Notable CBOR Tags

Notable CBOR Tags Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

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's original edition, RFC 7049, defined a basic set of 16 tags as well as a registry that can be used to contribute additional tag definitions . Since RFC 7049 was published, at the time of writing some 250 definitions of tags and ranges of tags have been added to that registry. The present document provides a roadmap to a large subset of these tag definitions. Where applicable, it points to an IETF standards or standard development document that specifies the tag. Where no such document exists, the intention is to collect specification information from the sources of the registrations. After some more development, the present document is intended to be useful as a reference document for the IANA registrations of the CBOR tags the definitions of which have been collected. Note to Readers This is an individual submission to the CBOR working group of the IETF, https://datatracker.ietf.org/wg/cbor/about/. Discussion currently takes places on the github repository https://github.com/cabo/notable-tags. If the CBOR WG believes this is a useful document, discussion is likely to move to the CBOR WG mailing list and a github repository at the CBOR WG github organization, https://github.com/cbor-wg. The current version is true work in progress; some of the sections haven't been filled in yet, and in particular, permission has not been obtained from all authors of registered tag definitions to copy over their text.

Introduction (TO DO, expand on text from abstract here; move references here and neuter them in the abstract as per .) The selection of the tags presented here is somewhat arbitrary; considerations such as how wide the scope and area of application of a tag definition is combine with an assessment how "ready to use" the tag definition is (i.e., is the tag specification in a state where it can be used). This document can only be a snapshot of a subset of the current registrations. The most up to date set of registrations is always available in the registry "" .

The CBOR Tags Registry CBOR tags are an extension point of CBOR, and that extension point is managed through an IANA registry ( as set up in Section of RFC 8949 ), not through publishing documents. Some tag ranges require a stable specification to be publicly available under the "Specification Required" policy (Section of RFC 8126 ). That stable availability can be provided by any suitable means, such as a (specific revision of a) GitHub repository, a document published by an SDO or other entity, or even by a specific revision of an Internet-Draft, or an RFC if that happens to be in the pipeline. (In the latter case, while the RFC is being developed, the allocation will probably be made when that RFC still is in Internet-Draft state, which is then referenced in the registry and is usually updated to the RFC number when that is published.) An example for this is provided by of the present document. A few allocations were made for these tags (101, 121-127, 1280-1400) that are now a permanent part of the CBOR Tags registry. These allocations do reference revision –07 of the present Internet-Draft, because that contains an explanation of what these tags are supposed to be used for. That document revision –07 is part of the permanent record of the IETF and is not going away, so we are able to use it as a stable part of the information provided with the allocation. (In more recent revisions of the present document, some glue text was updated and a clerical error in illustrative CDDL code was fixed; eventually, there may be some incentive to update the registry entry to a newer revision of the present document.) As with other stable specifications referenced this way, there is no need for the present document to be a published RFC before the tags become useful; i.e., the publication action that makes the tag numbers available for wide use in interchange is the acceptance of the tags into the IANA CBOR Tags registry.

Terminology The definitions of apply. Specifically: The term "byte" is used in its now customary sense as a synonym for "octet"; "byte strings" are CBOR data items carrying a sequence of zero or more (binary) bytes, while "text strings" are CBOR data items carrying a sequence of zero or more Unicode scalar values (code points that can be part of a string of Unicode characters), encoded in UTF-8 . 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 superscript notation (example for two to the power of 64: 2⁶⁴) denotes exponentiation; in the plain text version of this document, superscript notation is rendered in paragraph text by C-incompatible surrogate notation as seen in this example. Ranges expressed using .. are inclusive of the limits given. Type names such as "int", "bigint" or "decfrac" are taken from , the Concise Data Definition Language (CDDL).

RFC 7049 (original CBOR specification) defines a number of tags that are listed here for convenience only. Tag numbers defined in RFC 7049

Tag number	Tag content	Short Description	Section of RFC 7049
0	UTF-8 string	Standard date/time string	2.4.1
1	multiple	Epoch-based date/time	2.4.1
2	byte string	Positive bignum	2.4.2
3	byte string	Negative bignum	2.4.2
4	array	Decimal fraction	2.4.3
5	array	Bigfloat	2.4.3
21	multiple	Expected conversion to base64url encoding	2.4.4.2
22	multiple	Expected conversion to base64 encoding	2.4.4.2
23	multiple	Expected conversion to base16 encoding	2.4.4.2
24	byte string	Encoded CBOR data item	2.4.4.1
32	UTF-8 string	URI	2.4.4.3
33	UTF-8 string	base64url	2.4.4.3
34	UTF-8 string	base64	2.4.4.3
35	UTF-8 string	Regular expression	2.4.4.3
36	UTF-8 string	MIME message	2.4.4.3
55799	multiple	Self-describe CBOR	2.4.5

Tags Related to Those Defined in RFC 7049 Separately registered tags that are directly related to the tags predefined in RFC 7049 include:

