Reading the looking glass: what RTT actually tells you

Light in fibre covers roughly 200 kilometres every millisecond, so a round trip is close to the distance divided by a hundred. That is the whole model behind the looking glass: great-circle distance, switching overhead, and a small allowance for the fact that routes are never straight.

It predicts one thing very well — which of our core sites you should ask for. If Helsinki comes back 40 ms lower than Frankfurt for your site, that ordering will hold in the real world even if the absolute numbers move by a few milliseconds.

What it cannot see is your own transit: a congested peering point or a provider that routes your traffic the long way round will not show up until we probe from your prefix. That is free and takes an afternoon — ask the NOC before you sign anything with an SLA attached.