Nice explanation!
Thanks Mike. Appreciate it. On Thu, Feb 2, 2012 at 6:08 AM, Mike Jones <m...@mikejones.in> wrote: > On 1 February 2012 20:25, Anurag Bhatia <m...@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
