Tuesday, 25 October 2011

Cannot trust WMBC policers...


This is one for the techies, and mainly for other ISPs, but I'll try and explain it anyway

We buy bandwidth to link to our favourite telco for broadband services. We have to commit to a level of usage in advance for which we pay. We can then use up to that level for that price. We try not to be the bottleneck in our services where possible, but there are events where demand exceeds the level we have. The IOS5 update was an example where we would have needed about 50% extra capacity for a couple of hours. We could allow cases like this to simply use the extra, it the problem is that not only do we pay extra per megabit when that happens, we have to pay for a whole month's worth even if we only use it for 15 minutes. Therefore, like most ISPs, we limit the bandwidth to the commit level, but we increase the level regularly as demand increases generally.

The problem is how to manage the usage so as not to exceed the agreed level. It is different levels for different services on the same link, which makes it complicated. We can do this on the FireBrick. But many ISPs have to rely on our favorite telco to manage the bandwidth for them. They will police the usage for you, and you can choose to police at the commit level, or, say, allow 5% burst or 10% burst or even more if you like. Even though we have our own policers we use them as a safety net setting a 5% burst limit. Many ISPs will set to not allow any burst on the basis that they are charged for a whole month for as little as 15 minutes, so may as well just buy more commit in the first place than make use of bursting.

When this crazy scheme was first announced during the trial we immediately gave them feedback saying they would have a problem if they could not synchronise the policer change with the billing. If they increase the policer one day and the billing later, then they would assume you were incorrectly bursting. They ignored us and we got unexpected burst charges for exactly this, even for over 5% which should not be possible if we have set that as the limit. Bills still in dispute for many months.

So, how do they fix this? Do they start changing the bills before the policer? No, they add a note to the email saying the policer has been changed now, but not to use any extra bandwidth until 11pm as doing so will be charged as burst.

So that means ISPs can no longer rely on our favourite telco to police the service for them. If they up the bandwidth there is a good chance they will get usage to the new limit before the billing change and so pay for a whole month of burst changes.

So this makes the whole point of them policing for you a waste of time. Every ISP has to have their own policers or face expensive burst charges.

All they had to do is ensure billing before policer change for increases (and other way for decreases) and it would be fine.

Arrrrrg!

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