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Network Time Protocol (NTP)

[Originally published October 16, 2017]

NTP is a component of ISC's Network Names & Numbers service.

PennNet NTP Service
Information Systems & Computing operates the central Network Time Protocol (NTP) service.   
NTP is a mechanism for synchronizing computer clocks across the network. NTP servers provide network-encoded timing signals to clients interested in setting and maintaining their clocks. What time is it now? How fast is time passing?
The NTP service at Penn is distributed across three servers in multiple, distinct data centers across the Penn campus. Each server has its own, local, high-performance source of reference time, and receives authenticated NTP data from the National Institute of Standards and Technology (NIST). In addition, all three servers share timing information with each other in a secure manner.
Using the PennNet NTP service
Virtually all platforms are capable of using NTP, and of receiving NTP information from multiple sources simultaneously. Configure your system with the NTP servers shown on the right. Below are some hints for the most common platforms.
  • Apple OS X - In the Date & Time section of the System Preferences, choose the Date & Time tab, check Set date and time automatically, and enter the three server hostnames separated by commas, like this:,, timeserv
  • Microsoft Windows - From a command prompt (Start → type 'cmd' → right click the shortcut and run as administrator), issue the following commands to set the server list and force the computer to resynchronize.
  1. w32tm /config /syncfromflags:manual /manualpeerlist:""
  2. w32tm /config /update
  • Linux/BSD/*nix - Refer to the documentation for your platform (e.g. Ubuntu, Red Hat, FreeBSD, or Solaris). In most cases, you will be able to add the following lines to ntp.conf, and restart the ntpd service.
server iburst
server iburst
server iburst

PennNet NTP Service

NTP Service Architecture

Each of the three NTP servers is equipped with an embedded reference clock, which is set and disciplined by a radio receiver. Two units receive L-band radio signals from the Global Positioning System (GPS) satellites, while the third unit receives AM radio signals from the WWVB transmitter in Fort Collins, CO.

In addition to the on-board timing electronics, each of the three servers acts as an NTP client to a pair of NTP servers operated by the National Institute of Standards and Technology (NIST) as part of their Internet Time Service (ITS). These protocol associations are secured by Penn-specific symmetric keys assigned to us by NIST.

Finally, the three servers are all associated with each other as peers— timing information can flow in either direction—using independent, symmetric keys for security.

NTP Server Specifications

Frequently Asked Questions

If your systems are configured to receive NTP service from any of the following names or IP addresses, you should update your configuration as soon as possible to use active, supported time servers.

The retired NTP servers were:


Yes and no. The central DHCP servers will return NTP server IP addresses to clients that request them (DHCP option 42). The NTP servers do not perform broadcast or multicast protocol operations.

The Network Time Foundation maintains a site with copious reference material and links to additional information.

Visit the Time and Frequency Division of the National Institute of Standards and Technology (NIST) to learn more about the Internet Time Service and WWVB Radio Station, which are among the time references that the PennNet NTP Service uses to maintain reliable, secure, accurate time for Penn.

Though we tend to think of its positioning and navigation applications, the Global Positioning System (GPS) also provides high-performance timing signals, which represent another key component of the PennNet NTP Service.