netdev - Re: [PATCH net] tcp: disallow cwnd undo when switching congestion control

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Date:   Wed, 31 May 2017 12:47:32 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net] tcp: disallow cwnd undo when switching congestion control

On Wed, May 31, 2017 at 11:47 AM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> On Wed, 31 May 2017 11:21:27 -0700
> Yuchung Cheng <ycheng@...gle.com> wrote:
>
> > When the sender switches its congestion control during loss
> > recovery, if the recovery is spurious then it may incorrectly
> > revert cwnd and ssthresh to the older values set by a previous
> > congestion control. Consider a congestion control (like BBR)
> > that does not use ssthresh and keeps it infinite: the connection
> > may incorrectly revert cwnd to an infinite value when switching
> > from BBR to another congestion control.
> >
> > This patch fixes it by disallowing such cwnd undo operation
> > upon switching congestion control.  Note that undo_marker
> > is not reset s.t. the packets that were incorrectly marked
> > lost would be corrected. We only avoid undoing the cwnd in
> > tcp_undo_cwnd_reduction().
> >
> > Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> > Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> > Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> That looks correct. Are there other values of congestion state
> that should be reset?
>
Nothing in particular comes up. We found this bug by internally
checking insane cwnd values on Google hosts. We might find more :-)