net-tcp: Fast Open client - detecting SYN-data drops

On paths with firewalls dropping SYN with data or experimental TCP options,
Fast Open connections will have experience SYN timeout and bad performance.
The solution is to track such incidents in the cookie cache and disables
Fast Open temporarily.

Since only the original SYN includes data and/or Fast Open option, the
SYN-ACK has some tell-tale sign (tcp_rcv_fastopen_synack()) to detect
such drops. If a path has recurring Fast Open SYN drops, Fast Open is
disabled for 2^(recurring_losses) minutes starting from four minutes up to
roughly one and half day. sendmsg with MSG_FASTOPEN flag will succeed but
it behaves as connect() then write().

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 3 deletions

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index d02ff3777785..99779ae44f64 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -32,6 +32,8 @@ enum tcp_metric_index {
 
 struct tcp_fastopen_metrics {
 	u16	mss;
+	u16	syn_loss:10;		/* Recurring Fast Open SYN losses */
+	unsigned long	last_syn_loss;	/* Last Fast Open SYN loss */
 	struct	tcp_fastopen_cookie	cookie;
 };
 
@@ -125,6 +127,7 @@ static void tcpm_suck_dst(struct tcp_metrics_block *tm, struct dst_entry *dst)
 	tm->tcpm_ts = 0;
 	tm->tcpm_ts_stamp = 0;
 	tm->tcpm_fastopen.mss = 0;
+	tm->tcpm_fastopen.syn_loss = 0;
 	tm->tcpm_fastopen.cookie.len = 0;
 }
 
@@ -644,7 +647,8 @@ bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw)
 static DEFINE_SEQLOCK(fastopen_seqlock);
 
 void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
-			    struct tcp_fastopen_cookie *cookie)
+			    struct tcp_fastopen_cookie *cookie,
+			    int *syn_loss, unsigned long *last_syn_loss)
 {
 	struct tcp_metrics_block *tm;
 
@@ -659,14 +663,15 @@ void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
 			if (tfom->mss)
 				*mss = tfom->mss;
 			*cookie = tfom->cookie;
+			*syn_loss = tfom->syn_loss;
+			*last_syn_loss = *syn_loss ? tfom->last_syn_loss : 0;
 		} while (read_seqretry(&fastopen_seqlock, seq));
 	}
 	rcu_read_unlock();
 }
 
-
 void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
-			    struct tcp_fastopen_cookie *cookie)
+			    struct tcp_fastopen_cookie *cookie, bool syn_lost)
 {
 	struct tcp_metrics_block *tm;
 
@@ -679,6 +684,11 @@ void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
 		tfom->mss = mss;
 		if (cookie->len > 0)
 			tfom->cookie = *cookie;
+		if (syn_lost) {
+			++tfom->syn_loss;
+			tfom->last_syn_loss = jiffies;
+		} else
+			tfom->syn_loss = 0;
 		write_sequnlock_bh(&fastopen_seqlock);
 	}
 	rcu_read_unlock();