Re: PMTUD for IPv4 Multicast - How?

[sthaug () nethelp no]

> > > At first, I thought this was a bug, but then learned that RFCs 1112, 1122
> > > and 1812 all specify that ICMP unreachables not be sent in response to
> > > multicast packets.
> >
> > > I'm struggling to grok the rationale behind not sending unreachables in
> > > response to multicast packets. It seems to me that our networks put IPv4
> > > multicast speakers in a position where it's impossible for them to do the
> > > right thing.
> >
> > 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".
>
> 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?

ICMP replies to multicast packets can cause ICMP "implosion". This is
not a new discussion - see for instance

http://mailman.nanog.org/pipermail/nanog/2012-June/048685.html

Steinar Haug, Nethelp consulting, sthaug () nethelp no