Internet DRAFT - draft-ietf-ntp-packet-timestamps
draft-ietf-ntp-packet-timestamps
NTP Working Group T. Mizrahi
Internet-Draft Huawei Smart Platforms iLab
Intended status: Informational J. Fabini
Expires: September 12, 2020 TU Wien
A. Morton
AT&T Labs
March 11, 2020
Guidelines for Defining Packet Timestamps
draft-ietf-ntp-packet-timestamps-09
Abstract
Various network protocols make use of binary-encoded timestamps that
are incorporated in the protocol packet format, referred to as packet
timestamps for short. This document specifies guidelines for
defining packet timestamp formats in networking protocols at various
layers. It also presents three recommended timestamp formats. The
target audience of this document includes network protocol designers.
It is expected that a new network protocol that requires a packet
timestamp will, in most cases, use one of the recommended timestamp
formats. If none of the recommended formats fits the protocol
requirements, the new protocol specification should specify the
format of the packet timestamp according to the guidelines in this
document.
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
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Drafts is at https://datatracker.ietf.org/drafts/current/.
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 September 12, 2020.
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Copyright Notice
Copyright (c) 2020 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Scope of this Document . . . . . . . . . . . . . . . . . 3
1.3. How to Use This Document . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Terms used in this Document . . . . . . . . . . . . . . . 4
3. Packet Timestamp Specification Template . . . . . . . . . . . 5
4. Recommended Timestamp Formats . . . . . . . . . . . . . . . . 6
4.1. Using a Recommended Timestamp Format . . . . . . . . . . 7
4.2. NTP Timestamp Formats . . . . . . . . . . . . . . . . . . 7
4.2.1. NTP 64-bit Timestamp Format . . . . . . . . . . . . . 7
4.2.2. NTP 32-bit Timestamp Format . . . . . . . . . . . . . 9
4.3. The PTP Truncated Timestamp Format . . . . . . . . . . . 10
5. Synchronization Aspects . . . . . . . . . . . . . . . . . . . 11
6. Timestamp Use Cases . . . . . . . . . . . . . . . . . . . . . 12
6.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Packet Timestamp Control Field . . . . . . . . . . . . . . . 14
7.1. High-level Control Field Requirements . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
1.1. Background
Timestamps are widely used in network protocols for various purposes:
timestamps are used for logging or reporting the time of an event,
delay measurement and clock synchronization protocols both make use
of timestamped messages, and in security protocols a timestamp is
often used as part of a value that is unlikely to repeat (nonce).
Timestamps are represented in the RFC series in one of two forms:
text-based timestamps, and packet timestamps. Text-based timestamps
[RFC3339] are represented as user-friendly strings, and are widely
used in the RFC series, for example in information objects and data
models, e.g., [RFC5646], [RFC6991], and [RFC7493]. Packet
timestamps, on the other hand, are represented by a compact binary
field that has a fixed size, and are not intended to have a human-
friendly format. Packet timestamps are also very common in the RFC
series, and are used for example for measuring delay and for
synchronizing clocks, e.g., [RFC5905], [RFC4656], and [RFC7323].
1.2. Scope of this Document
This document presents guidelines for defining a packet timestamp
format in network protocols. Three recommended timestamp formats are
presented. It is expected that a new network protocol that requires
a packet timestamp will, in most cases, use one of these recommended
timestamp formats. In some cases a network protocol may use more
than one of the recommended timestamp formats. However, if none of
the recommended formats fits the protocol requirements, the new
protocol specification should specify the format of the packet
timestamp according to the guidelines in this document.
The rationale behind defining a relatively small set of recommended
formats is that it enables significant reuse; network protocols can
typically reuse the timestamp format of the Network Time Protocol
(NTP) or the Precision Time Protocol (PTP), allowing a
straightforward integration with an NTP or a PTP-based timer.
Moreover, since accurate timestamping mechanisms are often
implemented in hardware, a new network protocol that reuses an
existing timestamp format can be quickly deployed using existing
hardware timestamping capabilities.
