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authorYuchung Cheng <ycheng@google.com>2012-05-02 13:30:03 +0000
committerDavid S. Miller <davem@davemloft.net>2012-05-02 20:56:10 -0400
commiteed530b6c67624db3f2cf477bac7c4d005d8f7ba (patch)
treec07096807ead2adb9d85e85d1a9cd1ada85755ac /net
parent1fbc340514fc3003514bd681b372e1f47ae6183f (diff)
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: Yuchung Cheng <ycheng@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/sysctl_net_ipv4.c10
-rw-r--r--net/ipv4/tcp.c3
-rw-r--r--net/ipv4/tcp_input.c15
-rw-r--r--net/ipv4/tcp_minisocks.c1
4 files changed, 29 insertions, 0 deletions
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 33417f84e07..ef32956ed65 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -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 };
@@ -677,6 +678,15 @@ static struct ctl_table ipv4_table[] = {
.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,
.maxlen = sizeof(sysctl_udp_mem),
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 9670af34193..6802c89bc44 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -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:
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index be8e09d2c6b..e042cabb695 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -99,6 +99,7 @@ int sysctl_tcp_thin_dupack __read_mostly;
int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
int sysctl_tcp_abc __read_mostly;
+int sysctl_tcp_early_retrans __read_mostly = 2;
#define FLAG_DATA 0x01 /* Incoming frame contained data. */
#define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a window update. */
@@ -906,6 +907,7 @@ static void tcp_init_metrics(struct sock *sk)
if (dst_metric(dst, RTAX_REORDERING) &&
tp->reordering != dst_metric(dst, RTAX_REORDERING)) {
tcp_disable_fack(tp);
+ tcp_disable_early_retrans(tp);
tp->reordering = dst_metric(dst, RTAX_REORDERING);
}
@@ -988,6 +990,9 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
#endif
tcp_disable_fack(tp);
}
+
+ if (metric > 0)
+ tcp_disable_early_retrans(tp);
}
/* This must be called before lost_out is incremented */
@@ -2492,6 +2497,16 @@ static int tcp_time_to_recover(struct sock *sk)
tcp_is_sack(tp) && !tcp_send_head(sk))
return 1;
+ /* Trick#6: TCP early retransmit, per RFC5827. To avoid spurious
+ * retransmissions due to small network reorderings, we implement
+ * Mitigation A.3 in the RFC and delay the retransmission for a short
+ * interval if appropriate.
+ */
+ if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out &&
+ (tp->packets_out == (tp->sacked_out + 1) && tp->packets_out < 4) &&
+ !tcp_may_send_now(sk))
+ return 1;
+
return 0;
}
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 3cabafb5cdd..6f6a9183282 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -482,6 +482,7 @@ struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req,
newtp->sacked_out = 0;
newtp->fackets_out = 0;
newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
+ tcp_enable_early_retrans(newtp);
/* So many TCP implementations out there (incorrectly) count the
* initial SYN frame in their delayed-ACK and congestion control