Network Time Protocol Version 5
draft-ietf-ntp-ntpv5-05
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| Last updated | 2025-07-02 (Latest revision 2025-04-10) | ||
| Replaces | draft-mlichvar-ntp-ntpv5 | ||
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draft-ietf-ntp-ntpv5-05
Internet Engineering Task Force M. Lichvar
Internet-Draft Red Hat
Intended status: Standards Track T. Mizrahi
Expires: 3 January 2026 Huawei
2 July 2025
Network Time Protocol Version 5
draft-ietf-ntp-ntpv5-05
Abstract
The Network Time Protocol (NTP) is a widely deployed protocol that
allows hosts to obtain the current time of day from time servers.
This document specifies version 5 of the protocol (NTPv5), which
adopts a client-server model as its sole mode of operation. Legacy
operational modes supported in earlier versions have been removed to
improve protocol robustness and clarity. While this specification
defines the protocol used for time distribution, it does not define
the algorithms or heuristics employed by clients to determine or
adjust their local time.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 January 2026.
Copyright Notice
Copyright (c) 2025 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 (https://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
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and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Main Differences between NTPv5 and NTPv4 . . . . . . . . . . 4
4. Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. The NTP Message Exchange . . . . . . . . . . . . . . . . 6
4.2. Hierarchical Time Distribution . . . . . . . . . . . . . 7
4.3. Leap Seconds . . . . . . . . . . . . . . . . . . . . . . 8
5. Data Types . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Message Format . . . . . . . . . . . . . . . . . . . . . . . 9
7. Extension Fields . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Draft Identification Extension Field . . . . . . . . . . 14
7.2. Padding Extension Field . . . . . . . . . . . . . . . . . 15
7.3. Message Authentication Code Extension Field . . . . . . . 15
7.4. Reference IDs Request and Response Extension Fields . . . 15
7.5. Server Information Extension Field . . . . . . . . . . . 18
7.6. Correction Extension Field . . . . . . . . . . . . . . . 19
7.7. Reference Timestamp Extension Field . . . . . . . . . . . 22
7.8. Monotonic Receive Timestamp Extension Field . . . . . . . 22
7.9. Secondary Receive Timestamp Extension Field . . . . . . . 23
8. Measurement Modes . . . . . . . . . . . . . . . . . . . . . . 24
9. Client Operation . . . . . . . . . . . . . . . . . . . . . . 27
10. Server Operation . . . . . . . . . . . . . . . . . . . . . . 29
11. Network Time Security with NTPv5 . . . . . . . . . . . . . . 32
12. NTPv5 Negotiation in previous NTP versions . . . . . . . . . 33
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
15. Security Considerations . . . . . . . . . . . . . . . . . . . 35
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
16.1. Normative References . . . . . . . . . . . . . . . . . . 36
16.2. Informative References . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
The Network Time Protocol (NTP) is a widely used protocol that
enables hosts to synchronize their clocks over packet-switched,
variable-latency data networks. Time is distributed hierarchically,
beginning with primary time servers, often synchronized to
Coordinated Universal Time (UTC) via external sources such as GNSS,
and propagating through a network of clients. These clients may
themselves act as servers for downstream systems, forming a time
distribution tree. To enhance reliability, stability, and accuracy,
clients can query multiple servers concurrently and apply local
algorithms to select and combine time sources.
This document specifies version 5 of the protocol (NTPv5), which
defines only the on-the-wire protocol used for time exchange. It
does not cover client-side algorithms such as source selection,
filtering of time measurements, or clock discipline. These aspects
are considered out of scope.
NTPv5 adopts a simplified client-server model as its sole operational
mode. For security and robustness, legacy modes from previous
versions, such as symmetric active, symmetric passive, broadcast,
control, and private modes, have been removed. Only client and
server modes are retained in this version.
To support optional features and future extensibility, NTPv5 makes
use of extension fields. This document defines several such fields,
which may be included in protocol messages to convey additional
information beyond the core header.
NTPv5 supports secure operation through two possible mechanisms
originally defined for NTPv4. The first is the NTP Message
Authentication Code (MAC) [RFC8573] mechanism, which provides basic
message integrity. The second is Network Time Security (NTS)
[RFC8915], which offers stronger security guarantees, including
server authentication, replay protection, and secure key
establishment.
This specification also introduces a feature aimed at improving
synchronization accuracy. The Correction Extension Field allows
clients and servers to communicate variable delays introduced by
intermediate network devices such as switches and routers.
Backward compatibility is supported through version negotiation. A
server that implements multiple protocol versions responds using the
same version as the client's request, provided that version is
supported.
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2. Conventions
2.1. Terminology
Abbreviations used in this document:
MAC Message Authentication Code
NTP Network Time Protocol
NTS Network Time Security
NTS-KE Network Time Security Key Establishment
ppm parts per million
PTP Precision Time Protocol
SI International System of Units
TAI International Atomic Time
TC Transparent Clock
UT1 Universal Time 1
UTC Coordinated Universal Time
2.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Main Differences between NTPv5 and NTPv4
NTPv5 is very similar to NTPv4 [RFC5905]. The main differences are:
1. The protocol specification (this document) describes only the
on-wire protocol. Filtering of measurements, security
mechanisms, source selection, clock control, and other
algorithms, are out of scope.
2. For security reasons, NTPv5 drops support for the symmetric
active, symmetric passive, broadcast, control, and private
modes. The symmetric and broadcast modes are vulnerable to
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replay attacks. The control and private modes can be exploited
for denial-of-service traffic amplification attacks. Only the
client and server modes remain in NTPv5.
3. The NTPv5 message format differs from that of NTPv4. However,
the Version Number field remains at the same offset in both
formats, enabling protocol implementations to distinguish
between the two versions.
4. Timestamps are clearly separated from values used as cookies.
5. Synchronized server clock is indicated in a separate flag
instead of the leap indicator.
6. NTPv5 messages can be extended only with extension fields. The
MAC field is wrapped in an extension field to avoid an ambiguity
in parsing of the NTP message, which was later addressed for
NTPv4 in RFC 7822 [RFC7822].
7. Extension fields can be of any length, even indivisible by 4,
but are padded to a multiple of 4 octets. Extension fields
specified for NTPv4 can be included in NTPv5 messages as
specified for NTPv4.
8. NTPv5 adds support for other timescales than UTC.
9. The NTP era number is exchanged in the protocol, which extends
the unambiguous time interval from 136 years to about 35000
years.
10. NTPv5 adds a flag to clearly identify the use of interleaved
mode instead of comparing timestamps or cookies. The server's
receive timestamps do not need to be unique in order to support
the interleaved mode.
11. NTPv5 works with sets of reference IDs to prevent
synchronization loops over multiple hosts, even if they form
over other NTP sources than the system peer. Reference IDs have
120 bits instead of 32 bits to minimize the rate of false
positives.
12. Resolution of the root delay and root dispersion fields is
improved from about 15 microseconds to about 4 nanoseconds.
13. Clients do not leak information about their clock (e.g.
timestamps, estimated accuracy).
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4. Basic Concepts
4.1. The NTP Message Exchange
An NTP client exchanges messages with one or more NTP servers; the
client sends a request and the server sends a response. Both the
client and server measure the time of transmission and reception of
every message they send and receive. The servers provide their
timestamps to the client.
