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Chapter 25 - Configuring NTP
Chapter 25 - Configuring NTP
Introduction
This chapter familiarizes the user with:
• Enabling/Disabling NTP
• Setting servers and peers
• Setting generic NTP options
• NTP Tools
NTP Fundamentals
NTP (Network Time Protocol) is an Internet protocol used to synchronize the clocks
of computers to some time reference. Variants of NTP such as SNTP (Simple NTP, a
reduced functionality NTP) and XNTP (Experimental NTP) exist. NTP itself is
available in versions 3 and 4 (RuggedRouter includes version 4).
NTP is a fault-tolerant protocol that allows an NTP daemon program to automatically
select the best of several available time sources, or reference clocks, to synchronize to.
Multiple candidates can be combined to minimize the accumulated error. Temporarily
or permanently wrong time sources are detected and avoided.
The NTP daemon achieves synchronization by making small and frequent changes to
the router hardware clock.
The NTP daemon operates in a client-server mode, both synchronizing from servers
and providing synchronization to peers.
If NTP has a number of servers to choose from, it will synchronize with the lowest
stratum server. The stratum is a measure of the number of servers to the (most highly
accurate) reference clock. A reference clock itself appears at stratum 0. A server
synchronized to a stratum n server will be running at stratum n + 1.
You will generally configure lower stratum NTP hosts as servers and other NTP hosts
at the same stratum as peers. If all your configured servers fail, a configured peer will
help in providing the NTP time. It is generally a good idea to configure one at least
one server and peer.
The NTP daemon will know about the NTP servers and peers to use in three ways.
• It can be configured manually with a list of servers to poll from,
• It can be configured manually with a list of peers to send to,
• It can look at advertisements issued by other servers on multicast or broadcast
addresses.
Note that if multicasting or broadcasting is used, it is strongly recommended to enable
authentication unless you trust all hosts on the network.
NTP uses UDP/IP packets for data transfer because of the fast connection setup and
response times UDP offers. The NTP protocol uses port UDP port 123. Note that if
your router employs a firewall and acts as a client it must open UDP port 123.
Additionally, if the router acts as a server the firewall must allow connection requests
on port 123 as well.
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