Content Delivery Networks

Michal Krsek michal at
Tue Aug 7 14:05:07 UTC 2007

Hi Patrick,

>> 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)

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.

> Application redirection is far, far slower.  (I am assuming you are 
> talking about something like HTTP level redirects.  Did you mean 
> something else?)

Yes, it is slower, but in some scenarios (streaming hours of content for 
thousands of spectators) works nicely.

Currently I'm using both mechanisms. Typically I'm using application level 
redirets and when I know there will be significant peak (e.g. ten times more 
than average), I'll provide DNS FQDN :-)

> As for anycast, with your own backbone, you don't need any  cooperation. 
> Even if you don't, the cooperation you need from your  providers & peers 
> is minimal at worst.  (At least relative to writing  the code for, say, 
> DNS redirection.)  But anycast assumes "BGP knows  best", and we all know 
> that is not even close to the truth.

I'm definitelly not talking about situation when you are running your own 

Anycast in general works for small transactions (root DNS fits perfectly), 
but Internet is "living beast" and if you provide live stream that lasts two 
hours, one flipping BGP session makes the content almost unusable for part 
of your users.


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