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

Tue Nov 30 19:55:20 UTC 2004

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.