Re: How to game the system (was Re: What does 95th %tile mean?)
On Thu, 19 April 2001, Greg A. Woods wrote:
From the provider's perspective this doesn't really matter a whole lot and they can reduce the effect of your bursts on their other customers by simply narrowing their window. There's no need to do silly sliding windows, multiple windows, etc. -- the averages are just fine.
From a provider's perspective it does matter. The most costly part of any network is the last 1% because most of the fixed cost of the network is based on the peak capacity, not average capacity. Billing based on average usage works best if you have a large population and excess capacity. Generally its the largest of the large customers who have the most to gain by gaming the system. A few T1 customers gaming the system is just noise.
But you might have an OC48 customer who pegs their link requiring you to add more peak capacity to your network. But times their traffic, so their average usage never exceeds an OC24, and pays the average price. This is the type of customer which nailed the electric companies. If you can save tens of thousands dollars by starting an electric smelter at 30 minutes past the hour instead of exactly on the hour, its noticable. The electric companies reserve their fanciest meters for large power consumers like smelters and colo providers :-) This was fine in the past, when we were doubling the Internet's capacity every X days. I have a feeling providers will start managing/reducing the excess capacity headroom in their networks. And will start looking at ways to wring more money out of their current customers. Some rocket science accountant somewhere should be working on a way to maximize the chargable usage by just shifting the window bins back and forth until he finds the maximum. Is there any commercial provider not trying to maximize billable usage? By carefully choosing window bins, a provider could generate 1%, 25%, 50% more revenue across all his usage-based customers. And if you got this idea from me, I expect 5% of the take....
[ On , April 19, 2001 at 21:35:30 (-0700), Sean Donelan wrote: ]
Subject: Re: How to game the system (was Re: What does 95th %tile mean?)
From a provider's perspective it does matter. The most costly part of any network is the last 1% because most of the fixed cost of the network is based on the peak capacity, not average capacity. Billing based on average usage works best if you have a large population and excess capacity. Generally its the largest of the large customers who have the most to gain by gaming the system. A few T1 customers gaming the system is just noise.
Hold on a moment here. We cannot eat our cake and have it too! If an ISP is billing by 95th percentile peak bandwidth usage then they cannot, by definition, be billing by 100th percentile peak bandwidth usage at the same time (or even 99th percentle peak)! That last 1%, or even 5% must get lost in the noise lest you over-charge your customers! Indeed when the customer's pipe is much larger than his required capacity a Nth percentile metering scheme is the only known fair way of defining what the peak usage is! Secondly if you don't have any excess upstream capacity then it doesn't matter how hard any customer tries to pump, or pull, full line rate through their port -- it's just not going to happen. When you're upstreams are flat-lined then everyone gets his fair share of what meager pickings are to be had and everything else either gets dropped on the floor or is flow-controlled by higher level protocols. Obviously 95th percentile peak usage billing works best for the ISP when that's how your upstreams bill you too because then you'll likely have excess upstream capacity, almost by definition. That way you make sure you get paid for the peak usage you'll be paying for. However even if your upstreams are already pegged (and you're not buying more bandwidth because the numbers just don't make it affordable) then it'll still work out OK (though you'll undoubtably be wanting to adjust your prices so that you can afford more capacity). In the "pegged pipe" scenario your customer's can't draw blood from a stone, not even the largest of the large. That's what makes Nth percentile billing fair for both parties, in fact -- they don't pay for what they can't get no matter how fat a pipe you hand them. On the other hand if a customer's trying to avoid paying for 100mbit of usage even though all he could get from you was a 100mbit ethernet port then he's almost infinitely more likely to use some form of rate limiter (even if it's just ALTQ in a BSD router) to even things out properly than he is to try and play games with some form of window juggling against your counter sampler. A real rate limiter will adjust the packet rates in such a way that higher level protocols won't complain or suffer from packet loss, etc.. Heck unless your game-playing customer is going to write some custom application to transfer bulk data using UDP or similar they're only going to do themselves more damage than good by bursting their pipe on and off to try to fool any sampling window. Indeed you can probably even make your customers happier by selling them a proper rate limiter that they can control! In any case, just as I said, if your sample rate is around 5 seconds or so then there's not bloody much anyone can do to push more data through any pipe in such a way that they'll fool your rate calculations and that'll adversely affect any other customers, unless maybe you've made the mistake of selling a 100mbit pipe to someone who really does want at least 50mbits at peak times and you've only got two or three other customers total who only want a megabit or so each. That's about the only scenario where 50% sample-rate traffic bursts would cause any noticable effect on other customers. Even in that case those two or three tiny customers are such a small part of your business that you just can't be bothered to care about their streaming sessions looking a little jerky. In this case you can probably even give your one big fish a real rate limiter just to make everyone happier all around! If you can't afford to sampe every five seconds then do it every minute and change your Nth percentile to 97 or 98 instead of 95!
But you might have an OC48 customer who pegs their link requiring you to add more peak capacity to your network. But times their traffic, so their average usage never exceeds an OC24, and pays the average price. This is the type of customer which nailed the electric companies.
That just does not compute. If their average use is OC24 then it does not matter whether they push/pull OC48 for 2.5 seconds and zero for 2.5 seconds, or OC24 for 5 seconds. It all gets lost in the noise because either way your other customers still get to use the other half of that OC48's worth of bandwidth. You do not have to buy more peak capacity for your upstreams if the peaks are only 50% of the width of your sample period, at least not if your sample period is less than 5 minutes or so. This is the Internet -- it's not a circuit-based telephone connection network, and it's not ATM (though either may be used to transport IP, of course). It's certainly not equivalent to a power distribution grid where excess demand starts to look like a short circuit and causes real damage. The traffic on the Internet is little bunches of packets that can interleave between each other. Real-world traffic flows on IP networks are incredibly variable and resilient, partly due to the heroic efforts of higher level protocols such as TCP. -- 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>
I have a feeling providers will start managing/reducing the excess capacity headroom in their networks. And will start looking at ways to wring more money out of their current customers. Some rocket science accountant somewhere should be working on a way to maximize the chargable usage by just shifting the window bins back and forth until he finds the maximum.
This would work well for end-users. Buy outbound time on inbound-heavy networks, and vice-versa. Route updates are too slow to move the content around, otherwise a really savvy and cheap buyer could keep thier "current" server in the dark part of the planet full time and get excess bandwidth for next to nothing all day. -- Eric A. Hall http://www.ehsco.com/ Internet Core Protocols http://www.oreilly.com/catalog/coreprot/
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