Most Unix and Linux operating systems insert leap seconds by repeating the last second of the day. This can cause problems with some software. Leap smearing involves slowing clocks for a period of time before and after the actual leap second. This prevents leap seconds from being potentially disruptive events.
However, leap smearing servers will provide a slightly different time to other servers during the insertion period. For this reason, Google recommends that its public NTP servers are not used in conjunction with other non leap-smearing servers.
Open the Control Panel. If so, it will synchronize to the domain controller. In this instance, you will need to configure the domain controller to synchronize with an NTP server. The ntpd daemon is configured from a configuration file ntp. Note: Remember that it is not recommended to configure smeared and non-smeared NTP servers. You should avoid excessive use of public NTP servers. Only query servers at reasonable intervals. This may vary from once a day to a few times an hour, depending on your system requirements.
NTP clients should never be configured to request time from a NTP server more frequently than once every four seconds. Clients that exceed this rate may be flagged as attempting a denial of service DoS attack and may be refused service. It is generally accepted good practice to choose a relatively local time reference.
The Windows event viewer application can be used to inspect such log entries. To find out which process has that specific process ID, you can for exampleopen a PowerShell command line window and type the command.
So whenever ntpd had to set the system time you find an associated system log entry with the process ID of ntpd mentioned in the event details. Similarly, if another process has set the system time you can identify that process.
For example, if the time in a VM is periodicallyset by the VMware tools then the process ID may belong to a process named vmwared , so you know you have to change the parameter Time Sync in the virtual machine settings and set it to Off.
Please keep in mind that a process is assigned a new process ID if another instance is started,so also if a service is restarted the new instance of the service has a different process IDthat may not match the process ID found in older system events. Also, if a program runs, sets the system time, and then terminates, it will not be shown in the process list anymore after it has terminated.
The NTP reference implementation ntpd uses a different approach for redundancy as usually known from other server approaches. However, as explained earlier, you can simply configure several time sources at the client,so the client ntpd itself checks all servers periodically, and selects the ones to use. If one of the configured reference time sources becomes unreachable,this time source is automatically discarded by the selection algorithm. Specifically, if the system peer becomes unreachable then simply a new system peer is selected from the remaining candidates,as long as at least one candidate is available.
Since the system clock adjustment has been derived from the previous system peer and the candidates ,switching can be done very smoothly. Also, if the time provided by a specific source starts to drift away from the time provided by other sources,the drifting time source becomes a falseticker and is also discarded. So even if a GPS clock is spoofedby some bad guys this can be detected and the GPS clock can be discarded and overvoted as long asthere are other time sources available which provide and agree on the right time.
A special case is when one or more configured time source have been reachable for some time,and then suddenly all time sources become unreachable. This means that e. In this case ntpd normally does not change its leap bits back to 11 , and does not change its stratumback to Instead, it keeps the stratum value it had before, and just starts to increase its so-called root dispersion value over time.
This state is called holdover mode. The root dispersion can be interpreted as a very coarse estimate of how much the local time has drifted away from some reference time. Normally it increases at a constant rate, but is reset to a low value whenever the time could be queried successfully from a reference time source. The value to which the root dispersion is reset depends on the precision of the reference time source. Anyway, in holdover mode there are no more successful queries to a reference time source, so the root dispersion keeps increasing continuously over time.
The root dispersion is also put into the NTP packets sent to clients, so a client can see that the root dispersion is increasingand thus the time of the server has started drifting, and each client itself can decide what to do:. Generally, ntpd disciplines its own system time as long as the time sources are accepted,and starts sending the freewheeling system time when all configured time sources become unreachable.
On the other hand, if a refclock e. So it usually make sense in this case to let ntpd accept the GPS receiver for quite some time even after GPS reception has failed. The parse refclock driver driver 8 from the NTP software package which is used for Meinberg GPS receiverssupports the concept of a trust time. Please note that only the parse refclock driver supports this,other refclock drivers which might be used for different GPS receivers e.
NMEA don't support this. The trust time interval starts when GPS reception suddenly fails, and only after the trust time has expired, ntpd notices that the GPS receiver has failed, and is unsynchronized. So ntpd discards the GPS time sourceonly after the trust time interval. This feature provides a stable time for a much longer holdover interval than the freewheeling clockof an embedded microprocessor board, or a standard PC.
The trust time interval needs to be determined according to the quality of the oscillator, and the acceptable time offsetdue to the clock drift after reception has failed, which is a requirement of the specific application. For example, if the acceptable drift is 10 milliseconds the trust time interval can be much longer than if the acceptable drift is only microseconds.
A basic question is why a client should stop accepting that server if there is no alternate time source available. Usually, the time on a client drifts much more if the clients stops synchronizing to a dedicated NTP serversince the server provides much more stable time even when in holdover.
So in most cases a better approach is to let clients still accept the time from a stable time source,but generate an alert e. So there's pretty much time for investigation, and to fix the reception problem. Described above is the default behavior of the NTP reference implementation in client and server role. However, other clients, specifically simple SNTP clients may behave differently. There are SNTP implementations our there which only look at the stratum value received from the NTP server,and expect the stratum to change back to 16 if the time sources of the server aren't synchronized anymore.
With some specific configuration you can force this behavior for the NTP server, e. In this case the server ntpd discards its time source when it becomes unreachable, and switches to the configuredsubstitute time source which has stratum 15 , and thus becomes stratum 15 plus 1, i. So in special cases the trust time can be set to a very short interval only, so that the stratum changes quickly to 16,as mentioned above.
However, as explained before,the basic question is whether this is the best approach for the application. If there are simple NTP clients which don't provide the powerful functionality of ntpd ,but rely on a time source which is always available, then 2 or more LANTIMEs can be configured as clusterwhich share an additional, common cluster IP address.
So a client which synchronizes to the cluster IP address doesn't even notice if one device failssince the service is taken over by another device. Unfortunately the high quality oscillator which often even includes an oven OCXO requires much more power than can be provided by a small backup battery,and thus accuracy is lost after power cycling.
This also means that after power cycling the GPS receiver claims to be not synchronized ,and also ntpd has its stratum set to 16 and its leap bits to 11 , so clients don't acceptthe ntpd running on the LANTIME as time source after power cyclinguntil the GPS receiver is synchronized again to the satellites,so that ntpd can accept it as time source and synchronize to the GPS receiver.
Startup Behavior Right after startup, ntpd sets its internal status to stratum 16 and leap bits 11 to indicate that it is not yet synchronized. Polling And Accepting Time Sources It's a policy of ntpd that any reference time source is only accepted if the time source claims to be synchronized ,no matter if the time source is a so-called hardware refclock , e.
An operator has changed the system time. This requires admin rights, so it's the operator's own problem if thinks he has to mess up the timekeeping. Another time synchronization software is running. It's never a good idea to have more than one program running in parallel to discipline the system time, so all programs but a single one should be disabled. The system timekeeping is broken. There have been cases where the time on a Windows server lost more than 30 seconds whenever a huge database application ran some maintenance tasks in the night.
A program like ntpd is unable to compensate this, so the bad programs should be fixed instead. It supports IPv4 only, but not IPv6. For current Windows installations the current stable NTP version should be used which also contains current security patches.
Step 3a without initial configuration file : for experienced users - the configuration file must be setup manually. Step 3b: initial configuration with external time server : specification of up to 9 external NTP servers.
Contains short lists of the most important NTP configuration parameters, command line options and file formats used by NTP, e. Home Software Download. Note: If the NTP service fails to start after installation on Windows XP or Windows 7, the Visual Studio Redistributable package my be missing, althogh it should be available by default on all current Windows installations.
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