RFC 5905
Network Time Protocol Version 4: Protocol and Algorithms Specification

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Internet Engineering Task Force (IETF)                          D. Mills
Request for Comments: 5905                                   U. Delaware
Obsoletes: 1305, 4330                                     J. Martin, Ed.
Category: Standards Track                                            ISC
ISSN: 2070-1721                                               J. Burbank
                                                                W. Kasch
                                                               June 2010

Network Time Protocol Version 4: Protocol and Algorithms Specification


   The Network Time Protocol (NTP) is widely used to synchronize
   computer clocks in the Internet.  This document describes NTP version
   4 (NTPv4), which is backwards compatible with NTP version 3 (NTPv3),
   described in RFC 1305, as well as previous versions of the protocol.
   NTPv4 includes a modified protocol header to accommodate the Internet
   Protocol version 6 address family.  NTPv4 includes fundamental
   improvements in the mitigation and discipline algorithms that extend
   the potential accuracy to the tens of microseconds with modern
   workstations and fast LANs.  It includes a dynamic server discovery
   scheme, so that in many cases, specific server configuration is not
   required.  It corrects certain errors in the NTPv3 design and
   implementation and includes an optional extension mechanism.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Requirements Notation ......................................5
   2. Modes of Operation ..............................................6
   3. Protocol Modes ..................................................6
      3.1. Dynamic Server Discovery ...................................7
   4. Definitions .....................................................8
   5. Implementation Model ...........................................10
   6. Data Types .....................................................12
   7. Data Structures ................................................16
      7.1. Structure Conventions .....................................16
      7.2. Global Parameters .........................................16
      7.3. Packet Header Variables ...................................17
      7.4. The Kiss-o'-Death Packet ..................................24
      7.5. NTP Extension Field Format ................................25
   8. On-Wire Protocol ...............................................26
   9. Peer Process ...................................................30
      9.1. Peer Process Variables ....................................31
      9.2. Peer Process Operations ...................................33
   10. Clock Filter Algorithm ........................................37
   11. System Process ................................................39
      11.1. System Process Variables .................................40
      11.2. System Process Operations ................................41
           11.2.1. Selection Algorithm ...............................43
           11.2.2. Cluster Algorithm .................................44
           11.2.3. Combine Algorithm .................................45
      11.3. Clock Discipline Algorithm ...............................47
   12. Clock-Adjust Process ..........................................51
   13. Poll Process ..................................................51
      13.1. Poll Process Variables ...................................51
      13.2. Poll Process Operations ..................................52
   14. Simple Network Time Protocol (SNTP) ...........................54
   15. Security Considerations .......................................55
   16. IANA Considerations ...........................................58
   17. Acknowledgements ..............................................59
   18. References ....................................................59
      18.1. Normative References .....................................59
      18.2. Informative References ...................................59
   Appendix A.  Code Skeleton  .......................................61
     A.1.  Global Definitions  .......................................61
       A.1.1. Definitions, Constants, Parameters .....................61
       A.1.2. Packet Data Structures .................................65
       A.1.3. Association Data Structures ............................66
       A.1.4. System Data Structures .................................68
       A.1.5. Local Clock Data Structures ............................69
       A.1.6. Function Prototypes ....................................69
     A.2. Main Program and Utility Routines ..........................70
     A.3. Kernel Input/Output Interface ..............................73
     A.4. Kernel System Clock Interface ..............................74
     A.5. Peer Process ...............................................76
       A.5.1. receive() ..............................................77
       A.5.2. clock_filter() .........................................85
       A.5.3. fast_xmit() ............................................88
       A.5.4. access() ...............................................89
       A.5.5. System Process .........................................90
       A.5.6. Clock Adjust Process ..................................103
       A.5.7. Poll Process ..........................................104

1.  Introduction Go to the top of this page...top

   This document defines the Network Time Protocol version 4 (NTPv4),
   which is widely used to synchronize system clocks among a set of
   distributed time servers and clients.  It describes the core
   architecture, protocol, state machines, data structures, and
   algorithms.  NTPv4 introduces new functionality to NTPv3, as
   described in [RFC1305], and functionality expanded from Simple NTP
   version 4 (SNTPv4) as described in [RFC4330] (SNTPv4 is a subset of
   NTPv4).  This document obsoletes [RFC1305] and [RFC4330].  While
   certain minor changes have been made in some protocol header fields,
   these do not affect the interoperability between NTPv4 and previous
   versions of NTP and SNTP.

   The NTP subnet model includes a number of widely accessible primary
   time servers synchronized by wire or radio to national standards.
   The purpose of the NTP protocol is to convey timekeeping information
   from these primary servers to secondary time servers and clients via
   both private networks and the public Internet.  Precisely tuned
   algorithms mitigate errors that may result from network disruptions,
   server failures, and possible hostile actions.  Servers and clients
   are configured such that values flow towards clients from the primary
   servers at the root via branching secondary servers.

   The NTPv4 design overcomes significant shortcomings in the NTPv3
   design, corrects certain bugs, and incorporates new features.  In
   particular, expanded NTP timestamp definitions encourage the use of
   the floating double data type throughout the implementation.  As a
   result, the time resolution is better than one nanosecond, and
   frequency resolution is less than one nanosecond per second.
   Additional improvements include a new clock discipline algorithm that
   is more responsive to system clock hardware frequency fluctuations.
   Typical primary servers using modern machines are precise within a
   few tens of microseconds.  Typical secondary servers and clients on
   fast LANs are within a few hundred microseconds with poll intervals
   up to 1024 seconds, which was the maximum with NTPv3.  With NTPv4,
   servers and clients are precise within a few tens of milliseconds
   with poll intervals up to 36 hours.

   The main body of this document describes the core protocol and data
   structures necessary to interoperate between conforming
   implementations.  Appendix A contains a full-featured example in the
   form of a skeleton program, including data structures and code
   segments for the core algorithms as well as the mitigation algorithms
   used to enhance reliability and accuracy.  While the skeleton program
   and other descriptions in this document apply to a particular
   implementation, they are not intended as the only way the required
   functions can be implemented.  The contents of Appendix A are non-
   normative examples designed to illustrate the protocol's operation
   and are not a requirement for a conforming implementation.  While the
   NTPv3 symmetric key authentication scheme described in this document
   has been carried over from NTPv3, the Autokey public key
   authentication scheme new to NTPv4 is described in [RFC5906].

   The NTP protocol includes modes of operation described in Section 2
   using data types described in Section 6 and data structures described
   in Section 7.  The implementation model described in Section 5 is
   based on a threaded, multi-process architecture, although other
   architectures could be used as well.  The on-wire protocol described
   in Section 8 is based on a returnable-time design that depends only
   on measured clock offsets, but does not require reliable message
   delivery.  Reliable message delivery such as TCP [RFC0793] can
   actually make the delivered NTP packet less reliable since retries
   would increase the delay value and other errors.  The synchronization
   subnet is a self-organizing, hierarchical, master-slave network with
   synchronization paths determined by a shortest-path spanning tree and
   defined metric.  While multiple masters (primary servers) may exist,
   there is no requirement for an election protocol.

   This document includes material from [ref9], which contains flow
   charts and equations unsuited for RFC format.  There is much
   additional information in [ref7], including an extensive technical
   analysis and performance assessment of the protocol and algorithms in
   this document.  The reference implementation is available at

   The remainder of this document contains numerous variables and
   mathematical expressions.  Some variables take the form of Greek
   characters, which are spelled out by their full case-sensitive name.
   For example, DELTA refers to the uppercase Greek character, while
   delta refers to the lowercase character.  Furthermore, subscripts are
   denoted with '_'; for example, theta_i refers to the lowercase Greek
   character theta with subscript i, or phonetically theta sub i.  In
   this document, all time values are in seconds (s), and all
   frequencies will be specified as fractional frequency offsets (FFOs)
   (pure number).  It is often convenient to express these FFOs in parts
   per million (ppm).

1.1.  Requirements Notation Go to the top of this page...top

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Modes Of Operation Go to the top of this page...top

   An NTP implementation operates as a primary server, secondary server,
   or client.  A primary server is synchronized to a reference clock
   directly traceable to UTC (e.g., GPS, Galileo, etc.).  A client
   synchronizes to one or more upstream servers, but does not provide
   synchronization to dependent clients.  A secondary server has one or
   more upstream servers and one or more downstream servers or clients.
   All servers and clients who are fully NTPv4-compliant MUST implement
   the entire suite of algorithms described in this document.  In order
   to maintain stability in large NTP subnets, secondary servers SHOULD
   be fully NTPv4-compliant.  Alternative algorithms MAY be used, but
   their output MUST be identical to the algorithms described in this

3.  Protocol Modes Go to the top of this page...top

   There are three NTP protocol variants: symmetric, client/server, and
   broadcast.  Each is associated with an association mode (a
   description of the relationship between two NTP speakers) as shown in
   Figure 1.  In addition, persistent associations are mobilized upon
   startup and are never demobilized.  Ephemeral associations are
   mobilized upon the arrival of a packet and are demobilized upon error
   or timeout.

          |  Association Mode | Assoc. Mode Value | Packet Mode Value|
          | Symmetric Active  |         1         | 1 or 2           |
          | Symmetric Passive |         2         | 1                |
          | Client            |         3         | 4                |
          | Server            |         4         | 3                |
          | Broadcast Server  |         5         | 5                |
          | Broadcast Client  |         6         | N/A              |

                  Figure 1: Association and Packet Modes

   In the client/server variant, a persistent client sends packet mode 4
   packets to a server, which returns packet mode 3 packets.  Servers
   provide synchronization to one or more clients, but do not accept
   synchronization from them.  A server can also be a reference clock
   driver that obtains time directly from a standard source such as a
   GPS receiver or telephone modem service.  In this variant, clients
   pull synchronization from servers.
   In the symmetric variant, a peer operates as both a server and client
   using either a symmetric active or symmetric passive association.  A
   persistent symmetric active association sends symmetric active (mode
   1) packets to a symmetric active peer association.  Alternatively, an
   ephemeral symmetric passive association can be mobilized upon the
   arrival of a symmetric active packet with no matching association.
   That association sends symmetric passive (mode 2) packets and
   persists until error or timeout.  Peers both push and pull
   synchronization to and from each other.  For the purposes of this
   document, a peer operates like a client, so references to client
   imply peer as well.

   In the broadcast variant, a persistent broadcast server association
   sends periodic broadcast server (mode 5) packets that can be received
   by multiple clients.  Upon reception of a broadcast server packet
   without a matching association, an ephemeral broadcast client (mode
   6) association is mobilized and persists until error or timeout.  It
   is useful to provide an initial volley where the client operating in
   client mode exchanges several packets with the server, so as to
   calibrate the propagation delay and to run the Autokey security
   protocol, after which the client reverts to broadcast client mode.  A
   broadcast server pushes synchronization to clients and other servers.

   Loosely following the conventions established by the telephone
   industry, the level of each server in the hierarchy is defined by a
   stratum number.  Primary servers are assigned stratum one; secondary
   servers at each lower level are assigned stratum numbers one greater
   than the preceding level.  As the stratum number increases, its
   accuracy degrades depending on the particular network path and system
   clock stability.  Mean errors, measured by synchronization distances,
   increase approximately in proportion to stratum numbers and measured
   round-trip delay.

   As a standard practice, timing network topology should be organized
   to avoid timing loops and minimize the synchronization distance.  In
   NTP, the subnet topology is determined using a variant of the
   Bellman-Ford distributed routing algorithm, which computes the
   shortest-path spanning tree rooted on the primary servers.  As a
   result of this design, the algorithm automatically reorganizes the
   subnet, so as to produce the most accurate and reliable time, even
   when there are failures in the timing network.

3.1.  Dynamic Server Discovery Go to the top of this page...top

   There are two special associations, manycast client and manycast
   server, which provide a dynamic server discovery function.  There are
   two types of manycast client associations: persistent and ephemeral.
   The persistent manycast client sends client (mode 3) packets to a
   designated IPv4 or IPv6 broadcast or multicast group address.
   Designated manycast servers within range of the time-to-live (TTL)
   field in the packet header listen for packets with that address.  If
   a server is suitable for synchronization, it returns an ordinary
   server (mode 4) packet using the client's unicast address.  Upon
   receiving this packet, the client mobilizes an ephemeral client (mode
   3) association.  The ephemeral client association persists until
   error or timeout.

   A manycast client continues sending packets to search for a minimum
   number of associations.  It starts with a TTL equal to one and
   continuously adding one to it until the minimum number of
   associations is made or when the TTL reaches a maximum value.  If the
   TTL reaches its maximum value and yet not enough associations are
   mobilized, the client stops transmission for a time-out period to
   clear all associations, and then repeats the search cycle.  If a
   minimum number of associations has been mobilized, then the client
   starts transmitting one packet per time-out period to maintain the
   associations.  Field constraints limit the minimum value to 1 and the
   maximum to 255.  These limits may be tuned for individual application

   The ephemeral associations compete among themselves.  As new
   ephemeral associations are mobilized, the client runs the mitigation
   algorithms described in Sections 10 and 11.2 for the best candidates
   out of the population, the remaining ephemeral associations are timed
   out and demobilized.  In this way, the population includes only the
   best candidates that have most recently responded with an NTP packet
   to discipline the system clock.

4.  Definitions Go to the top of this page...top

   A number of technical terms are defined in this section.  A timescale
   is a frame of reference where time is expressed as the value of a
   monotonically increasing binary counter with an indefinite number of
   bits.  It counts in seconds and fractions of a second, when a decimal
   point is employed.  The Coordinated Universal Time (UTC) timescale is
   defined by ITU-R TF.460 [ITU-R_TF.460].  Under the auspices of the
   Metre Convention of 1865, in 1975 the CGPM [CGPM] strongly endorsed
   the use of UTC as the basis for civil time.

   The Coordinated Universal Time (UTC) timescale represents mean solar
   time as disseminated by national standards laboratories.  The system
   time is represented by the system clock maintained by the hardware
   and operating system.  The goal of the NTP algorithms is to minimize
   both the time difference and frequency difference between UTC and the
   system clock.  When these differences have been reduced below nominal
   tolerances, the system clock is said to be synchronized to UTC.
   The date of an event is the UTC time at which the event takes place.
   Dates are ephemeral values designated with uppercase T.  Running time
   is another timescale that is coincident to the synchronization
   function of the NTP program.

   A timestamp T(t) represents either the UTC date or time offset from
   UTC at running time t.  Which meaning is intended should be clear
   from the context.  Let T(t) be the time offset, R(t) the frequency
   offset, and D(t) the aging rate (first derivative of R(t) with
   respect to t).  Then, if T(t_0) is the UTC time offset determined at
   t = t_0, the UTC time offset at time t is

   T(t) = T(t_0) + R(t_0)(t-t_0) + 1/2 * D(t_0)(t-t_0)^2 + e,

   where e is a stochastic error term discussed later in this document.
   While the D(t) term is important when characterizing precision
   oscillators, it is ordinarily neglected for computer oscillators.  In
   this document, all time values are in seconds (s) and all frequency
   values are in seconds-per-second (s/s).  It is sometimes convenient
   to express frequency offsets in parts-per-million (ppm), where 1 ppm
   is equal to 10^(-6) s/s.

   It is important in computer timekeeping applications to assess the
   performance of the timekeeping function.  The NTP performance model
   includes four statistics that are updated each time a client makes a
   measurement with a server.  The offset (theta) represents the
   maximum-likelihood time offset of the server clock relative to the
   system clock.  The delay (delta) represents the round-trip delay
   between the client and server.  The dispersion (epsilon) represents
   the maximum error inherent in the measurement.  It increases at a
   rate equal to the maximum disciplined system clock frequency
   tolerance (PHI), typically 15 ppm.  The jitter (psi) is defined as
   the root-mean-square (RMS) average of the most recent offset
   differences, and it represents the nominal error in estimating the

   While the theta, delta, epsilon, and psi statistics represent
   measurements of the system clock relative to each server clock
   separately, the NTP protocol includes mechanisms to combine the
   statistics of several servers to more accurately discipline and
   calibrate the system clock.  The system offset (THETA) represents the
   maximum-likelihood offset estimate for the server population.  The
   system jitter (PSI) represents the nominal error in estimating the
   system offset.  The delta and epsilon statistics are accumulated at
   each stratum level from the reference clock to produce the root delay
   (DELTA) and root dispersion (EPSILON) statistics.  The
   synchronization distance (LAMBDA) equal to EPSILON + DELTA / 2
   represents the maximum error due to all causes.  The detailed
   formulations of these statistics are given in Section 11.2.  They are
   available to the dependent applications in order to assess the
   performance of the synchronization function.

5.  Implementation Model Go to the top of this page...top

   Figure 2 shows the architecture of a typical, multi-threaded
   implementation.  It includes two processes dedicated to each server,
   a peer process to receive messages from the server or reference
   clock, and a poll process to transmit messages to the server or
   reference clock.

   . Remote   .   Peer/Poll  .              System          .  Clock   .
   . Servers  .   Processes  .              Process         .Discipline.
   .          .              .                              . Process  .
   .+--------+. +-----------+. +------------+               .          .
   .|        |->|           |. |            |               .          .
   .|Server 1|  |Peer/Poll 1|->|            |               .          .
   .|        |<-|           |. |            |               .          .
   .+--------+. +-----------+. |            |               .          .
   .          .       ^      . |            |               .          .
   .          .       |      . |            |               .          .
   .+--------+. +-----------+. |            |  +-----------+.          .
   .|        |->|           |. | Selection  |->|           |. +------+ .
   .|Server 2|  |Peer/Poll 2|->|    and     |  | Combine   |->| Loop | .
   .|        |<-|           |. | Cluster    |  | Algorithm |. |Filter| .
   .+--------+. +-----------+. | Algorithms |->|           |. +------+ .
   .          .       ^      . |            |  +-----------+.    |     .
   .          .       |      . |            |               .    |     .
   .+--------+. +-----------+. |            |               .    |     .
   .|        |->|           |. |            |               .    |     .
   .|Server 3|  |Peer/Poll 3|->|            |               .    |     .
   .|        |<-|           |. |            |               .    |     .
   .+--------+. +-----------+. +------------+               .    |     .
                       |                                    .    V     .
                       |                                    . +-----+  .
                       +--------------------------------------| VFO |  .
                                                            . +-----+  .
                                                            .  Clock   .
                                                            .  Adjust  .
                                                            .  Process .

                      Figure 2: Implementation Model
   These processes operate on a common data structure, called an
   association, which contains the statistics described above along with
   various other data described in Section 9.  A client sends packets to
   one or more servers and then processes returned packets when they are
   received.  The server interchanges source and destination addresses
   and ports, overwrites certain fields in the packet and returns it
   immediately (in the client/server mode) or at some time later (in the
   symmetric modes).  As each NTP message is received, the offset theta
   between the peer clock and the system clock is computed along with
   the associated statistics delta, epsilon, and psi.

   The system process includes the selection, cluster, and combine
   algorithms that mitigate among the various servers and reference
   clocks to determine the most accurate and reliable candidates to
   synchronize the system clock.  The selection algorithm uses Byzantine
   fault detection principles to discard the presumably incorrect
   candidates called "falsetickers" from the incident population,
   leaving only good candidates called "truechimers".  A truechimer is a
   clock that maintains timekeeping accuracy to a previously published
   and trusted standard, while a falseticker is a clock that shows
   misleading or inconsistent time.  The cluster algorithm uses
   statistical principles to find the most accurate set of truechimers.
   The combine algorithm computes the final clock offset by
   statistically averaging the surviving truechimers.

   The clock discipline process is a system process that controls the
   time and frequency of the system clock, here represented as a
   variable frequency oscillator (VFO).  Timestamps struck from the VFO
   close the feedback loop that maintains the system clock time.
   Associated with the clock discipline process is the clock-adjust
   process, which runs once each second to inject a computed time offset
   and maintain constant frequency.  The RMS average of past time offset
   differences represents the nominal error or system clock jitter.  The
   RMS average of past frequency offset differences represents the
   oscillator frequency stability or frequency wander.  These terms are
   given precise interpretation in Section 11.3.

   A client sends messages to each server with a poll interval of 2^tau
   seconds, as determined by the poll exponent tau.  In NTPv4, tau
   ranges from 4 (16 s) to 17 (36 h).  The value of tau is determined by
   the clock discipline algorithm to match the loop-time constant T_c =
   2^tau.  In client/server mode, the server responds immediately;
   however, in symmetric modes, each of two peers manages tau as a
   function of current system offset and system jitter, so they may not
   agree with the same value.  It is important that the dynamic behavior
   of the clock discipline algorithm be carefully controlled in order to
   maintain stability in the NTP subnet at large.  This requires that
   the peers agree on a common tau equal to the minimum poll exponent of
   both peers.  The NTP protocol includes provisions to properly
   negotiate this value.

