From: kwe@6SigmaNets.COM (Kent W. England) I'm skeptical of any end-to-end availability figures over 97%. I don't think they reflect the reality of leased line circuits today, or else they don't include the leaf node circuits and only report backbone availability. For a highly redundant backbone, almost any definition of availability should result in a number like 99.mumble%. Remember 99.9% availability means less than 9 hours outage per year. Routing hiccups take that much. One or two leased lines outages is all you get for 9 hours. The real world is a lot less available than that.

Thank you! I thought I was living in a twilight zone with people reporting 99.9% network availability. This is the rathole end-to-end network useablity. The customer is interested in end-to-end useability. While the network operator can only easily measure intra-network modules. I can't tell you the answer, but there is definitely something happening with customer perceptions of Internet useability. Looking at the numbers I would agree a single leased circuit should be less reliable (single point of failure) than a highly redundant backbone. But by our customer perceptions, that isn't the case. Either we have better than "normal" leased circuits, or the highly redundant backbones aren't, or our customers needs are based something we aren't directly measuring. Highly redundant backbones remain extremely vunerable to the "glitch." Human glitches, software glitches, "impossible" data glitches. Redundant backbones do protect against the backhoe "glitch."

But since half the web servers I try to talk to refuse me half the time, I'm not sure that network availability per se (HWB's complaints duly acknowledged) is the tallest pole in the tent.

Part of this problem is the growing number of interdepencies (complexity, chaos?). Even if each individual module is working 99.9% of the time, the probabilities start looking pretty bad when all need to be working at the same time. To make a web connection, you have a string of name servers, a string of networks to the name servers, a string of routers on those networks, another string of networks to the web server, another string of routers, more strings of networks and routers and servers on the return path. I'm amazed it even works 50% of the time. Unfortunately our customers aren't always as understanding. Since error reporting sucks in most network applications, it becomes the fault of whatever help desk happens to take the customers phone call. -- Sean Donelan, Data Research Associates, Inc, St. Louis, MO Affiliation given for identification not representation