RFC9581: Concise Binary Object Representation (CBOR) Tags for Time, Duration, and Period
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(Clock accuracy)
(Duration)
(Network Time Protocol)
(NTP)
(Period in time)
(Point in time)
(precision time protocol)
(ptp)
(tai)
(Timescale)
(UTC)
Internet Engineering Task Force (IETF) C. Bormann
Request for Comments: 9581 Universität Bremen TZI
Category: Standards Track B. Gamari
ISSN: 2070-1721 Well-Typed
H. Birkholz
Fraunhofer SIT
August 2024
Concise Binary Object Representation (CBOR) Tags for Time, Duration, and
Period
Abstract
The Concise Binary Object Representation (CBOR, RFC 8949) is a data
format whose design goals include the possibility of extremely small
code size, fairly small message size, and extensibility without the
need for version negotiation.
In CBOR, one point of extensibility is the definition of CBOR tags.
RFC 8949 defines two tags for time: CBOR tag 0 (RFC 3339 time as a
string) and tag 1 (POSIX time as int or float). Since then,
additional requirements have become known. The present document
defines a CBOR tag for time that allows a more elaborate
representation of time, as well as related CBOR tags for duration and
time period. This document is intended as the reference document for
the IANA registration of the CBOR tags defined.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9581.
Copyright Notice
Copyright (c) 2024 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
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in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Terminology
2. Objectives
3. Time Format
3.1. Key 1
3.2. Keys 4 and 5
3.3. Keys -3, -6, -9, -12, -15, and -18
3.4. Keys -1, -13, and 13: Timescale
3.5. Clock Quality
3.5.1. ClockClass (Key -2)
3.5.2. ClockAccuracy (Key -4)
3.5.3. OffsetScaledLogVariance (Key -5)
3.5.4. Uncertainty (Key -7)
3.5.5. Guarantee (Key -8)
3.6. Keys -10, 10: Time Zone Hint
3.7. Keys -11, 11: IXDTF Suffix Information
4. Duration Format
5. Period Format
6. CDDL Type Names
7. IANA Considerations
7.1. CBOR Tags
7.2. Timescales Registry
7.3. Time Tag Map Keys Registry
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Collected CDDL
Acknowledgements
Authors' Addresses
1. Introduction
The Concise Binary Object Representation (CBOR) [RFC8949] provides
for the interchange of structured data without a requirement for a
pre-agreed schema. RFC 8949 defines a basic set of data types, as
well as a tagging mechanism that enables extending the set of data
types supported via an IANA registry for CBOR tags (see
[IANA.cbor-tags] and Section 9.2 of [RFC8949]).
RFC 8949 defines two tags for time: CBOR tag 0 ([RFC3339] time as a
string) and tag 1 (POSIX time as int or float). Since then,
additional requirements have become known. The present document
defines a CBOR tag for time that allows a more elaborate
representation of time, as well as related CBOR tags for durations
and time periods. This document is intended as the reference
document for the IANA registration of the CBOR tags defined.
1.1. Terminology
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.
The term "byte" is used in its now customary sense as a synonym for
"octet".
Superscript notation denotes exponentiation. For example, 2 to the
power of 64 is notated: 2^64. In the plain-text rendition of this
specification, superscript notation is not available and
exponentiation is therefore rendered by the surrogate notation seen
here in the plain-text rendition.
CBOR diagnostic notation is defined in Section 8 of [RFC8949] and
Appendix G of [RFC8610]. A machine-processable model of the data
structures defined in this specification is provided throughout the
text using the Concise Data Definition Language (CDDL) [RFC8610];
Appendix A provides the collected model information.
Several time-related terms, such as UTC and International Atomic Time
(TAI), are discussed in [RFC9557], which may be a useful companion
document beyond its direct use in Sections 3.6 and 3.7.
2. Objectives
For the time tag, the present specification addresses the following
objectives that go beyond the original tags 0 and 1 (defined in
Sections 3.4.1 and 3.4.2 of [RFC8949]):
* Additional resolution for epoch-based time (as in tag 1). CBOR
tag 1 only provides for representation of time as an integer and
as up to a binary64 floating-point value [IEEE754], which limits
the resolution to approximately microseconds at the time of
writing (progressively becoming worse over time).
* Indication of timescale. Tags 0 and 1 are defined for UTC;
however, some interchanges are better performed on TAI. Other
timescales may be registered once they become relevant (e.g., one
of the proposed successors to UTC that might no longer use leap
seconds or a scale based on smeared leap seconds).
