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RFC 7049

Concise Binary Object Representation (CBOR)

Pages: 54
Obsoleted by:  8949
Part 1 of 3 – Pages 1 to 20
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Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 7049                       Universitaet Bremen TZI
Category: Standards Track                                     P. Hoffman
ISSN: 2070-1721                                           VPN Consortium
                                                            October 2013


              Concise Binary Object Representation (CBOR)

Abstract

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. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7049. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2. Specification of the CBOR Encoding . . . . . . . . . . . . . 6 2.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Indefinite Lengths for Some Major Types . . . . . . . . . 9 2.2.1. Indefinite-Length Arrays and Maps . . . . . . . . . . 9 2.2.2. Indefinite-Length Byte Strings and Text Strings . . . 11 2.3. Floating-Point Numbers and Values with No Content . . . . 12 2.4. Optional Tagging of Items . . . . . . . . . . . . . . . . 14 2.4.1. Date and Time . . . . . . . . . . . . . . . . . . . . 16 2.4.2. Bignums . . . . . . . . . . . . . . . . . . . . . . . 16 2.4.3. Decimal Fractions and Bigfloats . . . . . . . . . . . 17 2.4.4. Content Hints . . . . . . . . . . . . . . . . . . . . 18 2.4.4.1. Encoded CBOR Data Item . . . . . . . . . . . . . 18 2.4.4.2. Expected Later Encoding for CBOR-to-JSON Converters . . . . . . . . . . . . . . . . . . . 18 2.4.4.3. Encoded Text . . . . . . . . . . . . . . . . . . 19 2.4.5. Self-Describe CBOR . . . . . . . . . . . . . . . . . 19 3. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 20 3.1. CBOR in Streaming Applications . . . . . . . . . . . . . 20 3.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 21 3.3. Syntax Errors . . . . . . . . . . . . . . . . . . . . . . 21 3.3.1. Incomplete CBOR Data Items . . . . . . . . . . . . . 22 3.3.2. Malformed Indefinite-Length Items . . . . . . . . . . 22 3.3.3. Unknown Additional Information Values . . . . . . . . 23 3.4. Other Decoding Errors . . . . . . . . . . . . . . . . . . 23 3.5. Handling Unknown Simple Values and Tags . . . . . . . . . 24 3.6. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.7. Specifying Keys for Maps . . . . . . . . . . . . . . . . 25 3.8. Undefined Values . . . . . . . . . . . . . . . . . . . . 26 3.9. Canonical CBOR . . . . . . . . . . . . . . . . . . . . . 26 3.10. Strict Mode . . . . . . . . . . . . . . . . . . . . . . . 28 4. Converting Data between CBOR and JSON . . . . . . . . . . . . 29 4.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 29 4.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 30 5. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 31 5.1. Extension Points . . . . . . . . . . . . . . . . . . . . 32 5.2. Curating the Additional Information Space . . . . . . . . 33 6. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 33 6.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 34 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 7.1. Simple Values Registry . . . . . . . . . . . . . . . . . 35 7.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 35 7.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 36 7.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 37
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     7.5.  The +cbor Structured Syntax Suffix Registration . . . . .  37
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  38
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     10.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  41
   Appendix B.  Jump Table . . . . . . . . . . . . . . . . . . . . .  45
   Appendix C.  Pseudocode . . . . . . . . . . . . . . . . . . . . .  48
   Appendix D.  Half-Precision . . . . . . . . . . . . . . . . . . .  50
   Appendix E.  Comparison of Other Binary Formats to CBOR's Design
                Objectives . . . . . . . . . . . . . . . . . . . . .  51
     E.1.  ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . .  52
     E.2.  MessagePack . . . . . . . . . . . . . . . . . . . . . . .  52
     E.3.  BSON  . . . . . . . . . . . . . . . . . . . . . . . . . .  53
     E.4.  UBJSON  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     E.5.  MSDTP: RFC 713  . . . . . . . . . . . . . . . . . . . . .  53
     E.6.  Conciseness on the Wire . . . . . . . . . . . . . . . . .  53

