CBOR time tags: Concise Binary Object Representation (CBOR) Tags for Time, Duration, and Period

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.

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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