netdev - Re: RACK not getting disabled

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Message-ID: <20170918214509.GD28186@strugglingcoder.info>
Date:   Mon, 18 Sep 2017 14:45:09 -0700
From:   hiren panchasara <hiren@...ugglingcoder.info>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: RACK not getting disabled

On 09/18/17 at 02:29P, hiren panchasara wrote:
> On 09/18/17 at 02:18P, Eric Dumazet wrote:
> > On Mon, 2017-09-18 at 13:14 -0700, hiren panchasara wrote:
> > > Hi all, I am trying to disable rack to see 3dupacks in action during
> > > loss-detection but based on the pcap, I see that it's still trigger
> > > loss-recovery on the first SACK (as if RACK is still enabled/active).
> > > 
> > > Here is what I did to disable rack:
> > > net.ipv4.tcp_recovery = 0
> > > 
> > > I've also disabled metrics:
> > > net.ipv4.tcp_no_metrics_save = 1 
> > > And also flushed existing entries with 'ip tcp_metrics flush' just to be
> > > on a safer side.
> > > 
> > > Not really relevant here but I've also switched to reno.
> > > 
> > > I am on: 4.10.0-33-generic
> > > pcap: https://transfer.sh/mfoiN/reno_no_rack.pcap
> > > 
> > > What am I missing? I can provide any additional info.
> > > 
> > > Cheers,
> > > Hiren
> > 
> > 
> > A single SACK can contains enough information to trigger a retransmit.
> 
> Bah, right. FACK!
> > 
> > If you absolutely want to see the old 3 dupack in action, you also want
> > to disable SACK.
> 
> I believe net.ipv4.tcp_fack = 0 would achieve that without disabling
> sack.

Ah, what you probably meant was that this could still be < 3 dupacks
triggering loss-recovery based on reordering.

I believe I got what I was looking for.

Thanks a ton,
Hiren

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