How billing on the 95th percentile works is covered in 95th percentile billing explained: a measurement every five minutes, the highest 5% drops out, and the commit is what you agree to. The model is fair, but it has one drawback. The final figure is only fixed at the end of the period, and if that is the first time you look, you discover an overrun on the invoice. This article is about the weeks in between: what you see, when we warn you and how to read an interim figure.
An interim figure is not a forecast
The misconception is that the 95th percentile halfway through the month is a good estimate of the final figure. It is not, for an arithmetic reason. The 95th percentile throws away the busiest 5% of the measurements. Over a whole month that is about 36 hours; after two days it is not even 2.5 hours. One busy evening at the start of the period therefore weighs heavily, and only fades as the month goes on.
Early in the period the interim figure is therefore often higher than the final one. It can go the other way too: a quiet first week followed by a busy last week ends higher than it looked halfway. So read an interim figure as a direction, not an outcome, and all the more carefully the younger the period is.
What you see in the dashboard
Under Network → Network traffic you find the graph of your transit traffic over the current period, with below it:
- Use of your commit so far — the 95th percentile of this period as a percentage of your commit, with a bar that fills up.
- Your commit — as it is in your contract. We take that value from our accounts every day, so what you see here is what the invoice uses too.
- Overage so far — only shown once you are over it.
Previous period lets you look back one period. That is the comparison that matters most: not "where are we now", but "is this pattern above or below last month".
A period follows the invoice date of your contract, so it does not always coincide with the calendar month. Go by the period in the dashboard rather than the date on the calendar.
When we warn you
You do not have to keep checking the page. As soon as the 95th percentile of the running period reaches 80 percent of your commit, the account owner gets a mail, and once more when it passes 100 percent. The mail gives the percentage, the usage and your commit, and a link to the traffic page.
Two choices are deliberate:
- Not in the first days — for the reason above. A 95th percentile over two days is mostly your peak, and a warning on it would usually be a false alarm. We only start looking from the third day of the period.
- Once per threshold — per period at most one mail at 80 and one at 100 percent. Someone hovering around 85 percent all month does not get the same message every day.
The percentage in the mail is about the busiest direction, inbound or outbound. That is also the direction billing uses.
What to do with a warning
A mail at 80 percent is not an alarm, but a moment to take a look. Three questions help:
- Is this an outlier or a trend? — compare with the previous period. A migration, a campaign or a one-off large transfer pushes the figure up without anything changing structurally.
- How far into the period are we? — on day four, 80 percent says less than on day twenty.
- Is it traffic you want? — traffic you do not expect is always worth a look. An attack shows up separately in the DDoS overview.
If you go over once, you pay overage according to your agreement. For a single month that is often cheaper than a higher commit you pay every month. If your traffic grows structurally, a higher commit is usually better value; that is a conversation we would rather have up front than afterwards on the invoice.
IP transit at Xyphen IT
The traffic page and the warnings come with every IP transit connection with a commit. Unsure whether your commit still fits, or want to work out an upgrade? Get in touch or request a proposal.