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[ih] fragmentation (Re: Could it have been different? [was Re: vm vs. memory])

On 10/25/2017 6:42 PM, Paul Vixie wrote:
> where we are today is that fragmentation is completely hosed. pmtud does 
> not work in practice, 
I do agree there are problems with fragmentation, but some layer of
fragmentation/reassembly is required when any layer below establishes a
"maximum" on message sizes, otherwise no amount of data origin guessing
will overcome a given level of layering and encapsulation.

Especially with PMTUD, but I thought that's why we've shifted to PLPMTUD.

IP fragmentation isn't working as intended, IMO because most routers
don't stay in their lanes - by forwarding only on IP header info.
Devices that peek into contents (DPI) necessarily need to reassemble
(even if as a shadow copy); claiming that "reassembly doesn't belong in
routers" while doing DPI is cherry-picking which parts of the router
requirements to follow and which to break.

(yes, I know PLPMTUD doesn't currently work with UDP, which is why I've
been working on extensions to UDP to support transport-layer fragmentation)