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NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for high stratum NTP servers is atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There's nothing magical about GPS or atomic clocks that make them tell you exactly what time it is; because of how atomic clocks work, they are simply very good at, having once been told what time it is, keep telling accurate time (since the second is defined in terms of atomic effects). In fact, it's worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks in turn are synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time but also the time.

Once you have an exact time on one system connected to a network like the Internet, it's a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a stratum 2 (or lower) NTP server is no different from your desktop system syncing against a set of NTP servers.

By the time you have a few accurate times (as obtained from NTP servers or elsewhere) and know the rate of advancement of your local clock (which is easy to determine), you can calculate your local clock's drift rate relative to the "believed accurate" passage of time. Once locked in, this value can then be used to continuously adjust the local clock to make it report values very close to the accurate passage of time, even if the local real-time clock itself is highly inaccurate; as long as your local clock is not highly erratic, this should allow keeping accurate time for some time even if your upstream time source becomes unavailable for any reason. Some NTP client implementations (probably most ntpd daemon or system service implementations) do this, and others (like ntpd's companion ntpdate which simply sets the clock once) do not. This is commonly referred to as a drift file because it stores persistently a measure of clock drift, but strictly speaking it doesn't have to be stored as a specific file on disk.

Note that "high stratum" here means a low stratum number. Stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a stratum 0 time source as its time source (and is thus slightly less accurate than the stratum 0 time source). Stratum 2 again is slightly less accurate than stratum 1 because it is syncing its time against the stratum 1 source. And so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases.

NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for central NTP servers is atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There's nothing magical about GPS or atomic clocks that make them tell you exactly what time it is; because of how atomic clocks work, they are simply very good at, having once been told what time it is, keep telling accurate time (since the second is defined in terms of atomic effects). In fact, it's worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks in turn are synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time but also the time.

Once you have an exact time on one system connected to a network like the Internet, it's a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a stratum 2 (or farther from the actual time source) NTP server is no different from your desktop system syncing against a set of NTP servers.

By the time you have a few accurate times (as obtained from NTP servers or elsewhere) and know the rate of advancement of your local clock (which is easy to determine), you can calculate your local clock's drift rate relative to the "believed accurate" passage of time. Once locked in, this value can then be used to continuously adjust the local clock to make it report values very close to the accurate passage of time, even if the local real-time clock itself is highly inaccurate; as long as your local clock is not highly erratic, this should allow keeping accurate time for some time even if your upstream time source becomes unavailable for any reason. Some NTP client implementations (probably most ntpd daemon or system service implementations) do this, and others (like ntpd's companion ntpdate which simply sets the clock once) do not. This is commonly referred to as a drift file because it stores persistently a measure of clock drift, but strictly speaking it doesn't have to be stored as a specific file on disk.

In NTP, stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a stratum 0 time source as its time source (and is thus slightly less accurate than the stratum 0 time source). Stratum 2 again is slightly less accurate than stratum 1 because it is syncing its time against the stratum 1 source. And so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases.

NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for high stratum NTP servers is atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There's nothing magical about GPS or atomic clocks that make them tell you exactly what time it is; because of how atomic clocks work, they are simply very good at, having once been told what time it is, keep telling accurate time (since the second is defined in terms of atomic effects). In fact, it's worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks in turn are synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time but also the time.

Once you have an exact time on one system connected to a network like the Internet, it's a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a stratum 2 (or lower) NTP server is no different from your desktop system syncing against a set of NTP servers.

By the time you have a few accurate times (as obtained from NTP servers) and know the rate of advancement of your local clock (which is easy to determine), you can calculate your local clock's drift rate relative to the "believed accurate" passage of time. Once locked in, this value can then be used to continuously adjust the local clock to make it report values very close to the accurate passage of time, even if the local real-time clock itself is highly inaccurate; as long as your local clock is not highly erratic, this should allow keeping accurate time for some time even if your upstream time source becomes unavailable for any reason. Some NTP implementations (probably most ntpd daemon or system service implementations) do this, and others (like ntpd's companion ntpdate which simply sets the clock once) do not.

Note that "high stratum" here means a low stratum number. Stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a stratum 0 time source as its time source (and is thus slightly less accurate than the stratum 0 time source). Stratum 2 again is slightly less accurate than stratum 1 because it is syncing its time against the stratum 1 source. And so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases.

NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for high stratum NTP servers is atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There's nothing magical about GPS or atomic clocks that make them tell you exactly what time it is; because of how atomic clocks work, they are simply very good at, having once been told what time it is, keep telling accurate time (since the second is defined in terms of atomic effects). In fact, it's worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks in turn are synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time but also the time.

Once you have an exact time on one system connected to a network like the Internet, it's a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a stratum 2 (or lower) NTP server is no different from your desktop system syncing against a set of NTP servers.

By the time you have a few accurate times (as obtained from NTP servers or elsewhere) and know the rate of advancement of your local clock (which is easy to determine), you can calculate your local clock's drift rate relative to the "believed accurate" passage of time. Once locked in, this value can then be used to continuously adjust the local clock to make it report values very close to the accurate passage of time, even if the local real-time clock itself is highly inaccurate; as long as your local clock is not highly erratic, this should allow keeping accurate time for some time even if your upstream time source becomes unavailable for any reason. Some NTP client implementations (probably most ntpd daemon or system service implementations) do this, and others (like ntpd's companion ntpdate which simply sets the clock once) do not. This is commonly referred to as a drift file because it stores persistently a measure of clock drift, but strictly speaking it doesn't have to be stored as a specific file on disk.

Note that "high stratum" here means a low stratum number. Stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a stratum 0 time source as its time source (and is thus slightly less accurate than the stratum 0 time source). Stratum 2 again is slightly less accurate than stratum 1 because it is syncing its time against the stratum 1 source. And so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases.

NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for high stratum NTP servers is atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There's nothing magical about GPS or atomic clocks that make them tell you exactly what time it is; because of how atomic clocks work, they are simply very good at, having once been told what time it is, keep telling accurate time (since the second is defined in terms of atomic effects). In fact, it's worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks in turn are synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time but also the time.

Once you have an exact time on one system connected to a network like the Internet, it's a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a stratum 2 (or lower) NTP server is no different from your desktop system syncing against a set of NTP servers.

Note that "high stratum" here means a low stratum number. Stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a stratum 0 time source as its time source (and is thus slightly less accurate than the stratum 0 time source). Stratum 2 again is slightly less accurate than stratum 1 because it is syncing its time against the stratum 1 source. And so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases.

NTP servers rely on highly accurate clocks for precision timekeeping. A common time source for high stratum NTP servers is atomic clocks, or GPS receivers (remember that GPS satellites have atomic clocks onboard). These clocks are defined as accurate since they provide a highly exact time reference. There's nothing magical about GPS or atomic clocks that make them tell you exactly what time it is; because of how atomic clocks work, they are simply very good at, having once been told what time it is, keep telling accurate time (since the second is defined in terms of atomic effects). In fact, it's worth noting that GPS time is distinct from the UTC that we are more used to seeing. These atomic clocks in turn are synchronized against International Atomic Time or TAI in order to not only accurately tell the passage of time but also the time.

Once you have an exact time on one system connected to a network like the Internet, it's a matter of protocol engineering enabling transfer of precise times between hosts over an unreliable network. In this regard a stratum 2 (or lower) NTP server is no different from your desktop system syncing against a set of NTP servers.

By the time you have a few accurate times (as obtained from NTP servers) and know the rate of advancement of your local clock (which is easy to determine), you can calculate your local clock's drift rate relative to the "believed accurate" passage of time. Once locked in, this value can then be used to continuously adjust the local clock to make it report values very close to the accurate passage of time, even if the local real-time clock itself is highly inaccurate; as long as your local clock is not highly erratic, this should allow keeping accurate time for some time even if your upstream time source becomes unavailable for any reason. Some NTP implementations (probably most ntpd daemon or system service implementations) do this, and others (like ntpd's companion ntpdate which simply sets the clock once) do not.

Note that "high stratum" here means a low stratum number. Stratum 0 is by definition an accurate time source. Stratum 1 is a system that uses a stratum 0 time source as its time source (and is thus slightly less accurate than the stratum 0 time source). Stratum 2 again is slightly less accurate than stratum 1 because it is syncing its time against the stratum 1 source. And so on. In practice, this loss of accuracy is so small that it is completely negligible in all but the most extreme of cases.