Content Delivery Networks

Patrick W.Gilmore patrick at
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  
> globe)

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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