Question regarding anycasting in CDN setup

Anurag Bhatia me at anuragbhatia.com
Wed Feb 8 13:51:17 CST 2012


Nice explanation!


Thanks Mike.

Appreciate it.

On Thu, Feb 2, 2012 at 6:08 AM, Mike Jones <mike at mikejones.in> wrote:

> On 1 February 2012 20:25, Anurag Bhatia <me at 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
>



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Anurag Bhatia
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