Concise Binary Object Representation (CBOR)

1.1. Objectives

The objectives of CBOR, roughly in decreasing order of importance, are:¶

1. The representation must be able to unambiguously encode most common data formats used in Internet standards.¶

   • It must represent a reasonable set of basic data types and structures using binary encoding. "Reasonable" here is largely influenced by the capabilities of JSON, with the major addition of binary byte strings. The structures supported are limited to arrays and trees; loops and lattice-style graphs are not supported.¶
   • There is no requirement that all data formats be uniquely encoded; that is, it is acceptable that the number "7" might be encoded in multiple different ways.¶

2. The code for an encoder or decoder must be able to be compact in order to support systems with very limited memory, processor power, and instruction sets.¶

   • An encoder and a decoder need to be implementable in a very small amount of code (for example, in class 1 constrained nodes as defined in [RFC7228]).¶
   • The format should use contemporary machine representations of data (for example, not requiring binary-to-decimal conversion).¶

3. Data must be able to be decoded without a schema description.¶

   • Similar to JSON, encoded data should be self-describing so that a generic decoder can be written.¶

4. The serialization must be reasonably compact, but data compactness is secondary to code compactness for the encoder and decoder.¶

   • "Reasonable" here is bounded by JSON as an upper bound in size, and by implementation complexity maintaining a lower bound. Using either general compression schemes or extensive bit-fiddling violates the complexity goals.¶

5. The format must be applicable to both constrained nodes and high-volume applications.¶

   • This means it must be reasonably frugal in CPU usage for both encoding and decoding. This is relevant both for constrained nodes and for potential usage in applications with a very high volume of data.¶

6. The format must support all JSON data types for conversion to and from JSON.¶

   • It must support a reasonable level of conversion as long as the data represented is within the capabilities of JSON. It must be possible to define a unidirectional mapping towards JSON for all types of data.¶

7. The format must be extensible, and the extended data must be decodable by earlier decoders.¶

   • The format is designed for decades of use.¶
   • The format must support a form of extensibility that allows fallback so that a decoder that does not understand an extension can still decode the message.¶
   • The format must be able to be extended in the future by later IETF standards.¶

1.2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

The term "byte" is used in its now-customary sense as a synonym for "octet". All multi-byte values are encoded in network byte order (that is, most significant byte first, also known as "big-endian").¶

This specification makes use of the following terminology:¶

Data item:

A single piece of CBOR data. The structure of a data item may contain zero, one, or more nested data items. The term is used both for the data item in representation format and for the abstract idea that can be derived from that by a decoder; the former can be addressed specifically by using "encoded data item".¶

Decoder:

A process that decodes a well-formed encoded CBOR data item and makes it available to an application. Formally speaking, a decoder contains a parser to break up the input using the syntax rules of CBOR, as well as a semantic processor to prepare the data in a form suitable to the application.¶

Encoder:

A process that generates the (well-formed) representation format of a CBOR data item from application information.¶

Data Stream:

A sequence of zero or more data items, not further assembled into a larger containing data item. The independent data items that make up a data stream are sometimes also referred to as "top-level data items".¶

Well-formed:

A data item that follows the syntactic structure of CBOR. A well-formed data item uses the initial bytes and the byte strings and/or data items that are implied by their values as defined in CBOR and does not include following extraneous data. CBOR decoders by definition only return contents from well-formed data items.¶

Valid:

A data item that is well-formed and also follows the semantic restrictions that apply to CBOR data items.¶

Expected:

Besides its normal English meaning, the term "expected" is used to describe requirements beyond CBOR validity that an application has on its input data. Well-formed (processable at all), valid (checked by a validity-checking generic decoder), and expected (checked by the application) form a hierarchy of layers of acceptability.¶

Stream decoder:

A process that decodes a data stream and makes each of the data items in the sequence available to an application as they are received.¶

Where bit arithmetic or data types are explained, this document uses the notation familiar from the programming language C, except that "**" denotes exponentiation. Similar to the "0x" notation for hexadecimal numbers, numbers in binary notation are prefixed with "0b". Underscores can be added to a number solely for readability, so 0b00100001 (0x21) might be written 0b001_00001 to emphasize the desired interpretation of the bits in the byte; in this case, it is split into three bits and five bits. Encoded CBOR data items are sometimes given in the "0x" or "0b" notation; these values are first interpreted as numbers as in C and are then interpreted as byte strings in network byte order, including any leading zero bytes expressed in the notation.¶

2.1. Extended Generic Data Models

This basic generic data model comes pre-extended by the registration of a number of simple values and tag numbers right in this document, such as:¶

• false, true, null, and undefined (simple values identified by 20..23)¶
• integer and floating-point values with a larger range and precision than the above (tag numbers 2 to 5)¶
• application data types such as a point in time or an RFC 3339 date/time string (tag numbers 1, 0)¶

Further elements of the extended generic data model can be (and have been) defined via the IANA registries created for CBOR. Even if such an extension is unknown to a generic encoder or decoder, data items using that extension can be passed to or from the application by representing them at the interface to the application within the basic generic data model, i.e., as generic values of a simple type or generic tags.¶

In other words, the basic generic data model is stable as defined in this document, while the extended generic data model expands by the registration of new simple values or tag numbers, but never shrinks.¶