1.3. How to Use This Document
This document is intended as a reference for network protocol
designers. When defining a network protocol that uses a packet
timestamp, the recommended timestamp formats should be considered
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first (Section 4). If one of these formats is used, it should be
referenced along the lines of the examples in Section 6.1 and
Section 6.2. If none of the recommended formats fits the required
functionality, then a new timestamp format should be defined using
the template of Section 3.
2. Terminology
2.1. 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.
2.2. Abbreviations
NTP Network Time Protocol [RFC5905]
PTP Precision Time Protocol [IEEE1588]
TAI International Atomic Time
UTC Coordinated Universal Time
2.3. Terms used in this Document
Timestamp: A value that represents a point in time,
corresponding to an event that occurred or is
scheduled to occur.
Timestamp error: The difference between the timestamp value and
the value of a reference clock at the time of
the event that the timestamp was intended to
indicate.
Timestamp format: The specification of a timestamp, which is
represented by a set of attributes that
unambiguously define the syntax and semantics
of a timestamp.
Timestamp accuracy: The mean over an ensemble of measurements of
the timestamp error.
Timestamp precision: The variation over an ensemble of measurements
of the timestamp error.
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Timestamp resolution: The minimal time unit used for representing
the timestamp.
3. Packet Timestamp Specification Template
This document recommends to use the timestamp formats defined in
Section 4. In cases where these timestamp formats do not satisfy the
protocol requirements, the timestamp specification should clearly
state the reasons for defining a new format. Moreover, it is
recommended to derive the new timestamp format from an existing
timestamp format, either a timestamp format from this document, or
any other previously defined timestamp format.
The timestamp specification must unambiguously define the syntax and
the semantics of the timestamp. The current section defines the
minimum set of attributes, but it should be noted that in some cases
additional attributes or aspects will need to be defined in the
timestamp specification.
This section defines a template for specifying packet timestamps. A
timestamp format specification MUST include at least the following
aspects:
Timestamp syntax:
- Size: The number of bits (or octets) used to represent the
packet timestamp field. If the timestamp is comprised of more
than one field, the size of each field is specified. Network
order (big endian) is assumed by default; if this is not the case
then this section explicitly specifies the endianity.
Timestamp semantics:
- Units: The units used to represent the timestamp. If the
timestamp is comprised of more than one field, the units of each
field are specified. If a field is limited to a specific range of
values, this section specifies the permitted range of values.
- Resolution: The timestamp resolution; the resolution is equal to
the timestamp field unit. If the timestamp consists of two or
more fields using different time units, then the resolution is the
smallest time unit.
- Wraparound: The wraparound period of the timestamp; any further
wraparound-related considerations should be described here.
- Epoch: The origin of the timescale used for the timestamp; the
moment in time used as a reference for the timestamp value. For
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example, the epoch may be based on a standard time scale, such as
UTC. Another example is a relative timestamp, in which the epoch
could be the time at which the device using the timestamp was
powered up, and is not affected by leap seconds (see the next
attribute).
- Leap seconds: This subsection specifies whether the timestamp is
affected by leap seconds. If the timestamp is affected by leap
seconds, then it represents the time elapsed since the epoch minus
the number of leap seconds that have occurred since the epoch.
Synchronization aspects:
The specification of a network protocol that makes use of a packet
timestamp is expected to include the synchronization aspects of
using the timestamp. While the synchronization aspects are not
strictly part of the timestamp format specification, these aspects
provide the necessary context for using the timestamp within the
scope of the protocol. In some cases timestamps are used without
synchronization, e.g., a timestamp that indicates the number of
seconds since power up. In such cases the Synchronization Aspects
section will specify that the timestamp does not correspond to a
synchronized time reference, and may discuss how this affects the
usage of the timestamp. Further details about synchronization
aspects are discussed in Section 5.
4. Recommended Timestamp Formats
This document defines a set of recommended timestamp formats.
