One document matched: draft-ietf-ntp-ntpv4-proto-02.txt
Differences from draft-ietf-ntp-ntpv4-proto-01.txt
NTP WG J. Burbank, Ed.
Internet-Draft JHU/APL
Obsoletes: RFC 4330 (if approved) J. Martin, Ed.
Expires: September 2, 2006 Netzwert AG
D. Mills
U. Del.
March 2006
The Network Time Protocol Version 4 Protocol Specification
draft-ietf-ntp-ntpv4-proto-02
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Network Time Protocol (NTP) is widely used to synchronize
computer clocks in the Internet. This memorandum describes Version 4
of the NTP (NTPv4), introducing several changes from Version 3 of NTP
(NTPv3) described in RFC 1305, including the introduction of a
modified protocol header to accomodate Internet Protocol Version 6.
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NTPv4 also includes optional extensions to the NTPv3
protocol,including a dynamic server discovery mechanism.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
2. NTP Timestamp . . . . . . . . . . . . . . . . . . . . . . . . 4
3. NTP Message Formats . . . . . . . . . . . . . . . . . . . . . 6
3.1. Leap Indicator (LI) . . . . . . . . . . . . . . . . . . . 7
3.2. Version (VN) . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Stratum (Strat) . . . . . . . . . . . . . . . . . . . . . 9
3.5. Poll Interval (Poll) . . . . . . . . . . . . . . . . . . . 9
3.6. Precision (Prec) . . . . . . . . . . . . . . . . . . . . . 9
3.7. Root Delay . . . . . . . . . . . . . . . . . . . . . . . . 9
3.8. Root Dispersion . . . . . . . . . . . . . . . . . . . . . 10
3.9. Reference Identifier . . . . . . . . . . . . . . . . . . . 10
3.10. Reference Timestamp . . . . . . . . . . . . . . . . . . . 11
3.11. Originate Timestamp . . . . . . . . . . . . . . . . . . . 11
3.12. Receive Timestamp . . . . . . . . . . . . . . . . . . . . 11
3.13. Transmit Timestamp . . . . . . . . . . . . . . . . . . . . 11
3.14. NTPv4 Extension Fields . . . . . . . . . . . . . . . . . . 12
3.15. Authentication (optional) . . . . . . . . . . . . . . . . 13
4. NTP Protocol Operation . . . . . . . . . . . . . . . . . . . . 14
5. SNTP Protocol Operation . . . . . . . . . . . . . . . . . . . 17
6. NTP Server Operations . . . . . . . . . . . . . . . . . . . . 18
7. NTP Client Operations . . . . . . . . . . . . . . . . . . . . 20
8. NTP Symmetric Peer Operations . . . . . . . . . . . . . . . . 22
9. Dynamic Server Discovery . . . . . . . . . . . . . . . . . . . 22
10. The Kiss-o'-Death Packet . . . . . . . . . . . . . . . . . . . 23
11. Security Considerations . . . . . . . . . . . . . . . . . . . 24
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
14.1. Normative References . . . . . . . . . . . . . . . . . . . 25
14.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. NTP Control Messages . . . . . . . . . . . . . . . . 27
A.1. NTP Control Message Format . . . . . . . . . . . . . . . . 28
A.2. Status Words . . . . . . . . . . . . . . . . . . . . . . . 30
A.2.1. System Status Word . . . . . . . . . . . . . . . . . . 31
A.2.2. Peer Status Word . . . . . . . . . . . . . . . . . . . 33
A.2.3. Clock Status Word . . . . . . . . . . . . . . . . . . 34
A.2.4. Error Status Word . . . . . . . . . . . . . . . . . . 35
A.3. Commands . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . . . 40
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1. Introduction
The Network Time Protocol Version 3 (NTPv3) [1] has been widely used
to synchronize computer clocks in the global Internet. It provides
comprehensive mechanisms to access national time and frequency
dissemination services, organize the NTP subnet of servers and
clients and adjust the system clock in each participant. In most
places on the Internet of today, NTP provides accuracies of 1-50 ms,
depending on the characteristics of the synchronization source and
network paths.
NTP is designed for use by clients and servers with a wide range of
capabilities. Thus, the Simple Network Time Protocol Version 4
(SNTPv4) as described in [2] was developed for platforms that cannot
afford the size and complexity of NTP as a whole.
Since the standardization of NTPv3, there has been significant
development which has led to Version 4 of the Network Time Protocol
(NTPv4). This document describes NTPv4, which introduces new
functionality to NTPv3 as described in RFC 1305, and functionality
expanded from that of SNTPv4 as described in RFC 4330 (SNTPv4 is a
subset of NTPv4). This document obsoletes RFC 4330.
When operating with current and previous versions of NTP and SNTP,
NTPv4 requires no changes to the protocol or implementations now
running or likely to be implemented specifically for future NTP or
SNTP versions. The NTP and SNTP packet formats are the same and the
arithmetic operations to calculate the client time, clock offset and
round trip delay are the same. To a NTP or SNTP server, NTP and SNTP
clients are indistinguishable; to a NTP or SNTP client, NTP and SNTP
servers are indistinguishable.
An important provision in this memo is the interpretation of certain
NTP header fields which provide for IPv6 [3]and OSI [4] addressing.
The only significant difference between the NTPv3 and NTPv4 header
formats is the four-octet Reference Identifier field, which is used
primarily to detect and avoid synchronization loops. In all NTP and
SNTP versions providing IPv4 addressing, primary servers use a four-
character ASCII reference clock identifier in this field, while
secondary servers use the 32-bit IPv4 address of the synchronization
source. In NTPv4 providing IPv6 and OSI addressing, primary servers
use the same clock identifier, but secondary servers use the first 32
bits of the MD5 hash of the IPv6 or NSAP address of the
synchronization source. A further use of this field is when the
server sends a kiss-o'-death message documented later in this
document.
In the case of OSI, the Connectionless Transport Service (CLTS) is
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used as in [5]. Each NTP packet is transmitted as the TS- Userdata
parameter of a T-UNITDATA Request primitive. Alternately, the header
can be encapsulated in a TPDU which itself is transported using UDP,
as described in [6]. It is not advised that NTP be operated at the
upper layers of the OSI stack, such as might be inferred from [7], as
this could seriously degrade accuracy. With the header formats
defined in this memo, it is, in principle, possible to interwork
between servers and clients of one protocol family and another,
although the practical difficulties may make this inadvisable.
This document is organized as follows. Section 2 describes the NTP
timestamp format and Section 3 the NTP message format. Section 4
provides general NTP protocol details, with the subset SNTP described
in Section 5. This is followed by specific sections on Server
(Section 6), Client(Section 7), and Symmetric Peer(Section 8) modes
of operation. Section 9 defines the new mechanism for server
discovery. describes the control and management mechanism for NTP.
Section 10 describes the kiss-o'-death message, whose functionality
is similar to the ICMP Source Quench and ICMP Destination Unreachable
messages. Section 11 presents NTPv4 security considerations and
Section 12 discusses IANA Considerations.Appendix A presents optional
NTP control messages.