While there is a strong expectation that generic encoders and decoders can represent false, true, and null (undefined is intentionally omitted) in the form appropriate for their programming environment, implementation of the data model extensions created by tags is truly optional and a matter of implementation quality.¶

2.2. Specific Data Models

The specific data model for a CBOR-based protocol usually subsets the extended generic data model and assigns application semantics to the data items within this subset and its components. When documenting such specific data models, where it is desired to specify the types of data items, it is preferred to identify the types by the names they have in the generic data model ("negative integer", "array") instead of by referring to aspects of their CBOR representation ("major type 1", "major type 4").¶

Specific data models can also specify what values (including values of different types) are equivalent for the purposes of map keys and encoder freedom. For example, in the generic data model, a valid map MAY have both 0 and 0.0 as keys, and an encoder MUST NOT encode 0.0 as an integer (major type 0, Section 3.1). However, if a specific data model declares that floating-point and integer representations of integral values are equivalent, using both map keys 0 and 0.0 in a single map would be considered duplicates, even while encoded as different major types, and so invalid; and an encoder could encode integral-valued floats as integers or vice versa, perhaps to save encoded bytes.¶

3.1. Major Types

The following lists the major types and the additional information and other bytes associated with the type.¶

Major type 0:

an integer in the range 0..2**64-1 inclusive. The value of the encoded item is the argument itself. For example, the integer 10 is denoted as the one byte 0b000_01010 (major type 0, additional information 10). The integer 500 would be 0b000_11001 (major type 0, additional information 25) followed by the two bytes 0x01f4, which is 500 in decimal.¶

Major type 1:

a negative integer in the range -2**64..-1 inclusive. The value of the item is -1 minus the argument. For example, the integer -500 would be 0b001_11001 (major type 1, additional information 25) followed by the two bytes 0x01f3, which is 499 in decimal.¶

Major type 2:

a byte string. The number of bytes in the string is equal to the argument. For example, a byte string whose length is 5 would have an initial byte of 0b010_00101 (major type 2, additional information 5 for the length), followed by 5 bytes of binary content. A byte string whose length is 500 would have 3 initial bytes of 0b010_11001 (major type 2, additional information 25 to indicate a two-byte length) followed by the two bytes 0x01f4 for a length of 500, followed by 500 bytes of binary content.¶

Major type 3:

a text string (Section 2), encoded as UTF-8 ([RFC3629]). The number of bytes in the string is equal to the argument. A string containing an invalid UTF-8 sequence is well-formed but invalid. This type is provided for systems that need to interpret or display human-readable text, and allows the differentiation between unstructured bytes and text that has a specified repertoire and encoding. In contrast to formats such as JSON, the Unicode characters in this type are never escaped. Thus, a newline character (U+000A) is always represented in a string as the byte 0x0a, and never as the bytes 0x5c6e (the characters "\" and "n") or as 0x5c7530303061 (the characters "\", "u", "0", "0", "0", and "a").¶

Major type 4:

an array of data items. Arrays are also called lists, sequences, or tuples. The argument is the number of data items in the array. Items in an array do not need to all be of the same type. For example, an array that contains 10 items of any type would have an initial byte of 0b100_01010 (major type of 4, additional information of 10 for the length) followed by the 10 remaining items.¶

Major type 5:

a map of pairs of data items. Maps are also called tables, dictionaries, hashes, or objects (in JSON). A map is comprised of pairs of data items, each pair consisting of a key that is immediately followed by a value. The argument is the number of pairs of data items in the map. For example, a map that contains 9 pairs would have an initial byte of 0b101_01001 (major type of 5, additional information of 9 for the number of pairs) followed by the 18 remaining items. The first item is the first key, the second item is the first value, the third item is the second key, and so on. Because items in a map come in pairs, their total number is always even: A map that contains an odd number of items (no value data present after the last key data item) is not well-formed. A map that has duplicate keys may be well-formed, but it is not valid, and thus it causes indeterminate decoding; see also Section 5.6.¶

Major type 6:

a tagged data item ("tag") whose tag number is the argument and whose enclosed data item ("tag content") is the single encoded data item that follows the head. See Section 3.4.¶

Major type 7:

floating-point numbers and simple values, as well as the "break" stop code. See Section 3.3.¶

These eight major types lead to a simple table showing which of the 256 possible values for the initial byte of a data item are used (Table 6).¶

In major types 6 and 7, many of the possible values are reserved for future specification. See Section 9 for more information on these values.¶

Table 1 summarizes the major types defined by CBOR, ignoring the next section for now. The number N in this table stands for the argument, mt for the major type.¶

3.2. Indefinite Lengths for Some Major Types

Four CBOR items (arrays, maps, byte strings, and text strings) can be encoded with an indefinite length using additional information value 31. This is useful if the encoding of the item needs to begin before the number of items inside the array or map, or the total length of the string, is known. (The application of this is often referred to as "streaming" within a data item.)¶

Indefinite-length arrays and maps are dealt with differently than indefinite-length byte strings and text strings.¶

83        -- Array of length 3
   01     -- 1
   82     -- Array of length 2
      02  -- 2
      03  -- 3
   82     -- Array of length 2
      04  -- 4
      05  -- 5

0x9f018202039f0405ffff
9F        -- Start indefinite-length array
   01     -- 1
   82     -- Array of length 2
      02  -- 2
      03  -- 3
   9F     -- Start indefinite-length array
      04  -- 4
      05  -- 5
      FF  -- "break" (inner array)
   FF     -- "break" (outer array)

