At 3:43 PM 11/28/95, Sean Donelan wrote:

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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.

My analysis was assumed to apply within a single service providers backbone. I believe you can report end-to-end availability within a single service provider's backbone and have a meaningful number. But leased lines bring that figure down into the range of 99.7%. I think frame relay has the promise of getting that up to .8 or .85, if the regional bells get good at FR net mgmt. The dismal situation you are referring to is the 12-15 service provider backbones, all mashing together at 7-8 exchange points with lots of paths that wind back and forth over asymmetric routes from one coast to the other and back. It's a lot different than when NSFnet was the default. No more default. Today routing is more complex, business is lower margin, and engineer folks don't have time to look up from their router memory overload and route cache woes to figure out why their routes thru ISP#17 aren't optimal. --Kent