[dns-operations] DNS flag day 2020 update

Paul Vixie paul at redbarn.org
Wed Mar 25 17:26:24 UTC 2020

Willem Toorop wrote on 2020-03-25 04:08:
> Op 25-03-2020 om 09:27 schreef Paul Vixie:
>> ...
>> many of us are successfully using 1400 or larger. the MTU value of 1280 that
>> this calculation is based on, was arbitrarily chosen in the IPv6
>> specification, and no real network operates with this limit. the 48 byte
>> subtrahend was arbitrarily chosen without leaving room for IP6 options. this
>> never matters for TCP because TCP knows the size of the IP6 options that will
>> be used. this in turn never matters because the internet's effective MTU is
>> ~1500.
> Hi Paul,
> I did measure MTU's available to resolvers on RIPE Atlas in June 2013
> and presented results then at RIPE67:
> https://nlnetlabs.nl/downloads/presentations/20131016-RIPE67-pmtud4dns.pdf
> At that time there were 1029 RIPE Atlas probes, which combined had 863
> IPv6 capable resolvers.  411 of those (51%) had MTU smaller than 1500.
> 115 had an MTU of 1280.  On slide 17 of the presentation you can see the
> the different detected MTU's at that time.

in those days the smallness of the probe network and the self-selecting 
nature of who ran the probes made the data inconclusive.

> Maybe it's worthwhile to redo those measurements again with the 16000
> IPv6 capable resolvers that we can target on RIPE Atlas right now.

i am in favour of that proposal. see also attached message from a month 
ago, which followed my presentation at DNS-OARC SF on this very topic, 
and a hallway conversation between myself and the author (brian somers 
of cisco-umbrella "opendns"):

> - Routing equipment that fragments data to small sizes does not seem
>   to exist on this data path.
> ...
> - Our heuristic of 1500 - 60 - 8 - (fudge=22) works in practice!

P Vixie

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