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Stratum

Stratum is the term used to define the relative proximity of a node in a network to a time
source (such as a radio clock). Stratum 1 is the server connected to the time source itself. (In
most cases the time source and the stratum 1 server are in the same physical location.) An
NTP client or server connected to a stratum 1 source would be stratum 2. A client or server
connected to a stratum 2 machine would be stratum 3, and so on, as demonstrated in the
diagram below.
The farther away from stratum 1 a device is, the more likely there will be discrepancies or
errors in the time adjustments done by NTP. A list of stratum 1 and 2 sources available to the
public can be found on the Internet.
♦ Note ♦
It is not required that NTP be connected to an officially
recognized time source (for example, a radio clock).
NTP can use any time source to synchronize time in
the network.

Using NTP in a Network

NTP operates on the premise that there is one true standard time (defined by UTC), and that if
several servers claiming synchronization to the standard time are in disagreement, then one or
more of them must be out of synchronization or not functioning correctly.
The stratum gradiation is used to qualify the accuracy of a time source along with other
factors such as advertised precision and the length of the network path between connections.
NTP operates with a basic distrust of time information sent from other network entities, and is
most effective when multiple NTP time sources are integrated together for checks and cross-
checks.
To achieve this end, there are several modes of operation that an NTP entity can use when
synchronizing time in a network. These modes help predict how the entity behaves when
requesting or sending time information, listed below:
• A switch can be a client of an NTP server (usually of a lower stratum), receiving time infor-
mation from the server but not passing it on to other switches.
• A switch can be a client of an NTP server, and in turn be a server to another switch or
switches.
• A switch (regardless of its status as either a client or server) must be peered with another
switch. Peering allows NTP entities in the network of the same stratum to regard each other
as reliable sources of time and exchange time information.
Examples of these are shown in the simple network diagram on the following page:
Time Source Omni Switch/Routers running NTP
Stratum 1 Stratum 2 Stratum 3
(UTC)