From: SeongJae Park sjpark@amazon.de
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@gmai...) - 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@amazon...) - 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