[149333] in North American Network Operators' Group
Re: Question regarding anycasting in CDN setup
daemon@ATHENA.MIT.EDU (Jared Mauch)
Wed Feb 1 15:47:51 2012
From: Jared Mauch <jared@puck.nether.net>
In-Reply-To: <CAJ0+aXZiB3XbUqOtV0y+X4VJSQCuYc-ffvPgQ5J=TucXWhfrVA@mail.gmail.com>
Date: Wed, 1 Feb 2012 15:47:05 -0500
To: Anurag Bhatia <me@anuragbhatia.com>
Cc: NANOG Mailing List <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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.
>=20
> 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 - lh3.googleusercontent.com - 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=
