Why do you need your own time server although there are time servers available on the internet?

In principle it is possible to synchronize your computers with time servers on the internet.

However, a lot of our customers rely on their own time server in their network environment for security and/or maintainability reasons.

• Particularly in the case of our LANTIME you or a responsible person can be notified by mail or SNMP trap if there is a malfunction in your time synchronisation.
  
• The clients on the network do not depend on an active internet connection.
  
• The clients on the network do not depend on the availability of an external time server
  
• A test of other freely available time servers (not PTB!) reported that many NTP servers distributed a significantly wrong time, although they were classified as stratum-1 time servers. This is the responsibility of the server's administrators.
  
• If an internet connection is working properly then NTP can determine and account for the packet transmission delays quite reliable. However, if the internet connection is at its capacity limit, time synchronization can be significantly degraded due to high dispersion in packet transmission delays. Reasons for this may be hacker attacks, which must not address your own network, or new viruses causing a huge flood of emails, like it has already happened in the past.
  
• An own time server can not easily be compromised out of the internet. At the first moment this sounds trivially, however, there has been a case which really occurred and which caused some concern to the NTP community:
  A manufacturer of low-cost routers had the IP address of a public NTP server hard coded into the firmware in order to let the routers get their time from that server. Additionaly, the implementation of the firmware was faulty, which resulted in a huge flood of NTP queries being sent to the NTP server. This did not even constrain the function of the NTP server, but also caused large costs to the operator due to the huge network traffic. In this case not even the deactivation of the NTP server did help since the faulty routers kept sending request packet even at a higher rate if the NTP server did not respond. Details can be found under the following link:
  
  http://www.cs.wisc.edu/~plonka/netgear-sntp
  
  In the United States the U.S. Naval Observatory (USNO) has a similar function to spread the legal time as the PTB in Germany, and also operates publicly available NTP servers for a long time. Those NTP servers are more and more constrained by "bad" clients, which makes the future of the public service questionnable. There are already precautions to limit the affect of such clients. Dave Mills, the originator of NTP, cooperates with the USNO and has already adverted this in the NTP news group:
  
  http://groups.google.de/groups?q=ntp+clogging+defense

The topics outlined above should provide some arguments to install an own time server, if an accurate time is a requirement for the reliable operation of a local network.