rfc1918 ignorant

Date: Wed, 23 Jul 2003 08:59:18 -0400 (EDT)
From: Dave Temkin <dave@ordinaryworld.com>
Sender: owner-nanog@merit.edu

Is this really an issue? So long as they're not advertising the space I
see no issue with routing traffic through a 10. network as transit. If
you have no reason to reach their router directly (and after Cisco's last
exploit, I'd think no one would want anyone to reach their router directly
:slight_smile: ), what's the harm done?

RFC1918 merely states that it shouldn't be routed on the global internet,
not that it can't be used for transit space.

That's not what is in my copy of 1918.

"In order to use private address space, an enterprise needs to
determine which hosts do not need to have network layer connectivity
outside the enterprise in the foreseeable future and thus could be
classified as private. Such hosts will use the private address space
defined above. Private hosts can communicate with all other hosts
inside the enterprise, both public and private. However, they cannot
have IP connectivity to any host outside of the enterprise. While not
having external (outside of the enterprise) IP connectivity private
hosts can still have access to external services via mediating
gateways (e.g., application layer gateways)."

As I read this, packets with a source address in 19298 space should
NEVER appear outside the enterprise. Comcast and many others seem to
blithely ignore this for convenience sake. (It's not like they need a
huge amount of space to give private addresses to these links.)

You read this correctly. We also wrote:

   It is strongly recommended that routers which connect enterprises to
   external networks are set up with appropriate packet and routing
   filters at both ends of the link in order to prevent packet and
   routing information leakage. An enterprise should also filter any
   private networks from inbound routing information in order to protect
   itself from ambiguous routing situations which can occur if routes to
   the private address space point outside the enterprise.

I consider this quite explicit. I also consider this still very valid.

Imho the PMTU argument is moot. This should not be an issue at all other
than on the edges these days. Should it nonetheless be an issue it
can be done in the "boundary" routers which have interfaces numbered
public address space. I do not know all the details here, so if
I am wrong in detail, please tell me.