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Re: Someones being naughty again...

  • From: Alec H. Peterson
  • Date: Sat Mar 14 09:07:12 1998

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

You only run into this situation if:

1) The packets have DONT_FRAG flag set on them
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.

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.


|Alec Peterson - [email protected]  | Erols Internet, INC.             |
|Network Engineer                  | Springfield, VA.                 |