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rfc:rfc7049

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.

Bormann & Hoffman Standards Track [Page 1] RFC 7049 CBOR October 2013

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

Bormann & Hoffman Standards Track [Page 2] RFC 7049 CBOR October 2013

   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).

Bormann & Hoffman Standards Track [Page 3] RFC 7049 CBOR October 2013

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.

Bormann & Hoffman Standards Track [Page 4] RFC 7049 CBOR October 2013

 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.

Bormann & Hoffman Standards Track [Page 5] RFC 7049 CBOR October 2013

 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

Bormann & Hoffman Standards Track [Page 6] RFC 7049 CBOR October 2013

 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

Bormann & Hoffman Standards Track [Page 7] RFC 7049 CBOR October 2013

    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.

Bormann & Hoffman Standards Track [Page 8] RFC 7049 CBOR October 2013

 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.

Bormann & Hoffman Standards Track [Page 9] RFC 7049 CBOR October 2013

 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"

Bormann & Hoffman Standards Track [Page 10] RFC 7049 CBOR October 2013

 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,

Bormann & Hoffman Standards Track [Page 11] RFC 7049 CBOR October 2013

 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.

Bormann & Hoffman Standards Track [Page 12] RFC 7049 CBOR October 2013

  +-------------+--------------------------------------------------+
  | 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

Bormann & Hoffman Standards Track [Page 13] RFC 7049 CBOR October 2013

 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.

Bormann & Hoffman Standards Track [Page 14] RFC 7049 CBOR October 2013

 +--------------+------------------+---------------------------------+
 | 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                         |

Bormann & Hoffman Standards Track [Page 15] RFC 7049 CBOR October 2013

 |              |                  |                                 |
 | 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

Bormann & Hoffman Standards Track [Page 16] RFC 7049 CBOR October 2013

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:

Bormann & Hoffman Standards Track [Page 17] RFC 7049 CBOR October 2013

 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

Bormann & Hoffman Standards Track [Page 18] RFC 7049 CBOR October 2013

 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.

Bormann & Hoffman Standards Track [Page 19] RFC 7049 CBOR October 2013

 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.

3. Creating CBOR-Based Protocols

 Data formats such as CBOR are often used in environments where there
 is no format negotiation.  A specific design goal of CBOR is to not
 need any included or assumed schema: a decoder can take a CBOR item
 and decode it with no other knowledge.
 Of course, in real-world implementations, the encoder and the decoder
 will have a shared view of what should be in a CBOR data item.  For
 example, an agreed-to format might be "the item is an array whose
 first value is a UTF-8 string, second value is an integer, and
 subsequent values are zero or more floating-point numbers" or "the
 item is a map that has byte strings for keys and contains at least
 one pair whose key is 0xab01".
 This specification puts no restrictions on CBOR-based protocols.  An
 encoder can be capable of encoding as many or as few types of values
 as is required by the protocol in which it is used; a decoder can be
 capable of understanding as many or as few types of values as is
 required by the protocols in which it is used.  This lack of
 restrictions allows CBOR to be used in extremely constrained
 environments.
 This section discusses some considerations in creating CBOR-based
 protocols.  It is advisory only and explicitly excludes any language
 from RFC 2119 other than words that could be interpreted as "MAY" in
 the sense of RFC 2119.

3.1. 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.

Bormann & Hoffman Standards Track [Page 20] RFC 7049 CBOR October 2013

 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.

3.2. Generic Encoders and Decoders

 A generic CBOR decoder can decode all well-formed CBOR data and
 present them to an application.  CBOR data is well-formed if it uses
 the initial bytes, as well as the byte strings and/or data items that
 are implied by their values, in the manner defined by CBOR, and no
 extraneous data follows (Appendix C).
 Even though CBOR attempts to minimize these cases, not all well-
 formed CBOR data is valid: for example, the format excludes simple
 values below 32 that are encoded with an extension byte.  Also,
 specific tags may make semantic constraints that may be violated,
 such as by including a tag in a bignum tag or by following a byte
 string within a date tag.  Finally, the data may be invalid, such as
 invalid UTF-8 strings or date strings that do not conform to
 [RFC3339].  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 3.5).
 Generic decoders provide ways to present well-formed CBOR values,
 both valid and invalid, to an application.  The diagnostic notation
 (Section 6) 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.

3.3. Syntax Errors

 A decoder encountering a CBOR data item that is not well-formed
 generally can choose to completely fail the decoding (issue an error
 and/or stop processing altogether), substitute the problematic data
 and data items using a decoder-specific convention that clearly
 indicates there has been a problem, or take some other action.

Bormann & Hoffman Standards Track [Page 21] RFC 7049 CBOR October 2013

3.3.1. Incomplete CBOR Data Items

 The representation of a CBOR data item has a specific length,
 determined by its initial bytes and by the structure of any data
 items enclosed in the data items.  If less data is available, this
 can be treated as a syntax error.  A decoder may also implement
 incremental parsing, that is, decode the data item as far as it is
 available and present the data found so far (such as in an event-
 based interface), with the option of continuing the decoding once
 further data is available.
 Examples of incomplete data items include:
 o  A decoder expects a certain number of array or map entries but
    instead encounters the end of the data.
 o  A decoder processes what it expects to be the last pair in a map
    and comes to the end of the data.
 o  A decoder has just seen a tag and then encounters the end of the
    data.
 o  A decoder has seen the beginning of an indefinite-length item but
    encounters the end of the data before it sees the "break" stop
    code.

