Windows default ntp servers

Windows Time Service Processes and Interactions. This topic explains only how the Windows Time service W32Time works. Although the Windows Time service is not an exact implementation of the Network Time Protocol NTP , it uses the complex suite of algorithms that is defined in the NTP specifications to ensure that clocks on computers throughout a network are as accurate as possible. Ideally, all computer clocks in an AD DS domain are synchronized with the time of an authoritative computer.

Many factors can affect time synchronization on a network. The following factors often affect the accuracy of synchronization in AD DS:. Prior to Windows Server , the W32Time service was not designed to meet time-sensitive application needs. However, updates to Windows Server now allow you to implement a solution for 1ms accuracy in your domain.

See Windows Accurate Time and Support boundary to configure the Windows Time service for high-accuracy environments for more information. Computers that synchronize their time less frequently or are not joined to a domain are configured, by default, to synchronize with time. Therefore, it is impossible to guarantee time accuracy on computers that have intermittent or no network connections.

An AD DS forest has a predetermined time synchronization hierarchy. The Windows Time service synchronizes time between computers within the hierarchy, with the most accurate reference clocks at the top. If more than one time source is configured on a computer, Windows Time uses NTP algorithms to select the best time source from the configured sources based on the computer's ability to synchronize with that time source. The Windows Time service does not support network synchronization from broadcast or multicast peers.

Every computer that is running the Windows Time service uses the service to maintain the most accurate time. Computers that are members of a domain act as a time client by default, therefore, in most cases it is not necessary to configure the Windows Time Service.

However, the Windows Time Service can be configured to request time from a designated reference time source, and can also provide time to clients. The degree to which a computer's time is accurate is called a stratum. The most accurate time source on a network such as a hardware clock occupies the lowest stratum level, or stratum one. This accurate time source is called a reference clock. An NTP server that acquires its time directly from a reference clock occupies a stratum that is one level higher than that of the reference clock.

Resources that acquire time from the NTP server are two steps away from the reference clock, and therefore occupy a stratum that is two higher than the most accurate time source, and so on.

As a computer's stratum number increases, the time on its system clock may become less accurate. 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.

The output of this command displays a list of W32time configuration parameters that are set for the client. Windows Server has improved the time synchronization algorithms to align with RFC specifications. Therefore, if you want to set the local time client to point to multiple peers, we recommended that you prepare three or more different time servers. 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. These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience. Necessary Necessary. Necessary cookies are absolutely essential for the website to function properly.

This category only includes cookies that ensures basic functionalities and security features of the website. 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.

Using local time references will reduce network latency and round-trip delays and provide a more robust time service.

Public time servers are often maintained by volunteers. They provide no guarantee of availability or accuracy. You use them at your own risk. Also, many NTP authentication and security mechanisms cannot be used with public time servers, which may leave your systems open to abuse. If your organisation requires an accurate and reliable source of time, you should seriously consider installing a local time reference.