Commit 794200d6 authored Apr 29, 2019 by Yuchung Cheng Committed by David S. Miller May 01, 2019

tcp: undo cwnd on Fast Open spurious SYNACK retransmit



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>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 8c3cfe19

net/ipv4/tcp_input.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -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);

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