3.3.2. Malformed Indefinite-Length Items

 Examples of malformed indefinite-length data items include:
 o  Within an indefinite-length byte string or text, a decoder finds
    an item that is not of the appropriate major type before it finds
    the "break" stop code.
 o  Within an indefinite-length map, a decoder encounters the "break"
    stop code immediately after reading a key (the value is missing).
 Another error is finding a "break" stop code at a point in the data
 where there is no immediately enclosing (unclosed) indefinite-length
 item.

Bormann & Hoffman Standards Track [Page 22] RFC 7049 CBOR October 2013

3.3.3. Unknown Additional Information Values

 At the time of writing, some additional information values are
 unassigned and reserved for future versions of this document (see
 Section 5.2).  Since the overall syntax for these additional
 information values is not yet defined, a decoder that sees an
 additional information value that it does not understand cannot
 continue parsing.

3.4. Other Decoding Errors

 A CBOR data item may be syntactically well-formed but present a
 problem with interpreting the data encoded in it in the CBOR data
 model.  Generally speaking, a decoder that finds a data item with
 such a problem might issue a warning, might stop processing
 altogether, might handle the error and make the problematic value
 available to the application as such, or take some other type of
 action.
 Such problems might include:
 Duplicate keys in a map:  Generic decoders (Section 3.2) make data
    available to applications using the native CBOR data model.  That
    data model includes maps (key-value mappings with unique keys),
    not multimaps (key-value mappings where multiple entries can have
    the same key).  Thus, a generic decoder that gets a CBOR map item
    that has duplicate keys will decode to a map with only one
    instance of that key, or it might stop processing altogether.  On
    the other hand, a "streaming decoder" may not even be able to
    notice (Section 3.7).
 Inadmissible type on the value following a tag:  Tags (Section 2.4)
    specify what type of data item is supposed to follow the tag; for
    example, the tags for positive or negative bignums are supposed to
    be put on byte strings.  A decoder that decodes the tagged data
    item into a native representation (a native big integer in this
    example) is expected to check the type of the data item being
    tagged.  Even decoders that don't have such native representations
    available in their environment may perform the check on those tags
    known to them and react appropriately.
 Invalid UTF-8 string:  A decoder might or might not want to verify
    that the sequence of bytes in a UTF-8 string (major type 3) is
    actually valid UTF-8 and react appropriately.

Bormann & Hoffman Standards Track [Page 23] RFC 7049 CBOR October 2013

3.5. Handling Unknown Simple Values and Tags

 A decoder that comes across a simple value (Section 2.3) that it does
 not recognize, such as a value that was added to the IANA registry
 after the decoder was deployed or a value that the decoder chose not
 to implement, might issue a warning, might stop processing
 altogether, might handle the error by making the unknown value
 available to the application as such (as is expected of generic
 decoders), or take some other type of action.
 A decoder that comes across a tag (Section 2.4) that it does not
 recognize, such as a tag that was added to the IANA registry after
 the decoder was deployed or a tag that the decoder chose not to
 implement, might issue a warning, might stop processing altogether,
 might handle the error and present the unknown tag value together
 with the contained data item to the application (as is expected of
 generic decoders), might ignore the tag and simply present the
 contained data item only to the application, or take some other type
 of action.

3.6. Numbers

 For the purposes of this specification, all number representations
 for the same numeric value are equivalent.  This means that an
 encoder can encode a floating-point value of 0.0 as the integer 0.
 It, however, also means that an application that expects to find
 integer values only might find floating-point values if the encoder
 decides these are desirable, such as when the floating-point value is
 more compact than a 64-bit integer.
 An application or protocol that uses CBOR might restrict the
 representations of numbers.  For instance, a protocol that only deals
 with integers might say that floating-point numbers may not be used
 and that decoders of that protocol do not need to be able to handle
 floating-point numbers.  Similarly, a protocol or application that
 uses CBOR might say that decoders need to be able to handle either
 type of number.
 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.

Bormann & Hoffman Standards Track [Page 24] RFC 7049 CBOR October 2013

 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_11101 followed by two
 bytes of 0x00) as long as the application can decode an integer of
 the given size.

3.7. 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 objects, a key of integer 1 cannot be distinguished from a
 key of string "1".  This means that, if integer keys are used, the
 simultaneous use of string keys that look like numbers needs to be
 avoided.  Again, this leads to the conclusion that keys should be of
 a single CBOR type.
 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 should make an intentional decision about what
 to do when a receiving application does see multiple identical keys
 in a map.  The resulting rule in the protocol should respect the CBOR
 data model: it cannot prescribe a specific handling of the entries

Bormann & Hoffman Standards Track [Page 25] RFC 7049 CBOR October 2013

 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 are using strict mode
 (Section 3.10).
 The CBOR data model for maps does not allow ascribing semantics to
 the order of the key/value pairs in the map representation.
 Thus, it would be a very bad practice to define a CBOR-based protocol
 in such a way that changing the key/value pair order in a map would
 change the semantics, apart from trivial aspects (cache usage, etc.).
 (A CBOR-based protocol can prescribe a specific order of
 serialization, such as for canonicalization.)
 Applications for constrained devices that have maps with 24 or fewer
 frequently used keys should consider using small integers (and those
 with up to 48 frequently used keys should consider also using small
 negative integers) because the keys can then be encoded in a single
 byte.

3.8. Undefined Values

 In some CBOR-based protocols, the simple value (Section 2.3) 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.

