netdev - Re: [PATCH v2 1/2] tcp: add new tcp_mtu_probe

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Date:   Thu, 8 Aug 2019 08:02:37 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Josh Hunt <johunt@...mai.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 1/2] tcp: add new tcp_mtu_probe_floor sysctl

On Wed, Aug 7, 2019 at 7:55 PM Josh Hunt <johunt@...mai.com> wrote:
>
> The current implementation of TCP MTU probing can considerably
> underestimate the MTU on lossy connections allowing the MSS to get down to
> 48. We have found that in almost all of these cases on our networks these
> paths can handle much larger MTUs meaning the connections are being
> artificially limited. Even though TCP MTU probing can raise the MSS back up
> we have seen this not to be the case causing connections to be "stuck" with
> an MSS of 48 when heavy loss is present.
>
> Prior to pushing out this change we could not keep TCP MTU probing enabled
> b/c of the above reasons. Now with a reasonble floor set we've had it
> enabled for the past 6 months.
>
> The new sysctl will still default to TCP_MIN_SND_MSS (48), but gives
> administrators the ability to control the floor of MSS probing.
>
> Signed-off-by: Josh Hunt <johunt@...mai.com>
> ---

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Josh. I agree with Eric that it would be great if you are able
to share the value that you have found to work well.

neal