The NTP message exchange is illustrated in Figure 1, depicting four
timestamp values:
1. T1 - client's transmit timestamp of the request
2. T2 - server's receive timestamp of the request
3. T3 - server's transmit timestamp of the response
4. T4 - client's receive timestamp of the response
Server T2 T3
------------+----+--------
/ \
request / \ response ---> time
/ \
--------+------------+-----
Client T1 T4
Figure 1: NTP message exchange
The timestamps T1 and T4 are recorded locally by the client and are
not transmitted over the network. In contrast, the server includes
timestamps T2 and T3 in its response to the client. As a result, by
the end of the exchange, the client has access to all four
timestamps, commonly referred to as a sample.
The client can use the set of four timestamps to estimate both the
clock offset relative to the server and the network delay. The
offset of the server clock relative to the client clock can be
calculated using Eq. (1), and the two-way delay between the client
and server can be estimated using Eq. (2).
(1) offset = ((T2 + T3) - (T4 + T1)) / 2
(2) delay = |(T4 - T1) - (T3 - T2)|
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The offset and delay estimates given in the two equations above are
based on a single timestamp sample. In practice, a client typically
maintains ongoing estimates of offset and delay by aggregating
multiple samples collected over several message exchanges.
4.2. Hierarchical Time Distribution
NTP supports a hierarchical time distribution scheme. This hierarchy
is defined by stratum levels, which indicate the distance from the
reference time source. Stratum 1 servers are directly synchronized
with an external reference clock. Servers that synchronize with
stratum 1 servers are classified as stratum 2, and the hierarchy
continues in this manner.
An NTP response message contains two fields, root delay and root
dispersion, which together provide an estimate of the server's time
error accumulated along the synchronization path.
Root delay represents the cumulative delay along the path to the
reference time source used by the primary time server. Each server
in the synchronization chain adds its own delay estimate based on the
server it deems most accurate. This estimate includes both the
measured two-way delay towards the server and any processing delays
between the timestamping and actual sending or receiving of NTP
messages. To estimate the potential error caused by asymmetric
delays, half of the root delay is typically used as a bound on the
clock's maximum error.
Root dispersion provides an estimate, in time units, of the maximum
potential error in the clock due to both the instability of clocks
along the synchronization path and the variability in NTP
measurements. Each server in the chain contributes its own
dispersion value to the cumulative root dispersion. The method for
estimating dispersion can vary between implementations and often
depends on the underlying clock model. In a simple model, dispersion
may be calculated using a constant Dispersion Rate (DR), as shown in
Equation (3). The constant DR represents a relative frequency error,
typically within the range (0, 1). For instance, a DR value of
0.000015 (15 ppm) is suggested in [RFC5905].
(3) dispersion = |T4 - T1| * DR
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The root distance is defined as the sum of the root dispersion and
half of the root delay. It represents an estimate of the maximum
possible time error of the clock, accounting for the assumed clock
stability and the worst-case scenario of asymmetric network delays.
Although root distance is not transmitted in NTP messages, it can be
computed and used by the client to assess the accuracy of time
sources.
A synchronization loop can occur when clients attempt to synchronize
with each other--either directly or indirectly through other servers.
Such loops are undesirable in an NTP network, as they create positive
feedback cycles that can degrade synchronization stability. To help
detect and prevent these loops, servers use randomly generated
reference IDs, which are exchanged in NTP messages as described in
Section 7.4.
4.3. Leap Seconds
An NTPv5 server can distribute time using one of four timescales:
UTC, TAI, UT1, or leap-smeared UTC. The specific timescale in use is
indicated by the Timescale field in the NTP message. Of these
options, UTC and leap-smeared UTC are subject to leap second
adjustments.
A leap second is a one-second adjustment occasionally applied to UTC
to maintain alignment with solar time. This adjustment can be either
positive, by inserting an extra second, or negative, by removing one.
To date, only positive leap seconds have been introduced. Therefore,
the following discussion focuses on the more common case of a
positive leap second, though the behavior of a negative leap second
can be inferred.
When time is distributed using the UTC timescale, a system clock may
handle a positive leap second in one of two ways: it may either roll
back by one second at the end of the leap second, or it may pause,
holding the same time, for the duration of the leap second, as
outlined in [RFC8877]. As a result, any NTP message exchange that
occurs outside of a leap second will yield timestamps that reflect
server's clock according to the actual UTC time.
When using the leap-smeared timescale, the system clock is gradually
slowed down during the leap second and surrounding time intervals,
allowing the time to smoothly adjust until it aligns with the correct
value. This adjustment period, known as a leap smear, can span from
a few seconds to several hours before, during, and/or after the
scheduled leap second.
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The Leap Indicator (LI) field in an NTP message signals that a leap
second is scheduled to occur within the next 14 days.
5. Data Types
NTPv5 uses few different data types. They are all in the network
order. Beside signed and unsigned integers, it has also the
following fixed-point types:
time32
A 32-bit unsigned fixed-point type containing values in seconds.
It has 4 bits describing the unsigned integral part and 28 bits
describing the fractional part. The maximum value is 16 seconds
and the resolution is about 3.7 nanoseconds. Note that this is
different from the 32-bit time format in NTPv4.
timestamp64
A 64-bit unsigned fixed-point type containing a timestamp
specified in seconds. It has 32 signed integer bits and 32
fractional bits. It spans an interval of about 136 years and has
a resolution of about 0.23 nanoseconds. It can be used in
different timescales. In the UTC timescale it is the number of SI
seconds since 1 Jan 1972 plus 2272060800 (number of seconds since
1 Jan 1900 assuming 86400-second days), excluding leap second
adjustments. Timestamps in the TAI timescale are the same except
they include leap seconds and extra 10 seconds for the original
difference between TAI and UTC in 1972, when leap seconds were
introduced. A value of 0 indicates an unknown or invalid
timestamp. One interval covered by the type is called an NTP era.
The era starting at the epoch is era number 0, the following era
is number 1, and so on.
Some fields use a logarithmic scale, where an 8-bit signed integer
represents the rounded log2 value of seconds. For example, a log2
value of 4 is 2^4 (2 to the power of 4, 16) seconds, or a log2 value
of -2 is 2^-2 (0.25 seconds).
6. Message Format
NTPv5 servers and clients exchange messages as UDP datagrams.
Clients send requests to servers and servers send them back
responses. The server's UDP port in NTP messages is 123, as assigned
by IANA. The client's UDP port can be any number consistent with the
local policy. The format of the UDP payload is shown 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 | Stratum | Poll | Precision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timescale | Era | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Dispersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Server Cookie (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Client Cookie (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receive Timestamp (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Transmit Timestamp (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Extension Field 1 (variable) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Extension Field N (variable) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of NTPv5 messages
Each NTPv5 message has a header containing the following fields:
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Leap indicator (LI)
A 2-bit field indicating upcoming leap seconds. In requests it is
always 0. In responses it has one of the following values:
0: Normal
No leap second will be inserted or deleted in the next 14 days,
or the server is responding in a leap-smeared timescale (i.e.
the client is not expected to be handling leap seconds).
1: Insert leap second
A leap second will be inserted in the next 14 days at the end
of the current month.
2: Delete leap second
A leap second will be deleted in the next 14 days at the end of
the current month.
