DrPeering - 

Is there a way to guesstimate 95th percentile from average utilization?

Andy Bernard

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Mr. Bernard -

Most wholesale transit (and some CDN) services are metered on the 95th percentile. The typical measurement method is to take 5-minute samples across a month, rank the deltas from lowest to highest, and pick the 95th percentile measurement to calculate the monthly transit fee. This method allows the customer to burst the top 5% without paying for it.

I have spoken with hundreds of network engineers over the decade about traffic management and one observation comes up time and again:

The 95th percentile rule of thumb: Internet traffic tends to follow a sinusoidal curve such that 95th percentile is about 2 to 2.5 times the average.

Caveat: Of course there are a variety of traffic patterns that can skew the 95th percentile # higher or lower. For example, one scrappy Tier 2 ISP described a scenario where she would burst for less than 5% of the month and the rest of the month she would simply send/receive traffic to her commit rate. That is one extreme, where the 95th percentile tends towards the lower end of the traffic utilization spectrum. The other extreme is when the pipe is always saturated, so 95%=Average.

For most folks, this 95th percentile rule of thumb won't be too far off.

Dr Peering

DrPeering@DrPeering.net