Simple Network Time Protocol (SNTP)
RFC 1361

Document Type RFC - Informational (August 1992; Errata)
Obsoleted by RFC 1769
Last updated 2014-03-03
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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
   unlimited.

Abstract

   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

Mills                                                           [Page 1]
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
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