0x9f01820203820405ff
9F        -- Start indefinite-length array
   01     -- 1
   82     -- Array of length 2
      02  -- 2
      03  -- 3
   82     -- Array of length 2
      04  -- 4
      05  -- 5
   FF     -- "break"

0x83018202039f0405ff
83        -- Array of length 3
   01     -- 1
   82     -- Array of length 2
      02  -- 2
      03  -- 3
   9F     -- Start indefinite-length array
      04  -- 4
      05  -- 5
      FF  -- "break"

0x83019f0203ff820405
83        -- Array of length 3
   01     -- 1
   9F     -- Start indefinite-length array
      02  -- 2
      03  -- 3
      FF  -- "break"
   82     -- Array of length 2
      04  -- 4
      05  -- 5

0xbf6346756ef563416d7421ff
BF           -- Start indefinite-length map
   63        -- First key, UTF-8 string length 3
      46756e --   "Fun"
   F5        -- First value, true
   63        -- Second key, UTF-8 string length 3
      416d74 --   "Amt"
   21        -- Second value, -2
   FF        -- "break"

5F              -- Start indefinite-length byte string
   44           -- Byte string of length 4
      aabbccdd  -- Bytes content
   43           -- Byte string of length 3
      eeff99    -- Bytes content
   FF           -- "break"

3.3. Floating-Point Numbers and Values with No Content

Major type 7 is for two types of data: floating-point numbers and "simple values" that do not need any content. Each value of the 5-bit additional information in the initial byte has its own separate meaning, as defined in Table 2. Like the major types for integers, items of this major type do not carry content data; all the information is in the initial bytes.¶

As with all other major types, the 5-bit value 24 signifies a single-byte extension: it is followed by an additional byte to represent the simple value. (To minimize confusion, only the values 32 to 255 are used.) This maintains the structure of the initial bytes: as for the other major types, the length of these always depends on the additional information in the first byte. Table 3 lists the values assigned and available for simple types.¶

An encoder MUST NOT issue two-byte sequences that start with 0xf8 (major type = 7, additional information = 24) and continue with a byte less than 0x20 (32 decimal). Such sequences are not well-formed. (This implies that an encoder cannot encode false, true, null, or undefined in two-byte sequences, only the one-byte variants of these are well-formed.)¶

The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit IEEE 754 binary floating-point values [IEEE754]. These floating-point values are encoded in the additional bytes of the appropriate size. (See Appendix D for some information about 16-bit floating point.)¶

3.4. Tagging of Items

In CBOR, a data item can be enclosed by a tag to give it additional semantics while retaining its structure. The tag is major type 6, and represents an unsigned integer as indicated by the tag's argument (Section 3); the (sole) enclosed data item is carried as content data. If a tag requires structured data, this structure is encoded into the nested data item. The definition of a tag number usually restricts what kinds of nested data item or items are valid for tags using this tag number.¶

For example, assume that a byte string of length 12 is marked with a tag of number 2 to indicate it is a positive bignum (Section 3.4.3). This would be marked as 0b110_00010 (major type 6, additional information 2 for the tag number) followed by 0b010_01100 (major type 2, additional information of 12 for the length) followed by the 12 bytes of the bignum.¶

Decoders do not need to understand tags of every tag number, and tags may be of little value in applications where the implementation creating a particular CBOR data item and the implementation decoding that stream know the semantic meaning of each item in the data flow. Their primary purpose in this specification is to define common data types such as dates. A secondary purpose is to provide conversion hints when it is foreseen that the CBOR data item needs to be translated into a different format, requiring hints about the content of items. Understanding the semantics of tags is optional for a decoder; it can just jump over the initial bytes of the tag (that encode the tag number) and interpret the tag content itself, presenting both tag number and tag content to the application.¶

A tag applies semantics to the data item it encloses. Thus, if tag A encloses tag B, which encloses data item C, tag A applies to the result of applying tag B on data item C. That is, a tag is a data item consisting of a tag number and an enclosed value. The content of the tag (the enclosed data item) is the data item (the value) that is being tagged.¶

IANA maintains a registry of tag numbers as described in Section 9.2. Table 4 provides a list of tag numbers that were defined in [RFC7049], with definitions in the rest of this section. Note that many other tag numbers have been defined since the publication of [RFC7049]; see the registry described at Section 9.2 for the complete list.¶

Conceptually, tags are interpreted in the generic data model, not at (de-)serialization time. A small number of tags (specifically, tag number 25 and tag number 29) have been registered with semantics that may require processing at (de-)serialization time: The decoder needs to be aware and the encoder needs to be in control of the exact sequence in which data items are encoded into the CBOR data stream. This means these tags cannot be implemented on top of every generic CBOR encoder/decoder (which might not reflect the serialization order for entries in a map at the data model level and vice versa); their implementation therefore typically needs to be integrated into the generic encoder/decoder. The definition of new tags with this property is NOT RECOMMENDED.¶

Protocols using tag numbers 0 and 1 extend the generic data model (Section 2) with data items representing points in time; tag numbers 2 and 3, with arbitrarily sized integers; and tag numbers 4 and 5, with floating point values of arbitrary size and precision.¶

C2                        -- Tag 2
   49                     -- Byte string of length 9
      010000000000000000  -- Bytes content