   The implementation model includes some means to set and adjust the
   system clock.  The operating system is assumed to provide two
   functions: one to set the time directly, for example, the Unix
   settimeofday() function, and another to adjust the time in small
   increments advancing or retarding the time by a designated amount,
   for example, the Unix adjtime() function.  In this and following
   references, parentheses following a name indicate reference to a
   function rather than a simple variable.  In the intended design the
   clock discipline process uses the adjtime() function if the
   adjustment is less than a designated threshold, and the
   settimeofday() function if above the threshold.  The manner in which
   this is done and the value of the threshold as described in
   Section 10.

6.  Data Types Go to the top of this page...top

   All NTP time values are represented in twos-complement format, with
   bits numbered in big-endian (as described in Appendix A of [RFC0791])
   fashion from zero starting at the left, or high-order, position.
   There are three NTP time formats, a 128-bit date format, a 64-bit
   timestamp format, and a 32-bit short format, as shown in Figure 3.
   The 128-bit date format is used where sufficient storage and word
   size are available.  It includes a 64-bit signed seconds field
   spanning 584 billion years and a 64-bit fraction field resolving .05
   attosecond (i.e., 0.5e-18).  For convenience in mapping between
   formats, the seconds field is divided into a 32-bit Era Number field
   and a 32-bit Era Offset field.  Eras cannot be produced by NTP
   directly, nor is there need to do so.  When necessary, they can be
   derived from external means, such as the filesystem or dedicated
       0                   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
      |          Seconds              |           Fraction            |

                               NTP Short Format

       0                   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
      |                            Seconds                            |
      |                            Fraction                           |

                             NTP Timestamp Format

       0                   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
      |                           Era Number                          |
      |                           Era Offset                          |
      |                                                               |
      |                           Fraction                            |
      |                                                               |

                              NTP Date Format

                        Figure 3: NTP Time Formats

   The 64-bit timestamp format is used in packet headers and other
   places with limited word size.  It includes a 32-bit unsigned seconds
   field spanning 136 years and a 32-bit fraction field resolving 232
   picoseconds.  The 32-bit short format is used in delay and dispersion
   header fields where the full resolution and range of the other
   formats are not justified.  It includes a 16-bit unsigned seconds
   field and a 16-bit fraction field.

   In the date and timestamp formats, the prime epoch, or base date of
   era 0, is 0 h 1 January 1900 UTC, when all bits are zero.  It should
   be noted that strictly speaking, UTC did not exist prior to 1 January
   1972, but it is convenient to assume it has existed for all eternity,
   even if all knowledge of historic leap seconds has been lost.  Dates
   are relative to the prime epoch; values greater than zero represent
   times after that date; values less than zero represent times before
   it.  Note that the Era Offset field of the date format and the
   Seconds field of the timestamp format have the same interpretation.

   Timestamps are unsigned values, and operations on them produce a
   result in the same or adjacent eras.  Era 0 includes dates from the
   prime epoch to some time in 2036, when the timestamp field wraps
   around and the base date for era 1 is established.  In either format,
   a value of zero is a special case representing unknown or
   unsynchronized time.  Figure 4 shows a number of historic NTP dates
   together with their corresponding Modified Julian Day (MJD), NTP era,
   and NTP timestamp.

   | Date        | MJD        | NTP | NTP Timestamp | Epoch            |
   |             |            | Era | Era Offset    |                  |
   | 1 Jan -4712 | -2,400,001 | -49 | 1,795,583,104 | 1st day Julian   |
   | 1 Jan -1    | -679,306   | -14 | 139,775,744   | 2 BCE            |
   | 1 Jan 0     | -678,491   | -14 | 171,311,744   | 1 BCE            |
   | 1 Jan 1     | -678,575   | -14 | 202,939,144   | 1 CE             |
   | 4 Oct 1582  | -100,851   | -3  | 2,873,647,488 | Last day Julian  |
   | 15 Oct 1582 | -100,840   | -3  | 2,874,597,888 | First day        |
   |             |            |     |               | Gregorian        |
   | 31 Dec 1899 | 15019      | -1  | 4,294,880,896 | Last day NTP Era |
   |             |            |     |               | -1               |
   | 1 Jan 1900  | 15020      | 0   | 0             | First day NTP    |
   |             |            |     |               | Era 0            |
   | 1 Jan 1970  | 40,587     | 0   | 2,208,988,800 | First day UNIX   |
   | 1 Jan 1972  | 41,317     | 0   | 2,272,060,800 | First day UTC    |
   | 31 Dec 1999 | 51,543     | 0   | 3,155,587,200 | Last day 20th    |
   |             |            |     |               | Century          |
   | 8 Feb 2036  | 64,731     | 1   | 63,104        | First day NTP    |
   |             |            |     |               | Era 1            |

                 Figure 4: Interesting Historic NTP Dates

   Let p be the number of significant bits in the second fraction.  The
   clock resolution is defined as 2^(-p), in seconds.  In order to
   minimize bias and help make timestamps unpredictable to an intruder,
   the non-significant bits should be set to an unbiased random bit
   string.  The clock precision is defined as the running time to read
   the system clock, in seconds.  Note that the precision defined in
   this way can be larger or smaller than the resolution.  The term rho,
   representing the precision used in the protocol, is the larger of the
   The only arithmetic operation permitted on dates and timestamps is
   twos-complement subtraction, yielding a 127-bit or 63-bit signed
   result.  It is critical that the first-order differences between two
   dates preserve the full 128-bit precision and the first-order
   differences between two timestamps preserve the full 64-bit
   precision.  However, the differences are ordinarily small compared to
   the seconds span, so they can be converted to floating double format
   for further processing and without compromising the precision.

   It is important to note that twos-complement arithmetic does not
   distinguish between signed and unsigned values (although comparisons
   can take sign into account); only the conditional branch instructions
   do.  Thus, although the distinction is made between signed dates and
   unsigned timestamps, they are processed the same way.  A perceived
   hazard with 64-bit timestamp calculations spanning an era, such as is
   possible in 2036, might result in over-run.  In point of fact, if the
   client is set within 68 years of the server before the protocol is
   started, correct values are obtained even if the client and server
   are in adjacent eras.

   Some time values are represented in exponent format, including the
   precision, time constant, and poll interval.  These are in 8-bit
   signed integer format in log2 (log base 2) seconds.  The only
   arithmetic operations permitted on them are increment and decrement.
   For the purpose of this document and to simplify the presentation, a
   reference to one of these variables by name means the exponentiated
   value, e.g., the poll interval is 1024 s, while reference by name and
   exponent means the actual value, e.g., the poll exponent is 10.

   To convert system time in any format to NTP date and timestamp
   formats requires that the number of seconds s from the prime epoch to
   the system time be determined.  To determine the integer era and
   timestamp given s,

   era = s / 2^(32) and timestamp = s - era * 2^(32),

   which works for positive and negative dates.  To determine s given
   the era and timestamp,

   s = era * 2^(32) + timestamp.

   Converting between NTP and system time can be a little messy, and is
   beyond the scope of this document.  Note that the number of days in
   era 0 is one more than the number of days in most other eras, and
   this won't happen again until the year 2400 in era 3.
   In the description of state variables to follow, explicit reference
   to integer type implies a 32-bit unsigned integer.  This simplifies
   bounds checks, since only the upper limit needs to be defined.
   Without explicit reference, the default type is 64-bit floating
   double.  Exceptions will be noted as necessary.

7.  Data Structures Go to the top of this page...top

   The NTP state machines are defined in the following sections.  State
   variables are separated into classes according to their function in
   packet headers, peer and poll processes, the system process, and the
   clock discipline process.  Packet variables represent the NTP header
   values in transmitted and received packets.  Peer and poll variables
   represent the contents of the association for each server separately.
   System variables represent the state of the server as seen by its
   dependent clients.  Clock discipline variables represent the internal
   workings of the clock discipline algorithm.  An example is described
   in Appendix A.

7.1.  Structure Conventions Go to the top of this page...top

   In order to distinguish between different variables of the same name
   but used in different processes, the naming convention summarized in
   Figure 5 is adopted.  A receive packet variable v is a member of the
   packet structure r with fully qualified name r.v.  In a similar
   manner, x.v is a transmit packet variable, p.v is a peer variable,
   s.v is a system variable, and c.v is a clock discipline variable.
   There is a set of peer variables for each association; there is only
   one set of system and clock variables.

                   | Name | Description                     |
                   | r.   | receive packet header variable  |
                   | x.   | transmit packet header variable |
                   | p.   | peer/poll variable              |
                   | s.   | system variable                 |
                   | c.   | clock discipline variable       |

                       Figure 5: Prefix Conventions

7.2.  Global Parameters Go to the top of this page...top

   In addition to the variable classes, a number of global parameters
   are defined in this document, including those shown with values in
   Figure 6.
            | Name      | Value | Description                      |
            | PORT      | 123   | NTP port number                  |
            | VERSION   | 4     | NTP version number                   |
            | TOLERANCE | 15e-6 | frequency tolerance PHI (s/s)    |
            | MINPOLL   | 4     | minimum poll exponent (16 s)     |
            | MAXPOLL   | 17    | maximum poll exponent (36 h)     |
            | MAXDISP   | 16    | maximum dispersion (16 s)        |
            | MINDISP   | .005  | minimum dispersion increment (s) |
            | MAXDIST   | 1     | distance threshold (1 s)         |
            | MAXSTRAT  | 16    | maximum stratum number           |

                        Figure 6: Global Parameters

   While these are the only global parameters needed for
   interoperability, a larger collection is necessary in any
   implementation.  Appendix A.1.1 contains those used by the skeleton
   for the mitigation algorithms, clock discipline algorithm, and
   related implementation-dependent functions.  Some of these parameter
   values are cast in stone, like the NTP port number assigned by the
   IANA and the version number assigned NTPv4 itself.  Others, like the
   frequency tolerance (also called PHI), involve an assumption about
   the worst-case behavior of a system clock once synchronized and then
   allowed to drift when its sources have become unreachable.  The
   minimum and maximum parameters define the limits of state variables
   as described in later sections of this document.

   While shown with fixed values in this document, some implementations
   may make them variables adjustable by configuration commands.  For
   instance, the reference implementation computes the value of
   PRECISION as log2 of the minimum time in several iterations to read
   the system clock.

7.3.  Packet Header Variables Go to the top of this page...top

   The most important state variables from an external point of view are
   the packet header variables described in Figure 7 and below.  The NTP
   packet header consists of an integral number of 32-bit (4 octet)
   words in network byte order.  The packet format consists of three
   components: the header itself, one or more optional extension fields,
   and an optional message authentication code (MAC).  The header
   component is identical to the NTPv3 header and previous versions.
   The optional extension fields are used by the Autokey public key
   cryptographic algorithms described in [RFC5906].  The optional MAC is
   used by both Autokey and the symmetric key cryptographic algorithm
   described in this RFC.
               | Name      | Formula    | Description           |
               | leap      | leap       | leap indicator (LI)   |
               | version   | version    | version number (VN)   |
               | mode      | mode       | mode                  |
               | stratum   | stratum    | stratum               |
               | poll      | poll       | poll exponent         |
               | precision | rho        | precision exponent    |
               | rootdelay | delta_r    | root delay            |
               | rootdisp  | epsilon_r  | root dispersion       |
               | refid     | refid      | reference ID          |
               | reftime   | reftime    | reference timestamp   |
               | org       | T1         | origin timestamp      |
               | rec       | T2         | receive timestamp     |
               | xmt       | T3         | transmit timestamp    |
               | dst       | T4         | destination timestamp |
               | keyid     | keyid      | key ID                |
               | dgst      | dgst       | message digest        |

                     Figure 7: Packet Header Variables

   The NTP packet is a UDP datagram [RFC0768].  Some fields use multiple
   words and others are packed in smaller fields within a word.  The NTP
   packet header shown in Figure 8 has 12 words followed by optional
   extension fields and finally an optional message authentication code
   (MAC) consisting of the Key Identifier field and Message Digest
       0                   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 ID                         |
      |                                                               |
      +                     Reference Timestamp (64)                  +
      |                                                               |
      |                                                               |
      +                      Origin Timestamp (64)                    +
      |                                                               |
      |                                                               |
      +                      Receive Timestamp (64)                   +
      |                                                               |
      |                                                               |
      +                      Transmit Timestamp (64)                  +
      |                                                               |
      |                                                               |
      .                                                               .
      .                    Extension Field 1 (variable)               .
      .                                                               .
      |                                                               |
      |                                                               |
      .                                                               .
      .                    Extension Field 2 (variable)               .
      .                                                               .
      |                                                               |
      |                          Key Identifier                       |
      |                                                               |
      |                            dgst (128)                         |
      |                                                               |

                      Figure 8: Packet Header Format
   The extension fields are used to add optional capabilities, for
   example, the Autokey security protocol [RFC5906].  The extension
   field format is presented in order for the packet to be parsed
   without the knowledge of the extension field functions.  The MAC is
   used by both Autokey and the symmetric key authentication scheme.

   A list of the packet header variables is shown in Figure 7 and
   described in detail below.  Except for a minor variation when using
   the IPv6 address family, these fields are backwards compatible with
   NTPv3.  The packet header fields apply to both transmitted packets (x
   prefix) and received packets (r prefix).  In Figure 8, the size of
   some multiple-word fields is shown in bits if not the default 32
   bits.  The basic header extends from the beginning of the packet to
   the end of the Transmit Timestamp field.

   The fields and associated packet variables (in parentheses) are
   interpreted as follows:

   LI Leap Indicator (leap): 2-bit integer warning of an impending leap
   second to be inserted or deleted in the last minute of the current
   month with values defined in Figure 9.

           | Value | Meaning                                |
           | 0     | no warning                             |
           | 1     | last minute of the day has 61 seconds  |
           | 2     | last minute of the day has 59 seconds  |
           | 3     | unknown (clock unsynchronized)         |

                         Figure 9: Leap Indicator

   VN Version Number (version): 3-bit integer representing the NTP
   version number, currently 4.

   Mode (mode): 3-bit integer representing the mode, with values defined
   in Figure 10.
                      | Value | Meaning                  |
                      | 0     | reserved                 |
                      | 1     | symmetric active         |
                      | 2     | symmetric passive        |
                      | 3     | client                   |
                      | 4     | server                   |
                      | 5     | broadcast                |
                      | 6     | NTP control message      |
                      | 7     | reserved for private use |

                       Figure 10: Association Modes

   Stratum (stratum): 8-bit integer representing the stratum, with
   values defined in Figure 11.

        | Value  | Meaning                                             |
        | 0      | unspecified or invalid                              |
        | 1      | primary server (e.g., equipped with a GPS receiver) |
        | 2-15   | secondary server (via NTP)                          |
        | 16     | unsynchronized                                      |
        | 17-255 | reserved                                            |

                         Figure 11: Packet Stratum

   It is customary to map the stratum value 0 in received packets to
   MAXSTRAT (16) in the peer variable p.stratum and to map p.stratum
   values of MAXSTRAT or greater to 0 in transmitted packets.  This
   allows reference clocks, which normally appear at stratum 0, to be
   conveniently mitigated using the same clock selection algorithms used
   for external sources (see Appendix A.5.5.1 for an example).

   Poll: 8-bit signed integer representing the maximum interval between
   successive messages, in log2 seconds.  Suggested default limits for
   minimum and maximum poll intervals are 6 and 10, respectively.

   Precision: 8-bit signed integer representing the precision of the
   system clock, in log2 seconds.  For instance, a value of -18
   corresponds to a precision of about one microsecond.  The precision
   can be determined when the service first starts up as the minimum
   time of several iterations to read the system clock.
   Root Delay (rootdelay): Total round-trip delay to the reference
   clock, in NTP short format.

   Root Dispersion (rootdisp): Total dispersion to the reference clock,
   in NTP short format.

   Reference ID (refid): 32-bit code identifying the particular server
   or reference clock.  The interpretation depends on the value in the
   stratum field.  For packet stratum 0 (unspecified or invalid), this
   is a four-character ASCII [RFC1345] string, called the "kiss code",
   used for debugging and monitoring purposes.  For stratum 1 (reference
   clock), this is a four-octet, left-justified, zero-padded ASCII
   string assigned to the reference clock.  The authoritative list of
   Reference Identifiers is maintained by IANA; however, any string
   beginning with the ASCII character "X" is reserved for unregistered
   experimentation and development.  The identifiers in Figure 12 have
   been used as ASCII identifiers:

     | ID   | Clock Source                                             |
     | GOES | Geosynchronous Orbit Environment Satellite               |
     | GPS  | Global Position System                                   |
     | GAL  | Galileo Positioning System                               |
     | PPS  | Generic pulse-per-second                                 |
     | IRIG | Inter-Range Instrumentation Group                        |
     | WWVB | LF Radio WWVB Ft. Collins, CO 60 kHz                     |
     | DCF  | LF Radio DCF77 Mainflingen, DE 77.5 kHz                  |
     | HBG  | LF Radio HBG Prangins, HB 75 kHz                         |
     | MSF  | LF Radio MSF Anthorn, UK 60 kHz                          |
     | JJY  | LF Radio JJY Fukushima, JP 40 kHz, Saga, JP 60 kHz       |
     | LORC | MF Radio LORAN C station, 100 kHz                        |
     | TDF  | MF Radio Allouis, FR 162 kHz                             |
     | CHU  | HF Radio CHU Ottawa, Ontario                             |
     | WWV  | HF Radio WWV Ft. Collins, CO                             |
     | WWVH | HF Radio WWVH Kauai, HI                                  |
     | NIST | NIST telephone modem                                     |
     | ACTS | NIST telephone modem                                     |
     | USNO | USNO telephone modem                                     |
     | PTB  | European telephone modem                                 |

                     Figure 12: Reference Identifiers

   Above stratum 1 (secondary servers and clients): this is the
   reference identifier of the server and can be used to detect timing
   loops.  If using the IPv4 address family, the identifier is the four-
   octet IPv4 address.  If using the IPv6 address family, it is the
   first four octets of the MD5 hash of the IPv6 address.  Note that,
   when using the IPv6 address family on an NTPv4 server with a NTPv3
   client, the Reference Identifier field appears to be a random value
   and a timing loop might not be detected.

   Reference Timestamp: Time when the system clock was last set or
   corrected, in NTP timestamp format.

   Origin Timestamp (org): Time at the client when the request departed
   for the server, in NTP timestamp format.

   Receive Timestamp (rec): Time at the server when the request arrived
   from the client, in NTP timestamp format.

   Transmit Timestamp (xmt): Time at the server when the response left
   for the client, in NTP timestamp format.

   Destination Timestamp (dst): Time at the client when the reply
   arrived from the server, in NTP timestamp format.

   Note: The Destination Timestamp field is not included as a header
   field; it is determined upon arrival of the packet and made available
   in the packet buffer data structure.

   If the NTP has access to the physical layer, then the timestamps are
   associated with the beginning of the symbol after the start of frame.
   Otherwise, implementations should attempt to associate the timestamp
   to the earliest accessible point in the frame.

   The MAC consists of the Key Identifier followed by the Message
   Digest.  The message digest, or cryptosum, is calculated as in
   [RFC1321] over all NTP-header and optional extension fields, but not
   the MAC itself.

   Extension Field n: See Section 7.5 for a description of the format of
   this field.

   Key Identifier (keyid): 32-bit unsigned integer used by the client
   and server to designate a secret 128-bit MD5 key.

   Message Digest (digest): 128-bit MD5 hash computed over the key
   followed by the NTP packet header and extensions fields (but not the
   Key Identifier or Message Digest fields).

   It should be noted that the MAC computation used here differs from
   those defined in [RFC1305] and [RFC4330] but is consistent with how
   existing implementations generate a MAC.

