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

[Neal Cardwell]

On Sat, Feb 1, 2020 at 2:19 AM <sj38.park@xxxxxxxxx> wrote:
>
> 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
>
> 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@xxxxxxxxx>
> ---
>  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..980bd04b9d95 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_ATO_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,
> --

Scheduling a timer for TCP_ATO_MIN, typically 40ms, sounds like it
might be a bit on the slow side. How about TCP_TIMEOUT_MIN, which is
typically 2ms on a HZ=1000 kernel?

I think this would be closer to what Eric mentioned: "sending the SYN
a few ms after the RST seems way better than waiting 1 second as if we
received no packet at all."

neal