How CDNs and Alternative DNS Services Combine For Higher Latency

The_PHP_Jedi writes "Alternative DNS services, such as OpenDNS and Google Public DNS, are used to bypass the sluggishness often associated with local ISP DNS servers. However, as more websites, particularly smaller ones, use content distribution networks via embedded ads, widgets, and other assets, the effectiveness of non-ISP DNS servers may be undermined. Why? Because CDNs rely on the location of a user's DNS server to determine the closest server with the hosted content. Sajal Kayan published a series of test results which demonstrates the difference, and also provided the Python script used so you can test which is the most effective DNS service for your own Internet connection."

Comments

This is not accurate

[davidu]

I'm the founder of OpenDNS (and long-time slashdot reader).

This article is not very accurate for a number of reasons. First, both my service (OpenDNS) and Google's are co-located in similar POPs to all of the major CDNs which causes this problem to be largely avoided. The author of the blog post used a tiny sample size and tested mainly from EC2 instances, neither of which helps his cause.

1) EC2 instances are BY DESIGN not co-located in the same place as major peering infrastructure because that real estate costs more. They are one or two hops away. People use EC2 for compute power, not for routing performance. So he needs to use something like Keynote or Gomez to test from home connections. If he had, he'd see it doesn't impact anything, and often improves performance, especially in the US. We don't have POPs in Asia yet, though they are coming this year, and when we do, we'll improve things for him.

2) Akamai is the only CDN where this will ever be perceptible because their deployments are so dense. They have 3000+ pops which means they will also be able to target more precisely. But this is being worked on RIGHT NOW in the IETF -- http://tools.ietf.org/html/draft-vandergaast-edns-client-ip-01

Anyways, this is really not the issue the author makes it out to be, and for the edge cases, they are being worked on.

Thanks,
David

Re:This is not accurate

[arkhan_jg]

You know, I'd thought I'd actually try it out for myself with a rough and ready test. I have an ISP that gives me multiple real IP addresses, so I stuck my PC on the DMZ with a real IP, and tested each of the DNS servers as the sole DNS server in windows, without using either my local dnsmasq local cache or the one on my router. Obviously, I flushed windows own DNS cache between each ping test. The results are below, make of them what you will.

I also tested all DNS providers with both primary and secondary servers; since the 2ndary servers always gave me the same IP address as the primary, they're not included. Ping times are a simple 0DP average of two sets of 10 pings (and there were no odd spikes, with my connection otherwise idle)

First though, the response times of the DNS servers themselves, average uncached - tested using GRC's DNSBench.
aaisp is my own ISP, BT is a large ISP in my country, 4.2.2.3 is one which I'm using at the moment, having previously tested it as fastest.

google (8.8.8.8): 156ms
opendns (208.67.222.222): 176 ms
aaisp (217.169.20.20): 115 ms
BT (194.72.9.34): 71ms
level 3 (4.2.2.3): 95ms

Then, testing which CDN server each DNS server sends me to, and the ping times of those servers - I used the same CDN DNS names as the article;

First, cdn.thaindia.com (internap):

google resolves as 64.7.222.130, ping 167ms
opendns resolves as 77.242.194.130, ping 15ms (!)
aaisp resolves as 64.20.60.99, ping 82 ms
BT resolves as 64.20.60.106, ping 81ms
level 3 resolves as 64.20.60.106, ping 81ms

Then profile.ak.fbcdn.net (akamai):

google resolves it as 92,122,217,75, ping 22ms
opendns resolves as 195.59.150.152, ping 15ms
aaisp resolves as 92.122,208.106, ping 13ms
BT resolves as 88.221.94.242, ping 14ms
level 3 resolves as 195.59.150.144, ping 15 ms

However you slice it, google's public DNS is a bad choice for me. Longer to resolve addresses, and it sends me to non-optimal CDN servers. OpenDNS is a mixed bag; slower resolution than the rest, but sends me to easily the most optimal cdn.thaindia.com server (shame about the redirected NXDOMAIN problem). Yet BT are the fastest DNS resolver of all, and still return decent results. Go figure; I thought they'd be overloaded and well, crap.

I'm definitely going to have to further testing for my own personal use, using whole page rendering on my favourite sites to see what is actually the best option for me personally, as DNS resolver speed clearly isn't the whole story in this CDN world.