Unix Tip: What is NTP?

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For some of us, the word "synchronize" brings up memories of the characters
on Mission Impossible synchronizing their watches so that they can complete
a seemingly impossible mission with split-second accuracy. While time
synchronization on computer systems seems far less dramatic, it is no less
critical. Time differences between systems running a complex application can
lead to some very troublesome problems -- such as when these time differences
make transactions appear to have happened out of order. To avoid this kind
of problem, a number of solutions for keeping system clocks in synch, have
been devised. NTP is the larges scale and most effective of these.

NTP, however, isn't just about synchronizing systems with each other. NTP is
about setting system clocks accurately. When derived from the most accurate
clocks in the world, after all, the time on systems on a network -- all
getting time ticks from an accurate time keeper -- will be both accurate and
synchronized.

Time Accuracy

Before we delve into the network time protocol, let's consider what it means
for a clock to be accurate and just how accurate a clock can be.

The most accurate time measurement devices available today are atomic clocks.
More reliable than the movement of the stars, these clocks rely on the
oscillation of the nucleus and electrons of an atom to measure a unit of time.

To get a picture of what time accuracy means, let's consider a simple analogy.
When I was a kid, I was told that the way to measure seconds was to preceed
each number with the word "Mississippi". So, I would count "Mississippi-1,
Mississippi-2, Mississippi-3" and so on. Just counting "one, two, three" and
so on wouldn't be accurate, my teachers told me, because I would say these
words too quickly making each of my seconds far too short. Mississippi worked
quite well for me. In fact, I sometimes still count seconds in this way. If
I count to 20 using the Mississippi Method, I am probably not off more than
5-10%. So, at the end of my alleged twenty seconds, maybe only 18 or as many
as 22 seconds have actually passed. That's fine for hide and seek, but it
probably doesn't cut it when we're timing synapses in the brain to analyze
some nervous system dysfunction or tracing the sequence of operations in a
security attack on our network.

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