7.4.  The Kiss-o'-Death Packet Go to the top of this page...top

   If the Stratum field is 0, which implies unspecified or invalid, the
   Reference Identifier field can be used to convey messages useful for
   status reporting and access control.  These are called Kiss-o'-Death
   (KoD) packets and the ASCII messages they convey are called kiss
   codes.  The KoD packets got their name because an early use was to
   tell clients to stop sending packets that violate server access
   controls.  The kiss codes can provide useful information for an
   intelligent client, either NTPv4 or SNTPv4.  Kiss codes are encoded
   in four-character ASCII strings that are left justified and zero
   filled.  The strings are designed for character displays and log
   files.  A list of the currently defined kiss codes is given in
   Figure 13.  Recipients of kiss codes MUST inspect them and, in the
   following cases, take these actions:

   a.  For kiss codes DENY and RSTR, the client MUST demobilize any
       associations to that server and stop sending packets to that

   b.  For kiss code RATE, the client MUST immediately reduce its
       polling interval to that server and continue to reduce it each
       time it receives a RATE kiss code.

   c.  Kiss codes beginning with the ASCII character "X" are for
       unregistered experimentation and development and MUST be ignored
       if not recognized.

   d.  Other than the above conditions, KoD packets have no protocol
       significance and are discarded after inspection.
   | Code |                           Meaning                          |
   | ACST | The association belongs to a unicast server.               |
   | AUTH | Server authentication failed.                              |
   | AUTO | Autokey sequence failed.                                   |
   | BCST | The association belongs to a broadcast server.             |
   | CRYP | Cryptographic authentication or identification failed.     |
   | DENY | Access denied by remote server.                            |
   | DROP | Lost peer in symmetric mode.                               |
   | RSTR | Access denied due to local policy.                         |
   | INIT | The association has not yet synchronized for the first     |
   |      | time.                                                      |
   | MCST | The association belongs to a dynamically discovered server.|
   | NKEY | No key found. Either the key was never installed or is     |
   |      | not trusted.                                               |
   | RATE | Rate exceeded. The server has temporarily denied access    |
   |      | because the client exceeded the rate threshold.            |
   | RMOT | Alteration of association from a remote host running       |
   |      | ntpdc.                                                     |
   | STEP | A step change in system time has occurred, but the         |
   |      | association has not yet resynchronized.                    |

                           Figure 13: Kiss Codes

   The Receive Timestamp and the Transmit Timestamp (set by the server)
   are undefined when in a KoD packet and MUST NOT be relied upon to
   have valid values and MUST be discarded.

7.5.  NTP Extension Field Format Go to the top of this page...top

   In NTPv4, one or more extension fields can be inserted after the
   header and before the MAC, which is always present when an extension
   field is present.  Other than defining the field format, this
   document makes no use of the field contents.  An extension field
   contains a request or response message in the format shown in
   Figure 14.
       0                   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
      |          Field Type           |            Length             |
      .                                                               .
      .                            Value                              .
      .                                                               .
      |                       Padding (as needed)                     |

                     Figure 14: Extension Field Format

   All extension fields are zero-padded to a word (four octets)
   boundary.  The Field Type field is specific to the defined function
   and is not elaborated here.  While the minimum field length
   containing required fields is four words (16 octets), a maximum field
   length remains to be established.

   The Length field is a 16-bit unsigned integer that indicates the
   length of the entire extension field in octets, including the Padding

8.  On-Wire Protocol Go to the top of this page...top

   The heart of the NTP on-wire protocol is the core mechanism that
   exchanges time values between servers, peers, and clients.  It is
   inherently resistant to lost or duplicate packets.  Data integrity is
   provided by the IP and UDP checksums.  No flow control or
   retransmission facilities are provided or necessary.  The protocol
   uses timestamps, which are either extracted from packet headers or
   struck from the system clock upon the arrival or departure of a
   packet.  Timestamps are precision data and should be restruck in the
   case of link-level retransmission and corrected for the time to
   compute a MAC in transmit.

   NTP messages make use of two different communication modes, one-to-
   one and one-to-many, commonly referred to as unicast and broadcast.
   For the purposes of this document, the term broadcast is interpreted
   as any available one-to-many mechanism.  For IPv4, this equates to
   either IPv4 broadcast or IPv4 multicast.  For IPv6, this equates to
   IPv6 multicast.  For this purpose, IANA has allocated the IPv4
   multicast address and the IPv6 multicast address ending
   :101, with the prefix determined by scoping rules.  Any other non-
   allocated multicast address may also be used in addition to these
   allocated multicast addresses.
   The on-wire protocol uses four timestamps numbered t1 through t4 and
   three state variables org, rec, and xmt, as shown in Figure 15.  This
   figure shows the most general case where each of two peers, A and B,
   independently measure the offset and delay relative to the other.
   For purposes of illustration, the packet timestamps are shown in
   lowercase, while the state variables are shown in uppercase.  The
   state variables are copied from the packet timestamps upon arrival or
   departure of a packet.
             t2            t3           t6            t7
        +---------+   +---------+   +---------+   +---------+
        |    0    |   |    t1   |   |   t3    |   |    t5   |
        +---------+   +---------+   +---------+   +---------+
        |    0    |   |    t2   |   |   t4    |   |    t6   |  Packet
        +---------+   +---------+   +---------+   +---------+ Timestamps
        |   t1    |   |t3=clock |   |   t5    |   |t7=clock |
        +---------+   +---------+   +---------+   +---------+
        |t2=clock |                 |t6=clock |
        +---------+                 +---------+
                                                               Peer B
        +---------+   +---------+   +---------+   +---------+
   org  |   T1    |   |    T1   |   | t5<>T1? |   |    T5   |
        +---------+   +---------+   +---------+   +---------+   State
   rec  |   T2    |   |    T2   |   |   T6    |   |    T6   | Variables
        +---------+   +---------+   +---------+   +---------+
   xmt  |    0    |   |    T3   |   |  t3=T3? |   |    T7   |
        +---------+   +---------+   +---------+   +---------+

                  t2      t3                 t6          t7
                 /\         \                 /\            \
                 /           \                /              \
                /             \              /                \
               /               \/           /                 \/
             t1                t4         t5                  t8

            t1            t4            t5             t8
        +---------+   +---------+   +---------+   +---------+
        |    0    |   |    t1   |   |   t3    |   |    t5   |
        +---------+   +---------+   +---------+   +---------+
        |    0    |   |    t2   |   |   t4    |   |    t6   |  Packet
        +---------+   +---------+   +---------+   +---------+ Timestamps
        |t1=clock |   |    t3   |   |t5=clock |   |    t7   |
        +---------+   +---------+   +---------+   +---------+
                      |t4=clock |                 |t8=clock |
                      +---------+                 +---------+
                                                               Peer A
        +---------+   +---------+   +---------+   +---------+
   org  |    0    |   |  t3<>0? |   |   T3    |   | t7<>T3? |
        +---------+   +---------+   +---------+   +---------+   State
   rec  |    0    |   |    T4   |   |   T4    |   |    T8   | Variables
        +---------+   +---------+   +---------+   +---------+
   xmt  |   T1    |   |  t1=T1? |   |   T5    |   |  t5=T5? |
        +---------+   +---------+   +---------+   +---------+

                        Figure 15: On-Wire Protocol
   In the figure, the first packet transmitted by A contains only the
   origin timestamp t1, which is then copied to T1.  B receives the
   packet at t2 and copies t1 to T1 and the receive timestamp t2 to T2.
   At this time or some time later at t3, B sends a packet to A
   containing t1 and t2 and the transmit timestamp t3.  All three
   timestamps are copied to the corresponding state variables.  A
   receives the packet at t4 containing the three timestamps t1, t2, and
   t3 and the destination timestamp t4.  These four timestamps are used
   to compute the offset and delay of B relative to A, as described

   Before the xmt and org state variables are updated, two sanity checks
   are performed in order to protect against duplicate, bogus, or
   replayed packets.  In the exchange above, a packet is duplicate or
   replay if the transmit timestamp t3 in the packet matches the org
   state variable T3.  A packet is bogus if the origin timestamp t1 in
   the packet does not match the xmt state variable T1.  In either of
   these cases, the state variables are updated, then the packet is
   discarded.  To protect against replay of the last transmitted packet,
   the xmt state variable is set to zero immediately after a successful
   bogus check.

   The four most recent timestamps, T1 through T4, are used to compute
   the offset of B relative to A

   theta = T(B) - T(A) = 1/2 * [(T2-T1) + (T3-T4)]

   and the round-trip delay

   delta = T(ABA) = (T4-T1) - (T3-T2).

   Note that the quantities within parentheses are computed from 64-bit
   unsigned timestamps and result in signed values with 63 significant
   bits plus sign.  These values can represent dates from 68 years in
   the past to 68 years in the future.  However, the offset and delay
   are computed as sums and differences of these values, which contain
   62 significant bits and two sign bits, so they can represent
   unambiguous values from 34 years in the past to 34 years in the
   future.  In other words, the time of the client must be set within 34
   years of the server before the service is started.  This is a
   fundamental limitation with 64-bit integer arithmetic.

   In implementations where floating double arithmetic is available, the
   first-order differences can be converted to floating double and the
   second-order sums and differences computed in that arithmetic.  Since
   the second-order terms are typically very small relative to the
   timestamp magnitudes, there is no loss in significance, yet the
   unambiguous range is restored from 34 years to 68 years.

   In some scenarios where the initial frequency offset of the client is
   relatively large and the actual propagation time small, it is
   possible for the delay computation to become negative.  For instance,
   if the frequency difference is 100 ppm and the interval T4-T1 is 64
   s, the apparent delay is -6.4 ms.  Since negative values are
   misleading in subsequent computations, the value of delta should be
   clamped not less than s.rho, where s.rho is the system precision
   described in Section 11.1, expressed in seconds.

   The discussion above assumes the most general case where two
   symmetric peers independently measure the offsets and delays between
   them.  In the case of a stateless server, the protocol can be
   simplified.  A stateless server copies T3 and T4 from the client
   packet to T1 and T2 of the server packet and tacks on the transmit
   timestamp T3 before sending it to the client.  Additional details for
   filling in the remaining protocol fields are given in a Section 9 and
   following sections and in the appendix.

   Note that the on-wire protocol as described resists replay of a
   server response packet.  However, it does not resist replay of the
   client request packet, which would result in a server reply packet
   with new values of T2 and T3 and result in incorrect offset and
   delay.  This vulnerability can be avoided by setting the xmt state
   variable to zero after computing the offset and delay.

9.  Peer Process Go to the top of this page...top

   The process descriptions to follow include a listing of the important
   state variables followed by an overview of the process operations
   implemented as routines.  Frequent reference is made to the skeleton
   in the appendix.  The skeleton includes C-language fragments that
   describe the functions in more detail.  It includes the parameters,
   variables, and declarations necessary for a conforming NTPv4
   implementation.  However, many additional variables and routines may
   be necessary in a working implementation.

   The peer process is called upon arrival of a server or peer packet.
   It runs the on-wire protocol to determine the clock offset and round-
   trip delay and computes statistics used by the system and poll
   processes.  Peer variables are instantiated in the association data
   structure when the structure is initialized and updated by arriving
   packets.  There is a peer process, poll process, and association
   process for each server.

9.1.  Peer Process Variables Go to the top of this page...top

   Figures 16, 17, 18, and 19 summarize the common names, formula names,
   and a short description of the peer variables.  The common names and
   formula names are interchangeable; formula names are intended to
   increase readability of equations in this specification.  Unless
   noted otherwise, all peer variables have assumed prefix p.

                 | Name    | Formula  | Description           |
                 | srcaddr | srcaddr  | source address        |
                 | srcport | srcport  | source port           |
                 | dstaddr | dstaddr  | destination address   |
                 | dstport | destport | destination port      |
                 | keyid   | keyid    | key identifier key ID |

              Figure 16: Peer Process Configuration Variables

                | Name      | Formula    | Description         |
                | leap      | leap       | leap indicator      |
                | version   | version    | version number      |
                | mode      | mode       | mode                |
                | stratum   | stratum    | stratum             |
                | ppoll     | ppoll      | peer poll exponent  |
                | rootdelay | delta_r    | root delay          |
                | rootdisp  | epsilon_r  | root dispersion     |
                | refid     | refid      | reference ID        |
                | reftime   | reftime    | reference timestamp |

                 Figure 17: Peer Process Packet Variables

                     | Name | Formula | Description        |
                     | org  | T1      | origin timestamp   |
                     | rec  | T2      | receive timestamp  |
                     | xmt  | T3      | transmit timestamp |
                     | t    | t       | packet time        |

                Figure 18: Peer Process Timestamp Variables
                     | Name   | Formula | Description     |
                     | offset | theta   | clock offset    |
                     | delay  | delta   | round-trip delay|
                     | disp   | epsilon | dispersion      |
                     | jitter | psi     | jitter          |
                     | filter | filter  | clock filter    |
                     | tp     | t_p     | filter time     |

               Figure 19: Peer Process Statistics Variables

   The following configuration variables are normally initialized when
   the association is mobilized, either from a configuration file or
   upon the arrival of the first packet for an unknown association.

   srcaddr: IP address of the remote server or reference clock.  This
   becomes the destination IP address in packets sent from this

   srcport: UDP port number of the server or reference clock.  This
   becomes the destination port number in packets sent from this
   association.  When operating in symmetric modes (1 and 2), this field
   must contain the NTP port number PORT (123) assigned by the IANA.  In
   other modes, it can contain any number consistent with local policy.

   dstaddr: IP address of the client.  This becomes the source IP
   address in packets sent from this association.

   dstport: UDP port number of the client, ordinarily the NTP port
   number PORT (123) assigned by the IANA.  This becomes the source port
   number in packets sent from this association.

   keyid: Symmetric key ID for the 128-bit MD5 key used to generate and
   verify the MAC.  The client and server or peer can use different
   values, but they must map to the same key.

   The variables defined in Figure 17 are updated from the packet header
   as each packet arrives.  They are interpreted in the same way as the
   packet variables of the same names.  It is convenient for later
   processing to convert the NTP short format packet values r.rootdelay
   and r.rootdisp to floating doubles as peer variables.

   The variables defined in Figure 18 include the timestamps exchanged
   by the on-wire protocol in Section 8.  The t variable is the seconds
   counter c.t associated with these values.  The c.t variable is
   maintained by the clock-adjust process described in Section 12.  It
   counts the seconds since the service was started.  The variables
   defined in Figure 19 include the statistics computed by the
   clock_filter() routine described in Section 10.  The tp variable is
   the seconds counter associated with these values.

9.2.  Peer Process Operations Go to the top of this page...top

   The receive routine defines the process flow upon the arrival of a
   packet.  An example is described by the receive() routine in
   Appendix A.5.1.  There is no specific method required for access
   control, although it is recommended that implementations include such
   a scheme, which is similar to many others now in widespread use.  The
   access() routine in Appendix A.5.4 describes a method of implementing
   access restrictions using an access control list (ACL).  Format
   checks require correct field length and alignment, acceptable version
   number (1-4), and correct extension field syntax, if present.

   There is no specific requirement for authentication; however, if
   authentication is implemented, then the MD5-keyed hash algorithm
   described in [RFC1321] must be supported.

   Next, the association table is searched for matching source address
   and source port, for example, using the find_assoc() routine in
   Appendix A.5.1.  Figure 20 is a dispatch table where the columns
   correspond to the packet mode and rows correspond to the association
   mode.  The intersection of the association and packet modes
   dispatches processing to one of the following steps.

           |                  |              Packet Mode              |
           | Association Mode |   1   |   2   |   3   |   4   |   5   |
           | No Association 0 | NEWPS | DSCRD | FXMIT | MANY  | NEWBC |
           | Symm. Active   1 | PROC  | PROC  | DSCRD | DSCRD | DSCRD |
           | Symm. Passive  2 | PROC  | ERR   | DSCRD | DSCRD | DSCRD |
           | Client         3 | DSCRD | DSCRD | DSCRD | PROC  | DSCRD |
           | Server         4 | DSCRD | DSCRD | DSCRD | DSCRD | DSCRD |
           | Broadcast      5 | DSCRD | DSCRD | DSCRD | DSCRD | DSCRD |
           | Bcast Client   6 | DSCRD | DSCRD | DSCRD | DSCRD | PROC  |

                      Figure 20: Peer Dispatch Table

   DSCRD.  This indicates a non-fatal violation of protocol as the
   result of a programming error, long-delayed packet, or replayed
   packet.  The peer process discards the packet and exits.
   ERR.  This indicates a fatal violation of protocol as the result of a
   programming error, long-delayed packet, or replayed packet.  The peer
   process discards the packet, demobilizes the symmetric passive
   association, and exits.

   FXMIT.  This indicates a client (mode 3) packet matching no
   association (mode 0).  If the destination address is not a broadcast
   address, the server constructs a server (mode 4) packet and returns
   it to the client without retaining state.  The server packet header
   is constructed.  An example is described by the fast_xmit() routine
   in Appendix A.5.3.  The packet header is assembled from the receive
   packet and system variables as shown in Figure 21.  If the
   s.rootdelay and s.rootdisp system variables are stored in floating
   double, they must be converted to NTP short format first.

                   | Packet Variable -->   Variable    |
                   | r.leap        -->     p.leap      |
                   | r.mode        -->     p.mode      |
                   | r.stratum     -->     p.stratum   |
                   | r.poll        -->     p.ppoll     |
                   | r.rootdelay   -->     p.rootdelay |
                   | r.rootdisp    -->     p.rootdisp  |
                   | r.refid       -->     p.refid     |
                   | r.reftime     -->     p.reftime   |
                   | r.keyid       -->     p.keyid     |

                     Figure 21: Receive Packet Header

   Note that, if authentication fails, the server returns a special
   message called a crypto-NAK.  This message includes the normal NTP
   header data shown in Figure 8, but with a MAC consisting of four
   octets of zeros.  The client MAY accept or reject the data in the
   message.  After these actions, the peer process exits.

   If the destination address is a multicast address, the sender is
   operating in manycast client mode.  If the packet is valid and the
   server stratum is less than the client stratum, the server sends an
   ordinary server (mode 4) packet, but one which uses its unicast
   destination address.  A crypto-NAK is not sent if authentication
   fails.  After these actions, the peer process exits.

   MANY: This indicates a server (mode 4) packet matching no
   association.  Ordinarily, this can happen only as the result of a
   manycast server reply to a previously sent multicast client packet.
   If the packet is valid, an ordinary client (mode 3) association is
   mobilized and operation continues as if the association was mobilized
   by the configuration file.

   NEWBC.  This indicates a broadcast (mode 5) packet matching no
   association.  The client mobilizes either a client (mode 3) or
   broadcast client (mode 6) association.  Examples are shown in the
   mobilize() and clear() routines in Appendix A.2.  Then, the packet is
   validated and the peer variables initialized.  An example is provided
   by the packet() routine in Appendix A.5.1.1.

   If the implementation supports no additional security or calibration
   functions, the association mode is set to broadcast client (mode 6)
   and the peer process exits.  Implementations supporting public key
   authentication MAY run the Autokey or equivalent security protocol.
   Implementations SHOULD set the association mode to 3 and run a short
   client/server exchange to determine the propagation delay.  Following
   the exchange, the association mode is set to 6 and the peer process
   continues in listen-only mode.  Note the distinction between a mode-6
   packet, which is reserved for the NTP monitor and control functions,
   and a mode-6 association.

   NEWPS.  This indicates a symmetric active (mode 1) packet matching no
   association.  The client mobilizes a symmetric passive (mode 2)
   association.  An example is shown in the mobilize() and clear()
   routines in Appendix A.2.  Processing continues in the PROC section

   PROC.  This indicates a packet matching an existing association.  The
   packet timestamps are carefully checked to avoid invalid, duplicate,
   or bogus packets.  Additional checks are summarized in Figure 22.
   Note that all packets, including a crypto-NAK, are considered valid
   only if they survive these tests.
   | Packet Type              | Description                            |
   | 1 duplicate packet       | The packet is at best an old duplicate |
   |                          | or at worst a replay by a hacker.      |
   |                          | This can happen in symmetric modes if  |
   |                          | the poll intervals are uneven.         |
   | 2 bogus packet           |                                        |
   | 3 invalid                | One or more timestamp fields are       |
   |                          | invalid. This normally happens in      |
   |                          | symmetric modes when one peer sends    |
   |                          | the first packet to the other and      |
   |                          | before the other has received its      |
   |                          | first reply.                           |
   | 4 access denied          | The access controls have blacklisted   |
   |                          | the source.                            |
   | 5 authentication failure | The cryptographic message digest does  |
   |                          | not match the MAC.                     |
   | 6 unsynchronized         | The server is not synchronized to a    |
   |                          | valid source.                          |
   | 7 bad header data        | One or more header fields are invalid. |

                      Figure 22: Packet Error Checks

   Processing continues by copying the packet variables to the peer
   variables as shown in Figure 21.  An example is described by the
   packet() routine in Appendix A.5.1.1.  The receive() routine
   implements tests 1-5 in Figure 22; the packet() routine implements
   tests 6-7.  If errors are found, the packet is discarded and the peer
   process exits.

