netdev - Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not enabled

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Message-ID: <CAK6E8=dvxpc35FtbY+hU801H4-Kau+OnhQk=qKmLSw9jTQihwg@mail.gmail.com>
Date:   Wed, 7 Mar 2018 15:48:52 -0800
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
        Netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not enabled

On Wed, Mar 7, 2018 at 12:19 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
>
> On Wed, Mar 7, 2018 at 7:59 AM, Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:
> > A bogus undo may/will trigger when the loss recovery state is
> > kept until snd_una is above high_seq. If tcp_any_retrans_done
> > is zero, retrans_stamp is cleared in this transient state. On
> > the next ACK, tcp_try_undo_recovery again executes and
> > tcp_may_undo will always return true because tcp_packet_delayed
> > has this condition:
> >     return !tp->retrans_stamp || ...
> >
> > Check for the false fast retransmit transient condition in
> > tcp_packet_delayed to avoid bogus undos. Since snd_una may have
> > advanced on this ACK but CA state still remains unchanged,
> > prior_snd_una needs to be passed instead of tp->snd_una.
>
> This one also seems like a case where it would be nice to have a
> specific packet-by-packet example, or trace, or packetdrill scenario.
> Something that we might be able to translate into a test, or at least
> to document the issue more explicitly.
I am hesitate for further logic to make undo "perfect" on non-sack
cases b/c undo is very complicated and SACK is extremely
well-supported today. so a trace to demonstrate how severe this issue
is appreciated.

>
> Thanks!
> neal