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

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Date:   Thu, 8 Aug 2019 08:12:44 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Josh Hunt <johunt@...mai.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, edumazet@...gle.com, ncardwell@...gle.com
Subject: Re: [PATCH v2 1/2] tcp: add new tcp_mtu_probe_floor sysctl



On 8/8/19 1:52 AM, Josh Hunt 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.

I am still sad to see you do not share what is a reasonable value and let
everybody guess. 

It seems to be a top-secret value.

> 
> 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>

Signed-off-by: Eric Dumazet <edumazet@...gle.com>