3.9. Canonical CBOR

 Some protocols may want encoders to only emit CBOR in a particular
 canonical format; those protocols might also have the decoders check
 that their input is canonical.  Those protocols are free to define
 what they mean by a canonical format and what encoders and decoders
 are expected to do.  This section lists some suggestions for such
 protocols.
 If a protocol considers "canonical" to mean that two encoder
 implementations starting with the same input data will produce the
 same CBOR output, the following four rules would suffice:
 o  Integers must be as small as possible.
  • 0 to 23 and -1 to -24 must be expressed in the same byte as the

major type;

  • 24 to 255 and -25 to -256 must be expressed only with an

additional uint8_t;

Bormann & Hoffman Standards Track [Page 26] RFC 7049 CBOR October 2013

  • 256 to 65535 and -257 to -65536 must be expressed only with an

additional uint16_t;

  • 65536 to 4294967295 and -65537 to -4294967296 must be expressed

only with an additional uint32_t.

 o  The expression of lengths in major types 2 through 5 must be as
    short as possible.  The rules for these lengths follow the above
    rule for integers.
 o  The keys in every map must be sorted lowest value to highest.
    Sorting is performed on the bytes of the representation of the key
    data items without paying attention to the 3/5 bit splitting for
    major types.  (Note that this rule allows maps that have keys of
    different types, even though that is probably a bad practice that
    could lead to errors in some canonicalization implementations.)
    The sorting rules are:
  • If two keys have different lengths, the shorter one sorts

earlier;

  • If two keys have the same length, the one with the lower value

in (byte-wise) lexical order sorts earlier.

 o  Indefinite-length items must be made into definite-length items.
 If a protocol allows for IEEE floats, then additional
 canonicalization rules might need to be added.  One example rule
 might be to have all floats start as a 64-bit float, then do a test
 conversion to a 32-bit float; if the result is the same numeric
 value, use the shorter value and repeat the process with a test
 conversion to a 16-bit float.  (This rule selects 16-bit float for
 positive and negative Infinity as well.)  Also, there are many
 representations for NaN.  If NaN is an allowed value, it must always
 be represented as 0xf97e00.
 CBOR tags present additional considerations for canonicalization.
 The absence or presence of tags in a canonical format is determined
 by the optionality of the tags in the protocol.  In a CBOR-based
 protocol that allows optional tagging anywhere, the canonical format
 must not allow them.  In a protocol that requires tags in certain
 places, the tag needs to appear in the canonical format.  A CBOR-
 based protocol that uses canonicalization might instead say that all
 tags that appear in a message must be retained regardless of whether
 they are optional.

Bormann & Hoffman Standards Track [Page 27] RFC 7049 CBOR October 2013

3.10. Strict Mode

 Some areas of application of CBOR do not require canonicalization
 (Section 3.9) but may require that different decoders reach the same
 (semantically equivalent) results, even in the presence of
 potentially malicious data.  This can be required if one application
 (such as a firewall or other protecting entity) makes a decision
 based on the data that another application, which independently
 decodes the data, relies on.
 Normally, it is the responsibility of the sender to avoid ambiguously
 decodable data.  However, the sender might be an attacker specially
 making up CBOR data such that it will be interpreted differently by
 different decoders in an attempt to exploit that as a vulnerability.
 Generic decoders used in applications where this might be a problem
 need to support a strict mode in which it is also the responsibility
 of the receiver to reject ambiguously decodable data.  It is expected
 that firewalls and other security systems that decode CBOR will only
 decode in strict mode.
 A decoder in strict mode will reliably reject any data that could be
 interpreted by other decoders in different ways.  It will reliably
 reject data items with syntax errors (Section 3.3).  It will also
 expend the effort to reliably detect other decoding errors
 (Section 3.4).  In particular, a strict 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 following:
 o  a map (major type 5) that has more than one entry with the same
    key
 o  a tag that is used on a data item of the incorrect type
 o  a data item that is incorrectly formatted for the type given to
    it, such as invalid UTF-8 or data that cannot be interpreted with
    the specific tag that it has been tagged with
 A decoder in strict mode can do one of two things when it encounters
 a tag or simple value that it does not recognize:
 o  It can report an error (and not return data).
 o  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 or
    simple value.

Bormann & Hoffman Standards Track [Page 28] RFC 7049 CBOR October 2013

 The latter approach, which is also appropriate for non-strict
 decoders, supports 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 this processing may have an appreciable cost (in
 particular with duplicate detection for maps), support of strict mode
 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 unambiguously decodable CBOR results.  A generic
 encoder also may want to provide a strict mode where it reliably
 limits its output to unambiguously decodable CBOR, independent of
 whether or not its application is providing API-conformant data.

4. Converting Data between CBOR and JSON

 This section gives non-normative advice about converting between CBOR
 and JSON.  Implementations of converters are free to use whichever
 advice here they want.
 It is worth noting that a JSON text is a sequence of characters, not
 an encoded sequence of bytes, while a CBOR data item consists of
 bytes, not characters.

4.1. 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.
 o  An integer (major type 0 or 1) becomes a JSON number.
 o  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.
 o  A UTF-8 string (major type 3) becomes a JSON string.  Note that
    JSON requires escaping certain characters (RFC 4627, Section 2.5):
    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.
 o  An array (major type 4) becomes a JSON array.