1. Introduction

There are hundreds of standardized formats for binary representation of structured data (also known as binary serialization formats). Of those, some are for specific domains of information, while others are generalized for arbitrary data. In the IETF, probably the best-known formats in the latter category are ASN.1's BER and DER [ASN.1]. The format defined here follows some specific design goals that are not well met by current formats. The underlying data model is an extended version of the JSON data model [RFC4627]. It is important to note that this is not a proposal that the grammar in RFC 4627 be extended in general, since doing so would cause a significant backwards incompatibility with already deployed JSON documents. Instead, this document simply defines its own data model that starts from JSON. Appendix E lists some existing binary formats and discusses how well they do or do not fit the design objectives of the Concise Binary Object Representation (CBOR).
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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 [CNN-TERMS]). * 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.
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   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", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119, BCP 14 [RFC2119] and indicate requirement levels for compliant CBOR implementations. 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.
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   Decoder:  A process that decodes a 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 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 is not followed by extraneous data.

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

   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 such 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.

2. Specification of the CBOR Encoding

A CBOR-encoded data item is structured and encoded as described in this section. The encoding is summarized in Table 5. The initial byte of each data item contains both information about the major type (the high-order 3 bits, described in Section 2.1) and additional information (the low-order 5 bits). When the value of the additional information is less than 24, it is directly used as a small unsigned integer. When it is 24 to 27, the additional bytes for a variable-length integer immediately follow; the values 24 to 27 of the additional information specify that its length is a 1-, 2-, 4-, or 8-byte unsigned integer, respectively. Additional information
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   value 31 is used for indefinite-length items, described in
   Section 2.2.  Additional information values 28 to 30 are reserved for
   future expansion.

   In all additional information values, the resulting integer is
   interpreted depending on the major type.  It may represent the actual
   data: for example, in integer types, the resulting integer is used
   for the value itself.  It may instead supply length information: for
   example, in byte strings it gives the length of the byte string data
   that follows.

   A CBOR decoder implementation can be based on a jump table with all
   256 defined values for the initial byte (Table 5).  A decoder in a
   constrained implementation can instead use the structure of the
   initial byte and following bytes for more compact code (see
   Appendix C for a rough impression of how this could look).

2.1. Major Types

The following lists the major types and the additional information and other bytes associated with the type. Major type 0: an unsigned integer. The 5-bit additional information is either the integer itself (for additional information values 0 through 23) or the length of additional data. Additional information 24 means the value is represented in an additional uint8_t, 25 means a uint16_t, 26 means a uint32_t, and 27 means a uint64_t. 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. The encoding follows the rules for unsigned integers (major type 0), except that the value is then -1 minus the encoded unsigned integer. 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 string's length in bytes is represented following the rules for positive integers (major type 0). 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
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      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, specifically a string of Unicode
      characters that is encoded as UTF-8 [RFC3629].  The format of this
      type is identical to that of byte strings (major type 2), that is,
      as with major type 2, the length gives the number of bytes.  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 array's length follows the rules for
      byte strings (major type 2), except that the length denotes the
      number of data items, not the length in bytes that the array takes
      up.  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 map's length follows
      the rules for byte strings (major type 2), except that the length
      denotes the number of pairs, not the length in bytes that the map
      takes up.  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.
      A map that has duplicate keys may be well-formed, but it is not
      valid, and thus it causes indeterminate decoding; see also
      Section 3.7.

   Major type 6:  optional semantic tagging of other major types.  See
      Section 2.4.
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   Major type 7:  floating-point numbers and simple data types that need
      no content, as well as the "break" stop code.  See Section 2.3.

   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 5).

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

2.2. 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.

