Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI
RFC 4330

Network Working Group                                           D. Mills
Request for Comments: 4330                        University of Delaware
Obsoletes: 2030, 1769                                       January 2006
Category: Informational

             Simple Network Time Protocol (SNTP) Version 4
                         for IPv4, IPv6 and OSI

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memorandum describes the Simple Network Time Protocol Version 4
   (SNTPv4), which is a subset of the Network Time Protocol (NTP) used
   to synchronize computer clocks in the Internet.  SNTPv4 can be used
   when the ultimate performance of a full NTP implementation based on
   RFC 1305 is neither needed nor justified.  When operating with
   current and previous NTP and SNTP versions, SNTPv4 requires no
   changes to the specifications or known implementations, but rather
   clarifies certain design features that 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 1769, which describes SNTP Version 3
   (SNTPv3), and RFC 2030, which describes SNTPv4.  Its purpose is to
   correct certain inconsistencies in the previous documents and to
   clarify header formats and protocol operations for NTPv3 (IPv4) and
   SNTPv4 (IPv4, IPv6, and OSI), which are also used for SNTP.  A
   further purpose is to provide guidance for home and business client
   implementations for routers and other consumer devices to protect the
   server population from abuse.  A working knowledge of the NTPv3
   specification, RFC 1305, is not required for an implementation of
   SNTP.

Mills                        Informational                      [Page 1]
RFC 4330             SNTPv4 for IPv4, IPv6 and OSI          January 2006

Table of Contents

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................5
   2. Operating Modes and Addressing ..................................5
   3. NTP Timestamp Format ............................................6
   4. Message Format ..................................................8
   5. SNTP Client Operations .........................................13
   6. SNTP Server Operations .........................................16
   7. Configuration and Management ...................................19
   8. The Kiss-o'-Death Packet .......................................20
   9. On Being a Good Network Citizen ................................21
   10. Best Practices ................................................21
   11. Security Considerations .......................................24
   12. Acknowledgements ..............................................24
   13. Contributors ..................................................24
   14. Informative References ........................................25

1.  Introduction

   The Network Time Protocol Version 3 (NTPv3), specified in RFC 1305
   [MIL92], is widely used to synchronize computer clocks in the global
   Internet.  It provides comprehensive mechanisms to access national
   time and frequency dissemination services, organize the NTP subnet of
   servers and clients, and adjust the system clock in each participant.
   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.

   RFC 1305 specifies the NTP protocol machine in terms of events,
   states, transition functions and actions, and engineered algorithms
   to improve the timekeeping quality and to mitigate several
   synchronization sources, some of which may be faulty.  To achieve
   accuracies in the low milliseconds over paths spanning major portions
   of the Internet, these intricate algorithms, or their functional
   equivalents, are necessary.  In many applications, accuracies on the
   order of significant fractions of a second are acceptable.  In simple
   home router applications, accuracies of up to a minute may suffice.
   In such cases, simpler protocols, such as the Time Protocol specified
   in RFC 868 [POS83], have been used for this purpose.  These protocols
   involve an RPC exchange where the client requests the time of day and
   the server returns it in seconds past a 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 jitter and clock
   frequency wander characteristics.  Many users of NTP in the Internet