NTPv4 is hereafter referred to simply as NTP, unless explicitly
noted.
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 RFC 2119 [8].
2. NTP Timestamp
There are three NTP formats used to represent time values: a 128-bit
date format, a 64-bit timestamp format, and a 32-bit short format.
NTP data are specified as integer or fixed-point quantities, with
bits numbered in big-endian fashion from 0 starting at the left or
most significant end. Unless specified otherwise, all quantities are
unsigned and may occupy the full field width with an implied 0
preceding bit 0. Note that dates cannot be produced by NTP, but can
rather be obtained from external means and conveyed via the protocol.
Date values are represented in twos compliment arithmetic relative to
the base date of 0628:16h 7 February 2036 UTC (when all 128 bits are
zero). Values greater than zero represent times after the base date;
values less than zero represent times before the base date. Dates
are signed values. Timestamps are signed values. A value of zero is
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a special case representing unknown or unsynchronized time.
Figure 1 illustrates the three NTP time formats.
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 1: NTP Timestamp Format
Note that, since some time in 1968 (second 2,147,483,648) the most
significant bit (bit 0 of the integer part) has been set and that the
64-bit field will overflow some time in 2036 (second 4,294,967,296).
There will exist a 232-picosecond interval, henceforth ignored, every
136 years when the 64-bit field will be 0, which by convention is
interpreted as an invalid or unavailable timestamp.
If bit 0 is set, the UTC time is in the range 1968-2036 and UTC time
is reckoned from 0h 0m 0s UTC on 1 January 1900. If bit 0 is not
set, the time is in the range 2036-2104 and UTC time is calculated
from 6h 28m 16s UTC on 7 February 2036. Note that when calculating
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the correspondence, 2000 is a leap year and leap seconds are not
included in the reckoning.
3. NTP Message Formats
Both NTP and SNTP are layered above the User Datagram Protocol (UDP)
[9], which itself is layered on the Internet Protocol (IP) [10] [3].
The structure of the IP and UDP headers is described in the cited
specification documents and will not be detailed further here. The
UDP port number assigned to NTP is 123, which MUST be used in both
the Source Port and Destination Port fields in the UDP header. The
remaining UDP header fields should be set as described in the
specification.
Figure 2 provides a description of the NTPv4 message format. This
format is identical to that described in RFC 1305, with the exception
of the contents of the reference identifier field and optional
extension fields. The header fields are defined in Figure 2.
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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 | Strat | Poll | Prec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Dispersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Reference Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Origin Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receive Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Transmit Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Extension Field 1 (Optional) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Extension Field 2 (Optional) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Authentication .
. (Optional) (160 bits) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NTPv4 Message Format
3.1. Leap Indicator (LI)
This is a two-bit field indicating an impending leap second to be
inserted in the NTP timescale. The bits are set before 23:59 on the
day of insertion and reset after 00:00 on the following day. This
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causes the number of seconds (rollover interval) in the day of
insertion to be increased or decreased by one. A leap second is
inserted or deleted in the timescale on the last day of June or
December. The possible values of the LI field, and corresponding
meanings, are given in Table 1.
+----+--------------------------------------------+
| LI | Meaning |
+----+--------------------------------------------+
| 0 | no warning |
| 1 | last minute of the day has 61 seconds |
| 2 | last minute of the day has 59 seconds |
| 3 | alarm condition (clock never synchronized) |
+----+--------------------------------------------+
Table 1: Length Indicator Field Values
On startup, servers set this field to 3 (clock not synchronized) and
set this field to some other value when synchronized to the primary
reference clock. Once set to other than 3, the field is never set to
that value again, even if all synchronization sources become
unreachable or defective.
3.2. Version (VN)
This is a three-bit integer indicating the NTP/SNTP version number,
set to 4 for NTPv4. If necessary to distinguish between IPv4, IPv6
and OSI, the encapsulating context must be inspected.
3.3. Mode
This is a three-bit number indicating the protocol mode. The values
are defined in Table 2.
+------+--------------------------+
| Mode | Meaning |
+------+--------------------------+
| 0 | reserved |
| 1 | symmetric active |
| 2 | symmetric passive |
| 3 | client |
| 4 | server |
| 5 | broadcast |
| 6 | NTP control message |
| 7 | reserved for private use |
+------+--------------------------+
Table 2: Mode Field Values
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Mode 0 is reserved. Modes 1 and 2 are intended for use by symmetric
peers who set this mode to 1 or 2 depending on whether it is active
or passive mode. In unicast mode or discovery mode, the client sets
this field to 3 (client) in the request and the server sets it to 4
(server) in the reply. In broadcast mode, the server sets this field
to 5 (broadcast). A mode type of 6 is reserved for NTP control
messages. Mode 7 is reserved for private usage.
3.4. Stratum (Strat)
This is a eight-bit unsigned integer indicating the stratum. This
field is significant only in SNTP server messages, where the values
are defined in Table 3.
+---------+-------------------------------------------------------+
| Stratum | Meaning |
+---------+-------------------------------------------------------+
| 0 | kiss-o'-death message |
| 1 | primary reference (e.g., synchronized by radio clock) |
| 2-255 | secondary reference (synchronized by NTP or SNTP) |
+---------+-------------------------------------------------------+
Table 3: Stratum Field Values
3.5. Poll Interval (Poll)
This is an eight-bit unsigned integer indicating the maximum interval
between successive messages, in log2 seconds. A client SHOULD NOT
use a poll interval less than 15 seconds, except at initial startup
when it MAY send a sequence of 8 packets at 1 second intervals to
provide initial synchronization of the clients with each server. A
client SHOULD increase the poll interval as performance permits and
especially if the server does not respond within a reasonable time.
3.6. Precision (Prec)
This is an eight-bit signed integer indicating the precision of the
system clock in log2 seconds. Precision is normally determined when
the service is established as the minimum number of iterations of the
time to read the system clock. As an example, a value of -18
corresponds to a precision of about one microsecond.
3.7. Root Delay
This is a 32-bit signed fixed-point number indicating the total
roundtrip delay to the primary reference source, in 32-bit NTP short
format. Note that this variable can take on both positive and
negative values, depending on the relative time and frequency
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offsets. This field is significant only in server messages, where
the values range from negative values of a few milliseconds to
positive values of several hundred milliseconds.
3.8. Root Dispersion
This is a 32-bit unsigned fixed-point number indicating the nominal
error relative to the primary reference source in seconds, in 32-bit
NTP short format.
3.9. Reference Identifier
This is a 32-bit bitstring identifying the particular reference
source. The interpretation of this field depends on the value in the
stratum field. For stratum 0, this is a four-character ASCII string,
referred to as a 'kiss code' and is used for debugging and monitoring
purposes. For stratum 1, this is a four-octet, left-justified, zero-
padded ASCII string assigned to the reference source. Above stratum
1 (secondary servers and clients), this is the reference identifier
of the server. If employing IPv4, the value is the 32-bit IPv4
address of the synchronization source. For IPv6 and OSI, the value
is the first 32 bits of the MD5 hash of the IPv6 or NSAP address of
the synchronization source. The fASCII identifiers that are
currently defined are given in Table 4.
