[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, 1 deletions

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 9bf73fe50948..de4e83b6da4b 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -147,7 +147,8 @@ extern struct sock *inet_csk_clone(struct sock *sk,
 enum inet_csk_ack_state_t {
         ICSK_ACK_SCHED  = 1,
         ICSK_ACK_TIMER  = 2,
-        ICSK_ACK_PUSHED = 4
+        ICSK_ACK_PUSHED = 4,
+        ICSK_ACK_PUSHED2 = 8
 };
 
 extern void inet_csk_init_xmit_timers(struct sock *sk,