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

[Eric Dumazet]

On Fri, Jan 31, 2020 at 8:12 AM <sjpark@xxxxxxxxxx> wrote:
>
> On Fri, 31 Jan 2020 07:01:21 -0800 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> > On Fri, Jan 31, 2020 at 4:25 AM <sjpark@xxxxxxxxxx> wrote:
> >
> > > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> > > ---
> > >  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)

canonical fied would be icsk->icsk_retransmits (look in net/ipv4/tcp_timer.c )

AFAIK, it seems we forget to clear tp->total_retrans in tcp_disconnect()
I will send a patch for this tp->total_retrans thing.

> +                               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
> > >
> >