Primary (stratum 1) servers set this field to a code identifying the
external reference source according to Table 4.
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+-------+----------------------------------------------------+
| Code | External Reference Source |
+-------+----------------------------------------------------+
| GOES | Geosynchronous Orbit Environment Satellite |
| GPS | Global Position System |
| PPS | Generic pulse-per-second |
| IRIG | Inter-Range Instrumentation Group |
| WWVB | LF Radio WWVB Ft. Collins, CO 60 kHz |
| DCF77 | LF Radio DCF77 Mainflingen, DE 77.5 kHz |
| HBG | LF Radio HBG Prangins, HB 75 kHz |
| MSF | LF Radio MSF Rugby, UK 60 kHz |
| JJY | LF Radio JJY Fukushima, JP 40 kHz, Saga, JP 60 kHz |
| LORC | MF Radio LORAN C 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 |
| USNO | USNO telephone modem |
| PTB | European telephone modem |
+-------+----------------------------------------------------+
Table 4: Currently-defined Reference Identifiers
If the external reference is one of those listed, the associated code
should be used. Codes for sources not listed can be created as
appropriate (see IANA Considerations section of this document).
3.10. Reference Timestamp
This is a 64 bit signed integer indicating the time when the system
clock was last set or correctetd, in 64-bit NTP timestamp format.
3.11. Originate Timestamp
This is the time at which the request departed the client for the
server, in 64-bit NTP timestamp format.
3.12. Receive Timestamp
This is the time at which the request arrived at the server or the
reply arrived at the client, in 64-bit NTP timestamp format.
3.13. Transmit Timestamp
This is the time at which the request departed the client or the
reply departed the server, in 64-bit NTP timestamp format.
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3.14. NTPv4 Extension Fields
NTPv4 defines new extension field formats. The minimum extension
field length is 8 octets. The format of the NTP extension field is
given in Figure Figure 3.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Value .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Signature .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (as needed) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: NTP Extension Field Format
The Field Type field is a 16-bit integer which indicates the type of
extension message contained within the extension field.
The Length field is a 16-bit integer indicates the length of the
entire extension field in octets, including the Length and Padding
fields.
The 32-bit Association ID field is set by clients to the value
previously received from the server or 0 otherwise. The server sets
the Association ID field when sending a response as a handle for
subsequent exchanges. If the association ID value in a request does
not match the association ID of any association, the server returns
the request with the first two bits of the Field Type field set to 1.
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The Timestamp and Filestamp 32-bit fields carry the seconds field of
an NTP timestamp. The Timestamp field establishes the signature
epoch of the data field in the message, while the filestamp
establishes the generation epoch of the file that ultimately produced
the data.
The 32-bit Value Length field indicates the length of the Value field
in octets. The minimum length of the Value field is 0.
The 32-bit Signature Length field indicates the length of the
Signature field in octets.
Zero padding is applied, as necessary, to extend the extension field
to a word (4-octet) boundary. If multiple extension fields are
present, the last extension field is zero-padded to a double-word (8
octet) boundary.
3.15. Authentication (optional)
NTPv4 provides an optional 160-bit Authentication field. When
implemented, the 32-bit Key Identifier and 128-bit Message Digest
fields contain the Message Authentication Code (MAC) information
which uses an MD5 cryptosum of NTP header plus extension fields. The
authentication field format is shown in Figure Figure 4.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Message Digest +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: NTP Authentication Field
The 32-bit Key Identifier is an integer identifying the 128-bit
private key used to generate the MAC. The Message Digest field
contains the MD5 Message Digest. In NTPv4, the presence of one or
more extension fields requires the presence of an authentication
field. The presence of the Authentication field and extension fields
is determined from the Length field.
The Key Identifier is initialized to zero at the start of an
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association. The type of association then determines the key
identifier. If the association is active (modes 1, 3, 5) the key is
determined from the system key identifier. If the association is
passive (modes 2, 4) the key is determined from the peer key
identifier, if the authentic bit is set (see [1]), or as the default
key (zero) otherwise.
4. NTP Protocol Operation
The NTP protocol defines three operational roles, Client, Server, and
Symmetric Peer. Clients request or receive time from Servers
(solicited or unsolicited). Servers respond to requests or send
periodic time updates to Clients. Symmetric Peers exchange time data
bidirectionally. A given NTPv4 implementation can operate in any or
all of these modes.
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
to mean 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 prefix determined by scoping rules.
Except in broadcast mode, an NTP association is formed when two peers
exchange messages and one or both of them create and maintain an
instantiation of the protocol machine, called an association. The
association can operate in one of five modes as indicated by the
host- mode variable (peer.mode) (see [1] for a description of the NTP
variables): symmetric active, symmetric passive, client, server and
broadcast, which are defined as follows:
Symmetric Active (1): A host operating in this mode sends periodic
messages regardless of the reachability state or stratum of its peer.
By operating in this mode the host announces its willingness to
synchronize and be synchronized by the peer.
Symmetric Passive (2): This type of association is ordinarily created
upon arrival of a message from a peer operating in the symmetric
active mode and persists only as long as the peer is reachable and
operating at a stratum level less than or equal to the host;
otherwise, the association is dissolved. However, the association
will always persist until at least one message has been sent in
reply. By operating in this mode the host announces its willingness
to synchronize and be synchronized by the peer.
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Client (3): A host operating in this mode sends periodic messages
regardless of the reachability state or stratum of its peer. By
operating in this mode the host announces its willingness to be
synchronized by, but not to synchronize the peer.
Server (4): This type of association is ordinarily created upon
arrival of a client request message and exists only in order to reply
to that request, after which the association is dissolved. By
operating in this mode the host announces its willingness to
synchronize, but not to be synchronized by the peer.
Broadcast (5): A host operating in this mode sends periodic messages
regardless of the reachability state or stratum of the peers. By
operating in this mode the host announces its willingness to
synchronize all of the peers, but not to be synchronized by any of
them.
NTP messages are layered on top of UDP. All messages MUST be sent
with a destination port of 123, and SHOULD be sent with a source port
of 123.
The on-wire protocol uses four timestamps numbered T1 through T4 and
three state variables org, rec, and xmt, as shown in Figure Figure 5,
where T1 corresponds to the Reference Timestamp T2 corresponds to the
Originate Timestamp, T3 corresponds to the Receive Timestamp, and T4
corresponds to the Transmit Timestamp.
