tcp: up initial rmem to 128KB and SYN rwin to around 64KB

Previously TCP initial receive buffer is ~87KB by default and
the initial receive window is ~29KB (20 MSS). This patch changes
the two numbers to 128KB and ~64KB (rounding down to the multiples
of MSS) respectively. The patch also simplifies the calculations s.t.
the two numbers are directly controlled by sysctl tcp_rmem[1]:

  1) Initial receiver buffer budget (sk_rcvbuf): while this should
     be configured via sysctl tcp_rmem[1], previously tcp_fixup_rcvbuf()
     always override and set a larger size when a new connection
     establishes.

  2) Initial receive window in SYN: previously it is set to 20
     packets if MSS <= 1460. The number 20 was based on the initial
     congestion window of 10: the receiver needs twice amount to
     avoid being limited by the receive window upon out-of-order
     delivery in the first window burst. But since this only
     applies if the receiving MSS <= 1460, connection using large MTU
     (e.g. to utilize receiver zero-copy) may be limited by the
     receive window.

With this patch TCP memory configuration is more straight-forward and
more properly sized to modern high-speed networks by default. Several
popular stacks have been announcing 64KB rwin in SYNs as well.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 21 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index fe7855b090e4..059b67af28b1 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -195,21 +195,6 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
         inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
 }
 
-
-u32 tcp_default_init_rwnd(u32 mss)
-{
-        /* Initial receive window should be twice of TCP_INIT_CWND to
-         * enable proper sending of new unsent data during fast recovery
-         * (RFC 3517, Section 4, NextSeg() rule (2)). Further place a
-         * limit when mss is larger than 1460.
-         */
-        u32 init_rwnd = TCP_INIT_CWND * 2;
-
-        if (mss > 1460)
-                init_rwnd = max((1460 * init_rwnd) / mss, 2U);
-        return init_rwnd;
-}
-
 /* Determine a window scaling and initial window to offer.
  * Based on the assumption that the given amount of space
  * will be offered. Store the results in the tp structure.
@@ -244,7 +229,10 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
         if (sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows)
                 (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
         else
-                (*rcv_wnd) = space;
+                (*rcv_wnd) = min_t(u32, space, U16_MAX);
+
+        if (init_rcv_wnd)
+                *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
 
         (*rcv_wscale) = 0;
         if (wscale_ok) {
@@ -257,11 +245,6 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
                         (*rcv_wscale)++;
                 }
         }
-
-        if (!init_rcv_wnd) /* Use default unless specified otherwise */
-                init_rcv_wnd = tcp_default_init_rwnd(mss);
-        *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
-
         /* Set the clamp no higher than max representable value */
         (*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
 }