   The on-wire protocol calculates the clock offset theta and round-trip
   delay delta from the four most recent timestamps as described in
   Section 8.  While it is, in principle, possible to do all
   calculations except the first-order timestamp differences in fixed-
   point arithmetic, it is much easier to convert the first-order
   differences to floating doubles and do the remaining calculations in
   that arithmetic, and this will be assumed in the following

   Next, the 8-bit p.reach shift register in the poll process described
   in Section 13 is used to determine whether the server is reachable
   and the data are fresh.  The register is shifted left by one bit when
   a packet is sent and the rightmost bit is set to zero.  As valid
   packets arrive, the rightmost bit is set to one.  If the register
   contains any nonzero bits, the server is considered reachable;
   otherwise, it is unreachable.  Since the peer poll interval might
   have changed since the last packet, the host poll interval is
   reviewed.  An example is provided by the poll_update() routine in
   Appendix A.5.7.2.

   The dispersion statistic epsilon(t) represents the maximum error due
   to the frequency tolerance and time since the last packet was sent.
   It is initialized

   epsilon(t_0) = r.rho + s.rho + PHI * (T4-T1)

   when the measurement is made at t_0 according to the seconds counter.
   Here, r.rho is the packet precision described in Section 7.3 and
   s.rho is the system precision described in Section 11.1, both
   expressed in seconds.  These terms are necessary to account for the
   uncertainty in reading the system clock in both the server and the

   The dispersion then grows at constant rate PHI; in other words, at
   time t, epsilon(t) = epsilon(t_0) + PHI * (t-t_0).  With the default
   value PHI = 15 ppm, this amounts to about 1.3 s per day.  With this
   understanding, the argument t will be dropped and the dispersion
   represented simply as epsilon.  The remaining statistics are computed
   by the clock filter algorithm described in the next section.

10.  Clock Filter Algorithm Go to the top of this page...top

   The clock filter algorithm is part of the peer process.  It grooms
   the stream of on-wire data to select the samples most likely to
   represent accurate time.  The algorithm produces the variables shown
   in Figure 19, including the offset (theta), delay (delta), dispersion
   (epsilon), jitter (psi), and time of arrival (t).  These data are
   used by the mitigation algorithms to determine the best and final
   offset used to discipline the system clock.  They are also used to
   determine the server health and whether it is suitable for

   The clock filter algorithm saves the most recent sample tuples
   (theta, delta, epsilon, t) in the filter structure, which functions
   as an 8-stage shift register.  The tuples are saved in the order that
   packets arrive.  Here, t is the packet time of arrival according to
   the seconds counter and should not be confused with the peer variable

   The following scheme is used to ensure sufficient samples are in the
   filter and that old stale data are discarded.  Initially, the tuples
   of all stages are set to the dummy tuple (0, MAXDISP, MAXDISP, 0).
   As valid packets arrive, tuples are shifted into the filter causing
   old tuples to be discarded, so eventually only valid tuples remain.
   If the three low-order bits of the reach register are zero,
   indicating three poll intervals have expired with no valid packets
   received, the poll process calls the clock filter algorithm with a
   dummy tuple just as if the tuple had arrived from the network.  If
   this persists for eight poll intervals, the register returns to the
   initial condition.

   In the next step, the shift register stages are copied to a temporary
   list and the list sorted by increasing delta.  Let i index the stages
   starting with the lowest delta.  If the first tuple epoch t_0 is not
   later than the last valid sample epoch tp, the routine exits without
   affecting the current peer variables.  Otherwise, let epsilon_i be
   the dispersion of the ith entry, then

                     ---     epsilon_i
      epsilon =       \     ----------
                      /        (i+1)
                     ---     2

   is the peer dispersion p.disp.  Note the overload of epsilon, whether
   input to the clock filter or output, the meaning should be clear from

   The observer should note (a) if all stages contain the dummy tuple
   with dispersion MAXDISP, the computed dispersion is a little less
   than 16 s, (b) each time a valid tuple is shifted into the register,
   the dispersion drops by a little less than half, depending on the
   valid tuples dispersion, and (c) after the fourth valid packet the
   dispersion is usually a little less than 1 s, which is the assumed
   value of the MAXDIST parameter used by the selection algorithm to
   determine whether or not the peer variables are acceptable.

   Let the first stage offset in the sorted list be theta_0; then, for
   the other stages in any order, the jitter is the RMS average

                          +-----                 -----+^1/2
                          |  n-1                      |
                          |  ---                      |
                  1       |  \                     2  |
      psi   =  -------- * |  /    (theta_0-theta_j)   |
                (n-1)     |  ---                      |
                          |  j=1                      |
                          +-----                 -----+

   where n is the number of valid tuples in the filter (n > 1).  In
   order to ensure consistency and avoid divide exceptions in other
   computations, the psi is bounded from below by the system precision
   s.rho expressed in seconds.  While not in general considered a major
   factor in ranking server quality, jitter is a valuable indicator of
   fundamental timekeeping performance and network congestion state.  Of
   particular importance to the mitigation algorithms is the peer
   synchronization distance, which is computed from the delay and

   lambda = (delta / 2) + epsilon.

   Note that epsilon and therefore lambda increase at rate PHI.  The
   lambda is not a state variable, since lambda is recalculated at each
   use.  It is a component of the root synchronization distance used by
   the mitigation algorithms as a metric to evaluate the quality of time
   available from each server.

   It is important to note that, unlike NTPv3, NTPv4 associations do not
   show a timeout condition by setting the stratum to 16 and leap
   indicator to 3.  The association variables retain the values
   determined upon arrival of the last packet.  In NTPv4, lambda
   increases with time, so eventually the synchronization distance
   exceeds the distance threshold MAXDIST, in which case the association
   is considered unfit for synchronization.

   An example implementation of the clock filter algorithm is shown in
   the clock_filter() routine of Appendix A.5.2.

11.  System Process Go to the top of this page...top

   As each new sample (theta, delta, epsilon, jitter, t) is produced by
   the clock filter algorithm, all peer processes are scanned by the
   mitigation algorithms consisting of the selection, cluster, combine,
   and clock discipline algorithms in the system process.  The selection
   algorithm scans all associations and casts off the falsetickers,
   which have demonstrably incorrect time, leaving the truechimers as
   result.  In a series of rounds, the cluster algorithm discards the
   association statistically furthest from the centroid until a
   specified minimum number of survivors remain.  The combine algorithm
   produces the best and final statistics on a weighted average basis.
   The final offset is passed to the clock discipline algorithm to steer
   the system clock to the correct time.

   The cluster algorithm selects one of the survivors as the system
   peer.  The associated statistics (theta, delta, epsilon, jitter, t)
   are used to construct the system variables inherited by dependent
   servers and clients and made available to other applications running
   on the same machine.

11.1.  System Process Variables Go to the top of this page...top

   Figure 23 summarizes the common names, formula names, and a short
   description of each system variable.  Unless noted otherwise, all
   variables have assumed prefix s.

                | Name      | Formula    | Description            |
                | t         | t          | update time            |
                | p         | p          | system peer identifier |
                | leap      | leap       | leap indicator         |
                | stratum   | stratum    | stratum                |
                | precision | rho        | precision              |
                | offset    | THETA      | combined offset        |
                | jitter    | PSI        | combined jitter        |
                | rootdelay | DELTA      | root delay             |
                | rootdisp  | EPSILON    | root dispersion        |
                | v         | v          | survivor list          |
                | refid     | refid      | reference ID           |
                | reftime   | reftime    | reference time         |
                | NMIN      | 3          | minimum survivors      |
                | CMIN      | 1          | minimum candidates     |

                    Figure 23: System Process Variables

   Except for the t, p, offset, and jitter variables and the NMIN and
   CMIN constants, the variables have the same format and interpretation
   as the peer variables of the same name.  The NMIN and CMIN parameters
   are used by the selection and cluster algorithms described in the
   next section.

   The t variable is the seconds counter at the time of the last update.
   An example is shown by the clock_update() routine in
   Appendix A.5.5.4.  The p variable is the system peer identifier
   determined by the cluster() routine in Section 11.2.2.  The precision
   variable has the same format as the packet variable of the same name.
   The precision is defined as the larger of the resolution and time to
   read the clock, in log2 units.  For instance, the precision of a
   mains-frequency clock incrementing at 60 Hz is 16 ms, even when the
   system clock hardware representation is to the nanosecond.

   The offset and jitter variables are determined by the combine
   algorithm in Section 11.2.3.  These values represent the best and
   final offset and jitter used to discipline the system clock.
   Initially, all variables are cleared to zero, then the leap is set to
   3 (unsynchronized) and stratum is set to MAXSTRAT (16).  Remember
   that MAXSTRAT is mapped to zero in the transmitted packet.

11.2.  System Process Operations Go to the top of this page...top

   Figure 24 summarizes the system process operations performed by the
   clock select routine.  The selection algorithm described in
   Section 11.2.1 produces a majority clique of presumed correct
   candidates (truechimers) based on agreement principles.  The cluster
   algorithm described in Section 11.2.2 discards outliers to produce
   the most accurate survivors.  The combine algorithm described in
   Section 11.2.3 provides the best and final offset for the clock
   discipline algorithm.  An example is described in Appendix A.5.5.6.
   If the selection algorithm cannot produce a majority clique, or if it
   cannot produce at least CMIN survivors, the system process exits
   without disciplining the system clock.  If successful, the cluster
   algorithm selects the statistically best candidate as the system peer
   and its variables are inherited as the system variables.
                          | clock_select()  |
   .                               V          .
   .      yes +---------+ +-----------------+ .
   .       +--| accept? | | scan candidates | .
   .       |  +---------+ |                 | .
   .       V        no |  |                 | .
   .  +---------+      |  |                 | .
   .  | add peer|      |  |                 | .
   .  +----------      |  |                 | .
   .       |           V  |                 | .
   .       +---------->-->|                 | .
   .                      |                 | .
   . Selection Algorithm  +-----------------+ .
                       no +-------------------+
            +-------------|     survivors?    |
            |             +-------------------+
            |                       | yes
            |                       V
            |             +-------------------+
            |             | Cluster Algorithm |
            |             +-------------------+
            |                       |
            |                       V
            V         yes +-------------------+
            |<------------|     n < CMIN?     |
            |             +-------------------+
            V                       |
     +-----------------+            V no
     |   s.p = NULL    |  +-------------------+
     +-----------------+  |   s.p = v_0.p     |
            |             +-------------------+
            V                       |
     +-----------------+            V
     | return (UNSYNC) |  +-------------------+
     +-----------------+  |   return (SYNC)   |

                      Figure 24: Clock Select Routine

11.2.1.  Selection Algorithm Go to the top of this page...top

   Note that the selection and cluster algorithms are described
   separately, but combined in the code skeleton.  The selection
   algorithm operates to find an intersection interval containing a
   majority clique of truechimers using Byzantine agreement principles
   originally proposed by Marzullo [ref6], but modified to improve
   accuracy.  An overview of the algorithm is given below and described
   in the first half of the clock_select() routine in Appendix A.5.5.1.

   First, those servers that are unusable according to the rules of the
   protocol are detected and discarded as shown by the accept() routine
   in Appendix A.5.5.3.  Next, a set of tuples (p, type, edge) is
   generated for the remaining candidates.  Here, p is the association
   identifier and type identifies the upper (+1), middle (0), and lower
   (-1) endpoints of a correctness interval centered on theta for that
   candidate.  This results in three tuples, lowpoint (p, -1, theta -
   lambda), midpoint (p, 0, theta), and highpoint (p, +1, theta +
   lambda), where lambda is the root synchronization distance.  An
   example of this calculation is shown by the rootdist() routine in
   Appendix A.5.1.1.  The steps of the algorithm are:

   1.  For each of m associations, place three tuples as defined above
   on the candidate list.

   2.  Sort the tuples on the list by the edge component.  Order the
   lowpoint, midpoint, and highpoint of these intervals from lowest to
   highest.  Set the number of falsetickers f = 0.

   3.  Set the number of midpoints d = 0.  Set c = 0.  Scan from lowest
   endpoint to highest.  Add one to c for every lowpoint, subtract one
   for every highpoint, add one to d for every midpoint.  If c >= m - f,
   stop; set l = current lowpoint.

   4.  Set c = 0.  Scan from highest endpoint to lowest.  Add one to c
   for every highpoint, subtract one for every lowpoint, add one to d
   for every midpoint.  If c >= m - f, stop; set u = current highpoint.

   5.  Is d = f and l < u?  If yes, then follow step 5A; else, follow
   step 5B.

   5A. Success: the intersection interval is [l, u].

   5B. Add one to f.  Is f < (m / 2)?  If yes, then go to step 3 again.
   If no, then go to step 6.

   6.  Failure; a majority clique could not be found.  There are no
   suitable candidates to discipline the system clock.
   The algorithm is described in detail in Appendix A.5.5.1.  Note that
   it starts with the assumption that there are no falsetickers (f = 0)
   and attempts to find a non-empty intersection interval containing the
   midpoints of all correct servers, i.e., truechimers.  If a non-empty
   interval cannot be found, it increases the number of assumed
   falsetickers by one and tries again.  If a non-empty interval is
   found and the number of falsetickers is less than the number of
   truechimers, a majority clique has been found and the midpoint of
   each truechimer (theta) represents the candidates available to the
   cluster algorithm.

   If a majority clique is not found, or if the number of truechimers is
   less than CMIN, there are insufficient candidates to discipline the
   system clock.  CMIN defines the minimum number of servers consistent
   with the correctness requirements.  Suspicious operators would set
   CMIN to ensure multiple redundant servers are available for the
   algorithms to mitigate properly.  However, for historic reasons the
   default value for CMIN is one.

11.2.2.  Cluster Algorithm Go to the top of this page...top

   The candidates of the majority clique are placed on the survivor list
   v in the form of tuples (p, theta_p, psi_p, lambda_p), where p is an
   association identifier, theta_p, psi_p, and stratum_p the current
   offset, jitter and stratum of association p, respectively, and
   lambda_p is a merit factor equal to stratum_p * MAXDIST + lambda,
   where lambda is the root synchronization distance for association p.
   The list is processed by the cluster algorithm below.  An example is
   shown by the second half of the clock_select() algorithm in
   Appendix A.5.5.1.

   1.  Let (p, theta_p, psi_p, lambda_p) represent a survivor candidate.

   2.  Sort the candidates by increasing lambda_p.  Let n be the number
   of candidates and NMIN the minimum required number of survivors.

   3.  For each candidate, compute the selection jitter psi_s:

             +-----                       -----+^1/2
             |        n-1                      |
             |        ---                      |
             |   1    \                     2  |
     psi_s = | ---- * /  (theta_s - theta_j)   |
             |  n-1   ---                      |
             |        j=1                      |
             +-----                       -----+

   4.  Select psi_max as the candidate with maximum psi_s.
   5.  Select psi_min as the candidate with minimum psi_p.

   6.  Is psi_max < psi_min or n <= NMIN?  If yes, follow step 6A;
   otherwise, follow step 6B.

   6A. Done.  The remaining candidates on the survivor list are ranked
   in the order of preference.  The first entry on the list represents
   the system peer; its variables are used later to update the system

   6B. Delete the outlier candidate with psi_max; reduce n by one and go
   back to step 3.

   The algorithm operates in a series of rounds where each round
   discards the statistical outlier with maximum selection jitter psi_s.
   However, if psi_s is less than the minimum peer jitter psi_p, no
   improvement is possible by discarding outliers.  This and the minimum
   number of survivors represent the terminating conditions of the
   algorithm.  Upon termination, the final value of psi_max is saved as
   the system selection jitter PSI_s for use later.

11.2.3.  Combine Algorithm Go to the top of this page...top

   The clock combine route processes the remaining survivors to produce
   the best and final data for the clock discipline algorithm.  The
   routine processes peer offset and jitter statistics to produce the
   combined system offset THETA and system peer jitter PSI_p, where each
   server statistic is weighted by the reciprocal of the root
   synchronization distance and the result normalized.  An example is
   shown by the clock_combine() routine in Appendix A.5.5.5

   The combined THETA is passed to the clock update routine.  The first
   candidate on the survivor list is nominated as the system peer with
   identifier p.  The system peer jitter PSI_p is a component of the
   system jitter PSI.  It is used along with the selection jitter PSI_s
   to produce the system jitter:

   PSI = [(PSI_s)^2 + (PSI_p)^2]^1/2

   Each time an update is received from the system peer, the clock
   update routine is called.  By rule, an update is discarded if its
   time of arrival p.t is not strictly later than the last update used
   s.t.  The labels IGNOR, PANIC, ADJ, and STEP refer to return codes
   from the local clock routine described in the next section.

   IGNORE means the update has been ignored as an outlier.  PANIC means
   the offset is greater than the panic threshold PANICT (1000 s) and
   SHOULD cause the program to exit with a diagnostic message to the
   system log.  STEP means the offset is less than the panic threshold,
   but greater than the step threshold STEPT (125 ms).  In this case,
   the clock is stepped to the correct offset, but since this means all
   peer data have been invalidated, all associations MUST be reset and
   the client begins as at initial start.

   ADJ means the offset is less than the step threshold and thus a valid
   update.  In this case, the system variables are updated from the peer
   variables as shown in Figure 25.

                  | System Variable <-- System Peer Variable  |        |
                  | s.leap      <-- p.leap                    |
                  | s.stratum   <-- p.stratum + 1             |
                  | s.offset    <-- THETA                     |
                  | s.jitter    <-- PSI                       |
                  | s.rootdelay <-- p.delta_r + delta         |
                  | s.rootdisp  <-- p.epsilon_r + p.epsilon + |
                  |                 p.psi + PHI * (s.t - p.t) |
                  |                 + |THETA|                 |
                  | s.refid     <-- p.refid                   |
                  | s.reftime   <-- p.reftime                 |
                  | s.t         <-- p.t                       |

                    Figure 25: System Variables Update

   There is an important detail not shown.  The dispersion increment
   (p.epsilon + p.psi + PHI * (s.t - p.t) + |THETA|) is bounded from
   below by MINDISP.  In subnets with very fast processors and networks
   and very small delay and dispersion this forces a monotone-definite
   increase in s.rootdisp (EPSILON), which avoids loops between peers
   operating at the same stratum.

   The system variables are available to dependent application programs
   as nominal performance statistics.  The system offset THETA is the
   clock offset relative to the available synchronization sources.  The
   system jitter PSI is an estimate of the error in determining this
   value, elsewhere called the expected error.  The root delay DELTA is
   the total round-trip delay relative to the primary server.  The root
   dispersion EPSILON is the dispersion accumulated over the network
   from the primary server.  Finally, the root synchronization distance
   is defined as:

   which represents the maximum error due all causes and is designated
   the root synchronization distance.

   An example of the clock update routine is provided in
   Appendix A.5.5.4.

11.3.  Clock Discipline Algorithm Go to the top of this page...top

   The NTPv4 clock discipline algorithm, shortened to discipline in the
   following, functions as a combination of two quite philosophically
   different feedback control systems.  In a phase-locked loop (PLL)
   design, periodic phase updates at update intervals mu seconds are
   used directly to minimize the time error and indirectly the frequency
   error.  In a frequency-locked loop (FLL) design, periodic frequency
   updates at intervals mu are used directly to minimize the frequency
   error and indirectly the time error.  As shown in [ref7], a PLL
   usually works better when network jitter dominates, while an FLL
   works better when oscillator wander dominates.  This section contains
   an outline of how the NTPv4 design works.  An in-depth discussion of
   the design principles is provided in [ref7], which also includes a
   performance analysis.

   The discipline is implemented as the feedback control system shown in
   Figure 26.  The variable theta_r represents the combine algorithm
   offset (reference phase) and theta_c the VFO offset (control phase).
   Each update produces a signal V_d representing the instantaneous
   phase difference theta_r - theta_c.  The clock filter for each server
   functions as a tapped delay line, with the output taken at the tap
   selected by the clock filter algorithm.  The selection, cluster, and
   combine algorithms combine the data from multiple filters to produce
   the signal V_s.  The loop filter, with impulse response F(t),
   produces the signal V_c, which controls the VFO frequency omega_c and
   thus the integral of the phase theta_c which closes the loop.  The
   V_c signal is generated by the clock-adjust process in Section 12.
   The detailed equations that implement these functions are best
   presented in the routines of Appendices A.5.5.6 and A.5.6.1.
                theta_r + +---------\        +----------------+
            NTP --------->|  Phase   \  V_d  |                | V_s
                theta_c - | Detector  ------>|  Clock Filter  |----+
                +-------->|          /       |                |    |
                |         +---------/        +----------------+    |
                |                                                  |
              -----                                                |
             /     \                                               |
             | VFO |                                               |
             \     /                                               |
              -----    .......................................     |
                ^      .            Loop Filter              .     |
                |      . +---------+   x  +-------------+    .     |
                | V_c  . |         |<-----|             |    .     |
                +------.-|  Clock  |   y  | Phase/Freq  |<---------+
                       . | Adjust  |<-----| Prediction  |    .
                       . |         |      |             |    .
                       . +---------+      +-------------+    .