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t2 t3 t6 t7
+---------+ +---------+ +---------+ +---------+
T1 | 0 | | t2 | | t4 | | t6 |
+---------+ +---------+ +---------+ +---------+
T2 | 0 | | t1 | | t3 | | t5 | Packet
+---------+ +---------+ +---------+ +---------+ Variables
T3 |t2=clock | | t2 | |t6=clock | | t6 |
+---------+ +---------+ +---------+ +---------+
T4 | t1 | |t3=clock | | t5 | |t7=clock |
+---------+ +---------+ +---------+ +---------+
Peer B
+---------+ +---------+ +---------+ +---------+
org | t1 | | t1 | | T3<>t1? | | t5 |
+---------+ +---------+ +---------+ +---------+ State
rec | t2 | | t2 | | t6 | | t6 | Variables
+---------+ +---------+ +---------+ +---------+
xmt | 0 | | t3 | | T1<>t3? | | t7 |
+---------+ +---------+ +---------+ +---------+
t2 t3 t6 t7
---------------------------------------------------------
/\ \ /\ \
/ \ / \
/ \ / \
/ \/ / \/
---------------------------------------------------------
t1 t4 t5 t8
t1 t4 t5 t8
+---------+ +---------+ +---------+ +---------+
T1 | 0 | | t2 | | t4 | | t6 |
+---------+ +---------+ +---------+ +---------+
T2 | 0 | | t1 | | t3 | | t5 | Packet
+---------+ +---------+ +---------+ +---------+ Variables
T3 | 0 | |t4=clock | | t4 | |t8=clock |
+---------+ +---------+ +---------+ +---------+
T4 |t1=clock | | t3 | |t5=clock | | t7 |
+---------+ +---------+ +---------+ +---------+
Peer A
+---------+ +---------+ +---------+ +---------+
org | 0 | | T3<>0? | | t3 | | T3<>t3? |
+---------+ +---------+ +---------+ +---------+ State
rec | 0 | | t4 | | t4 | | t8 | Variables
+---------+ +---------+ +---------+ +---------+
xmt | t1 | | T1=t1? | | t5 | | T1<>t5? |
+---------+ +---------+ +---------+ +---------+
Figure 5: NTPState
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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 illustrative purposes, the individual timestamp values
are shown in lower case with subscripts indicating the order of
transmission and reception. In the figure, the first packet
transmitted by A contains only the transmit timestamp T4 with value
t1. B receives the packet at t2 and saves the originate timestamp T2
with value t1 in state variable org and the receive timestamp T3 with
value t2 in state variable rec. Afterwards, B sends a packet to A
containing the org and rec state variables in T2 and T1 respectively
and additionally the transmit timestamp T4 with value t3, which is
saved in the xmt state variable. When this packet arrives at A the
packet header variables T1, T2, T3, and T4 represent the four
timestampes necessary to compute the offset and delay of B relative
to A.
Before the A state variables are updated, two sanity checks are
performed in order to protect against duplicate or invalid packets.
A packet is a duplicate if the transmit timestamp T4 in the packet
matches the xmt state variable. A packet is invalid if the origin
timestamp T2 in the packet does not match the org state variable. In
either of these cases the state variables are updated, but the packet
is discarded.
The general rules that govern the updating of state variables and
packet variables are given in Figure 6.
+-------------------------------------------------------+
| Receive | Transmit |
+-------------------------------------------------------+
| org=T4 | org=unchanged |
| rec=Time of Receipt | rec=unchanged |
| xmt=unchanged | xmt=Time of transmission |
| | |
| T1=Received T3 | T1=rcv |
| T2=Received T2 | T2=org |
| T3=rec | T3=unchanged |
| T4=Received T4 | T4=xmt |
+-------------------------------------------------------+
Figure 6: Relationship between NTP State Variables and NTP Packet
Variables
5. SNTP Protocol Operation
SNTP operates using the same message formats, addresses, and ports as
NTP. However, it is stateless, operating only in the client or
server roles. Thus it is compatible with, and a subset of, NTP.
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6. NTP Server Operations
Fundamentally, the NTP Server role consists of listening for client
requests, and providing time and associated details as a response.
Additionally, a server can provide time and associated details
periodically via a broadcast mechanism.
An NTP server can communicate via unicast, broadcast, or both. A
server receiving a unicast request (NTP mode 3), modifies fields in
the NTP header as described below, and sends a reply (NTP mode 4),
possibly using the same message buffer as the request. When
operating in a broadcast mode, unsolicited messages (NTP mode 5) with
field values as described below are normally sent at intervals
ranging from 64 s to 1024 s, depending on the expected frequency
tolerance of the client clocks and the required accuracy.
A broadcast server may or may not send messages if not synchronized
to a correctly operating source, but the preferred option is to
transmit, since this allows reachability to be determined regardless
of synchronization state.
The Leap Indicator (LI) is set to 3 (unsynchronized) if the server
has never synchronized to a reference source. Once synchronized, the
LI field is set to one of the other three values and remains at the
last value set even if the reference source becomes unreachable or
turns faulty.
The Version (VN) is copied from the request packet, if responding to
a unicast request. For broadcast, this is set to 4.
The Mode is set to Server (4) if in response to a unicast request.
For broadcast, this is set to Broadcast (5).
The Stratum field is set to the server's current stratum, if
synchronized. If synchronized to a primary reference source the
Stratum field is set to 1. If unsynchronized this field is set to 0.
The Poll field is coppied from the request, if responding to a
unicast request. For broadcast, this is set to the nearest integer
log2 of the poll interval.
The Precision field is set to reflect the maximum reading error of
the system clock. The Root Delay and Root Dispersion fields are set
to 0 for a primary server; optionally, the Root Dispersion field can
be set to a value corresponding to the maximum error of the radio
clock itself.
If the server is synchronized to a reference source, the value of the
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Reference ID is set to a four-character ASCII string identifying the
source, left justified and zero padded to 32bits. For IPv4 secondary
servers,the value is the 32-bit IPv4 address of the synchronization
source. For IPv6 and OSI secondary servers, the value is the first
32 bits of the MD5 hash of the IPv6 or NSAP address of the
synchronization source. If unsynchronized, it is set to an ASCII
error identifier.
The timestamp fields in the server message are set as follows. If
the server is unsynchronized or first coming up, all timestamp fields
are set to zero with one exception. If the server is synchronized,
the Transmit Timestamp field of the request is copied unchanged to
the Originate Timestamp field of the reply.
If the server is synchronized, the Reference Timestamp is set to the
time the last update was received from the reference source. The
Originate Timestamp field is set as in the unsynchronized case above.
The Transmit Timestamp field is set to the time of day when the
message is sent. In broadcast messages the Receive Timestamp field
is set to zero and copied from the Transmit Timestamp field in other
messages.
Table 5 summarizes these actions.
+---------------+-----------+-------------------+-------------------+
| Field Name | Unicast | Unicast Reply | Broadcast |
| | Request | | |
+---------------+-----------+-------------------+-------------------+
| LI | ignore | as needed | as needed |
| VN | 1-4 | copied from | 4 |
| | | request | |
| Mode | 1 or 3 | 2 or 4 | 5 |
| Stratum | ignore | 1 | 1 |
| Poll | ignore | copied from | log2 poll |
| | | request | interval |
| Precision | ignore | -log2 server | -log2 server |
| | | significant bits | significant bits |
| Root Delay | ignore | 0 | 0 |
| Root | ignore | 0 | 0 |
| Dispersion | | | |
| Reference | ignore | source ident | source ident |
| Identifier | | | |
| Reference | ignore | time of last src. | time of last src. |
| Timestamp | | update | update |
| Originate | ignore | copied from xmit | 0 |
| Timestamp | | timestamp | |
| Receive | ignore | time of day | 0 |
| Timestamp | | | |
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| Transmit | (see | time of day | time of day |
| Timestamp | text) | | |
| Authenticator | optional | optional | optional |
+---------------+-----------+-------------------+-------------------+
Table 5: NTP Server Message Field Population
Broadcast servers should respond to client unicast requests, as
well as send unsolicited broadcast messages. Broadcast clients
may send unicast requests in order to measure the network
propagation delay between the server and client and then continue
operation in listen-only mode. However, broadcast servers may
choose not to respond to unicast requests, so unicast clients
should be prepared to abandon the measurement and assume a default
value for the delay.
