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

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 <sjpark@amazon.de>
---
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,
--
2.17.1
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