Bormann & Hoffman Standards Track [Page 29] RFC 7049 CBOR October 2013

 o  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.
 o  False (major type 7, additional information 20) becomes a JSON
    false.
 o  True (major type 7, additional information 21) becomes a JSON
    true.
 o  Null (major type 7, additional information 22) becomes a JSON
    null.
 o  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.
 o  Any other simple value (major type 7, any additional information
    value not yet discussed) is represented by the substitute value.
 o  A bignum (major type 6, tag value 2 or 3) is represented by
    encoding its byte string in base64url without padding and becomes
    a JSON string.  For tag value 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.)
 o  A byte string with an encoding hint (major type 6, tag value 21
    through 23) is encoded as described and becomes a JSON string.
 o  For all other tags (major type 6, any other tag value), the
    embedded CBOR item is represented as a JSON value; the tag value
    is ignored.
 o  Indefinite-length items are made definite before conversion.

4.2. 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:

Bormann & Hoffman Standards Track [Page 30] RFC 7049 CBOR October 2013

 o  JSON numbers without fractional parts (integer numbers) are
    represented as integers (major types 0 and 1, possibly major type
    6 tag value 2 and 3), choosing the shortest form; integers longer
    than an implementation-defined threshold (which is usually either
    32 or 64 bits) may instead be represented as floating-point
    values.  (If the JSON was generated from a JavaScript
    implementation, its precision is already limited to 53 bits
    maximum.)
 o  Numbers with fractional parts are represented as floating-point
    values.  Preferably, the shortest exact floating-point
    representation is used; 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).  There
    may be an implementation-defined limit to the precision that will
    affect the precision of the represented values.  Decimal
    representation should only be used 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.

5. Future Evolution of CBOR

 Successful protocols evolve over time.  New ideas appear,
 implementation platforms improve, related protocols are developed and
 evolve, and new requirements from applications and protocols are
 added.  Facilitating protocol evolution is therefore an important
 design consideration for any protocol development.
 For protocols that will use CBOR, CBOR provides some useful
 mechanisms to facilitate their evolution.  Best practices for this
 are well known, particularly from JSON format development of JSON-
 based protocols.  Therefore, such best practices are outside the
 scope of this specification.
 However, facilitating the evolution of CBOR itself is very well
 within its scope.  CBOR is designed to both provide a stable basis
 for development of CBOR-based protocols and to be able to evolve.

Bormann & Hoffman Standards Track [Page 31] RFC 7049 CBOR October 2013

 Since a successful protocol may live for decades, CBOR needs to be
 designed for decades of use and evolution.  This section provides
 some guidance for the evolution of CBOR.  It is necessarily more
 subjective than other parts of this document.  It is also necessarily
 incomplete, lest it turn into a textbook on protocol development.

5.1. 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:
 o  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 7.1
    is the appropriate way to address the extensibility of this
    codepoint space.
 o  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 can choose
    to simply ignore it or to process it as an unknown tag wrapping
    the following data item.  The IANA registry in Section 7.2 is the
    appropriate way to address the extensibility of this codepoint
    space.
 o  the "additional information" space.  An implementation receiving
    an unknown additional information value has no way to continue
    parsing, so allocating codepoints to this space is a major step.
    There are also very few codepoints left.

Bormann & Hoffman Standards Track [Page 32] RFC 7049 CBOR October 2013

5.2. 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.

6. Diagnostic Notation

 CBOR is a binary interchange format.  To facilitate documentation and
 debugging, and in particular to facilitate communication between
 entities cooperating in debugging, this section defines a simple
 human-readable diagnostic notation.  All actual interchange always
 happens in the binary format.
 Note that this truly is a diagnostic format; it is not meant to be
 parsed.  Therefore, no formal definition (as in ABNF) is given in
 this document.  (Implementers looking for a text-based format for
 representing CBOR data items in configuration files may also want to
 consider YAML [YAML].)
 The diagnostic notation is loosely based on JSON as it is defined in
 RFC 4627, extending it where needed.
 The notation borrows the JSON syntax for numbers (integer and
 floating point), True (>true<), False (>false<), Null (>null<), UTF-8
 strings, arrays, and maps (maps are called objects in JSON; the
 diagnostic notation extends JSON here by allowing any data item in
 the key position).  Undefined is written >undefined< as in
 JavaScript.  The non-finite floating-point numbers Infinity,
 -Infinity, and NaN are written exactly as in this sentence (this is
 also a way they can be written in JavaScript, although JSON does not
 allow them).  A tagged item is written as an integer number for the
 tag followed by the item in parentheses; for instance, an RFC 3339
 (ISO 8601) date could be notated as:

Bormann & Hoffman Standards Track [Page 33] RFC 7049 CBOR October 2013

    0("2013-03-21T20:04:00Z")
 or the equivalent relative time as
    1(1363896240)
 Byte strings are notated in one of the base encodings, without
 padding, enclosed in single quotes, prefixed by >h< for base16, >b32<
 for base32, >h32< for base32hex, >b64< for base64 or base64url (the
 actual encodings do not overlap, so the string remains unambiguous).
 For example, the byte string 0x12345678 could be written h'12345678',
 b32'CI2FM6A', or b64'EjRWeA'.
 Unassigned simple values are given as "simple()" with the appropriate
 integer in the parentheses.  For example, "simple(42)" indicates
 major type 7, value 42.

6.1. 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").

Bormann & Hoffman Standards Track [Page 34] RFC 7049 CBOR October 2013

7. IANA Considerations

 IANA has created two registries for new CBOR values.  The registries
 are separate, that is, not under an umbrella registry, and follow the
 rules in [RFC5226].  IANA has also assigned a new MIME media type and
 an associated Constrained Application Protocol (CoAP) Content-Format
 entry.

7.1. Simple Values Registry

 IANA has created the "Concise Binary Object Representation (CBOR)
 Simple Values" registry.  The initial values are shown in Table 2.
 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.

