rfc1361.txt

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RFC 1361                          SNTP                       August 1992


   Root Dispersion: This is a 32-bit unsigned fixed-point number
   indicating the maximum error relative to the primary reference
   source, in seconds with fraction point between bits 15 and 16. The
   values that normally appear in this field range from zero to several
   hundred milliseconds.

   Reference Clock Identifier: This is a 32-bit code identifying the
   particular reference clock. In the case of stratum 0 (unspecified) or
   stratum 1 (primary reference), this is a four-octet, left-justified,
   zero-padded ASCII string. While not enumerated as part of the NTP
   specification, the following are representative ASCII identifiers:

      Stratum Code  Meaning
      ------------------------------------------------------------
      0   ascii     generic time service other than NTP, such as ACTS
                    (Automated Computer Time Service), TIME (UDP/Time
                    Protocol), TSP (TSP Unix time protocol), DTSS
                    (Digital Time Synchronization Service), etc.
      1   ATOM      calibrated atomic clock
      1   VLF       VLF radio (OMEGA, etc.)
      1   callsign  Generic radio
      1   LORC      LORAN-C radionavigation system
      1   GOES      Geostationary Operational Environmental Satellite
      1   GPS       Global Positioning Service
      2   address   secondary reference (four-octet Internet address of
                    the NTP server)

   Reference Timestamp: This is the local time at which the local clock
   was last set or corrected, in 64-bit timestamp format.

   Originate Timestamp: This is the local time at which the request
   departed the client for the server, in 64-bit timestamp format.

   Receive Timestamp: This is the local time at which the request
   arrived at the server, in 64-bit timestamp format.

   Transmit Timestamp: This is the local time at which the reply
   departed the server for the client, in 64-bit timestamp format.

   Authenticator (optional): When the NTP authentication mechanism is
   implemented, this contains the authenticator information defined in
   Appendix C of RFC-1305. In SNTP this field is ignored for incoming
   messages and is not generated for outgoing messages.








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RFC 1361                          SNTP                       August 1992


4. SNTP Client Operations

   The model for an SNTP client operating with either an NTP or SNTP
   server is a RPC client with no persistent state. The client
   initializes the SNTP message header, sends the message to the server
   and strips the time of day from the reply. For this purpose all of
   the message-header fields shown above are set to zero, except the
   first octet. In this octet the Leap Indicator is set to zero (no
   warning) and the Mode to 3 (client). The Version Number must agree
   with the software version of the NTP or SNTP server; however, NTP
   Version 3 (RFC-1305) servers will also accept Version 2 (RFC-1119)
   and Version 1 (RFC-1059) messages, while NTP Version 2 servers will
   also accept NTP Version 1 messages. Version 0 (original NTP described
   in RFC-959) messages are no longer supported. Since there are NTP
   servers of all three versions operating in the Internet of today, it
   is recommended that the Version Number field be set to one.

   The server reply includes all the fields described above; however, in
   SNTP only the Transmit Timestamp has explicit meaning. The integer
   part of this field contains the server time of day in the same format
   as the Time Protocol. While the fraction part of this field will
   usually be valid, the accuracy achieved with the SNTP mode of access
   probably does not justify its use.

   The following table is a summary of the SNTP client operations. There
   are three recommended error checks shown in the table. In all NTP
   versions, if the Leap Indicator field is 3 or the Transmit Timestamp
   is zero (unsynchronized), the server has never synchronized or not
   synchronized to a valid timing source within the last 24 hours. If
   the Stratum field is 0 (unspecified or unavailable), the server has
   never synchronized, has lost reachability with all timing sources or
   is synchronized by some protocol other than NTP. Whether to believe
   the transmit timestamp or not in this case is at the discretion of
   the client implementation.

















