Technical data

5.1.1 Time Distributed Through a Hierarchy of Servers
In the NTP environment, time is distributed through a hierarchy of NTP time
servers. Each server adopts a stratum that indicates how far away it is operating
from an external source of UTC. NTP times are an offset of UTC. Stratum 1
servers have access to an external time source, usually a radio clock. A stratum 2
server is one that is currently obtaining time from a stratum 1 server; a stratum
3 server gets its time from a stratum 2 server, and so on. To avoid long-lived
synchronization loops, the number of strata is limited to 15.
Stratum 2 (and higher) hosts might be company or campus servers that obtain time
from some number of primary servers and provide time to many local clients. In
general:
• Lower-stratum hosts act as time servers.
• Higher-stratum hosts are clients that adjust their time clocks according to
the servers.
Internet time servers are usually stratum 1 servers. Other hosts connected to an
internet time server have stratum numbers of 2 or higher and may act as time
servers for other hosts on the network. Clients usually choose one of the lowest
accessible stratum servers from which to synchronize.
5.1.2 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, and repeats the
process.
The smaller the value loaded into this register, the more quickly it reaches zero
and triggers an update. In such an instance, the clock runs more quickly. A larger
value means more time between updates; therefore, the clock runs more slowly.
The amount of time between clock updates is known as a clock tick.
5.1.3 How NTP Adjusts 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 to
reduce the difference between the synchronization source and the local clock.
If the local system time is not reasonably correct, NTP does 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. For more details about event logging
and for help interpreting an NTP log file, refer to the Compaq TCP/IP Services for
OpenVMS Management guide.
Fore information regarding operating system and daylight saving time issues, refer
to the OpenVMS documentation set.
5–2 Network Server Services