North American Network Operators Group
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Re: Someones being naughty again...

  • From: Marc Slemko
  • Date: Sat Mar 14 18:19:52 1998 
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.