2.2.1. Indefinite-Length Arrays and Maps

Indefinite-length arrays and maps are simply opened without indicating the number of data items that will be included in the array or map, using the additional information value of 31. The initial major type and additional information byte is followed by the elements of the array or map, just as they would be in other arrays or maps. The end of the array or map is indicated by encoding a "break" stop code in a place where the next data item would normally have been included. The "break" is encoded with major type 7 and additional information value 31 (0b111_11111) but is not itself a data item: it is just a syntactic feature to close the array or map. That is, the "break" stop code comes after the last item in the array or map, and it cannot occur anywhere else in place of a data item. In this way, indefinite-length arrays and maps look identical to other arrays and maps except for beginning with the additional information value 31 and ending with the "break" stop code. Arrays and maps with indefinite lengths allow any number of items (for arrays) and key/value pairs (for maps) to be given before the "break" stop code. There is no restriction against nesting indefinite-length array or map items. A "break" only terminates a single item, so nested indefinite-length items need exactly as many "break" stop codes as there are type bytes starting an indefinite- length item.
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   For example, assume an encoder wants to represent the abstract array
   [1, [2, 3], [4, 5]].  The definite-length encoding would be
   0x8301820203820405:

   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

   Indefinite-length encoding could be applied independently to each of
   the three arrays encoded in this data item, as required, leading to
   representations such as:

   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"
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   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


   An example of an indefinite-length map (that happens to have two
   key/value pairs) might be:

   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        -- -2
      FF        -- "break"

2.2.2. Indefinite-Length Byte Strings and Text Strings

Indefinite-length byte strings and text strings are actually a concatenation of zero or more definite-length byte or text strings ("chunks") that are together treated as one contiguous string. Indefinite-length strings are opened with the major type and additional information value of 31, but what follows are a series of byte or text strings that have definite lengths (the chunks). The end of the series of chunks is indicated by encoding the "break" stop code (0b111_11111) in a place where the next chunk in the series would occur. The contents of the chunks are concatenated together,
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   and the overall length of the indefinite-length string will be the
   sum of the lengths of all of the chunks.  In summary, an indefinite-
   length string is encoded similarly to how an indefinite-length array
   of its chunks would be encoded, except that the major type of the
   indefinite-length string is that of a (text or byte) string and
   matches the major types of its chunks.

   For indefinite-length byte strings, every data item (chunk) between
   the indefinite-length indicator and the "break" MUST be a definite-
   length byte string item; if the parser sees any item type other than
   a byte string before it sees the "break", it is an error.

   For example, assume the sequence:

   0b010_11111 0b010_00100 0xaabbccdd 0b010_00011 0xeeff99 0b111_11111

   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"

   After decoding, this results in a single byte string with seven
   bytes: 0xaabbccddeeff99.

   Text strings with indefinite lengths act the same as byte strings
   with indefinite lengths, except that all their chunks MUST be
   definite-length text strings.  Note that this implies that the bytes
   of a single UTF-8 character cannot be spread between chunks: a new
   chunk can only be started at a character boundary.

2.3. 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 1. Like the major types for integers, items of this major type do not carry content data; all the information is in the initial bytes.
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    +-------------+--------------------------------------------------+
    | 5-Bit Value | Semantics                                        |
    +-------------+--------------------------------------------------+
    | 0..23       | Simple value (value 0..23)                       |
    |             |                                                  |
    | 24          | Simple value (value 32..255 in following byte)   |
    |             |                                                  |
    | 25          | IEEE 754 Half-Precision Float (16 bits follow)   |
    |             |                                                  |
    | 26          | IEEE 754 Single-Precision Float (32 bits follow) |
    |             |                                                  |
    | 27          | IEEE 754 Double-Precision Float (64 bits follow) |
    |             |                                                  |
    | 28-30       | (Unassigned)                                     |
    |             |                                                  |
    | 31          | "break" stop code for indefinite-length items    |
    +-------------+--------------------------------------------------+

        Table 1: Values for Additional Information in Major Type 7

   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 2 lists the values
   assigned and available for simple types.

