Computer Time Synchronization
A Beginner's Guide to Network Time Protocol (NTP)
Warning! This page contains obsolete information.
Selecting Public NTP Time servers
An important factor in getting a network correctly set up with
NTP is the selection of servers from which time is obtained.
Depending on your network size you will be using either public
stratum 1 or 2 time servers or may create your own private or
public stratum 1 time server with the appropriate receiving
device. You want time servers that are close to you in network
delay which loosely correlates to geographic proximity. You also
want diverse sources. If all your NTP servers (or clients) use
the same sources then any error in one of them will be magnified.
If you are using stratum 2 time servers, it is desirable to use
stratum 2 time servers that use different stratum 1 servers as
any errors from the top level time server may be propagated
through the next level servers.
The NTP software package includes an
utility that gives the offset and network distance of NTP servers
as well as their parent time servers. (One thing I don't understand
is that you're supposed to use multiple servers at each level but
ntptrace only shows one server at each level above the initial
server being queried. How does it know which one to use? From
my own servers, it appears to change dynamically sometimes
reporting one and sometimes other higher level servers; this may
be the server with smallest current synch distance.)
*NOTE* Since this page was written four years ago in 2001, a new
type of server called a pool server has become available. These are
available without restrictions and it is generally easier and quicker
to find pool servers close to you than unrestricted stratum 1 or 2
time servers. You should start with the appropriate
server list. If you use pool servers you don't
need to manually search for stratum 2 or 1
time servers near you geographically with conditions
that are acceptable to you. Additionally, some stratum 1 or 2 servers
require prior notification or arrangements to get through a firewall.
The pool servers may not be as optimal as carefully selected stratum
1 or 2 servers but they are much simpler. In 2001 I easily found more
than 15 US stratum 2 servers that met my needs to select from. Four years
later I could not find 4.
After you've picked a prospective list of time servers, run ntptrace
on them and pick those that are nearby in terms of "synch
distance" and use different stratum 1 servers if you are using
stratum 2 servers and to the extent practical, are accessed via
different network paths. Finding the best servers was much more
time consuming than installing the software (a good reason for
using pool servers).
Ntptrace does no averaging or multiple passes. I wrote a tiny
. . .
that was nothing but date and ntptrace followed by prospective
server names on each line. The output was redirected to a
file and then printed. This helped me see patterns more easily
than manually running ntptrace many times. Once I saw the
stratum 1 servers, I grouped all stratum 2 servers with the same
stratum 1 server so I could see which was best of each group.
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