3: Unknown
The server does not have a time source or other source
providing information about leap seconds.
Version Number (VN)
A 3-bit field containing the value 5.
Mode
A 3-bit field containing the value 3 (request) or 4 (response).
Stratum
An 8-bit field containing the stratum of the server. Primary time
servers have a stratum of 1, their clients have a stratum of 2,
and so on. The value of 0 indicates an unknown or infinite
stratum. In requests it is always 0. When 0 in a response, it
indicates the server was unable or unwilling to provide valid time
information. Servers advertising a stratum above 16 should not be
synchronized to, except when the client is explicitly configured
to do so by the end-user.
Poll
An 8-bit signed integer containing the minimum allowed polling
interval as a log2 value in seconds. In requests it is always 0.
In responses it is the server's value that the client is expected
to follow. This field is valid even when stratum is 0. A value
of 127 indicates the server does not want to hear from the client
again. Note that the poll value has a different meaning than in
NTPv4. It supersedes the NTPv4 Kiss-o'-Death (KoD) RATE and DENY
codes.
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Precision
An 8-bit signed integer containing the precision of the timestamps
included in the message as a rounded log2 value in seconds. In
requests, which do not contain any timestamps, it is always 0.
Timescale
An 8-bit identifier of the timescale. In requests it is the
requested timescale. In responses it is the timescale of the
receive and transmit timestamps. Defined values are:
0: UTC
1: TAI
2: UT1
3: Leap-smeared UTC
Era
An 8-bit unsigned NTP era number corresponding to the receive
timestamp. In requests it is always 0.
Flags
A 16-bit integer that can contain the following flags:
0x1: Synchronized
In requests it is 0. In responses a value of 1 indicates the
server's clock is synchronized and the provided timestamps can
be used by the client for its own synchronization.
0x2: Interleaved mode
In requests a value of 1 is a request for a response in the
interleaved mode. In responses a value of 1 indicates the
response is in the interleaved mode.
0x4: Authentication NAK
In requests it is 0. In responses a value of 1 indicates that
the server failed to authenticate the request. A server
setting this bit SHOULD also set the stratum of the message to
0.
Root Delay
A field using the time32 type. In responses it is the server's
root delay. In requests it is always 0.
Root Dispersion
A field using the time32 type. In responses it is the server's
root dispersion. In requests it is always 0.
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Server Cookie
A 64-bit field containing a number generated by the server which
enables the interleaved mode. In requests it is 0, or a copy of
the server cookie from the last response.
Client Cookie
A 64-bit field containing a random number generated by the client.
Responses contain a copy of the field from the corresponding
request, which allows the client to verify that the responses are
related to the requests.
Receive Timestamp
A field using the timestamp64 type. In requests it is always 0.
In responses it is the time when the request was received by the
server. The timestamp corresponds to the end of the reception.
Transmit Timestamp
A field using the timestamp64 type. In requests it is always 0.
In responses it is the server's time denoting the beginning of the
transmission of a response to the client. Which response it
refers to depends on the selected mode (basic or interleaved).
See Measurement Modes (Section 8) for detail.
The header has 48 octets, which is the minimum length of a valid
NTPv5 message. A message can contain optional extension fields (zero
or more). The maximum length is not specified, but the length MUST
be divisible by 4 octets.
7. Extension Fields
The format of NTPv5 extension fields is shown in 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Data (variable) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of NTPv5 extension fields
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Each extension field has a header which contains a 16-bit type and
16-bit length. The length is in octets and it includes the header.
The minimum length is 4, i.e. an extension field does not have to
contain any data. If the length is not divisible by 4, the extension
field is padded with zeros to the smallest multiple of 4 octets.
If a request contains an extension field, the server MUST include
this extension field in the response unless the specification of the
extension field states otherwise, or the server does not support the
extension field. If the server excludes one or more extension fields
in the response, it MUST include a Padding extension field that
compensates for the excluded fields, making the request and response
symmetric in length. A client can interpret the absence of an
expected extension field in a response as an indication that the
server does not support the extension field.
Extension fields specified for NTPv4 can be included in NTPv5
messages as specified for NTPv4.
The rest of this section describes extension fields specified for
NTPv5. Clients are not required to use or support any of these
extension fields, but servers are required to support the following
extension fields: Padding, Server Information, Reference IDs Request,
Reference IDs Response.
7.1. Draft Identification Extension Field
Note to the editors: this section must be removed before final
publication.
This field, with type 0xF5FF, is used to indicate which draft of the
specification an implementation is based upon. It MUST be included
in NTPv5 requests produced by an implementation based on a draft of
this specification, and MUST NOT be included in NTPv5 requests
produced by an implementation based on the final version of this
specification. Server MUST use this field if and only if responding
to a request containing this field and the server is a draft
implementation.
The contents of this field MUST be the full name, including version
number, of the draft upon which the implementation is based, encoded
as an ASCII string. If the server string is longer than the client
string, the server MUST NOT respond in that version to prevent
traffic amplification.
A server MUST NOT respond to requests with a draft identification it
does not recognize. If it responds, it SHOULD respond according to
the same draft specification as given by the client.
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Note: the content of this field MUST NOT be null-terminated (the
extension field in the NTP message may need to be padded with up to 3
octets). When comparing the strings be compared in their full
length, i.e. a longer string containing a shorter string is not
sufficient.
7.2. Padding Extension Field
This field, with type [[TBD]] (draft: 0xF501), can have any length.
The data field within this extension field SHOULD contain zeros and
it MUST be ignored by the receiver.
Servers can use this extension field to pad the response to match the
length of the request if the response does not contain all requested
extension fields, or some have a variable length. If the request
message includes a padding extension, the server can increase its
length if necessary. If the request message does not include a
padding extension, the server can add it to the response. Clients
can include the padding extension in the request message, allowing
the extension to be used for internal purposes. For instance, an NTP
client receiving a response message can use this extension field to
transfer the reception time from a hardware module to a software
module.
This field MUST be supported on servers.
7.3. Message Authentication Code Extension Field
This field, with type [[TBD]] (draft: 0xF502), authenticates the
NTPv5 message with a symmetric key. Implementations SHOULD use the
Message Authentication Code (MAC) specified in RFC8573 [RFC8573].
The MAC MUST be computed over the NTP header and all the extension
fields, if present, located prior to the Message Authentication
Extension Field. The extension field MUST be the last extension
field in the message unless an extension field is specifically
allowed to be placed after a MAC or another authenticator field, such
as the Correction Extension Field (Section 7.6).
7.4. Reference IDs Request and Response Extension Fields
Each NTPv5 server has a randomly generated 120-bit reference ID (it
will be split into 10 12-bit values). The extension fields described
in this section are used to exchange sets of reference IDs in order
to detect synchronization loops, i.e. when a client is synchronizing
(directly or indirectly) to one of its own clients, or more generally
detect presence of a specific time source in the synchronization
chain.
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As each client can be synchronized to an unlimited number of servers
(and there can be up to 15 strata of servers), the reference IDs are
exchanged as a Bloom filter [Bloom] instead of a list to limit the
amount of data that needs to be exchanged.
The Bloom filter is an array of 4096 bits. When empty, all bits are
zero. To add a reference ID to the filter, the 120-bit value of the
reference ID is split into 10 12-bit values and the bits of the array
at the 10 positions given by the 12-bit values are set to one.
