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68 CHAPTER 5: SYSTEM TIME
What is the
Network Time
Protocol?
The Network Time Protocol (NTP) is used to synchronize the time of client
machines and servers with other well-known, highly accurate servers or
reference time sources. It maintains a consistent Coordinated Universal
Time (UTC) within your network which is far more accurate than the
internal system clocks of client machines and prevents time drift from
occurring on the Webcache.
NTP provides client machine and server time accuracies typically within a
millisecond on LANs, relative to a primary NTP server synchronized to UTC
via a Global Positioning Service (GPS) receiver. Such accurate
time-keeping is an essential part of the operation of the Webcache.
Choosing a
Network Time
Protocol Server
You can choose to use one of the many public NTP servers that are
available on the Internet or set up your own NTP server. When you have
access to an NTP server, you can configure the Webcache to determine
the current time using NTP; see “Configuring the System Time Using the
Network Time Protocol” on page 69 for more information.
Public NTP servers are grouped into stratums. The NTP primary (stratum 1)
servers are connected to a reference clock, which is typically an expensive
cesium clock or cheaper GPS receiver. Servers operating at stratum 1 are
the most accurate available, but also the fewest in number because of
the prohibitive cost of reference clocks.
The NTP secondary (stratum 2) servers are in turn connected to a stratum
1 server and are therefore less accurate but greater in number. Stratum 3
servers are connected to stratum 2 servers, and so on, up to an imposed
limit of 15 strata. You should not use a high level public stratum server
because of their limited number and because the load placed on them is
increasingly heavy.
3Com recommends that if your network has an internal NTP server, you
should use this rather than a public stratum server. If not, you should use
the lowest stratum public NTP server available to you.