|
North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: 95th Percentile again (was RE: C&W Peering Problem?)
On Sat, Jun 02, 2001 at 10:15:35PM -0400, Richard A. Steenbergen wrote: > On Sat, 2 Jun 2001, Joe Abley wrote: > > > On Sat, Jun 02, 2001 at 05:28:52PM -0400, Timothy Brown wrote: > > > As an interesting aside to this discussion, Digital Island bills for > > > total traffic transmitted per month (in GB increments). Does anyone > > > using them have any comments on this approach besides the obvious? Does > > > anyone else do a similar deal? > > > > This may be obvious, but billing by volume (bytes transferred) places far > > greater availability requirements on the measurement system than rate-based > > charging schemes. > > > > If I am charging by the byte, I have to count every packet. If my measurement > > system breaks, I lose money until it is fixed. > > > > If I am charging by the 95%tile of five-minute average throughput > > measurements obtained during a calendar month, I can make do with much > > more coarse-grained sampling. Measurement system breaks, I'm quite > > possibly going to bill the same amount as if it hadn't broken. > > No, you are confused. No, just viewing the world from a strange perspective :) > A rate based billing system polls a byte counter on > a switch or router at set intervals (ex: every 5 mins), subtracts the > previously recorded value, and divides by the number of seconds in that > interval. Yes. I referred to the result of that calculation as the "five-minute average throughput measurement", but I was being more general about the mechanics of measurement -- in some cases there are no counters to poll (see below). > If the polling system cannot reach the device it is monitoring, > samples can be missed, this is a very old problem of rate-based > monitoring. Every rate-based system of which I am aware orders these > "samples" to calculate 95th percentile, so a missed sample is equivilent > to a 0 sample. No. If you are missing a "five-minute average throughput measurement" for some reason, you just have fewer samples to sort at the end of the month. Chances are you still have a reasonable approximation of the 95%ile sample value, if you don't miss too many. > A rate can be interpolated for the missing time, I agree, that would be yucky. > [...] > > A volume based billing system on the other hand, could theoretically poll > only once a billing period. In reality it would probably poll more often, > both to keep the customer apprised of their currently used amount, and to > prevent the possibility of counter rollovers, but it would never "miss" a > billing sample. If you have bytes-in/bytes-out counters to poll, then you're totally right. [you also have to deal with counters being reset to zero due to mysteriously exploding router issues]. There are cases where there are no such counters, however, such as customers who obtain transit through a shared ethernet/FDDI/ATM interface, and where equivalent counters are not available at layer-2 (e.g. someone else runs the switches, switches suck, etc). The last time I worried about this we were using an ATM network to aggregate customer PVCs, and it was not possible to obtain per-PVC stats from the routers or the switches, for various disgusting reasons. We were carrying sufficiently little international transit traffic (this was NZ) that we were able to make measurements using NeTraMeT meters to sniff-n-count all international traffic through span ports on ethernet switches. In such an environment, billing 95%ile reliably is easier than billing volume accurately. Joe
|