CCIE Practical Studies, Volume I

NTP Overview

NTP was designed to provide an accurate and stable clock to remote sources across an unmanaged, global Internet environment. Before NTP, other protocols such as Daytime protocol, Time protocol, and ICMP timestamp provided this service. Digital Time Service (DTS) accomplished many of the same objectives as NTP. However, NTP provides for the use of stratum information in clock selection and provides an accurate compensation for inherent clock frequency errors. DTS does not use a stratum or compensate for inherent frequency errors.

The concept of a stratum was conceived directly from the telephone industry, under BELL 86. The accuracy of each NTP server was defined with a stratum number. The most accurate server stratum starts at 1 and increments from there. The implementation of a stratum allows NTP to select from multiple clock sources and judge which one to synchronize with.

The stratum value of 1 requires the accuracy provided by atomic clocks. You will not be able to configure a Cisco router to supply or have a stratum 1 clock, for obvious reasons.

NOTE

Atomic clocks have oscillators that can maintain extremely precise frequencies that correspond to natural phenomenon . The earliest and ultimate oscillators are our celestial bodies. But because of their vastness and our lack of scientific knowledge about them, scientists base atomic clocks on the orbital states of an electron instead of the orbital states of planets and our solar system. The atom provides a stable and accurate model on which scientists can perform accurate measurements. Atomic oscillators are based on the transitions of hydrogen, cesium, and rubidium atoms .

NOTE

To test NTP, I used the shareware TARDIS2000 V1.2, available at c/net's DOWNLOAD.COM or http://download.cnet.com. Two public NTP/SNTP clocks' addresses that you can use in your models are zeus.tamu.edu or 128.194.103.14, and tmc.edu or 128.249.1.1.

After you synchronize with them, most atomic clocks provide a stratum level of 3 or greater to the client. Consider 3 to be a highly reliable stratum level when using NTP.

NTP was specifically designed to produce three products:

• Clock offset ” Clock offset is the amount by which to adjust the local clock so that it corresponds to the reference clock.

• Round-trip delay ” Round-trip delay makes it possible for a reference clock to launch a message to arrive back at the reference clock at a specific time.

• Dispersion ” Dispersion is the maximum error of the local clock relative to the reference or NTP server.

These products are all produced in relation to the local clock, and they use Greenwich Mean Time, UTC, or Coordinated Universal Time as a common reference in time. If the local clock is unreliable, it will affect NTP synchronization, as well as the values provided by NTP, which also provides these values with a relatively simple data flow and little overhead to the network. NTP accomplishes this by using User Datagram Protocol (UDP) port 123. Data integrity is provided by UDP checksums; no flow control or retransmission facilities are provided or necessary.

NTP can be used in a variety of ways. Perhaps the most common use of NTP is for an NTP client to obtain a valid clock from an external source through IP. If multiple clients are synchronizing off the same source, this allows for entire network synchronization. Network clock synchronization can provide several useful functions:

• As the tragic story of Bob illustrates, NTP can be a useful protocol when trying to correlate network anomalies, such as link or neighbor failures.

• Network clock synchronization helps to correlate time accurate logs and debug information from multiple routers.

• Network-management platforms, such as CiscoWorks and HP OpenView, will be more effective and accurate in reporting network statistics.

NTP was first described in RFC 958, but since its first release, it has evolved many times. NTP version 3 is the dominant version of NTP today. RFC 1305 outlines NTP and obsoletes RFCs 1119, 1059, and 958. RFC 2030 outlines the Simple Network Time Protocol (SNTP), which is an adaptation of NTP Version 3. The only significant change in SNTP Version 4 is its adaptation to properly interpret an IPv6 header and OSI addressing.

NOTE

You can find all RFCs online at www.isi.edu/in-notes/rfc xxxx.txt, where xxxx is the number of the RFC.