By incorporating a way to transport [RFC9557] suffix information (see
Sections 3.6 and 3.7), additional indications of intents about the
interpretation of the time given can be provided; in particular, for
instances of time that, at the time they are being described, are in
the future. Intents might include information about time zones,
daylight saving times, preferred calendar representations, etc.
Semantics not covered by this document can be added by registering
additional map keys for the map that is the content of the tag (see
etime-detailed in Figure 1), the specification for which is
referenced by the registry entry (see Section 3).
For example, map keys could be registered for direct representations
of natural platform time formats. Some platforms use epoch-based
time formats that require some computation to convert them into the
representations allowed by tag 1; these computations can also lose
precision and cause ambiguities. (The present specification does not
take a position on whether tag 1 can be "fixed" to include, e.g.,
Decimal or Bigfloat representations. It does define how to use these
representations with the extended time format.)
Additional tags are defined for durations and periods.
3. Time Format
An extended time is indicated by CBOR tag 1001, the content of which
is a map data item (CBOR major type 5). The map may contain integer
(major types 0 and 1) or text string (major type 3) keys, with the
value type determined by each specific key. For negative integer
keys and text string values of the key, implementations MUST ignore
key/value pairs they do not understand; these keys are "elective", as
the extended time as a whole is still usable without the information
they carry if an implementation elects not to implement them.
Conversely, implementations MUST signal an error when encountering
key/value pairs that use unsigned integer keys they do not understand
or implement (these are either "base time" or "critical", see below).
The map MUST contain exactly one unsigned integer key that specifies
the "base time" and MAY also contain one or more negative integer or
text-string keys, which may encode supplementary information.
Supplementary information MAY also be provided by additional unsigned
integer keys that are explicitly defined to provide supplementary
information (we say these keys are defined to be "critical"); as
these are required to be understood, there can be no confusion with
base time keys.
Negative integer and text string keys always supply supplementary
information (they are "elective", and this will not be explicitly
stated below).
Supplementary information may include:
* a higher precision time offset to be added to the base time,
* a reference timescale and epoch different from the default UTC and
1970-01-01, and
* information about clock quality parameters, such as source,
accuracy, and uncertainty.
Additional keys can be defined by registering them in the "Time Tag
Map Keys" registry (Section 7.3). Registered keys may, for instance,
add intent information such as time zone, daylight saving time, and/
or possibly positioning coordinates to express information that would
indicate a local time.
This document does not define supplementary text keys. A number of
both unsigned and negative-integer keys are defined in the following
subsections.
Figure 1 provides a formal definition of tag 1001 in CDDL.
Etime = #6.1001(etime-detailed)
etime-framework = {
uint => any ; at least one base time
* (nint/text) => any ; elective supplementary information
* uint => any ; critical supplementary information
}
etime-detailed = ({
$$ETIME-BASETIME
ClockQuality-group
* $$ETIME-ELECTIVE
* $$ETIME-CRITICAL
* ((nint/text) .feature "etime-elective-extension") => any
* (uint .feature "etime-critical-extension") => any
}) .within etime-framework
Figure 1: CDDL Definition of Tag 1001
3.1. Key 1
Key 1 indicates a base time value that is exactly like the data item
that would be tagged by CBOR tag 1 (POSIX time [TIME_T] as int or
float). As described above, the time value indicated by the value
under this key can be further modified by other keys.
$$ETIME-BASETIME //= (1: ~time)
3.2. Keys 4 and 5
Keys 4 and 5 indicate a base time value and are like key 1, except
that the data item is an array as defined for CBOR tag 4 or 5,
respectively. This can be used to include a Decimal or Bigfloat
epoch-based float [TIME_T] in an extended time, e.g., to achieve
higher resolution or to avoid rounding errors.
$$ETIME-BASETIME //= (4: ~decfrac)
$$ETIME-BASETIME //= (5: ~bigfloat)
3.3. Keys -3, -6, -9, -12, -15, and -18
The keys -3, -6, -9, -12, -15, and -18 indicate additional decimal
fractions by giving an unsigned integer (major type 0) and scaling
this with the scale factor 1e-3, 1e-6, 1e-9, 1e-12, 1e-15, and 1e-18,
respectively (see Table 1). Each extended time data item MUST NOT
contain more than one of these keys. These additional fractions are
added to a base time in seconds [SI-SECOND] indicated by key 1, which
then MUST also be present and MUST have an integer value.
