[PATCH v3 0/2] Fix reconnection latency caused by FIN/ACK handling race

[sj38 . park]

From: SeongJae Park <sjpark@xxxxxxxxx>

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

Patchset Organization
---------------------

The first patch fixes the problem by adjusting the first resend delay of
the SYN in the case.  The second one adds a user space test to reproduce
this problem.

The patches are based on the v5.5.  You can also clone the complete git
tree:

    $ git clone git://github.com/sjp38/linux -b patches/finack_lat/v3

The web is also available:
https://github.com/sjp38/linux/tree/patches/finack_lat/v3


Patchset History
----------------

>From v2
(https://lore.kernel.org/linux-kselftest/20200201071859.4231-1-sj38.park@xxxxxxxxx/)
 - Use TCP_TIMEOUT_MIN as reduced delay (Neal Cardwall)
 - Add Reviewed-by and Signed-off-by from Eric Dumazet

>From v1
(https://lore.kernel.org/linux-kselftest/20200131122421.23286-1-sjpark@xxxxxxxxxx/)
 - Drop the trivial comment fix patch (Eric Dumazet)
 - Limit the delay adjustment to only the first SYN resend (Eric Dumazet)
 - selftest: Avoid use of hard-coded port number (Eric Dumazet)
 - Explain RST/ACK and FIN/ACK has no big difference (Neal Cardwell)

SeongJae Park (2):
  tcp: Reduce SYN resend delay if a suspicous ACK is received
  selftests: net: Add FIN_ACK processing order related latency spike
    test

 net/ipv4/tcp_input.c                       |   8 +-
 tools/testing/selftests/net/.gitignore     |   1 +
 tools/testing/selftests/net/Makefile       |   2 +
 tools/testing/selftests/net/fin_ack_lat.c  | 151 +++++++++++++++++++++
 tools/testing/selftests/net/fin_ack_lat.sh |  35 +++++
 5 files changed, 196 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/net/fin_ack_lat.c
 create mode 100755 tools/testing/selftests/net/fin_ack_lat.sh

-- 
2.17.1