Clearly, different network protocols may have different requirements
and constraints, and consequently may use different timestamp
formats. The choice of the specific timestamp format for a given
protocol may depend on a various factors. A few examples of factors
that may affect the choice of the timestamp format:
o Timestamp size: while some network protocols use a large timestamp
field, in some cases there may be constraints with respect to the
timestamp size, affecting the choice of the timestamp format.
o Resolution: the time resolution is another factor that may
directly affect the selected timestamp format. A potentially
important factor in this context is extensibility; it may be
desirable to allow a timestamp format to be extensible to a higher
resolution by extending the field. For example, the resolution of
the NTP 32-bit timestamp format can be improved by extending it to
the NTP 64-bit timestamp format in a straightforward way.
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o Wraparound period: the length of the time interval in which the
timestamp is unique may also be an important factor in choosing
the timestamp format. Along with the timestamp resolution, these
two factors determine the required number of bits in the
timestamp.
o Common format for multiple protocols: if there are two or more
network protocols that use timestamps and are often used together
in typical systems, using a common timestamp format should be
preferred if possible. For example, if the network protocol that
is being defined typically runs on a PC, then an NTP-based
timestamp format may allow easier integration with an NTP-
synchronized timer. In contrast, a protocol that is typically
deployed on a hardware-based platform, may make better use of a
PTP-based timestamp, allowing more efficient integration with a
PTP-synchronized timer.
4.1. Using a Recommended Timestamp Format
A specification that uses one of the recommended timestamp formats
should specify explicitly that this is a recommended timestamp
format, and point to the relevant section in the current document.
4.2. NTP Timestamp Formats
4.2.1. NTP 64-bit Timestamp Format
The Network Time Protocol (NTP) 64-bit timestamp format is defined in
[RFC5905]. This timestamp format is used in several network
protocols, including [RFC6374], [RFC4656], and [RFC5357]. Since this
timestamp format is used in NTP, this timestamp format should be
preferred in network protocols that are typically deployed in concert
with NTP.
The format is presented in this section according to the template
defined in Section 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NTP [RFC5905] 64-bit Timestamp Format
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Timestamp field format:
Seconds: specifies the integer portion of the number of seconds
since the epoch.
- Size: 32 bits.
- Units: seconds.
Fraction: specifies the fractional portion of the number of
seconds since the epoch.
- Size: 32 bits.
- Units: the unit is 2^(-32) seconds, which is roughly equal to
233 picoseconds.
Epoch:
The epoch is 1 January 1900 at 00:00 UTC.
Note: As pointed out in [RFC5905], strictly speaking, UTC did not
exist prior to 1 January 1972, but it is convenient to assume it
has existed for all eternity. The current epoch implies that the
timestamp specifies the number of seconds since 1 January 1972 at
00:00 UTC plus 2272060800 (which is the number of seconds between
1 January 1900 and 1 January 1972).
Leap seconds:
This timestamp format is affected by leap seconds. The timestamp
represents the number of seconds elapsed since the epoch minus the
number of leap seconds. Thus, during and possibly before and/or
after the occurrence of a leap second, the value of the timestamp
may temporarily be ambiguous, as further discussed in Section 5.
Resolution:
The resolution is 2^(-32) seconds.
Wraparound:
This time format wraps around every 2^32 seconds, which is roughly
136 years. The next wraparound will occur in the year 2036.
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4.2.2. NTP 32-bit Timestamp Format
The Network Time Protocol (NTP) 32-bit timestamp format is defined in
[RFC5905]. This timestamp format is used in
[I-D.ietf-ippm-initial-registry] and
[I-D.ietf-sfc-nsh-dc-allocation]. This timestamp format should be
preferred in network protocols that are typically deployed in concert
with NTP. The 32-bit format can be used either when space
constraints do not allow the use of the 64-bit format, or when the
32-bit format satisfies the resolution and wraparound requirements.
The format is presented in this section according to the template
defined in Section 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds | Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NTP [RFC5905] 32-bit Timestamp Format
Timestamp field format:
Seconds: specifies the integer portion of the number of seconds
since the epoch.
- Size: 16 bits.
- Units: seconds.
Fraction: specifies the fractional portion of the number of
seconds since the epoch.