                       +---------+-----------------+
                       | Value   | Semantics       |
                       +---------+-----------------+
                       | 0..19   | (Unassigned)    |
                       |         |                 |
                       | 20      | False           |
                       |         |                 |
                       | 21      | True            |
                       |         |                 |
                       | 22      | Null            |
                       |         |                 |
                       | 23      | Undefined value |
                       |         |                 |
                       | 24..31  | (Reserved)      |
                       |         |                 |
                       | 32..255 | (Unassigned)    |
                       +---------+-----------------+

                          Table 2: Simple Values
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   The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit
   IEEE 754 binary floating-point values.  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.)

2.4. Optional Tagging of Items

In CBOR, a data item can optionally be preceded by a tag to give it additional semantics while retaining its structure. The tag is major type 6, and represents an integer number as indicated by the tag's integer value; the (sole) 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 usually restricts what kinds of nested data item or items can be carried by a tag. The initial bytes of the tag follow the rules for positive integers (major type 0). The tag is followed by a single data item of any type. For example, assume that a byte string of length 12 is marked with a tag to indicate it is a positive bignum (Section 2.4.2). This would be marked as 0b110_00010 (major type 6, additional information 2 for the tag) 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, and thus 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 allow optional tagging when the decoder is a generic CBOR decoder that might be able to benefit from hints about the content of items. Understanding the semantic tags is optional for a decoder; it can just jump over the initial bytes of the tag and interpret the tagged data item itself. A tag always applies to the item that is directly followed by it. Thus, if tag A is followed by tag B, which is followed by data item C, tag A applies to the result of applying tag B on data item C. That is, a tagged item is a data item consisting of a tag and a value. The content of the tagged item is the data item (the value) that is being tagged. IANA maintains a registry of tag values as described in Section 7.2. Table 3 provides a list of initial values, with definitions in the rest of this section.
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   +--------------+------------------+---------------------------------+
   | Tag          | Data Item        | Semantics                       |
   +--------------+------------------+---------------------------------+
   | 0            | UTF-8 string     | Standard date/time string; see  |
   |              |                  | Section 2.4.1                   |
   |              |                  |                                 |
   | 1            | multiple         | Epoch-based date/time; see      |
   |              |                  | Section 2.4.1                   |
   |              |                  |                                 |
   | 2            | byte string      | Positive bignum; see Section    |
   |              |                  | 2.4.2                           |
   |              |                  |                                 |
   | 3            | byte string      | Negative bignum; see Section    |
   |              |                  | 2.4.2                           |
   |              |                  |                                 |
   | 4            | array            | Decimal fraction; see Section   |
   |              |                  | 2.4.3                           |
   |              |                  |                                 |
   | 5            | array            | Bigfloat; see Section 2.4.3     |
   |              |                  |                                 |
   | 6..20        | (Unassigned)     | (Unassigned)                    |
   |              |                  |                                 |
   | 21           | multiple         | Expected conversion to          |
   |              |                  | base64url encoding; see         |
   |              |                  | Section 2.4.4.2                 |
   |              |                  |                                 |
   | 22           | multiple         | Expected conversion to base64   |
   |              |                  | encoding; see Section 2.4.4.2   |
   |              |                  |                                 |
   | 23           | multiple         | Expected conversion to base16   |
   |              |                  | encoding; see Section 2.4.4.2   |
   |              |                  |                                 |
   | 24           | byte string      | Encoded CBOR data item; see     |
   |              |                  | Section 2.4.4.1                 |
   |              |                  |                                 |
   | 25..31       | (Unassigned)     | (Unassigned)                    |
   |              |                  |                                 |
   | 32           | UTF-8 string     | URI; see Section 2.4.4.3        |
   |              |                  |                                 |
   | 33           | UTF-8 string     | base64url; see Section 2.4.4.3  |
   |              |                  |                                 |
   | 34           | UTF-8 string     | base64; see Section 2.4.4.3     |
   |              |                  |                                 |
   | 35           | UTF-8 string     | Regular expression; see         |
   |              |                  | Section 2.4.4.3                 |
   |              |                  |                                 |
   | 36           | UTF-8 string     | MIME message; see Section       |
   |              |                  | 2.4.4.3                         |
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   |              |                  |                                 |
   | 37..55798    | (Unassigned)     | (Unassigned)                    |
   |              |                  |                                 |
   | 55799        | multiple         | Self-describe CBOR; see         |
   |              |                  | Section 2.4.5                   |
   |              |                  |                                 |
   | 55800+       | (Unassigned)     | (Unassigned)                    |
   +--------------+------------------+---------------------------------+

