It’s the Speed of Light, Dude!

At the recent Sharkfest conference where Apposite was exhibiting our PCAP replay feature, a number of people walked up to our booth and said, “You’re the dudes who inject latency to simulate all the router hops along the network?”
Well… kind of. We certainly make products that inject latency to simulate WAN links, but the latency is primarily caused by the speed of light.
The Speed of LightMost people refused to believe us. “The speed of light is, like, close to infinite, dude. It’s all the routers holding up the packets,” was the common reply.
And our answer: “Nope, it’s the speed of light.” Router latency is on the order of tens of microseconds, not milliseconds (though to give them partial credit, they probably meant queuing delay which can be significant if there is any congestion).
Indeed, the speed of light is pretty darned fast, so fast it’s awfully hard to measure. On human scales, it’s essentially infinite. Of course everyone knows it takes years for light to reach other stars, but they’re zillions of miles away. New York we can drive to from here without a rocket-propelled spaceship, although my new car can go pretty fast if there are no cops around.
But at computer scales over continental distances, the speed of light is significant and is the immediate cause of WAN delay. Let’s take the example of San Francisco to New York City. With a straight shot, that’s a distance of approximately 3000 miles or 5000 km. And the speed of light through fiber is approximately 2000 km / sec, or 5 ms per 1000 km (note that this is a good deal less than the 3000 km/sec speed of light through a vacuum cleaner.) So SF to NY takes 25 ms, and NY back to SF is another 25 ms, for a round-trip latency of 50 ms.
Of course, backbone networks are not a straight shot, and the actual path, even a relatively straight one, may be closer to 6000 or 7000 km in each direction for a round trip latency of around 70 ms. Add in some transmission delay and maybe some queuing delay along the way for a typical value of 100 ms RTT. But the majority of that is the speed of light. Dude.

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