C4             -- Tag 4
   82          -- Array of length 2
      21       -- -2
      19 6ab3  -- 27315

C5             -- Tag 5
   82          -- Array of length 2
      20       -- -1
      03       -- 3

4.1. Preferred Serialization

For some values at the data model level, CBOR provides multiple serializations. For many applications, it is desirable that an encoder always chooses a preferred serialization (preferred encoding); however, the present specification does not put the burden of enforcing this preference on either encoder or decoder.¶

Some constrained decoders may be limited in their ability to decode non-preferred serializations: For example, if only integers below 1_000_000_000 are expected in an application, the decoder may leave out the code that would be needed to decode 64-bit arguments in integers. An encoder that always uses preferred serialization ("preferred encoder") interoperates with this decoder for the numbers that can occur in this application. More generally speaking, it therefore can be said that a preferred encoder is more universally interoperable (and also less wasteful) than one that, say, always uses 64-bit integers.¶

Similarly, a constrained encoder may be limited in the variety of representation variants it supports in such a way that it does not emit preferred serializations ("variant encoder"): Say, it could be designed to always use the 32-bit variant for an integer that it encodes even if a short representation is available (again, assuming that there is no application need for integers that can only be represented with the 64-bit variant). A decoder that does not rely on only ever receiving preferred serializations ("variation-tolerant decoder") can there be said to be more universally interoperable (it might very well optimize for the case of receiving preferred serializations, though). Full implementations of CBOR decoders are by definition variation-tolerant; the distinction is only relevant if a constrained implementation of a CBOR decoder meets a variant encoder.¶

The preferred serialization always uses the shortest form of representing the argument (Section 3)); it also uses the shortest floating-point encoding that preserves the value being encoded (see Section 5.5). Definite length encoding is preferred whenever the length is known at the time the serialization of the item starts.¶

4.2. Deterministically Encoded CBOR

Some protocols may want encoders to only emit CBOR in a particular deterministic format; those protocols might also have the decoders check that their input is in that deterministic format. Those protocols are free to define what they mean by a "deterministic format" and what encoders and decoders are expected to do. This section defines a set of restrictions that can serve as the base of such a deterministic format.¶

5.1. CBOR in Streaming Applications

In a streaming application, a data stream may be composed of a sequence of CBOR data items concatenated back-to-back. In such an environment, the decoder immediately begins decoding a new data item if data is found after the end of a previous data item.¶

Not all of the bytes making up a data item may be immediately available to the decoder; some decoders will buffer additional data until a complete data item can be presented to the application. Other decoders can present partial information about a top-level data item to an application, such as the nested data items that could already be decoded, or even parts of a byte string that hasn't completely arrived yet.¶

Note that some applications and protocols will not want to use indefinite-length encoding. Using indefinite-length encoding allows an encoder to not need to marshal all the data for counting, but it requires a decoder to allocate increasing amounts of memory while waiting for the end of the item. This might be fine for some applications but not others.¶

5.2. Generic Encoders and Decoders

A generic CBOR decoder can decode all well-formed CBOR data and present them to an application. See Appendix C.¶

Even though CBOR attempts to minimize these cases, not all well-formed CBOR data is valid: for example, the encoded text string 0x62c0ae does not contain valid UTF-8 and so is not a valid CBOR item. Also, specific tags may make semantic constraints that may be violated, such as a bignum tag enclosing another tag, or an instance of tag number 0 containing a byte string, or containing a text string with contents that do not match [RFC3339]'s date-time production. There is no requirement that generic encoders and decoders make unnatural choices for their application interface to enable the processing of invalid data. Generic encoders and decoders are expected to forward simple values and tags even if their specific codepoints are not registered at the time the encoder/decoder is written (Section 5.4).¶

Generic decoders provide ways to present well-formed CBOR values, both valid and invalid, to an application. The diagnostic notation (Section 8) may be used to present well-formed CBOR values to humans.¶

Generic encoders provide an application interface that allows the application to specify any well-formed value, including simple values and tags unknown to the encoder.¶

5.3. Validity of Items

A well-formed but invalid CBOR data item presents a problem with interpreting the data encoded in it in the CBOR data model. A CBOR-based protocol could be specified in several layers, in which the lower layers don't process the semantics of some of the CBOR data they forward. These layers can't notice any validity errors in data they don't process and MUST forward that data as-is. The first layer that does process the semantics of an invalid CBOR item MUST take one of two choices:¶

1. Replace the problematic item with an error marker and continue with the next item, or¶
2. Issue an error and stop processing altogether.¶

A CBOR-based protocol MUST specify which of these options its decoders take, for each kind of invalid item they might encounter.¶

Such problems might occur at the basic validity level of CBOR or in the context of tags (tag validity).¶

5.4. Validity and Evolution

A decoder with validity checking will expend the effort to reliably detect data items with validity errors. For example, such a decoder needs to have an API that reports an error (and does not return data) for a CBOR data item that contains any of the validity errors listed in the previous subsection.¶

The set of tags defined in the tag registry (Section 9.2), as well as the set of simple values defined in the simple values registry (Section 9.1), can grow at any time beyond the set understood by a generic decoder. A validity-checking decoder can do one of two things when it encounters such a case that it does not recognize:¶

• It can report an error (and not return data). Note that this error is not a validity error per se. This kind of error is more likely to be raised by a decoder that would be performing validity checking if this were a known case.¶
• It can emit the unknown item (type, value, and, for tags, the decoded tagged data item) to the application calling the decoder, with an indication that the decoder did not recognize that tag number or simple value.¶

