The NIST Internet Time Service (ITS) allows users to synchronize computer clocks via the Internet. The time information provided by the service is directly traceable to UTC(NIST). The service responds to time requests from any Internet client in several formats including the DAYTIME, TIME, and NTP protocols.

Requests in these formats generally do not support authentication, and no keys or passwords are needed to use these services.

In addition to these services (which will not be modified), we have begun testing a server that will provide authenticated NTP messages using a symmetric-key encryption algorithm. This additional service is being offered on an experimental basis only. See the authenticated NTP description for more details.

The NIST Internet Time Service uses multiple stratum-1 time servers. Here are the server names, locations, and IP addresses.

Internet time code protocols are defined by a series of documents called Request for Comments, or RFCs. These documents are available on-line from several sites on the Internet. The protocols supported by the NIST Internet Time Service are:

Network Time Protocol (RFC-1305)

The Network Time Protocol (NTP) is the most commonly used Internet time protocol, and the one that provides the best performance. Large computers and workstations often include NTP software with their operating systems. The client software runs continuously as a background task that periodically gets updates from one or more servers. The client software ignores responses from servers that appear to be sending the wrong time, and averages the results from those that appear to be correct.

Many of the available NTP software clients for personal computers don’t do any averaging at all. Instead, they make a single timing request to a signal server (just like a Daytime or Time client) and then use this information to set their computer’s clock. The proper name for this type of client is SNTP (Simple Network Time Protocol).

The NIST servers listen for a NTP request on port 123, and respond by sending a udp/ip data packet in the NTP format. The data packet includes a 64-bit timestamp containing the time in UTC seconds since January 1, 1900 with a resolution of 200 ps.

Most of the NIST time servers do not require any authentication when requesting the time in NTP format, and no keys or passwords are needed to use this service. In addition to this standard NTP service (which will not be modified), we have begun testing an authenticated version of NTP using a single time server that implements the symmetric key encryption method defined in the NTP documentation. In order to use this server, you must apply to NIST for an encryption key, which will be linked to the network address of your system. This service is being offered on an experimental basis only, and it may not be continued after the initial testing period. For more details, please see the authenticated ntp description.

Daytime Protocol (RFC-867)

This protocol is widely used by small computers running MS-DOS and similar operating systems. The server listens on port 13, and responds to requests in either tcp/ip or udp/ip formats. The standard does not specify an exact format for the Daytime Protocol, but requires that the time is sent using standard ASCII characters. NIST chose a time code format similar to the one used by its dial-up Automated Computer Time Service (ACTS), as shown below:

JJJJJ YR-MO-DA HH:MM:SS TT L H msADV UTC(NIST) OTM

where:

• JJJJJ is the Modified Julian Date (MJD). The MJD has a starting point of midnight on November 17, 1858. You can obtain the MJD by subtracting exactly 2 400 000.5 days from the Julian Date, which is an integer day number obtained by counting days from the starting point of noon on 1 January 4713 B.C. (Julian Day zero).
  
  
• YR-MO-DA is the date. It shows the last two digits of the year, the month, and the current day of month.
  
  
• HH:MM:SS is the time in hours, minutes, and seconds. The time is always sent as Coordinated Universal Time (UTC). An offset needs to be applied to UTC to obtain local time. For example, Mountain Time in the U. S. is 7 hours behind UTC during Standard Time, and 6 hours behind UTC during Daylight Saving Time.
  
  
• TT is a two digit code (00 to 99) that indicates whether the United States is on Standard Time (ST) or Daylight Saving Time (DST). It also indicates when ST or DST is approaching. This code is set to 00 when ST is in effect, or to 50 when DST is in effect. During the month in which the time change actually occurs, this number will decrement every day until the change occurs. For example, during the month of November, the U.S. changes from DST to ST. On November 1, the number will change from 50 to the actual number of days until the time change. It will decrement by 1 every day until the change occurs at 2 a.m. local time when the value is 1. Likewise, the spring change is at 2 a.m. local time when the value reaches 51.
  
  
• L is a one-digit code that indicates whether a leap second will be added or subtracted at midnight on the last day of the current month. If the code is 0, no leap second will occur this month. If the code is 1, a positive leap second will be added at the end of the month. This means that the last minute of the month will contain 61 seconds instead of 60. If the code is 2, a second will be deleted on the last day of the month. Leap seconds occur at a rate of about one per year. They are used to correct for irregularity in the earth's rotation. The correction is made just before midnight UTC (not local time).
  
  
• H is a health digit that indicates the health of the server. If H = 0, the server is healthly. If H = 1, then the server is operating properly but its time may be in error by up to 5 seconds. This state should change to fully healthy within 10 minutes. If H = 2, then the server is operating properly but its time is known to be wrong by more than 5 seconds. If H = 3, then a hardware or software failure has occurred and the amount of the time error is unknown. If H = 4 the system is operating in a special maintenance mode and both its accuracy and its response time may be degraded. This value is not used for production servers except in special circumstances. The transmitted time will still be correct to within ±1 second in this mode.
  
  
• msADV displays the number of milliseconds that NIST advances the time code to partially compensate for network delays. The advance is currently set to 50.0 milliseconds.
  
  
• The label UTC(NIST) is contained in every time code. It indicates that you are receiving Coordinated Universal Time (UTC) from the National Institute of Standards and Technology (NIST).
  
  
• OTM (on-time marker) is an asterisk (*). The time values sent by the time code refer to the arrival time of the OTM. In other words, if the time code says it is 12:45:45, this means it is 12:45:45 when the OTM arrives.

Time Protocol (RFC-868)

This simple protocol is now used by only about 1% of ITS customers. It returns a 32-bit unformatted binary number that represents the time in UTC seconds since January 1, 1900. The server listens for Time Protocol requests on port 37, and responds in either tcp/ip or udp/ip formats. Conversion to local time (if necessary) is the responsibility of the client program. The 32-bit binary format can represent times over a span of about 136 years with a resolution of 1 second. There is no provision for increasing the resolution or increasing the range of years.

The strength of the time protocol is its simplicity. Since many computers keep time internally as the number of seconds since January 1, 1970 (or another date), converting the received time to the necessary format is often a simple matter of binary arithmetic. However, the format does not allow any additional information to be transmitted, such as advance notification of leap seconds or daylight saving time, or information about the health of the server.

For questions or more information about the NIST Internet Time Service, contact Judah Levine: jlevine@boulder.nist.gov