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Re: [tcpm] PLPMTUD for all protocols

Mikael Abrahamsson <swmike@swm.pp.se> Wed, 28 March 2018 15:51 UTC

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Date: Wed, 28 Mar 2018 17:51:34 +0200
From: Mikael Abrahamsson <swmike@swm.pp.se>
To: William Herrin <bill@herrin.us>
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Subject: Re: [tcpm] PLPMTUD for all protocols
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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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