On Wed, 29 Aug 2001 12:21:23 EDT, "Martin, Christian" <cmartin@gnilink.net> said:
Since I wasn't there during your problems in 1994 and 1996, I ask you, "Did you not know that you were approaching a resource constraint?" In regards
Ever been driving down an unfamiliar road late at night, seen 2 headlights in the distance, and then been very surprised when you realize that instead of 2 headlights a normal distance apart a long way off, it's a Jeep with two very closely-set headlights RIGHT ON TOP OF YOU? Yes, you know you're approaching, but your estimates of arrival time may be very poor.... The problem is that we're basically talking about queueing theory, where the queue is "incoming routing updates". As anybody who's done a lot of driving in a major city will tell you, if the traffic is 80% of the road's capacity, things are good, at 90% they're good, at 98% they're tolerable, and at 101% you're screwed. Traffic engineers (the highway kind) have long known about the equivalent of "ringing" on major highways - if there's a temporary congestion, there's a backlog. When the congestion is cleared, you end up with a traffic jam moving backwards down the road, as cars at the front come back up to speed, but cars arriving at the back have to slow down. Such backwardly mobile jams have been observed to go around the Washington DC beltway 2 to 3 times before finally dissipating. The difference is that the highway traffic engineers have close to a century of experience, and have gotten very familiar with the behavior patterns of congestion (take 30 major cities in the US, and in 1 year you have 10,000 examples of meltdown-level congestion to base your numbers on, assuming that each city only has 1 traffic jam per day). We've been at it in a serious way only a decade or two - and we've intentionally avoided meltdowns (notice that Sean Doran only cited 2 examples for a decade). This means we're on a lot shakier ground when trying to make predictions - we *dont* have tens of thousands of data points.... -- Valdis Kletnieks Operating Systems Analyst Virginia Tech