]> Universität Bremen TZI

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Well-Typed

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Fraunhofer Institute for Secure Information Technology

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Internet-Draft 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 (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 duration and time period. It is intended as the reference document for the IANA registration of the CBOR tags defined. The present version (-10) addresses comments and WG discussion during the AD review. About This Document Status information for this document may be found at . Discussion of this document takes place on the CBOR Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

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. In CBOR, one point of extensibility is the definition of CBOR tags. 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 duration and time period. It is intended as the reference document for the IANA registration of the CBOR tags defined.

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 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". 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. CBOR diagnostic notation is defined in and . A machine-processable model of the data structures defined in this specification is provided throughout the text using the Concise Data Definition Language, CDDL ; provides the collected model information.

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 timescale. Tags 0 and 1 are 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 suffix information (, ), additional indications can be provided of intents about the interpretation of the time given, in particular for future times. Intents might include information about time zones, daylight savings times, preferred calendar representations, etc. Semantics not covered by this document can be added by registering additional map keys for the map inside the tag, the specification for which is referenced by the registry entry (, ). For example, map keys could be registered for:

• 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. (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.

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. Conversely, for unsigned integer keys, implementations MUST signal as an error key/value pairs 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 ("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 ("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
• information about clock quality parameters, such as source, accuracy, and uncertainty

Additional keys can be defined by registering them in the Map Key Registry (). 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.

This document does not define supplementary text keys. A number of both unsigned and negative-integer keys are defined in the following subsections. A formal definition of Tag 1001 in CDDL is: 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 ]]>

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 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 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 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)

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 (compare Section 7.27.1 of ). 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. E.g., for C timespec, the fractional part tv_nsec needs to be between 0 inclusive and 10⁹ exclusive, which can be achieved by also adjusting the base time appropriately.

Key -1: Timescale Key -1 is used to indicate a timescale. The value 0 indicates UTC, with the POSIX epoch ; the value 1 indicates TAI, with the PTP (Precision Time Protocol) epoch (1 January 1970 00:00:00 TAI, see or ). $ETIME-TIMESCALE) $ETIME-TIMESCALE /= &(etime-utc: 0) $ETIME-TIMESCALE /= &(etime-tai: 1) ]]> If key -1 is not present, timescale value 0 is implied. Additional values can be registered in the Timescale Registry (); values MUST be integers or text strings. (Note that there should be no timescales "GPS" or "NTP" — instead, the time should be converted to TAI or UTC using a single addition or subtraction.)

Converting Common Offset Timescales

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. 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 ) ]]>

ClockClass (Key -2) Key -2 (ClockClass) can be used to indicate the clock class as per (which is based on Table 5 in Section 7.6.2.4 of ; this has updated language as Table 4 in Section 7.6.2.5 of ). It is defined as a one-byte unsigned integer as that is the range defined there.

ClockAccuracy (Key -4) Key -4 (ClockAccuracy) can be used to indicate the clock accuracy as per (which is based on Table 6 in Section 7.6.2.5 of ; additional values have been defined in Table 5 in Section 7.6.2.6 of ). 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 (in the range was a subset of that, 32 to 47 for 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 and the same section in .

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. 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 an uncertainty (which is already elective) to the information they can process.

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. 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.

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 . 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.

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 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: suffix-values } suffix-key = tstr .abnf ("suffix-key" .det IXDTF) suffix-values = one-or-more one-or-more = 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 [IXDTF]; update as needed! ]]> When keys -11 and 11 both are present, the two maps MUST NOT have entries with the same map keys. Figure 4 of gives an example for an extended date-time with both time zone and suffix information: A time tag that is approximating this example, in CBOR diagnostic notation, would be: Note that both -10 and -11 are using negative keys and therefore provide elective information, as in the IXDTF form. Note also that in this example the time numeric offset (-08:00) is lost in translating from the information in the IXDTF into a Posix time that can be included under Key 1 in a time tag.

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 timescale 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. Without such context, durations are subject to some uncertainties underlying the timescale used. E.g., for durations intended as a determinant of future time periods, there is some uncertainty of what irregularities (such as leap seconds, 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.

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: 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 somewhat verbosely expressed in CDDL as:

CDDL typenames When detailed validation is not needed, the type names defined in are recommended:

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

IANA Considerations RFC Editor: please replace RFCthis with the RFC number of this RFC, and remove this note.

CBOR tags In the "CBOR Tags" registry , IANA has allocated the tags in from what was at the time 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".