- Size: 16 bits.
- Units: the unit is 2^(-16) seconds, which is roughly equal to
15.3 microseconds.
Epoch:
The epoch is 1 January 1900 at 00:00 UTC.
Note: As pointed out in [RFC5905], strictly speaking, UTC did not
exist prior to 1 January 1972, but it is convenient to assume it
has existed for all eternity. The current epoch implies that the
timestamp specifies the number of seconds since 1 January 1972 at
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00:00 UTC plus 2272060800 (which is the number of seconds between
1 January 1900 and 1 January 1972).
Leap seconds:
This timestamp format is affected by leap seconds. The timestamp
represents the number of seconds elapsed since the epoch minus the
number of leap seconds. Thus, during and possibly after the
occurrence of a leap second, the value of the timestamp may
temporarily be ambiguous, as further discussed in Section 5.
Resolution:
The resolution is 2^(-16) seconds.
Wraparound:
This time format wraps around every 2^16 seconds, which is roughly
18 hours.
4.3. The PTP Truncated Timestamp Format
The Precision Time Protocol (PTP) [IEEE1588] uses an 80-bit timestamp
format. The truncated timestamp format is a 64-bit field, which is
the 64 least significant bits of the 80-bit PTP timestamp. Since
this timestamp format is similar to the one used in PTP, this
timestamp format should be preferred in network protocols that are
typically deployed in PTP-capable devices.
The PTP truncated timestamp format was defined in [IEEE1588v1] and is
used in several protocols, such as [RFC6374], [RFC7456], [RFC8186]
and [ITU-T-Y.1731].
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nanoseconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: PTP [IEEE1588] Truncated Timestamp Format
Timestamp field format:
Seconds: specifies the integer portion of the number of seconds
since the epoch.
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- Size: 32 bits.
- Units: seconds.
Nanoseconds: specifies the fractional portion of the number of
seconds since the epoch.
- Size: 32 bits.
- Units: nanoseconds. The value of this field is in the range 0
to (10^9)-1.
Epoch:
The PTP [IEEE1588] epoch is 1 January 1970 00:00:00 TAI.
Leap seconds:
This timestamp format is not affected by leap seconds.
Resolution:
The resolution is 1 nanosecond.
Wraparound:
This time format wraps around every 2^32 seconds, which is roughly
136 years. The next wraparound will occur in the year 2106.
5. Synchronization Aspects
A specification that defines a new timestamp format or uses one of
the recommended timestamp formats should include a section on
Synchronization Aspects. Note that the recommended timestamp formats
defined in this document (Section 4) do not include the
synchronization aspects of these timestamp formats, but it is
expected that specifications of network protocols that make use of
these formats should include the synchronization aspects. Examples
of a Synchronization Aspects section can be found in Section 6.
The Synchronization Aspects section should specify all the
assumptions and requirements related to synchronization. For
example, the synchronization aspects may specify whether nodes
populating the timestamps should be synchronized among themselves,
and whether the timestamp is measured with respect to a central
reference clock such as an NTP server. If time is assumed to be
synchronized to a time standard such as UTC or TAI, it should be
specified in this section. Further considerations may be discussed
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in this section, such as the required timestamp accuracy and
precision.
Another aspect that should be discussed in this section is leap
second [RFC5905] considerations. The timestamp specification
template (Section 3) specifies whether the timestamp is affected by
leap seconds. It is often the case that further details about leap
seconds will need to be defined in the Synchronization Aspects
section. Generally speaking, a leap second is a one-second
adjustment that is occasionally applied to UTC in order to keep it
aligned to the solar time. A leap second may be either positive or
negative, i.e., the clock may either be shifted one second forwards
or backwards. All leap seconds that have occurred up to the
publication of this document have been in the backwards direction,
and although forward leap seconds are theoretically possible, the
text throughout this document focuses on the common case, which is
the backward leap second. In a timekeeping system that considers
leap seconds, the system clock may be affected by a leap second in
one of three possible ways:
o The clock is turned backwards one second at the end of the leap
second.
o The clock is frozen during the duration of the leap second.
o The clock is slowed down during the leap second and adjacent time
intervals until the new time value catches up. The interval for
this process, commonly referred to as leap smear, can range from
several seconds to several hours before, during, and/or after the
occurrence of the leap second.
