Simple Network Time Protocol (SNTP)
RFC 1769

Document Type RFC - Informational (March 1995; No errata)
Obsoleted by RFC 2030, RFC 4330
Obsoletes RFC 1361
Author David Mills 
Last updated 2013-03-02
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Network Working Group                                           D. Mills
Request for Comments: 1769                        University of Delaware
Obsoletes: 1361                                               March 1995
Category: Informational

                  Simple Network Time Protocol (SNTP)

Status of this Memo

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


   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 can operate in both unicast
   modes (point to point) and broadcast modes (point to multipoint). It
   can also operate in IP multicast mode where this service is
   available. SNTP 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 remote-procedure call (RPC) mode with accuracy
   and reliability expectations similar to the UDP/TIME protocol
   described in RFC-868.

   This memorandum obsoletes RFC-1361 of the same title. Its purpose is
   to explain the protocol model for operation in broadcast mode, to
   provide additional clarification in some places and to correct a few
   typographical errors. A working knowledge of the NTP Version 3
   specification RFC-1305 is not required for an implementation of SNTP.
   Distribution of this memorandum is unlimited.

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 characteristics of the synchronization source and
   network paths.

Mills                                                           [Page 1]
RFC 1769                          SNTP                       March 1995

   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
   in the order of large fractions of the second, is sufficient. In such
   cases simpler protocols such as the Time Protocol [POS83], have been
   used for this purpose. These protocols usually involve an 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 users of the Internet NTP synchronization
   subnet of today use a software package 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 less stringent
   accuracy expectations.

   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
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