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. Thanks, -mark