North American Network Operators Group
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Re: Content Delivery Networks

  • From: Michal Krsek
  • Date: Tue Aug 07 15:43:06 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.

Regards
MK