29 Sep

95th Percentile Monitoring Explained

When shopping for Internet bandwidth, you will find that the most common billing method is the so-called 95th Percentile Monitoring, or more simply,  the 95% Method. So what does that mean? To illustrate, we’ll use the following “monthly view” traffic graph, which shows the traffic in an out of a network interface for a 4-5 week period:

mrtg monthly - 95th Percentile Monitoring Explained

As you can see, this graph has the data rate on the vertical axis, and the passage of time on the horizontal access. About five weeks worth of data is graphed here both in (green) and out (blue) of the interface, with  the sample rate consolidated from the industry-typical five minutes intervals down to two hour intervals for this graph.

So how would a service provider compute bandwidth usage for the month, using this data? You might assume that it would be fair to add the Average In and Average Out values (remember, internet traffic travels in both directions) – a total of 36.3 Mbps in this case – and declare that to be the official usage for the month shown. In fact, many data centers will do just that. However, there is another way: the 95th Percentile Method. Here’s how it works:

  • A log of 30 days worth of traffic samples at the default sample frequency (every 5 minutes is typical) is accumulated;
  • The log is sorted in descending order, which places the biggest traffic peaks at the top;
  • The top 5% of the log data is discarded;
  • The value of the largest remaining peak becomes the effective bandwidth usage value for the month.

The advantage of the 95th Percentile method is that, over the course of a 30 day ‘month’ you can have up to 36 hours of peak traffic that will be completely ignored for billing purposes, no matter how high the peaks are. In the example of the monthly graph given above, the actual 95th Percentile traffic number for that data is 25.7 Mbps; 10.6 Mbps lower than using the ‘sum of the averages’ method. Additionally, for fractional commits on larger bandwidth pipes (i.e. 1o0Mbps on a GigE interface), a substantial amount of transfer above one’s prepaid allotment is essentially free for the taking each month – many Terabytes in the case of a GigE connection!

As you can see, 95th Percentile Monitoring can be quite advantageous when getting Colocation bandwidth.