                 Figure 26: Clock Discipline Feedback Loop

   Ordinarily, the pseudo-linear feedback loop described above operates
   to discipline the system clock.  However, there are cases where a
   non-linear algorithm offers considerable improvement.  One case is
   when the discipline starts without knowledge of the intrinsic clock
   frequency.  The pseudo-linear loop takes several hours to develop an
   accurate measurement and during most of that time the poll interval
   cannot be increased.  The non-linear loop described below does this
   in 15 minutes.  Another case is when occasional bursts of large
   jitter are present due to congested network links.  The state machine
   described below resists error bursts lasting less than 15 minutes.

   Figure 27 contains a summary of the variables and parameters
   including the variable (lowercase) or parameter (uppercase) name,
   formula name, and short description.  Unless noted otherwise, all
   variables have assumed prefix c.  The variables t, tc, state, hyster,
   and count are integers; the remaining variables are floating doubles.
   The function of each will be explained in the algorithm descriptions
                | Name   | Formula    | Description              |
                | t      | timer      | seconds counter          |
                | offset | theta      | combined offset          |
                | resid  | theta_r    | residual offset          |
                | freq   | phi        | clock frequency          |
                | jitter | psi        | clock offset jitter      |
                | wander | omega      | clock frequency wander   |
                | tc     | tau        | time constant (log2)     |
                | state  | state      | state                    |
                | adj    | adj        | frequency adjustment     |
                | hyster | hyster     | hysteresis counter       |
                | STEPT  | 125        | step threshold (.125 s)  |
                | WATCH  | 900        | stepout thresh(s)        |
                | PANICT | 1000       | panic threshold (1000 s) |
                | LIMIT  | 30         | hysteresis limit         |
                | PGATE  | 4          | hysteresis gate          |
                | TC     | 16         | time constant scale      |
                | AVG    | 8          | averaging constant       |

           Figure 27: Clock Discipline Variables and Parameters

   The process terminates immediately if the offset is greater than the
   panic threshold PANICT (1000 s).  The state transition function is
   described by the rstclock() function in Appendix A.5.5.7.  Figure 28
   shows the state transition function used by this routine.  It has
   four columns showing, respectively, the state name, predicate and
   action if the offset theta is less than the step threshold, the
   predicate and actions otherwise, and finally some comments.
      | State | theta < STEP        | theta > STEP      | Comments     |
      | NSET  | ->FREQ              | ->FREQ            | no frequency |
      |       | adjust time         | step time         | file         |
      | FSET  | ->SYNC              | ->SYNC            | frequency    |
      |       | adjust time         | step time         | file         |
      | SPIK  | ->SYNC              | if < 900 s ->SPIK | outlier      |
      |       | adjust freq         | else ->SYNC       | detected     |
      |       | adjust time         | step freq         |              |
      |       |                     | step time         |              |
      | FREQ  | if < 900 s ->FREQ   | if < 900 s ->FREQ | initial      |
      |       | else ->SYNC         | else ->SYNC       | frequency    |
      |       | step freq           | step freq         |              |
      |       | adjust time         | adjust time       |              |
      | SYNC  | ->SYNC              | if < 900 s ->SPIK | normal       |
      |       | adjust freq         | else ->SYNC       | operation    |
      |       | adjust time         | step freq         |              |
      |       |                     | step time         |              |

                   Figure 28: State Transition Function

   In the table entries, the next state is identified by the arrow ->
   with the actions listed below.  Actions such as adjust time and
   adjust frequency are implemented by the PLL/FLL feedback loop in the
   local_clock() routine.  A step clock action is implemented by setting
   the clock directly, but this is done only after the stepout threshold
   WATCH (900 s) when the offset is more than the step threshold STEPT
   (.125 s).  This resists clock steps under conditions of extreme
   network congestion.

   The jitter (psi) and wander (omega) statistics are computed using an
   exponential average with weight factor AVG.  The time constant
   exponent (tau) is determined by comparing psi with the magnitude of
   the current offset theta.  If the offset is greater than PGATE (4)
   times the clock jitter, the hysteresis counter hyster is reduced by
   two; otherwise, it is increased by one.  If hyster increases to the
   upper limit LIMIT (30), tau is increased by one; if it decreases to
   the lower limit -LIMIT (-30), tau is decreased by one.  Normally, tau
   hovers near MAXPOLL, but quickly decreases if a temperature spike
   causes a frequency surge.

12.  Clock-Adjust Process Go to the top of this page...top

   The actual clock-adjust process runs at one-second intervals to add
   the frequency correction and a fixed percentage of the residual
   offset theta_r.  The theta_r is, in effect, the exponential decay of
   the theta value produced by the loop filter at each update.  The TC
   parameter scales the time constant to match the poll interval for
   convenience.  Note that the dispersion EPSILON increases by PHI at
   each second.

   The clock-adjust process includes a timer interrupt facility driving
   the seconds counter c.t.  It begins at zero when the service starts
   and increments once each second.  At each interrupt, the
   clock_adjust() routine is called to incorporate the clock discipline
   time and frequency adjustments, then the associations are scanned to
   determine if the seconds counter equals or exceeds the p.next state
   variable defined in the next section.  If so, the poll process is
   called to send a packet and compute the next p.next value.

   An example of the clock-adjust process is shown by the clock_adjust()
   routine in Appendix A.5.6.1.

13.  Poll Process Go to the top of this page...top

   Each association supports a poll process that runs at regular
   intervals to construct and send packets in symmetric, client, and
   broadcast server associations.  It runs continuously, whether or not
   servers are reachable in order to manage the clock filter and reach

13.1.  Poll Process Variables Go to the top of this page...top

   Figure 29 summarizes the common names, formula names, and a short
   description of the poll process variables (lowercase) and parameters
   (uppercase).  Unless noted otherwise, all variables have assumed
   prefix p.
                   | Name    | Formula | Description        |
                   | hpoll   | hpoll   | host poll exponent |
                   | last    | last    | last poll time     |
                   | next    | next    | next poll time     |
                   | reach   | reach   | reach register     |
                   | unreach | unreach | unreach counter    |
                   | UNREACH | 24      | unreach limit      |
                   | BCOUNT  | 8       | burst count        |
                   | BURST   | flag    | burst enable       |
                   | IBURST  | flag    | iburst enable      |

             Figure 29: Poll Process Variables and Parameters

   The poll process variables are allocated in the association data
   structure along with the peer process variables.  The following is a
   detailed description of the variables.  The parameters will be called
   out in the following text.

   hpoll: signed integer representing the poll exponent, in log2 seconds

   last: integer representing the seconds counter when the most recent
   packet was sent

   next: integer representing the seconds counter when the next packet
   is to be sent

   reach: 8-bit integer shift register shared by the peer and poll

   unreach: integer representing the number of seconds the server has
   been unreachable

13.2.  Poll Process Operations Go to the top of this page...top

   As described previously, once each second the clock-adjust process is
   called.  This routine calls the poll routine for each association in
   turn.  If the time for the next poll message is greater than the
   seconds counter, the routine returns immediately.  Symmetric (modes
   1, 2), client (mode 3), and broadcast server (mode 5) associations
   routinely send packets.  A broadcast client (mode 6) association runs
   the routine to update the reach and unreach variables, but does not
   send packets.  The poll process calls the transmit process to send a
   packet.  If in a burst (burst > 0), nothing further is done except
   call the poll update routine to set the next poll interval.
   If not in a burst, the reach variable is shifted left by one bit,
   with zero replacing the rightmost bit.  If the server has not been
   heard for the last three poll intervals, the clock filter routine is
   called to increase the dispersion.  An example is shown in
   Appendix A.5.7.3.

   If the BURST flag is lit and the server is reachable and a valid
   source of synchronization is available, the client sends a burst of
   BCOUNT (8) packets at each poll interval.  The interval between
   packets in the burst is two seconds.  This is useful to accurately
   measure jitter with long poll intervals.  If the IBURST flag is lit
   and this is the first packet sent when the server has been
   unreachable, the client sends a burst.  This is useful to quickly
   reduce the synchronization distance below the distance threshold and
   synchronize the clock.

   If the P_MANY flag is lit in the p.flags word of the association,
   this is a manycast client association.  Manycast client associations
   send client mode packets to designated multicast group addresses at
   MINPOLL intervals.  The association starts out with a TTL of 1.  If
   by the time of the next poll there are fewer than MINCLOCK servers
   have been mobilized, the ttl is increased by one.  If the ttl reaches
   the limit TTLMAX, without finding MINCLOCK servers, the poll interval
   increases until reaching BEACON, when it starts over from the

   The poll() routine includes a feature that backs off the poll
   interval if the server becomes unreachable.  If reach is nonzero, the
   server is reachable and unreach is set to zero; otherwise, unreach is
   incremented by one for each poll to the maximum UNREACH.  Thereafter
   for each poll hpoll is increased by one, which doubles the poll
   interval up to the maximum MAXPOLL determined by the poll_update()
   routine.  When the server again becomes reachable, unreach is set to
   zero, hpoll is reset to the tc system variable, and operation resumes

   A packet is sent by the transmit process.  Some header values are
   copied from the peer variables left by a previous packet and others
   from the system variables.  Figure 30 shows which values are copied
   to each header field.  In those implementations, using floating
   double data types for root delay and root dispersion, these must be
   converted to NTP short format.  All other fields are either copied
   intact from peer and system variables or struck as a timestamp from
   the system clock.
                   | Packet Variable <--   Variable    |
                   | x.leap        <--     s.leap      |
                   | x.version     <--     s.version   |
                   | x.mode        <--     s.mode      |
                   | x.stratum     <--     s.stratum   |
                   | x.poll        <--     s.poll      |
                   | x.precision   <--     s.precision |
                   | x.rootdelay   <--     s.rootdelay |
                   | x.rootdisp    <--     s.rootdisp  |
                   | x.refid       <--     s.refid     |
                   | x.reftime     <--     s.reftime   |
                   | x.org         <--     p.xmt       |
                   | x.rec         <--     p.dst       |
                   | x.xmt         <--     clock       |
                   | x.keyid       <--     p.keyid     |
                   | x.digest      <--     md5 digest  |

                   Figure 30: xmit_packet Packet Header

   The poll update routine is called when a valid packet is received and
   immediately after a poll message has been sent.  If in a burst, the
   poll interval is fixed at 2 s; otherwise, the host poll exponent
   hpoll is set to the minimum of ppoll from the last packet received
   and hpoll from the poll routine, but not less than MINPOLL or greater
   than MAXPOLL.  Thus, the clock discipline can be oversampled but not
   undersampled.  This is necessary to preserve subnet dynamic behavior
   and protect against protocol errors.

   The poll exponent is converted to an interval, which, when added to
   the last poll time variable, determines the value of the next poll
   time variable.  Finally, the last poll time variable is set to the
   current seconds counter.

14.  Simple Network Time Protocol (SNTP) Go to the top of this page...top

   Primary servers and clients complying with a subset of NTP, called
   the Simple Network Time Protocol (SNTPv4) [RFC4330], do not need to
   implement the mitigation algorithms described in Section 9 and
   following sections.  SNTP is intended for primary servers equipped
   with a single reference clock, as well as for clients with a single
   upstream server and no dependent clients.  The fully developed NTPv4
   implementation is intended for secondary servers with multiple
   upstream servers and multiple downstream servers or clients.  Other
   than these considerations, NTP and SNTP servers and clients are
   completely interoperable and can be intermixed in NTP subnets.
   An SNTP primary server implementing the on-wire protocol described in
   Section 8 has no upstream servers except a single reference clock.
   In principle, it is indistinguishable from an NTP primary server that
   has the mitigation algorithms and therefore capable of mitigating
   between multiple reference clocks.

   Upon receiving a client request, an SNTP primary server constructs
   and sends the reply packet as described in Figure 31.  Note that the
   dispersion field in the packet header must be updated as described in
   Section 5.

                   | Packet Variable <--   Variable    |
                   | x.leap        <--     s.leap      |
                   | x.version     <--     r.version   |
                   | x.mode        <--     4           |
                   | x.stratum     <--     s.stratum   |
                   | x.poll        <--     r.poll      |
                   | x.precision   <--     s.precision |
                   | x.rootdelay   <--     s.rootdelay |
                   | x.rootdisp    <--     s.rootdisp  |
                   | x.refid       <--     s.refid     |
                   | x.reftime     <--     s.reftime   |
                   | x.org         <--     r.xmt       |
                   | x.rec         <--     r.dst       |
                   | x.xmt         <--     clock       |
                   | x.keyid       <--     r.keyid     |
                   | x.digest      <--     md5 digest  |

                    Figure 31: fast_xmit Packet Header

   An SNTP client implementing the on-wire protocol has a single server
   and no dependent clients.  It can operate with any subset of the NTP
   on-wire protocol, the simplest approach using only the transmit
   timestamp of the server packet and ignoring all other fields.
   However, the additional complexity to implement the full on-wire
   protocol is minimal so that a full implementation is encouraged.

15.  Security Considerations Go to the top of this page...top

   NTP security requirements are even more stringent than most other
   distributed services.  First, the operation of the authentication
   mechanism and the time synchronization mechanism are inextricably
   intertwined.  Reliable time synchronization requires cryptographic
   keys that are valid only over a designated time interval; but, time
   intervals can be enforced only when participating servers and clients
   are reliably synchronized to UTC.  In addition, the NTP subnet is
   hierarchical by nature, so time and trust flow from the primary
   servers at the root through secondary servers to the clients at the

   An NTP client can claim to have authentic time to dependent
   applications only if all servers on the path to the primary servers
   are authenticated.  In NTP each server authenticates the next lower
   stratum servers and authenticates by induction the lowest stratum
   (primary) servers.  It is important to note that authentication in
   the context of NTP does not necessarily imply the time is correct.
   An NTP client mobilizes a number of concurrent associations with
   different servers and uses a crafted agreement algorithm to pluck
   truechimers from the population possibly including falsetickers.

   The NTP specification assumes that the goal of the intruder is to
   inject false time values, disrupt the protocol, or clog the network,
   servers, or clients with spurious packets that exhaust resources and
   deny service to legitimate applications.  There are a number of
   defense mechanisms already built in the NTP architecture, protocol,
   and algorithms.  The on-wire timestamp exchange scheme is inherently
   resistant to spoofing, packet-loss, and replay attacks.  The
   engineered clock filter, selection and clustering algorithms are
   designed to defend against evil cliques of Byzantine traitors.  While
   not necessarily designed to defeat determined intruders, these
   algorithms and accompanying sanity checks have functioned well over
   the years to deflect improperly operating but presumably friendly
   scenarios.  However, these mechanisms do not securely identify and
   authenticate servers to clients.  Without specific further
   protection, an intruder can inject any or all of the following

   1.  An intruder can intercept and archive packets forever, as well as
       all the public values ever generated and transmitted over the

   2.  An intruder can generate packets faster than the server, network
       or client can process them, especially if they require expensive
       cryptographic computations.

   3.  In a wiretap attack, the intruder can intercept, modify, and
       replay a packet.  However, it cannot permanently prevent onward
       transmission of the original packet; that is, it cannot break the
       wire, only tell lies and congest it.  Generally, the modified
       packet cannot arrive at the victim before the original packet,
       nor does it have the server private keys or identity parameters.
   4.  In a middleman or masquerade attack, the intruder is positioned
       between the server and client, so it can intercept, modify and
       replay a packet and prevent onward transmission of the original
       packet.  However, the middleman does not have the server private

   The NTP security model assumes the following possible limitations:

   1.  The running times for public key algorithms are relatively long
       and highly variable.  In general, the performance of the time
       synchronization function is badly degraded if these algorithms
       must be used for every NTP packet.

   2.  In some modes of operation, it is not feasible for a server to
       retain state variables for every client.  It is however feasible
       to regenerated them for a client upon arrival of a packet from
       that client.

   3.  The lifetime of cryptographic values must be enforced, which
       requires a reliable system clock.  However, the sources that
       synchronize the system clock must be trusted.  This circular
       interdependence of the timekeeping and authentication functions
       requires special handling.

   4.  Client security functions must involve only public values
       transmitted over the net.  Private values must never be disclosed
       beyond the machine on which they were created, except in the case
       of a special trusted agent (TA) assigned for this purpose.

   Unlike the Secure Shell (SSH) security model, where the client must
   be securely authenticated to the server, in NTP the server must be
   securely authenticated to the client.  In SSH, each different
   interface address can be bound to a different name, as returned by a
   reverse-DNS query.  In this design, separate public/private key pairs
   may be required for each interface address with a distinct name.  A
   perceived advantage of this design is that the security compartment
   can be different for each interface.  This allows a firewall, for
   instance, to require some interfaces to authenticate the client and
   others not.

   In the case of NTP as specified herein, NTP broadcast clients are
   vulnerable to disruption by misbehaving or hostile SNTP or NTP
   broadcast servers elsewhere in the Internet.  Such disruption can be
   minimized by several approaches.  Filtering can be employed to limit
   the access of NTP clients to known or trusted NTP broadcast servers.
   Such filtering will prevent malicious traffic from reaching the NTP
   clients.  Cryptographic authentication at the client will only allow
   timing information from properly signed NTP messages to be utilized
   in synchronizing its clock.  Higher levels of authentication may be
   gained by the use of the Autokey mechanism [RFC5906].

   Section 8 describes a potential security concern with the replay of
   client requests.  Following the recommendations in that section
   provides protection against such attacks.

   It should be noted that this specification is describing an existing
   implementation.  While the security shortfalls of the MD5 algorithm
   are well-known, its use in the NTP specification is consistent with
   widescale deployment in the Internet community.

16.  IANA Considerations Go to the top of this page...top

   UDP/TCP Port 123 was previously assigned by IANA for this protocol.
   The IANA has assigned the IPv4 multicast group address and
   the IPv6 multicast address ending :101 for NTP.  This document
   introduces NTP extension fields allowing for the development of
   future extensions to the protocol, where a particular extension is to
   be identified by the Field Type sub-field within the extension field.
   IANA has established and will maintain a registry for Extension Field
   Types associated with this protocol, populating this registry with no
   initial entries.  As future needs arise, new Extension Field Types
   may be defined.  Following the policies outlined in [RFC5226], new
   values are to be defined by IETF Review.

   The IANA has created a new registry for NTP Reference Identifier
   codes.  This includes the current codes defined in Section 7.3, and
   may be extended on a First-Come-First-Serve (FCFS) basis.  The format
   of the registry is:

     | ID   | Clock Source                                             |
     | GOES | Geosynchronous Orbit Environment Satellite               |
     | GPS  | Global Position System                                   |
     | ...  | ...                                                      |

                   Figure 32: Reference Identifier Codes

   The IANA has created a new registry for NTP Kiss-o'-Death codes.
   This includes the current codes defined in Section 7.4, and may be
   extended on a FCFS basis.  The format of the registry is:
   | Code |                           Meaning                          |
   | ACST | The association belongs to a unicast server.               |
   | AUTH | Server authentication failed.                              |
   | ...  | ...                                                        |

                           Figure 33: Kiss Codes

   For both Reference Identifiers and Kiss-o'-Death codes, IANA is
   requested to never assign a code beginning with the character "X", as
   this is reserved for experimentation and development.

17.  Acknowledgements Go to the top of this page...top

   The editors would like to thank Karen O'Donoghue, Brian Haberman,
   Greg Dowd, Mark Elliot, Harlan Stenn, Yaakov Stein, Stewart Bryant,
   and Danny Mayer for technical reviews and specific text contributions
   to this document.

18.  References Go to the top of this page...top

18.1.  Normative References Go to the top of this page...top

   [RFC0768]       Postel, J., "User Datagram Protocol", STD 6, RFC 768,
                   August 1980.

   [RFC0791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                   September 1981.

   [RFC0793]       Postel, J., "Transmission Control Protocol", STD 7,
                   RFC 793, September 1981.

   [RFC1321]       Rivest, R., "The MD5 Message-Digest Algorithm",
                   RFC 1321, April 1992.

   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

18.2.  Informative References Go to the top of this page...top

   [CGPM]          Bureau International des Poids et Mesures, "Comptes
                   Rendus de la 15e CGPM", 1976.

   [ITU-R_TF.460]  International Telecommunications Union, "ITU-R TF.460
                   Standard-frequency and time-signal emissions",
                   February 2002.
   [RFC1305]       Mills, D., "Network Time Protocol (Version 3)
                   Specification, Implementation and Analysis",
                   RFC 1305, March 1992.

