On 8/7/07, Patrick W.Gilmore <patrick@ianai.net> wrote:

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.

--
TTFN,
patrick