel and noticed
the indicator from the user side showed worse results than before. It is
caused by the limitation of tp->rcv_wnd.

In 2018 commit a337531b ("tcp: up initial rmem to 128KB and SYN rwin
to around 64KB") limited the initial value of tp->rcv_wnd to 65535, most
CDN teams 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. Small rcv_wnd means slow transfer speed, to some extent. It's
the side effect for the latency/time-sensitive users.

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. The change doesn't violate
RFC.

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.

When we apply such a patch, having a large rcv_wnd if the user tweak this
knob can help transfer data more rapidly and save some rtts.

Fixes: a337531b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20240521134220.12510-1-kerneljasonxing@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>