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Re: Someones being naughty again...
On Sat, 14 Mar 1998, Alec H. Peterson wrote: > At 02:36 3/14/98 , Marc Slemko wrote: > > > >For example? A router with one ATM interface going to the world with a > >high MTU with an ethernet on the other side. Say you use private IP space > >for links on that router. Say someone on the Internet filters traffic > >from private netblocks; lots of people do. There _can_ be machines that > >are completely unable to transfer data (eg. download a web page) from > >another because you just broken path MTU discovery. This is not a made up > >situation, this is a real example that I have had to deal with of how > >using private IP space for > >network interfaces used for public traffic does break things in some > >situations. > > You only run into this situation if: > > 1) The packets have DONT_FRAG flag set on them As I said, path MTU discovery. That imples DF. > 2) The ATM interface is in fact set with a small MTU. > > 1 is very possible nowadays, but 2 is prolly not. Yeah, ATM has a small > cell size, but most IP over ATM interfaces I've ever seen have an MTU of > something like 4470. Yes, they cheat and do break down the packet into > cells, but can you imagine trying to put a TCP download into 48 bytes? > You'd use most if not all of that for the IP header. No, the whole point is the ATM interface has a large MTU and the ethernet has a small MTU, which means that large segments coming from the "outside" to the "inside" don't fit. This is just a simple example from my life; there are many situations that can cause this which aren't always obvious. > > It is still an open debate about whether or not RFC1918 space is wise to > use, but I'd say it's a sign of a commendable effort on @Home's part that > they are trying to conserve IP space, even though they do have lots of > routable addresses. You can call it an open debate until you actually try using it. Of course, most people don't notice the things that break when they do.