Tag 63, registered by this document (), is a parallel to tag 24, with the single difference that its byte string tag content carries a CBOR Sequence instead of a single encoded CBOR data item.
Tag 108, registered by this document (), is a parallel to tag 23, with the single difference that the hexadecimal form uses lowercase instead of uppercase letters.
- - Note that tag 23 has a serialization that is one byte shorter than tag 108, so if all else is equal, tag 23 (and thus upper case hex) would be chosen as it is slightly more efficient than tag 108. However, designers of CBOR protocols that use one of these tags often have reason to prefer lowercase hex in the application they are trying to be compatible with, in which case 108 provides a solution that is only one byte more expensive.
Tag 257, registered by Peter Occil with a specification in http://peteroupc.github.io/CBOR/binarymime.html, is a parallel to tag 36, except that the tag content is a byte string, which therefore can also carry binary MIME messages as per .
Tag 21065, having been registered by this document (), is a parallel to tag 35, with the difference that its text string tag content carries an I-Regexp regular expression instead of a regexp of a more unspecified flavor. Companion tag 21066, having been registered by Joe Hildebrand with a specification in https://github.com/hildjj/cbor-specs/blob/main/regexp.md, is the equivalent for JavaScript (ECMA262), but besides the regular expression itself also can include the regular expression flags as a separate item.

Tags from RFC 7049 not listed in RFC 8949 Appendix of RFC 8949 states:

Tag 35 is not defined by this document; the registration based on the definition in RFC 7049 remains in place.

The reason for this exclusion is that the definition of Tag 35 in , leaves too much open to ensure interoperability:

Tag 35 is for regular expressions in Perl Compatible Regular Expressions (PCRE) / JavaScript syntax [ECMA262].

Not only are two partially incompatible specifications given for the semantics, JavaScript regular expressions have also developed significantly within the decade since JavaScript 5.1 (which was referenced as "ECMA262" by ), making it less reliable to assume that a producing application will manage to stay within that 2011 subset. Nonetheless, the registration is in place, so it is available for applications that simply want to mark a text string as being a regular expression roughly of the PCRE/Javascript flavor families. See also Tag 21065 and 21066 above.

Security A number of CBOR tags are defined in security specifications that make use of CBOR.

COSE CBOR Object Signing and Encryption (COSE) is defined in a number of RFCs. was the initial specification, set up the registries, and populated them with an initial set of assignments. A revision split this specification into the data structure definitions (), an Internet Standard , and a separate document defining the representation for the algorithms employed , which is expected to be updated more frequently than the COSE format itself. added a separate set of algorithms for cryptographic hash functions (Hash functions have been a component of some combined algorithms but weren't originally assigned separate codepoints themselves). A revised COSE counter signature structure was defined in , another part of ; this also defines a tag for these. Tag numbers defined in RFC9052, COSE, and RFC 9338

Tag number	Tag content	Short Description
16	COSE_Encrypt0	COSE Single Recipient Encrypted Data Object
17	COSE_Mac0	COSE Mac w/o Recipients Object
18	COSE_Sign1	COSE Single Signer Data Object
19	COSE_Countersignature	COSE standalone V2 countersignature ()
96	COSE_Encrypt	COSE Encrypted Data Object
97	COSE_Mac	COSE MACed Data Object
98	COSE_Sign	COSE Signed Data Object

Tags for Bare Hash Values does not define CBOR tags for cryptographic Hash values; it rightly notes that Hash values are often used in structures that are application-specific and should be defined with those applications. However, there are many cases where just a bare hash value is required, and for these cases common tags are useful. In one use case, these tags occur in a data structure that is specified to indicate elision by using one of these tags as an alternative to some other data structure. To enable agility, tags need to indicate the hash function used, preferably using the COSE algorithms registry as populated by . The codepoint range available for the COSE algorithms registry is large, but the most likely range to be used for standard Hash functions is "Integer values between -256 and 255", which have the registry policy "Standards Action With Expert Review" (, Registry "" ). To this end, the present document registers a range of 512 tags from 18300 to 18811 (inclusive), paralleling the algorithm identifier range of -256 .. 255 (inclusive). The tag number for COSE algorithm number N is then defined to be 18556+N, except for N = 0 (see below). The tag value is a CBOR byte string, with the exception N = 0. For example, in SHA-256 has the COSE algorithm identifier -16. This is in the range -256 .. 255 (inclusive range). Therefore, tag 18540 (= 18556 + (-16)) is the tag for a byte string containing a SHA-256 hash. As a special case, there is one exception: Tag 18556 (= 18556 + 0) stands for the combination of a an explicit numeric COSE algorithm identifier with a hash value in an array, analogous to the use of COSE_CertHash in :

Generic CDDL for Tags for Bare Hash Values = #6.(value) General_COSE_Hash = #6.([ hashAlg: alg .within (int .ne directhash / tstr), hashValue: value .within bstr ]) hashmiddle = 18556 directhash = (-256 .. -1) / (1 .. 255) ]]> An example for the SHA-256 hash of "hello world" in CBOR diagnostic notation: The same in CBOR pretty printed hex: As none has been registered, no real example can be given for a hash algorithm with an identifier not in the standard range, but if -4711 were such an identifier, a hash with an explicit algorithm number could look like: Note that not all tags assigned in this section do parallel an algorithm that is a cryptographic hash algorithm. Where this is not the case, there currently is no defined semantics for this tag, but the tags are assigned anyway. The semantics of tags that parallel algorithm assignments other than for cryptographic hash functions could be defined by a future version of this specification. Note also that the cryptographic hashes in the content of the tag are not protected; any further protection (confidentiality, integrity) needs to be provided in the surrounding data structure, storage system, or communication channel.

