Question regarding anycasting in CDN setup

Jared Mauch jared at
Wed Feb 1 20:47:05 UTC 2012

On Feb 1, 2012, at 3:25 PM, Anurag Bhatia wrote:

> 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 - - this one is further a cname
> string and at the end you will find different A records when checked from
> different locations.

The real answer to this is highly variable based on criteria that are unknown
by many people outside of the operators at these networks.

what is fairly well known:

1) Anycast can be used to provide low latency queries for stateless (UDP) and
   state full protocols (TCP).
2) Query responses will vary based on node hit and/or source IP address the
   query comes from.  Source address is used to attempt traffic localization.

   This can be defeated by using another resolver on purpose, or inadvertently
   (eg: corporate VPN may cause you to use a CDN node that is non-local by using
    corp DNS).
3) CDNs vary the response based upon uptime/load and other unknown policy criteria.
   They don't want to send you to a server that is down, nor one that is overloaded.

The secret is in the sauce here and is complex enough that it's not easy to perfect.

Also, be careful equating Anycast w/ CDN.  They are not the same thing but sometimes
are related.  (e.g.: cousins)

	- Jared

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