[15667] in North American Network Operators' Group
Re: Someones being naughty again...
daemon@ATHENA.MIT.EDU (Marc Slemko)
Sat Mar 14 18:12:03 1998
Date: Sat, 14 Mar 1998 15:58:30 -0700 (MST)
From: Marc Slemko <marcs@znep.com>
To: "Alec H. Peterson" <ahp@hilander.com>
cc: nanog@merit.edu
In-Reply-To: <199803141358.IAA27672@ramirez.hilander.com>
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.