The latter approach, which is also appropriate for decoders that do not support validity checking, provides forward compatibility with newly registered tags and simple values without the requirement to update the encoder at the same time as the calling application. (For this, the API for the decoder needs to have a way to mark unknown items so that the calling application can handle them in a manner appropriate for the program.)¶

Since some of the processing needed for validity checking may have an appreciable cost (in particular with duplicate detection for maps), support of validity checking is not a requirement placed on all CBOR decoders.¶

Some encoders will rely on their applications to provide input data in such a way that valid CBOR results from the encoder. A generic encoder also may want to provide a validity-checking mode where it reliably limits its output to valid CBOR, independent of whether or not its application is indeed providing API-conformant data.¶

5.5. Numbers

CBOR-based protocols should take into account that different language environments pose different restrictions on the range and precision of numbers that are representable. For example, the JavaScript number system treats all numbers as floating point, which may result in silent loss of precision in decoding integers with more than 53 significant bits. A protocol that uses numbers should define its expectations on the handling of non-trivial numbers in decoders and receiving applications.¶

A CBOR-based protocol that includes floating-point numbers can restrict which of the three formats (half-precision, single-precision, and double-precision) are to be supported. For an integer-only application, a protocol may want to completely exclude the use of floating-point values.¶

A CBOR-based protocol designed for compactness may want to exclude specific integer encodings that are longer than necessary for the application, such as to save the need to implement 64-bit integers. There is an expectation that encoders will use the most compact integer representation that can represent a given value. However, a compact application should accept values that use a longer-than-needed encoding (such as encoding "0" as 0b000_11001 followed by two bytes of 0x00) as long as the application can decode an integer of the given size.¶

The preferred encoding for a floating-point value is the shortest floating-point encoding that preserves its value, e.g., 0xf94580 for the number 5.5, and 0xfa45ad9c00 for the number 5555.5, unless the CBOR-based protocol specifically excludes the use of the shorter floating-point encodings. For NaN values, a shorter encoding is preferred if zero-padding the shorter significand towards the right reconstitutes the original NaN value (for many applications, the single NaN encoding 0xf97e00 will suffice).¶

5.6. Specifying Keys for Maps

The encoding and decoding applications need to agree on what types of keys are going to be used in maps. In applications that need to interwork with JSON-based applications, keys probably should be limited to UTF-8 strings only; otherwise, there has to be a specified mapping from the other CBOR types to Unicode characters, and this often leads to implementation errors. In applications where keys are numeric in nature and numeric ordering of keys is important to the application, directly using the numbers for the keys is useful.¶

If multiple types of keys are to be used, consideration should be given to how these types would be represented in the specific programming environments that are to be used. For example, in JavaScript Maps [ECMA262], a key of integer 1 cannot be distinguished from a key of floating-point 1.0. This means that, if integer keys are used, the protocol needs to avoid use of floating-point keys the values of which happen to be integer numbers in the same map.¶

Decoders that deliver data items nested within a CBOR data item immediately on decoding them ("streaming decoders") often do not keep the state that is necessary to ascertain uniqueness of a key in a map. Similarly, an encoder that can start encoding data items before the enclosing data item is completely available ("streaming encoder") may want to reduce its overhead significantly by relying on its data source to maintain uniqueness.¶

A CBOR-based protocol MUST define what to do when a receiving application does see multiple identical keys in a map. The resulting rule in the protocol MUST respect the CBOR data model: it cannot prescribe a specific handling of the entries with the identical keys, except that it might have a rule that having identical keys in a map indicates a malformed map and that the decoder has to stop with an error. Duplicate keys are also prohibited by CBOR decoders that enforce validity (Section 5.4).¶

The CBOR data model for maps does not allow ascribing semantics to the order of the key/value pairs in the map representation. Thus, a CBOR-based protocol MUST NOT specify that changing the key/value pair order in a map would change the semantics, except to specify that some, orders are disallowed, for example where they would not meet the requirements of a deterministic encoding (Section 4.2). (Any secondary effects of map ordering such as on timing, cache usage, and other potential side channels are not considered part of the semantics but may be enough reason on its own for a protocol to require a deterministic encoding format.)¶

Applications for constrained devices that have maps where a small number of frequently used keys can be identified should consider using small integers as keys; for instance, a set of 24 or fewer frequent keys can be encoded in a single byte as unsigned integers, up to 48 if negative integers are also used. Less frequently occurring keys can then use integers with longer encodings.¶

5.7. Undefined Values

In some CBOR-based protocols, the simple value (Section 3.3) of Undefined might be used by an encoder as a substitute for a data item with an encoding problem, in order to allow the rest of the enclosing data items to be encoded without harm.¶

6.1. Converting from CBOR to JSON

Most of the types in CBOR have direct analogs in JSON. However, some do not, and someone implementing a CBOR-to-JSON converter has to consider what to do in those cases. The following non-normative advice deals with these by converting them to a single substitute value, such as a JSON null.¶