+=====+==============+=================+
| Key | Meaning | Example Usage |
+=====+==============+=================+
| -3 | milliseconds | Java time |
+-----+--------------+-----------------+
| -6 | microseconds | (old) UNIX time |
+-----+--------------+-----------------+
| -9 | nanoseconds | (new) UNIX time |
+-----+--------------+-----------------+
| -12 | picoseconds | Haskell time |
+-----+--------------+-----------------+
| -15 | femtoseconds | (future) |
+-----+--------------+-----------------+
| -18 | attoseconds | (future) |
+-----+--------------+-----------------+
Table 1: Keys for Decimally Scaled
Fractions
$$ETIME-ELECTIVE //= (-3: uint)
$$ETIME-ELECTIVE //= (-6: uint)
$$ETIME-ELECTIVE //= (-9: uint)
$$ETIME-ELECTIVE //= (-12: uint)
$$ETIME-ELECTIVE //= (-15: uint)
$$ETIME-ELECTIVE //= (-18: uint)
Note that these keys have been provided to facilitate representing
pairs of the form second/decimal fraction of a second, as found for
instance in C timespec (Section 7.27.1 of [C]). When ingesting a
timestamp with one of these keys into a type provided by the target
platform, care has to be taken to meet its invariants. For example,
for C timespec, the fractional part tv_nsec needs to be between 0
inclusive and 10^9 exclusive, which can be achieved by also adjusting
the base time appropriately.
3.4. Keys -1, -13, and 13: Timescale
Keys -1, -13, and 13 are used to indicate a timescale, where key 13
is critical. Keys -1 and -13 have identical semantics (both are
assigned because key -1 was chosen first and then, when key 13 was
added, it appeared desirable to have a negative equivalent). Each
extended time data item MUST NOT contain more than one of these keys.
The value 0 indicates UTC, with the POSIX epoch [TIME_T]; the value 1
indicates TAI, with the Precision Time Protocol (PTP) epoch (1
January 1970 00:00:00 TAI, see [IEEE1588-2019] or [IEEE1588-2008]).
$$ETIME-ELECTIVE //= (-1 => $ETIME-TIMESCALE)
$$ETIME-ELECTIVE //= (-13 => $ETIME-TIMESCALE)
$$ETIME-CRITICAL //= (13 => $ETIME-TIMESCALE)
$ETIME-TIMESCALE /= &(etime-utc: 0)
$ETIME-TIMESCALE /= &(etime-tai: 1)
If none of the keys are present, the default timescale value 0 is
implied.
Timescale values MUST be unsigned integers or text strings; text
strings are provided for experimentation and MUST NOT be used between
parties that are not both part of the experiment. Additional
unsigned integer values can be registered in the "Timescales"
registry (Section 7.2). (Note that there should be no timescales
"GPS" or "NTP" [RFC5905] -- instead, the time should be converted to
TAI or UTC using a single addition or subtraction.)
t = t - 2208988800
utc ntp
t = t + 315964819
tai gps
Figure 2: Converting Common Offset Timescales
| Editor's note: This initial set of timescales was deliberately
| chosen to be frugal, as the specification of the tag provides
| an extension point where additional timescales can be
| registered at any time. Registrations are clearly needed for
| earth-referenced timescales (such as UT1 and TT), as well as
| possibly for specific realizations of abstract timescales (such
| as TAI(USNO), the specific realization obtained at the United
| States Naval Observatory, which is more accurate as a constant
| offset basis for GPS times). While the registration process
| itself is trivial, these registrations need to be made based on
| a solid specification of their actual definition.
3.5. Clock Quality
A number of keys are defined to indicate the quality of the clock
that was used to determine the point in time.
The first three are analogous to clock-quality-grouping in [RFC8575],
which is in turn based on the definitions in [IEEE1588-2008]; the
last two are specific to this document.
ClockQuality-group = (
? &(ClockClass: -2) => uint .size 1 ; PTP/RFC8575
? &(ClockAccuracy: -4) => uint .size 1 ; PTP/RFC8575
? &(OffsetScaledLogVariance: -5) => uint .size 2 ; PTP/RFC8575
? &(Uncertainty: -7) => ~time/~duration
? &(Guarantee: -8) => ~time/~duration
)
3.5.1. ClockClass (Key -2)
Key -2 (ClockClass) can be used to indicate the clock class as per
[RFC8575] (which is based on Table 5 in Section 7.6.2.4 of
[IEEE1588-2008]; Table 4 in Section 7.6.2.5 of [IEEE1588-2019] has
updated language). It is defined as a one-byte unsigned integer as
that is the range defined in IEEE 1588.