7.2. Tags Registry

 IANA has created the "Concise Binary Object Representation (CBOR)
 Tags" registry.  The initial values are shown in Table 3.
 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:
 o  Data item
 o  Semantics (short form)
 In addition, First Come First Served requests should include:
 o  Point of contact
 o  Description of semantics (URL)
    This description is optional; the URL can point to something like
    an Internet-Draft or a web page.

Bormann & Hoffman Standards Track [Page 35] RFC 7049 CBOR October 2013

7.3. Media Type ("MIME Type")

 The Internet media type [RFC6838] for CBOR data is application/cbor.
 Type name: application
 Subtype name: cbor
 Required parameters: n/a
 Optional parameters: n/a
 Encoding considerations:  binary
 Security considerations:  See Section 8 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>

Bormann & Hoffman Standards Track [Page 36] RFC 7049 CBOR October 2013

7.4. CoAP Content-Format

 Media Type: application/cbor
 Encoding: -
 Id: 60
 Reference: [RFC7049]

7.5. The +cbor Structured Syntax Suffix Registration

 Name: Concise Binary Object Representation (CBOR)
 +suffix: +cbor
 References: [RFC7049]
 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 8 of this document
 Contact:
    Apps Area Working Group (apps-discuss@ietf.org)

Bormann & Hoffman Standards Track [Page 37] RFC 7049 CBOR October 2013

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

8. Security Considerations

 A network-facing application can exhibit vulnerabilities in its
 processing logic for incoming data.  Complex parsers are well known
 as a likely source of such vulnerabilities, such as the ability to
 remotely crash a node, or even remotely execute arbitrary code on it.
 CBOR attempts to narrow the opportunities for introducing such
 vulnerabilities by reducing parser complexity, by giving the entire
 range of encodable values a meaning where possible.
 Resource exhaustion attacks might attempt to lure a decoder into
 allocating very big data items (strings, arrays, maps) or exhaust the
 stack depth by setting up deeply nested items.  Decoders need to have
 appropriate resource management to mitigate these attacks.  (Items
 for which very large sizes are given can also attempt to exploit
 integer overflow vulnerabilities.)
 Applications where a CBOR data item is examined by a gatekeeper
 function and later used by a different application may exhibit
 vulnerabilities when multiple interpretations of the data item are
 possible.  For example, an attacker could make use of duplicate keys
 in maps and precision issues in numbers to make the gatekeeper base
 its decisions on a different interpretation than the one that will be
 used by the second application.  Protocols that are used in a
 security context should be defined in such a way that these multiple
 interpretations are reliably reduced to a single one.  To facilitate
 this, encoder and decoder implementations used in such contexts
 should provide at least one strict mode of operation (Section 3.10).

9. Acknowledgements

 CBOR was inspired by MessagePack.  MessagePack was developed and
 promoted by Sadayuki Furuhashi ("frsyuki").  This reference to
 MessagePack is solely for attribution; CBOR is not intended as a
 version of or replacement for MessagePack, as it has different design
 goals and requirements.
 The need for functionality beyond the original MessagePack
 Specification became obvious to many people at about the same time
 around the year 2012.  BinaryPack is a minor derivation of
 MessagePack that was developed by Eric Zhang for the binaryjs
 project.  A similar, but different, extension was made by Tim Caswell

Bormann & Hoffman Standards Track [Page 38] RFC 7049 CBOR October 2013

 for his msgpack-js and msgpack-js-browser projects.  Many people have
 contributed to the recent discussion about extending MessagePack to
 separate text string representation from byte string representation.
 The encoding of the additional information in CBOR was inspired by
 the encoding of length information designed by Klaus Hartke for CoAP.
 This document also incorporates suggestions made by many people,
 notably Dan Frost, James Manger, Joe Hildebrand, Keith Moore, Matthew
 Lepinski, Nico Williams, Phillip Hallam-Baker, Ray Polk, Tim Bray,
 Tony Finch, Tony Hansen, and Yaron Sheffer.

10. References

10.1. Normative References

 [ECMA262]  European Computer Manufacturers Association, "ECMAScript
            Language Specification 5.1 Edition", ECMA Standard
            ECMA-262, June 2011, <http://www.ecma-international.org/
            publications/files/ecma-st/ECMA-262.pdf>.
 [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
            Internet: Timestamps", RFC 3339, July 2002.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.
 [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
            Syndication Format", RFC 4287, December 2005.
 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, October 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.

Bormann & Hoffman Standards Track [Page 39] RFC 7049 CBOR October 2013

 [TIME_T]   The Open Group Base Specifications, "Vol. 1: Base
            Definitions, Issue 7", Section 4.15 'Seconds Since the
            Epoch', IEEE Std 1003.1, 2013 Edition, 2013,
            <http://pubs.opengroup.org/onlinepubs/9699919799/
            basedefs/V1_chap04.html#tag_04_15>.

10.2. Informative References

 [ASN.1]    International Telecommunication Union, "Information
            Technology -- ASN.1 encoding rules: Specification of Basic
            Encoding Rules (BER), Canonical Encoding Rules (CER) and
            Distinguished Encoding Rules (DER)", ITU-T Recommendation
            X.690, 1994.
 [BSON]     Various, "BSON - Binary JSON", 2013,
            <http://bsonspec.org/>.
 [CNN-TERMS]
            Bormann, C., Ersue, M., and A. Keranen, "Terminology for
            Constrained Node Networks", Work in Progress, July 2013.
 [MessagePack]
            Furuhashi, S., "MessagePack", 2013, <http://msgpack.org/>.
 [RFC0713]  Haverty, J., "MSDTP-Message Services Data Transmission
            Protocol", RFC 713, April 1976.
 [RFC4627]  Crockford, D., "The application/json Media Type for
            JavaScript Object Notation (JSON)", RFC 4627, July 2006.
 [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13, RFC
            6838, January 2013.
 [UBJSON]   The Buzz Media, "Universal Binary JSON Specification",
            2013, <http://ubjson.org/>.
 [YAML]     Ben-Kiki, O., Evans, C., and I. Net, "YAML Ain't Markup
            Language (YAML[TM]) Version 1.2", 3rd Edition, October
            2009, <http://www.yaml.org/spec/1.2/spec.html>.

