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TNS Services

Functional Description

Suggested Configurations
End User devices
Server devices

Technical Specifications

Online Resources

Future Services

Time Reference Services Offered by TNS

Telecommunications and Networking Services (TNS) provides many Networking Services including Network Time Protocol (NTP) services. Other groups at Penn State provide higher level services which rely on these underlying services. There are no charges for these services.

This document describes the features, configuration methods, expected accuracy, and reference material for implementing use of the services. It also offers a structural model of additional future functions that can be built on the existing services.

Functional Description

    Telecommunications and Networking Services (TNS) provides time services based on the Network Time Protocol (NTP) that were established to provide accurate time references for computer users at Penn State. The services are synchronized to The National Institute of Standards and Technology (NIST), formerly The National Bureau of Standards (NBS). The connections are via High Frequency (HF) radio and Global Positioning System (GPS) satellite receivers as well as multiple Internet paths to computers across the nation that have similar receivers or calibrated atomic clocks.

    A simplified description of NTP follows:

    1. NIST or other time service provides a reference time. This is referred to as a "Stratum 0" source; it is not possible to get down to a closer level of accuracy for time. This device only outputs a time reference; it does not listen to other sources.
    2. A device that is synchronized to a Stratum 0 reference time clock is referred to as a "Stratum 1" source. It is typically attached to the directly synchronized clock via radio, GPS or portable atomic clock. The stratum 1 device answers queries from other devices that request its time information. This device can be configured to use other sources if its primary reference (from the Stratum 0 clock) fails in some manner.
    3. Devices that connect to Stratum 1 sources via NTP are referred to as "Stratum 2" clocks. Many stratum 2 clocks may derive time from the same stratum 1 clock. Stratum 2 (and above) clocks can also "peer" with each other to make use of divergent network paths, which leads to a more stable and robust time for all devices in the peer group. The server clock.psu.edu is a stratum 2 clock.
    4. A "Stratum 3" device has retrieved its time from a stratum 2 server. These may be departmental network servers with many clients, or they could be individual user devices.

    In any of the above steps to provide time, the NTP algorithm may decide to "throw out" a server which appears to be "crazy" or otherwise providing the wrong time. A device running NTP is able to make this decision if it has at least three sources to compare. Thus the method of configuration of an NTP client starts by selecting at least three NTP servers (ref. next section).

    The above structure is in place as shown in this simplified drawing of NTP servers at Penn State.

Suggested Configurations

For the simplest configuration, any device at Penn State should point to clock.psu.edu as a minimum time source. Even devices expected to operate at stratum 2 or lower would be advised to configure that host to provide robustness in case of lower stratum server failures. The notable exception to this suggestion is the case where a network of user devices has a time source provided by a local network server that either filters packets or provides a proxy function (for example: a secure LAN, computer lab, restricted-bandwidth connection, etc.).

End User devices:

Many users at Penn State are clients of some form of "Network" operating system. The Network server machine may provide time automatically to its clients, so no time service configuration is required beyond that. However, if the Network Administrator has not configured the Network server to retrieve the correct time (see Server devices below) then the users should point that person to this page for information on accomplishing that service implementation.

It is suggested that the configuration of NTP clients on end user devices (which serve no additional networks of time clients) be set to use no lower than a stratum 2 source such as clock.psu.edu. Multiple stratum 2 (or higher) devices may be used, but the preference would be to select servers on the same LAN or in Penn State at large to increase reliability over those which are elsewhere on the internet. Check the Online Resources for locations of software for particular operating systems to set the local device time.

Server devices:

Devices that are expected to serve more than 10 clients for time services may be configured to use the TNS stratum 1 servers. Use of other internet NTP servers is constrained by the acceptable use parameters listed in the Public NTP Servers sub-page from the NTP Server information page (see Online Resources section).

A device running NTP is only able to make intelligent time source decisions if it has at least three sources to compare. Thus the method of configuration of an NTP client starts by selecting at least three NTP servers. If multiple servers are to be configured, especially in peer mode, the sources should be semi-overlapping to prevent common failure modes and to increase robustness.

The NTP software suite includes tools to check the peer and server status of a particular device. If many servers use the same eventual stratum 0 source, the entire tree structure will be easily subject to a single point of failure. The configuration should use diverse sources and diverse network paths if possible. (Note that multiple GPS time sources are a diverse source, unlike multiple WWV receivers which all receive time from the same source.)

Start with at least two Penn State servers and one Internet server. An example starter configuration will be provided here after approval by administrators of the involved systems. Then monitor the parameters to decide if more sources are needed, or if one or more should be replaced. Details on configuration are included on the NTP Server web page (in the Online Resources section below).

Devices that use Distributed Computing Environment (DCE) may configure their Distributed Time Service (DTS) to point at NTP servers. The current DCE environment at Penn State requires only a 5-minute window of time accuracy, but some heavily-loaded devices must run a time protocol like NTP to attain even that.

Other servers at Penn State:

The following NTP servers are a subset of those which provide time to Penn State devices:

clock.aset.psu.edu
clock.cse.psu.edu
clock1.math .psu.edu
spain.pop.psu.edu

Technical Specifications

    The following table provides the expected accuracy relative to Universal Coordinated Time (UTC sic), formerly known as Greenwich Mean Time (GMT) of each type of time service at TNS.

    Service

    Stratum

    Accuracy

    clock.psu.edu

    2

    10 ms (0.01 Seconds)

    wwv.tns.its.psu.edu

    1

    20 ms (0.02 Seconds)

    Gps1.tns.its.psu.edu

    1

    1 ms (0.001 Seconds)

    Note that lower Stratum level does not imply a better time accuracy. The lower stratum devices (level 1 in this table) are intended to provide different source material for a higher stratum device (level 2 in this table) to choose from. In fact, the stratum 2 server clock.psu.edu uses the TNS stratum 1 servers as well as other off-campus stratum 1 servers to derive its time.

    A sanity check is in order at this point. If a selection of source clocks is being considered for a particular configuration, the use of time on the device(s) should also be considered. For simple email and spreadsheet work, an accuracy of 1 second may be sufficient. However, if the devices are being used in a multiple-machine software computation, it may be necessary to work for better accuracy.

Online Resources
Future Services

    Semi-Annual meetings of NTP administrators for server devices would be advisable to ensure reliability and accuracy of time services at Penn State. If interest exists, an email to mr-atoz@math.psu.edu could initiate a meeting discussion.

    It is possible to distribute time from NTP via Multicast and TNS could configure that service if a Penn State group that would benefit from its availability requested it. Note that few operating systems currently support multicast NTP as a method of time synchronization. If more devices become capable of using the service, the existence of NTP Over Multicast could save time for network administrators who would otherwise have to configure clients to receive time via another method.

    An additional output from the clock which provides Gps1.tns.its.psu.edu will eventually be adapted to a workstation that is directly connected to an Internet2 style network at Penn State. This will provide the overall accuracy of the GPS receiver to Internet2 style networks at Penn State and elsewhere. Resolution better than 1 microsecond (0.000001 seconds) will be attainable if the configuration is carefully assembled and maintained. With this accuracy, researchers at Internet2 sites will be able to study network latency and other physical phenomenon, where previously no method existed to provide the needed time references.

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