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.'

Although 0 and 16 sit at opposite ends of the numbering, their meanings are not symmetrical. Stratum 0 refers to the time source itself at the very top of the hierarchy, whereas Stratum 16 does not denote a seventeenth layer but rather a marker indicating that a device is not synchronized. The valid synchronized range runs only from 0 to 15 (RFC 5905).

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 servers worldwide, and the stratum of participating servers is not uniform. 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 is a managed NTP service that EC2 instances can reach at the link-local address 169.254.169.123.