The way leap seconds are handled depends on the synchronization
protocol, and is thus not specified in this document. However, if a
timestamp format is defined with respect to a timescale that is
affected by leap seconds, the Synchronization Aspects section should
specify how the use of leap seconds affects the timestamp usage.
6. Timestamp Use Cases
Packet timestamps are used in various network protocols. Typical
applications of packet timestamps include delay measurement, clock
synchronization, and others. The following table presents a (non-
exhaustive) list of protocols that use packet timestamps, and the
timestamp formats used in each of these protocols.
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+----------------------+-----------------------------------+-----------+
| | Recommended formats | Other |
+----------------------+-----------+-----------+-----------+-----------+
| Protocol |NTP 64-bit |NTP 32-bit |PTP Trunc. | |
+----------------------+-----------+-----------+-----------+-----------+
| NTP [RFC5905] | + | | | |
+----------------------+-----------+-----------+-----------+-----------+
| OWAMP [RFC4656] | + | | | |
+----------------------+-----------+-----------+-----------+-----------+
| TWAMP [RFC5357] | + | | | |
| TWAMP [RFC8186] | + | | + | |
+----------------------+-----------+-----------+-----------+-----------+
| TRILL [RFC7456] | | | + | |
+----------------------+-----------+-----------+-----------+-----------+
| MPLS [RFC6374] | | | + | |
+----------------------+-----------+-----------+-----------+-----------+
| TCP [RFC7323] | | | | + |
+----------------------+-----------+-----------+-----------+-----------+
| RTP [RFC3550] | + | | | + |
+----------------------+-----------+-----------+-----------+-----------+
| IPFIX [RFC7011] | | | | + |
+----------------------+-----------+-----------+-----------+-----------+
| BinaryTime [RFC6019] | | | | + |
+----------------------+-----------+-----------+-----------+-----------+
| [I-D.ietf-ippm- | + | + | | |
| initial-registry] | | | | |
+----------------------+-----------+-----------+-----------+-----------+
| [I-D.ietf-sfc-nsh | | + | + | |
| -dc-allocation] | | | | |
+----------------------+-----------+-----------+-----------+-----------+
Figure 4: Protocols that use Packet Timestamps
The rest of this section presents two hypothetic examples of network
protocol specifications that use one of the recommended timestamp
formats. The examples include the text that specifies the
information related to the timestamp format.
6.1. Example 1
Timestamp:
The timestamp format used in this specification is the NTP
[RFC5905] 64-bit format, as specified in Section 4.2.1 of
[I-D.ietf-ntp-packet-timestamps].
Synchronization aspects:
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It is assumed that nodes that run this protocol are synchronized
to UTC using a synchronization mechanism that is outside the scope
of this document. In typical deployments this protocol will run
on a machine that uses NTP [RFC5905] for synchronization. Thus,
the timestamp may be derived from the NTP-synchronized clock,
allowing the timestamp to be measured with respect to the clock of
an NTP server. Since the NTP time format is affected by leap
seconds, the current timestamp format is similarly affected.
Thus, the value of a timestamp during or slightly after a leap
second may be temporarily inaccurate.
6.2. Example 2
Timestamp:
The timestamp format used in this specification is the PTP
[IEEE1588] Truncated format, as specified in Section 4.3 of
[I-D.ietf-ntp-packet-timestamps].
Synchronization aspects:
It is assumed that nodes that run this protocol are synchronized
among themselves. Nodes may be synchronized to a global reference
time. Note that if PTP [IEEE1588] is used for synchronization,
the timestamp may be derived from the PTP-synchronized clock,
allowing the timestamp to be measured with respect to the clock of
an PTP Grandmaster clock.
