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6 NTP Service
Network Time Protocol (NTP) is a network protocol used to synchronize the clocks of
computers on your network to a time reference clock. NTP is used to ensure that all the
computers on a network are reporting the same time.
If an isolated network, or even a single computer, is running on wrong time, services
that use time and date stamps (like mail service, or web service with timed cookies) will
send wrong time and date stamps and be out of synchronization with other computers
across the Internet. For example, an email message could arrive minutes or years before
it was sent (according to the time stamp), and a reply to that message could come
before the original was sent.
How NTP Works
NTP uses Universal Time Coordinated (UTC) as its reference time. UTC is based on an
atomic resonance, and clocks that run according to UTC are often called “atomic
clocks.”
Internet-wide, authoritative NTP servers (called Stratum 1 servers) keep track of the
current UTC time. Other subordinate servers (called Stratum 2 and 3 servers) query the
Stratum 1 servers on a regular basis and estimate the time taken across the network to
send and receive the query. They then factor this estimate with the query result to set
the Stratum 2 or 3 servers own time. The estimates are accurate to the nanosecond.
Your local network can then query the Stratum 3 servers for the time. Then it repeats
the process. An NTP client computer on your network then takes the UTC time
reference and converts it, through its own time zone setting to local time, and sets its
internal clock accordingly.
LL2351.Book Page 79 Monday, September 8, 2003 2:47 PM