7. NTP Client Operations
The role of an NTP client is to determine the current time (and
associated information) from an NTP server. This can be done
actively, by sending a unicast request to a configured server, or
passively by listening on a known address for periodic server
messages.
An NTP client can operate in unicast or broadcast modes. In unicast
mode the client sends a request (NTP mode 3) to a designated unicast
server and expects a reply (NTP mode 4) from that server. In
broadcast client mode it sends no request and waits for a broadcast
(NTP mode 5) from one or more broadcast servers.
A unicast client initializes the NTP message header, sends the
request to the server and strips the time of day from the Transmit
Timestamp field of the reply. For this purpose, all of the NTP
header fields shown in Section 3 are set to 0, except the Mode, VN
and optional Transmit Timestamp fields.
NTP and SNTP clients set the mode field to 3 (client) for unicast
requests. They set the VN field to any version number supported by
the server selected by configuration or discovery and can
interoperate with all previous version NTP and SNTP servers. Servers
reply with the same version as the request, so the VN field of the
request also specifies the VN field of the reply. An NTP client can
specify the earliest acceptable version on the expectation that any
server of that or later version will respond. NTPv4 servers are
backwards compatible with NTPv3 as defined in RFC 1305, NTPv2 as
defined in [11], and NTPv1 as defined in [12]. NTPv0 defined in [13]
is not supported.
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In unicast mode, the Transmit Timestamp field in the request SHOULD
be set to the time of day according to the client clock in NTP
timestamp format. This allows for the determination of the
propagation delay between the server and client and to align the
system clock relative to the server. In addition, this provides a
simple method to verify that the server reply is in fact a legitimate
response to the specific client request and avoid replays. Note that
in broadcast mode, the client cannot necessarily calculate the
propagation delay or determine the validity of the server.
There is some latitude on the part of most clients to forgive invalid
timestamps, such as might occur when first coming up or during
periods when the reference source is inoperative. The most important
indicator of an unhealthy server is the Stratum field, in which a
value of 0 indicates an unsynchronized condition. When this value is
displayed, clients should discard the server message, regardless of
the contents of other fields.
Table 6 summarizes the required NTP client operations in unicast and
broadcast modes
+-------------------+---------------+-------------------+-----------+
| Field Name | Unicast | Unicast Reply | Broadcast |
| | Request | | |
+-------------------+---------------+-------------------+-----------+
| LI | 0 | 0-3 | 0-3 |
| VN | 1-4 | copied from | 1-4 |
| | | request | |
| Mode | 1 or 3 | 2 or 4 | 5 |
| Stratum | 0 | 0-15 | 0-15 |
| Poll | 0 | ignore | ignore |
| Precision | 0 | ignore | ignore |
| Root Delay | 0 | ignore | ignore |
| Root Dispersion | 0 | ignore | ignore |
| Reference | 0 | ignore | ignore |
| Identifier | | | |
| Reference | 0 | ignore | ignore |
| Timestamp | | | |
| Originate | 0 | (see text) | ignore |
| Timestamp | | | |
| Receive Timestamp | 0 | (see text) | ignore |
| Transmit | (see text) | nonzero | nonzero |
| Timestamp | | | |
| Authenticator | optional | optional | optional |
+-------------------+---------------+-------------------+-----------+
Table 6: NTP Client Message Field Population
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8. NTP Symmetric Peer Operations
NTP Symmetric Peer mode is intended for configurations where a set of
low-stratum peers operate as mutual backups for each other. Each
peer normally operates with one or more sources, such as a reference
clock, or a subset of primary or secondry servers known to be
reliable or authentic.
Symmetric Peer mode is exclusive to the NTP protocol and is
specifically excluded from SNTP operation. For the purposes of this
document, an NTP peer operates like a client.
9. Dynamic Server Discovery
NTPv4 provides a mechanism, commonly known as "Manycast", for a
client to dynamically discover the existance of one or more servers
with no a-priori knowledge. Once servers are discovered, they are
then treated as any other unicast server.
A client employing server discovery is configured with MinServers,
the minimum number of desired servers and MaxServers, the maximum
number of desired servers. The discovery mechanism is a simple
expanding ring search, using IP multicast with increasing TTLs or Hop
Counts. The multicast address used MUST be scoped to the local site,
as defined by [14].
The client initiates the discovery process by sending an NTP message
to the configured multicast address (224.0.1.1 for IPv4 and a
multicast address ending :101 for IPv6 with proper scoping.) with an
IP TTL or Hop Count of 1. This message has all of the NTP header
fields set to 0, except the Mode, VN and optional Transmit Timestamp
fields. The Mode is set to 3. It then starts a retry timer
(Default: 64 seconds) and listens for unicast responses from servers.
The source address of any server responses are treated as newly
configured unicast servers, up to a limit of MaxServers. If the
number of discovered servers is less than MinServers when the retry
timer expires, an identical NTP message is sent with an increased
TTL/Hop Count, and the retry timer is restarted. This continues
until either MinServers servers have been discovered or a configured
maximum TTL/Hop Count is reached. If the configured maximum TTL/Hop
Count is reached, packets continue to be periodically sent at the
maximum TTL/Hop Count. If at some subsequent time, the number of
valid servers drops below MinServers, the process restarts at the
initial state.
A server configured to provide server discovery will listen on the
specified multicast address for discovery messages from clients. If
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the server is in scope of the current TTL and is itself synchronized
to a valid source it replies to the discovery message from the client
with an ordinary unicast server message as described in Section 6
10. The Kiss-o'-Death Packet
According to the NTPv3 specification [1], if the Stratum field in the
NTP header is 1, indicating a primary server, the Reference
Identifier field contains an ASCII string identifying the particular
reference clock type. However, in [1] nothing is said about the
Reference Identifier field if the Stratum field is 0, which is called
out as "unspecified". However, if the Stratum field is 0, the
Reference Identifier field can be used to convey messages useful for
status reporting and access control. In NTPv4 and SNTPv4, packets of
this kind 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. These codes are encoded in four-character ASCII strings 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 Table 7.
+------+------------------------------------------------------------+
| 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. |
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| STEP | A step change in system time has occurred, but the |
| | association has not yet resynchronized |
+------+------------------------------------------------------------+
Table 7: Currently-defined NTP Kiss Codes
In general, an NTP client should stop sending to a particular server
if that server returns a reply with a Stratum field of 0, regardless
of kiss code, and an alternate server is available. If no alternate
server is available, the client SHOULD increase the poll interval as
performance permits.
