My colleague Nick Gall pointed out an article in the ACM Queue that I’d missed: Improving Performance on the Internet, by Akamai’s chief scientist, Tom Leighton.

There is certainly some amount of marketing spin in that article, but it is nonetheless a very good read. If you are looking for a primer on why there are CDNs, or are interested in understanding how the application delivery network service works, this is a great article. Even if you’re not interested in CDNs, the section called “Highly Distributed Network Design” has a superb set of principles for fault-tolerant distributed systems, which I’ll quote here:

1. Ensure significant redundancy in all systems to facilitate failover.
   
2. Use software logic to provide message reliability.
   
3. Use distributed control for coordination.
   
4. Fail cleanly and restart.
   
5. Phase software releases.
   
6. Notice and proactively quarantine faults.
   

One niggle: The article says, “The top 30 networks combined deliver only 50 percent of end-user traffic, and it drops off quickly from there, with a very-long-tail distribution over the Internet’s 13,000 networks.” That statement needs a very important piece of context: the fact that most of those networks do not belong to network operators (i.e., carriers, cable companies, etc.). Many of them are simply “autonomous systems” (in Internet parlance) owned by enterprises, or which belong to Web hosters, and so forth. That’s why the top 30 account for so much of the traffic, and that percentage would be sharply increased if you allocated them the enterprises who buy transit from them. (Those interested in looking at data to do a deeper dive should check out the Routing Report site.)