Network Working Group                                        D. Mills
Request for Comments:  1059                    University of Delaware
                                                            July 1988

                   Network Time Protocol (Version 1)
                    Specification and Implementation

Status of this Memo

   This memo describes the Network Time Protocol (NTP), specifies its
   formal structure and summarizes information useful for its
   implementation.  NTP provides the mechanisms to synchronize time and
   coordinate time distribution in a large, diverse internet operating
   at rates from mundane to lightwave.  It uses a returnable-time design
   in which a distributed subnet of time servers operating in a self-
   organizing, hierarchical master-slave configuration synchronizes
   logical clocks within the subnet and to national time standards via
   wire or radio.  The servers can also redistribute reference time via
   local routing algorithms and time daemons.

   The NTP architectures, algorithms and protocols which have evolved
   over several years of implementation and refinement are described in
   this document.  The prototype system, which has been in regular
   operation in the Internet for the last two years, is described in an
   Appendix along with performance data which shows that timekeeping
   accuracy throughout most portions of the Internet can be ordinarily
   maintained to within a few tens of milliseconds, even in cases of
   failure or disruption of clocks, time servers or nets.  This is a
   Draft Standard for an Elective protocol.  Distribution of this memo
   is unlimited.

                             Table of Contents

   1.      Introduction                                               3
   1.1.    Related Technology                                         4
   2.      System Architecture                                        6
   2.1.    Implementation Model                                       7
   2.2.    Network Configurations                                     9
   2.3.    Time Scales                                               10
   3.      Network Time Protocol                                     12
   3.1.    Data Formats                                              12
   3.2.    State Variables and Parameters                            13
   3.2.1.  Common Variables                                          15
   3.2.2.  System Variables                                          17
   3.2.3.  Peer Variables                                            18
   3.2.4.  Packet Variables                                          19
   3.2.5.  Clock Filter Variables                                    19
   3.2.6.  Parameters                                                20




 
RFC 1059                 Network Time Protocol                 July 1988


   3.3.    Modes of Operation                                        21
   3.4.    Event Processing                                          22
   3.4.1.  Timeout Procedure                                         23
   3.4.2.  Receive Procedure                                         24
   3.4.3.  Update Procedure                                          27
   3.4.4.  Initialization Procedures                                 29
   4.      Filtering and Selection Algorithms                        29
   4.1.    Clock Filter Algorithm                                    29
   4.2     Clock Selection Algorithm                                 30
   4.3.    Variable-Rate Polling                                     32
   5.      Logical Clocks                                            33
   5.1.    Uniform Phase Adjustments                                 35
   5.2.    Nonuniform Phase Adjustments                              36
   5.3.    Maintaining Date and Time                                 37
   5.4.    Calculating Estimates                                     37
   6.      References                                                40

   Appendices
   Appendix A. UDP Header Format                                     43
   Appendix B. NTP Data Format                                       44
   Appendix C. Timeteller Experiments                                47
   Appendix D. Evaluation of Filtering Algorithms                    49
   Appendix E. NTP Synchronization Networks                          56

   List of Figures
   Figure 2.1. Implementation Model                                   8
   Figure 3.1. Calculating Delay and Offset                          26
   Figure 5.1. Clock Registers                                       34
   Figure D.1. Calculating Delay and Offset                          50
   Figure E.1. Primary Service Network                               57

   List of Tables
   Table 2.1. Dates of Leap-Second Insertion                         11
   Table 3.1. System Variables                                       14
   Table 3.2. Peer Variables                                         14
   Table 3.3. Packet Variables                                       15
   Table 3.4. Parameters                                             15
   Table 4.1. Outlyer Selection Procedure                            32
   Table 5.1. Clock Parameters                                       35
   Table C.1. Distribution Functions                                 47
   Table D.1. Delay and Offset Measurements (UMD)                    52
   Table D.2.a Delay and Offset Measurements (UDEL)                  52
   Table D.2.b Offset Measurements (UDEL)                            53
   Table D.3. Minimum Filter (UMD - NCAR)                            54