Before you configure NTP, you must be aware of some basic computer clock and NTP-related concepts.
This section describes how computers keep time. This is a high-level introduction and is not meant to be all inclusive.
When the system is not running the Solaris OS, the time-of-day chip maintains basic 24-hour time. This time is copied into a 64-bit counter used by the kernel to maintain 24-hour time for a running system.
Sun systems use a combination of an oscillator and a 64-bit counter to keep track of time. A specific number of oscillations cause an interrupt that, if processed, will cause the counter to increment.
The Sun system central processing units (CPUs) generate the regular interrupts. By default, 100 interrupts are generated per second. For the system’s counter to increment, the CPUs interrupt must be processed by the kernel. Each interrupt that gets processed is known as a clock tick. However, not all interrupts get processed. This is often due to high system loads and higher priority tasks that take precedence within the kernel. Therefore, gradually, a clock will fall slightly behind because not all time interrupts are processed. However, the controller boards in Sun FireTM 12k to 25k high-end servers use a real-time clock, not the normal 100 interrupts per second method. This makes them excellent NTP servers, since the clock does not drift as it does on a regular server or workstation. However, making them an NTP client can cause issues with the SMS software.
| Note: The 32-bit time counter would reach its limit in the year 2038. The 64-bit time counter was started at 0 at midnight, January 1, 1970 Greenwich Mean Time (GMT). The counter will reach its limit in about 290 million years. |
Variation in the frequency of the oscillator and delays to the kernel interrupt routine cause clock drifts. NTP disciplines the system clock frequency and time, producing more accurate timing mechanisms for the system.
Many network applications need synchronized clocks to properly function. For example:
- Encryption - This application often uses time as a component of encryption keys.
- Network management - This application uses time to determine exactly when something took place.
- Logging - The syslog facility uses time to display system events.
- File systems - Applications time stamp files when they are created or modified. Many backup applications are configured to use time as a criteria for determining backups, so that clock synchronization between the backup server and other systems is important.
- Cluster Nodes - Individual nodes in a Sun Cluster configuration use NTP to ensure that they all agree on the time.
Several terms are used when describing time-related topics. These terms are described in The table.
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NTP Terms |
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Term |
Description |
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Reference clock |
A clock that provides current time by accurately following a time standard, such as Coordinated Universal Time (UTC). |
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Strata |
NTP servers are arranged in a hierarchy of levels, called strata. A stratum-1 server is more accurate than a stratum-10 server. There are 16 strata. |
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Stratum-1 server |
A highly available NTP server that has its own reference clock. |
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Resolution |
The smallest increment in time that a clock offers. For example, a wristwatch usually has a resolution of one second. |
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Precision |
The smallest increase in time that a computer program can use. |
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Jitter |
The difference of the differences experienced when repeatedly measuring time. |
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Accuracy |
How close a clock follows an official time reference, such as UTC. |
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Reliability |
The length of time that a clock can remain accurate within a specified range. |
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Wander |
All clocks suffer from frequency variations. This variation is called wander. |
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Drift file |
A file that contains the frequency offset of the local system’s clock oscillator. Drift file contents can be used by protocols, like NTP, to cause a system’s clock to be more accurate. The default location for Sun’s NTP drift file is /var/ntp/ntp.drift. |
xntpd |
The NTP daemon. |
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The ntp.conf file |
A file that causes the xntpd daemon to start in either the client or the server mode and provides configuration statements that control the behavior of the xntpd daemon. |
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The fudge command |
You can use the fudge command in the ntp.conf file as a keyword to configure reference clocks in special ways, such as defining calibration constants to force a time offset to a particular external-time standard. |
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Discipline |
A general term used for various actions carried out by some protocol, which helps keep a local clock better synchronized to an official time source, such as UTC. |