7. Packet Timestamp Control Field
In some cases it is desirable to have a control field that describes
structure, format, content, and properties of timestamps. Control
information about the timestamp format can be conveyed in some
protocols using a dedicated control plane protocol, or may be made
available at the management plane, for example using a YANG data
model. An optional control field allows some of the control
information to be attached to the timestamp.
An example of a packet timestamp control field is the Error Estimate
field, defined by Section 4.1.2 in [RFC4656], which is used in OWAMP
[RFC4656] and TWAMP [RFC5357]. The Root Dispersion and Root Delay
fields in the NTP header [RFC5905] are two examples of fields that
provide information about the timestamp precision. Another example
of an auxiliary field is the Correction Field in the PTP header
[IEEE1588]; its value is used as a correction to the timestamp, and
may be assigned by the sender of the PTP message and updated by
transit nodes (Transparent Clocks) in order to account for the delay
along the path.
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This section defines high-level guidelines for defining packet
timestamp control fields in network protocols that can benefit from
such timestamp-related control information. The word 'requirements'
is used in its informal context in this section.
7.1. High-level Control Field Requirements
A control field for packet timestamps must offer an adequate feature
set and fulfill a series of requirements to be usable and accepted.
The following list captures the main high-level requirements for
timestamp fields.
1. Extensible Feature Set: protocols and applications depend on
various timestamp characteristics. A timestamp control field
must support a variable number of elements (components) that
either describe or quantify timestamp-specific characteristics or
parameters. Examples of potential elements include timestamp
size, encoding, accuracy, leap seconds, reference clock
identifiers, etc.
2. Size: Essential for an efficient use of timestamp control fields
is the trade-off between supported features and control field
size. Protocols and applications may select the specific control
field elements that are needed for their operation from the set
of available elements.
3. Composition: Applications may depend on specific control field
elements being present in messages. The status of these elements
may be either mandatory, conditional mandatory, or optional,
depending on the specific application and context. A control
field specification must support applications in conveying or
negotiating (a) the set of control field elements along with (b)
the status of any element (i.e., mandatory, conditional
mandatory, or optional) by defining appropriate data structures
and identity codes.
4. Category: Control field elements can characterize either static
timestamp information (like, e.g., timestamp size in bytes and
timestamp semantics: NTP 64 bit format) or runtime timestamp
information (like, e.g., estimated timestamp accuracy at the time
of sampling: 20 microseconds to UTC). For efficiency reason it
may be meaningful to support separation of these two concepts:
while the former (static) information is typically valid
throughout a protocol session and may be conveyed only once, at
session establishment time, the latter (runtime) information
augments any timestamp instance and may cause substantial
overhead for high-traffic protocols.
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Proposals for timestamp control fields will be defined in separate
documents and are out of scope of this document.
8. IANA Considerations
This document includes no request to IANA.
9. Security Considerations
A network protocol that uses a packet timestamp MUST specify the
security considerations that result from using the timestamp. This
section provides an overview of some of the common security
considerations of using timestamps.
Any metadata that is attached to control or data packets, and
specifically packet timestamps, can facilitate network
reconnaissance; by passively eavesdropping to timestamped packets an
attacker can gather information about the network performance, and
about the level of synchronization between nodes.
In some cases timestamps could be spoofed or modified by on-path
attackers, thus attacking the application that uses the timestamps.
For example, if timestamps are used in a delay measurement protocol,
an attacker can modify en route timestamps in a way that manipulates
the measurement results. Integrity protection mechanisms, such as
Message Authentication Codes (MAC), can mitigate such attacks. The
specification of an integrity protection mechanism is outside the
scope of this document, as typically integrity protection will be
defined on a per-network-protocol basis, and not specifically for the
timestamp field.
Another potential threat that can have a similar impact is delay
attacks. An attacker can maliciously delay some or all of the en
route messages, with the same harmful implications as described in
the previous paragraph. Mitigating delay attacks is a significant
challenge; in contrast to spoofing and modification attacks, the
delay attack cannot be prevented by cryptographic integrity
protection mechanisms. In some cases delay attacks can be mitigated
by sending the timestamped information through multiple paths,
allowing to detect and to be resilient to an attacker that has access
to one of the paths.
