[ On Friday, April 20, 2001 at 08:03:02 (+1000), Geoff Huston wrote: ]
Subject: Re: What does 95th %tile mean?
The 95% reading always struck me as a randomly generated number in any case.
Huh? It's a simple and mathematically sound and highly repeatable and auditable way of drawing a line on the usage graph that says something like: If you were to have had a fixed-rate connection this is the bandwidth that you would have required over the previous billing period in order to have obtained effectively the same level of performance as you actually enjoyed over that period. The only trick (from the customer P.O.V.) is in understanding that this is what you're buying and in realising that if you use it then you will pay for it. It probably works best for links that have aggregated traffic (eg. for 1st and 2nd tier providers).
Depending on the synchronization between the burst pattern and the sampling system, and the sampling technique itself, the 95% reading can be zero, half the line rate, or the line rate, and all answers are equally valid in some sense.
Perhaps you need to learn that the "bit rate" values used in deriving an N'th percentile value are first calculated by counting the number of octets that crossed an interface since the last sample was taken and dividing by the amount of time since that last sample was taken (and then adjusting with a multiplier for different units, eg. octets vs. bits or whatever). In other words the bit rate values are taken as the average rate over the specified sample time. No data is thrown away or ignored -- every single byte is counted and every count is critical to finding the correct N'th percentile value. There's absolutely nothing in the way of synchronisation required and indeed there's no such thing as a "burst pattern" when you consider that at any given instant in time an octet will cross a (to pick a specific example) 10-mbit interface at ten megabits per second! How else can you imagine measuring the bit rate utilisation of a fixed-rate pipe? The same N'th percentile measurement can always be calculated from either end of a pipe so long as the sample interval is the same at both ends, and so long as the pipe has no (measurable) loss. If there's measurable loss then you'd better measure it and take it into account or else you will end up with unfair billing. In fact the very same octet-count measurements are needed for any kind of usage-based billing. The only difference with N'th percentile metering is that the sample time needs to be short enough to catch user-noticable bursts (i.e. to avoid averaging out bursts that were they to be flattened out to the average rate would be noticable to the user). For most currently used IP services this might be somewhere between 5 seconds and 60 seconds. For straight bulk throughput billing you only need to sample often enough to aoivd missing counter roll-over or counter reset events. -- Greg A. Woods +1 416 218-0098 VE3TCP <gwoods@acm.org> <woods@robohack.ca> Planix, Inc. <woods@planix.com>; Secrets of the Weird <woods@weird.com>