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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."
Seven or so years ago, before I retired from one of the large cable companies, CDNs were hosting the relatively static parts for a surprisingly large number of broadly popular sites. I had an opportunity to see the list when we were approached by the then-largest CDN, who wanted to place servers in many of our head-end locations for the obvious performance benefit. I was the one who pointed out that all of our internal DNS requests were routed to one of two data centers, one on the East Coast and one on the West, creating exactly the situation described in the OP: the CDN would have no idea where the original request came from, so would be unable to direct the end user to the appropriate server.
I was one of the few engineers who argued for less centralization in our network. I wanted broader distribution for reliability purposes: at that time, the massive centralized mail servers had a tendency to fail at the drop of a hat. But it would also have given us the ability to work with companies like the CDNs in order to provide better service.
This is exactly the problem. Most people have probably not heard about a little company called Akamai, but chances are if you're downloading content from a large site, you're using Akamai's content delivery network. Go view a trailer on Apple's site for instance and you'll see the host is actually served off edgesuite.net (which is Akamai). They use a distributed system of caching mirror servers to serve up content to a server closest to you geographically.
The one reason I use an open DNS server instead of my cable provider's (Cox Cable) servers is because they have an Akamai server for Cox and it was horribly overloaded. I was getting 512 Kbps anytime I was trying to download something from Apple. I switched my DNS to a combination of Level3's and Cisco's open DNS servers and I started hitting another Akamai server outside Cox and started getting 15 Mbps. It was night and day going from barely being able to watch a standard definition movie trailer on Apple's site while it buffered buffered, played, buffered, play buffered, etc. to being able to watch a 1080p HDTV stream with the buffer way ahead of my realtime viewing.