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, 11 insertions, 2 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 88693281da4c..c5cfd5ec3184 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2860,10 +2860,19 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_fastopen_request *fo = tp->fastopen_req;
-        int space, i, err = 0, iovlen = fo->data->msg_iovlen;
+        int syn_loss = 0, space, i, err = 0, iovlen = fo->data->msg_iovlen;
         struct sk_buff *syn_data = NULL, *data;
+        unsigned long last_syn_loss = 0;
+
+        tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
+                               &syn_loss, &last_syn_loss);
+        /* Recurring FO SYN losses: revert to regular handshake temporarily */
+        if (syn_loss > 1 &&
+            time_before(jiffies, last_syn_loss + (60*HZ << syn_loss))) {
+                fo->cookie.len = -1;
+                goto fallback;
+        }
 
-        tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie);
         if (fo->cookie.len <= 0)
                 goto fallback;