[183361] in North American Network Operators' Group
Re: PMTUD for IPv4 Multicast - How?
daemon@ATHENA.MIT.EDU (sthaug@nethelp.no)
Mon Aug 31 15:49:50 2015
X-Original-To: nanog@nanog.org
Date: Mon, 31 Aug 2015 21:49:43 +0200 (CEST)
To: chris@marget.com
From: sthaug@nethelp.no
In-Reply-To: <CAAG5dDD+oNOEK0b57j8EnVNrZ09oEHzFy9ruATyAzfeDs=furw@mail.gmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces@nanog.org
> > > 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
