[funsec] [email@example.com: [Outages] Theoretical vs
real-world speed limit of Ping]
ge at linuxbox.org
ge at linuxbox.org
Sun Jun 3 05:32:20 CDT 2007
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From: virendra rode // <virendra.rode at gmail.com>
To: outages at isotf.org
Subject: [Outages] Theoretical vs real-world speed limit of Ping
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I thought some of you might be interested in this stats.
Theoretical response time limits of Ping
So, if we assume that a ?ping? travels with the speed of light, which
are the best possible response times we can get?
10 km -> 0,067 milliseconds
100 km -> 0,67 milliseconds
1,000 km -> 6,7 milliseconds
10,000 km -> 67 milliseconds
(Note that Ping shows the time for a roundtrip, i.e. 2 x the distance.)
Pinging between Europe and USA
The distance between New York and Paris is 5,839 km. For light to travel
from New York to Paris and then back again would take 40 milliseconds.
Maximum distance between two places on Earth
If you choose the shortest route, the maximum distance between two
locations will never be more than halfway around the planet. Halfway
around Earth is about 20,000 km.
Considering that Ping goes to a destination and then back again, the
packet sent by Ping would travel 40,000 km, the equivalent of a trip
That is 133 milliseconds.
Anyone who has tried to ping various servers across the world will know
that this is a way better response time than what you can realistically
get. So why is ping so slow?
Why Ping is slower in the real world
The examples above take place under ideal conditions that don?t exist in
the real world.
1. The actual distance traveled will be longer, more like zig zag
than a straight line.
2. Repeaters, switches and routers will slow down transfer speeds.
The more equipment the signal has to pass through (for example routers),
the longer it will take to reach its target.
3. The actual speed of the signal will never quite match the speed of
light. Even with fiber optics (glass) the speed of light is about 30%
slower than through vacuum or air, and most of the distance covered will
be through fiber.
4. A beam of light bounces around like a pinball when it travels
along the thin (admittedly extremely thin) fiber, and that will of
course make the distance traveled even longer.
With all this in mind, you should probably double the ?ideal? response
times shown above for a more realistic target to aim at. It?s useful to
know when there is room to push for better network performance, and when
the actual physical limits set in.
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