netdev - Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received

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Date:   Fri, 31 Jan 2020 07:01:21 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     sjpark@...zon.com
Cc:     David Miller <davem@...emloft.net>, Shuah Khan <shuah@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, sj38.park@...il.com,
        aams@...zon.com, SeongJae Park <sjpark@...zon.de>
Subject: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received

On Fri, Jan 31, 2020 at 4:25 AM <sjpark@...zon.com> wrote:

> Signed-off-by: SeongJae Park <sjpark@...zon.de>
> ---
>  net/ipv4/tcp_input.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 2a976f57f7e7..b168e29e1ad1 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5893,8 +5893,12 @@ 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 ignore. */
> +                       inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
> +                                                 TCP_ATO_MIN, TCP_RTO_MAX);

This is not what I suggested.

I suggested implementing a strategy where only the _first_ retransmit
would be done earlier.

So you need to look at the current counter of retransmit attempts,
then reset the timer if this SYN_SENT
socket never resent a SYN.

We do not want to trigger packet storms, if for some reason the remote
peer constantly sends
us the same packet.

Thanks.

>                         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
>