![]() If the time offset at system start is greater than 1000 seconds then the user, or an installation script, should make a manual adjustment. If the time offset of the clock is more than 128ms (the default setting), ntpd can “step” the clock forwards or backwards. This slow change is referred to as slewing and cannot go backwards. It will take at least 2000 seconds to adjust the clock by 1 second using this method. The system clock is adjusted slowly, at most at a rate of 0.5ms per second, to reduce this offset by changing the frequency of the counter being used. This is then compared to the system clock to determine the time offset, the difference between the system clock’s time and what ntpd has determined the time should be. ![]() When a packet containing time information is received it is not immediately responded to, but is first subject to validation checks and then processed together with several other time samples to arrive at an estimate of the time. The designation Stratum 16 is used to indicate that the server is not currently synchronized to a reliable time source. Servers of the same stratum can exchange packets with each other but are still designated as belonging to just the one stratum, the stratum one below the best reference they are synchronized to. A server which is synchronized by means of packets marked stratum n belongs to the next, lower, stratum and will mark its packets as stratum n+1. ![]() These servers send out packets marked as Stratum 1. Atomic clocks are referred to as Stratum 0 as this is the source, but no Stratum 0 packet is sent on the Internet, all stratum 0 atomic clocks are attached to a server which is referred to as stratum 1. Hence the word stratum is used when referring to a specific layer. The servers are thought of as being arranged in layers, or strata, from 1 at the top down to 15. NTP servers are classified according to their synchronization distance from the atomic clocks which are the source of the time signals. When the system clock is being synchronized by ntpd, the kernel will in turn update the RTC every 11 minutes automatically. Hence the need for the system clock to be constantly synchronized with external time references. The time kept by the RTC will drift away from actual time by up to 5 minutes per month due to temperature variations. On system start, the system clock reads the time and date from the RTC. It is very fast, has a high resolution, and there are no interrupts. The TSC is a CPU register which counts the number of cycles since it was last reset. Usually, the Time Stamp Counter ( TSC) is used. The system clock can keep time by using various clock sources. See the rtc(4) and hwclock(8) man pages for information on hardware clocks. Linux uses a software clock as its system clock for better resolution than the typical embedded hardware clock referred to as the “ Real Time Clock” (RTC). Information about these time servers can found at The user space daemon updates the system clock, which is a software clock running in the kernel. These NTP servers provide “ Coordinated Universal Time” ( UTC). However for most situations, a range of publicly accessible time servers connected to the Internet at geographically dispersed locations can be used. Many devices which obtain the time and date via low frequency radio transmissions or the Global Position System (GPS) exist. An ideal situation would be where each site has a server, with its own reference clock attached, to act as a site-wide time server. Their signals can be deliberately degraded for military reasons. The satellites that make up the Global Position System contain more than one atomic clock, making their time signals potentially very accurate. Many standards bodies around the world have atomic clocks which may be made available as a reference. The Network Time Protocol ( NTP) enables the accurate dissemination of time and date information in order to keep the time clocks on networked computer systems synchronized to a common reference over the network or the Internet. ![]() Fedora Diversity, Equity, and Inclusion (DEI).Kernel, Module and Driver Configuration.System Locale and Keyboard Configuration.
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