                         Table 3: Values for Tags

2.4.1. Date and Time

Tag value 0 is for date/time strings that follow the standard format described in [RFC3339], as refined by Section 3.3 of [RFC4287]. Tag value 1 is for numerical representation of seconds relative to 1970-01-01T00:00Z in UTC time. (For the non-negative values that the Portable Operating System Interface (POSIX) defines, the number of seconds is counted in the same way as for POSIX "seconds since the epoch" [TIME_T].) The tagged item can be a positive or negative integer (major types 0 and 1), or a floating-point number (major type 7 with additional information 25, 26, or 27). Note that the number can be negative (time before 1970-01-01T00:00Z) and, if a floating- point number, indicate fractional seconds.

2.4.2. Bignums

Bignums are integers that do not fit into the basic integer representations provided by major types 0 and 1. They are encoded as a byte string data item, which is interpreted as an unsigned integer n in network byte order. For tag value 2, the value of the bignum is n. For tag value 3, the value of the bignum is -1 - n. Decoders that understand these tags MUST be able to decode bignums that have leading zeroes. For example, the number 18446744073709551616 (2**64) is represented as 0b110_00010 (major type 6, tag 2), followed by 0b010_01001 (major type 2, length 9), followed by 0x010000000000000000 (one byte 0x01 and eight bytes 0x00). In hexadecimal: C2 -- Tag 2 29 -- Byte string of length 9 010000000000000000 -- Bytes content
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2.4.3. Decimal Fractions and Bigfloats

Decimal fractions combine an integer mantissa with a base-10 scaling factor. They are most useful if an application needs the exact representation of a decimal fraction such as 1.1 because there is no exact representation for many decimal fractions in binary floating point. Bigfloats combine an integer mantissa with a base-2 scaling factor. They are binary floating-point values that can exceed the range or the precision of the three IEEE 754 formats supported by CBOR (Section 2.3). Bigfloats may also be used by constrained applications that need some basic binary floating-point capability without the need for supporting IEEE 754. A decimal fraction or a bigfloat is represented as a tagged array that contains exactly two integer numbers: an exponent e and a mantissa m. Decimal fractions (tag 4) use base-10 exponents; the value of a decimal fraction data item is m*(10**e). Bigfloats (tag 5) use base-2 exponents; the value of a bigfloat data item is m*(2**e). The exponent e MUST be represented in an integer of major type 0 or 1, while the mantissa also can be a bignum (Section 2.4.2). An example of a decimal fraction is that the number 273.15 could be represented as 0b110_00100 (major type of 6 for the tag, additional information of 4 for the type of tag), followed by 0b100_00010 (major type of 4 for the array, additional information of 2 for the length of the array), followed by 0b001_00001 (major type of 1 for the first integer, additional information of 1 for the value of -2), followed by 0b000_11001 (major type of 0 for the second integer, additional information of 25 for a two-byte value), followed by 0b0110101010110011 (27315 in two bytes). In hexadecimal: C4 -- Tag 4 82 -- Array of length 2 21 -- -2 19 6ab3 -- 27315 An example of a bigfloat is that the number 1.5 could be represented as 0b110_00101 (major type of 6 for the tag, additional information of 5 for the type of tag), followed by 0b100_00010 (major type of 4 for the array, additional information of 2 for the length of the array), followed by 0b001_00000 (major type of 1 for the first integer, additional information of 0 for the value of -1), followed by 0b000_00011 (major type of 0 for the second integer, additional information of 3 for the value of 3). In hexadecimal:
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   C5             -- Tag 5
      82          -- Array of length 2
         20       -- -1
         03       -- 3

