[dns-operations] Announcement - DNS flag day on 2019-02-01

[Florian Weimer]

* Mark Andrews:

>> On 15 Jun 2018, at 1:30 pm, Florian Weimer <fw at deneb.enyo.de> wrote:
>> 
>> * Mark Andrews:
>> 
>>> No, a you just fragment at network MTU. The IETF even specified a
>>> setsockopt in the advanced socket API to tell the kernel to do that.
>> 
>> As specified, IPv6 does not have a network MTU, just like IPv4.
>
> Network minimum MTU (1280 RFC 2460 section 5.5) then if you want to be
> semantically correct. See RFC 3542 for IPV6_USE_MIN_MTU for the description
> of the setsockopt which followed from draft-ietf-ipngwg-bsd-frag.

Please read what I wrote below.  According to the specification, nodes
still need to deal with lower MTUs than that.

>> | In response to an IPv6 packet that is sent to an IPv4 destination
>> | (i.e., a packet that undergoes translation from IPv6 to IPv4), the
>> | originating IPv6 node may receive an ICMP Packet Too Big message
>> | reporting a Next-Hop MTU less than 1280.  In that case, the IPv6 node
>> | is not required to reduce the size of subsequent packets to less than
>> | 1280, but must include a Fragment header in those packets so that the
>> | IPv6-to-IPv4 translating router can obtain a suitable Identification
>> | value to use in resulting IPv4 fragments.
>> 
>> <https://tools.ietf.org/html/rfc2460#section-5>
>> 
>> RFC 6946 affirms this bizarre behavior.
>> 
>> Therefore, if you want to avoid state, you need to send atomic
>> fragments unconditionally, but that causes interoperability problems,
>> so you cannot do this in practice.