Re: PMTUD for IPv4 Multicast - How?

[Masataka Ohta <mohta () necom830 hpcl titech ac jp>]

Chris Marget wrote:

> > For the exact same reason that replying to an ICMP Echo Request sent to
> > your broadcast address is generally considered a Bad Idea.
> >
> > The obvious solution is "Doctor, it hurts when I do that" "Don't do that
> > anymore".

And, it implies that some ISPs will filter all the ICMPv6 PTB including
those generated against unicast ones, which means PMTUDv6 won't work.

Filtering ICMPv6 PTB generated against multicast packets but not unicast
ones is not very easy.

> It's not as obvious to me as it is to you. I mean, v6 *requires* exactly
> this behavior, so it can't be all that bad, can it?

Yes, of course.

See

        https://en.wikipedia.org/wiki/Design_by_committee

which is why we should avoid IPv6 entirely, especially because NAT,
with its 48bit effective address space, is fair enough and, for
theoretical purity, NAT can be modified to have full end to end
transparency (https://tools.ietf.org/html/draft-ohta-e2e-nat-00),
or, UPnP capable NAT already practically have the transparency.

> I'll probably come around, but I've not yet concluded that "screw it,
> fragment my traffic, I don't care" is the stance that a conscientious
> application should be taking.

Don't you care, for routers, generating ICMP PTB is as burdensome
as generating fragments?

                                                        Masataka Ohta

PS

Pages 87-101 of

        ftp://chacha.hpcl.titech.ac.jp/2014/infra5.ppt

is my presentation at APNIC32 on the problem.