TitleSimple Network Time Protocol (SNTP)
AuthorD. Mills
DateAugust 1992
Format:TXT, HTML
Obsoleted byRFC1769

Network Working Group                                           D. Mills
Request for Comments: 1361                        University of Delaware
                                                             August 1992

                  Simple Network Time Protocol (SNTP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is


   This memorandum describes the Simple Network Time Protocol (SNTP),
   which is an adaptation of the Network Time Protocol (NTP) used to
   synchronize computer clocks in the Internet. SNTP can be used when
   the ultimate performance of the full NTP implementation described in
   RFC-1305 is not needed or justified. It involves no change to the
   current or previous NTP specification versions or known
   implementations, but rather a clarification of certain design
   features of NTP which allow operation in a simple, stateless RPC mode
   with accuracy and reliability expectations similar to the UDP/TIME
   protocol described in RFC-868.

   This memorandum does not obsolete or update any RFC. A working
   knowledge of RFC-1305 is not required for an implementation of SNTP.

1. Introduction

   The Network Time Protocol (NTP) specified in RFC-1305 [MIL92] is used
   to synchronize computer clocks in the global Internet. It provides
   comprehensive mechanisms to access national time and frequency
   dissemination services, organize the time-synchronization subnet and
   adjust the local clock in each participating subnet peer. In most
   places of the Internet of today, NTP provides accuracies of 1-50 ms,
   depending on the jitter characteristics of the synchronization source
   and network paths.

   RFC-1305 specifies the NTP protocol machine in terms of events,
   states, transition functions and actions and, in addition, optional
   algorithms to improve the timekeeping quality and mitigate among
   several, possibly faulty, synchronization sources. To achieve
   accuracies in the low milliseconds over paths spanning major portions
   of the Internet of today, these intricate algorithms, or their
   functional equivalents, are necessary. However, in many cases
   accuracies of this order are not required and something less, perhaps

RFC 1361                          SNTP                       August 1992

   in the order of one second, is sufficient. In such cases simpler
   protocols such as the Time Protocol [POS83], have been used for this
   purpose. These protocols usually involve a remote-procedure call
   (RPC) exchange where the client requests the time of day and the
   server returns it in seconds past some known reference epoch.

   NTP is designed for use by clients and servers with a wide range of
   capabilities and over a wide range of network delays and jitter
   characteristics. Most members of the Internet NTP synchronization
   subnet of today use software packages including the full suite of NTP
   options and algorithms, which are relatively complex, real-time
   applications. While the software has been ported to a wide variety of
   hardware platforms ranging from supercomputers to personal computers,
   its sheer size and complexity is not appropriate for many
   applications. Accordingly, it is useful to explore alternative access
   strategies using far simpler software appropriate for accuracy
   expectations in the order of a second.

   This memorandum describes the Simple Network Time Protocol (SNTP),
   which is a simplified access strategy for servers and clients using
   NTP as now specified and deployed in the Internet. There are no
   changes to the protocol or implementations now running or likely to
   be implemented in the near future. The access paradigm is identical
   to the UDP/Time Protocol and, in fact, it should be easily possible
   to adapt a UDP/Time client implementation, say for a personal
   computer, to operate using SNTP. Moreover, SNTP is also designed to
   operate in a dedicated server configuration including an integrated
   radio clock. With careful design and control of the various latencies
   in the system, which is practical in a dedicated design, it is
   possible to deliver time accurate to the order of microseconds.

   It is strongly recommended that SNTP be used only at the extremities
   of the synchronization subnet. SNTP clients should operate only at
   the leaves (highest stratum) of the subnet and in configurations
   where no SNTP client is dependent on another SNTP client for
   synchronization. SNTP servers should operate only at the root
   (stratum 1) of the subnet and then only in configurations where no
   other source of synchronization other than a reliable radio clock is
   available. The full degree of reliability ordinarily expected of
   primary servers is possible only using the redundant sources, diverse
   subnet paths and crafted algorithms of a full NTP implementation.
   This extends to the primary source of synchronization itself in the
   form of multiple radio clocks and backup paths to other primary
   servers should the radio clock fail or become faulty. Therefore, the
   use of SNTP rather than NTP in primary servers should be carefully

RFC 1361                          SNTP                       August 1992

2. NTP Timestamp Format

   SNTP uses the standard NTP timestamp format described in RFC-1305 and
   previous versions of that document. In conformance with standard
   Internet practice, NTP data are specified as integer or fixed-point
   quantities, with bits numbered in big-endian fashion from zero
   starting at the left, or high-order, position. Unless specified
   otherwise, all quantities are unsigned and may occupy the full field
   width with an implied zero preceding bit zero.