RFC 8392 (CWT) defines the CBOR Web Token (CWT), making use of COSE to define a CBOR variant of the JOSE Web Token (JWT), , a standardized security token that has found use in the area of web applications, but is not technically limited to those. Tag number defined for RFC 8392 CBOR Web Token (CWT)

Tag number	Tag content	Short Description
61	CBOR Web Token (CWT)	CBOR Web Token (CWT)

CBOR-based Representation Formats Representation formats can be built on top of CBOR.

YANG-CBOR YANG is a data modeling language originally designed in the context of the Network Configuration Protocol (NETCONF) , now widely used for modeling management and configuration information. defines an XML-based representation format, and defines a JSON-based representation format for YANG. YANG-CBOR is a representation format for YANG data in CBOR. Tag numbers defined for YANG-CBOR

Tag number	Tag content	Short Description	Section of YANG-CBOR
43	byte string	YANG bits datatype	6.7
44	unsigned integer	YANG enumeration datatype	6.6
45	unsigned integer or text string	YANG identityref datatype	6.10
46	unsigned integer or text string or array	YANG instance-identifier datatype	6.13
47	unsigned integer	YANG Schema Item iDentifier (sid)	3.2

proposes to employ additional tags to enable the use of efficient binary encodings for certain frequently used YANG data types .

Protocols Protocols may want to allocate CBOR tag numbers to identify specific protocol elements.

DOTS DDoS Open Threat Signaling (DOTS) defines tag number 271 for the DOTS signal channel object in .

RAINS As an example for how experimental protocols can make use of CBOR tag definitions, the RAINS (Another Internet Naming Service) Protocol Specification defines tag number 15309736 for a RAINS Message . (The seemingly random tag number was chosen so that, when represented as an encoded CBOR tag argument, it contains the Unicode character 雨 in UTF-8, which represents rain in a number of languages.)

Datatypes

Advanced arithmetic General information about the representation of numbers in CBOR can be found in as well as . A number of tags have been registered for arithmetic representations beyond those built into CBOR and defined by tags in . These are all documented under http://peteroupc.github.io/CBOR/; the last pathname component for the URL is given in the column "Reference" of . Tags for advanced arithmetic

Tag number	Tag content	Short Description	Reference
30	array	Rational number	rational.html
264	array	Decimal fraction with arbitrary exponent	bigfrac.html
265	array	Bigfloat with arbitrary exponent	bigfrac.html
268	array	Extended decimal fraction	extended.html
269	array	Extended bigfloat	extended.html
270	array	Extended rational number	extended.html

CBOR's basic generic data model (Section of RFC 8949 ) has a number system with limited-range integers (major types 0 and 1: -2⁶⁴..2⁶⁴-1) and floating point numbers that cover binary16, binary32, and binary64 (including non-finites) from . With the tags defined with , the extended generic data model (Section of RFC 8949 ) adds unlimited-range integers (tag numbers 2 and 3, "bigint" in CDDL) as well as floating point values using the bases 2 (tag number 5, "bigfloat") and 10 (tag number 4, "decfrac"). This pre-defined number system has a number of limitations that are addressed in three of the tags discussed here:

Tag number 30 allows the representation of rational numbers as a ratio of two integers: a numerator (usually written as the top part of a fraction), and a denominator (the bottom part), where both integers can be limited-range basic and unlimited-range integers. The mathematical value of a rational number is the numerator divided by the denominator. This tag can express all numbers that the extended generic data model of can express, except for non-finites ; it also can express rational numbers that cannot be expressed with denominators that are a power of 2 or a power of 10. For example, the rational number 1/3 is encoded: Many programming languages have built-in support for rational numbers or support for them is included in their standard libraries; tag number 30 is a way for these platforms to interchange these rational numbers in CBOR.
Tag numbers 4 and 5 are limited in the range of the (base 10 or base 2) exponents by the limited-range integers in the basic generic data model. Tag numbers 264 and 265 are exactly equivalent to 4 and 5, respectively, but also allow unlimited-range integers as exponents. While applications for floating point numbers with exponents outside the CBOR basic integer range are limited, tags 264 and 265 allow unlimited roundtripping with other formats that allow very large or very small exponents, such as those JSON can provide if the limitations of I-JSON do not apply.

The tag numbers 268..270 extend these tags further by providing a way to express non-finites within a tag with this number. This does not increase the expressiveness of the data model (the non-finites can already be expressed using major type 7 floating point numbers), but does allow both finite and non-finite values to carry the same tag. In most applications, a choice that includes some of the three tags 30, 264, 265 for finite values and major type 7 floating point values for non-finites (as well as possibly other parts of the CBOR number system) will be the preferred solution. This document suggests using the CDDL typenames defined in for the three most useful tag numbers in this section.

CDDL for extended arithmetic tags = #6.30([numerator: N, denominator: D]) ; the value 1/3 can be notated as rational_of<1, 3> extended_decfrac = #6.264([e10: integer, m: integer]) extended_bigfloat = #6.265([e2: integer, m: integer]) ]]>

Variants of undefined https://github.com/svaarala/cbor-specs/blob/master/cbor-absent-tag.rst defines tag 31 to be applied to the CBOR value Undefined (0xf7), slightly modifying its semantics to stand for an absent value in a CBOR Array. (TO DO: Obtain permission to copy the definitions here.)