Bormann & Hoffman Standards Track [Page 40] RFC 7049 CBOR October 2013

Appendix A. Examples

 The following table provides some CBOR-encoded values in hexadecimal
 (right column), together with diagnostic notation for these values
 (left column).  Note that the string "\u00fc" is one form of
 diagnostic notation for a UTF-8 string containing the single Unicode
 character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (u umlaut).
 Similarly, "\u6c34" is a UTF-8 string in diagnostic notation with a
 single character U+6C34 (CJK UNIFIED IDEOGRAPH-6C34, often
 representing "water"), and "\ud800\udd51" is a UTF-8 string in
 diagnostic notation with a single character U+10151 (GREEK ACROPHONIC
 ATTIC FIFTY STATERS).  (Note that all these single-character strings
 could also be represented in native UTF-8 in diagnostic notation,
 just not in an ASCII-only specification like the present one.)  In
 the diagnostic notation provided for bignums, their intended numeric
 value is shown as a decimal number (such as 18446744073709551616)
 instead of showing a tagged byte string (such as
 2(h'010000000000000000')).
 +------------------------------+------------------------------------+
 | Diagnostic                   | Encoded                            |
 +------------------------------+------------------------------------+
 | 0                            | 0x00                               |
 |                              |                                    |
 | 1                            | 0x01                               |
 |                              |                                    |
 | 10                           | 0x0a                               |
 |                              |                                    |
 | 23                           | 0x17                               |
 |                              |                                    |
 | 24                           | 0x1818                             |
 |                              |                                    |
 | 25                           | 0x1819                             |
 |                              |                                    |
 | 100                          | 0x1864                             |
 |                              |                                    |
 | 1000                         | 0x1903e8                           |
 |                              |                                    |
 | 1000000                      | 0x1a000f4240                       |
 |                              |                                    |
 | 1000000000000                | 0x1b000000e8d4a51000               |
 |                              |                                    |
 | 18446744073709551615         | 0x1bffffffffffffffff               |
 |                              |                                    |
 | 18446744073709551616         | 0xc249010000000000000000           |
 |                              |                                    |
 | -18446744073709551616        | 0x3bffffffffffffffff               |
 |                              |                                    |

Bormann & Hoffman Standards Track [Page 41] RFC 7049 CBOR October 2013

 | -18446744073709551617        | 0xc349010000000000000000           |
 |                              |                                    |
 | -1                           | 0x20                               |
 |                              |                                    |
 | -10                          | 0x29                               |
 |                              |                                    |
 | -100                         | 0x3863                             |
 |                              |                                    |
 | -1000                        | 0x3903e7                           |
 |                              |                                    |
 | 0.0                          | 0xf90000                           |
 |                              |                                    |
 | -0.0                         | 0xf98000                           |
 |                              |                                    |
 | 1.0                          | 0xf93c00                           |
 |                              |                                    |
 | 1.1                          | 0xfb3ff199999999999a               |
 |                              |                                    |
 | 1.5                          | 0xf93e00                           |
 |                              |                                    |
 | 65504.0                      | 0xf97bff                           |
 |                              |                                    |
 | 100000.0                     | 0xfa47c35000                       |
 |                              |                                    |
 | 3.4028234663852886e+38       | 0xfa7f7fffff                       |
 |                              |                                    |
 | 1.0e+300                     | 0xfb7e37e43c8800759c               |
 |                              |                                    |
 | 5.960464477539063e-8         | 0xf90001                           |
 |                              |                                    |
 | 0.00006103515625             | 0xf90400                           |
 |                              |                                    |
 | -4.0                         | 0xf9c400                           |
 |                              |                                    |
 | -4.1                         | 0xfbc010666666666666               |
 |                              |                                    |
 | Infinity                     | 0xf97c00                           |
 |                              |                                    |
 | NaN                          | 0xf97e00                           |
 |                              |                                    |
 | -Infinity                    | 0xf9fc00                           |
 |                              |                                    |
 | Infinity                     | 0xfa7f800000                       |
 |                              |                                    |
 | NaN                          | 0xfa7fc00000                       |
 |                              |                                    |
 | -Infinity                    | 0xfaff800000                       |
 |                              |                                    |