   Decimal fractions and bigfloats provide no representation of
   Infinity, -Infinity, or NaN; if these are needed in place of a
   decimal fraction or bigfloat, the IEEE 754 half-precision
   representations from Section 2.3 can be used.  For constrained
   applications, where there is a choice between representing a specific
   number as an integer and as a decimal fraction or bigfloat (such as
   when the exponent is small and non-negative), there is a quality-of-
   implementation expectation that the integer representation is used
   directly.

2.4.4. Content Hints

The tags in this section are for content hints that might be used by generic CBOR processors.
2.4.4.1. Encoded CBOR Data Item
Sometimes it is beneficial to carry an embedded CBOR data item that is not meant to be decoded immediately at the time the enclosing data item is being parsed. Tag 24 (CBOR data item) can be used to tag the embedded byte string as a data item encoded in CBOR format.
2.4.4.2. Expected Later Encoding for CBOR-to-JSON Converters
Tags 21 to 23 indicate that a byte string might require a specific encoding when interoperating with a text-based representation. These tags are useful when an encoder knows that the byte string data it is writing is likely to be later converted to a particular JSON-based usage. That usage specifies that some strings are encoded as base64, base64url, and so on. The encoder uses byte strings instead of doing the encoding itself to reduce the message size, to reduce the code size of the encoder, or both. The encoder does not know whether or not the converter will be generic, and therefore wants to say what it believes is the proper way to convert binary strings to JSON. The data item tagged can be a byte string or any other data item. In the latter case, the tag applies to all of the byte string data items contained in the data item, except for those contained in a nested data item tagged with an expected conversion. These three tag types suggest conversions to three of the base data encodings defined in [RFC4648]. For base64url encoding, padding is not used (see Section 3.2 of RFC 4648); that is, all trailing equals
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   signs ("=") are removed from the base64url-encoded string.  Later
   tags might be defined for other data encodings of RFC 4648 or for
   other ways to encode binary data in strings.

2.4.4.3. Encoded Text
Some text strings hold data that have formats widely used on the Internet, and sometimes those formats can be validated and presented to the application in appropriate form by the decoder. There are tags for some of these formats. o Tag 32 is for URIs, as defined in [RFC3986]; o Tags 33 and 34 are for base64url- and base64-encoded text strings, as defined in [RFC4648]; o Tag 35 is for regular expressions in Perl Compatible Regular Expressions (PCRE) / JavaScript syntax [ECMA262]. o Tag 36 is for MIME messages (including all headers), as defined in [RFC2045]; Note that tags 33 and 34 differ from 21 and 22 in that the data is transported in base-encoded form for the former and in raw byte string form for the latter.

2.4.5. Self-Describe CBOR

In many applications, it will be clear from the context that CBOR is being employed for encoding a data item. For instance, a specific protocol might specify the use of CBOR, or a media type is indicated that specifies its use. However, there may be applications where such context information is not available, such as when CBOR data is stored in a file and disambiguating metadata is not in use. Here, it may help to have some distinguishing characteristics for the data itself. Tag 55799 is defined for this purpose. It does not impart any special semantics on the data item that follows; that is, the semantics of a data item tagged with tag 55799 is exactly identical to the semantics of the data item itself. The serialization of this tag is 0xd9d9f7, which appears not to be in use as a distinguishing mark for frequently used file types. In particular, it is not a valid start of a Unicode text in any Unicode encoding if followed by a valid CBOR data item.
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   For instance, a decoder might be able to parse both CBOR and JSON.
   Such a decoder would need to mechanically distinguish the two
   formats.  An easy way for an encoder to help the decoder would be to
   tag the entire CBOR item with tag 55799, the serialization of which
   will never be found at the beginning of a JSON text.



(page 20 continued on part 2)

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