M A G N U M 6 K S W I T C H E S , M N S - 6 K U S E R G U I D ETime or Temps Atomique International (TAI) by inserting leap seconds at intervals of about 18months. UTC time is disseminated by various means, including radio and satellite navigationsystems, telephone modems and portable clocks.In 1981 the time synchronization technology was documented in the now historic InternetEngineering Note series as IEN-173. The first specification of a public protocol developed fromit appeared in RFC-778. The first deployment of the technology in a local network was as anintegral function of the Hello routing protocol documented in RFC-891, which survived for manyyears in a network prototyping and test bed operating system called the Fuzzball. There wasconsiderable discussion during 1989 about the newly announced Digital Time SynchronizationSer-vice (DTSS), which was adopted for the Enterprise network. The DTSS and NTPcommunities had much the same goals, but somewhat different strategies for achieving them.One problem with DTSS, as viewed by the NTP community, was a possibly serious loss ofaccuracy, since the DTSS design did not discipline the clock frequency. The problem with theNTP design, as viewed from the DTSS community, was the lack of formal correctness principlesin the design process.Simple Network Protocol (SNTP) is described in RFC-1769 as well as in RFC-2030. SNTP iscompatible with NTP as implemented for the IPv4, IPv6 and OSI protocol stacks. SNTP hasbeen used in several standalone NTP servers integrated with GPS receivers.The article from NIST http://tf.nist.gov/timefreq/service/pdf/computertime.pdf providesdetails on time synchronization services as well as ports time synchronization services need tocommunicate on. http://physics.nist.gov/GenInt/Time/time.html provides a walk through thehistory of time and time synchronization on the NIST site. There are many other interestingarticles available on Internet.Stratum clocksNTP uses a hierarchical system of "clock strata". The stratum levels define the distance from thereference clock and exist to prevent cycles in the hierarchy. (Note that this is different from thenotion of clock strata used in telecommunications systems.)Stratum 0These are devices such as atomic (cesium, rubidium) clocks, GPS clocks or other radioclocks. Stratum-0 devices are not attached to the network; instead they are locallyconnected to computers (e.g. via an RS-232 connection.) The atomic clock at the NISTDenver facility is an example of the Stratum 0 clock.Stratum 1These are computers attached to Stratum 0 devices. Normally they act as time servers fortiming requests from Stratum 2 servers via NTP. These computers are also referred to astime servers. Time servers from NIST and USNO are examples of Stratum 1 servers.Stratum 2These are computers that send NTP requests to Stratum 1 servers. Normally a Stratum 2computer will reference a number of Stratum 1 servers and use the NTP algorithm togather the best data sample, dropping any Stratum 1 servers that seem obviously wrong.85