Bormann & Hoffman Standards Track [Page 42] RFC 7049 CBOR October 2013

 | Infinity                     | 0xfb7ff0000000000000               |
 |                              |                                    |
 | NaN                          | 0xfb7ff8000000000000               |
 |                              |                                    |
 | -Infinity                    | 0xfbfff0000000000000               |
 |                              |                                    |
 | false                        | 0xf4                               |
 |                              |                                    |
 | true                         | 0xf5                               |
 |                              |                                    |
 | null                         | 0xf6                               |
 |                              |                                    |
 | undefined                    | 0xf7                               |
 |                              |                                    |
 | simple(16)                   | 0xf0                               |
 |                              |                                    |
 | simple(24)                   | 0xf818                             |
 |                              |                                    |
 | simple(255)                  | 0xf8ff                             |
 |                              |                                    |
 | 0("2013-03-21T20:04:00Z")    | 0xc074323031332d30332d32315432303a |
 |                              | 30343a30305a                       |
 |                              |                                    |
 | 1(1363896240)                | 0xc11a514b67b0                     |
 |                              |                                    |
 | 1(1363896240.5)              | 0xc1fb41d452d9ec200000             |
 |                              |                                    |
 | 23(h'01020304')              | 0xd74401020304                     |
 |                              |                                    |
 | 24(h'6449455446')            | 0xd818456449455446                 |
 |                              |                                    |
 | 32("http://www.example.com") | 0xd82076687474703a2f2f7777772e6578 |
 |                              | 616d706c652e636f6d                 |
 |                              |                                    |
 | h''                          | 0x40                               |
 |                              |                                    |
 | h'01020304'                  | 0x4401020304                       |
 |                              |                                    |
 | ""                           | 0x60                               |
 |                              |                                    |
 | "a"                          | 0x6161                             |
 |                              |                                    |
 | "IETF"                       | 0x6449455446                       |
 |                              |                                    |
 | "\"\\"                       | 0x62225c                           |
 |                              |                                    |
 | "\u00fc"                     | 0x62c3bc                           |
 |                              |                                    |

Bormann & Hoffman Standards Track [Page 43] RFC 7049 CBOR October 2013

 | "\u6c34"                     | 0x63e6b0b4                         |
 |                              |                                    |
 | "\ud800\udd51"               | 0x64f0908591                       |
 |                              |                                    |
 | []                           | 0x80                               |
 |                              |                                    |
 | [1, 2, 3]                    | 0x83010203                         |
 |                              |                                    |
 | [1, [2, 3], [4, 5]]          | 0x8301820203820405                 |
 |                              |                                    |
 | [1, 2, 3, 4, 5, 6, 7, 8, 9,  | 0x98190102030405060708090a0b0c0d0e |
 | 10, 11, 12, 13, 14, 15, 16,  | 0f101112131415161718181819         |
 | 17, 18, 19, 20, 21, 22, 23,  |                                    |
 | 24, 25]                      |                                    |
 |                              |                                    |
 | {}                           | 0xa0                               |
 |                              |                                    |
 | {1: 2, 3: 4}                 | 0xa201020304                       |
 |                              |                                    |
 | {"a": 1, "b": [2, 3]}        | 0xa26161016162820203               |
 |                              |                                    |
 | ["a", {"b": "c"}]            | 0x826161a161626163                 |
 |                              |                                    |
 | {"a": "A", "b": "B", "c":    | 0xa5616161416162614261636143616461 |
 | "C", "d": "D", "e": "E"}     | 4461656145                         |
 |                              |                                    |
 | (_ h'0102', h'030405')       | 0x5f42010243030405ff               |
 |                              |                                    |
 | (_ "strea", "ming")          | 0x7f657374726561646d696e67ff       |
 |                              |                                    |
 | [_ ]                         | 0x9fff                             |
 |                              |                                    |
 | [_ 1, [2, 3], [_ 4, 5]]      | 0x9f018202039f0405ffff             |
 |                              |                                    |
 | [_ 1, [2, 3], [4, 5]]        | 0x9f01820203820405ff               |
 |                              |                                    |
 | [1, [2, 3], [_ 4, 5]]        | 0x83018202039f0405ff               |
 |                              |                                    |
 | [1, [_ 2, 3], [4, 5]]        | 0x83019f0203ff820405               |
 |                              |                                    |
 | [_ 1, 2, 3, 4, 5, 6, 7, 8,   | 0x9f0102030405060708090a0b0c0d0e0f |
 | 9, 10, 11, 12, 13, 14, 15,   | 101112131415161718181819ff         |
 | 16, 17, 18, 19, 20, 21, 22,  |                                    |
 | 23, 24, 25]                  |                                    |
 |                              |                                    |
 | {_ "a": 1, "b": [_ 2, 3]}    | 0xbf61610161629f0203ffff           |
 |                              |                                    |

Bormann & Hoffman Standards Track [Page 44] RFC 7049 CBOR October 2013

 | ["a", {_ "b": "c"}]          | 0x826161bf61626163ff               |
 |                              |                                    |
 | {_ "Fun": true, "Amt": -2}   | 0xbf6346756ef563416d7421ff         |
 +------------------------------+------------------------------------+
             Table 4: Examples of Encoded CBOR Data Items

Appendix B. Jump Table

 For brevity, this jump table does not show initial bytes that are
 reserved for future extension.  It also only shows a selection of the
 initial bytes that can be used for optional features.  (All unsigned
 integers are in network byte order.)
 +-----------------+-------------------------------------------------+
 | Byte            | Structure/Semantics                             |
 +-----------------+-------------------------------------------------+
 | 0x00..0x17      | Integer 0x00..0x17 (0..23)                      |
 |                 |                                                 |
 | 0x18            | Unsigned integer (one-byte uint8_t follows)     |
 |                 |                                                 |
 | 0x19            | Unsigned integer (two-byte uint16_t follows)    |
 |                 |                                                 |
 | 0x1a            | Unsigned integer (four-byte uint32_t follows)   |
 |                 |                                                 |
 | 0x1b            | Unsigned integer (eight-byte uint64_t follows)  |
 |                 |                                                 |
 | 0x20..0x37      | Negative integer -1-0x00..-1-0x17 (-1..-24)     |
 |                 |                                                 |
 | 0x38            | Negative integer -1-n (one-byte uint8_t for n   |
 |                 | follows)                                        |
 |                 |                                                 |
 | 0x39            | Negative integer -1-n (two-byte uint16_t for n  |
 |                 | follows)                                        |
 |                 |                                                 |
 | 0x3a            | Negative integer -1-n (four-byte uint32_t for n |
 |                 | follows)                                        |
 |                 |                                                 |
 | 0x3b            | Negative integer -1-n (eight-byte uint64_t for  |
 |                 | n follows)                                      |
 |                 |                                                 |
 | 0x40..0x57      | byte string (0x00..0x17 bytes follow)           |
 |                 |                                                 |
 | 0x58            | byte string (one-byte uint8_t for n, and then n |
 |                 | bytes follow)                                   |
 |                 |                                                 |
 | 0x59            | byte string (two-byte uint16_t for n, and then  |
 |                 | n bytes follow)                                 |