3.5.2. ClockAccuracy (Key -4)
Key -4 (ClockAccuracy) can be used to indicate the clock accuracy as
per [RFC8575] (which is based on Table 6 in Section 7.6.2.5 of
[IEEE1588-2008]; additional values have been defined in Table 5 in
Section 7.6.2.6 of [IEEE1588-2019]). It is defined as a one-byte
unsigned integer as that is the range defined there. The range
between 23 and 47 is a slightly distorted logarithmic scale from 1 ps
to 1 s in [IEEE1588-2019] (in [IEEE1588-2008], the range was a subset
of that, 32 to 47 for 25 ns to 1 s) -- see Figure 3; the number 254
is the value to be used if an unknown accuracy needs to be expressed.
acc
enum ≈ 48 + ⌊2 ⋅log ──── - ε⌋
acc 10 s
Figure 3: Approximate Conversion from Accuracy to Accuracy
Enumeration Value
3.5.3. OffsetScaledLogVariance (Key -5)
Key -5 (OffsetScaledLogVariance) can be used to represent the
variance exhibited by the clock when it has lost its synchronization
with an external reference clock. The details for the computation of
this characteristic are defined in Section 7.6.3 of [IEEE1588-2019]
and the same section in [IEEE1588-2008].
3.5.4. Uncertainty (Key -7)
Key -7 (Uncertainty) can be used to represent a known uncertainty of
measurement for the clock as a numeric value in seconds or as a
duration (Section 4).
For this document, uncertainty is defined as in Section 2.2.3 of
[GUM]: "parameter, associated with the result of a measurement, that
characterizes the dispersion of the values that could reasonably be
attributed to the measurand". More specifically, the value for this
key represents the expanded uncertainty for k = 2 (Section 6.2.1 of
[GUM]) in seconds.
Note that the additional information that can be meaningfully
provided with the duration that represents an uncertainty is limited,
e.g., it is not customary to provide an uncertainty for a duration
representing an uncertainty. Implementations are free to reduce the
information contained in an uncertainty (which is already elective)
to the information they can process.
For example, a timestamp that is given to a resolution of 10^-6
seconds (microseconds) but only has an uncertainty of 10^-3 seconds
(milliseconds) could be expressed by one of the extended time tags in
Figure 4 (note the slight rounding error in the third case, which is
probably inconsequential for an uncertainty value):
1001({1: 1697724754, -6: 873294, -7: {1: 0, -6: 1000}}),
1001({1: 1697724754, -6: 873294, -7: {1: 0, -3: 1}}),
1001({1: 1697724754, -6: 873294, -7: {1: 0.001}})
Figure 4: Examples Using Uncertainty
3.5.5. Guarantee (Key -8)
Key -8 (Guarantee) can be used to represent a stated guarantee for
the accuracy of the point in time as a numeric value in seconds or as
a duration (Section 4) representing the maximum allowed deviation
from the true value.
While such a guarantee is unattainable in theory, existing standards
such as [RFC3161] stipulate the representation of such guarantees,
and therefore this format provides a way to represent them as well;
the time value given is nominally guaranteed to not deviate from the
actual time by more than the value of the guarantee in seconds.
Note that the additional information that can be meaningfully
provided with the duration that represents a guarantee is limited,
e.g., it is not meaningful to provide a guarantee of accuracy for the
duration representing a guarantee of accuracy. Implementations are
free to reduce a guarantee (which is already elective) to the
information they can process.
3.6. Keys -10, 10: Time Zone Hint
Keys -10 and 10 supply supplementary information, where key 10 is
critical.
They can be used to provide a hint about the time zone that would
best fit for displaying the time given to humans, using a text string
in the format defined for time-zone-name or time-numoffset in
[RFC9557]. Key -10 is equivalent to providing this information as an
elective hint, while key 10 provides this information as critical
(i.e., it MUST be used when interpreting the entry with this key).
Keys -10 and 10 MUST NOT both be present.
$$ETIME-ELECTIVE //= (-10: time-zone-info)
$$ETIME-CRITICAL //= (10: time-zone-info)
time-zone-info = tstr .abnf
("time-zone-name / time-numoffset" .det IXDTFtz)
IXDTFtz = '
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-zone-initial = ALPHA / "." / "_"
time-zone-char = time-zone-initial / DIGIT / "-" / "+"
time-zone-part = time-zone-initial *time-zone-char
; but not "." or ".."
time-zone-name = time-zone-part *("/" time-zone-part)
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
' ; extracted from [RFC9557] and [RFC3339]
3.7. Keys -11, 11: IXDTF Suffix Information
Keys -11 and 11 supply supplementary information, where key 11 is
critical.
