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

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Date:   Fri, 31 Jan 2020 17:12:00 +0100
From:   <sjpark@...zon.com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     <sjpark@...zon.com>, 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: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received

On Fri, 31 Jan 2020 07:01:21 -0800 Eric Dumazet <edumazet@...gle.com> wrote:

> 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.

You're right, I missed the important point, thank you for pointing it.  Among
retransmission related fields of 'tcp_sock', I think '->total_retrans' would
fit for this check.  How about below change?

```
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2a976f57f7e7..29fc0e4da931 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 (tp->total_retrans == 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,
```

Thanks,
SeongJae Park

> 
> 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
> >
>