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, 9 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 38b6a811edfc..c49a4fc175bd 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5652,6 +5652,7 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *data = tcp_write_queue_head(sk);
         u16 mss = tp->rx_opt.mss_clamp;
+        bool syn_drop;
 
         if (mss == tp->rx_opt.user_mss) {
                 struct tcp_options_received opt;
@@ -5664,7 +5665,14 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                 mss = opt.mss_clamp;
         }
 
-        tcp_fastopen_cache_set(sk, mss, cookie);
+        /* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
+         * the remote receives only the retransmitted (regular) SYNs: either
+         * the original SYN-data or the corresponding SYN-ACK is lost.
+         */
+        syn_drop = (cookie->len <= 0 && data &&
+                    inet_csk(sk)->icsk_retransmits);
+
+        tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);
 
         if (data) { /* Retransmit unacked data in SYN */
                 tcp_retransmit_skb(sk, data);