Typed and Homogeneous Arrays defines tags for various kinds of arrays. A summary is reproduced in . Tag numbers defined for Arrays

Tag	Data Item	Semantics
64	byte string	uint8 Typed Array
65	byte string	uint16, big endian, Typed Array
66	byte string	uint32, big endian, Typed Array
67	byte string	uint64, big endian, Typed Array
68	byte string	uint8 Typed Array, clamped arithmetic
69	byte string	uint16, little endian, Typed Array
70	byte string	uint32, little endian, Typed Array
71	byte string	uint64, little endian, Typed Array
72	byte string	sint8 Typed Array
73	byte string	sint16, big endian, Typed Array
74	byte string	sint32, big endian, Typed Array
75	byte string	sint64, big endian, Typed Array
76	byte string	(reserved)
77	byte string	sint16, little endian, Typed Array
78	byte string	sint32, little endian, Typed Array
79	byte string	sint64, little endian, Typed Array
80	byte string	IEEE 754 binary16, big endian, Typed Array
81	byte string	IEEE 754 binary32, big endian, Typed Array
82	byte string	IEEE 754 binary64, big endian, Typed Array
83	byte string	IEEE 754 binary128, big endian, Typed Array
84	byte string	IEEE 754 binary16, little endian, Typed Array
85	byte string	IEEE 754 binary32, little endian, Typed Array
86	byte string	IEEE 754 binary64, little endian, Typed Array
87	byte string	IEEE 754 binary128, little endian, Typed Array
40	array of two arrays*	Multi-dimensional Array, row-major order
1040	array of two arrays*	Multi-dimensional Array, column-major order
41	array	Homogeneous Array

Domain-Specific (TO DO: Obtain permission to copy the definitions here; explain how tags 52 and 54 essentially obsolete 260/261.) Select Domain-Specific Tags

Tag number	Tag content	Short Description	Reference	Author
37	byte string	Binary UUID ()	https://github.com/lucas-clemente/cbor-specs/blob/master/uuid.md	Lucas_Clemente
48	byte string	IEEE MAC Address
52	byte string or array	IPv4, [prefixlen,IPv4], [IPv4,prefixpart]
54	byte string or array	IPv6, [prefixlen,IPv6], [IPv6,prefixpart]
257	byte string	Binary MIME message	http://peteroupc.github.io/CBOR/binarymime.html	Peter Occil
260	byte string	Network Address (IPv4 or IPv6 or MAC Address)	http://www.employees.org/~ravir/cbor-network.txt	Ravi Raju
261	map	Network Address Prefix (IPv4 or IPv6 Address + Mask Length)	https://github.com/toravir/CBOR-Tag-Specs/blob/master/networkPrefix.md	Ravi Raju
263	byte string	Hexadecimal string	https://github.com/toravir/CBOR-Tag-Specs/blob/master/hexString.md	Ravi Raju
266	text string	Internationalized resource identifier (IRI)	https://peteroupc.github.io/CBOR/iri.html	Peter Occil
267	text string	Internationalized resource identifier reference (IRI reference)	https://peteroupc.github.io/CBOR/iri.html	Peter Occil
1048	byte string	IEEE OUI/CID

Notes:

In the registration for Tag 37, the reference for UUID has been updated from to . The new RFC has a somewhat different internal structure; the definition of the layout of a binary UUID is now distributed over .
Tags 48, 52, and 54 are recommended for new designs that need to represent MAC addresses, IP addresses, and IP address prefixes; they essentially replace tags 260 and 261, which continue to be available for their existing applications.
Tag 263 describes a different kind of Hexadecimal string (sequence of hex digit pairs, same layout as tag 23) from the hex-string defined by the YANG data type hex-string (sequence of hex digit pairs separated by colons, ).

Human-readable Text Select Tags for Human-readable Text

Tag	Data Item	Semantics	Reference
38	array	Language-tagged string

Tag 38 was originally registered by Peter Occil in http://peteroupc.github.io/CBOR/langtags.html; it has since been adopted and extended in , where a detailed definition of the tag and a few simple examples for its use are provided. The problem that this tag was designed to solve is that text strings often need additional information to be properly presented to a human. While Unicode (and the UTF-8 form of Unicode used in CBOR) define the characters, additional information about the human language in use and the writing direction appropriate for the text given are often required. The need to provide language information with text has been well-known for a while and led to a common form for this information, the language tag, defined in . Less well-known is the need to provide separate directionality information as well. The need for this information is demonstrated in , which points out that it is "actually a bad idea to rely on language information to apply direction" and points out further reference information on this. shows more examples for language tags and directionality, while provides an introduction to the way browsers, where "the order of characters in memory (logical) is not the same as the order in which they are displayed (visual)", "produce the correct order at the time of display" (Unicode Bidirectional Algorithm). Tag 38 meets the requirements of its specific application in , which could be summarized as: Supplying the necessary information to present isolated, linear, comparatively small pieces of human-readable text. It neither addresses more complex requirements of specific languages such as , nor does it address requirements for more complex structure in texts such as emphasis, lists, or tables. These more complex requirements are typically met by specific media types such as HTML .