A server maintains a copy of the filter for each server it acquires
time from. The filter provided by the server to clients is the union
of the filters (using the bitwise OR operation) of the server's
sources selected for synchronization and the server's own reference
ID.
If the server uses a previous version of NTP for some of its sources,
the reference IDs added to the filter are generated from their IP
addresses as the first 120 bits of the MD5 [RFC1321] sum of the
address in network order. If the server uses a reference clock, the
reference ID is the first 120 bits of the MD5 sum of the 4-octet
zero-padded ASCII string from the NTP Reference Identifier Codes
registry maintained by IANA, or a string beginning with the uppercase
letter X, which are reserved for private and experimental use.
A client checking whether the server's set of reference IDs contains
the client's own reference ID checks whether the bits at the 10
positions corresponding to the 12-bit values from the reference ID
are all set to one.
When a client that also serves time to other clients as an NTPv5
server detects its reference ID in a server's set of reference IDs,
it SHOULD assume it is in a synchronization loop, and stop using that
server for synchronization. When the client's reference ID is no
longer detected in the server's filter, it SHOULD wait for a random
number of polling intervals (e.g. between 0 and 4) before selecting
the server again. The random delay helps with stabilization of the
selection in longer loops. If a recurring loop is detected, it is
recommended to increase the random delay in order to avoid livelock
scenarios.
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False positives are possible. The probability of a collision grows
with the number of reference IDs in the filter. With 26 reference
IDs it is about 1e-12. With 118 IDs it is about 1e-6. The client
MAY avoid selecting a server which has too many bits set in the
filter (e.g. more than half) to reduce the probability of the
collision for its own clients. A client which detected a
synchronization loop MAY change its own reference ID to limit the
duration of the potential collision.
Bloom filters are free from false negatives. However, there is a
transient period between the addition of a reference ID to a server's
Bloom filter and the propagation of this information through a chain
of clients. During this period, the new reference ID may not be
detected, potentially causing a temporary synchronization loop. For
instance, if two servers inquire each other about the list of
reference IDs roughly at the same time, neither will detect its
reference ID in the other's Bloom filter, resulting in a temporary
loop.
Generally, when a server updates the Bloom filter it distributes to
clients, temporary synchronization loops might occur before
converging to an acyclic distribution tree.
The filter can be exchanged as a single 512-octet array, or it can be
exchanged in smaller chunks over multiple NTP messages, making them
shorter, but delaying the detection of the synchronization loop.
The request extension field specifies the offset of the requested
chunk in the filter as a number of octets. The requested length of
the chunk is given by the length of data in the request extension
field. The response extension field MUST have the same length as the
request extension field. If the request contains an invalid offset
for the length, i.e. it is larger than 512 minus length of data in
the extension field, the extension field MUST be ignored.
When a filter is sent to a client in multiple chunks, the server
might update the Bloom filter to a new value after some chunks have
been sent. This can cause the subsequent chunks to be inconsistent
with the previously sent ones. The partially updated Bloom filter
might cause false negative or false positive detections. This
transient issue is resolved once the server completes sending the
updated Bloom filter.
The client SHOULD use requests of a constant length for the
association to avoid adding a variation to the measured NTP delay.
The format of the Reference IDs Request is shown in Figure 4.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF503) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
. .
. Padding (variable) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of Reference IDs Request Extension Field
The format of the Reference IDs Response is shown in Figure 5.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF504) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Bloom filter chunk (variable) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Format of Reference IDs Response Extension Field
These fields MUST be supported on servers which can be synchronized
to other NTP servers (i.e. they can be in a synchronization loop).
7.5. Server Information Extension Field
This field provides clients with information about which NTP versions
are supported by the server, i.e. whether it can respond to requests
conforming to the specific version. It contains a 16-bit field with
flags indicating support for NTP versions in the range of 1 to 16,
where the least significant bit corresponds to the version 1. The
extension field has a fixed length of 8 octets. In requests, all
data fields of the extension are 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF505) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported NTP versions | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 6: Format of Server Information Extension Field
This field MUST be supported on servers.
7.6. Correction Extension Field
Processing and queueing delays in network switches and routers may be
a significant source of jitter and asymmetry in network delay, which
has a negative impact on accuracy and stability of clocks
synchronized by NTP. A solution to this problem is defined in the
Precision Time Protocol (PTP) [IEEE1588], which is a different
protocol for synchronization of clocks in networks. In PTP a special
type of switch or router, called a Transparent Clock (TC), updates a
correction field in PTP messages to account for the time messages
spend in the TC. This is accomplished by timestamping the message at
the ingress and egress ports, taking the difference to determine time
in the TC and adding this to the Delay Correction. Clients can
account for the accumulated Delay Correction to determine a more
accurate clock offset and network delay.
The NTPv5 Delay Correction has the same format as the PTP
correctionField to make it easier for manufacturers of switches and
routers to implement NTP corrections. The format of the Correction
Extension Field is shown in Figure 7.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF506) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Origin Correction +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Origin Path ID | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Delay Correction +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay Path ID | Checksum Complement |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format of Correction Extension Field
Field Type
The type which identifies the Correction extension field (value
TBD).
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Length
The length of the extension field, which is 28 octets.
Origin Correction
A field which contains a copy of the accumulated delay correction
from the request packet in the NTP exchange.
Origin Path ID
A field which contains a copy of the final path ID from the
request packet in the NTP exchange.
Reserved
16 bit reserved for future specification by the IETF. Transmit
with all zeros.
Delay Correction
A signed fixed-point number of nanoseconds with 48 integer bits
and 16 binary fractional bits, which represents the current
correction of the network delay that has accumulated for this
packet on the path from the source to the destination. A value of
one in all bits, except the most significant, of the field
indicates that the correction is too big to be represented. The
format of this field is identical to the PTP correctionField.
Path ID
A 16-bit identification number of the path where the delay
correction was updated.
Checksum Complement
A field which can be modified in order to keep the UDP checksum of
the packet valid. This allows the UDP checksum to be transmitted
before the Correction Field is received and modified. The same
field is described in RFC 7821 [RFC7821].
An NTP client with enabled support for network corrections SHOULD add
the Correction Extension Field to its requests, with all fields of
the extension field set to zero. It MUST be the last extension field
in the NTP message.
A network device forwarding packets (e.g. a switch or router) with
enabled support for NTP corrections MUST modify only packets which
meet all of the following requirements:
1. It is an IPv4 or IPv6 packet
2. The IP protocol number is 17 (UDP)
3. The UDP source port or destination port is 123 (NTP)
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4. The NTP version is 5
5. The NTP message contains the Correction Extension Field
The network device SHOULD add the difference between the beginning of
the NTP message retransmission and the end of the message reception
to the Delay Correction value in the Correction Extension Field.
Note that this time difference might be negative, e.g. in a cut-
through switch. If the packet is transmitted at the same speed as it
was received and the length of the packet does not change (e.g. due
to adding or removing a VLAN tag), the beginning and end of the
interval may correspond to any point of the reception and
transmission as long as it is consistent for all forwarded packets of
the same length. If the transmission speed or length of the packet
is different, the beginning and end of the interval SHOULD correspond
to the end of the reception and beginning of the transmission
respectively.
