Therefore, the stratum level of any computer is an indicator of how closely that computer is synchronized with the most accurate time source.
When the W32Time Manager receives time samples, it uses special algorithms in NTP to determine which of the time samples is the most appropriate for use. The time service also uses another set of algorithms to determine which of the configured time sources is the most accurate.
When the time service has determined which time sample is best, based on the above criteria, it adjusts the local clock rate to allow it to converge toward the correct time.
If the time difference between the local clock and the selected accurate time sample also called the time skew is too large to correct by adjusting the local clock rate, the time service sets the local clock to the correct time.
This adjustment of clock rate or direct clock time change is known as clock discipline. The Windows Time Service Manager is responsible for initiating the action of the NTP time providers included with the operating system.
The Windows Time Service Manager controls all functions of the Windows Time service and the coalescing of all time samples. In addition to providing information about the current system state, such as the current time source or the last time the system clock was updated, the Windows Time Service Manager is also responsible for creating events in the event log.
These time samples are then passed to the Windows Time Service Manager, which collects all the samples and passes them to the clock discipline subcomponent.
The clock discipline subcomponent applies the NTP algorithms which results in the selection of the best time sample. The clock discipline subcomponent adjusts the time of the system clock to the most accurate time by either adjusting the clock rate or directly changing the time.
If a computer has been designated as a time server, it can send the time on to any computer requesting time synchronization at any point in this process. Time protocols determine how closely two computers' clocks are synchronized. A time protocol is responsible for determining the best available time information and converging the clocks to ensure that a consistent time is maintained on separate systems.
NTP is an Internet time protocol that includes the discipline algorithms necessary for synchronizing clocks. NTP is a fault-tolerant, highly scalable time protocol and is the protocol used most often for synchronizing computer clocks by using a designated time reference. NTP time synchronization takes place over a period of time and involves the transfer of NTP packets over a network. NTP packets contain time stamps that include a time sample from both the client and the server participating in time synchronization.
NTP relies on a reference clock to define the most accurate time to be used and synchronizes all clocks on a network to that reference clock.
UTC is independent of time zones and enables NTP to be used anywhere in the world regardless of time zone settings. NTP includes two algorithms, a clock-filtering algorithm and a clock-selection algorithm, to assist the Windows Time service in determining the best time sample. The clock-filtering algorithm is designed to sift through time samples that are received from queried time sources and determine the best time samples from each source. The clock-selection algorithm then determines the most accurate time server on the network.
This information is then passed to the clock discipline algorithm, which uses the information gathered to correct the local clock of the computer, while compensating for errors due to network latency and computer clock inaccuracy.
The NTP algorithms are most accurate under conditions of light-to-moderate network and server loads. As with any algorithm that takes network transit time into account, NTP algorithms might perform poorly under conditions of extreme network congestion. The Windows Time service is a complete time synchronization package that can support a variety of hardware devices and time protocols. To enable this support, the service uses pluggable time providers.
A time provider is responsible for either obtaining accurate time stamps from the network or from hardware or for providing those time stamps to other computers over the network. The NTP provider is the standard time provider included with the operating system.
NtpServer output provider. This is a time server that responds to client time requests on the network. NtpClient input provider. This is a time client that obtains time information from another source, either a hardware device or an NTP server, and can return time samples that are useful for synchronizing the local clock. Although the actual operations of these two providers are closely related, they appear independent to the time service.
Starting with Windows Server, when a Windows computer is connected to a network, it is configured as an NTP client. Also, computers running the Windows Time service only attempt to synchronize time with a domain controller or a manually specified time source by default. These are the preferred time providers because they are automatically available, secure sources of time. Within an AD DS forest, the Windows Time service relies on standard domain security features to enforce the authentication of time data.
The security of NTP packets that are sent between a domain member computer and a local domain controller that is acting as a time server is based on shared key authentication. The Windows Time service uses the computer's Kerberos session key to create authenticated signatures on NTP packets that are sent across the network. NTP packets are not transmitted inside the Net Logon secure channel.
Instead, when a computer requests the time from a domain controller in the domain hierarchy, the Windows Time service requires that the time be authenticated. The domain controller then returns the required information in the form of a bit value that has been authenticated with the session key from the Net Logon service. The bold one is hardcoded in some apps or OSes. I use nslookup to get their IP addresses. Some others like the xxx. Are you aware if there's a list which indicates whether an ntp-server supports NTS on port ?
I've just upgrade to chronyd-nts on my router which functions as local NTP-server but which I wish to add to the pool, if accurate enough, but the amount of servers that can be found is still quite limited. Thanks for this list. Do you know if there is a list which can be downloaded automatically from the web? I mean to develop a tool which would can display a list of server to select. It would be awesome if you could remove the following time server because it itsn't working anymore and the domain seems to be for sale :.
