tcp: Reduce SYN resend delay if a suspicous ACK is received

When closing a connection, the two acks that required to change closing
socket's status to FIN_WAIT_2 and then TIME_WAIT could be processed in
reverse order.  This is possible in RSS disabled environments such as a
connection inside a host.

For example, expected state transitions and required packets for the
disconnection will be similar to below flow.

	 00 (Process A)				(Process B)
	 01 ESTABLISHED				ESTABLISHED
	 02 close()
	 03 FIN_WAIT_1
	 04 		---FIN-->
	 05 					CLOSE_WAIT
	 06 		<--ACK---
	 07 FIN_WAIT_2
	 08 		<--FIN/ACK---
	 09 TIME_WAIT
	 10 		---ACK-->
	 11 					LAST_ACK
	 12 CLOSED				CLOSED

In some cases such as LINGER option applied socket, the FIN and FIN/ACK
will be substituted to RST and RST/ACK, but there is no difference in
the main logic.

The acks in lines 6 and 8 are the acks.  If the line 8 packet is
processed before the line 6 packet, it will be just ignored as it is not
a expected packet, and the later process of the line 6 packet will
change the status of Process A to FIN_WAIT_2, but as it has already
handled line 8 packet, it will not go to TIME_WAIT and thus will not
send the line 10 packet to Process B.  Thus, Process B will left in
CLOSE_WAIT status, as below.

	 00 (Process A)				(Process B)
	 01 ESTABLISHED				ESTABLISHED
	 02 close()
	 03 FIN_WAIT_1
	 04 		---FIN-->
	 05 					CLOSE_WAIT
	 06 				(<--ACK---)
	 07	  			(<--FIN/ACK---)
	 08 				(fired in right order)
	 09 		<--FIN/ACK---
	 10 		<--ACK---
	 11 		(processed in reverse order)
	 12 FIN_WAIT_2

Later, if the Process B sends SYN to Process A for reconnection using
the same port, Process A will responds with an ACK for the last flow,
which has no increased sequence number.  Thus, Process A will send RST,
wait for TIMEOUT_INIT (one second in default), and then try
reconnection.  If reconnections are frequent, the one second latency
spikes can be a big problem.  Below is a tcpdump results of the problem:

    14.436259 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [S], seq 2560603644
    14.436266 IP 127.0.0.1.4242 > 127.0.0.1.45150: Flags [.], ack 5, win 512
    14.436271 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [R], seq 2541101298
    /* ONE SECOND DELAY */
    15.464613 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [S], seq 2560603644

This commit mitigates the problem by reducing the delay for the next SYN
if the suspicous ACK is received while in SYN_SENT state.

Following commit will add a selftest, which can be also helpful for
understanding of this issue.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_input.c

@@ -5908,8 +5908,14 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 		 *        the segment and return)"
 		 */
 		if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
-		    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
+		    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
+			/* Previous FIN/ACK or RST/ACK might be ignored. */
+			if (icsk->icsk_retransmits == 0)
+				inet_csk_reset_xmit_timer(sk,
+						ICSK_TIME_RETRANS,
+						TCP_TIMEOUT_MIN, TCP_RTO_MAX);
 			goto reset_and_undo;
+		}
 
 		if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
 		    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,