[TCP]: Send ACKs each 2nd received segment.

It does not affect either mss-sized connections (obviously) or
connections controlled by Nagle (because there is only one small
segment in flight).

The idea is to record the fact that a small segment arrives on a
connection, where one small segment has already been received and
still not-ACKed. In this case ACK is forced after tcp_recvmsg() drains
receive buffer.

In other words, it is a "soft" each-2nd-segment ACK, which is enough
to preserve ACK clock even when ABC is enabled.

Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 511b738f118a..b3def0df14fb 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -156,6 +156,8 @@ static void tcp_measure_rcv_mss(struct sock *sk,
                                 return;
                         }
                 }
+                if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
+                        icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
                 icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
         }
 }