[149331] in North American Network Operators' Group
Re: Question regarding anycasting in CDN setup
daemon@ATHENA.MIT.EDU (Charles Gucker)
Wed Feb 1 15:37:31 2012
In-Reply-To: <CAJ0+aXZiB3XbUqOtV0y+X4VJSQCuYc-ffvPgQ5J=TucXWhfrVA@mail.gmail.com>
Date: Wed, 1 Feb 2012 15:36:46 -0500
From: Charles Gucker <cgucker@onesc.net>
To: Anurag Bhatia <me@anuragbhatia.com>
Cc: NANOG Mailing List <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Wed, Feb 1, 2012 at 3:25 PM, Anurag Bhatia <me@anuragbhatia.com> wrote:
> Hello everyone!
>
> I have a small question and was wondering if someone could help me with
> that.
>
> Question is - why companies like Google, Amazon are having partial
> anycasting in CDN setups? E.g if we pick a random hostname from url of
> Picasa picture - lh3.googleusercontent.com - this one is further a cname
> string and at the end you will find different A records when checked from
> different locations.
The simple answer for this is, Google cannot be expected to have a
local cache of every image supplied to them globally on every server.
So they use unicast servers behind a DNS based geo load balancer
configuration. As for DNS, every anycasted node is expected to be
able to resolve any DNS request that is made.
It's all a matter of disk and acceptable delay in providing the data
from the "closest" disk.
charles