The receive timestamp of the ingress port and transmit timestamp of
the egress port MUST be from the same clock, or two clocks that are
synchronized to each other. The clocks do not need to be
synchronized to an external reference if their frequency is accurate
enough for the accuracy of measured NTP delay required by the
application. The correction field is updated before or during the
transmission of the message. It is a one-step end-to-end transparent
clock in the PTP terminology.
The network device SHOULD have a randomly generated 16-bit ID number
assigned to each of its ports. When it modifies the Correction
Extension Field, it SHOULD update the Path ID field of the extension
field by adding to it the values of the incoming and outgoing port
ID. The Path ID values can be used by clients to determine if the
NTP request and response have likely traversed the same network path.
If the network device modified any fields of the Correction Extension
Field, it MUST also update the Checksum Complement field in order to
keep the existing UDP checksum valid, or update the UDP checksum in
the UDP header itself. The network device MUST NOT modify any other
data in the UDP payload.
If an NTP server supports the Correction Extension Field and receives
a request which contains this extension field, it SHOULD include the
extension field in the response. If it is included, it MUST be the
last extension field in the message. It MUST copy the Delay
Correction and Path ID from the request to the Origin Correction and
Origin Path ID fields in the response respectively. It SHOULD set
the Delay Correction and Path ID fields of the response to zero.
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The Correction Extension Field is required to be the last extension
field in the message to provide an indicator at a fixed offset from
the ending of the message reception (which might simplify hardware
implementation of the correction) and to avoid being covered by the
Message Authentication Code Extension Field, or other authenticator
extension fields (e.g. Network Time Security). It is the exception
in the requirement specified for the Message Authentication Code
Extension Field. If the corrections were covered by the
authenticator fields, the network devices would need to have access
to the keys and have a significant additional complexity in order to
update the authenticator fields when they modify the Correction
Extension Field.
As the Correction Extension Field is not protected, NTP clients MUST
validate the corrections before their application as specified in
Measurement Modes (Section 8). Clients MUST ignore an unexpected
Correction Extension Field in the response, i.e. if it was not
included in the request.
7.7. Reference Timestamp Extension Field
This field contains the time of the last update of the clock. It has
a fixed length of 12 octets. In requests, that timestamp is always
0.
(Is this really needed? It was mostly unused in NTPv4.)
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF507) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Reference Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Format of Reference Timestamp Extension Field
7.8. Monotonic Receive Timestamp Extension Field
When a clock is synchronized to a time source, there is a compromise
between time (phase) accuracy and frequency accuracy, because the
frequency of the clock has to be adjusted to correct time errors that
accumulate due to the frequency error (e.g. caused by changes in the
temperature of the crystal). Faster corrections of time can minimize
the time error, but increase the frequency error, which transfers to
clients using that clock as a time source and increases their
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frequency and time errors. This issue can be avoided by transferring
time and frequency separately using different clocks.
The Monotonic Receive Timestamp Extension Field contains an extra
receive timestamp with a 32-bit epoch ID captured by a clock which
does not have corrected phase and can better transfer frequency than
the clock which captures the receive and transmit timestamps in the
header. The extension field has a constant length of 16 octets. In
requests, the counter and timestamp are always 0.
The epoch ID is a random number which is changed when frequency
transfer needs to be restarted, e.g. due to a step of the clock.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF508) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Monotonic Receive Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of Monotonic Receive Timestamp Extension Field
The client can determine the frequency-transfer offset from the time-
transfer offset and difference between the two receive timestamps in
the response. It can use the frequency-transfer offset to better
control the frequency of its clock, avoiding the frequency error in
the server's time-transfer clock.
7.9. Secondary Receive Timestamp Extension Field
This extension field provides an additional receive timestamp of the
client request in a selected timescale. It enables the client to get
the same receive timestamp in different timescales in order to
calculate the current offset between the timescales.
In requests, the Timescale field selects the requested timescale.
The other data fields in the extension field MUST be set to 0.
The Timescale, Era, and Secondary Receive Timestamp fields in a
response have the same meaning as the Timescale, Era, and Receive
Timestamp fields in the header respectively.
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If the server does not support the requested timescale, it MUST
ignore the extension field in the request. If the server supports
the timescale, but does not have a reliable timestamp (e.g. due to
being close to a leap second), it SHOULD set the timestamp field to
0.
The server MAY provide in this extension field timestamps in
timescales which it does not provide in the header, e.g. it can
provide UTC in addition to leap-smeared UTC to enable its clients to
measure the current smearing offset.
A request MAY contain multiple instances of this extension field, but
each timescale MUST be requested at most once, not counting the
timescale in the header. The server SHOULD include in its response
timestamps in all requested timescales it supports.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = [[TBD]] (draft 0xF509) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timescale | Era | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Secondary Receive Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Format of Secondary Receive Timestamp Extension Field
8. Measurement Modes
At the end of an NTP message exchange, the client has access to four
timestamps, which it can use to estimate the clock offset and the
network delay, as described in Section 4.1.
If the Correction Extension Field is used and the corrections are
known for both the request and response, a corrected offset and delay
is calculated:
* offset_c = offset + (Cd - Co) / 2
* delay_c = delay - (Cd + Co - Drx - Dtx) * (1 - FC)|
where
* Co is the Origin Correction from the response to the request
corresponding to timestamps T1 and T2
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* Cd is the Delay Correction from the response corresponding to
timestamps T3 and T4
* FC is the maximum expected frequency error of devices providing
the delay corrections (e.g. 100 ppm)
* Drx is the time it took to receive the frame containing the
response corresponding to T3 and T4
* Dtx is the time it took to transmit the frame containing the
request corresponding to T1 and T2. If unknown, it SHOULD be set
to Drx.
The corrected offset and delay MUST NOT be accepted if any of
delay_c, Co and Cr is negative. The uncorrected delay MUST always be
used for calculation of root delay.
The client can make measurements in the basic mode, or interleaved
mode if supported on the server. In the basic mode, the transmit
timestamp in the server response corresponds to the message which
contains the timestamp itself. In the interleaved mode it
corresponds to a previous response identified by the server cookie.
The interleaved mode enables the server to provide the client with a
more accurate transmit timestamp which is available only after the
response was formed or sent.
An example of cookies and timestamps in an NTPv5 exchange using the
basic mode is shown in Figure 11.
Server t2 t3 t6 t7 t10 t11
-----+----+----------------+----+----------------+----+-----
/ \ / \ / \
Client / \ / \ / \
--+----------+----------+----------+----------+----------+--
t1 t4 t5 t8 t9 t12
+----+ +----+ +----+ +----+ +----+ +----+
SC | 0 | | s1 | | 0 | | s2 | | 0 | | s3 |
CC | c1 | | c1 | | c2 | | c2 | | c3 | | c3 |
Rx | 0 | | t2 | | 0 | | t6 | | 0 | |t10 |
Tx | 0 | | t3 | | 0 | | t7 | | 0 | |t11 |
+----+ +----+ +----+ +----+ +----+ +----+
Figure 11: Cookies and timestamps in basic mode
From the three exchanges in this example, the client would use the
the following sets of timestamps:
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* (t1, t2, t3, t4)
* (t5, t6, t7, t8)
* (t9, t10, t11, t12)
The NTPv4 interleaved client-server mode is described in RFC 9769
[RFC9769]. The difference between NTPv5 and NTPv4 is that in NTPv5
the interleaved mode is enabled explicitly by a flag and the previous
transmit timestamp on the server is identified by the server cookie
instead of the receive timestamp, which avoids the requirements for
receive timestamps to be unique and not equal to transmit timestamps.
