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Keywords: [--------], NTP, SNTP, Synchronization
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
JHU/APL
June 2010
Network Time Protocol Version 4: Protocol and Algorithms Specification
Abstract
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
http://www.rfc-editor.org/info/rfc5905.
Mills, et al. Standards Track [Page 1]
RFC 5905 NTPv4 Specification June 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
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
Mills, et al. Standards Track [Page 2]
RFC 5905 NTPv4 Specification June 2010
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
Mills, et al. Standards Track [Page 3]
RFC 5905 NTPv4 Specification June 2010
1. Introduction
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-
Mills, et al. Standards Track [Page 4]
RFC 5905 NTPv4 Specification June 2010
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
www.ntp.org.
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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Mills, et al. Standards Track [Page 5]
RFC 5905 NTPv4 Specification June 2010
2. Modes of Operation
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
specification.
3. Protocol Modes
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.
Mills, et al. Standards Track [Page 6]
RFC 5905 NTPv4 Specification June 2010
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
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
Mills, et al. Standards Track [Page 7]
RFC 5905 NTPv4 Specification June 2010
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
needs.
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
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.
Mills, et al. Standards Track [Page 8]
RFC 5905 NTPv4 Specification June 2010
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
offset.
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
Mills, et al. Standards Track [Page 9]
RFC 5905 NTPv4 Specification June 2010
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
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
Mills, et al. Standards Track [Page 10]
RFC 5905 NTPv4 Specification June 2010
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
Mills, et al. Standards Track [Page 11]
RFC 5905 NTPv4 Specification June 2010
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
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
hardware.
Mills, et al. Standards Track [Page 12]
RFC 5905 NTPv4 Specification June 2010
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
Mills, et al. Standards Track [Page 13]
RFC 5905 NTPv4 Specification June 2010
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
two.
Mills, et al. Standards Track [Page 14]
RFC 5905 NTPv4 Specification June 2010
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.
Mills, et al. Standards Track [Page 15]
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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
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
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
In addition to the variable classes, a number of global parameters
are defined in this document, including those shown with values in
Figure 6.
Mills, et al. Standards Track [Page 16]
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+-----------+-------+----------------------------------+
| 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
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.
Mills, et al. Standards Track [Page 17]
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+-----------+------------+-----------------------+
| 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
field.
Mills, et al. Standards Track [Page 18]
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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
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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.
Mills, et al. Standards Track [Page 20]
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+-------+--------------------------+
| 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.
Mills, et al. Standards Track [Page 21]
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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
Mills, et al. Standards Track [Page 22]
RFC 5905 NTPv4 Specification June 2010
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.
Mills, et al. Standards Track [Page 23]
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7.4. The Kiss-o'-Death Packet
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
server;
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.
Mills, et al. Standards Track [Page 24]
RFC 5905 NTPv4 Specification June 2010
+------+------------------------------------------------------------+
| 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
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.
Mills, et al. Standards Track [Page 25]
RFC 5905 NTPv4 Specification June 2010
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
field.
8. On-Wire Protocol
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 224.0.1.1 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.
Mills, et al. Standards Track [Page 26]
RFC 5905 NTPv4 Specification June 2010
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.
Mills, et al. Standards Track [Page 27]
RFC 5905 NTPv4 Specification June 2010
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
Mills, et al. Standards Track [Page 28]
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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
below.
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
Mills, et al. Standards Track [Page 29]
RFC 5905 NTPv4 Specification June 2010
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
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.
Mills, et al. Standards Track [Page 30]
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9.1. Peer Process Variables
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
Mills, et al. Standards Track [Page 31]
RFC 5905 NTPv4 Specification June 2010
+--------+---------+-----------------+
| 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
association.
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
Mills, et al. Standards Track [Page 32]
RFC 5905 NTPv4 Specification June 2010
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
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.
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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.
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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
below.
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.
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+--------------------------+----------------------------------------+
| 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
description.
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
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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
client.
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
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
synchronization.
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
tp.
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.
Mills, et al. Standards Track [Page 37]
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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
i=n-1
--- epsilon_i
epsilon = \ ----------
/ (i+1)
--- 2
i=0
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
context.
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
Mills, et al. Standards Track [Page 38]
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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
dispersion.
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
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.
Mills, et al. Standards Track [Page 39]
RFC 5905 NTPv4 Specification June 2010
11.1. System Process Variables
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.
