Re: Responding to Ron's comment about removing fragmentation

["C. M. Heard" <[e-mail]>]

On Fri, 14 Nov 2014, Jeroen Massar wrote:
> Fragmentation is not needed. Packets that are larger than 1280 where the
> MTU is 1280 should be split up by the tool/protocol sending those packets.
> 
> That is what TCP does. UDP rejects them so that the app can fix the size
> and chunk them up properly.

Some application protocols do not perform chunking but require that 
a message be encapsulated in a single UDP datagram, i.e., they rely 
on UDP to provide message framing.  DNS is one of those (unless it 
is in expensive TCP-fallback mode).  And it is an established fact 
that if DNSSEC is turned on, it's not difficult for UDP-encapsulated 
replies to exceed the 1280-byte guaranteed minimum MTU.  See Section 
3.2 of https://tools.ietf.org/html/draft-bonica-6man-frag-deprecate-02

If there were some incrementally deployable way of adding a 
fragmentation capability to UDP so that it does not have to rely on 
IP fragmentation for large datagrams, I think the case for removing 
fragmentation from IPv6 would be a lot stronger.  However, I've not 
seen such a proposal.  That's not for want of trying on my part, 
though it may well be from lack of imagination.

Mike Heard