Technical data

Configuring and Managing NTP
12.1 Key Concepts
12.1.3 How the OpenVMS System Maintains the System Clock
The OpenVMS system clock is maintained as a software timer with a resolution of
100 nanoseconds, updated at 10-millisecond intervals. A clock update is triggered
when a register, loaded with a predefined value, has decremented to zero. Upon
reaching zero, an interrupt is triggered that reloads the register, thus repeating
the process.
The smaller the value loaded into this register, the more quickly it reaches zero
and triggers an update. The clock runs more quickly in such an instance. A
larger value means more time between updates; therefore, the clock runs more
slowly. A clock tick is the amount of time between clock updates.
12.1.4 How NTP Makes Adjustments to System Time
Once NTP has selected a suitable synchronization source, NTP compares the
source’s time with that of the local clock. If NTP determines that the local clock
is running ahead of or behind the synchronization source, NTP uses a general
drift mechanism to slow down or speed up the clock as needed. NTP accomplishes
this by issuing a series of new clock ticks. For example, if NTP detects that the
local clock is drifting ahead by +0.1884338 second, it issues a series of new ticks
in an effort to reduce the difference between the synchronization source and the
local clock.
If the local system time is not reasonably correct, NTP will not set the local clock.
For example, if the new time is more than 1000 seconds off in either direction,
NTP does not set the clock. In this case, NTP logs the error and shuts down.
NTP maintains a record of the resets it makes along with informational messages
in the NTP log file, TCPIP$NTP_RUN.LOG. See Section 12.6 for more details
about event logging and help in interpreting an NTP log file.
12.1.5 Configuring the Local Host
As the system manager of the local host, you determine which network hosts to
use for synchronization and populate an NTP configuration file with a list of the
participating hosts.
NTP hosts may be configured in one or more of the following modes:
• Client/server mode
This mode indicates that the local host wants to obtain time from the remote
server and is willing to supply time to the remote server if necessary. This
mode is appropriate in configurations involving a number of redundant time
servers interconnected through diverse network paths. Internet time servers
generally use this mode.
Indicate this mode with a
peer declaration in the configuration file. For
example:
peer 18.72.0.3
• Client mode
This mode indicates that the local host wants to obtain time from the remote
server but it is not willing to provide time to the remote server. Client mode
is appropriate for file server and workstation clients that do not provide
synchronization to other local clients. A host with higher stratum generally
uses this mode.
Configuring and Managing NTP 12–3