11. Security Considerations
NTPv4 provides an optional authentication field that utilizes the MD5
algorithm. MD5, as the case for SHA-1, is derived from MD4, which
has long been known to be weak. In 2004, techniques for efficiently
finding collisions in MD5 were announced. A summary of the weakness
of MD5 can be found in [15].
In the case of NTP as specified herein, there is a vulnerability that
NTP broadcast clients can be disrupted by misbehaving or hostile SNTP
or NTP broadcast servers elsewhere in the Internet. Access controls
and/or cryptographic authentication means should be provided for
additional security in such cases.
While not required in a conforming NTP client implementation, there
are a variety of recommended checks that an NTP client can perform
that are designed to avoid various types of abuse that might happen
as the result of server implementation errors or malicious attack.
These recommended checks are as follows:
When the IP source and destination addresses are available for the
client request, they should match the interchanged addresses in
the server reply.
When the UDP source and destination ports are available for the
client request, they should match the interchanged ports in the
server reply.
The Originate Timestamp in the server reply should match the
Transmit Timestamp used in the client request.
A client can check the Root Delay and Root Dispersion fields are
each greater than or equal to 0 and less than infinity, where
infinity is is on the order of 15-20 seconds. This check avoids
using a server whose synchronization source has expired for a very
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long time.
12. 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 identifies the set of defined 4-character (ASCII)
Reference Identifier values. This document also defines the set of
defined Kiss Codes. This document also 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 is requested to establish and maintain a registry for Reference
Identifiers, Kiss codes, and Extension Field Types associated with
this protocol, populating this registry from the Reference
Identifiers given in Section 3.9 and Kiss Codes given in Section 11
as the initial entries. The Extension Field Types registry will have
no initial entries. As future needs arise, new Reference
Identifiers, Kiss Codes, and Extension Field Types may be defined.
Following the policies outlined in [16], new values are to be defined
by IETF Consensus.
13. Acknowledgements
This document has drawn material from RFC 4330, "Simple Network Time
Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI." As a result, the
authors would like to acknowledge D. Plonka of the University of
Wisconsin and J. Montgomery of Netgear, who were contributors. The
authors would also like to thank B. Haberman for providing rigorous
reviews of this document.
14. References
14.1. Normative References
[1] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.
14.2. Informative References
[2] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI", RFC 4330, January 2006.
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[3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[4] Colella, R., Callon, R., Gardner, E., and Y. Rekhter,
"Guidelines for OSI NSAP Allocation in the Internet", RFC 1629,
May 1994.
[5] International Standards Organization, "International Standards
8602 - Information Processing Systems - OSI: Connectionless
Transport Protocol Specification.", NDSS , December 1986.
[6] Shue, C., Haggerty, W., and K. Dobbins, "OSI connectionless
transport services on top of UDP: Version 1", RFC 1240,
June 1991.
[7] Furniss, P., "Octet Sequences for Upper-Layer OSI to Support
Basic Communications Applications", RFC 1698, October 1994.
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[9] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[10] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[11] Mills, D., "Network Time Protocol (version 2) specification and
implementation", STD 12, RFC 1119, September 1989.
[12] Mills, D., "Network Time Protocol (version 1) specification and
implementation", RFC 1059, July 1988.
[13] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
RFC 959, October 1985.
[14] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
[15] Bellovin, S. and E. Rescorla, "Deploying a New Hash Algorithm",
Proceedings of the 13th Annual ISOC Network and Distributed
System Security Symposium (NDSS) , February 2006.
[16] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
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Appendix A. NTP Control Messages
In a comprehensive network-management environment, facilities are
presumed available to perform routine NTP control and monitoring
functions, such as setting the leap-indicator bits at the primary
servers, adjusting the various system parameters and monitoring
regular operations. Ordinarily, these functions can be implemented
using a network-management protocol such as SNMP and suitable
extensions to the MIB database. However, in those cases where such
facilities are not available, these functions can be implemented
using special NTP control messages described herein. These messages
are intended for use only in systems where no other management
facilities are available or appropriate, such as in dedicated-
function bus peripherals. Support for these messages is not required
in order to conform to this specification.
The NTP Control Message has the value 6 specified in the mode field
of the first octet of the NTP header and is formatted as shown in
Section 10.1. The format of the data field is specific to each
command or response; however, in most cases the format is designed to
be constructed and viewed by humans and so is coded in free-form
ASCII. This facilitates the specification and implementation of
simple management tools in the absence of fully evolved network-
management facilities. As in ordinary NTP messages, the
authenticator field follows the data field. If the authenticator is
used the data field is zero-padded to a 32-bit boundary, but the
padding bits are not considered part of the data field and are not
included in the field count.
IP hosts are not required to reassemble datagrams larger than 576
octets; however, some commands or responses may involve more data
than will fit into a single datagram. Accordingly, a simple
reassembly feature is included in which each octet of the message
data is numbered starting with zero. As each fragment is transmitted
the number of its first octet is inserted in the offset field and the
number of octets is inserted in the count field. The more-data (M)
bit is set in all fragments except the last.
Most control functions involve sending a command and receiving a
response, perhaps involving several fragments. The sender chooses a
distinct, nonzero sequence number and sets the status field and R and
E bits to zero. The responder interprets the opcode and additional
information in the data field, updates the status field, sets the R
bit to one and returns the three 32-bit words of the header along
with additional information in the data field. In case of invalid
message format or contents the responder inserts a code in the status
field, sets the R and E bits to one and, optionally, inserts a
diagnostic message in the data field.
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Some commands read or write system variables and peer variables for
an association identified in the command. Others read or write
variables associated with a radio clock or other device directly
connected to a source of primary synchronization information. To
identify which type of variable and association a 16-bit association
identifier is used. System variables are indicated by the identifier
zero. As each association is mobilized a unique, nonzero identifier
is created for it. These identifiers are used in a cyclic fashion,
so that the chance of using an old identifier which matches a newly
created association is remote. A management entity can request a
list of current identifiers and subsequently use them to read and
write variables for each association. An attempt to use an expired
identifier results in an exception response, following which the list
can be requested again.
Some exception events, such as when a peer becomes reachable or
unreachable, occur spontaneously and are not necessarily associated
with a command. An implementation may elect to save the event
information for later retrieval or to send an asynchronous response
(called a trap) or both. In case of a trap the IP address and port
number is determined by a previous command and the sequence field is
set as described below. Current status and summary information for
the latest exception event is returned in all normal responses. Bits
in the status field indicate whether an exception has occurred since
the last response and whether more than one exception has occurred.
Commands need not necessarily be sent by an NTP peer, so ordinary
access-control procedures may not apply; however, the optional mask/
match mechanism suggested elsewhere in this document provides the
capability to control access by mode number, so this could be used to
limit access for control messages (mode 6) to selected address
ranges.
