shim6 @ NANOG (forwarded note from John Payne)

Iljitsch van Beijnum iljitsch at
Wed Mar 1 18:23:53 UTC 2006

On 1-mrt-2006, at 17:22, David Barak wrote:

> I think that we could spend
> our time better in coming up with a different approach
> to addressing hierarchy instead.

I agree.

The address space is one dimensional. This means you can encode a  
single thing in it in a hierarchical manner "for free". With PA,  
that's the ISP: for any address, it's very easy to determine which  
ISP it belongs to and thus route the packet to that ISP. (We're so  
used to this that we don't even notice anymore.)

However, this doesn't work for multihoming because rather than a  
linear space starting with ISP A and ending with ISP Z we now have a  
matrix: A-A, A-B, A-C ... Z-X, Z-Y, Z-Z. (Worse with more than two  
ISPs.) You can't do a longest match first lookup on a  
multidimensional space, so in routing, every end-user becomes his own  
ISP and occupies a slot at the top of the hierarchy.

The thing is, it's not even hard to aggregate differently: just have  
router A hold the first quarter of the global routing table (0/2 with  
v4), router B the second quarter (64/2), router C the second quarter  
(128/2) and router D the fourth quarter (192/2), for example.

There is one snag, though: either you need four routers in each  
location, or you have to bring the traffic to the place where the  
router handling that part of the table is located.

Now I happen to think that we can massage this such that it's not  
necessary to add extra routers to speak of or backhaul traffic  
through places where it doesn't belong so basically all of this is  
free (no new protocols!), but unfortunately, I'm having a hard time  
convincing others that this is a workable approach.

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