Extended Time Formats Additional tag definitions have been provided for date and time values. Tag numbers for date and time

Tag	Data Item	Semantics
100	integer	date in number of days since epoch
1004	text string	RFC 3339 full-date string
1001	map	extended time
1002	map	duration
1003	map	period

Note that tags 100 and 1004 are for calendar dates that are not anchored to a specific time zone; they are meant to specify calendar dates as perceived by humans, e.g. for use in personal identification documents. Converting such a calendar date into a specific point in time needs the addition of a time-of-day (for which a CBOR tag is outstanding) and timezone information (also outstanding). Alternatively, a calendar date plus timezone information can be converted into a time period (range of time values given by the starting and the ending time); note that these time periods are not always exactly 24 h (86400 s) long. does not suggest CDDL type names for the two tags. We suggest copying the definitions in into application-specific CDDL as needed.

CDDL for calendar date tags (RFC8943) Tag 1001 extends tag 1 by additional information (such as picosecond resolution) and allows the use of Decimal and Bigfloat numbers for the time.

Platform-oriented

Perl (These are actually not as Perl-specific as the title of this section suggests. See also the penultimate paragraph of Section of RFC 8949 .) These are all documented under http://cbor.schmorp.de/; the last pathname component is given in . (TO DO: Obtain permission to copy the definitions here.) Tag numbers that aid the Perl platform

Tag	Data Item	Semantics	Reference
256	multiple	mark value as having string references	stringref
25	unsigned integer	reference the nth previously seen string	stringref
26	array	Serialized Perl object with classname and constructor arguments	perl-object
27	array	Serialized language-independent object with type name and constructor arguments	generic-object
28	multiple	mark value as (potentially) shared	value-sharing
29	unsigned integer	reference nth marked value	value-sharing
22098	multiple	hint that indicates an additional level of indirection	indirection

JSON (TO DO: Obtain permission to copy the definitions here.) Tag number 262 has been registered to identify byte strings that carry embedded JSON text (https://github.com/toravir/CBOR-Tag-Specs/blob/master/embeddedJSON.md). Tag number 275 can be used to identify maps that contain keys that are all of type Text String, as they would occur in JSON (https://github.com/ecorm/cbor-tag-text-key-map).

Weird text encodings (TO DO: Obtain permission to copy the definitions here.) Some variants of UTF-8 are in use in specific areas of application. Tags have been registered to be able to carry around strings identified as to which of these variants is used, in case they are not also valid UTF-8 and can therefore not be represented as a CBOR text string (https://github.com/svaarala/cbor-specs/blob/master/cbor-nonutf8-string-tags.rst). Tag numbers for UTF-8 variants

Tag Number	Data Item	Semantics
272	byte string	Non-UTF-8 CESU-8 string
273	byte string	Non-UTF-8 WTF-8 string
274	byte string	Non-UTF-8 MUTF-8 string

Application-specific (TO DO: Obtain permission to copy the definitions here.) Application-specific Tags

Tag number	Tag content	Short Description	Reference	Author
39	multiple	Identifier	https://github.com/lucas-clemente/cbor-specs/blob/master/id.md	Lucas Clemente
42	byte string	IPLD content identifier	https://github.com/ipld/cid-cbor/	Volker Mische
103	array	Geographic Coordinates	https://github.com/allthingstalk/cbor/blob/master/CBOR-Tag103-Geographic-Coordinates.md	Danilo Vidovic
104	multiple	Geographic Coordinate Reference System WKT or EPSG number
120	multiple	Internet of Things Data Point	https://github.com/allthingstalk/cbor/blob/master/CBOR-Tag120-Internet-of-Things-Data-Points.md	Danilo Vidovic
258	array	Mathematical finite set	https://github.com/input-output-hk/cbor-sets-spec/blob/master/CBOR_SETS.md	Alfredo Di Napoli
259	map	Map datatype with key-value operations (e.g. `.get()`/`.set()`/`.delete()`)	https://github.com/shanewholloway/js-cbor-codec/blob/master/docs/CBOR-259-spec--explicit-maps.md	Shane Holloway

Enumerated Alternative Data Items (Original Text for this section was contributed by Duncan Coutts and Michael Peyton Jones; all errors are the author's.) A set of CBOR tag numbers has been allocated () for encoding data composed of enumerated alternatives: Tags for Enumerated Alternative Data Items

Tags	Data Item	Meaning
121..127	any	alternatives 0..6, 1+1 encoding
1280..1400	any	alternatives 7..127, 1+2 encoding
101	array [uint, any]	alternatives as given by the uint + 128

The tags defined in this section are for encoding data that can be in one of a number of different enumerated forms. For example data representing the result of some action might be either a failure with some failure detail, or a success with some result. In this example there are two cases, the failure case and the success case, and we can enumerate them as 0 and 1. In general the number of alternatives, and what data is expected in each alternative case is entirely application dependent. The tags defined in this specification allow the encoding of any number of alternatives, but provide compact encoding for the common cases of low numbers of alternatives:

Alternatives 0..6 can be encoded in 2 bytes;
Alternatives 7..127 can be encoded in 3 bytes;
Alternatives 128+ can be encoded in 3-12 bytes.

There are no special considerations for deterministic encoding Section of RFC 8949 : The case numbers covered by each tag do not overlap; particularly, tag 101 encoding starts where the more compact special encodings for 0..6 and 7..127 end. For cases 0..6 and 7..127, the tag value indicates the value of the alternative. For cases 128+, a single tag number is used with an enclosed two-element array that contains the case number and the value of the alternative.