   [RFC1345]       Simonsen, K., "Character Mnemonics and Character
                   Sets", RFC 1345, June 1992.

   [RFC4330]       Mills, D., "Simple Network Time Protocol (SNTP)
                   Version 4 for IPv4, IPv6 and OSI", RFC 4330,
                   January 2006.

   [RFC5226]       Narten, T. and H. Alvestrand, "Guidelines for Writing
                   an IANA Considerations Section in RFCs", BCP 26,
                   RFC 5226, May 2008.

   [RFC5906]       Haberman, B., Ed. and D. Mills, "Network Time
                   Protocol Version 4: Autokey Specification", RFC 5906,
                   June 2010.

   [ref6]          Marzullo and S. Owicki, "Maintaining the time in a
                   distributed system", ACM Operating Systems Review 19,
                   July 1985.

   [ref7]          Mills, D.L., "Computer Network Time Synchronization -
                   the Network Time Protocol", CRC Press, 304 pp, 2006.

   [ref9]          Mills, D.L., Electrical and Computer Engineering
                   Technical Report 06-6-1, NDSS, June 2006, "Network
                   Time Protocol Version 4 Reference and Implementation
                   Guide", 2006.

Appendix A.  Code Skeleton Go to the top of this page...top

   This appendix is intended to describe the protocol and algorithms of
   an implementation in a general way using what is called a code
   skeleton program.  This consists of a set of definitions, structures,
   and code fragments that illustrate the protocol operations without
   the complexities of an actual implementation of the protocol.  This
   program is not an executable and is not designed to run in the
   ordinary sense.

   Most of the features of the reference implementation are included
   here, with the following exceptions: there are no provisions for
   reference clocks or public key (Autokey) cryptography.  There is no
   huff-n'-puff filter, anti-clockhop hysteresis, or monitoring
   provisions.  Many of the values that can be tinkered in the reference
   implementation are assumed constants here.  There are only minimal
   provisions for the kiss-o'-death packet and no responding code.

   The program is not intended to be fast or compact, just to
   demonstrate the algorithms with sufficient fidelity to understand how
   they work.  The code skeleton consists of eight segments, a header
   segment included by each of the other segments, plus a code segment
   for the main program, kernel I/O and system clock interfaces, and
   peer, system, clock_adjust, and poll processes.  These are presented
   in order below along with definitions and variables specific to each

A.1.  Global Definitions Go to the top of this page...top

A.1.1.  Definitions, Constants, Parameters

#include <math.h>               /* avoids complaints about sqrt() */

#include <sys/time.h>           /* for gettimeofday() and friends */

#include <stdlib.h>             /* for malloc() and friends */

#include <string.h>             /* for memset() */

 * Data types
 * This program assumes the int data type is 32 bits and the long data
 * type is 64 bits.  The native data type used in most calculations is
 * floating double.  The data types used in some packet header fields
 * require conversion to and from this representation.  Some header
 * fields involve partitioning an octet, here represented by individual
 * octets.
 * The 64-bit NTP timestamp format used in timestamp calculations is
 * unsigned seconds and fraction with the decimal point to the left of
 * bit 32.  The only operation permitted with these values is
 * subtraction, yielding a signed 31-bit difference.  The 32-bit NTP
 * short format used in delay and dispersion calculations is seconds and
 * fraction with the decimal point to the left of bit 16.  The only
 * operations permitted with these values are addition and
 * multiplication by a constant.
 * The IPv4 address is 32 bits, while the IPv6 address is 128 bits.  The
 * message digest field is 128 bits as constructed by the MD5 algorithm.
 * The precision and poll interval fields are signed log2 seconds.

typedef unsigned long long tstamp;   /* NTP timestamp format */

typedef unsigned int tdist;     /* NTP short format */

typedef unsigned long ipaddr;   /* IPv4 or IPv6 address */

typedef unsigned long digest;   /* md5 digest */

typedef signed char s_char;     /* precision and poll interval (log2) */

 * Timestamp conversion macroni

#define FRIC        65536.                  /* 2^16 as a double */

#define D2FP(r)     ((tdist)((r) * FRIC))   /* NTP short */

#define FP2D(r)     ((double)(r) / FRIC)

#define FRAC       4294967296.             /* 2^32 as a double */

#define D2LFP(a)   ((tstamp)((a) * FRAC))  /* NTP timestamp */

#define LFP2D(a)   ((double)(a) / FRAC)

#define U2LFP(a)   (((unsigned long long) \
                       ((a).tv_sec + JAN_1970) << 32) + \
                       (unsigned long long) \
                       ((a).tv_usec / 1e6 * FRAC))

 * Arithmetic conversions

#define LOG2D(a)        ((a) < 0 ? 1. / (1L << -(a)) : \
                            1L << (a))          /* poll, etc. */

#define SQUARE(x)       (x * x)

#define SQRT(x)         (sqrt(x))

 * Global constants.  Some of these might be converted to variables
 * that can be tinkered by configuration or computed on-the-fly.  For
 * instance, the reference implementation computes PRECISION on-the-fly
 * and provides performance tuning for the defines marked with % below.

#define VERSION         4       /* version number */

#define MINDISP         .01     /* % minimum dispersion (s) */

#define MAXDISP         16      /* maximum dispersion (s) */

#define MAXDIST         1       /* % distance threshold (s) */

#define NOSYNC          0x3     /* leap unsync */

#define MAXSTRAT        16      /* maximum stratum (infinity metric) */

#define MINPOLL         6       /* % minimum poll interval (64 s)*/

#define MAXPOLL         17      /* % maximum poll interval (36.4 h) */

#define MINCLOCK        3       /* minimum manycast survivors */

#define MAXCLOCK        10      /* maximum manycast candidates */

#define TTLMAX          8       /* max ttl manycast */

#define BEACON          15      /* max interval between beacons */

#define PHI             15e-6   /* % frequency tolerance (15 ppm) */

#define NSTAGE          8       /* clock register stages */

#define NMAX            50      /* maximum number of peers */

#define NSANE           1       /* % minimum intersection survivors */

#define NMIN            3       /* % minimum cluster survivors */

 * Global return values

#define TRUE            1       /* boolean true */

#define FALSE           0       /* boolean false */

 * Local clock process return codes

#define IGNORE          0       /* ignore */

#define SLEW            1       /* slew adjustment */

#define STEP            2       /* step adjustment */

#define PANIC           3       /* panic - no adjustment */

 * System flags

#define S_FLAGS         0       /* any system flags */

#define S_BCSTENAB      0x1     /* enable broadcast client */

 * Peer flags

#define P_FLAGS         0       /* any peer flags */

#define P_EPHEM         0x01    /* association is ephemeral */

#define P_BURST         0x02    /* burst enable */

#define P_IBURST        0x04    /* intial burst enable */

#define P_NOTRUST       0x08    /* authenticated access */

#define P_NOPEER        0x10    /* authenticated mobilization */

#define P_MANY          0x20    /* manycast client */

 * Authentication codes

#define A_NONE          0       /* no authentication */

#define A_OK            1       /* authentication OK */

#define A_ERROR         2       /* authentication error */

#define A_CRYPTO        3       /* crypto-NAK */

 * Association state codes

#define X_INIT          0       /* initialization */

#define X_STALE         1       /* timeout */

#define X_STEP          2       /* time step */

#define X_ERROR         3       /* authentication error */

#define X_CRYPTO        4       /* crypto-NAK received */

#define X_NKEY          5       /* untrusted key */

 * Protocol mode definitions

#define M_RSVD          0       /* reserved */

#define M_SACT          1       /* symmetric active */

#define M_PASV          2       /* symmetric passive */

#define M_CLNT          3       /* client */

#define M_SERV          4       /* server */

#define M_BCST          5       /* broadcast server */

#define M_BCLN          6       /* broadcast client */

 * Clock state definitions

#define NSET            0       /* clock never set */

#define FSET            1       /* frequency set from file */

#define SPIK            2       /* spike detected */

#define FREQ            3       /* frequency mode */

#define SYNC            4       /* clock synchronized */

#define min(a, b)   ((a) < (b) ? (a) : (b))

#define max(a, b)   ((a) < (b) ? (b) : (a))

A.1.2.  Packet Data Structures

 * The receive and transmit packets may contain an optional message
 * authentication code (MAC) consisting of a key identifier (keyid) and
 * message digest (mac in the receive structure and dgst in the transmit
 * structure).  NTPv4 supports optional extension fields that
 * are inserted after the header and before the MAC, but these are
 * not described here.
 * Receive packet
 * Note the dst timestamp is not part of the packet itself.  It is
 * captured upon arrival and returned in the receive buffer along with
 * the buffer length and data.  Note that some of the char fields are
 * packed in the actual header, but the details are omitted here.

struct r {
        ipaddr  srcaddr;        /* source (remote) address */
        ipaddr  dstaddr;        /* destination (local) address */
        char    version;        /* version number */
        char    leap;           /* leap indicator */
        char    mode;           /* mode */
        char    stratum;        /* stratum */
        char    poll;           /* poll interval */
        s_char  precision;      /* precision */
        tdist   rootdelay;      /* root delay */
        tdist   rootdisp;       /* root dispersion */
        char    refid;          /* reference ID */
        tstamp  reftime;        /* reference time */
        tstamp  org;            /* origin timestamp */
        tstamp  rec;            /* receive timestamp */
        tstamp  xmt;            /* transmit timestamp */
        int     keyid;          /* key ID */
        digest  mac;            /* message digest */
        tstamp  dst;            /* destination timestamp */

} r;

 * Transmit packet

struct x {
        ipaddr  dstaddr;        /* source (local) address */
        ipaddr  srcaddr;        /* destination (remote) address */
        char    version;        /* version number */
        char    leap;           /* leap indicator */
        char    mode;           /* mode */
        char    stratum;        /* stratum */
        char    poll;           /* poll interval */
        s_char  precision;      /* precision */
        tdist   rootdelay;      /* root delay */
        tdist   rootdisp;       /* root dispersion */
        char    refid;          /* reference ID */
        tstamp  reftime;        /* reference time */
        tstamp  org;            /* origin timestamp */
        tstamp  rec;            /* receive timestamp */
        tstamp  xmt;            /* transmit timestamp */
        int     keyid;          /* key ID */
        digest  dgst;           /* message digest */

} x;

A.1.3.  Association Data Structures

    * Filter stage structure.  Note the t member in this and other
    * structures refers to process time, not real time.  Process time
    * increments by one second for every elapsed second of real time.
   struct f {
           tstamp  t;              /* update time */
           double  offset;         /* clock ofset */
           double  delay;          /* roundtrip delay */
           double  disp;           /* dispersion */
   } f;

    * Association structure.  This is shared between the peer process
    * and poll process.
   struct p {

            * Variables set by configuration
           ipaddr  srcaddr;        /* source (remote) address */
           ipaddr  dstaddr;        /* destination (local) address */
           char    version;        /* version number */
           char    hmode;          /* host mode */
           int     keyid;          /* key identifier */
           int     flags;          /* option flags */

            * Variables set by received packet
           char    leap;           /* leap indicator */
           char    pmode;          /* peer mode */
           char    stratum;        /* stratum */
           char    ppoll;          /* peer poll interval */
           double  rootdelay;      /* root delay */
           double  rootdisp;       /* root dispersion */
           char    refid;          /* reference ID */
           tstamp  reftime;        /* reference time */
   #define begin_clear org         /* beginning of clear area */
           tstamp  org;            /* originate timestamp */
           tstamp  rec;            /* receive timestamp */
           tstamp  xmt;            /* transmit timestamp */

            * Computed data
           double  t;              /* update time */
           struct f f[NSTAGE];     /* clock filter */
           double  offset;         /* peer offset */
           double  delay;          /* peer delay */
           double  disp;           /* peer dispersion */
           double  jitter;         /* RMS jitter */

            * Poll process variables
           char    hpoll;          /* host poll interval */
           int     burst;          /* burst counter */
           int     reach;          /* reach register */
           int     ttl;            /* ttl (manycast) */
   #define end_clear unreach       /* end of clear area */
           int     unreach;        /* unreach counter */
           int     outdate;        /* last poll time */
           int     nextdate;       /* next poll time */
   } p;

A.1.4.  System Data Structures

    * Chime list.  This is used by the intersection algorithm.
   struct m {                      /* m is for Marzullo */
           struct p *p;            /* peer structure pointer */
           int     type;           /* high +1, mid 0, low -1 */
           double  edge;           /* correctness interval edge */
   } m;

    * Survivor list.  This is used by the clustering algorithm.
   struct v {
           struct p *p;            /* peer structure pointer */
           double  metric;         /* sort metric */
   } v;

    * System structure
   struct s {
           tstamp  t;              /* update time */
           char    leap;           /* leap indicator */
           char    stratum;        /* stratum */
           char    poll;           /* poll interval */
           char    precision;      /* precision */
           double  rootdelay;      /* root delay */
           double  rootdisp;       /* root dispersion */
           char    refid;          /* reference ID */
           tstamp  reftime;        /* reference time */
           struct m m[NMAX];       /* chime list */
           struct v v[NMAX];       /* survivor list */
           struct p *p;            /* association ID */
           double  offset;         /* combined offset */
           double  jitter;         /* combined jitter */
           int     flags;          /* option flags */
           int     n;              /* number of survivors */
   } s;

A.1.5.  Local Clock Data Structures

    * Local clock structure
   struct c {
           tstamp  t;              /* update time */
           int     state;          /* current state */
           double  offset;         /* current offset */
           double  last;           /* previous offset */
           int     count;          /* jiggle counter */
           double  freq;           /* frequency */
           double  jitter;         /* RMS jitter */
           double  wander;         /* RMS wander */
   } c;

A.1.6.  Function Prototypes

   * Peer process
  void    receive(struct r *);    /* receive packet */
  void    packet(struct p *, struct r *); /* process packet */
  void    clock_filter(struct p *, double, double, double); /* filter */
  double  root_dist(struct p *);  /* calculate root distance */
  int     fit(struct p *);        /* determine fitness of server */
  void    clear(struct p *, int); /* clear association */
  int     access(struct r *);     /* determine access restrictions */

   * System process
  int     main();                 /* main program */
  void    clock_select();         /* find the best clocks */
  void    clock_update(struct p *); /* update the system clock */
  void    clock_combine();        /* combine the offsets */

   * Local clock process
  int     local_clock(struct p *, double); /* clock discipline */
  void    rstclock(int, double, double); /* clock state transition */

   * Clock adjust process
  void    clock_adjust();         /* one-second timer process */
   * Poll process
  void    poll(struct p *);               /* poll process */
  void    poll_update(struct p *, int); /* update the poll interval */
  void    peer_xmit(struct p *);  /* transmit a packet */
  void    fast_xmit(struct r *, int, int); /* transmit a reply packet */

   * Utility routines
  digest  md5(int);               /* generate a message digest */
  struct p *mobilize(ipaddr, ipaddr, int, int, int, int); /* mobilize */
  struct p *find_assoc(struct r *); /* search the association table */

   * Kernel interface
  struct r *recv_packet();        /* wait for packet */
  void    xmit_packet(struct x *); /* send packet */
  void    step_time(double);      /* step time */
  void    adjust_time(double);    /* adjust (slew) time */
  tstamp  get_time();             /* read time */

A.2.  Main Program and Utility Routines

 * Definitions

#define PRECISION       -18     /* precision (log2 s)  */

#define IPADDR          0       /* any IP address */

#define MODE            0       /* any NTP mode */

#define KEYID           0       /* any key identifier */

 * main() - main program


        struct p *p;            /* peer structure pointer */
        struct r *r;            /* receive packet pointer */
         * Read command line options and initialize system variables.
         * The reference implementation measures the precision specific
         * to each machine by measuring the clock increments to read the
         * system clock.
        memset(&s, sizeof(s), 0);
        s.leap = NOSYNC;
        s.stratum = MAXSTRAT;
        s.poll = MINPOLL;
        s.precision = PRECISION;
        s.p = NULL;

         * Initialize local clock variables
        memset(&c, sizeof(c), 0);
        if (/* frequency file */ 0) {
                c.freq = /* freq */ 0;
                rstclock(FSET, 0, 0);
        } else {
                rstclock(NSET, 0, 0);
        c.jitter = LOG2D(s.precision);

         * Read the configuration file and mobilize persistent
         * associations with specified addresses, version, mode, key ID,
         * and flags.
        while (/* mobilize configurated associations */ 0) {
                p = mobilize(IPADDR, IPADDR, VERSION, MODE, KEYID,

         * Start the system timer, which ticks once per second.  Then,
         * read packets as they arrive, strike receive timestamp, and
         * call the receive() routine.
        while (0) {
                r = recv_packet();
                r->dst = get_time();


 * mobilize() - mobilize and initialize an association

struct p

        ipaddr  srcaddr,        /* IP source address */
        ipaddr  dstaddr,        /* IP destination address */
        int     version,        /* version */
        int     mode,           /* host mode */
        int     keyid,          /* key identifier */
        int     flags           /* peer flags */
        struct p *p;            /* peer process pointer */

         * Allocate and initialize association memory
        p = malloc(sizeof(struct p));
        p->srcaddr = srcaddr;
        p->dstaddr = dstaddr;
        p->version = version;
        p->hmode = mode;
        p->keyid = keyid;
        p->hpoll = MINPOLL;
        clear(p, X_INIT);
        p->flags = flags;
        return (p);

 * find_assoc() - find a matching association

struct p                        /* peer structure pointer or NULL */

        struct r *r             /* receive packet pointer */
        struct p *p;            /* dummy peer structure pointer */

         * Search association table for matching source
         * address, source port and mode.
        while (/* all associations */ 0) {
                if (r->srcaddr == p->srcaddr && r->mode == p->hmode)
        return (NULL);

 * md5() - compute message digest


       int     keyid           /* key identifier */
        * Compute a keyed cryptographic message digest.  The key
        * identifier is associated with a key in the local key cache.
        * The key is prepended to the packet header and extension fields
        * and the result hashed by the MD5 algorithm as described in
        * RFC 1321.  Return a MAC consisting of the 32-bit key ID
        * concatenated with the 128-bit digest.
       return (/* MD5 digest */ 0);

A.3.  Kernel Input/Output Interface

    * Kernel interface to transmit and receive packets.  Details are
    * deliberately vague and depend on the operating system.
    * recv_packet - receive packet from network
   struct r                        /* receive packet pointer*/
   *recv_packet() {
           return (/* receive packet r */ 0);

    * xmit_packet - transmit packet to network
           struct x *x             /* transmit packet pointer */
           /* send packet x */

A.4.  Kernel System Clock Interface

 * System clock utility functions
 * There are three time formats: native (Unix), NTP, and floating
 * double.  The get_time() routine returns the time in NTP long format.
 * The Unix routines expect arguments as a structure of two signed
 * 32-bit words in seconds and microseconds (timeval) or nanoseconds
 * (timespec).  The step_time() and adjust_time() routines expect signed
 * arguments in floating double.  The simplified code shown here is for
 * illustration only and has not been verified.

