Content Delivery Networks

Michal Krsek michal at
Tue Aug 7 19:15:55 UTC 2007

>>>> 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.

Sure, but 10% of 5 Gb/s is 500 Mb/s. In my streaming scenario.

I respect CDN for HTTP delivery has probably other experience. Also I'm 
using housing contracts for "deliver only to ISP users" and use no transit 
connectivity of housing ISPs (frankly - this is much cheaper).

> 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.

10% is tiny minority, but in real world with real costs, this minority can 
squeeze my profit :-)

>> 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.

I've tested (year ago) real scenario and got very disappointing feedback. It 
seemed that some corporate gateways here don't respect zone TTL.

I'm so far to recommend my solutions to the community. I think that every 
CND provider has to choose its own solution that fits it's own services.


More information about the NANOG mailing list