In many cases timestamping relies on an underlying synchronization
mechanism. Thus, any attack that compromises the synchronization
mechanism can also compromise protocols that use timestamping.
Attacks on time protocols are discussed in detail in [RFC7384].
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10. Acknowledgments
The authors thank Russ Housley, Yaakov Stein, Greg Mirsky, Warner
Losh, Rodney Cummings, Miroslav Lichvar, Denis Reilly, Daniel Franke,
Eric Vyncke, Ben Kaduk, Ian Swett, Francesca Palombini, Watson Ladd,
and other members of the NTP working group for many helpful comments.
The authors gratefully acknowledge Harlan Stenn and the people from
the Network Time Foundation for sharing their thoughts and ideas.
11. References
11.1. Normative References
[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>.
11.2. Informative References
[I-D.ietf-ippm-initial-registry]
Morton, A., Bagnulo, M., Eardley, P., and K. D'Souza,
"Initial Performance Metrics Registry Entries", draft-
ietf-ippm-initial-registry-16 (work in progress), March
2020.
[I-D.ietf-ntp-packet-timestamps]
Mizrahi, T., Fabini, J., and A. Morton, "Guidelines for
Defining Packet Timestamps", draft-ietf-ntp-packet-
timestamps-08 (work in progress), February 2020.
[I-D.ietf-sfc-nsh-dc-allocation]
Guichard, J., Smith, M., Kumar, S., Majee, S., and T.
Mizrahi, "Network Service Header (NSH) MD Type 1: Context
Header Allocation (Data Center)", draft-ietf-sfc-nsh-dc-
allocation-02 (work in progress), September 2018.
[IEEE1588]
IEEE, "IEEE 1588 Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems Version 2", 2008.
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[IEEE1588v1]
IEEE, "IEEE 1588 Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", 2002.
[ITU-T-Y.1731]
ITU-T, "OAM functions and mechanisms for Ethernet based
Networks", 2013.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
<https://www.rfc-editor.org/info/rfc4656>.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[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>.
[RFC6019] Housley, R., "BinaryTime: An Alternate Format for
Representing Date and Time in ASN.1", RFC 6019,
DOI 10.17487/RFC6019, September 2010,
<https://www.rfc-editor.org/info/rfc6019>.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<https://www.rfc-editor.org/info/rfc6374>.
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[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/info/rfc6991>.
[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export (IPFIX)
Protocol for the Exchange of Flow Information", STD 77,
RFC 7011, DOI 10.17487/RFC7011, September 2013,
<https://www.rfc-editor.org/info/rfc7011>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[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>.
[RFC7456] Mizrahi, T., Senevirathne, T., Salam, S., Kumar, D., and
D. Eastlake 3rd, "Loss and Delay Measurement in
Transparent Interconnection of Lots of Links (TRILL)",
RFC 7456, DOI 10.17487/RFC7456, March 2015,
<https://www.rfc-editor.org/info/rfc7456>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC8186] Mirsky, G. and I. Meilik, "Support of the IEEE 1588
Timestamp Format in a Two-Way Active Measurement Protocol
(TWAMP)", RFC 8186, DOI 10.17487/RFC8186, June 2017,
<https://www.rfc-editor.org/info/rfc8186>.
Authors' Addresses
Tal Mizrahi
Huawei Smart Platforms iLab
8-2 Matam
Haifa 3190501
Israel
Email: tal.mizrahi.phd@gmail.com
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Joachim Fabini
TU Wien
Gusshausstrasse 25/E389
Vienna 1040
Austria
Phone: +43 1 58801 38813
Fax: +43 1 58801 38898
Email: Joachim.Fabini@tuwien.ac.at
URI: http://www.tc.tuwien.ac.at/about-us/staff/joachim-fabini/
Al Morton
AT&T Labs
200 Laurel Avenue South
Middletown,, NJ 07748
USA
Phone: +1 732 420 1571
Fax: +1 732 368 1192
Email: acmorton@att.com
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