Technical data

FastIron Ethernet Switch Administration Guide 21
53-1002637-02
Network Time Protocol Version 4 (NTPv4)
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else in the world. It allows clients to use different time zone and daylight-saving properties. Primary
servers are synchronized by wire or radio to national standards such as GPS. Timing information is
conveyed from primary servers to secondary servers and clients in the network. NTP runs on UDP,
which in turn runs on IP.
NTP has a hierarchical structure. NTP uses the concept of a stratum to describe how many NTP
hops away a machine is from an authoritative time source. A stratum 1 time server typically has an
authoritative time source such as a radio or atomic clock, or a Global Positioning System [GPS]
time source directly attached. A stratum 2 time server receives its time through NTP from a stratum
1 time server and so on. As the network introduces timing discrepancies, lower stratum devices are
a factor less accurate. A hierarchical structure allows the overhead of providing time to many
clients to be shared among many time servers. Not all clients need to obtain time directly from a
stratum 1 reference, but can use stratum 2 or 3 references.
NTP operates on a client-server basis. The current implementation runs NTP as a secondary server
and/or a NTP Client. As a secondary server, the device operates with one or more upstream servers
and one or more downstream servers or clients. A client device synchronizes to one or more
upstream servers, but does not provide synchronization to dependant clients. Secondary servers at
each lower level are assigned stratum numbers one greater than the preceding level. As stratum
number increases, the accuracy decreases. Stratum one is assigned to Primary servers.
NTP uses the concept of associations to describe communication between two machines running
NTP. NTP associations are statistically configured. On startup or on the arrival of NTP packets,
associations are created. Multiple associations are created by the protocol to communicate with
multiple servers. NTP maintains a set of statistics for each of the server or the client it is associated
with. The statistics represent measurements of the system clock relative to each server clock
separately. NTP then determines the most accurate and reliable candidates to synchronize the
system clock. The final clock offset applied for clock adjustment is a statistical average derived
from the set of accurate sources.
When multiple sources of time (hardware clock, manual configuration) are available, NTP is always
considered to be more authoritative. NTP time overrides the time that is set by any other method.
NTPv4 obsoletes NTPv3 (RFC1305) and SNTP (RFC4330). SNTP is a subset of NTPv4. RFC 5905
describes NTPv4.
To keep the time in your network current, it is recommended that each device have its time
synchronized with at least four external NTP servers. External NTP servers should be synchronized
among themselves to maintain time synchronization.