Mills, et al. Standards Track [Page 40]
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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
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.
Mills, et al. Standards Track [Page 41]
RFC 5905 NTPv4 Specification June 2010
+-----------------+
| clock_select() |
+-----------------+
................................|...........
. V .
. yes +---------+ +-----------------+ .
. +--| accept? | | scan candidates | .
. | +---------+ | | .
. V no | | | .
. +---------+ | | | .
. | add peer| | | | .
. +---------- | | | .
. | V | | .
. +---------->-->| | .
. | | .
. Selection Algorithm +-----------------+ .
.................................|..........
V
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
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11.2.1. Selection Algorithm
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.
Mills, et al. Standards Track [Page 43]
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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
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.
Mills, et al. Standards Track [Page 44]
RFC 5905 NTPv4 Specification June 2010
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
variables.
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
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
Mills, et al. Standards Track [Page 45]
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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:
Mills, et al. Standards Track [Page 46]
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LAMBDA = EPSILON + DELTA / 2,
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
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.
Mills, et al. Standards Track [Page 47]
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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
below.
Mills, et al. Standards Track [Page 48]
RFC 5905 NTPv4 Specification June 2010
+--------+------------+--------------------------+
| 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.
Mills, et al. Standards Track [Page 49]
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+-------+---------------------+-------------------+--------------+
| 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.
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12. Clock-Adjust Process
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
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
register.
13.1. Poll Process Variables
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.
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+---------+---------+--------------------+
| 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
processes
unreach: integer representing the number of seconds the server has
been unreachable
13.2. Poll Process Operations
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.
Mills, et al. Standards Track [Page 52]
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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
beginning.
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
normally.
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.
Mills, et al. Standards Track [Page 53]
RFC 5905 NTPv4 Specification June 2010
+-----------------------------------+
| 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)
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.
Mills, et al. Standards Track [Page 54]
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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
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
Mills, et al. Standards Track [Page 55]
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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
leaves.
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
attacks:
1. An intruder can intercept and archive packets forever, as well as
all the public values ever generated and transmitted over the
net.
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.
Mills, et al. Standards Track [Page 56]
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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
keys.
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
Mills, et al. Standards Track [Page 57]
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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
UDP/TCP Port 123 was previously assigned by IANA for this protocol.
The IANA has assigned the IPv4 multicast group address 224.0.1.1 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:
Mills, et al. Standards Track [Page 58]
RFC 5905 NTPv4 Specification June 2010
+------+------------------------------------------------------------+
| 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
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
18.1. Normative References
[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
[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.
Mills, et al. Standards Track [Page 59]
RFC 5905 NTPv4 Specification June 2010
[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.
Mills, et al. Standards Track [Page 60]
RFC 5905 NTPv4 Specification June 2010
Appendix A. Code Skeleton
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
process.
A.1. Global Definitions
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
Mills, et al. Standards Track [Page 61]
RFC 5905 NTPv4 Specification June 2010
* 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) */
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RFC 5905 NTPv4 Specification June 2010
#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 */
Mills, et al. Standards Track [Page 63]
RFC 5905 NTPv4 Specification June 2010
/*
* 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))
Mills, et al. Standards Track [Page 64]
RFC 5905 NTPv4 Specification June 2010
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 */
Mills, et al. Standards Track [Page 65]
RFC 5905 NTPv4 Specification June 2010
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 */
Mills, et al. Standards Track [Page 66]
RFC 5905 NTPv4 Specification June 2010
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;
Mills, et al. Standards Track [Page 67]
RFC 5905 NTPv4 Specification June 2010
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;
Mills, et al. Standards Track [Page 68]
RFC 5905 NTPv4 Specification June 2010
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 */
Mills, et al. Standards Track [Page 69]
RFC 5905 NTPv4 Specification June 2010
/*
* 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
*/
int
main()
{
struct p *p; /* peer structure pointer */
struct r *r; /* receive packet pointer */
Mills, et al. Standards Track [Page 70]
RFC 5905 NTPv4 Specification June 2010
/*
* 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,
P_FLAGS);
}
/*
* 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();
receive(r);
}
return(0);
}
Mills, et al. Standards Track [Page 71]
RFC 5905 NTPv4 Specification June 2010
/*
* mobilize() - mobilize and initialize an association
*/
struct p