#define JAN_1970        2208988800UL /* 1970 - 1900 in seconds */

 * get_time - read system time and convert to NTP format


        struct timeval unix_time;

         * There are only two calls on this routine in the program.  One
         * when a packet arrives from the network and the other when a
         * packet is placed on the send queue.  Call the kernel time of
         * day routine (such as gettimeofday()) and convert to NTP
         * format.
        gettimeofday(&unix_time, NULL);
        return (U2LFP(unix_time));

 * step_time() - step system time to given offset value


        double  offset          /* clock offset */
        struct timeval unix_time;
        tstamp  ntp_time;

         * Convert from double to native format (signed) and add to the
         * current time.  Note the addition is done in native format to
         * avoid overflow or loss of precision.
        gettimeofday(&unix_time, NULL);
        ntp_time = D2LFP(offset) + U2LFP(unix_time);
        unix_time.tv_sec = ntp_time >> 32;
        unix_time.tv_usec = (long)(((ntp_time - unix_time.tv_sec) <<
            32) / FRAC * 1e6);
        settimeofday(&unix_time, NULL);

 * adjust_time() - slew system clock to given offset value


        double  offset          /* clock offset */
        struct timeval unix_time;
        tstamp  ntp_time;

         * Convert from double to native format (signed) and add to the
         * current time.
        ntp_time = D2LFP(offset);
        unix_time.tv_sec = ntp_time >> 32;
        unix_time.tv_usec = (long)(((ntp_time - unix_time.tv_sec) <<
            32) / FRAC * 1e6);
        adjtime(&unix_time, NULL);

A.5.  Peer Process

    * A crypto-NAK packet includes the NTP header followed by a MAC
    * consisting only of the key identifier with value zero.  It tells
    * the receiver that a prior request could not be properly
    * authenticated, but the NTP header fields are correct.
    * A kiss-o'-death packet is an NTP header with leap 0x3 (NOSYNC) and
    * stratum 16 (MAXSTRAT).  It tells the receiver that something
    * drastic has happened, as revealed by the kiss code in the refid
    * field.  The NTP header fields may or may not be correct.
    * Peer process parameters and constants
   #define SGATE           3       /* spike gate (clock filter */
   #define BDELAY          .004    /* broadcast delay (s) */

    * Dispatch codes
   #define ERR             -1      /* error */
   #define DSCRD           0       /* discard packet */
   #define PROC            1       /* process packet */
   #define BCST            2       /* broadcast packet */
   #define FXMIT           3       /* client packet */
   #define MANY            4       /* manycast packet */
   #define NEWPS           5       /* new symmetric passive client */
   #define NEWBC           6       /* new broadcast client */

    * Dispatch matrix
    *              active  passv  client server bcast */
   int table[7][5] = {
   /* nopeer  */   { NEWPS, DSCRD, FXMIT, MANY, NEWBC },
   /* active  */   { PROC,  PROC,  DSCRD, DSCRD, DSCRD },
   /* passv   */   { PROC,  ERR,   DSCRD, DSCRD, DSCRD },
   /* client  */   { DSCRD, DSCRD, DSCRD, PROC,  DSCRD },
   /* server  */   { DSCRD, DSCRD, DSCRD, DSCRD, DSCRD },
   /* bcast   */   { DSCRD, DSCRD, DSCRD, DSCRD, DSCRD },
   /* bclient */   { DSCRD, DSCRD, DSCRD, DSCRD, PROC}
    * Miscellaneous macroni
    * This macro defines the authentication state.  If x is 0,
    * authentication is optional; otherwise, it is required.
   #define AUTH(x, y)      ((x) ? (y) == A_OK : (y) == A_OK || \
                               (y) == A_NONE)

    * These are used by the clear() routine
   #define BEGIN_CLEAR(p)  ((char *)&((p)->begin_clear))
   #define END_CLEAR(p)    ((char *)&((p)->end_clear))
   #define LEN_CLEAR       (END_CLEAR((struct p *)0) - \
                               BEGIN_CLEAR((struct p *)0))

A.5.1.  receive()

 * receive() - receive packet and decode modes


        struct r *r             /* receive packet pointer */
        struct p *p;            /* peer structure pointer */
        int     auth;           /* authentication code */
        int     has_mac;        /* size of MAC */
        int     synch;          /* synchronized switch */

         * Check access control lists.  The intent here is to implement
         * a whitelist of those IP addresses specifically accepted
         * and/or a blacklist of those IP addresses specifically
         * rejected.  There could be different lists for authenticated
         * clients and unauthenticated clients.
        if (!access(r))
                return;                 /* access denied */

         * The version must not be in the future.  Format checks include
         * packet length, MAC length and extension field lengths, if
         * present.
        if (r->version > VERSION /* or format error */)
                return;                 /* format error */

         * Authentication is conditioned by two switches that can be
         * specified on a per-client basis.
         * P_NOPEER     do not mobilize an association unless
         *              authenticated.
         * P_NOTRUST    do not allow access unless authenticated
         *              (implies P_NOPEER).
         * There are four outcomes:
         * A_NONE       the packet has no MAC.
         * A_OK         the packet has a MAC and authentication
         *               succeeds.
         * A_ERROR      the packet has a MAC and authentication fails.
         * A_CRYPTO     crypto-NAK.  The MAC has four octets only.
         * Note: The AUTH (x, y) macro is used to filter outcomes.  If x
         * is zero, acceptable outcomes of y are NONE and OK.  If x is
         * one, the only acceptable outcome of y is OK.

        has_mac = /* length of MAC field */ 0;
        if (has_mac == 0) {
                auth = A_NONE;          /* not required */
        } else if (has_mac == 4) {
                auth = A_CRYPTO;       /* crypto-NAK */
        } else {
                if (r->mac != md5(r->keyid))
                        auth = A_ERROR; /* auth error */
                        auth = A_OK;    /* auth OK */

         * Find association and dispatch code.  If there is no
         * association to match, the value of p->hmode is assumed NULL.
        p = find_assoc(r);
        switch(table[(unsigned int)(p->hmode)][(unsigned int)(r->mode)])
         * Client packet and no association.  Send server reply without
         * saving state.
        case FXMIT:

                 * If unicast destination address, send server packet.
                 * If authentication fails, send a crypto-NAK packet.

                /* not multicast dstaddr */
                if (0) {
                        if (AUTH(p->flags & P_NOTRUST, auth))
                                fast_xmit(r, M_SERV, auth);
                        else if (auth == A_ERROR)
                                fast_xmit(r, M_SERV, A_CRYPTO);
                        return;         /* M_SERV packet sent */

                 * This must be manycast.  Do not respond if we are not
                 * synchronized or if our stratum is above the
                 * manycaster.
                if (s.leap == NOSYNC || s.stratum > r->stratum)

                 * Respond only if authentication is OK.  Note that the
                 * unicast address is used, not the multicast.
                if (AUTH(p->flags & P_NOTRUST, auth))
                        fast_xmit(r, M_SERV, auth);

         * New manycast client ephemeral association.  It is mobilized
         * in the same version as in the packet.  If authentication
         * fails, ignore the packet.  Verify the server packet by
         * comparing the r->org timestamp in the packet with the p->xmt
         * timestamp in the multicast client association.  If they
         * match, the server packet is authentic.  Details omitted.
        case MANY:
                if (!AUTH(p->flags & (P_NOTRUST | P_NOPEER), auth))
                        return;         /* authentication error */

                p = mobilize(r->srcaddr, r->dstaddr, r->version, M_CLNT,
                    r->keyid, P_EPHEM);

        * New symmetric passive association.  It is mobilized in the
        * same version as in the packet.  If authentication fails,
        * send a crypto-NAK packet.  If restrict no-moblize, send a
        * symmetric active packet instead.
        case NEWPS:
                if (!AUTH(p->flags & P_NOTRUST, auth)) {
                        if (auth == A_ERROR)
                                fast_xmit(r, M_SACT, A_CRYPTO);
                        return;         /* crypto-NAK packet sent */
                if (!AUTH(p->flags & P_NOPEER, auth)) {
                        fast_xmit(r, M_SACT, auth);
                        return;         /* M_SACT packet sent */
                p = mobilize(r->srcaddr, r->dstaddr, r->version, M_PASV,
                    r->keyid, P_EPHEM);

         * New broadcast client association.  It is mobilized in the
         * same version as in the packet.  If authentication fails,
         * ignore the packet.  Note this code does not support the
         * initial volley feature in the reference implementation.
        case NEWBC:
                if (!AUTH(p->flags & (P_NOTRUST | P_NOPEER), auth))
                        return;         /* authentication error */

                if (!(s.flags & S_BCSTENAB))
                        return;         /* broadcast not enabled */

                p = mobilize(r->srcaddr, r->dstaddr, r->version, M_BCLN,
                    r->keyid, P_EPHEM);
                break;                  /* processing continues */
         * Process packet.  Placeholdler only.
        case PROC:
                break;                  /* processing continues */

         * Invalid mode combination.  We get here only in case of
         * ephemeral associations, so the correct action is simply to
         * toss it.
        case ERR:
                clear(p, X_ERROR);
                return;                 /* invalid mode combination */

         * No match; just discard the packet.
        case DSCRD:
                return;                 /* orphan abandoned */

         * Next comes a rigorous schedule of timestamp checking.  If the
         * transmit timestamp is zero, the server is horribly broken.
        if (r->xmt == 0)
                return;                 /* invalid timestamp */

         * If the transmit timestamp duplicates a previous one, the
         * packet is a replay.
        if (r->xmt == p->xmt)
                return;                 /* duplicate packet */

         * If this is a broadcast mode packet, skip further checking.
         * If the origin timestamp is zero, the sender has not yet heard
         * from us.  Otherwise, if the origin timestamp does not match
         * the transmit timestamp, the packet is bogus.
        synch = TRUE;
        if (r->mode != M_BCST) {
                if (r->org == 0)
                        synch = FALSE;  /* unsynchronized */

                else if (r->org != p->xmt)
                        synch = FALSE;  /* bogus packet */

         * Update the origin and destination timestamps.  If
         * unsynchronized or bogus, abandon ship.
        p->org = r->xmt;
        p->rec = r->dst;
        if (!synch)
                return;                 /* unsynch */

         * The timestamps are valid and the receive packet matches the
         * last one sent.  If the packet is a crypto-NAK, the server
         * might have just changed keys.  We demobilize the association
         * and wait for better times.
        if (auth == A_CRYPTO) {
                clear(p, X_CRYPTO);
                return;                 /* crypto-NAK */

         * If the association is authenticated, the key ID is nonzero
         * and received packets must be authenticated.  This is designed
         * to avoid a bait-and-switch attack, which was possible in past
         * versions.
        if (!AUTH(p->keyid || (p->flags & P_NOTRUST), auth))
                return;                 /* bad auth */
         * Everything possible has been done to validate the timestamps
         * and prevent bad guys from disrupting the protocol or
         * injecting bogus data.  Earn some revenue.
        packet(p, r);

A.5.1.1.  packet()

 * packet() - process packet and compute offset, delay, and
 * dispersion.


        struct p *p,            /* peer structure pointer */
        struct r *r             /* receive packet pointer */
        double  offset;         /* sample offsset */
        double  delay;          /* sample delay */
        double  disp;           /* sample dispersion */

         * By golly the packet is valid.  Light up the remaining header
         * fields.  Note that we map stratum 0 (unspecified) to MAXSTRAT
         * to make stratum comparisons simpler and to provide a natural
         * interface for radio clock drivers that operate for
         *  convenience at stratum 0.
        p->leap = r->leap;
        if (r->stratum == 0)
                p->stratum = MAXSTRAT;
                p->stratum = r->stratum;
        p->pmode = r->mode;
        p->ppoll = r->poll;
        p->rootdelay = FP2D(r->rootdelay);
        p->rootdisp = FP2D(r->rootdisp);
        p->refid = r->refid;
        p->reftime = r->reftime;

         * Verify the server is synchronized with valid stratum and
         * reference time not later than the transmit time.
        if (p->leap == NOSYNC || p->stratum >= MAXSTRAT)
                return;                 /* unsynchronized */

         * Verify valid root distance.
        if (r->rootdelay / 2 + r->rootdisp >= MAXDISP || p->reftime >
                return;                 /* invalid header values */

        poll_update(p, p->hpoll);
        p->reach |= 1;

         * Calculate offset, delay and dispersion, then pass to the
         * clock filter.  Note carefully the implied processing.  The
         * first-order difference is done directly in 64-bit arithmetic,
         * then the result is converted to floating double.  All further
         * processing is in floating-double arithmetic with rounding
         * done by the hardware.  This is necessary in order to avoid
         * overflow and preserve precision.
         * The delay calculation is a special case.  In cases where the
         * server and client clocks are running at different rates and
         * with very fast networks, the delay can appear negative.  In
         * order to avoid violating the Principle of Least Astonishment,
         * the delay is clamped not less than the system precision.
        if (p->pmode == M_BCST) {
                offset = LFP2D(r->xmt - r->dst);
                delay = BDELAY;
                disp = LOG2D(r->precision) + LOG2D(s.precision) + PHI *
                    2 * BDELAY;
        } else {
                offset = (LFP2D(r->rec - r->org) + LFP2D(r->dst -
                    r->xmt)) / 2;
                delay = max(LFP2D(r->dst - r->org) - LFP2D(r->rec -
                    r->xmt), LOG2D(s.precision));
                disp = LOG2D(r->precision) + LOG2D(s.precision) + PHI *
                    LFP2D(r->dst - r->org);
        clock_filter(p, offset, delay, disp);

A.5.2.  clock_filter()

 * clock_filter(p, offset, delay, dispersion) - select the best from the
 * latest eight delay/offset samples.


        struct p *p,            /* peer structure pointer */
        double  offset,         /* clock offset */
        double  delay,          /* roundtrip delay */
        double  disp            /* dispersion */
        struct f f[NSTAGE];     /* sorted list */
        double  dtemp;
        int     i;

         * The clock filter contents consist of eight tuples (offset,
         * delay, dispersion, time).  Shift each tuple to the left,
         * discarding the leftmost one.  As each tuple is shifted,
         * increase the dispersion since the last filter update.  At the
         * same time, copy each tuple to a temporary list.  After this,
         * place the (offset, delay, disp, time) in the vacated
         * rightmost tuple.
        for (i = 1; i < NSTAGE; i++) {
                p->f[i] = p->f[i - 1];
                p->f[i].disp += PHI * (c.t - p->t);
                f[i] = p->f[i];
        p->f[0].t = c.t;
        p->f[0].offset = offset;
        p->f[0].delay = delay;
        p->f[0].disp = disp;
        f[0] = p->f[0];

         * Sort the temporary list of tuples by increasing f[].delay.
         * The first entry on the sorted list represents the best
         * sample, but it might be old.
        dtemp = p->offset;
        p->offset = f[0].offset;
        p->delay = f[0].delay;
        for (i = 0; i < NSTAGE; i++) {
                p->disp += f[i].disp / (2 ^ (i + 1));
                p->jitter += SQUARE(f[i].offset - f[0].offset);
        p->jitter = max(SQRT(p->jitter), LOG2D(s.precision));

         * Prime directive: use a sample only once and never a sample
         * older than the latest one, but anything goes before first
         * synchronized.
        if (f[0].t - p->t <= 0 && s.leap != NOSYNC)

         * Popcorn spike suppressor.  Compare the difference between the
         * last and current offsets to the current jitter.  If greater
         * than SGATE (3) and if the interval since the last offset is
         * less than twice the system poll interval, dump the spike.
         * Otherwise, and if not in a burst, shake out the truechimers.
        if (fabs(p->offset - dtemp) > SGATE * p->jitter && (f[0].t -
            p->t) < 2 * s.poll)

        p->t = f[0].t;
        if (p->burst == 0)

 * fit() - test if association p is acceptable for synchronization


        struct p *p             /* peer structure pointer */
         * A stratum error occurs if (1) the server has never been
         * synchronized, (2) the server stratum is invalid.
        if (p->leap == NOSYNC || p->stratum >= MAXSTRAT)
                return (FALSE);
         * A distance error occurs if the root distance exceeds the
         * distance threshold plus an increment equal to one poll
         * interval.
        if (root_dist(p) > MAXDIST + PHI * LOG2D(s.poll))
                return (FALSE);

         * A loop error occurs if the remote peer is synchronized to the
         * local peer or the remote peer is synchronized to the current
         * system peer.  Note this is the behavior for IPv4; for IPv6
         * the MD5 hash is used instead.
        if (p->refid == p->dstaddr || p->refid == s.refid)
                return (FALSE);

         * An unreachable error occurs if the server is unreachable.
        if (p->reach == 0)
                return (FALSE);

        return (TRUE);

 * clear() - reinitialize for persistent association, demobilize
 * for ephemeral association.


        struct p *p,            /* peer structure pointer */
        int     kiss            /* kiss code */
        int i;

         * The first thing to do is return all resources to the bank.
         * Typical resources are not detailed here, but they include
         * dynamically allocated structures for keys, certificates, etc.
         * If an ephemeral association and not initialization, return
         * the association memory as well.
        /* return resources */
        if (s.p == p)
                s.p = NULL;
        if (kiss != X_INIT && (p->flags & P_EPHEM)) {

         * Initialize the association fields for general reset.
        memset(BEGIN_CLEAR(p), LEN_CLEAR, 0);
        p->leap = NOSYNC;
        p->stratum = MAXSTRAT;
        p->ppoll = MAXPOLL;
        p->hpoll = MINPOLL;
        p->disp = MAXDISP;
        p->jitter = LOG2D(s.precision);
        p->refid = kiss;
        for (i = 0; i < NSTAGE; i++)
                p->f[i].disp = MAXDISP;

         * Randomize the first poll just in case thousands of broadcast
         * clients have just been stirred up after a long absence of the
         * broadcast server.
        p->outdate = p->t = c.t;
        p->nextdate = p->outdate + (random() & ((1 << MINPOLL) - 1));

A.5.3.  fast_xmit()

 * fast_xmit() - transmit a reply packet for receive packet r


        struct r *r,            /* receive packet pointer */
        int     mode,           /* association mode */
        int     auth            /* authentication code */
        struct x x;

         * Initialize header and transmit timestamp.  Note that the
         * transmit version is copied from the receive version.  This is
         * for backward compatibility.
        x.version = r->version;
        x.srcaddr = r->dstaddr;
        x.dstaddr = r->srcaddr;
        x.leap = s.leap;
        x.mode = mode;
        if (s.stratum == MAXSTRAT)
                x.stratum = 0;
                x.stratum = s.stratum;
        x.poll = r->poll;
        x.precision = s.precision;
        x.rootdelay = D2FP(s.rootdelay);
        x.rootdisp = D2FP(s.rootdisp);
        x.refid = s.refid;
        x.reftime = s.reftime;
        x.org = r->xmt;
        x.rec = r->dst;
        x.xmt = get_time();

         * If the authentication code is A.NONE, include only the
         * header; if A.CRYPTO, send a crypto-NAK; if A.OK, send a valid
         * MAC.  Use the key ID in the received packet and the key in
         * the local key cache.
        if (auth != A_NONE) {
                if (auth == A_CRYPTO) {
                        x.keyid = 0;
                } else {
                        x.keyid = r->keyid;
                        x.dgst = md5(x.keyid);

A.5.4.  access()

  * access() - determine access restrictions
         struct r *r             /* receive packet pointer */
          * The access control list is an ordered set of tuples
          * consisting of an address, mask, and restrict word containing
          * defined bits.  The list is searched for the first match on
          * the source address (r->srcaddr) and the associated restrict
          * word is returned.
         return (/* access bits */ 0);

A.5.5.  System Process

A.5.5.1.  clock_select()

 * clock_select() - find the best clocks


clock_select() {
       struct p *p, *osys;     /* peer structure pointers */
       double  low, high;      /* correctness interval extents */
       int     allow, found, chime; /* used by intersection algorithm */
       int     n, i, j;

         * We first cull the falsetickers from the server population,
         * leaving only the truechimers.  The correctness interval for
         * association p is the interval from offset - root_dist() to
         * offset + root_dist().  The object of the game is to find a
         * majority clique; that is, an intersection of correctness
         * intervals numbering more than half the server population.
         * First, construct the chime list of tuples (p, type, edge) as
         * shown below, then sort the list by edge from lowest to
         * highest.
        osys = s.p;
        s.p = NULL;
        n = 0;
        while (fit(p)) {
                s.m[n].p = p;
                s.m[n].type = +1;
                s.m[n].edge = p->offset + root_dist(p);
                s.m[n].p = p;
                s.m[n].type = 0;
                s.m[n].edge = p->offset;
                s.m[n].p = p;
                s.m[n].type = -1;
                s.m[n].edge = p->offset - root_dist(p);

         * Find the largest contiguous intersection of correctness
         * intervals.  Allow is the number of allowed falsetickers;
         * found is the number of midpoints.  Note that the edge values
         * are limited to the range +-(2 ^ 30) < +-2e9 by the timestamp
         * calculations.
        low = 2e9; high = -2e9;
        for (allow = 0; 2 * allow < n; allow++) {

                 * Scan the chime list from lowest to highest to find
                 * the lower endpoint.
                found = 0;
                chime = 0;
                for (i = 0; i < n; i++) {
                        chime -= s.m[i].type;
                        if (chime >= n - found) {
                                low = s.m[i].edge;
                        if (s.m[i].type == 0)

                 * Scan the chime list from highest to lowest to find
                 * the upper endpoint.
                chime = 0;
                for (i = n - 1; i >= 0; i--) {
                        chime += s.m[i].type;
                        if (chime >= n - found) {
                                high = s.m[i].edge;
                        if (s.m[i].type == 0)
                 * If the number of midpoints is greater than the number
                 * of allowed falsetickers, the intersection contains at
                 * least one truechimer with no midpoint.  If so,
                 * increment the number of allowed falsetickers and go
                 * around again.  If not and the intersection is
                 * non-empty, declare success.
                if (found > allow)

                if (high > low)

         * Clustering algorithm.  Construct a list of survivors (p,
         * metric) from the chime list, where metric is dominated first
         * by stratum and then by root distance.  All other things being
         * equal, this is the order of preference.
        s.n = 0;
        for (i = 0; i < n; i++) {
                if (s.m[i].edge < low || s.m[i].edge > high)

                p = s.m[i].p;
                s.v[n].p = p;
                s.v[n].metric = MAXDIST * p->stratum + root_dist(p);

         * There must be at least NSANE survivors to satisfy the
         * correctness assertions.  Ordinarily, the Byzantine criteria
         * require four survivors, but for the demonstration here, one
         * is acceptable.
        if (s.n < NSANE)

         * For each association p in turn, calculate the selection
         * jitter p->sjitter as the square root of the sum of squares
         * (p->offset - q->offset) over all q associations.  The idea is
         * to repeatedly discard the survivor with maximum selection
         * jitter until a termination condition is met.
        while (1) {
                struct p *p, *q, *qmax; /* peer structure pointers */
                double  max, min, dtemp;

                max = -2e9; min = 2e9;
                for (i = 0; i < s.n; i++) {
                        p = s.v[i].p;
                        if (p->jitter < min)
                                min = p->jitter;
                        dtemp = 0;
                        for (j = 0; j < n; j++) {
                                q = s.v[j].p;
                                dtemp += SQUARE(p->offset - q->offset);
                        dtemp = SQRT(dtemp);
                        if (dtemp > max) {
                                max = dtemp;
                                qmax = q;

                 * If the maximum selection jitter is less than the
                 * minimum peer jitter, then tossing out more survivors
                 * will not lower the minimum peer jitter, so we might
                 * as well stop.  To make sure a few survivors are left
                 * for the clustering algorithm to chew on, we also stop
                 * if the number of survivors is less than or equal to
                 * NMIN (3).
                if (max < min || n <= NMIN)

                 * Delete survivor qmax from the list and go around
                 * again.

