netdev - Re: [PATCH v3 net 2/5] tcp: prevent bogus FRTO undos with non-SACK flows

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Date:   Wed, 28 Mar 2018 15:26:28 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:     netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: Re: [PATCH v3 net 2/5] tcp: prevent bogus FRTO undos with non-SACK flows

On Tue, Mar 13, 2018 at 3:25 AM, Ilpo Järvinen
<ilpo.jarvinen@...sinki.fi> wrote:
>
> If SACK is not enabled and the first cumulative ACK after the RTO
> retransmission covers more than the retransmitted skb, a spurious
> FRTO undo will trigger (assuming FRTO is enabled for that RTO).
> The reason is that any non-retransmitted segment acknowledged will
> set FLAG_ORIG_SACK_ACKED in tcp_clean_rtx_queue even if there is
> no indication that it would have been delivered for real (the
> scoreboard is not kept with TCPCB_SACKED_ACKED bits in the non-SACK
> case so the check for that bit won't help like it does with SACK).
> Having FLAG_ORIG_SACK_ACKED set results in the spurious FRTO undo
> in tcp_process_loss.
>
> We need to use more strict condition for non-SACK case and check
> that none of the cumulatively ACKed segments were retransmitted
> to prove that progress is due to original transmissions. Only then
> keep FLAG_ORIG_SACK_ACKED set, allowing FRTO undo to proceed in
> non-SACK case.
>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> ---
>  net/ipv4/tcp_input.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 4a26c09..c60745c 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3166,6 +3166,15 @@ static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
>                                 pkts_acked = rexmit_acked + newdata_acked;
>
>                         tcp_remove_reno_sacks(sk, pkts_acked);
> +
> +                       /* If any of the cumulatively ACKed segments was
> +                        * retransmitted, non-SACK case cannot confirm that
> +                        * progress was due to original transmission due to
> +                        * lack of TCPCB_SACKED_ACKED bits even if some of
> +                        * the packets may have been never retransmitted.
> +                        */
> +                       if (flag & FLAG_RETRANS_DATA_ACKED)
> +                               flag &= ~FLAG_ORIG_SACK_ACKED;

How about keeping your excellent comment but move the fix to F-RTO
code directly so it's more clear? this way the flag remains clear that
indicates some never-retransmitted data are acked/sacked.

// pseudo code for illustration

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 8d480542aa07..f7f3357de618 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2629,8 +2629,15 @@ static void tcp_process_loss(struct sock *sk,
int flag, bool is_dupack,
        if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
                /* Step 3.b. A timeout is spurious if not all data are
                 * lost, i.e., never-retransmitted data are (s)acked.
+                *
+                * If any of the cumulatively ACKed segments was
+                * retransmitted, non-SACK case cannot confirm that
+                * progress was due to original transmission due to
+                * lack of TCPCB_SACKED_ACKED bits even if some of
+                * the packets may have been never retransmitted.
                 */
                if ((flag & FLAG_ORIG_SACK_ACKED) &&
+                   (tcp_is_sack(tp) || !FLAG_RETRANS_DATA_ACKED) &&
                    tcp_try_undo_loss(sk, true))
                        return;





>                 } else {
>                         int delta;
>
> --
> 2.7.4
>