Therefore, the NTPv5 interleaved mode is easier to implement. A
server implementation that should support both NTPv4 and NTPv5 does
not need to process interleaved requests and save timestamps
separately for the different NTP versions. It can reuse the NTPv4
support in NTPv5 by setting the server cookie to the (unique) receive
timestamp.
An example of an NTPv5 exchange using the interleaved mode is shown
in Figure 12. The messages in the basic and interleaved mode are
indicated with B and I respectively. The timestamps t3' and t11'
correspond to the same transmissions as t3 and t11, but they may be
less accurate (e.g. due to being captured in software before the
transmission). The first exchange is in the basic mode followed by a
second exchange in the interleaved mode. For the third exchange, the
client request is in the interleaved mode, but the server response is
in the basic mode, because the server no longer had the timestamp t7
(e.g. it was dropped to save timestamps for other clients using the
interleaved mode).
Server t2 t3 t6 t7 t10 t11
-----+----+----------------+----+----------------+----+-----
/ \ / \ / \
Client / \ / \ / \
--+----------+----------+----------+----------+----------+--
t1 t4 t5 t8 t9 t12
Mode: B B I I I B
+----+ +----+ +----+ +----+ +----+ +----+
SC | 0 | | s1 | | s1 | | s2 | | s2 | | s3 |
CC | c1 | | c1 | | c2 | | c2 | | c3 | | c3 |
Rx | 0 | | t2 | | 0 | | t6 | | 0 | |t10 |
Tx | 0 | | t3'| | 0 | | t3 | | 0 | |t11'|
+----+ +----+ +----+ +----+ +----+ +----+
Figure 12: Cookies and timestamps in interleaved mode
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From the three exchanges in this example, the client would use the
following sets of timestamps:
* (t1, t2, t3', t4)
* (t1, t2, t3, t4) or (t5, t6, t3, t4)
* (t9, t10, t11', t12)
9. Client Operation
An NTPv5 client can use one or multiple servers. It has a separate
association with each server. It makes periodic measurements of its
offset and delay to the server. It can filter the measurements and
compare measurements from different servers to select and combine the
best servers for synchronization. It can adjust its clock in order
to minimize its offset and keep the clock synchronized. These
algorithms are not specified in this document. The client MAY use
the NTPv4 [RFC5905] algorithms, or any more or less advanced
algorithms optimized for the same or different conditions.
The polling interval can be adjusted for the network conditions and
stability of the clock. When polling a public server on Internet,
the client SHOULD use a polling interval of at least 2^6 (64)
seconds, increasing in normal conditions up to at least 2^10 (1024)
seconds to avoid excessive load on the server in case the
implementation is used on a very large number of systems. On start,
the client MAY send a burst of up to 8 requests using a poll interval
of only 2^1 (2) seconds in order to make the initial correction of
the clock sooner.
The polling interval SHOULD contain a small random part (e.g. up to
2% of the target interval in a uniform distribution) in order to
spread the load on the server if a large number of identical clients
is (re)started at the same time.
The client SHOULD follow the minimum allowed polling interval
provided by the server in the poll field of the last valid response,
but only up to a reasonable maximum value to limit the impact of
misconfigurations, bugs, and potential denial-of-service attacks
where the client would accept a spoofed response. For example, the
client could limit the accepted minimum interval to 2^15 (about 9
hours) if authentication is disabled, or 2^18 (about 3 days) if
authentication is enabled.
Each successful measurement provides the client with an offset, delay
and dispersion. When combined with the server's root delay and
dispersion, it gives the client an estimate of the maximum error.
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On each poll, the client:
1. Generates a new random cookie.
2. Formats a request with necessary extension fields and the fields
in the header all zero except:
* Version is set to 5.
* Mode is set to 3.
* Timescale is set to the timescale in which the client wants to
operate.
* Flags are set according to the requested mode. The
interleaved mode flag requests the server to save the transmit
timestamp of the response and provide the transmit timestamp
of a previous response corresponding to the server cookie (if
not zero).
* Server cookie is set only in the interleaved mode. It is set
to the server cookie from the last valid response, or zero if
no such response was received yet or the transmit timestamp of
that response would no longer be useful to the client (e.g.
after missing too many responses).
* Client cookie is set to the newly generated cookie.
3. Sends the request to the server to the UDP port 123 and captures
a transmit timestamp for the packet.
4. Waits for a valid response from the server and captures a receive
timestamp. A valid response has version 5, mode 4, client cookie
equal to the cookie from the request, and passes authentication
if enabled. The client MUST ignore all invalid responses and
accept at most one valid response.
5. Checks whether the response is usable for synchronization of the
clock. Such a response has the Synchronized flag set, stratum
between 0 and 16, root delay and dispersion both smaller than a
specific value, e.g. 16 seconds, and timescale equal to the
requested timescale. If the response is in a different
timescale, the client can switch to the provided timescale,
convert the timestamps if the offset between the timescales is
known from the Secondary Receive Timestamp Extension Field or
other sources, or ignore the response.
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6. Saves the server's receive and transmit timestamps. If the
client internally counts seconds using a type wider than 32 bits,
it SHOULD expand the timestamps with the provided NTP era.
7. Calculates the offset, delay, and dispersion as specified in
Measurement Modes (Section 8).
A client which operates as a server for other clients MUST include
the Reference IDs Request Extension Field in its requests in order to
track reference IDs of its sources. If the server's set of reference
IDs contains the client's own reference ID, it SHOULD not select the
server for synchronization to avoid a synchronization loop. If the
client is requesting the reference IDs in multiple chunks, it SHOULD
NOT select the server for synchronization until it received the whole
set.
A client which uses multiple servers MUST be able to handle servers
providing timestamps in different timescales. It can ignore servers
not using the most common or preferred timescale, or convert them to
a common timescale if it knows the offsets between them.
If the client synchronizes its clock to a leap-smeared timescale, it
MUST NOT apply leap seconds and it SHOULD provide the same timescale
to its own clients if it is a server.
The client SHOULD periodically (e.g. every two weeks) refresh IP
addresses of all servers specified by hostname to limit the duration
when an IP address of a migrated or decommissioned server will still
be receiving NTP traffic from long-running clients. The client
SHOULD ignore the TTL value of the DNS record that provided the IP
address to avoid excessive DNS traffic for pools of NTP servers using
a short TTL for better load balancing.
10. Server Operation
A server receives requests on the UDP port 123. The server MUST
support measurements in the basic mode. It MAY support the
interleaved mode.
For the basic mode the server does not need to keep any client-
specific state. For the interleaved mode it needs to save transmit
timestamps and be able to identify them by a cookie.
The server maintains its leap indicator, stratum, root delay, and
root dispersion:
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* Leap indicator MUST be 3 if the clock is not synchronized or its
maximum error cannot be estimated by root delay and dispersion.
