icmp rpf

Mark Kent mark at noc.mainstreet.net
Mon Sep 25 17:09:00 UTC 2006

In response to this:
> Mark Smith wrote:
> >> The non-announcers, because they're also breaking PMTUD.
> Really?   How? 

Mark Smith replied with two paragraphs, but it's not 100% clear to me
that he got the reason why I asked.   I asked because his initial statement
boiled down to "numbering on un-announced space breaks PMTUD"...
but it doesn't, not by itself (which he later expanded).

It only does so in the presence of filtering.

I think this is an important point to make because of my interaction
with small.net.  When I pointed out the timeouts they said that it was
because they don't announce the router IP addresses, which is true but
not the whole story.  I mentioned that some providers in the past
numbered on rfc1918 space and traceroute still worked, so that alone
was not enough.

Then they said "it's because of the asymmetric path," and that also is
true, but again not enough.  A large proportion of traffic is
asymmetric, but traceroute still works.

Then they gave me an explanation that rested on the fact that the
routers will not respond to pings because they are unannounced outside
of their world.  That too is true, but irrelevant and I told them how
Jacobson's traceroute works and told them that *someone* was
dropping/filtering the return packets and I'ld like to know who/why.

They somewhat implied that it was my fault, and this situation was
unique to my net, so I used the big.net looking glass to show how the
same things happens from space not associated with my network.
(Yes, I should have done this from the outset.)

With that they asked big.net, and big.net said they filtered, 
and that's where we are.  

My point here is that it took me ten (10) emails with small.net to get
this information partly because the small.net support staff had notions
in their head premised on too simplistic statements like "numbering on
un-announced space breaks PMTUD."  

I wanted to clear this up because this list is likely read by support
people at various networks, and it's pretty clear that not all of them
are well versed even on something as thoroughly discussed over the ages
as traceroute.


More information about the NANOG mailing list