Symmetricom S100 manual Basic NTP Configuration

SyncServer S100

Basic NTP Configuration

No special configuration is required for a machine with a running NTP to be used by other network nodes as a standard server (as opposed to a broadcast server or peer). However, access control is needed to prevent a machine from acting as an NTP server to clients.

Further, operating as a broadcast server or a peer server involves additional configuration. Basic guidelines for architecting an NTP solution, should include:

1.Limiting single points of failure and maximizing independence

2.Controlling network impact

3.Enabling access control

4.Selecting appropriate reference clocks

Single points of failure can be reduced by assuring that client servers are as independent as possible. Using a number of independent servers reduces the effectiveness of an incorrectly configured server spoofing the time, and thus increases security. Verifying NTP server independence can be difficult. To effectively map the dependencies on an NTP subnet, each of the peers and servers must be mapped. Use ntptrace to determine the hierarchy of time sources used by a client. It can then be easily identified if two machines share common time servers. A client should always receive time from at least four servers. This will reduce the chances of it losing synchronization when a server fails. If fewer than four servers are used, the agreement algorithm cannot reliably detect a clique including a majority of trusted sources. An easy solution is to use three servers from a lower stratum number and one unrelated peer from the same stratum.

The goals of an NTP architect are two-fold: to limit NTP’s network activity and increase the accuracy of the clocks. To achieve high clock accuracy, the network latency needs to be low. NTP can achieve a high level of accuracy and remain a good network citizen if local NTP servers are used and NTP servers use the appropriate modes.

The easiest way to increase accuracy in an NTP configuration is to reduce the latency between the connections by putting NTP servers on the same LAN as their clients. If a LAN is very large, it is a good idea to have multiple servers in different geographic or network segments. However, if several independent servers are used, the NTP clock selection algorithms will probably help mitigate the effects of any increased latency. Another advantage to using local servers is that they tend to reduce the load on the WAN, though NTP is unlikely to be a big source of network load.

Another way of reducing NTP traffic, while keeping clock accuracy, is to use appropriate server modes. Central servers (generally stratum 1 and 2 servers) should use non-broadcast server/client mode or peer mode, which allows more accurate time distribution. These servers are generally geographically distributed; therefore, the accuracy of the time distribution is critical.

Broadcasting over high latency links can lead to very inaccurate time, both because of the latency and because it is likely the latency will be variable and unpredictable. Using broadcasting or multicasting over relatively local connections is acceptable. In fact, for a local server with a large number of clients and a fairly constant network latency broadcasting or

96 S100 User Guide – Rev. D – June 2005