Otherwise, it MUST be 0, 1, 2, depending on whether a leap second
is pending in the next 14 days and, if it is, whether it will be
inserted or deleted.
* Stratum SHOULD be 1 if the server is synchronized to a local
reference clock, or be one larger than the stratum of the NTP
server it selected as best for synchronization. It MUST be
greater than the minimum stratum of all servers selected for
synchronization, as they provided in their last accepted response,
or 0 to indicate infinity.
* Root delay MUST be an estimate of the double of the maximum error
of the server's clock due to asymmetries in the network delay and
other delays (e.g. timestamping) on the path to the primary time
source. For a server that is synchronized to other NTP servers,
one possibility is to set it to the latest measured delay to the
server it considers best plus the root delay provided in that
response.
* Root dispersion MUST be an estimate of the maximum error of the
server's clock from other causes than those covered by root delay,
e.g. instability of the clock and measurements by which it is
synchronized. If the server is synchronized to other NTP servers,
it MUST include root dispersion from one of the servers'
responses.
The server has a randomly generated 120-bit reference ID. It MUST
track reference IDs of its servers in order to be able to respond
with a Reference IDs Response Extension Field.
For each received request, the server:
1. Captures a receive timestamp.
2. Checks the version in the request. If it is not equal to 5, it
MUST either drop the request, or handle it according to the
specification corresponding to the protocol version.
3. Drops the request if the format is not valid, mode is not 3, or
authentication fails with the Message Authentication Code
Extension Field or another authenticator which does not have a
specified response for failed authentication. The server MUST
ignore unknown extension fields.
4. Server forms a response with requested extension fields and sets
the fields in the header as follows:
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* Leap Indicator, Stratum, Root delay, and Root dispersion, are
set to the current server's values.
* Version is set to 5.
* Poll is set to the minimum allowed polling interval of the
client (e.g. specified in the server configuration, using the
same value for all clients).
* Precision is set to the expected precision of the receive and
transmit timestamps included in the response (e.g. estimated
on the start of the server or specified in its configuration).
* Timescale is set to the client's requested timescale if it is
supported by the server. If not, the server SHOULD respond in
any timescale it supports.
* The flags are set as follows:
Synchronized is set if the server's clock is considered to be
synchronized, and the receive and transmit timestamps
provided in the response are usable for synchronization of
the client.
Interleaved mode is set if the interleaved mode is
implemented, was requested, and a response in the
interleaved mode is possible (i.e. a transmit timestamp is
associated with the server cookie).
* Era is set to the NTP era of the receive timestamp.
* Server Cookie is set when the interleaved mode is requested
and it is supported by the server, even if the response cannot
be in the requested mode due to the request having an unknown
or zero server cookie. The cookie identifies a more accurate
transmit timestamp of the response, which can be retrieved by
the client later with another request. The cookie MUST be
unique in a sufficiently long interval to prevent a client
from accepting a transmit timestamp that does not correspond
to the previous response it received. The cookie generation
is implementation-specific. For example, it can be a counter
incremented on each received request, a randomly generated
value, a timestamp, or an encrypted counter or timestamp
making the value unpredictable.
* Client Cookie is set to the Client Cookie from the request.
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* Receive Timestamp is set to the server's receive timestamp of
the request.
* Transmit Timestamp is set to a value which depends on the
measurement mode. In the basic mode it is the server's
current time when the message is formed. In the interleaved
mode it is the transmit timestamp of the previous response
identified by the server cookie in the request, captured at
some point after the message was formed.
5. The length of the response MUST be equal to the length of the
request. The server adds the Padding Extension Field or
increases its length if necessary to make the length of the
response equal to the length of the request.
6. Drops the response if it is longer than the request to prevent
traffic amplification.
7. Sends the response.
8. Saves the transmit timestamp and server cookie, if the
interleaved mode was requested and is supported by the server.
11. Network Time Security with NTPv5
The Network Time Security [RFC8915] mechanism uses the NTS-KE
protocol to establish keys and negotiate the next protocol. NTPv5
can be indicated as the next protocol with identifier [[TBD]] (draft
use 0x8001). This can be used by clients and servers to negotiate
NTPv5 for an NTS session.
No new NTS-KE records are specified for NTPv5. The records that were
specified for NTPv4 (i.e. NTPv4 New Cookie, NTPv4 Server Negotiation,
and NTPv4 Port Negotiation) are reused for NTPv5.
The NTS extension fields specified for NTPv4 are compatible with
NTPv5. No new extension fields are specified.
(Note to editor: remove this paragraph before publishing.) Client
implementations of a draft of this specification MUST provide the
identity of the draft implemented as data in an NTS-KE record of type
0x4001, which does not have the critical bit set. The draft identity
MUST be encoded as ascii and MUST not contain any trailing 0 bytes.
Servers that implement a draft MUST not accept NTPv5 as an option
unless they support the specific draft version identified.
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12. NTPv5 Negotiation in previous NTP versions
Servers that support multiple versions of NTP MUST send a response in
the same version as the request, to support backwards compatibility.
This does not preclude servers from acting as a client in one version
of NTP and a server in another.
NTPv5 messages are not compatible with NTPv4 and other previous
versions of NTP, even if they do not contain any extension fields.
Some widely used server NTPv4 implementations are known to accept
requests indicating a higher version, interpreting them as NTPv4, and
copying the version number from the request to its NTPv4 response.
Responses to NTPv5 requests have a zero client cookie, which causes
them to fail the NTPv5 validation and be ignored by the client.
The implementations are also known to not respond to requests with an
unknown extension field, which prevents an NTPv4 extension field to
be specified for NTPv5 negotiation. Instead, the negotiation reuses
the reference timestamp field.
An NTP server which supports NTPv5 and also NTPv4, NTPv3, NTPv2, or
NTPv1, SHOULD check the reference timestamp in received client-mode
requests of the previous NTP versions. If the reference timestamp
contains the value 0x4E5450354E545035 ("NTP5NTP5" in ASCII), it
SHOULD respond with the same reference timestamp to indicate it
supports NTPv5.
Note to the editor: Remove this paragraph before publication.
Implementations of a draft of this specification SHOULD use
0x4E54503544524654 ("NTP5DRFT" in ASCII) instead of
0x4E5450354E545035.
When NTPv5 is not expected to be widely supported on servers yet, an
NTP client which supports both NTPv5 and a previous NTP version, is
not configured to use a specific NTP version, and does not use NTS or
other mechanism including negotiation of the NTP version, SHOULD
start with the previous version and set the reference timestamp to
0x4e5450354e545035. If the server responds with the same reference
timestamp, the client SHOULD switch to NTPv5. If no valid response
is received for a number of requests (e.g. 2), the client SHOULD
switch back to the orignal NTP version and stick with it for a larger
number of requests (e.g. 256) before trying NTPv5 again.
The special value of the reference timestamp corresponds to
1941-08-24T01:13:25Z in NTP era 0 and 2077-09-29T07:41:41Z in NTP era
1. The negotiation will probably not be needed at that time anymore.
If NTPv5 servers and NTPv4-or-older-only clients are still in use and
they send a request with the special value by chance, they will get
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an acceptable response with no side effects. If an NTPv5 client gets
the special value by chance from an NTPv4-or-older-only server, it
will switch to NTPv5 and back after missing a valid response.
