Content Delivery Networks
patrick at ianai.net
Tue Aug 7 18:13:11 UTC 2007
On Aug 7, 2007, at 10:05 AM, Michal Krsek wrote:
>>> 5) User redirection
>>> - You have to implement a scalable mechanisms that redirects
>>> users to the closes POP. You can use application redirect (fast,
>>> but not so much scalable), DNS redirect (scalable, but not so
>>> fast) or anycasting (this needs cooperation with ISP).
>> What is slow about handing back different answers to the same
>> query via DNS, especially when they are pre-calculated? Seems
>> very fast to me.
> Yes DNS-based redirection scales very pretty.
> But there are two problems:
> 1) Client may not be in same network as DNS server (I'm using my
> home DNS server even if I'm at IETF or I2 meeting on other side of
This has been discussed. Operational experience posted here by Owen
shows < 10% of users are "far" from their recursive NS.
You are the tiny minority. (Don't feel bad, so am I. :) Most
"users" either use the NS handed out by their local DHCP server, or
they are VPN'ing anyway.
> 2) DNS TTL makes realtime traffic management inpossible. Remember
> you may not distribute network traffic, but sometimes also server
> load. If one server/POP fails or is overloaded, you need to
> redirect users to another one in realtime.
Define "real time"? To do it in 1 second or less is nigh
impossible. But I challenge you to fail anything over in 1 second
when IP communication with end users not on your LAN is involved.
I've seen TTLs as low as 20s, giving you a mean fail-over time of 10
seconds. That's more than fast enough for most applications these days.
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