What is NTP?

ITworld.com, Unix in the Enterprise 10/7/04

Sandra Henry-Stocker, ITworld.com



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.

What we need to measure a unit of time with great accuracy is a some mechanism for measuring each second so closely that, at the end of a week, month or year we still wouldn't be off by more than a couple seconds. One way to do this is by finding something that occurs in time with extremely reliable precision. The time it takes me to drive to work? Not even close. The time it takes a golf ball to drop from the top of the Empire State Building to the street below? Better, but dangerous. The time that it takes an atom to complete an oscillation? Ah, we're getting there!

The particular frequency of the oscillations in an atomic clock is determined by the mass of the nucleus and the gravity or "electronic spring" between the nucleus and the electron cloud around it. These oscillations are reminiscent of the pendulum of a grandfather clock, but they happen at an alarming frequency -- more than 9 billion times per second.

While atomic clocks can use cesium, hydrogen or rubidium atoms, the cesium clocks are the most accurate. Some of these clocks are accurate to five parts in ten to the 15th power. That's five parts in 10,000,000,000,000,000. That isn't perfect, of course. But if we lose one second in 6 million years, not many people are going to notice.

Synchronizing Networks



NTP, which stands for "network time protocol", synchronizes the clocks on systems and routers all over the world. The National Institute of Standards and Technology (NIST) estimates that there are between 10 and 20 million NTP servers and clients in the world.

Synchronized to an NTP server, systems on a network are as close in time as their connection to their source of accurate time information allows. On a wide area network (WAN), systems may be tens of milliseconds off. On a local area network, the difference may be much smaller than a millisecond.

Atomic Clocks and NTP Servers



Most systems running NTP -- either an NTP daemon such as ntpd or a command such as ntpdate which adjusts the system time when it is run -- do not synch up to an atomic clock but to an NTP server that itself gets accurate time information from an atomic clock or another NTP server. In order to make the NTP system work well, servers fall into a hierarchy in which each tier of servers is further from the original source of timing information.

Primary and Secondary Servers



Primary servers are those that synchronize to the time standards using radio, satellite or modem. Secondaries synchronize to the primaries. Tertiaries synchronize to the primaries and so on.

Secondaries allow thousands of systems to synchronize, keeping them from overloading the primary servers and distributing accurate time to the much of the Internet.

Organizations connected to the Internet can then, in turn, set up tertiary servers to provide time information to their own networks.

What time is it?



If you'd like to know what time it is in any state in the US right now, you can browse over to http://www.time.gov and click on the state of your choice. Of course, with only eight time zones to choose from (the map also includes places like Guam and Puerto Rico), you don't have to click on your own particular state -- just any state in the correct time zone. Sandra Henry-Stocker has been administering Unix systems for nearly 18 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She currently works for TeleCommunication Systems, a wireless communications company, in Annapolis, Maryland, where no one else necessarily shares any of her opinions. She lives with her second family on a small farm on Maryland's Eastern Shore. Send comments and suggestions to sstocker@itworld.com.