Simple Network Time Protocol (SNTP)
RFC 1361

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

Mills                                                           [Page 8]
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.

Mills                                                           [Page 9]
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: (302) 831-8247

   EMail: mills@udel.edu

Mills                                                          [Page 10]