Commit eed530b6 authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

tcp: early retransmit 

This patch implements RFC 5827 early retransmit (ER) for TCP.
It reduces DUPACK threshold (dupthresh) if outstanding packets are
less than 4 to recover losses by fast recovery instead of timeout.

While the algorithm is simple, small but frequent network reordering
makes this feature dangerous: the connection repeatedly enter
false recovery and degrade performance. Therefore we implement
a mitigation suggested in the appendix of the RFC that delays
entering fast recovery by a small interval, i.e., RTT/4. Currently
ER is conservative and is disabled for the rest of the connection
after the first reordering event. A large scale web server
experiment on the performance impact of ER is summarized in
section 6 of the paper "Proportional Rate Reduction for TCP”,
IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf



Note that Linux has a similar feature called THIN_DUPACK. The
differences are THIN_DUPACK do not mitigate reorderings and is only
used after slow start. Currently ER is disabled if THIN_DUPACK is
enabled. I would be happy to merge THIN_DUPACK feature with ER if
people think it's a good idea.

ER is enabled by sysctl_tcp_early_retrans:
  0: Disables ER

  1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4.

  2: (Default) reduce dupthresh like mode 1. In addition, delay
     entering fast recovery by RTT/4.

Note: mode 2 is implemented in the third part of this patch series.

Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 1fbc3405

Documentation/networking/ip-sysctl.txt

+14 −0
Original line number Diff line number Diff line
@@ -190,6 +190,20 @@ tcp_cookie_size - INTEGER 
tcp_dsack - BOOLEAN

	Allows TCP to send "duplicate" SACKs.



tcp_early_retrans - INTEGER

	Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold

	for triggering fast retransmit when the amount of outstanding data is

	small and when no previously unsent data can be transmitted (such

	that limited transmit could be used).

	Possible values:

		0 disables ER

		1 enables ER

		2 enables ER but delays fast recovery and fast retransmit

		  by a fourth of RTT. This mitigates connection falsely

		  recovers when network has a small degree of reordering

		  (less than 3 packets).

	Default: 2



tcp_ecn - INTEGER

	Enable Explicit Congestion Notification (ECN) in TCP. ECN is only

	used when both ends of the TCP flow support it. It is useful to

include/linux/tcp.h

+1 −0
Original line number Diff line number Diff line
@@ -372,6 +372,7 @@ struct tcp_sock { 
		repair      : 1,

		unused      : 1;

	u8	repair_queue;

	u8	do_early_retrans:1;/* Enable RFC5827 early-retransmit  */



/* RTT measurement */

	u32	srtt;		/* smoothed round trip time << 3	*/

include/net/tcp.h

+15 −0
Original line number Diff line number Diff line
@@ -252,6 +252,7 @@ extern int sysctl_tcp_max_ssthresh; 
extern int sysctl_tcp_cookie_size;

extern int sysctl_tcp_thin_linear_timeouts;

extern int sysctl_tcp_thin_dupack;

extern int sysctl_tcp_early_retrans;



extern atomic_long_t tcp_memory_allocated;

extern struct percpu_counter tcp_sockets_allocated;
@@ -797,6 +798,20 @@ static inline void tcp_enable_fack(struct tcp_sock *tp) 
	tp->rx_opt.sack_ok |= TCP_FACK_ENABLED;

}



/* TCP early-retransmit (ER) is similar to but more conservative than

 * the thin-dupack feature.  Enable ER only if thin-dupack is disabled.

 */

static inline void tcp_enable_early_retrans(struct tcp_sock *tp)

{

	tp->do_early_retrans = sysctl_tcp_early_retrans &&

		!sysctl_tcp_thin_dupack && sysctl_tcp_reordering == 3;

}



static inline void tcp_disable_early_retrans(struct tcp_sock *tp)

{

	tp->do_early_retrans = 0;

}



static inline unsigned int tcp_left_out(const struct tcp_sock *tp)

{

	return tp->sacked_out + tp->lost_out;

net/ipv4/sysctl_net_ipv4.c

+10 −0
Original line number Diff line number Diff line
@@ -27,6 +27,7 @@ 
#include <net/tcp_memcontrol.h>



static int zero;

static int two = 2;

static int tcp_retr1_max = 255;

static int ip_local_port_range_min[] = { 1, 1 };

static int ip_local_port_range_max[] = { 65535, 65535 };
@@ -676,6 +677,15 @@ static struct ctl_table ipv4_table[] = { 
		.mode           = 0644,

		.proc_handler   = proc_dointvec

	},

	{

		.procname	= "tcp_early_retrans",

		.data		= &sysctl_tcp_early_retrans,

		.maxlen		= sizeof(int),

		.mode		= 0644,

		.proc_handler	= proc_dointvec_minmax,

		.extra1		= &zero,

		.extra2		= &two,

	},

	{

		.procname	= "udp_mem",

		.data		= &sysctl_udp_mem,

net/ipv4/tcp.c

+3 −0
Original line number Diff line number Diff line
@@ -395,6 +395,7 @@ void tcp_init_sock(struct sock *sk) 
	tp->mss_cache = TCP_MSS_DEFAULT;



	tp->reordering = sysctl_tcp_reordering;

	tcp_enable_early_retrans(tp);

	icsk->icsk_ca_ops = &tcp_init_congestion_ops;



	sk->sk_state = TCP_CLOSE;
@@ -2495,6 +2496,8 @@ static int do_tcp_setsockopt(struct sock *sk, int level, 
			err = -EINVAL;

		else

			tp->thin_dupack = val;

			if (tp->thin_dupack)

				tcp_disable_early_retrans(tp);

		break;



	case TCP_REPAIR:

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