Re: Re: [PATCH v2.1 1/2] tcp: Reduce SYN resend delay if a suspicous ACK is received

On Sat, 1 Feb 2020 10:23:43 -0800 Eric Dumazet [off-list ref] wrote:

On 2/1/20 6:53 AM, sj38.park@gmail.com wrote:

quoted

From: SeongJae Park <redacted>

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: SeongJae Park <redacted>
---
 net/ipv4/tcp_input.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2a976f57f7e7..baa4fee117f9 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -5893,8 +5893,14 @@ 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,

Please add my

Signed-off-by: Eric Dumazet <edumazet@google.com>

Please resend the whole patch series as requested by netdev maintainers.


vi +134 Documentation/networking/netdev-FAQ.rst

Q: I made changes to only a few patches in a patch series should I resend only those changed?
---------------------------------------------------------------------------------------------
A: No, please resend the entire patch series and make sure you do number your
patches such that it is clear this is the latest and greatest set of patches
that can be applied.

Thank you, just sent it: https://lore.kernel.org/linux-kselftest/20200202033827.16304-1-sj38.park@gmail.com/ (local)

Also, appreciate for kindly noticing the rule :)


Thanks,
SeongJae Park