Re: Question regarding anycasting in CDN setup

[Anurag Bhatia <me () anuragbhatia com>]

Mike

 I can also have a single DNS
> server give 192.0.2.80 out to queries sourced from a US IP Address,
> 198.51.100.80 for queries sourced from a German IP Address and
> 203.0.113.80 to queries sourced from a Chinese address (djbdns has a
> module for this for example).


I have never did such setup, but I assume it works as you say. I wonder how
it finds a US based system from IP quickly (since it's DNS server)?


Thanks.
On Thu, Feb 9, 2012 at 1:21 AM, Anurag Bhatia <me () anuragbhatia com> wrote:

> Nice explanation!
>
>
> Thanks Mike.
>
> Appreciate it.
>
> On Thu, Feb 2, 2012 at 6:08 AM, Mike Jones <mike () mikejones in> wrote:
>
> > On 1 February 2012 20:25, Anurag Bhatia <me () anuragbhatia com> wrote:
> > <snip>
> > > Now my question here is - why this setup and not simply using having a A
> > > record for googlehosted.l.googleusercontent.com. which comes from any
> > > anycasted IP address space? Why not anycasting at CDN itself rather then
> > > only at DNS layer?
> >
> > You are confusing anycasting with offering different results.
> >
> > I can have an anycast DNS setup where all my servers give the same
> > response (example: most DNS providers), I can also have a single DNS
> > server give 192.0.2.80 out to queries sourced from a US IP Address,
> > 198.51.100.80 for queries sourced from a German IP Address and
> > 203.0.113.80 to queries sourced from a Chinese address (djbdns has a
> > module for this for example).
> >
> > I would guess that google probably have a highly customised algorithm
> > which uses a combination of source IP and the node that your query
> > arrived at as part of the process for deciding what answer to give
> > you, along with dozens of other internal factors.
> >
> > Although I do sometimes wonder why they use CNAME chains in cases
> > where the same servers are authoritative for the target name anyway.
> >
> > If you were wondering why they direct you to the unicast addresses for
> > the local datacentre instead of just giving an anycast address which
> > your nearest datacentre would answer, well their algorithm might
> > decide that it wants to serve you content from the second closest
> > datacentre because the closest one is near capacity, anycast can't do
> > that.
> >
> > - Mike
>
>
>
> --
>
> Anurag Bhatia
> anuragbhatia.com
> or simply - http://[2001:470:26:78f::5] if you are on IPv6 connected
> network!
>
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-- 

Anurag Bhatia
anuragbhatia.com
or simply - http://[2001:470:26:78f::5] if you are on IPv6 connected
network!

Twitter: @anurag_bhatia <https://twitter.com/#!/anurag_bhatia>
Linkedin: http://linkedin.anuragbhatia.com