NTP (Network Time Protocol)
ntp
A networking protocol for clock synchronization between computer systems over packet-switched networks.
Stratum
stratum
Definition
Stratum is the numeric level assigned to each node in the NTP (Network Time Protocol) hierarchy. A lower number indicates closer proximity to the reference time source and higher accuracy. Stratum 0 denotes the physical time sources themselves, such as atomic clocks and GPS receivers, which are not directly attached to the network. Stratum 16 is a special value meaning 'unsynchronized.'
The Hierarchy
Stratum 0 comprises the physical reference clocks (atomic clocks, GPS receivers). Stratum 1 servers connect directly to a Stratum 0 source and act as primary reference clocks. Stratum 2 servers obtain time from Stratum 1, and each subsequent level draws from the one above. Accuracy degrades slightly at each hop, yet Stratum 3 or 4 servers typically still maintain millisecond-level precision.
Practical Usage
Ordinary servers and desktop computers usually synchronize against Stratum 2 or 3 servers. The pool.ntp.org project aggregates volunteer-run Stratum 2 and 3 servers worldwide. Within enterprise networks, organizations commonly operate their own Stratum 1 or 2 server and have all internal devices synchronize from it. AWS's Amazon Time Sync Service (169.254.169.123) provides a Stratum 1-equivalent service accessible to EC2 instances.
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Related Terms
Atomic Clock
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.