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RFC 1361                          SNTP                       August 1992


      Field Name              Request        Reply
      -------------------------------------------------------------
      Leap Indicator (LI)     0              if 3 (unsynchronized),
                                             disregard
      Version Number (VN)     (see text)     ignore
      Mode                    3 (client)     ignore
      Stratum                 0              if 0 (unspecified),
                                             disregard
      Poll                    0              ignore
      Precision               0              ignore
      Root Delay              0              ignore
      Root Dispersion         0              ignore
      Reference Identifier    0              ignore
      Reference Timestamp     0              ignore
      Originate Timestamp     0              ignore
      Receive Timestamp       0              ignore
      Transmit Timestamp      0              time of day (seconds only);
                                             if 0 (unsynchronized),
                                             disregard
      Authenticator           (not used)     ignore

5. SNTP Server Operations

   The model for an SNTP server operating with either an NTP or SNTP
   client is an RPC server with no persistent state. The SNTP server
   ignores all header fields except the first octet, modifies certain
   fields and returns the message to the sender. Since an SNTP server
   ordinarily does not implement the full set of NTP algorithms intended
   to support the highest quality service, it is recommended that an
   SNTP server be operated only in conjunction with a source of outside
   synchronization, such as a radio clock. In this case the server
   always operates at stratum 1.

   The first octet is interpreted as follows. The Leap Indicator and
   Version Number fields are ignored. Optionally, messages with version
   numbers other than 1, 2, or 3 can be discarded. For primary servers
   connected to a functioning radio clock, the Leap Indicator field is
   set to zero and the Stratum field is set to one in the reply.
   otherwise, these fields are set to 3 and zero, respectively. In any
   case the Version Number and Poll fields are copied intact to the
   reply message header. If The Mode field is set to 3 (client), it is
   changed to 4 (server) in the reply; otherwise, this field is set to 2
   (symmetric passive).

   The Stratum field is set to reflect the maximum reading error of the
   local clock. For all practical cases it is computed as the negative
   of the number of significant bits to the right of the decimal point
   in the NTP timestamp format. The Root Delay and Root Dispersion



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RFC 1361                          SNTP                       August 1992


   fields are set to zero for a primary server; optionally, the Root
   Dispersion can be set to a value corresponding to the expected
   (constant) maximum expected error of the primary reference source.
   The Reference Identifier is set to designate the primary reference
   source, as indicated in the table above. If this information is
   unspecified or unavailable, the field is set to zero.

   The timestamp fields are set as follows. The Reference Timestamp,
   Receive Timestamp and Transmit Timestamp fields are set to the time
   of day at the server. The Originate Timestamp field is copied
   unchanged from the request. The following table summarizes these
   actions.

      Field Name              Request        Reply
      ----------------------------------------------------------
      Leap Indicator (LI)     ignore         0 (normal), 3
                                             (unsynchronized)
      Version Number (VN)     ignore         copied from request
      Mode                    (see text)     (see text)
      Stratum                 ignore         server stratum (1)
      Poll                    ignore         copied from request
      Precision               ignore         server precision
      Root Delay              ignore         0
      Root Dispersion         ignore         0 (see text)
      Reference Identifier    ignore         source identifier or 0
      Reference Timestamp     ignore         time of day or 0
      Originate Timestamp     ignore         copied from request
      Receive Timestamp       ignore         time of day or 0
      Transmit Timestamp      ignore         time of day or 0
      Authenticator           ignore         (not used)

6. References

   [DAR81] Postel, J., "Internet Protocol - DARPA Internet Program
   Protocol Specification", RFC 791, DARPA, September 1981.

   [MIL92] Mills, D., "Network Time Protocol (Version 3) Specification,
   Implementation and Analysis", RFC 1305, University of Delaware,
   March 1992.

   [POS80] Postel, J., "User Datagram Protocol", RFC 768,
   USC/Information Sciences Institute, August 1980.

   [POS83] Postel, J., and K. Harrenstien, "Time Protocol", RFC 868,
   USC/Information Sciences Institute, SRI, May 1983.






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RFC 1361                          SNTP                       August 1992


Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   David L. Mills
   Electrical Engineering Department
   University of Delaware
   Newark, DE 19716

   Phone: [phone]

   EMail: [e-mail]





































Mills                                                          [Page 10]