PMTUD for IPv4 Multicast - How?

Mon Aug 31 21:34:27 UTC 2015

> > 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

Thanks very much for the pointer to that discussion. "ICMP implosion"
has been a helpful search term.

The position taken there appears to boil down to:
- The IPv6 requirement to generate "too big" messages *really is a problem*
- RFC2463 should not have made the exception which allows sending these messages
- Multicast PMTUD should not be a thing
- Multicast speakers should send un-fragmentable minimum-sized packets

I remain fuzzy on exactly the nature of the implosion problem. Is the
concern that I might DDoS myself by sending un-fragmentable traffic?

It's hard for me to recognize this as a problem, but I'm working on
it. It seems to me that as a multicast speaker, the influx of ICMP
errors is both desirable (I set DF because I intend to react) and
under my control.

It certainly beats sending minimum-sized packets, which appears to be
the recommendation in the linked discussion.

If somebody would be so kind as to detail the disastrous nature of the
implosion, that would be helpful.