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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 ntptrace 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 public 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 script

ntptrace prospective_server1
ntptrace prospective_server2
ntptrace prospective_server3
. . .
ntptrace prospective_server15

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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