• An integer (major type 0 or 1) becomes a JSON number.¶
• A byte string (major type 2) that is not embedded in a tag that specifies a proposed encoding is encoded in base64url without padding and becomes a JSON string.¶
• A UTF-8 string (major type 3) becomes a JSON string. Note that JSON requires escaping certain characters ([RFC8259], Section 7): quotation mark (U+0022), reverse solidus (U+005C), and the "C0 control characters" (U+0000 through U+001F). All other characters are copied unchanged into the JSON UTF-8 string.¶
• An array (major type 4) becomes a JSON array.¶
• A map (major type 5) becomes a JSON object. This is possible directly only if all keys are UTF-8 strings. A converter might also convert other keys into UTF-8 strings (such as by converting integers into strings containing their decimal representation); however, doing so introduces a danger of key collision. Note also that, if tags on UTF-8 strings are ignored as proposed below, this will cause a key collision if the tags are different but the strings are the same.¶
• False (major type 7, additional information 20) becomes a JSON false.¶
• True (major type 7, additional information 21) becomes a JSON true.¶
• Null (major type 7, additional information 22) becomes a JSON null.¶
• A floating-point value (major type 7, additional information 25 through 27) becomes a JSON number if it is finite (that is, it can be represented in a JSON number); if the value is non-finite (NaN, or positive or negative Infinity), it is represented by the substitute value.¶
• Any other simple value (major type 7, any additional information value not yet discussed) is represented by the substitute value.¶
• A bignum (major type 6, tag number 2 or 3) is represented by encoding its byte string in base64url without padding and becomes a JSON string. For tag number 3 (negative bignum), a "~" (ASCII tilde) is inserted before the base-encoded value. (The conversion to a binary blob instead of a number is to prevent a likely numeric overflow for the JSON decoder.)¶
• A byte string with an encoding hint (major type 6, tag number 21 through 23) is encoded as described and becomes a JSON string.¶
• For all other tags (major type 6, any other tag number), the enclosed CBOR item is represented as a JSON value; the tag number is ignored.¶
• Indefinite-length items are made definite before conversion.¶

6.2. Converting from JSON to CBOR

All JSON values, once decoded, directly map into one or more CBOR values. As with any kind of CBOR generation, decisions have to be made with respect to number representation. In a suggested conversion:¶

• JSON numbers without fractional parts (integer numbers) are represented as integers (major types 0 and 1, possibly major type 6 tag number 2 and 3), choosing the shortest form; integers longer than an implementation-defined threshold may instead be represented as floating-point values. The default range that is represented as integer is -2**53+1..2**53-1 (fully exploiting the range for exact integers in the binary64 representation often used for decoding JSON [RFC7493]). A CBOR-based protocol, or a generic converter implementation, may choose -2**32..2**32-1 or -2**64..2**64-1 (fully using the integer ranges available in CBOR with uint32_t or uint64_t, respectively) or even -2**31..2**31-1 or -2**63..2**63-1 (using popular ranges for two's complement signed integers). (If the JSON was generated from a JavaScript implementation, its precision is already limited to 53 bits maximum.)¶
• Numbers with fractional parts are represented as floating-point values, performing the decimal-to-binary conversion based on the precision provided by IEEE 754 binary64. Then, when encoding in CBOR, the preferred serialization uses the shortest floating-point representation exactly representing this conversion result; for instance, 1.5 is represented in a 16-bit floating-point value (not all implementations will be capable of efficiently finding the minimum form, though). Instead of using the default binary64 precision, there may be an implementation-defined limit to the precision of the conversion that will affect the precision of the represented values. Decimal representation should only be used on the CBOR side if that is specified in a protocol.¶

CBOR has been designed to generally provide a more compact encoding than JSON. One implementation strategy that might come to mind is to perform a JSON-to-CBOR encoding in place in a single buffer. This strategy would need to carefully consider a number of pathological cases, such as that some strings represented with no or very few escapes and longer (or much longer) than 255 bytes may expand when encoded as UTF-8 strings in CBOR. Similarly, a few of the binary floating-point representations might cause expansion from some short decimal representations (1.1, 1e9) in JSON. This may be hard to get right, and any ensuing vulnerabilities may be exploited by an attacker.¶

7.1. Extension Points

In a protocol design, opportunities for evolution are often included in the form of extension points. For example, there may be a codepoint space that is not fully allocated from the outset, and the protocol is designed to tolerate and embrace implementations that start using more codepoints than initially allocated.¶

Sizing the codepoint space may be difficult because the range required may be hard to predict. An attempt should be made to make the codepoint space large enough so that it can slowly be filled over the intended lifetime of the protocol.¶

CBOR has three major extension points:¶

• the "simple" space (values in major type 7). Of the 24 efficient (and 224 slightly less efficient) values, only a small number have been allocated. Implementations receiving an unknown simple data item may be able to process it as such, given that the structure of the value is indeed simple. The IANA registry in Section 9.1 is the appropriate way to address the extensibility of this codepoint space.¶
• the "tag" space (values in major type 6). Again, only a small part of the codepoint space has been allocated, and the space is abundant (although the early numbers are more efficient than the later ones). Implementations receiving an unknown tag number can choose to simply ignore it or to process it as an unknown tag number wrapping the enclosed data item. The IANA registry in Section 9.2 is the appropriate way to address the extensibility of this codepoint space.¶
• the "additional information" space. An implementation receiving an unknown additional information value has no way to continue decoding, so allocating codepoints to this space is a major step. There are also very few codepoints left.¶

7.2. Curating the Additional Information Space

The human mind is sometimes drawn to filling in little perceived gaps to make something neat. We expect the remaining gaps in the codepoint space for the additional information values to be an attractor for new ideas, just because they are there.¶