13. Acknowledgements
Some ideas were taken from a different NTPv5 design proposed by
Daniel Franke.
The author would like to thank Doug Arnold and David Venhoek for
their contributions and Dan Drown, Watson Ladd, Hal Murray, Kurt
Roeckx, and Ulrich Windl for their suggestions and comments.
14. IANA Considerations
IANA is requested to allocate the following entries in the NTP
Extension Field Types Registry [RFC9748]:
+============+=====================+=============================+
| Field Type | Meaning | Reference |
+============+=====================+=============================+
| [[TBD]] | Padding | [[this memo]] (Section 7.2) |
+------------+---------------------+-----------------------------+
| [[TBD]] | Message | [[this memo]] (Section 7.3) |
| | Authentication Code | |
+------------+---------------------+-----------------------------+
| [[TBD]] | Reference IDs | [[this memo]] (Section 7.4) |
| | Request | |
+------------+---------------------+-----------------------------+
| [[TBD]] | Reference IDs | [[this memo]] (Section 7.4) |
| | Response | |
+------------+---------------------+-----------------------------+
| [[TBD]] | Server Information | [[this memo]] (Section 7.5) |
+------------+---------------------+-----------------------------+
| [[TBD]] | Correction | [[this memo]] (Section 7.6) |
+------------+---------------------+-----------------------------+
| [[TBD]] | Reference Timestamp | [[this memo]] (Section 7.7) |
+------------+---------------------+-----------------------------+
| [[TBD]] | Monotonic Receive | [[this memo]] (Section 7.8) |
| | Timestamp | |
+------------+---------------------+-----------------------------+
| [[TBD]] | Secondary Receive | [[this memo]] (Section 7.9) |
| | Timestamp | |
+------------+---------------------+-----------------------------+
Table 1
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IANA is requested to allocate the following entry in the Network Time
Security Next Protocols Registry [RFC8915]:
+============================+=======================+==============+
| Protocol ID | Protocol Name | Reference |
+============================+=======================+==============+
| [[TBD]], selected by IANA | Network Time | [[this |
| from the IETF Review range | Protocol version | memo]] |
| | 5 (NTPv5) | (Section 11) |
+----------------------------+-----------------------+--------------+
Table 2
15. Security Considerations
The security considerations of time protocols in general are
discussed in [RFC7384]. A successful attack on the time distribution
can result in one of three outcomes: Denial of Service (DoS), reduced
accuracy, or obtaining incorrect time. NTP can be compromised
through various methods, such as altering or delaying NTP messages
during transit, injecting malicious NTP messages or replaying valid
ones, or impersonating an NTP server.
The Message Authentication Code Extension Field can be used to
provide integrity protection, thus mitigating in-transit NTP message
modification and malicious packet injection.
Using NTS with NTPv5 provides enhanced security properties, including
server identity verification, improved replay protection, and secure
key establishment.
NTPv5 was designed to minimize the necessary on-the-wire data that is
included in the NTPv5 header in order to limit the amount of
information that is exposed to the network and minimize the potential
effect of network reconnaissance. For example, the client's
transmission timestamp is not included in the NTPv5 header.
Extension fields are used for conveying optional information.
The protocol operates in a client-server mode. Other modes of
operation, which were supported in the previous version of the
protocol have known security vulnerabilities. The symmetric and
broadcast modes are vulnerable to replay attacks. The control and
private modes can be exploited for denial-of-service traffic
amplification attacks. These modes are not supported in NTPv5.
The NTP response message has the same length as the corresponding
request message. This symmetric approach reduces the potential of
amplification attacks.
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The protocol does not inherently mitigate delay attacks. An on-path
attacker can delay NTP messages and compromise the accuracy. Delay
attacks can be mitigated by limiting the maximum acceptable delay.
In each request-response exchange the client computes the delay and
can discard the response if the delay exceeds a maximum expected
value. A more fine-grained mitigation approach is acquiring time
from multiple sources and selecting the sources that are most likely
to be accurate. A detailed scheme was defined in [RFC5905] and can
be used in NTPv5. This scheme defines a selection algorithm, which
identifies the most accurate and reliable time sources from a pool of
candidates. These sources are then grouped by the clustering
algorithm to minimize the impact of outliers. Finally, the combining
algorithm averages the time data from these clustered sources to
produce a final, stable time estimate. An enhancement to the
selection algorithm was proposed in [RFC9523]. An alternative
approach to using multiple time sources is using multiple diverse
paths between the client and server [RFC8039], as it is assumed that
only a subset of the paths is compromised by an on-path attacker.
The corrections provided by network devices in the Correction
Extension Field are not authenticated. Man-in-the-middle attackers
can modify the correction values. In order to mitigate such attacks,
the client validates the correction values as described in Section 8.
Thus, only corrections smaller than the measured delay are accepted
by clients. The security impact is comparable to the impact of
delaying unmodified NTP messages, as described above.
16. References
16.1. Normative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8573] Malhotra, A. and S. Goldberg, "Message Authentication Code
for the Network Time Protocol", RFC 8573,
DOI 10.17487/RFC8573, June 2019,
<https://www.rfc-editor.org/info/rfc8573>.
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16.2. Informative References
[Bloom] Bloom, B. H., "Space/Time Trade-Offs in Hash Coding with
Allowable Errors", June 1970,
<https://doi.org/10.1145/362686.362692>.
[IEEE1588] Institute of Electrical and Electronics Engineers, "IEEE
std. 1588-2019, "IEEE Standard for a Precision Clock
Synchronization for Networked Measurement and Control
Systems."", November 2019, <https://www.ieee.org>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[RFC7821] Mizrahi, T., "UDP Checksum Complement in the Network Time
Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
2016, <https://www.rfc-editor.org/info/rfc7821>.
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, <https://www.rfc-editor.org/info/rfc7822>.
[RFC8039] Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi,
"Multipath Time Synchronization", RFC 8039,
DOI 10.17487/RFC8039, December 2016,
<https://www.rfc-editor.org/info/rfc8039>.
[RFC8877] Mizrahi, T., Fabini, J., and A. Morton, "Guidelines for
Defining Packet Timestamps", RFC 8877,
DOI 10.17487/RFC8877, September 2020,
<https://www.rfc-editor.org/info/rfc8877>.
[RFC8915] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time
Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
<https://www.rfc-editor.org/info/rfc8915>.
[RFC9523] Rozen-Schiff, N., Dolev, D., Mizrahi, T., and M. Schapira,
"A Secure Selection and Filtering Mechanism for the
Network Time Protocol with Khronos", RFC 9523,
DOI 10.17487/RFC9523, February 2024,
<https://www.rfc-editor.org/info/rfc9523>.
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[RFC9748] Salz, R., "Updating the NTP Registries", RFC 9748,
DOI 10.17487/RFC9748, February 2025,
<https://www.rfc-editor.org/info/rfc9748>.
[RFC9769] Lichvar, M. and A. Malhotra, "NTP Interleaved Modes",
RFC 9769, DOI 10.17487/RFC9769, May 2025,
<https://www.rfc-editor.org/info/rfc9769>.
Authors' Addresses
Miroslav Lichvar
Red Hat
Email: mlichvar@redhat.com
Tal Mizrahi
Huawei
Email: tal.mizrahi.phd@gmail.com
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