fedora-docs-l10n/fedora-system-administrat/f29/pages/servers/Configuring_NTP_Using_ntpd

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	Configuring NTP Using ntpd
	Introduction to NTP
	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. Many standards bodies around the world have atomic clocks which may be made available as a reference. The satellites that make up the Global Position System contain more than one atomic clock, making their time signals potentially very accurate. Their signals can be deliberately degraded for military reasons. 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. Many devices which obtain the time and date via low frequency radio transmissions or the Global Position System (GPS) exist. However for most situations, a range of publicly accessible time servers connected to the Internet at geographically dispersed locations can be used. These `NTP` servers provide "pass:attributes[{blank}]_Coordinated Universal Time_pass:attributes[{blank}]" (UTC). Information about these time servers can found at [citetitle]_www.pool.ntp.org_.
	Accurate time keeping is important for a number of reasons in IT. In networking for example, accurate time stamps in packets and logs are required. Logs are used to investigate service and security issues and so time stamps made on different systems must be made by synchronized clocks to be of real value. As systems and networks become increasingly faster, there is a corresponding need for clocks with greater accuracy and resolution. In some countries there are legal obligations to keep accurately synchronized clocks. Please see [citetitle]_www.ntp.org_ for more information. In Linux systems, `NTP` is implemented by a daemon running in user space. The default `NTP` user space daemon in {MAJOROSVER} is `chronyd`. It must be disabled if you want to use the `ntpd` daemon. See xref:servers/Configuring_NTP_Using_the_chrony_Suite.adoc#ch-Configuring_NTP_Using_the_chrony_Suite[Configuring NTP Using the chrony Suite] for information on [application]chrony.
	The user space daemon updates the system clock, which is a software clock running in the kernel. Linux uses a software clock as its system clock for better resolution than the typical embedded hardware clock referred to as the "pass:attributes[{blank}]_Real Time Clock_pass:attributes[{blank}]" (RTC). See the `rtc(4)` and `hwclock(8)` man pages for information on hardware clocks. The system clock can keep time by using various clock sources. Usually, the _Time Stamp Counter_ (TSC) is used. The TSC is a CPU register which counts the number of cycles since it was last reset. It is very fast, has a high resolution, and there are no interrupts. On system start, the system clock reads the time and date from the RTC. The time kept by the RTC will drift away from actual time by up to 5 minutes per month due to temperature variations. Hence the need for the system clock to be constantly synchronized with external time references. When the system clock is being synchronized by `ntpd`, the kernel will in turn update the RTC every 11 minutes automatically.
	NTP Strata
	`NTP` servers are classified according to their synchronization distance from the atomic clocks which are the source of the time signals. The servers are thought of as being arranged in layers, or strata, from 1 at the top down to 15. Hence the word stratum is used when referring to a specific layer. 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. These servers send out packets marked as Stratum 1. 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`. 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. The designation Stratum 16 is used to indicate that the server is not currently synchronized to a reliable time source.
	Note that by default `NTP` clients act as servers for those systems in the stratum below them.
	Here is a summary of the `NTP` Strata:
	Stratum 0
	Atomic Clocks and their signals broadcast over Radio and GPS
	GPS (Global Positioning System)
	Mobile Phone Systems
	Low Frequency Radio Broadcasts+ WWVB (Colorado, USA.), JJY-40 and JJY-60 (Japan), DCF77 (Germany), and MSF (United Kingdom)
	These signals can be received by dedicated devices and are usually connected by RS-232 to a system used as an organizational or site-wide time server.
	Stratum 1
	Computer with radio clock, GPS clock, or atomic clock attached
	Stratum 2
	Reads from stratum 1; Serves to lower strata
	Stratum 3
	Reads from stratum 2; Serves to lower strata
	Stratum _n+1_
	Reads from stratum _n_pass:attributes[{blank}]; Serves to lower strata
	Stratum 15
	Reads from stratum 14; This is the lowest stratum.

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