PMTUD for IPv4 Multicast - How?

Mon Aug 31 16:51:17 UTC 2015

On Mon, Aug 31, 2015 at 12:37 PM, <Valdis.Kletnieks at vt.edu> wrote:

> On Mon, 31 Aug 2015 12:12:16 -0400, Chris Marget said:
>
> > 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?

> Don't send multicast packets with DF set.
>

Are you asserting that the default behavior of the Linux kernel (setting DF
on multicast packets) is wrong then?

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.

/chris