Question regarding anycasting in CDN setup

Anurag Bhatia me at anuragbhatia.com
Wed Feb 8 13:58:07 CST 2012


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 at anuragbhatia.com> wrote:

> 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
>>
>
>
>
> --
>
> 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
>
>


-- 

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


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