Re: operational stoppers to > 1500 MTU deployment in real life (Re: Path MTU Hop-by-Hop Option)

[George Michaelson <[e-mail]>]

dreams become nightmares very quickly.

I will observe that moving to jumbo for a locally attached file-store
was painful, so I do know even in the purely local context I've hit
pain.

However, in constructing GPON common-bearer services (NZ, AU) the
issue of a layer-2 which can encompass 1500 end to end, with a mix of
different overlay provider mechanisms (PPPoE, PPPoATM, Native,
VLAN...) It also becomes clear, the engineers who run the fibre are
well aware they have technology in the wide, talking from CPE to ISP
router, which *could* do it. They have to. They are pretty much
obliged to handle 2k, to allow the interior to be encap/decap for
1500. Customrs still get 14xx but the actual back-end is now north of
that. I think it could be more north.

Its not the glass and the layer-2 won't do "big" any more, its how the
upper layers are configured in equipment which is the problem.

What worries me is the people who are going to chime in about chipsets
and SFP designs which *cannot* do it. I suspect there is far more
transmission equipment stuck in the past than I would like.

I'll shut up now. I just wish as an SDO we could segment this problem
off and get a rational conversation about pits and poles to fix this.

-G

On Thu, Oct 18, 2018 at 2:36 PM Mikael Abrahamsson <[e-mail]> wrote:
>
> On Wed, 17 Oct 2018, George Michaelson wrote:
>
> > dreaming of a larger packet future...
>
> It's completely undeployable in real life without something like
> https://tools.ietf.org/html/draft-van-beijnum-multi-mtu-05
>
> It's not realistic to expect a whole lan of managed and unmanaged devices
> to show up at a LAN and communicate with each other on a large MTU LAN.
>
> Right now my only tools are to crank up the L2 switch to 9000 MTU and
> advertise MTU=9000 in RA. Ok, I show up with my device that only has 1500
> MTU/MRU, I see that RA and I'm like "well, I can't do higher than 1500
> anyway, so I'll continue doing that". I need to send a UDP packet to the
> other guy on the same LAN, that's fine, he has 9000 MRU/MTU. He then needs
> to send me 2000 bytes back, so he does that and because my MRU is only
> 1500, my NIC now silently drops that packet. No PMTUD, just silent drop.
> *poof* gone.
>
> There are LOTS of similar failure modes that can happen. We need
> per-destination PMTU both on/off-link. I need to be able to send RIO and
> say "ok, ::/0 has 1492 PMTU" while at the same time send PIO with
> "MTU 9000". Devices then need to understand if their neighbours actually
> can do 9000 or not.
>
> I've even seen hardware/software that will have interface MTU of 1500,
> take the RA MTU=9000, set it's IPv6 interface MTU to 9000, set MSS to
> 89xx, send a large packet (because it can), then it borks when a response
> comes back because it only has ~2000 in MRU.
>
> Yes, real life is that broken.
>
> --
> Mikael Abrahamsson    email: [e-mail]