   Since NTP timestamps are cherished data and, in fact, represent the
   main product of the protocol, a special timestamp format has been
   established. NTP timestamps are represented as a 64-bit unsigned
   fixed-point number, in seconds relative to 0h on 1 January 1900. The
   integer part is in the first 32 bits and the fraction part in the
   last 32 bits. This format allows convenient multiple-precision
   arithmetic and conversion to Time Protocol representation (seconds),
   but does complicate the conversion to ICMP Timestamp message
   representation (milliseconds). The precision of this representation
   is about 200 picoseconds, which should be adequate for even the most
   exotic requirements.

   Note that since some time in 1968 the most significant bit (bit 0 of
   the integer part) has been set and that the 64-bit field will
   overflow some time in 2036. Should NTP or SNTP be in use in 2036,
   some external means will be necessary to qualify time relative to
   1900 and time relative to 2036 (and other multiples of 136 years).
   Timestamped data requiring such qualification will be so precious
   that appropriate means should be readily available. There will exist
   a 200-picosecond interval, henceforth ignored, every 136 years when
   the 64-bit field will be zero, which by convention is interpreted as
   an invalid timestamp.

3. NTP Message Format

   Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
   [POS83], which itself is a client of the Internet Protocol (IP)
   [DAR81]. The structure of the IP and UDP headers is described in the
   relevant specification documents and will not be described further in
   this memorandum. Following is a description of the SNTP message
   format, which follows the IP and UDP headers. The SNTP message format
   is identical to the NTP format described in RFC-1305, with the
   exception that some of the data fields are "canned," that is,
   initialized to prespecified values. The format of the NTP message
   data area, which immediately follows the UDP header, is shown below.

RFC 1361                          SNTP                       August 1992

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |LI | VN  |Mode |    Stratum    |     Poll      |   Precision   |
      |                          Root Delay                           |
      |                       Root Dispersion                         |
      |                    Reference Identifier                       |
      |                                                               |
      |                   Reference Timestamp (64)                    |
      |                                                               |
      |                                                               |
      |                   Originate Timestamp (64)                    |
      |                                                               |
      |                                                               |
      |                    Receive Timestamp (64)                     |
      |                                                               |
      |                                                               |
      |                    Transmit Timestamp (64)                    |
      |                                                               |
      |                                                               |
      |                                                               |
      |                  Authenticator (optional) (96)                |
      |                                                               |
      |                                                               |

   As described in the next section, in SNTP most of these fields are
   initialized with prespecified data. For completeness, the function of
   each field is briefly summarized below.

   Leap Indicator (LI): This is a two-bit code warning of an impending
   leap second to be inserted/deleted in the last minute of the current
   day, with bit 0 and bit 1, respectively, coded as follows:

      LI       Value     Meaning
      00       0         no warning
      01       1         last minute has 61 seconds
      10       2         last minute has 59 seconds)
      11       3         alarm condition (clock not synchronized)

RFC 1361                          SNTP                       August 1992

   Version Number (VN): This is a three-bit integer indicating the NTP
   version number, currently 3.

   Mode: This is a three-bit integer indicating the mode, with values
   defined as follows:

      Mode     Meaning
      0        reserved
      1        symmetric active
      2        symmetric passive
      3        client
      4        server
      5        broadcast
      6        reserved for NTP control message
      7        reserved for private use

   The use of this field will be discussed in the next section.

   Stratum: This is a eight-bit integer indicating the stratum level of
   the local clock, with values defined as follows:

      Stratum  Meaning
      0        unspecified or unavailable
      1        primary reference (e.g., radio clock)
      2-15     secondary reference (via NTP or SNTP)
      16-255   reserved

   Poll Interval: This is an eight-bit signed integer indicating the
   maximum interval between successive messages, in seconds to the
   nearest power of two. The values that normally appear in this field
   range from 6 to 10, inclusive.

   Precision: This is an eight-bit signed integer indicating the
   precision of the local clock, in seconds to the nearest power of two.
   The values that normally appear in this field range from -6 for
   mains-frequency clocks to -18 for microsecond clocks found in some

   Root Delay: This is a 32-bit signed fixed-point number indicating the
   total roundtrip delay to the primary reference source, in seconds
   with fraction point between bits 15 and 16. Note that this variable
   can take on both positive and negative values, depending on the
   relative time and frequency errors. The values that normally appear
   in this field range from negative values of a few milliseconds to
   positive values of several hundred milliseconds.

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.

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.

RFC 1361                          SNTP                       August 1992

      Field Name              Request        Reply
      Leap Indicator (LI)     0              if 3 (unsynchronized),
      Version Number (VN)     (see text)     ignore
      Mode                    3 (client)     ignore
      Stratum                 0              if 0 (unspecified),
      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),
      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

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

      Field Name              Request        Reply
      Leap Indicator (LI)     ignore         0 (normal), 3
      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.

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