DNS Anycast as traffic optimizer?

Wed Sep 1 21:24:19 UTC 2004

On 2 Sep 2004, at 06:05, Bill Woodcock wrote:

> If you want nearest server, anycast will give you that
> essentially 100% of the time.

Just to clarify this slightly, since I've known people to misinterpret 
this point: a clear, contextual understanding of the word "nearest" is 
important in understanding this sentence.

Here's an example: France Telecom was an early supporter of F-root's 
anycast deployment in Hong Kong. Due to the peering between OpenTransit 
and F at the HKIX, the nearest F-root server to OT customers in Paris 
was in Asia, despite the fact that there were other F-root nodes 
deployed in Europe. Those OT customers were indeed reaching the nearest 
F-root node, or maybe they weren't, depending on what you understand by 
the word "near".

Another one: where anycast nodes are deployed within the scope of an 
IGP, topological nearness does not necessarily indicate best 
performance (since not all circuits will have the same loading, in 
general, and maybe a short, congested hop is not as "near" as several 
uncongested hops).

For F, we don't worry too much about which flavour of "near" we achieve 
for every potential client: 
redundancy/diversity/reliability/availability is more important than 
minimising the time to do a lookup, and the fact that the "near" we 
achieve in many cases corresponds to what human users expect it to mean 
is really just a bonus.

However, in the general case it's important to understand what kind of 
"near" you need, and to deploy accordingly.

Joe