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SubjectRe: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received
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On Fri, 31 Jan 2020 07:01:21 -0800 Eric Dumazet <edumazet@google.com> wrote:

> On Fri, Jan 31, 2020 at 4:25 AM <sjpark@amazon.com> wrote:
>
> > Signed-off-by: SeongJae Park <sjpark@amazon.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
> >
>

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