China - Aliyun time1. China - Tencent Cloud time1. China - NTP. CN cn. US - Apple time1. Apple seems to have sunset their secondary time servers, time. This meant it could not sync as there were no static routes or port exceptions created for this IP. There is a similar location for but I cannot find anything anywhere for Time. Any help would be greatly appreciated. This posting is provided AS IS with no warranties or guarantees , and confers no rights. My Linkedin Profile.
My MVP Profile. Please remember to mark the replies as answers if they help and unmark them if they provide no help. If you have only two time servers, you should specify the Ntpserver UseAsFallbackOnly flag 0x2 to de-prioritize one of them. For example, if you want to prioritize ntpserver. Additionally, you can run the following command and read the value of NtpServer in the output:. In order for W32tm. Then, to adjust the computer clock by using the clock rate, W32tm.
This algorithm varies depending on the version of Windows:. MaxAllowedPhaseOffset is configurable in the registry. However, the registry parameter is measured in seconds instead of clock ticks. This command produces output that resembles the following. The output presents the poll interval in both clock ticks and in seconds. The equations use the value measured in seconds the value in parentheses.
The output presents the clock rate in seconds. To see the SystemClockRate value in clock ticks, use the following formula:. For example, if SystemClockRate is 0. For full descriptions of the configurable parameters and their default values, see Config entries later in this article.
The following examples show how to apply these calculations for Windows Server R2 and earlier versions. In this case, if you want to set the clock back slowly, you would also have to adjust the values of PhaseCorrectRate or UpdateInterval in the registry to make sure that the equation result is TRUE. The Windows Time service stores a number of configuration properties as registry entries. It stores configuration information that the policies define in the Windows registry, and then uses those registry entries to configure the registry entries specific to the Windows Time service.
As a result, the values defined by Group Policy overwrite any pre-existing values in the Windows Time service section of the registry. Some of the preset GPO settings differ from the corresponding default Windows Time service registry entries. Windows loads these settings into the policy area of the registry under the following subkey:. Then Windows uses the policy settings to configure the related Windows Time service registry entries under the following subkey:.
The following table lists the policies that you can configure for the Windows Time service, and the registry subkeys that those policies affect. When you remove a Group Policy setting, Windows removes the corresponding entry from the policy area of the registry. This information is provided as a reference for use in troubleshooting and validation.
Windows registry keys are used by W32Time to store critical information. Don't change these values. Modifications to the registry are not validated by the registry editor or by Windows before they are applied.
If the registry contains invalid values, Windows may experience unrecoverable errors. Some of the parameters in the registry are measured in clock ticks and some are measured in seconds. To convert the time from clock ticks to seconds, use these conversion factors:. Note Zero is not a valid value for the FrequencyCorrectRate registry entry.
HoldPeriod All versions Controls the period of time for which spike detection is disabled in order to bring the local clock into synchronization quickly. A spike is a time sample indicating that time is off a number of seconds, and is usually received after good time samples have been returned consistently. The default value on domain members is 5. The default value on stand-alone clients and servers is 5. LargePhaseOffset All versions Specifies that a time offset greater than or equal to this value in 10 -7 seconds is considered a spike.
A network disruption such as a large amount of traffic might cause a spike. A spike will be ignored unless it persists for a long period of time. The default value on domain members is The default value on stand-alone clients and servers is It contains reserved data that is used by the Windows operating system, and any changes to this setting can cause unpredictable results. MaxAllowedPhaseOffset All versions Specifies the maximum offset in seconds for which W32Time attempts to adjust the computer clock by using the clock rate.
When the offset exceeds this rate, W32Time sets the computer clock directly. The default value for domain members is The default value for stand-alone clients and servers is 1. The default value for stand-alone clients and servers is MaxNegPhaseCorrection All versions Specifies the largest negative time correction, in seconds, that the service makes. If the service determines that a change larger than this is required, it logs an event instead.
This value means that the service always corrects the time. The default value for domain controllers is , 48 hrs. The default value for stand-alone clients and servers is 54, 15 hrs. MaxPollInterval All versions Specifies the largest interval, in log2 seconds, allowed for the system polling interval. Note that while a system must poll according to the scheduled interval, a provider can refuse to produce samples when requested to do so.
The default value for domain controllers is MaxPosPhaseCorrection All versions Specifies the largest positive time correction in seconds that the service makes. MinPollInterval All versions Specifies the smallest interval, in log base 2 seconds, allowed for the system polling interval.
Note that while a system does not request samples more frequently than this, a provider can produce samples at times other than the scheduled interval. The default value for domain controllers is 6. PhaseCorrectRate All versions Controls the rate at which the phase error is corrected. Specifying a small value corrects the phase error quickly, but might cause the clock to become unstable.
If the value is too large, it takes a longer time to correct the phase error.
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