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

[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.

> | 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.

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka at isc.org