Similar to keys -10 and 10, keys -11 (elective) and 11 (critical) can
be used to provide additional information in the style of Internet
Extended Date/Time Format (IXDTF) suffixes, such as the calendar that
would best fit for displaying the time given to humans. The key's
value is a map that has IXDTF suffix-key names as keys and
corresponding suffix values as values, specifically:
$$ETIME-ELECTIVE //= (-11: suffix-info-map)
$$ETIME-CRITICAL //= (11: suffix-info-map)
suffix-info-map = { * suffix-key => suffix-values }
suffix-key = tstr .abnf ("suffix-key" .det IXDTF)
suffix-values = one-or-more<suffix-value>
one-or-more<T> = T / [ 2* T ]
suffix-value = tstr .abnf ("suffix-value" .det IXDTF)
IXDTF = '
key-initial = lcalpha / "_"
key-char = key-initial / DIGIT / "-"
suffix-key = key-initial *key-char
suffix-value = 1*alphanum
alphanum = ALPHA / DIGIT
lcalpha = %x61-7A
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
' ; extracted from [RFC9557]
When keys -11 and 11 are both present, the two maps MUST NOT have
entries with the same map keys.
Figure 4 of [RFC9557] gives an example for an extended date-time with
both time zone and suffix information:
1996-12-19T16:39:57-08:00[America/Los_Angeles][u-ca=hebrew]
A time tag that is approximating this example, in CBOR diagnostic
notation, would be:
/ 1996-12-19T16:39:57-08:00[America//Los_Angeles][u-ca=hebrew] /
1001({ 1: 851042397,
-10: "America/Los_Angeles",
-11: { "u-ca": "hebrew" }
})
Note that both -10 and -11 are using negative keys and therefore
provide elective information, as in the IXDTF form given in the
comment. Also note that, in this example, the time numeric offset
(-08:00) is lost in translating from the [RFC3339] information in the
IXDTF into a POSIX time that can be included under key 1 in a time
tag.
4. Duration Format
A duration is the length of an interval of time. Durations in this
format are given in International System of Units (SI) seconds,
possibly adjusted for conventional corrections of the timescale given
(e.g., leap seconds).
Except for using tag 1002 instead of 1001, durations are structurally
identical to time values.
Duration = #6.1002(etime-detailed)
Semantically, they do not measure the time elapsed from a given epoch
but from the start to the end of an (otherwise unspecified) interval
of time.
In combination with an epoch identified in the context, a duration
can also be used to express an absolute time.
Without such context, durations are subject to some uncertainties
underlying the timescale used. For example, for durations intended
as a determinant of future time periods, there is some uncertainty of
what irregularities (such as leap seconds and timescale corrections)
will be exhibited by the timescale in that period. For durations as
measurements of past periods, abstracting the period to a duration
loses some detail about timescale irregularities. For many
applications, these uncertainties are acceptable and thus the use of
durations is appropriate.
| Note that the durations defined in [ISO8601:1988] and
| [ISO8601-1:2019] are rather different from the ones defined in
| the present specification; there is no intention to support ISO
| 8601 durations here.
5. Period Format
A period is a specific interval of time, specified as either two
extended times giving the start and the end of that interval or as
one of these two plus a duration.
This is represented as an array of unwrapped time and duration
elements, tagged with tag 1003, one of:
* a start and end time, in which case the tag content is an array of
two unwrapped extended time elements, or
* a start time with duration or an end time with duration. The tag
content is an array of 3 elements: the first two as above but
either the start or end time MUST be set to null and the third one
then is an unwrapped duration.
A simple CDDL definition that does not capture all the constraints
is:
simple-Period = #6.1003([
start: ~Etime / null
end: ~Etime / null
? duration: ~Duration
])
Exactly two out of the three elements must be present and non-null;
this can be somewhat more verbosely expressed in CDDL as:
Period = #6.1003([
(start: ~Etime,
((end: ~Etime) //
(end: null,
duration: ~Duration))) //
(start: null,
end: ~Etime,
duration: ~Duration)
])
6. CDDL Type Names
When detailed validation is not needed, the type names defined in
Figure 5 are recommended:
etime = #6.1001({* (int/tstr) => any})
duration = #6.1002({* (int/tstr) => any})
period = #6.1003([~etime/null, ~etime/null, ?~duration])
Figure 5: Recommended Type Names for CDDL
7. IANA Considerations
7.1. CBOR Tags
In the "CBOR Tags" registry [IANA.cbor-tags], IANA has allocated the
tags in Table 2.
