On Mon, Feb 8, 2016 at 9:05 AM David Borman
<dab(_at_)weston(_dot_)borman(_dot_)com> wrote:
On Feb 8, 2016, at 10:08 AM, Alexey Eromenko <al4321(_at_)gmail(_dot_)com>
wrote:
On Mon, Feb 8, 2016 at 5:52 PM, John Levine <johnl(_at_)taugh(_dot_)com>
wrote:
2. What kind of UDP applications use such big packets, over 1280 bytes ?
Um, the DNS with EDNS0, and particularly with DNSSEC? Is this a trick
question?
Yes.
The trick lies right there in IPv6 specification:
"A node must be able to accept a fragmented packet that, after
reassembly, is as large as 1500 octets."
... which is not much greater than 1280 bytes defined as minimum MTU for
IPv6.
Basically, Fragmentation, as specified in RFC-2460 (IPv6), solves the
problem ONLY for packet-sized between 1280 and 1500 bytes. Which is why I
doubt if it is useful at all.
That’s the minimum required implementation, which guarantees that you can
send at least a basic ethernet sized packet to any host. What is omitted
in the above qoute is next 2 sentences:
"A node is permitted to
accept fragmented packets that reassemble to more than 1500 octets.
An upper-layer protocol or application that depends on IPv6
fragmentation to send packets larger than the MTU of a path should
not send packets larger than 1500 octets unless it has assurance that
the destination is capable of reassembling packets of that larger
size.”
So if you are writing an application that needs >1500 octets, use an IPv6
implementation that supports >1500 octet fragmentation and reassembly.
... but as an application writer (or, basically anyone else), I have no
control over the "IPv6 implementation". Even if I'm in an environment where
I do control the OS / model of all devices, and I know they support >1500
octet, it seems like a bad idea to *rely* on that. Sometime I'm going to
want to change OS / add some other device, be able to interact with some
other system. This sounds like vendor lock at its worst...
W
-David Borman
(I also understand, that IPv4 fragmentation is much more flexible, yes.
It can go as small as 68 byte packets.)