Timescale Registry This specification defines a new registry titled "Timescales" in the "CBOR Tags" registry group , with a combination of "Expert Review" and "RFC Required" as the Registration Procedure (Sections and of ). 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), and brief description of the semantics, and a specification reference (RFC). The initial contents are shown in . Initial Content of Timescale Registry

Timescale	Value	Semantics	Reference
UTC	0	UTC with POSIX Epoch	[RFCthis]
TAI	1	TAI with PTP Epoch	[RFCthis]

Time Tag Map Key Registry This specification defines a new registry titled "Time Tag Map Keys" in the "CBOR Tags" registry group , with "Specification Required" as the Registration Procedure (). 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), a brief description of the semantics, and a specification reference (RFC). The initial contents are shown in . Initial Content of Time Tag Map Keys Registry

Value	Semantics	Reference
-18	attoseconds	[RFCthis]
-15	femtoseconds	[RFCthis]
-12	picoseconds	[RFCthis]
-11	IXDTF Suffix Information (elective)	[RFCthis],
-10	IXDTF Time Zone Hint (elective)	[RFCthis],
-9	nanoseconds	[RFCthis]
-8	Guarantee	[RFCthis]
-7	Uncertainty	[RFCthis]
-6	microseconds	[RFCthis]
-5	Offset-Scaled Log Variance	[RFCthis]
-4	Clock Accuracy	[RFCthis]
-3	milliseconds	[RFCthis]
-2	Clock Class	[RFCthis]
1	Base Time value as in CBOR Tag 1	[RFCthis]
4	Base Time value as in CBOR Tag 4	[RFCthis]
5	Base Time value as in CBOR Tag 5	[RFCthis]
10	IXDTF Time Zone Hint (critical)	[RFCthis],
11	IXDTF Suffix Information (critical)	[RFCthis],

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).

References Normative References The Concise Binary Object Representation (CBOR) 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. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. The Open Group Base Specifications International Organization for Standardization (ISO) This document defines a YANG data model for the configuration of devices and clocks using the Precision Time Protocol (PTP) as specified in IEEE Std 1588-2008. It also defines the retrieval of the configuration information, the data sets and the running states of PTP clocks. The YANG module in this document conforms to the Network Management Datastore Architecture (NMDA). IEEE Often called PTP v2, as it replaced the earlier 2002 version of this standard by a non-backwards compatible protocol. IEEE Often called PTP v2.1, as it has been designed so it can be used in way that is fully backwards compatible to IEEE1588-2008. Joint Committee for Guides in Metrology IANA This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Igalia, S.L. Universität Bremen TZI This document defines an extension to the timestamp format defined in RFC3339 for representing additional information including a time zone. It updates RFC3339 in the specific interpretation of the local offset Z, which is no longer understood to "imply that UTC is the preferred reference point for the specified time"; see Section 2. // (This "cref" paragraph will be removed by the RFC editor:) The // present version (-09) addresses comments received during IETF last // call. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document describes the format of a request sent to a Time Stamping Authority (TSA) and of the response that is returned. It also establishes several security-relevant requirements for TSA operation, with regards to processing requests to generate responses. [STANDARDS-TRACK] This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar. International Organization for Standardization Also available from <⁠https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub4-1-1991.pdf>. International Organization for Standardization International Organization for Standardization Contents available via <⁠https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf> Fourth Edition

Collected CDDL This appendix collects the CDDL rules spread over the document into one convenient place.

Collected CDDL rules from this specification 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) $$ETIME-ELECTIVE //= (-1 => $ETIME-TIMESCALE) $ETIME-TIMESCALE /= &(etime-utc: 0) $ETIME-TIMESCALE /= &(etime-tai: 1) ClockQuality-group = ( ? &(ClockClass: -2) => uint .size 1 ; PTP/RFC8575 ? &(ClockAccuracy: -4) => uint .size 1 ; PTP/RFC8575 ? &(OffsetScaledLogVariance: -5) => uint .size 2 ; PTP/RFC8575 ? &(Uncertainty: -7) => ~time/~duration ? &(Guarantee: -8) => ~time/~duration ) $$ETIME-ELECTIVE //= (-10: time-zone-info) $$ETIME-CRITICAL //= (10: time-zone-info) time-zone-info = tstr .abnf ("time-zone-name / time-numoffset" .det IXDTFtz) IXDTFtz = ' time-hour = 2DIGIT ; 00-23 time-minute = 2DIGIT ; 00-59 time-numoffset = ("+" / "-") time-hour ":" time-minute time-zone-initial = ALPHA / "." / "_" time-zone-char = time-zone-initial / DIGIT / "-" / "+" time-zone-part = time-zone-initial *13(time-zone-char) ; but not "." or ".." time-zone-name = time-zone-part *("/" time-zone-part) ALPHA = %x41-5A / %x61-7A ; A-Z / a-z DIGIT = %x30-39 ; 0-9 ' ; extracted from [IXDTF] and [RFC3339]; update as needed $$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 one-or-more = 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 [IXDTF]; update as needed! Duration = #6.1002(etime-detailed) Period = #6.1003([ start: ~Time / null end: ~Time / null ? duration: ~Duration / null ]) clumsy-Period = #6.1003([ (start: ~Time, ((end: ~Time, ? duration: null) // (end: null, duration: ~Duration))) // (start: null, end: ~Time, duration: ~Duration) ]) etime = #6.1001({* (int/tstr) => any}) duration = #6.1002({* (int/tstr) => any}) period = #6.1003([~etime/null, ~etime/null, ~duration/null]) ]]>

Acknowledgements The authors would like to acknowledge the many comments from members of the CBOR WG, as well as Francesca Palombini for her AD review.