The present specification does not manage the additional information codepoint space by an IANA registry. Instead, allocations out of this space can only be done by updating this specification.¶

For an additional information value of n >= 24, the size of the additional data typically is 2**(n-24) bytes. Therefore, additional information values 28 and 29 should be viewed as candidates for 128-bit and 256-bit quantities, in case a need arises to add them to the protocol. Additional information value 30 is then the only additional information value available for general allocation, and there should be a very good reason for allocating it before assigning it through an update of this protocol.¶

8.1. Encoding Indicators

Sometimes it is useful to indicate in the diagnostic notation which of several alternative representations were actually used; for example, a data item written >1.5< by a diagnostic decoder might have been encoded as a half-, single-, or double-precision float.¶

The convention for encoding indicators is that anything starting with an underscore and all following characters that are alphanumeric or underscore, is an encoding indicator, and can be ignored by anyone not interested in this information. Encoding indicators are always optional.¶

A single underscore can be written after the opening brace of a map or the opening bracket of an array to indicate that the data item was represented in indefinite-length format. For example, [_ 1, 2] contains an indicator that an indefinite-length representation was used to represent the data item [1, 2].¶

An underscore followed by a decimal digit n indicates that the preceding item (or, for arrays and maps, the item starting with the preceding bracket or brace) was encoded with an additional information value of 24+n. For example, 1.5_1 is a half-precision floating-point number, while 1.5_3 is encoded as double precision. This encoding indicator is not shown in Appendix A. (Note that the encoding indicator "_" is thus an abbreviation of the full form "_7", which is not used.)¶

As a special case, byte and text strings of indefinite length can be notated in the form (_ h'0123', h'4567') and (_ "foo", "bar").¶

9.1. Simple Values Registry

IANA has created the "Concise Binary Object Representation (CBOR) Simple Values" registry at [IANA.cbor-simple-values]. The initial values are shown in Table 3.¶

New entries in the range 0 to 19 are assigned by Standards Action. It is suggested that these Standards Actions allocate values starting with the number 16 in order to reserve the lower numbers for contiguous blocks (if any).¶

New entries in the range 32 to 255 are assigned by Specification Required.¶

9.2. Tags Registry

IANA has created the "Concise Binary Object Representation (CBOR) Tags" registry at [IANA.cbor-tags]. The tags that were defined in [RFC7049] are described in detail in Section 3.4, but other tags have already been defined.¶

New entries in the range 0 to 23 are assigned by Standards Action. New entries in the range 24 to 255 are assigned by Specification Required. New entries in the range 256 to 18446744073709551615 are assigned by First Come First Served. The template for registration requests is:¶

• Data item¶
• Semantics (short form)¶

In addition, First Come First Served requests should include:¶

• Point of contact¶
• Description of semantics (URL) - This description is optional; the URL can point to something like an Internet-Draft or a web page.¶

9.3. Media Type ("MIME Type")

The Internet media type [RFC6838] for a single encoded CBOR data item is application/cbor.¶

Type name: application¶

Subtype name: cbor¶

Required parameters: n/a¶

Optional parameters: n/a¶

Encoding considerations:

binary¶

Security considerations:

See Section 10 of this document¶

Interoperability considerations: n/a¶

Published specification: This document¶

Applications that use this media type:

None yet, but it is expected that this format will be deployed in protocols and applications.¶

Additional information:
  Magic number(s): n/a
  File extension(s): .cbor
  Macintosh file type code(s): n/a

Person & email address to contact for further information:
  Carsten Bormann
  cabo@tzi.org

Intended usage: COMMON

Restrictions on usage: none

Author:
  Carsten Bormann <cabo@tzi.org>

Change controller:
  The IESG <iesg@ietf.org>

9.4. CoAP Content-Format

Media Type: application/cbor¶

Encoding: -¶

Id: 60¶

Reference: [RFCthis]¶

9.5. The +cbor Structured Syntax Suffix Registration

Name: Concise Binary Object Representation (CBOR)¶

+suffix: +cbor¶

References: [RFCthis]¶

Encoding Considerations: CBOR is a binary format.¶

Interoperability Considerations: n/a¶

Fragment Identifier Considerations:
  The syntax and semantics of fragment identifiers specified for
  +cbor SHOULD be as specified for "application/cbor".  (At
  publication of this document, there is no fragment identification
  syntax defined for "application/cbor".)

  The syntax and semantics for fragment identifiers for a specific
  "xxx/yyy+cbor" SHOULD be processed as follows:

  For cases defined in +cbor, where the fragment identifier resolves
  per the +cbor rules, then process as specified in +cbor.

  For cases defined in +cbor, where the fragment identifier does
  not resolve per the +cbor rules, then process as specified in
  "xxx/yyy+cbor".

  For cases not defined in +cbor, then process as specified in
  "xxx/yyy+cbor".

Security Considerations:

See Section 10 of this document¶

Contact:
  Apps Area Working Group (apps-discuss@ietf.org)

Author/Change Controller:
  The Apps Area Working Group.
  The IESG has change control over this registration.

