Network Time Protocol (NTP)

Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in use. NTP was originally designed by David L. Mills of the University of Delaware, who still develops and maintains it with a team of volunteers.

NTP uses a hierarchical, semi-layered system of levels of clock sources. Each level of this hierarchy is termed a stratum and is assigned a layer number starting with 0 (zero) at the top. The stratum level defines its distance from the reference clock and exists to prevent cyclical dependencies in the hierarchy. It is important to note that the stratum is not an indication of quality or reliability, it is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources. This definition of stratum is also different from the notion of clock strata used in telecommunication systems.

Stratum 0

These are devices such as atomic (cesium, rubidium) clocks, GPS clocks or other radio clocks. Stratum-0 devices are traditionally not attached to the network; instead they are locally connected to computers (e.g., via an RS-232 connection using a pulse per second signal).

Stratum 1

These are computers attached to Stratum 0 devices. Normally they act as servers for timing requests from Stratum 2 servers via NTP. These computers are also referred to as time servers.

Stratum 2

These are computers that send NTP requests to Stratum 1 servers. Normally a Stratum 2 computer will reference a number of Stratum 1 servers and use the NTP algorithm to gather the best data sample, dropping any Stratum 1 servers that seem obviously wrong. Stratum 2 computers will peer with other Stratum 2 computers to provide more stable and robust time for all devices in the peer group. Stratum 2 computers normally act as servers for Stratum 3 NTP requests.

Stratum 3

These computers employ exactly the same algorithms for peering and data sampling as Stratum 2, and can themselves act as servers for stratum 4 computers, and so on.

While NTP (depending on what version of NTP protocol in use) supports up to 256 strata, only the first 16 are employed and any device at Stratum 16 is considered to be unsynchronized