Another application for this driver is if a particular server clock is to be used as the clock of last resort when all other normal synchronization sources have gone away. This is especially useful if that server has an ovenized oscillator. For this you would configure this driver at a stratum greater than any other likely sources of time (say 3 or 4) to prevent the server taking over when legitimate sources are still available.
A third application for this driver is when an external discipline source is available, such as the NIST lockclock program, which synchronizes the local clock via a telephone modem and the NIST Automated Computer Time Service (ACTS), or the Digital Time Synchronization Service (DTSS), which runs on DCE machines. In this case the stratum should be set at zero, indicating a bona fide stratum-1 source. In the case of DTSS, the local clock can have a rather large jitter, depending on the interval between corrections and the intrinsic frequency error of the clock oscillator. In extreme cases, this can cause clients to exceed the 128-ms slew window and drop off the NTP subnet.
In the case where a NTP time server is synchronized to some device or protocol that is not external to the NTP daemon itself, some means should be provided to pass such things as error and health values to the NTP daemon for dissemination to its clients. If this is not done, there is a very real danger that the device or protocol could fail and with no means to tell NTP clients of the mishap. When ordinary Unix system calls like adjtime() are used to discipline the kernel clock, there is no obvious way this can be done without modifying the code for each case. However, when a modified kernel with the ntp_adjtime() system call is available, that routine can be used for the same purpose as the adjtime() routine and in addition provided with the estimated error, maximum error, and leap-indicator values. This is the preferred way to synchronize the kernel clock and pass information to the NTP clients.
In the default mode the behavior of the clock selection algorithm is modified when this driver is in use. The algorithm is designed so that this driver will never be selected unless no other discipline source is available. This can be overridden with the prefer keyword of the server configuration command, in which case only this driver will be selected for synchronization and all other discipline sources will be ignored. This behavior is intended for use when an external discipline source controls the system clock. See the Mitigation Rules and the prefer Keyword page for a detailed description of the exact behavior.
The stratum for this driver is set at 3 by default, but can be changed by the fudge configuration command and/or the ntpdc utility. The reference ID is LCL by default, but can be changed using the same mechanisms. *NEVER* configure this driver to operate at a stratum which might possibly disrupt a client with access to a bona fide primary server, unless the local clock oscillator is reliably disciplined by another source. *NEVER NEVER* configure a server which might devolve to an undisciplined local clock to use multicast mode.
This driver provides a mechanism to trim the local clock in both time and frequency, as well as a way to manipulate the leap bits. The fudge time1 parameter adjusts the time (in seconds) and the fudge time2 parameter adjusts the frequency (in parts per million). Both parameters are additive and operate only once; that is, each command (as from ntpdc) adds signed increments in time or frequency to the nominal local clock time and frequency.
Reference Clock Drivers