Re: Regionally aggregatable address space for multihoming

[Sean Doran <smd@ebone.net>]

Iljitsch van Beijnum <iljitsch@muada.com> writes:

> All networks announce their customer's routes everywhere, but peers simply
> filter out these announcements on exchange points that do not fall within
> their idea of the region.

So what happens in the case where a site connects to a
network which does not actually connect to the local
exchange point at all?  

X-Corp acquires geographically-assigned addresses from 
a local registry, then buys transit from ANet and BNet; 
ANet is connected to the local IX but BNet is not.  
How shall BNet advertise reachability to X-Corp?

ANet and CNet, both present at the local IX, have a
commercial falling-out.  Or a fibre cut.  Or the IX fails.
Or something.  This leads to a long-term outage in
_direct_ local connectivity between ANet and CNet; however
ANet purchases backup connectivity from DNet, one of
CNet's peers.  Y-Corp, a customer of CNet in another city,
now wants to send traffic to X-Corp (and vice-versa).
How can this happen?

The question in a nutshell, is: how does one deal with
"exception routing" in the event one cannot guarantee a
always fully-interconnected geographical area?

That is, if one draws an addressing abstraction boundary
around a geographical area rather than a topological area,
must one/can one/how can one guarantee that the enclosed
area can always be abstracted?

This is the riddle of the CIDR Sphinx.

        Sean.