netdev - [PATCH] tcp: make TCP quick ACK behavior modifiable

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Date:	Mon, 23 Aug 2010 21:00:37 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	netdev@...r.kernel.org
Cc:	Hagen Paul Pfeifer <hagen@...u.net>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: [PATCH] tcp: make TCP quick ACK behavior modifiable

The TCP quick ACK mechanism analyze if a connection is interactive or
not. Per default the quick ACK mechanism is enabled and ACK packets are
triggered instantly to raise the CWND fast - which is clever for
bulk data (non-interactive) flows. On the other hand interactive protocols
like HTTP, SMTP or XMPP will suffer from the quick ACK mechanism
because one additional packets is generated. A simple heuristic
detects if a connection is interactive (pingpong) and if so,
disable the quick ACK. But, the mechanism is not in the ability to
blindly guess if a connection is interactive, and so it must wait for at
least one return packet with payload.

For the server side this requires one additional packet because (packet
number 5 and 6 can be combined):

192.168.1.35.44833 > 78.47.222.210.80: Flags [S], seq 2854340018, win 5840, options [mss 1460,sackOK,TS val 4382726 ecr 0,nop,wscale 7], length 0
78.47.222.210.80 > 192.168.1.35.44833: Flags [S.], seq 719041385, ack 2854340019, win 5792, options [mss 1452,sackOK,TS val 2606891996 ecr 4382726,nop,wscale 7], length 0
192.168.1.35.44833 > 78.47.222.210.80: Flags [.], ack 1, win 46, options [nop,nop,TS val 4382730 ecr 2606891996], length 0
192.168.1.35.44833 > 78.47.222.210.80: Flags [P.], seq 1:682, ack 1, win 46, options [nop,nop,TS val 4382730 ecr 2606891996], length 681
78.47.222.210.80 > 192.168.1.35.44833: Flags [.], ack 682, win 56, options [nop,nop,TS val 2606892002 ecr 4382730], length 0
78.47.222.210.80 > 192.168.1.35.44833: Flags [.], seq 1:1441, ack 682, win 56, options [nop,nop,TS val 2606892002 ecr 4382730], length 1440
192.168.1.35.44833 > 78.47.222.210.80: Flags [.], ack 1441, win 69, options [nop,nop,TS val 4382737 ecr 2606892002], length 0

This patch provides a sysctl interface for the administrator to globally
enable or disable TCP quick ACKs. Short lived protocols like HTTP will
save a non unimportant portion of packets!

Disable TCP Quick ACK:
$ echo 0 > /proc/sys/net/ipv4/tcp_quickack

Enable TCP Quick ACK:
$ echo 1 > /proc/sys/net/ipv4/tcp_quickack

Signed-off-by: Hagen Paul Pfeifer <hagen@...u.net>
Cc: David S. Miller <davem@...emloft.net>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
---
 include/net/inet_connection_sock.h |    3 +++
 net/ipv4/sysctl_net_ipv4.c         |    7 +++++++
 net/ipv4/tcp.c                     |    2 ++
 3 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index b6d3b55..da2fbaf 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -170,9 +170,12 @@ static inline int inet_csk_ack_scheduled(const struct sock *sk)
 	return inet_csk(sk)->icsk_ack.pending & ICSK_ACK_SCHED;
 }
 
+extern int sysctl_tcp_quickack;
+
 static inline void inet_csk_delack_init(struct sock *sk)
 {
 	memset(&inet_csk(sk)->icsk_ack, 0, sizeof(inet_csk(sk)->icsk_ack));
+	inet_csk(sk)->icsk_ack.pingpong = sysctl_tcp_quickack ? 0 : 1;
 }
 
 extern void inet_csk_delete_keepalive_timer(struct sock *sk);
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index d96c1da..8923ca8 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -394,6 +394,13 @@ static struct ctl_table ipv4_table[] = {
 		.proc_handler	= proc_dointvec
 	},
 	{
+		.procname	= "tcp_quickack",
+		.data		= &sysctl_tcp_quickack,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec
+	},
+	{
 		.procname	= "tcp_mem",
 		.data		= &sysctl_tcp_mem,
 		.maxlen		= sizeof(sysctl_tcp_mem),
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 176e11a..5161689 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -279,6 +279,8 @@
 
 int sysctl_tcp_fin_timeout __read_mostly = TCP_FIN_TIMEOUT;
 
+int sysctl_tcp_quickack __read_mostly = 1;
+
 struct percpu_counter tcp_orphan_count;
 EXPORT_SYMBOL_GPL(tcp_orphan_count);
 
-- 
1.7.2.1.95.g3d045.dirty

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