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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:
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 |