Subject: [PATCH 13/26] tcp: disable Fast Open on timeouts after handshake

From: Yuchung Cheng <[email-masked]>
Date: Wed, 18 Nov 2015 18:17:30 -0800

[ Upstream commit 0e45f4da5981895e885dd72fe912a3f8e32bae73 ]

Some middle-boxes black-hole the data after the Fast Open handshake
(https://www.ietf.org/proceedings/94/slides/slides-94-tcpm-13.pdf).
The exact reason is unknown. The work-around is to disable Fast Open
temporarily after multiple recurring timeouts with few or no data
delivered in the established state.

Signed-off-by: Yuchung Cheng <[email-masked]>
Signed-off-by: Eric Dumazet <[email-masked]>
Reported-by: Christoph Paasch <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/tcp_timer.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 8c65dc1..c8f9785 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -176,6 +176,18 @@ static int tcp_write_timeout(struct sock *sk)
 		syn_set = true;
 	} else {
 		if (retransmits_timed_out(sk, sysctl_tcp_retries1, 0, 0)) {
+			/* Some middle-boxes may black-hole Fast Open _after_
+			 * the handshake. Therefore we conservatively disable
+			 * Fast Open on this path on recurring timeouts with
+			 * few or zero bytes acked after Fast Open.
+			 */
+			if (tp->syn_data_acked &&
+			    tp->bytes_acked <= tp->rx_opt.mss_clamp) {
+				tcp_fastopen_cache_set(sk, 0, NULL, true, 0);
+				if (icsk->icsk_retransmits == sysctl_tcp_retries1)
+					NET_INC_STATS_BH(sock_net(sk),
+							 LINUX_MIB_TCPFASTOPENACTIVEFAIL);
+			}
 			/* Black hole detection */
 			tcp_mtu_probing(icsk, sk);
 
-- 
2.4.1