Updating system time
Often a stratum 2 computer will query several stratum 1 servers.
Other related network tools were available both then and now.
In 1991, the NTPv1 architecture, protocol and algorithms were brought to the attention of a wider engineering audience with the publication of an article by David L. In 1989, RFC 1119 was published defining NTPv2 by means of a state machine, with pseudocode to describe its operation.
It introduced a management protocol and cryptographic authentication scheme which have both survived into NTPv4.
The upper limit for stratum is 15; stratum 16 is used to indicate that a device is unsynchronized.
The NTP algorithms on each computer interact to construct a Bellman-Ford shortest-path spanning tree, to minimize the accumulated round-trip delay to the stratum 1 servers for all the clients.
Each level of this hierarchy is termed a "stratum" and is assigned a number starting with zero at the top.