E.1. ASN.1 DER, BER, and PER

[ASN.1] has many serializations. In the IETF, DER and BER are the most common. The serialized output is not particularly compact for many items, and the code needed to decode numeric items can be complex on a constrained device.¶

Few (if any) IETF protocols have adopted one of the several variants of Packed Encoding Rules (PER). There could be many reasons for this, but one that is commonly stated is that PER makes use of the schema even for parsing the surface structure of the data stream, requiring significant tool support. There are different versions of the ASN.1 schema language in use, which has also hampered adoption.¶

E.2. MessagePack

[MessagePack] is a concise, widely implemented counted binary serialization format, similar in many properties to CBOR, although somewhat less regular. While the data model can be used to represent JSON data, MessagePack has also been used in many remote procedure call (RPC) applications and for long-term storage of data.¶

MessagePack has been essentially stable since it was first published around 2011; it has not yet had a transition. The evolution of MessagePack is impeded by an imperative to maintain complete backwards compatibility with existing stored data, while only few bytecodes are still available for extension. Repeated requests over the years from the MessagePack user community to separate out binary and text strings in the encoding recently have led to an extension proposal that would leave MessagePack's "raw" data ambiguous between its usages for binary and text data. The extension mechanism for MessagePack remains unclear.¶

E.3. BSON

[BSON] is a data format that was developed for the storage of JSON-like maps (JSON objects) in the MongoDB database. Its major distinguishing feature is the capability for in-place update, which prevents a compact representation. BSON uses a counted representation except for map keys, which are null-byte terminated. While BSON can be used for the representation of JSON-like objects on the wire, its specification is dominated by the requirements of the database application and has become somewhat baroque. The status of how BSON extensions will be implemented remains unclear.¶

E.4. MSDTP: RFC 713

Message Services Data Transmission (MSDTP) is a very early example of a compact message format; it is described in [RFC0713], written in 1976. It is included here for its historical value, not because it was ever widely used.¶

E.5. Conciseness on the Wire

While CBOR's design objective of code compactness for encoders and decoders is a higher priority than its objective of conciseness on the wire, many people focus on the wire size. Table 7 shows some encoding examples for the simple nested array [1, [2, 3]]; where some form of indefinite-length encoding is supported by the encoding, [_ 1, [2, 3]] (indefinite length on the outer array) is also shown.¶

G.1. Examples for CBOR data items that are not well-formed

This subsection shows a few examples for CBOR data items that are not well-formed. Each example is a sequence of bytes each shown in hexadecimal; multiple examples in a list are separated by commas.¶

Examples for well-formedness error kind 1 (too much data) can easily be formed by adding data to a well-formed encoded CBOR data item.¶

Similarly, examples for well-formedness error kind 2 (too little data) can be formed by truncating a well-formed encoded CBOR data item. In test suites, it may be beneficial to specifically test with incomplete data items that would require large amounts of addition to be completed (for instance by starting the encoding of a string of a very large size).¶

A premature end of the input can occur in a head or within the enclosed data, which may be bare strings or enclosed data items that are either counted or should have been ended by a break stop code.¶

• End of input in a head: 18, 19, 1a, 1b, 19 01, 1a 01 02, 1b 01 02 03 04 05 06 07, 38, 58, 78, 98, 9a 01 ff 00, b8, d8, f8, f9 00, fa 00 00, fb 00 00 00¶
• Definite length strings with short data: 41, 61, 5a ff ff ff ff 00, 5b ff ff ff ff ff ff ff ff 01 02 03, 7a ff ff ff ff 00, 7b 7f ff ff ff ff ff ff ff 01 02 03¶
• Definite length maps and arrays not closed with enough items: 81, 81 81 81 81 81 81 81 81 81, 82 00, a1, a2 01 02, a1 00, a2 00 00 00¶
• Indefinite length strings not closed by a break stop code: 5f 41 00, 7f 61 00¶
• Indefinite length maps and arrays not closed by a break stop code: 9f, 9f 01 02, bf, bf 01 02 01 02, 81 9f, 9f 80 00, 9f 9f 9f 9f 9f ff ff ff ff, 9f 81 9f 81 9f 9f ff ff ff¶

A few examples for the five subkinds of well-formedness error kind 3 (syntax error) are shown below.¶

Subkind 1:¶

• Reserved additional information values: 1c, 1d, 1e, 3c, 3d, 3e, 5c, 5d, 5e, 7c, 7d, 7e, 9c, 9d, 9e, bc, bd, be, dc, dd, de, fc, fd, fe,¶

Subkind 2:¶

• Reserved two-byte encodings of simple types: f8 00, f8 01, f8 18, f8 1f¶

Subkind 3:¶

• Indefinite length string chunks not of the correct type: 5f 00 ff, 5f 21 ff, 5f 61 00 ff, 5f 80 ff, 5f a0 ff, 5f c0 00 ff, 5f e0 ff, 7f 41 00 ff¶
• Indefinite length string chunks not definite length: 5f 5f 41 00 ff ff, 7f 7f 61 00 ff ff¶

Subkind 4:¶

• Break occurring on its own outside of an indefinite length item: ff¶
• Break occurring in a definite length array or map or a tag: 81 ff, 82 00 ff, a1 ff, a1 ff 00, a1 00 ff, a2 00 00 ff, 9f 81 ff, 9f 82 9f 81 9f 9f ff ff ff ff¶
• Break in indefinite length map would lead to odd number of items (break in a value position): bf 00 ff, bf 00 00 00 ff¶

Subkind 5:¶

• Major type 0, 1, 6 with additional information 31: 1f, 3f, df¶