A.1. NTP Control Message Format
The format of the NTP Control Message header, which immediately
follows the UDP header, is shown in Figure 7. Following is a
description of its fields. Bit positions marked as zero are reserved
and should always be transmitted as zero.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|00 | VN | 6 | REM | Op | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Association ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset | Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Data (468 Octets Max) .
. .
| | Padding (zeros) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authenticator (optional)(96) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: NTP Control Message Format
Version Number (VN): This is a three-bit integer indicating the NTP
version number, currently four (4)
Mode: This is a three-bit integer indicating the mode. It must have
the value 6, indicating an NTP control message.
Response Bit (R): Set to zero for commands, one for responses.
Error Bit (E): Set to zero for normal response, one for error
response.
More Bit (M): Set to zero for last fragment, one for all others.
Operation Code (Op): This is a five-bit integer specifying the
command function. Values currently defined are given in Table 8.
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+-------+----------------------------------------+
| Value | Meaning |
+-------+----------------------------------------+
| 0 | reserved |
| 1 | read status command/response |
| 2 | read variables command/response |
| 3 | write variables command/response |
| 4 | read clock variables command/response |
| 5 | write clock variables command/response |
| 6 | set trap address/port command/response |
| 7 | trap response |
| 8-31 | reserved |
+-------+----------------------------------------+
Table 8: Currently-defined Operation Codes
Sequence: This is a 16-bit integer indicating the sequence number of
the command or response.
Status: This is a 16-bit code indicating the current status of the
system, peer or clock, with values coded as described in following
sections.
Association ID: This is a 16-bit integer identifying a valid
association.
Offset: This is a 16-bit integer indicating the offset, in octets, of
the first octet in the data area.
Count: This is a 16-bit integer indicating the length of the data
field, in octets.
Data: This contains the message data for the command or response.
The maximum number of data octets is 468.
Authenticator (optional): When the NTP authentication mechanism is
implemented, this contains the authenticator information.
A.2. Status Words
Status words indicate the present status of the system, associations
and clock. They are designed to be interpreted by network-monitoring
programs and are in one of four 16-bit formats described in this
section. System and peer status words are associated with responses
for all commands except the read clock variables, write clock
variables and set trap address/port commands. The association
identifier zero specifies the system status word, while a nonzero
identifier specifies a particular peer association. The status word
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returned in response to read clock variables and write clock
variables commands indicates the state of the clock hardware and
decoding software. A special error status word is used to report
malformed command fields or invalid values.
A.2.1. System Status Word
The system status word appears in the status field of the response to
a read status or read variables command with a zero association
identifier. The format of the system status word is given in
Figure 8.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| LI | Clock Source | Count | Code |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 8: System Status Word Format
Leap Indicator (LI): This is a two-bit code warning of an impending
leap second to be inserted/deleted in the last minute of the current
day, with bit 0 and bit 1, respectively, coded as shown in Table 9.
+-------+------------------------------------------+
| Value | Meaning |
+-------+------------------------------------------+
| 00 | no warning |
| 01 | last minute has 61 seconds |
| 10 | last minute has 59 seconds |
| 11 | alarm condition (clock not synchronized) |
+-------+------------------------------------------+
Table 9: Leap Indicator Field
Clock Source: This is a six-bit integer indicating the current
synchronization source, with values coded as shown in Table 10.
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+-------+---------------------------------------------------------+
| Value | Meaning |
+-------+---------------------------------------------------------+
| 0 | unspecified or unknown |
| 1 | Calibrated atomic clock (e.g.,, HP 5061) |
| 2 | VLF (band 4) or LF (band 5) radio (e.g.,, OMEGA,, WWVB) |
| 3 | HF (band 7) radio (e.g.,, CHU,, MSF,, WWV/H) |
| 4 | UHF (band 9) satellite (e.g.,, GOES,, GPS) |
| 5 | local net (e.g.,, DCN,, TSP,, DTS) |
| 6 | UDP/NTP |
| 7 | UDP/TIME |
| 8 | wall time |
| 9 | telephone modem (e.g. NIST) |
| 10-31 | reserved |
| 32 | PPS signal |
| 33-63 | reserved |
+-------+---------------------------------------------------------+
Table 10: Clock Source Field Values
System Event Counter: This is a four-bit integer indicating the
number of system exception events occurring since the last time the
system status word was returned in a response or included in a trap
message. The counter is cleared when returned in the status field of
a response and freezes when it reaches the value 15.
System Event Code: This is a four-bit integer identifying the latest
system exception event, with new values overwriting previous values,
and coded as shown in Table 11.
+-------+-----------------------------------------------------------+
| Value | Meaning |
+-------+-----------------------------------------------------------+
| 0 | unspecified |
| 1 | system restart |
| 2 | system or hardware fault |
| 3 | system new status word (leap bits or synchronization |
| | change) |
| 4 | system new synchronization source or stratum (sys.peer or |
| | sys.stratum) change |
| 5 | system clock reset (offset correction exceeds CLOCK.MAX) |
| 6 | system invalid time or date |
| 7 | system clock exception (see system clock status word) |
| 8-15 | reserved |
+-------+-----------------------------------------------------------+
Table 11: System Event Code Values
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A.2.2. Peer Status Word
A peer status word is returned in the status field of a response to a
read status, read variables or write variables command and appears
also in the list of association identifiers and status words returned
by a read status command with a zero association identifier. The
format of a peer status word is shown in Figure 9.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Peer Status | Sel | Count | Code |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Peer Status Word
Figure 9: Peer Status Word Format
Peer Status: This is a five-bit code indicating the status of the
peer determined by the packet procedure, with bits assigned as shown
in Table 12.
+-------+------------------------------------------+
| Value | Meaning |
+-------+------------------------------------------+
| 0 | configured (peer.config) |
| 1 | authentication enabled (peer.authenable) |
| 2 | authentication okay (peer.authentic) |
| 3 | reachability okay (peer.reach) |
| 4 | reserved |
+-------+------------------------------------------+
Table 12: Peer Status Values
Peer Selection (Sel): This is a three-bit integer indicating the
status of the peer determined by the clock-selection procedure, with
values coded as shown in Table 13.
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+-------+-----------------------------------------------------------+
| Value | Meaning |
+-------+-----------------------------------------------------------+
| 0 | rejected |
| 1 | passed sanity checks |
| 2 | passed correctness checks |
| 3 | passed candidate checks (if limit check implemented) |
| 4 | passed outlyer checks |
| 5 | current synchronization source; max distance exceeded (if |
| | limit check implemented) |
| 6 | current synchronization source; max distance okay |
| 7 | reserved |
+-------+-----------------------------------------------------------+
Table 13: Peer Selection Field Values
Peer Event Counter: This is a four-bit integer indicating the number
of peer exception events that occurred since the last time the peer
status word was returned in a response or included in a trap message.
The counter is cleared when returned in the status field of a
response and freezes when it reaches the value 15.
Peer Event Code: This is a four-bit integer identifying the latest
peer exception event, with new values overwriting previous values,
and coded as shown in Table 14.