Semantics The value consists of a case number and a case body. The case number is an unsigned integer that indicates which case out of the set of alternatives is used. The case body is any CBOR data value. In a setting where the application uses a schema (formally or informally), then there will be an appropriate sub-schema for each case in the set of alternatives. The representation of the case body should comply with the schema corresponding to the case number used. To continue the example above about representing failure or success, suppose that the failure detail consists of an integer code and a string, and suppose that the successful result is a byte string. A failure value will use case 0 and the case body will be a CBOR list containing an integer and a text string. Alternatively, a success value will use case 1 and the body will be a single CBOR byte string. Decoders that enforce a schema must check the case number is within the range of cases allowed, and that the case body follows the schema for the supplied case number. Generic decoders should allow any case number and any CBOR data value for the case body.

Rationale CBOR has direct support for combinations of multiple values but not for alternatives of multiple values. Combinations are expressed in CBOR using lists or maps. Most programming languages have a notion of data consisting of combinations of data values, often called records, structs or objects. Many programming languages also have a notion of data consisting of multiple alternative data values. For example C has unions, and other languages have "tagged" unions (where it is always clear which alternative is in use). Crucially for this set of tags, the set of alternatives must be closed and ordered. This allows encoding using an unsigned number to distinguish each case. Note that this does not correspond to the notion in some programming languages of classes and subclasses since in that context the set of alternatives is open and unordered. Alternatives of this kind are well-supported by tag 27 "Serialized language-independent object with type name and constructor arguments". In functional programming languages, the primary way of forming new data types is to enumerate a set of alternatives (each of which may be a record). Such forms of data are also supported in hybrid functional languages or languages with functional features. Thus, in some applications, it is very common to have data making use of alternatives, and it is worth finding a compact encoding, at least for the common cases. Just as most records are small, most alternatives are also small. In this specification we reserve 7 values in the 2-byte part of the available tag encoding space for alternatives 0..6 which are by far the most common. We reserve a range of 121 values in the 3-bytes tag encoding space. To cover the general case we use an encoding using a pair consisting of an unsigned integer and the case body, the first 24 of which also result in a 3-byte encoding.

Examples To elaborate on the example from the introduction, we have a "result" that is a failure or success, where:

the failure detail consists of an integer code and a string;
the successful result is a byte string.

This corresponds to the following schema, in CDDL notation: Example values: As a second example, here is one based on a data type defined within the Haskell programming language, representing a simple expression tree. In CDDL notation, and using the tags in this specification, such data could be encoded using this schema:

Implementation aids

Invalid Tag The present document registers tag numbers 65535, 4294967295, and 18446744073709551615 (16-bit 0xffff, 32-bit 0xffffffff, and 64-bit 0xffffffffffffffff) as Invalid Tags, tags that are always invalid, independent of the tag content provided. The purpose of these tag number registrations is to enable the tag numbers to be reserved for internal use by implementations to note the absence of a tag on a data item where a tag could also be expected with that data item as tag content. The Invalid Tags are not intended to ever occur in interchanged CBOR data items. Generic CBOR decoder implementations are encouraged to raise an error if an Invalid Tag occurs in a CBOR data item even if there is no validity checking implemented otherwise.

Programming Aid for Simple Values In a similar way, the present document also registers tag number 21334 as a fourth Invalid Tag. This tag is set aside specifically for use by CBOR implementations that have no natural way to represent Simple Values (Section of RFC 8949 ) beyond false, true, or null in their programming interface. For instance, such implementations can represent simple(123) as 21334(123) in the programming interface.

IANA Considerations In the registry "" , IANA has allocated the first to third tag in from the FCFS space, with the present document as the specification reference. IANA also has allocated the tags in the next seven rows from the Specification Required space, with the present document as the specification reference. Values for Tags

Tag	Data Item	Semantics	Reference
65535	(none valid)	always invalid	draft-bormann-cbor-notable-tags,
4294967295	(none valid)	always invalid	draft-bormann-cbor-notable-tags,
18446744073709551615	(none valid)	always invalid	draft-bormann-cbor-notable-tags,
63	byte string	Encoded CBOR Sequence	draft-bormann-cbor-notable-tags,
21065	text string	I-Regexp	draft-bormann-cbor-notable-tags, ;
18300 to 18555 (inclusive)	byte string	Bare Hash value (COSE algorithm -256 to -1)	draft-bormann-cbor-notable-tags,
18556	array	[COSE algorithm identifier, Bare Hash value]	draft-bormann-cbor-notable-tags,
18557 to 18811 (inclusive)	byte string	Bare Hash value (COSE algorithm 1 to 255)	draft-bormann-cbor-notable-tags,
108	byte string	Expected conversion to base16 encoding (lowercase)	draft-bormann-cbor-notable-tags,
21334	uint	(always invalid in interchange) programming aid for simple values	draft-bormann-cbor-notable-tags,

In addition, IANA has allocated the tags from , with a reference to the present document.

Security Considerations The security considerations of apply; the tags discussed here may also have specific security considerations that are mentioned in their specific sections above.

