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

Introduction The Concise Binary Object Representation (CBOR, ) 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. (TBD: Expand on text from abstract here.)

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 . The term "byte" is used in its now customary sense as a synonym for "octet". Where bit arithmetic is explained, this document uses the notation familiar from the programming language C (including C++14's 0bnnn binary literals), except that the operator "**" stands for exponentiation.

Objectives For the time tag, the present specification addresses the following objectives that go beyond the original tags 0 and 1:

Additional resolution for epoch-based time (as in tag 1). CBOR tag 1 only provides for integer and up to binary64 floating point representation of times, limiting resolution to approximately microseconds at the time of writing (and progressively becoming worse over time).
Indication of time scale. Tags 0 and 1 are for UTC; however, some interchanges are better performed on TAI. Other time scales 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).

Not currently addressed, but possibly covered by the definition of additional map keys for the map inside the tag:

Direct representation 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. (TBD: 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 with the extended time format.)
Additional indication of intents about the interpretation of the time given, in particular for future times. Intents might include information about time zones, daylight savings times, etc.

Additional tags are defined for durations and periods.

Time Format An extended time is indicated by CBOR tag 1001, which tags 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. Implementations MUST ignore key/value types they do not understand for negative integer and text string values of the key. Not understanding key/value for unsigned keys is an error. The map must contain exactly one unsigned integer key, which specifies the "base time", and may also contain one or more negative integer or text-string keys, which may encode supplementary information such as:

a higher precision time offset to be added to the base time,
a reference time scale and epoch different from the default UTC and 1970-01-01
information about clock quality parameters, such as source, accuracy, and uncertainty

Future keys may add:

intent information such as timezone and daylight savings time, and/or possibly positioning coordinates, to express information that would indicate a local time.

While this document does not define supplementary text keys, a number of unsigned and negative-integer keys are defined below.

Key 1 Key 1 indicates a value that is exactly like the data item that would be tagged by CBOR tag 1 (Posix time as int or float). The time value indicated by the value under this key can be further modified by other keys.

Keys 4 and 5 Keys 4 and 5 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 in an extended time.

Keys -3, -6, -9, -12, -15, -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 ). More than one of these keys MUST NOT be present in one extended time data item. These additional fractions are added to a base time in seconds indicated by a Key 1, which then MUST also be present and MUST have an integer value. Key for decimally scaled Fractions

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)

Key -1: Time Scale Key -1 is used to indicate a time scale. The value 0 indicates UTC, with the POSIX epoch ; the value 1 indicates TAI, with the PTP (Precision Time Protocol) epoch . If key -1 is not present, time scale value 0 is implied. Additional values can be registered in the (TBD define name for time scale registry); values MUST be integers or text strings. (Note that there should be no time scales "GPS" or "NTP" -- instead, the time should be converted to TAI or UTC using a single addition or subtraction.)

Converting Common Offset Time Scales

Clock Quality A number of keys are defined to indicate the quality of clock that was used to determine the point in time. The first three are analogous to clock-quality-grouping in , which is in turn based on the definitions in ; two more are specific to this document. ClockQuality-group = ( ? ClockClass => uint .size 1 ; PTP/RFC8575 ? ClockAccuracy => uint .size 1 ; PTP/RFC8575 ? OffsetScaledLogVariance => uint .size 2 ; PTP/RFC8575 ? Uncertainty => ~time/~duration ? Guarantee => ~time/~duration ) ClockClass = -2 ClockAccuracy = -4 OffsetScaledLogVariance = -5 Uncertainty = -7 Guarantee = -8

ClockClass (Key -2) Key -2 (ClockClass) can be used to indicate the clock class as per Table 5 of . It is defined as a one-byte integer as that is the ranged defined there.

ClockAccuracy (Key -4) Key -4 (ClockAccuracy) can be used to indicate the clock accuracy as per Table 6 of . It is defined as a one-byte integer as that is the ranged defined there. The range between 32 and 47 is a slightly distorted logarithmic scale from 25 ns to 1 s (see ); the number 254 is the value to be used if an unknown accuracy needs to be expressed.

Approximate conversion from accuracy to accuracy enumeration value

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 .

Uncertainty (Key -7) Key -7 (Uncertainty) can be used to represent a known measurement uncertainty for the clock, as a numeric value in seconds or as a duration (). For this document, uncertainty is defined as in Section 2.2.3 of : "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 extended uncertainty for k = 2, in seconds.

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 () representing the maximum allowed deviation from the true value. While such a guarantee is unattainable in theory, existing standards such as 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.

Duration Format A duration is the length of an interval of time. Durations in this format are given in SI seconds, possibly adjusted for conventional corrections of the time scale given (e.g., leap seconds). Except for using Tag 1002 instead of 1001, durations are structurally identical to time values. 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.

Period Format A period is a specific interval of time, specified as either two times giving the start and the end of that interval, or as one of these two plus a duration. They are given as an array of unwrapped time and duration elements, tagged with Tag 1003: Period = #6.1003([ start: ~Time / null end: ~Time / null ? duration: ~Duration / null ]) If the third array element is not given, the duration element is null. Exactly two out of the three elements must be non-null, this can be clumsily expressed in CDDL as: Period = #6.1003([ (start: ~Time, ((end: ~Time, ? duration: null) // (end: null, duration: ~Duration))) // (start: null, end: ~Time, duration: ~Duration) ])

CDDL typenames For the use with the CBOR Data Definition Language, CDDL , the type names defined in are recommended:

Recommended type names for CDDL etime = #6.1001({* (int/tstr) => any}) duration = #6.1002({* (int/tstr) => any}) period = #6.1003([~etime/null, ~etime/null, ~duration/null])

IANA Considerations In the registry , IANA has allocated the tags in from the FCFS space, with the present document as the specification reference. Values for Tags

Tag	Data Item	Semantics
1001	map	[RFCthis] extended time
1002	map	[RFCthis] duration
1003	array	[RFCthis] period

IANA is requested to change the "Data Item" column for Tag 1003 from "map" to "array".

Security Considerations The security considerations of RFC 8949 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).