+======+===========+===============+======================+
| Tag | Data Item | Semantics | Reference |
+======+===========+===============+======================+
| 1001 | map | extended time | [RFC9581, Section 3] |
+------+-----------+---------------+----------------------+
| 1002 | map | duration | [RFC9581, Section 4] |
+------+-----------+---------------+----------------------+
| 1003 | array | period | [RFC9581, Section 5] |
+------+-----------+---------------+----------------------+
Table 2: Values for Tags
IANA has updated the "Data Item" column for tag 1003 from "map" to
"array".
7.2. Timescales Registry
Per this specification, IANA has created a new "Timescales" registry
within the "Concise Binary Object Representation (CBOR) Tags"
registry group [IANA.cbor-tags]. The registration procedure requires
both "Expert Review" and "RFC Required" (Sections 4.5 and 4.7 of RFC
8126 [BCP26], respectively).
Each entry needs to provide a timescale name (a sequence of uppercase
ASCII characters and digits, where a digit may not occur at the
start: [A-Z][A-Z0-9]*), a value (CBOR unsigned integer, uint,
0..18446744073709551615), a brief description of the semantics, and a
specification reference (RFC). The initial contents are shown in
Table 3.
+===========+=======+======================+===========+
| Timescale | Value | Semantics | Reference |
+===========+=======+======================+===========+
| UTC | 0 | UTC with POSIX Epoch | [RFC9581] |
+-----------+-------+----------------------+-----------+
| TAI | 1 | TAI with PTP Epoch | [RFC9581] |
+-----------+-------+----------------------+-----------+
Table 3: Initial Content of Timescale Registry
7.3. Time Tag Map Keys Registry
Per this specification, IANA has created a new "Time Tag Map Keys"
registry within the "Concise Binary Object Representation (CBOR)
Tags" registry group [IANA.cbor-tags]. The registration procedure is
"Specification Required" (Section 4.6 of RFC 8126 [BCP26]).
The designated expert is requested to assign the key values with the
shortest encodings (1+0 and 1+1 encoding) to registrations that are
likely to enjoy wide use and can benefit from short encodings.
Each entry needs to provide a map key value (CBOR integer, int,
-18446744073709551616..18446744073709551615), a brief description of
the semantics, and a specification reference. Note that negative
integers indicate an elective key, while unsigned integers indicate a
key that either provides a base time or is critical. The designated
expert is requested to discuss with the registrant whether or not it
is desirable to register a pair of an elective and a critical key for
the same information, where the elective key value is the negative of
the critical key (similar to how for example -11 and 11 have been
assigned in Table 4). For the unsigned integers as keys, the choice
of base time or critical needs to be indicated in the brief semantics
description. (Elective map keys may be explicitly marked as such in
the description, e.g., to distinguish them from critical keys.)
The initial contents are shown in Table 4.
+=======+=====================================+============+
| Value | Semantics | Reference |
+=======+=====================================+============+
| -18 | attoseconds | [RFC9581] |
+-------+-------------------------------------+------------+
| -15 | femtoseconds | [RFC9581] |
+-------+-------------------------------------+------------+
| -13 | timescale (elective) | [RFC9581] |
+-------+-------------------------------------+------------+
| -12 | picoseconds | [RFC9581] |
+-------+-------------------------------------+------------+
| -11 | IXDTF Suffix Information (elective) | [RFC9581], |
| | | [RFC9557] |
+-------+-------------------------------------+------------+
| -10 | IXDTF Time Zone Hint (elective) | [RFC9581], |
| | | [RFC9557] |
+-------+-------------------------------------+------------+
| -9 | nanoseconds | [RFC9581] |
+-------+-------------------------------------+------------+
| -8 | Guarantee | [RFC9581] |
+-------+-------------------------------------+------------+
| -7 | Uncertainty | [RFC9581] |
+-------+-------------------------------------+------------+
| -6 | microseconds | [RFC9581] |
+-------+-------------------------------------+------------+
| -5 | Offset-Scaled Log Variance | [RFC9581] |
+-------+-------------------------------------+------------+
| -4 | Clock Accuracy | [RFC9581] |
+-------+-------------------------------------+------------+
| -3 | milliseconds | [RFC9581] |
+-------+-------------------------------------+------------+
| -2 | Clock Class | [RFC9581] |
+-------+-------------------------------------+------------+
| -1 | timescale (elective) legacy | [RFC9581] |
+-------+-------------------------------------+------------+
| 1 | base time value as in CBOR tag 1 | [RFC8949], |
| | | [RFC9581] |
+-------+-------------------------------------+------------+
| 4 | base time value as in CBOR tag 4 | [RFC8949], |
| | | [RFC9581] |
+-------+-------------------------------------+------------+
| 5 | base time value as in CBOR tag 5 | [RFC8949], |
| | | [RFC9581] |
+-------+-------------------------------------+------------+
| 10 | IXDTF Time Zone Hint (critical) | [RFC9557], |
| | | [RFC9581] |
+-------+-------------------------------------+------------+
| 11 | IXDTF Suffix Information (critical) | [RFC9557], |
| | | [RFC9581] |
+-------+-------------------------------------+------------+
| 13 | timescale (critical) | [RFC9581] |
+-------+-------------------------------------+------------+
Table 4: Initial Content of Time Tag Map Keys Registry
8. Security Considerations
The security considerations of [RFC8949] apply; the tags introduced
here are not expected to raise security considerations beyond those.
