[PATCH net-next 6/8] tcp: undo cwnd on Fast Open spurious SYNACK retransmit

[Yuchung Cheng <ycheng@google.com>]

This patch makes passive Fast Open reverts the cwnd to default
initial cwnd (10 packets) if the SYNACK timeout is spurious.

Passive Fast Open uses a full socket during handshake so it can
use the existing undo logic to detect spurious retransmission
by recording the first SYNACK timeout in key state variable
retrans_stamp. Upon receiving the ACK of the SYNACK, if the socket
has sent some data before the timeout, the spurious timeout
is detected by tcp_try_undo_recovery() in tcp_process_loss()
in tcp_ack().

But if the socket has not send any data yet, tcp_ack() does not
execute the undo code since no data is acknowledged. The fix is to
check such case explicitly after tcp_ack() during the ACK processing
in SYN_RECV state. In addition this is checked in FIN_WAIT_1 state
in case the server closes the socket before handshake completes.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/tcp_input.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 53b4c5a3113b..3a40584cb473 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6089,6 +6089,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		 * so release it.
 		 */
 		if (req) {
+			tcp_try_undo_loss(sk, false);
 			inet_csk(sk)->icsk_retransmits = 0;
 			reqsk_fastopen_remove(sk, req, false);
 			/* Re-arm the timer because data may have been sent out.
@@ -6143,6 +6144,8 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		 * our SYNACK so stop the SYNACK timer.
 		 */
 		if (req) {
+			tcp_try_undo_loss(sk, false);
+			inet_csk(sk)->icsk_retransmits = 0;
 			/* We no longer need the request sock. */
 			reqsk_fastopen_remove(sk, req, false);
 			tcp_rearm_rto(sk);
-- 
2.21.0.593.g511ec345e18-goog