References Normative References UTF-8, a transformation format of ISO 10646 ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. Concise Binary Object Representation (CBOR) 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. Concise Binary Object Representation (CBOR) Tags IANA CBOR Object Signing and Encryption (COSE) IANA Named Information IANA Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures 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. CBOR Object Signing and Encryption (COSE): Structures and Process Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. CBOR Object Signing and Encryption (COSE): Countersignatures Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. CBOR Object Signing and Encryption (COSE) defines a set of security services for CBOR. This document defines a countersignature algorithm along with the needed header parameters and CBOR tags for COSE. This document updates RFC 9052. CBOR Object Signing and Encryption (COSE): Initial Algorithms Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. CBOR Object Signing and Encryption (COSE): Hash Algorithms The CBOR Object Signing and Encryption (COSE) syntax (see RFC 9052) does not define any direct methods for using hash algorithms. There are, however, circumstances where hash algorithms are used, such as indirect signatures, where the hash of one or more contents are signed, and identification of an X.509 certificate or other object by the use of a fingerprint. This document defines hash algorithms that are identified by COSE algorithm identifiers. CBOR Object Signing and Encryption (COSE): Header Parameters for Carrying and Referencing X.509 Certificates The CBOR Object Signing and Encryption (COSE) message structure uses references to keys in general. For some algorithms, additional properties are defined that carry parameters relating to keys as needed. The COSE Key structure is used for transporting keys outside of COSE messages. This document extends the way that keys can be identified and transported by providing attributes that refer to or contain X.509 certificates. CBOR Web Token (CWT) CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. Distributed Denial-of-Service Open Threat Signaling (DOTS) Signal Channel Specification This document specifies the Distributed Denial-of-Service Open Threat Signaling (DOTS) signal channel, a protocol for signaling the need for protection against Distributed Denial-of-Service (DDoS) attacks to a server capable of enabling network traffic mitigation on behalf of the requesting client. A companion document defines the DOTS data channel, a separate reliable communication layer for DOTS management and configuration purposes. This document obsoletes RFC 8782. Concise Binary Object Representation (CBOR) Tags for Typed Arrays The Concise Binary Object Representation (CBOR), as defined in RFC 7049, 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. This document makes use of this extensibility to define a number of CBOR tags for typed arrays of numeric data, as well as additional tags for multi-dimensional and homogeneous arrays. It is intended as the reference document for the IANA registration of the CBOR tags defined. Informative References Guidelines for Writing an IANA Considerations Section in RFCs 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. CBOR Object Signing and Encryption (COSE) Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. Matching of Language Tags This document describes a syntax, called a "language-range", for specifying items in a user's list of language preferences. It also describes different mechanisms for comparing and matching these to language tags. Two kinds of matching mechanisms, filtering and lookup, are defined. Filtering produces a (potentially empty) set of language tags, whereas lookup produces a single language tag. Possible applications include language negotiation or content selection. This document, in combination with RFC 4646, replaces RFC 3066, which replaced RFC 1766. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Tags for Identifying Languages This document describes the structure, content, construction, and semantics of language tags for use in cases where it is desirable to indicate the language used in an information object. It also describes how to register values for use in language tags and the creation of user-defined extensions for private interchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Strings and bidi Use cases for bidi and language metadata on the Web Unicode Bidirectional Algorithm basics W3C Rules for Simple Placement of Japanese Ruby W3C First Public Working Draft HTML — Living Standard WHATWG Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] A Universally Unique IDentifier (UUID) URN Namespace This specification defines a Uniform Resource Name namespace for UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits long, and can guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System and later in the Open Software Foundation\'s (OSF) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the DCE specification with the kind permission of the OSF (now known as The Open Group). Information from earlier versions of the DCE specification have been incorporated into this document. [STANDARDS-TRACK] Universally Unique IDentifiers (UUIDs) This specification defines UUIDs (Universally Unique IDentifiers) -- also known as GUIDs (Globally Unique IDentifiers) -- and a Uniform Resource Name namespace for UUIDs. A UUID is 128 bits long and is intended to guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System (NCS), later in the Open Software Foundation's (OSF's) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the OSF DCE specification with the kind permission of the OSF (now known as "The Open Group"). Information from earlier versions of the OSF DCE specification have been incorporated into this document. This document obsoletes RFC 4122. Concise Binary Object Representation (CBOR) 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. Concise Binary Object Representation (CBOR) Sequences This document describes the Concise Binary Object Representation (CBOR) Sequence format and associated media type "application/cbor-seq". A CBOR Sequence consists of any number of encoded CBOR data items, simply concatenated in sequence. Structured syntax suffixes for media types allow other media types to build on them and make it explicit that they are built on an existing media type as their foundation. This specification defines and registers "+cbor-seq" as a structured syntax suffix for CBOR Sequences. The I-JSON Message Format I-JSON (short for "Internet JSON") is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results. The JavaScript Object Notation (JSON) Data Interchange Format JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. Concise Binary Object Representation (CBOR) Tags for Time, Duration, and Period 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. I-Regexp: An Interoperable Regular Expression Format This document specifies I-Regexp, a flavor of regular expression that is limited in scope with the goal of interoperation across many different regular expression libraries. Concise Problem Details for Constrained Application Protocol (CoAP) APIs This document defines a concise "problem detail" as a way to carry machine-readable details of errors in a Representational State Transfer (REST) response to avoid the need to define new error response formats for REST APIs for constrained environments. The format is inspired by, but intended to be more concise than, the problem details for HTTP APIs defined in RFC 7807. Common YANG Data Types This document defines a collection of common data types to be used with the YANG data modeling language. It includes several new type definitions and obsoletes RFC 6991. Encoding of Data Modeled with YANG in the Concise Binary Object Representation (CBOR) YANG (RFC 7950) is a data modeling language used to model configuration data, state data, parameters and results of Remote Procedure Call (RPC) operations or actions, and notifications. This document defines encoding rules for YANG in the Concise Binary Object Representation (CBOR) (RFC 8949). IANA Considerations and IETF Protocol and Documentation Usage for IEEE 802 Parameters Some IETF protocols make use of Ethernet frame formats and IEEE 802 parameters. This document discusses several aspects of such parameters and their use in IETF protocols, specifies IANA considerations for assignment of points under the IANA Organizationally Unique Identifier (OUI), and provides some values for use in documentation. This document obsoletes RFC 7042. Concise Binary Object Representation (CBOR) Tags for IPv4 and IPv6 Addresses and Prefixes This specification defines two Concise Binary Object Representation (CBOR) tags for use with IPv6 and IPv4 addresses and prefixes. On Numbers in CBOR Universität Bremen TZI The Concise Binary Object Representation (CBOR), as defined in STD 94 (RFC 8949), is a data representation format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. Among the kinds of data that a data representation format needs to be able to carry, numbers have a prominent role, but also have inherent complexity that needs attention from protocol designers and implementers of CBOR libraries and of the applications that use them. This document gives an overview over number formats available in CBOR and some notable CBOR tags registered, and it attempts to provide information about opportunities and potential pitfalls of these number formats. // This is a rather drafty initial revision, pieced together from // various components, so it has a higher level of redundancy than // ultimately desired. Stand-in Tags for YANG-CBOR Universität Bremen TZI CZ.NIC YANG (RFC 7950) is a data modeling language used to model configuration data, state data, parameters and results of Remote Procedure Call (RPC) operations or actions, and notifications. YANG-CBOR (RFC 9254) defines encoding rules for YANG in the Concise Binary Object Representation (CBOR) (RFC 8949). While the overall structure of YANG-CBOR is encoded in an efficient, binary format, YANG itself has its roots in XML and therefore traditionally encodes some information such as date/times and IP addresses/prefixes in a verbose text form. This document defines how to use existing CBOR tags for this kind of information in YANG-CBOR as a "stand-in" for the text-based information that would be found in the original form of YANG-CBOR. Information technology — Programming languages — C International Organization for Standardization Programming languages — C++ International Organization for Standardization ISO/IEC JTC1 SC22 WG21 N4860 IEEE Standard for Floating-Point Arithmetic IEEE RFC Style Guide This document describes the fundamental and unique style conventions and editorial policies currently in use for the RFC Series. It captures the RFC Editor's basic requirements and offers guidance regarding the style and structure of an RFC. Additional guidance is captured on a website that reflects the experimental nature of that guidance and prepares it for future inclusion in the RFC Style Guide. This document obsoletes RFC 2223, "Instructions to RFC Authors". Concise Binary Object Representation (CBOR) Tags for Time, Duration, and Period Universitaet Bremen TZI Well-Typed Fraunhofer SIT The Concise Binary Object Representation (CBOR, RFC 7049) 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 7049 defines two tags for time: CBOR tag 0 (RFC3339 time) and tag 1 (Posix time [TIME_T], 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, and anticipates the definition of related CBOR tags for duration and time period. It is intended as the reference document for the IANA registration of the CBOR tags defined. JSON Web Token (JWT) JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. The YANG 1.1 Data Modeling Language YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). Network Configuration Protocol (NETCONF) The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] JSON Encoding of Data Modeled with YANG This document defines encoding rules for representing configuration data, state data, parameters of Remote Procedure Call (RPC) operations or actions, and notifications defined using YANG as JavaScript Object Notation (JSON) text. RAINS (Another Internet Naming Service) Protocol Specification ETH Zurich ETH Zurich This document defines an alternate protocol for Internet name resolution, designed as a prototype to facilitate conversation about the evolution or replacement of the Domain Name System protocol. It attempts to answer the question: "how would we design DNS knowing what we do now," on the background of a set of properties of an idealized Internet naming service. Concise Binary Object Representation (CBOR) Tags for Date The Concise Binary Object Representation (CBOR), as specified in RFC 7049, 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 7049 defines two tags for time: CBOR tag 0 (date/time string as per RFC 3339) and tag 1 (POSIX "seconds since the epoch"). Since then, additional requirements have become known. This specification defines a CBOR tag for a date text string (as per RFC 3339) for applications needing a textual date representation within the Gregorian calendar without a time. It also defines a CBOR tag for days since the date 1970-01-01 in the Gregorian calendar for applications needing a numeric date representation without a time. This specification is the reference document for IANA registration of the CBOR tags defined. Concise Binary Object Representation (CBOR) Tag for Coordinate Reference System (CRS) Specification Ball Aerospace and Technologies Corp. The Concise Binary Object Representation (CBOR, RFC 7049) 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. An existing CBOR tag, 103, allows for the representation of geographic coordinates. Proper exploitation of geographic coordinates requires an associated reference frame. The present document defines a CBOR tag for referencing the coordinate reference system (CRS) for a geographic coordinate. It is intended as the reference document for the IANA registration of the CBOR tag defined.

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Acknowledgements (Many, TBD)

Contributors

poccil14 at gmail dot com

Peter Occil registered tags 30, 264, 265, 268–270 (), 38, 257, 266 and 267 (), and contributed much of the text about these tags in this document.

duncan@well-typed.com

me@michaelpj.com

To do

Further contributors will be listed here as text is added. Please stay tuned.