Time, of course, has significant security considerations; these
include the exploitation of ambiguities where time is security
relevant (e.g., for freshness or in a validity span) or the
disclosure of characteristics of the emitting system (e.g., time zone
or clock resolution and wall clock offset).
A more detailed discussion of security considerations emanating from
using a representation of time that allows the inclusion of complex
and possibly inconsistent information is available in "Security
Considerations" (Section 7 of [RFC9557]).
9. References
9.1. Normative References
[BCP26] Best Current Practice 26,
<https://www.rfc-editor.org/info/bcp26>.
At the time of writing, this BCP comprises the following:
Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[GUM] Joint Committee for Guides in Metrology, "Evaluation of
measurement data -- Guide to the expression of uncertainty
in measurement", JCGM 100:2008, September 2008,
<https://www.bipm.org/en/publications/guides/gum.html>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/cbor-tags>.
[IEEE1588-2008]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
IEEE 1588-2008, July 2008,
<https://standards.ieee.org/ieee/1588/4355/>. Often
called PTP v2, as it replaced the earlier 2002 version of
this standard by a non-backwards compatible protocol.
[IEEE1588-2019]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
IEEE 1588-2019, June 2020,
<https://standards.ieee.org/ieee/1588/6825/>. Often
called PTP v2.1, as it has been designed so it can be used
in a way that is fully backwards compatible to
IEEE1588-2008.
[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>.
[RFC8575] Jiang, Y., Ed., Liu, X., Xu, J., and R. Cummings, Ed.,
"YANG Data Model for the Precision Time Protocol (PTP)",
RFC 8575, DOI 10.17487/RFC8575, May 2019,
<https://www.rfc-editor.org/info/rfc8575>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC9557] Sharma, U. and C. Bormann, "Date and Time on the Internet:
Timestamps with Additional Information", RFC 9557,
DOI 10.17487/RFC9557, April 2024,
<https://www.rfc-editor.org/info/rfc9557>.
[SI-SECOND]
ISO, "Quantities and units -- Part 3: Space and time",
ISO 80000-3:2019, October 2019,
<https://www.iso.org/standard/64974.html>.
[TIME_T] IEEE, "The Open Group Base Specifications Issue 7",
Section 4.16 Seconds Since the Epoch, IEEE
Std 1003.1-2017, 2018,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16>.
9.2. Informative References
[C] ISO, "Information technology -- Programming languages --
C", Fourth Edition, ISO/IEC 9899:2018, June 2018,
<https://www.iso.org/standard/74528.html>. Contents
available via <https://www.open-
std.org/jtc1/sc22/wg14/www/docs/n2310.pdf>
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic",
IEEE 754-2019, DOI 10.1109/IEEESTD.2019.8766229, July
2019, <https://ieeexplore.ieee.org/document/8766229>.
[ISO8601-1:2019]
ISO, "Date and time -- Representations for information
interchange -- Part 1: Basic rules", ISO 8601-1:2019,
February 2019, <https://www.iso.org/standard/70907.html>.
[ISO8601:1988]
ISO, "Data elements and interchange formats -- Information
interchange -- Representation of dates and times",
ISO 8601:1988, June 1988,
<https://www.iso.org/standard/15903.html>. Also available
from <https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/
fipspub4-1-1991.pdf
(https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/
fipspub4-1-1991.pdf)>.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
2001, <https://www.rfc-editor.org/info/rfc3161>.