+-------+-----------------------------------------------------------+
| Value | Meaning |
+-------+-----------------------------------------------------------+
| 0 | unspecified |
| 1 | peer IP error |
| 2 | peer authentication failure (peer.authentic bit was one |
| | now zero) |
| 3 | peer unreachable (peer.reach was nonzero now zero) |
| 4 | peer reachable (peer.reach was zero now nonzero) |
| 5 | peer clock exception (see peer clock status word) |
| 6-15 | reserved |
+-------+-----------------------------------------------------------+
Table 14: Peer Event Codes
A.2.3. Clock Status Word
There are two ways a reference clock can be attached to a NTP service
host, as an dedicated device managed by the operating system and as a
synthetic peer managed by NTP. As in the read status command, the
association identifier is used to identify which one, zero for the
system clock and nonzero for a peer clock. Only one system clock is
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supported by the protocol, although many peer clocks can be
supported. A system or peer clock status word appears in the status
field of the response to a read clock variables or write clock
variables command. This word can be considered an extension of the
system status word or the peer status word as appropriate. The
format of the clock status word is shown in Figure 10.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Clock Status | Code |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 10: Clock Status Word Format
Clock Status: This is an eight-bit integer indicating the current
clock status, with values coded as shown in Table 15.
+-------+----------------------------+
| Value | Meaning |
+-------+----------------------------+
| 0 | clock nominally operating |
| 1 | reply timeout |
| 2 | bad reply format |
| 3 | hardware or software fault |
| 4 | propagation failure |
| 5 | bad date format or value |
| 6 | bad time format or value |
| 7-255 | reserved |
+-------+----------------------------+
Table 15: Clock Status Values
Clock Event Code: This is an eight-bit integer identifying the latest
clock exception event, with new values overwriting previous values.
When a change to any nonzero value occurs in the radio status field,
the radio status field is copied to the clock event code field and a
system or peer clock exception event is declared as appropriate.
A.2.4. Error Status Word
An error status word is returned in the status field of an error
response as the result of invalid message format or contents. Its
presence is indicated when the E (error) bit is set along with the
response (R) bit in the response. It consists of an eight-bit
integer coded as shown in Figure 11.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Error Code | Reserved |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 11: Error Status Word Format
Currently-defined error codes are given in Table 16.
+-------+----------------------------------+
| Value | Meaning |
+-------+----------------------------------+
| 0 | unspecified |
| 1 | authentication failure |
| 2 | invalid message length or format |
| 3 | invalid opcode |
| 4 | unknown association identifier |
| 5 | unknown variable name |
| 6 | invalid variable value |
| 7 | administratively prohibited |
| 8-255 | reserved |
+-------+----------------------------------+
Table 16: Error Code Values
A.3. Commands
Commands consist of the header and optional data field of the Status
Word. When present, the data field contains a list of identifiers or
assignments in the form
<<identifier>>[=<<value>>],<<identifier>>[=<<value>>],...
where <<identifier>> is the ASCII name of a system or peer variable
specified in Table 2 or Table 3 of [1] and <<value>> is expressed as
a decimal, hexadecimal or string constant in the syntax of the C
programming language. Where no ambiguity exists, the <169>sys.<170>
or <169>peer.<170> prefixes shown in Table 2 or Table 4 of [1] can be
suppressed. Whitespace (ASCII nonprinting format effectors) can be
added to improve readability for simple monitoring programs that do
not reformat the data field. Internet addresses are represented as
four octets in the form [n.n.n.n], where n is in decimal notation and
the brackets are optional. Timestamps, including reference,
originate, receive and transmit values, as well as the logical clock,
are represented in units of seconds and fractions, preferably in
hexadecimal notation, while delay, offset, dispersion and distance
values are represented in units of milliseconds and fractions,
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preferably in decimal notation.All other values are represented
as-is, preferably in decimal notation.
Implementations may define variables other than those listed in Table
2 or Table 3 of [1]. Called extramural variables, these are
distinguished by the inclusion of some character type other than
alphanumeric or <169>.<170> in the name. For those commands that
return a list of assignments in the response data field, if the
command data field is empty, it is expected that all available
variables defined in Table 3 or Table 4 of [1] will be included in
the response. For the read commands, if the command data field is
nonempty, an implementation may choose to process this field to
individually select which variables are to be returned.
Commands are interpreted as follows:
Read Status (1): The command data field is empty or contains a list
of identifiers separated by commas. The command operates in two ways
depending on the value of the association identifier. If this
identifier is nonzero, the response includes the peer identifier and
status word. Optionally, the response data field may contain other
information, such as described in the Read Variables command. If the
association identifier is zero, the response includes the system
identifier (0) and status word, while the data field contains a list
of binary-coded pairs
<<association identifier>> <<status word>>,
one for each currently defined association.
Read Variables (2): The command data field is empty or contains a
list of identifiers separated by commas. If the association
identifier is nonzero, the response includes the requested peer
identifier and status word, while the data field contains a list of
peer variables and values as described above. If the association
identifier is zero, the data field contains a list of system
variables and values. If a peer has been selected as the
synchronization source, the response includes the peer identifier and
status word; otherwise, the response includes the system identifier
(0) and status word.
Write Variables (3): The command data field contains a list of
assignments as described above. The variables are updated as
indicated. The response is as described for the Read Variables
command.
Read Clock Variables (4): The command data field is empty or contains
a list of identifiers separated by commas. The association
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identifier selects the system clock variables or peer clock variables
in the same way as in the Read Variables command. The response
includes the requested clock identifier and status word and the data
field contains a list of clock variables and values, including the
last timecode message received from the clock.
Write Clock Variables (5): The command data field contains a list of
assignments as described above. The clock variables are updated as
indicated. The response is as described for the Read Clock Variables
command.
Set Trap Address/Port (6): The command association identifier, status
and data fields are ignored. The address and port number for
subsequent trap messages are taken from the source address and port
of the control message itself. The initial trap counter for trap
response messages is taken from the sequence field of the command.
The response association identifier, status and data fields are not
significant. Implementations should include sanity timeouts which
prevent trap transmissions if the monitoring program does not renew
this information after a lengthy interval.
Trap Response (7): This message is sent when a system, peer or clock
exception event occurs. The opcode field is 7 and the R bit is set.
The trap counter is incremented by one for each trap sent and the
sequence field set to that value. The trap message is sent using the
IP address and port fields established by the set trap address/port
command. If a system trap the association identifier field is set to
zero and the status field contains the system status word. If a peer
trap the association identifier field is set to that peer and the
status field contains the peer status word. Optional ASCII-coded
information can be included in the data field.
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Authors' Addresses
Jack Burbank (editor)
The Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723-6099
US
Phone: +1 443 778 7127
Email: jack.burbank@jhuapl.edu
Jim Martin (editor)
Netzwert AG
An den Treptowers 1
Berlin 12435
Germany
Phone: +49.30/5 900 80-1180
Email: jim@netzwert.ag
Dr. David L. Mills
University of Delaware
Newark, DE 19716
US
Phone: +1 302 831 8247
Email: mills@udel.edu
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