         * Pick the best clock.  If the old system peer is on the list
         * and at the same stratum as the first survivor on the list,
         * then don't do a clock hop.  Otherwise, select the first
         * survivor on the list as the new system peer.
        if (osys->stratum == s.v[0].p->stratum)
                s.p = osys;
                s.p = s.v[0].p;

A.5.5.2.  root_dist()

 * root_dist() - calculate root distance


        struct p *p             /* peer structure pointer */

         * The root synchronization distance is the maximum error due to
         * all causes of the local clock relative to the primary server.
         * It is defined as half the total delay plus total dispersion
         * plus peer jitter.
        return (max(MINDISP, p->rootdelay + p->delay) / 2 +
            p->rootdisp + p->disp + PHI * (c.t - p->t) + p->jitter);

A.5.5.3.  accept()

 * accept() - test if association p is acceptable for synchronization


        struct p *p             /* peer structure pointer */
         * A stratum error occurs if (1) the server has never been
         * synchronized, (2) the server stratum is invalid.
        if (p->leap == NOSYNC || p->stratum >= MAXSTRAT)
                return (FALSE);

         * A distance error occurs if the root distance exceeds the
         * distance threshold plus an increment equal to one poll
         * interval.
        if (root_dist(p) > MAXDIST + PHI * LOG2D(s.poll))
                return (FALSE);

         * A loop error occurs if the remote peer is synchronized to the
         * local peer or the remote peer is synchronized to the current
         * system peer.  Note this is the behavior for IPv4; for IPv6
         * the MD5 hash is used instead.
        if (p->refid == p->dstaddr || p->refid == s.refid)
                return (FALSE);

         * An unreachable error occurs if the server is unreachable.
        if (p->reach == 0)
                return (FALSE);

        return (TRUE);

A.5.5.4.  clock_update()

 * clock_update() - update the system clock


        struct p *p             /* peer structure pointer */
        double dtemp;

         * If this is an old update, for instance, as the result of a
         * system peer change, avoid it.  We never use an old sample or
         * the same sample twice.
        if (s.t >= p->t)

         * Combine the survivor offsets and update the system clock; the
         * local_clock() routine will tell us the good or bad news.
        s.t = p->t;
        switch (local_clock(p, s.offset)) {
         * The offset is too large and probably bogus.  Complain to the
         * system log and order the operator to set the clock manually
         * within PANIC range.  The reference implementation includes a
         * command line option to disable this check and to change the
         * panic threshold from the default 1000 s as required.
        case PANIC:
                exit (0);

         * The offset is more than the step threshold (0.125 s by
         * default).  After a step, all associations now have
         * inconsistent time values, so they are reset and started
         * fresh.  The step threshold can be changed in the reference
         * implementation in order to lessen the chance the clock might
         * be stepped backwards.  However, there may be serious
         * consequences, as noted in the white papers at the NTP project
         * site.
        case STEP:
                while (/* all associations */ 0)
                        clear(p, X_STEP);
                s.stratum = MAXSTRAT;
                s.poll = MINPOLL;

         * The offset was less than the step threshold, which is the
         * normal case.  Update the system variables from the peer
         * variables.  The lower clamp on the dispersion increase is to
         * avoid timing loops and clockhopping when highly precise
         * sources are in play.  The clamp can be changed from the
         * default .01 s in the reference implementation.
        case SLEW:
                s.leap = p->leap;
                s.stratum = p->stratum + 1;
                s.refid = p->refid;
                s.reftime = p->reftime;
                s.rootdelay = p->rootdelay + p->delay;
                dtemp = SQRT(SQUARE(p->jitter) + SQUARE(s.jitter));
                dtemp += max(p->disp + PHI * (c.t - p->t) +
                    fabs(p->offset), MINDISP);
                s.rootdisp = p->rootdisp + dtemp;
         * Some samples are discarded while, for instance, a direct
         * frequency measurement is being made.
        case IGNORE:

A.5.5.5.  clock_combine()

 * clock_combine() - combine offsets


        struct p *p;            /* peer structure pointer */
        double x, y, z, w;
        int     i;

         * Combine the offsets of the clustering algorithm survivors
         * using a weighted average with weight determined by the root
         * distance.  Compute the selection jitter as the weighted RMS
         * difference between the first survivor and the remaining
         * survivors.  In some cases, the inherent clock jitter can be
         * reduced by not using this algorithm, especially when frequent
         * clockhopping is involved.  The reference implementation can
         * be configured to avoid this algorithm by designating a
         * preferred peer.
        y = z = w = 0;
        for (i = 0; s.v[i].p != NULL; i++) {
                p = s.v[i].p;
                x = root_dist(p);
                y += 1 / x;
                z += p->offset / x;
                w += SQUARE(p->offset - s.v[0].p->offset) / x;
        s.offset = z / y;
        s.jitter = SQRT(w / y);

A.5.5.6.  local_clock()

 * Clock discipline parameters and constants

#define STEPT           .128    /* step threshold (s) */

#define WATCH           900     /* stepout threshold (s) */

#define PANICT          1000    /* panic threshold (s) */

#define PLL             65536   /* PLL loop gain */

#define FLL             MAXPOLL + 1 /* FLL loop gain */

#define AVG             4       /* parameter averaging constant */

#define ALLAN           1500    /* compromise Allan intercept (s) */

#define LIMIT           30      /* poll-adjust threshold */

#define MAXFREQ         500e-6  /* frequency tolerance (500 ppm) */

#define PGATE           4       /* poll-adjust gate */

 * local_clock() - discipline the local clock

int                             /* return code */

        struct p *p,            /* peer structure pointer */
        double  offset          /* clock offset from combine() */
        int     state;          /* clock discipline state */
        double  freq;           /* frequency */
        double  mu;             /* interval since last update */
        int     rval;
        double  etemp, dtemp;

         * If the offset is too large, give up and go home.
        if (fabs(offset) > PANICT)
                return (PANIC);

         * Clock state machine transition function.  This is where the
         * action is and defines how the system reacts to large time
         * and frequency errors.  There are two main regimes: when the
         * offset exceeds the step threshold and when it does not.
        rval = SLEW;
        mu = p->t - s.t;
        freq = 0;
        if (fabs(offset) > STEPT) {
                switch (c.state) {
                 * In S_SYNC state, we ignore the first outlier and
                 * switch to S_SPIK state.
                case SYNC:
                        state = SPIK;
                        return (rval);

                 * In S_FREQ state, we ignore outliers and inliers.  At
                 * the first outlier after the stepout threshold,
                 * compute the apparent frequency correction and step
                 * the time.
                case FREQ:
                        if (mu < WATCH)
                                return (IGNORE);

                        freq = (offset - c.offset) / mu;
                        /* fall through to S_SPIK */

                 * In S_SPIK state, we ignore succeeding outliers until
                 * either an inlier is found or the stepout threshold is
                 * exceeded.
                case SPIK:
                        if (mu < WATCH)
                                return (IGNORE);

                        /* fall through to default */

                 * We get here by default in S_NSET and S_FSET states
                 * and from above in S_FREQ state.  Step the time and
                 * clamp down the poll interval.
                 * In S_NSET state, an initial frequency correction is
                 * not available, usually because the frequency file has
                 * not yet been written.  Since the time is outside the
                 * capture range, the clock is stepped.  The frequency
                 * will be set directly following the stepout interval.
                 * In S_FSET state, the initial frequency has been set
                 * from the frequency file.  Since the time is outside
                 * the capture range, the clock is stepped immediately,
                 * rather than after the stepout interval.  Guys get
                 * nervous if it takes 17 minutes to set the clock for
                 * the first time.
                 * In S_SPIK state, the stepout threshold has expired
                 * and the phase is still above the step threshold.
                 * Note that a single spike greater than the step
                 * threshold is always suppressed, even at the longer
                 * poll intervals.

                         * This is the kernel set time function, usually
                         * implemented by the Unix settimeofday() system
                         * call.
                        c.count = 0;
                        s.poll = MINPOLL;
                        rval = STEP;
                        if (state == NSET) {
                                rstclock(FREQ, p->t, 0);
                                return (rval);
                rstclock(SYNC, p->t, 0);
        } else {

                 * Compute the clock jitter as the RMS of exponentially
                 * weighted offset differences.  This is used by the
                 * poll-adjust code.
                etemp = SQUARE(c.jitter);
                dtemp = SQUARE(max(fabs(offset - c.last),
                c.jitter = SQRT(etemp + (dtemp - etemp) / AVG);
                switch (c.state) {

                 * In S_NSET state, this is the first update received
                 * and the frequency has not been initialized.  The
                 * first thing to do is directly measure the oscillator
                 * frequency.
                case NSET:
                        rstclock(FREQ, p->t, offset);
                        return (IGNORE);
                 * In S_FSET state, this is the first update and the
                 * frequency has been initialized.  Adjust the phase,
                 * but don't adjust the frequency until the next update.
                case FSET:
                        rstclock(SYNC, p->t, offset);

                 * In S_FREQ state, ignore updates until the stepout
                 * threshold.  After that, correct the phase and
                 * frequency and switch to S_SYNC state.
                case FREQ:
                        if (c.t - s.t < WATCH)
                                return (IGNORE);

                        freq = (offset - c.offset) / mu;

                 * We get here by default in S_SYNC and S_SPIK states.
                 * Here we compute the frequency update due to PLL and
                 * FLL contributions.

                         * The FLL and PLL frequency gain constants
                         * depending on the poll interval and Allan
                         * intercept.  The FLL is not used below one
                         * half the Allan intercept.  Above that the
                         * loop gain increases in steps to 1 / AVG.
                        if (LOG2D(s.poll) > ALLAN / 2) {
                                etemp = FLL - s.poll;
                                if (etemp < AVG)
                                        etemp = AVG;
                                freq += (offset - c.offset) / (max(mu,
                                    ALLAN) * etemp);
                         * For the PLL the integration interval
                         * (numerator) is the minimum of the update
                         * interval and poll interval.  This allows
                         * oversampling, but not undersampling.
                        etemp = min(mu, LOG2D(s.poll));
                        dtemp = 4 * PLL * LOG2D(s.poll);
                        freq += offset * etemp / (dtemp * dtemp);
                        rstclock(SYNC, p->t, offset);

         * Calculate the new frequency and frequency stability (wander).
         * Compute the clock wander as the RMS of exponentially weighted
         * frequency differences.  This is not used directly, but can,
         * along with the jitter, be a highly useful monitoring and
         * debugging tool.
        freq += c.freq;
        c.freq = max(min(MAXFREQ, freq), -MAXFREQ);
        etemp = SQUARE(c.wander);
        dtemp = SQUARE(freq);
        c.wander = SQRT(etemp + (dtemp - etemp) / AVG);

         * Here we adjust the poll interval by comparing the current
         * offset with the clock jitter.  If the offset is less than the
         * clock jitter times a constant, then the averaging interval is
         * increased; otherwise, it is decreased.  A bit of hysteresis
         * helps calm the dance.  Works best using burst mode.
        if (fabs(c.offset) < PGATE * c.jitter) {
                c.count += s.poll;
                if (c.count > LIMIT) {
                        c.count = LIMIT;
                        if (s.poll < MAXPOLL) {
                                c.count = 0;
        } else {
                c.count -= s.poll << 1;
                if (c.count < -LIMIT) {
                        c.count = -LIMIT;
                        if (s.poll > MINPOLL) {
                                c.count = 0;
        return (rval);

A.5.5.7.  rstclock()

   * rstclock() - clock state machine
          int     state,          /* new state */
          double  offset,         /* new offset */
          double  t               /* new update time */
           * Enter new state and set state variables.  Note, we use the
           * time of the last clock filter sample, which must be earlier
           * than the current time.
          c.state = state;
          c.last = c.offset = offset;
          s.t = t;

A.5.6.  Clock Adjust Process

A.5.6.1.  clock_adjust()

  * clock_adjust() - runs at one-second intervals
 clock_adjust() {
         double  dtemp;

          * Update the process time c.t.  Also increase the dispersion
          * since the last update.  In contrast to NTPv3, NTPv4 does not
          * declare unsynchronized after one day, since the dispersion
          * threshold serves this function.  When the dispersion exceeds
          * MAXDIST (1 s), the server is considered unfit for
          * synchronization.
         s.rootdisp += PHI;

          * Implement the phase and frequency adjustments.  The gain
          * factor (denominator) is not allowed to increase beyond the
          * Allan intercept.  It doesn't make sense to average phase
          * noise beyond this point and it helps to damp residual offset
          * at the longer poll intervals.
         dtemp = c.offset / (PLL * min(LOG2D(s.poll), ALLAN));
         c.offset -= dtemp;

          * This is the kernel adjust time function, usually implemented
          * by the Unix adjtime() system call.
         adjust_time(c.freq + dtemp);

          * Peer timer.  Call the poll() routine when the poll timer
          * expires.
         while (/* all associations */ 0) {
                 struct p *p;    /* dummy peer structure pointer */

                 if (c.t >= p->nextdate)

          * Once per hour, write the clock frequency to a file.
          * if (c.t % 3600 == 3599)
          *   write c.freq to file

A.5.7.  Poll Process

    * Poll process parameters and constants
   #define UNREACH         12      /* unreach counter threshold */
   #define BCOUNT          8       /* packets in a burst */
   #define BTIME           2       /* burst interval (s) */

A.5.7.1.  poll()

 * poll() - determine when to send a packet for association p->


        struct p *p             /* peer structure pointer */
        int     hpoll;
        int     oreach;

         * This routine is called when the current time c.t catches up
         * to the next poll time p->nextdate.  The value p->outdate is
         * the last time this routine was executed.  The poll_update()
         * routine determines the next execution time p->nextdate.
         * If broadcasting, just do it, but only if we are synchronized.
        hpoll = p->hpoll;
        if (p->hmode == M_BCST) {
                p->outdate = c.t;
                if (s.p != NULL)
                poll_update(p, hpoll);

         * If manycasting, start with ttl = 1.  The ttl is increased by
         * one for each poll until MAXCLOCK servers have been found or
         * ttl reaches TTLMAX.  If reaching MAXCLOCK, stop polling until
         * the number of servers falls below MINCLOCK, then start all
         * over.
        if (p->hmode == M_CLNT && p->flags & P_MANY) {
                p->outdate = c.t;
                if (p->unreach > BEACON) {
                        p->unreach = 0;
                        p->ttl = 1;
                } else if (s.n < MINCLOCK) {
                        if (p->ttl < TTLMAX)
                poll_update(p, hpoll);
        if (p->burst == 0) {

                 * We are not in a burst.  Shift the reachability
                 * register to the left.  Hopefully, some time before
                 * the next poll a packet will arrive and set the
                 * rightmost bit.
                oreach = p->reach;
                p->outdate = c.t;
                p->reach = p->reach << 1;
                if (!(p->reach & 0x7))
                        clock_filter(p, 0, 0, MAXDISP);
                if (!p->reach) {

                         * The server is unreachable, so bump the
                         * unreach counter.  If the unreach threshold
                         * has been reached, double the poll interval
                         * to minimize wasted network traffic.  Send a
                         * burst only if enabled and the unreach
                         * threshold has not been reached.
                        if (p->flags & P_IBURST && p->unreach == 0) {
                                p->burst = BCOUNT;
                        } else if (p->unreach < UNREACH)
                } else {

                         * The server is reachable.  Set the poll
                         * interval to the system poll interval.  Send a
                         * burst only if enabled and the peer is fit.
                        p->unreach = 0;
                        hpoll = s.poll;
                        if (p->flags & P_BURST && fit(p))
                                p->burst = BCOUNT;
        } else {
                 * If in a burst, count it down.  When the reply comes
                 * back the clock_filter() routine will call
                 * clock_select() to process the results of the burst.
         * Do not transmit if in broadcast client mode.
        if (p->hmode != M_BCLN)
        poll_update(p, hpoll);

A.5.7.2.  poll_update()

 * poll_update() - update the poll interval for association p
 * Note: This routine is called by both the packet() and poll() routine.
 * Since the packet() routine is executed when a network packet arrives
 * and the poll() routine is executed as the result of timeout, a
 * potential race can occur, possibly causing an incorrect interval for
 * the next poll.  This is considered so unlikely as to be negligible.


        struct p *p,            /* peer structure pointer */
        int     poll            /* poll interval (log2 s) */
         * This routine is called by both the poll() and packet()
         * routines to determine the next poll time.  If within a burst
         * the poll interval is two seconds.  Otherwise, it is the
         * minimum of the host poll interval and peer poll interval, but
         * not greater than MAXPOLL and not less than MINPOLL.  The
         * design ensures that a longer interval can be preempted by a
         * shorter one if required for rapid response.
        p->hpoll = max(min(MAXPOLL, poll), MINPOLL);
        if (p->burst > 0) {
                if (p->nextdate != c.t)
                        p->nextdate += BTIME;
        } else {
                 * While not shown here, the reference implementation
                 * randomizes the poll interval by a small factor.
                p->nextdate = p->outdate + (1 << max(min(p->ppoll,
                    p->hpoll), MINPOLL));

         * It might happen that the due time has already passed.  If so,
         * make it one second in the future.
        if (p->nextdate <= c.t)
                p->nextdate = c.t + 1;

A.5.7.3.  peer_xmit()

 * transmit() - transmit a packet for association p


        struct p *p             /* peer structure pointer */
        struct x x;             /* transmit packet */

         * Initialize header and transmit timestamp
        x.srcaddr = p->dstaddr;
        x.dstaddr = p->srcaddr;
        x.leap = s.leap;
        x.version = p->version;
        x.mode = p->hmode;
        if (s.stratum == MAXSTRAT)
                x.stratum = 0;
                x.stratum = s.stratum;
        x.poll = p->hpoll;
        x.precision = s.precision;
        x.rootdelay = D2FP(s.rootdelay);
        x.rootdisp = D2FP(s.rootdisp);
        x.refid = s.refid;
        x.reftime = s.reftime;
        x.org = p->org;
        x.rec = p->rec;
        x.xmt = get_time();
        p->xmt = x.xmt;

         * If the key ID is nonzero, send a valid MAC using the key ID
         * of the association and the key in the local key cache.  If
         * something breaks, like a missing trusted key, don't send the
         * packet; just reset the association and stop until the problem
         * is fixed.
        if (p->keyid)
                if (/* p->keyid invalid */ 0) {
                        clear(p, X_NKEY);
                x.dgst = md5(p->keyid);

Authors' Addresses

   Dr. David L. Mills
   University of Delaware
   Newark, DE  19716

   Phone: +1 302 831 8247
   EMail: mills@udel.edu

   Jim Martin (editor)
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1378
   EMail: jrmii@isc.org

   Jack Burbank
   The Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Road
   Laurel, MD  20723-6099

   Phone: +1 443 778 7127
   EMail: jack.burbank@jhuapl.edu

   William Kasch
   The Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Road
   Laurel, MD  20723-6099

   Phone: +1 443 778 7463
   EMail: william.kasch@jhuapl.edu