[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>.
[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>.
Appendix A. Collected CDDL
This appendix collects the CDDL rules spread over the document into
one convenient place.
Etime = #6.1001(etime-detailed)
etime-framework = {
uint => any ; at least one base time
* (nint/text) => any ; elective supplementary information
* uint => any ; critical supplementary information
}
etime-detailed = ({
$$ETIME-BASETIME
ClockQuality-group
* $$ETIME-ELECTIVE
* $$ETIME-CRITICAL
* ((nint/text) .feature "etime-elective-extension") => any
* (uint .feature "etime-critical-extension") => any
}) .within etime-framework
$$ETIME-BASETIME //= (1: ~time)
$$ETIME-BASETIME //= (4: ~decfrac)
$$ETIME-BASETIME //= (5: ~bigfloat)
$$ETIME-ELECTIVE //= (-3: uint)
$$ETIME-ELECTIVE //= (-6: uint)
$$ETIME-ELECTIVE //= (-9: uint)
$$ETIME-ELECTIVE //= (-12: uint)
$$ETIME-ELECTIVE //= (-15: uint)
$$ETIME-ELECTIVE //= (-18: uint)
$$ETIME-ELECTIVE //= (-1 => $ETIME-TIMESCALE)
$$ETIME-ELECTIVE //= (-13 => $ETIME-TIMESCALE)
$$ETIME-CRITICAL //= (13 => $ETIME-TIMESCALE)
$ETIME-TIMESCALE /= &(etime-utc: 0)
$ETIME-TIMESCALE /= &(etime-tai: 1)
ClockQuality-group = (
? &(ClockClass: -2) => uint .size 1 ; PTP/RFC8575
? &(ClockAccuracy: -4) => uint .size 1 ; PTP/RFC8575
? &(OffsetScaledLogVariance: -5) => uint .size 2 ; PTP/RFC8575
? &(Uncertainty: -7) => ~time/~duration
? &(Guarantee: -8) => ~time/~duration
)
$$ETIME-ELECTIVE //= (-10: time-zone-info)
$$ETIME-CRITICAL //= (10: time-zone-info)
time-zone-info = tstr .abnf
("time-zone-name / time-numoffset" .det IXDTFtz)
IXDTFtz = '
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-zone-initial = ALPHA / "." / "_"
time-zone-char = time-zone-initial / DIGIT / "-" / "+"
time-zone-part = time-zone-initial *time-zone-char
; but not "." or ".."
time-zone-name = time-zone-part *("/" time-zone-part)
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
' ; extracted from [RFC9557] and [RFC3339]
$$ETIME-ELECTIVE //= (-11: suffix-info-map)
$$ETIME-CRITICAL //= (11: suffix-info-map)
suffix-info-map = { * suffix-key => suffix-values }
suffix-key = tstr .abnf ("suffix-key" .det IXDTF)
suffix-values = one-or-more<suffix-value>
one-or-more<T> = T / [ 2* T ]
suffix-value = tstr .abnf ("suffix-value" .det IXDTF)
IXDTF = '
key-initial = lcalpha / "_"
key-char = key-initial / DIGIT / "-"
suffix-key = key-initial *key-char
suffix-value = 1*alphanum
alphanum = ALPHA / DIGIT
lcalpha = %x61-7A
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
' ; extracted from [RFC9557]
Duration = #6.1002(etime-detailed)
simple-Period = #6.1003([
start: ~Etime / null
end: ~Etime / null
? duration: ~Duration
])
Period = #6.1003([
(start: ~Etime,
((end: ~Etime) //
(end: null,
duration: ~Duration))) //
(start: null,
end: ~Etime,
duration: ~Duration)
])
etime = #6.1001({* (int/tstr) => any})
duration = #6.1002({* (int/tstr) => any})
period = #6.1003([~etime/null, ~etime/null, ?~duration])
Figure 6: Collected CDDL Rules from This Specification
Acknowledgements
The authors would like to acknowledge the many comments from members
of the CBOR WG, Francesca Palombini for her AD review, Thomas Fossati
and Qin Wu for their directorate reviews, and Rohan Mahy for one more
review late in the process.
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Ben Gamari
Well-Typed
117 Middle Rd.
Portsmouth, NH 03801
United States of America
Email: ben@well-typed.com
Henk Birkholz
Fraunhofer Institute for Secure Information Technology
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@ietf.contact