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Networking

What is Stratum 15?

In NTP, stratum 15 is the farthest valid level from a reference clock. A stratum 15 source still counts as synchronized; stratum 16 means unsynchronized.

What is stratum 15?

In the Network Time Protocol (NTP), a clock's stratum is its distance from a reference clock, counted in hops. Stratum 15 is the highest stratum number NTP still treats as a valid, synchronized time source. One more hop would make it stratum 16, which NTP uses to mean "not synchronized".

Stratum measures distance, not quality. A lower number usually means closer to true time, but a nearby stratum 3 server on a fast LAN can keep better time than a distant stratum 2 server across a slow WAN link.

The stratum levels

StratumWhat it is
0The reference clock itself, such as a GPS receiver, an atomic clock or a radio time signal; not reached directly over the network
1A primary time server connected straight to a stratum 0 clock
2A server that syncs from a stratum 1 server over the network
3 to 14Each further hop adds one
15The farthest level that is still valid
16Unsynchronized; must not be used as a time source

How stratum is assigned

A device doesn't choose its own stratum. When it syncs from a source, it takes that source's stratum plus one. A PC that syncs from a stratum 2 server becomes stratum 3, and anything that syncs from that PC becomes stratum 4. When choosing among several servers, NTP clients weigh stratum along with network delay and jitter.

Real networks rarely come anywhere near stratum 15. A typical design is only a few hops deep: a few internal servers sync from public stratum 1 or 2 servers, and everything else syncs from those. A device reporting a very high stratum usually points to a long or badly planned chain of time sources that is worth fixing.

For the CompTIA A+ exam

Core 1 (220-1201) objective 2.3 lists NTP as a server role. Stratum numbers aren't named in the objectives, but this course's NTP lesson covers them: stratum 0 is the reference clock, stratum 1 is attached to it, each hop adds one, 15 is the maximum valid level, and 16 means unsynchronized. Also remember that NTP uses UDP port 123.

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Related terms

atomic clock

An atomic clock is a highly precise timekeeper that uses the natural oscillations of atoms to measure time with extreme accuracy, typically to within a few nanoseconds per day.

GPS

GPS (Global Positioning System) is a satellite-based navigation system that provides location and time information to users with GPS receivers.

NTP

Network Time Protocol (NTP) is a networking protocol used to synchronize the clocks of computer systems over a network, ensuring accurate timekeeping and consistency across devices.

Stratum 0

Stratum 0 refers to the primary reference clock or time source in a time synchronization system, which provides the most authoritative and accurate time information.

Stratum 1

Stratum 1 refers to the highest level of precision in the Network Time Protocol (NTP) hierarchy, providing the most accurate and reliable time source for synchronizing computer clocks.

Stratum 16

In NTP, stratum 16 means a clock is unsynchronized, for example a server that lost its upstream source. Other devices must not use it as a time source.

Stratum 2

Stratum 2 is a category of clocks and timing devices that provide a higher level of accuracy and stability compared to consumer-grade clocks, but are not as precise as the most accurate atomic clocks.

UDP port 123

UDP port 123 is the port number used by the Network Time Protocol (NTP) to synchronize computer clocks over a network.

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