Sensible geographical addressing [Was: 16 vs 32 bit ASNs yadda, yadda]

Tue Nov 30 22:32:16 UTC 2004

I'm well aware that BGP is link speed agnostic.  That makes it even more
important (or less "not important"?) when looking at moving towards a
geographical routing concept.  If everything were equal, as I noted, then
geographical would make perfect sense.

But it isn't, so it doesn't.  :)

At large NAP points (the higher order ISP's) this may make some sense
because of the ubiquity of larger scale lines.  Throughout the entire bgp
structure though, this doesn't make as much sense.  The flip side, of
course, is that you rely on the higher-level ISPs to do some serious policy
upkeep.  This hasn't seemed to help much so far, and of course, as the
lower-tier ISPs or large-scale enterprises become multihomed, we still lose
out on what is being bantered.

Scott 

-----Original Message-----
From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On Behalf Of
Iljitsch van Beijnum
Sent: Tuesday, November 30, 2004 2:55 PM
To: NANOG list
Subject: Re: Sensible geographical addressing [Was: 16 vs 32 bit ASNs yadda,
yadda]

On 30-nov-04, at 16:29, Scott Morris wrote:

> In the interconnected world, geography is very much irrelevant to best 
> path routing.  It's all about speeds and feeds where a local-access 
> T-1 is obviously not preferable to a cross-country OC-3.

I have a very hard time seeing this as a realistic example in interdomain
routing. BGP has no idea about link speeds. I've seen many occasions where
BGP selects a path that is inferior because all the paths cross the same
number of ASes and that's the extent of BGP's knowledge.

When looking at a small scale, you're right that network topology and
geography are very different. For instance, I live in The Hague, which is in
a very small country very close to a major international fiber hub
(Amsterdam). This means that it's almost impossible for me to reach someone
else in The Hague (or the world, for that matter) without going through
Amsterdam. If you look at Holland as a whole, the picture is very different:
the vast majority of traffic between any two points within the country stays
within the country. If you look at a Western-European scale, there is almost
no traffic that leaves the region. And in 10 years, I've never seen any
traffic between two points in Holland go through Africa, Asia or South
America.

This means that with geographic aggregation in effect, 90% of Dutch more
specific routing information can be aggregated away elsewhere in Europe, 98%
in North America and (possibly) 100% elsewhere in the world.

Yes, there will always be exceptions. When you have a million entries in the
routing table, you don't worry about the 30000 special cases as long as you
can get the 970000 simple cases right.

Another misconception: the aggregation doesn't have to line up with the
fiber. If London needs two aggregates because one half is in the western
hemnisphere and the other half is in the eastern hemnisphere, who cares? And
it gets even better when you consider that an ISP will carry all of its
customer routes everywhere anyway: there is no need for two peers to agree
where the routing information for a certain geographic area is exchanged:
peer A simply listens for the information in the location that it finds most
suitable, and so does peer B. There is no requirement for this to happen in
the same location, or in the "target area" itself.