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Re: [tcpm] PLPMTUD for all protocols

"Maxim Proshin [GMAIL]" <mvproshin@gmail.com> Thu, 29 March 2018 09:45 UTC

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From: "Maxim Proshin [GMAIL]" <mvproshin@gmail.com>
Date: Thu, 29 Mar 2018 12:45:05 +0300
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To: Mikael Abrahamsson <swmike@swm.pp.se>
Cc: William Herrin <bill@herrin.us>, tcpm@ietf.org
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Subject: Re: [tcpm] PLPMTUD for all protocols
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Hi,

What is a status of classical PMTUD in Linux TCP? Is it enabled by default?
I guess it's enabled by default but TCP implementation actually relies on
IP for that.

BR, Maxim

On Wed, Mar 28, 2018 at 6:51 PM, Mikael Abrahamsson <swmike@swm.pp.se>
wrote:

> On Wed, 28 Mar 2018, William Herrin wrote:
>
> Sacrilege to suggest sending anything without the DF bit set, but if the
>> router's return has a black hole, maybe the answer is to let the router
>> communicate forward instead.
>>
>
> 1. IPv6 doesn't have a DF bit that can be on or off, it's always
> implicitly on.
> 2. Fragmentation doesn't usually happen if there is PMTUD blackhole.
>
> So I have no idea how to come to a best conclusion regarding the tradeoff
> between "doesn't work at all" and "doesn't work optimally in some cases".
> Generally, I'd be happy with a blackhole detect that falls back to low MTU
> (576 for IPv4, 1280 for IPv6) whenever there seems to be a real blackhole,
> and be very cautious in going back up again in MSS.
>
> Right now I would like things to be more resilient to complete failure
> than what TCP is today without blackhole detect.
>
> --
> Mikael Abrahamsson    email: swmike@swm.pp.se
>
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>



-- 
BR, Maxim

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