Bormann & Hoffman Standards Track [Page 45] RFC 7049 CBOR October 2013

 |                 |                                                 |
 | 0x5a            | byte string (four-byte uint32_t for n, and then |
 |                 | n bytes follow)                                 |
 |                 |                                                 |
 | 0x5b            | byte string (eight-byte uint64_t for n, and     |
 |                 | then n bytes follow)                            |
 |                 |                                                 |
 | 0x5f            | byte string, byte strings follow, terminated by |
 |                 | "break"                                         |
 |                 |                                                 |
 | 0x60..0x77      | UTF-8 string (0x00..0x17 bytes follow)          |
 |                 |                                                 |
 | 0x78            | UTF-8 string (one-byte uint8_t for n, and then  |
 |                 | n bytes follow)                                 |
 |                 |                                                 |
 | 0x79            | UTF-8 string (two-byte uint16_t for n, and then |
 |                 | n bytes follow)                                 |
 |                 |                                                 |
 | 0x7a            | UTF-8 string (four-byte uint32_t for n, and     |
 |                 | then n bytes follow)                            |
 |                 |                                                 |
 | 0x7b            | UTF-8 string (eight-byte uint64_t for n, and    |
 |                 | then n bytes follow)                            |
 |                 |                                                 |
 | 0x7f            | UTF-8 string, UTF-8 strings follow, terminated  |
 |                 | by "break"                                      |
 |                 |                                                 |
 | 0x80..0x97      | array (0x00..0x17 data items follow)            |
 |                 |                                                 |
 | 0x98            | array (one-byte uint8_t for n, and then n data  |
 |                 | items follow)                                   |
 |                 |                                                 |
 | 0x99            | array (two-byte uint16_t for n, and then n data |
 |                 | items follow)                                   |
 |                 |                                                 |
 | 0x9a            | array (four-byte uint32_t for n, and then n     |
 |                 | data items follow)                              |
 |                 |                                                 |
 | 0x9b            | array (eight-byte uint64_t for n, and then n    |
 |                 | data items follow)                              |
 |                 |                                                 |
 | 0x9f            | array, data items follow, terminated by "break" |
 |                 |                                                 |
 | 0xa0..0xb7      | map (0x00..0x17 pairs of data items follow)     |
 |                 |                                                 |
 | 0xb8            | map (one-byte uint8_t for n, and then n pairs   |
 |                 | of data items follow)                           |
 |                 |                                                 |

Bormann & Hoffman Standards Track [Page 46] RFC 7049 CBOR October 2013

 | 0xb9            | map (two-byte uint16_t for n, and then n pairs  |
 |                 | of data items follow)                           |
 |                 |                                                 |
 | 0xba            | map (four-byte uint32_t for n, and then n pairs |
 |                 | of data items follow)                           |
 |                 |                                                 |
 | 0xbb            | map (eight-byte uint64_t for n, and then n      |
 |                 | pairs of data items follow)                     |
 |                 |                                                 |
 | 0xbf            | map, pairs of data items follow, terminated by  |
 |                 | "break"                                         |
 |                 |                                                 |
 | 0xc0            | Text-based date/time (data item follows; see    |
 |                 | Section 2.4.1)                                  |
 |                 |                                                 |
 | 0xc1            | Epoch-based date/time (data item follows; see   |
 |                 | Section 2.4.1)                                  |
 |                 |                                                 |
 | 0xc2            | Positive bignum (data item "byte string"        |
 |                 | follows)                                        |
 |                 |                                                 |
 | 0xc3            | Negative bignum (data item "byte string"        |
 |                 | follows)                                        |
 |                 |                                                 |
 | 0xc4            | Decimal Fraction (data item "array" follows;    |
 |                 | see Section 2.4.3)                              |
 |                 |                                                 |
 | 0xc5            | Bigfloat (data item "array" follows; see        |
 |                 | Section 2.4.3)                                  |
 |                 |                                                 |
 | 0xc6..0xd4      | (tagged item)                                   |
 |                 |                                                 |
 | 0xd5..0xd7      | Expected Conversion (data item follows; see     |
 |                 | Section 2.4.4.2)                                |
 |                 |                                                 |
 | 0xd8..0xdb      | (more tagged items, 1/2/4/8 bytes and then a    |
 |                 | data item follow)                               |
 |                 |                                                 |
 | 0xe0..0xf3      | (simple value)                                  |
 |                 |                                                 |
 | 0xf4            | False                                           |
 |                 |                                                 |
 | 0xf5            | True                                            |
 |                 |                                                 |
 | 0xf6            | Null                                            |
 |                 |                                                 |
 | 0xf7            | Undefined                                       |
 |                 |                                                 |

Bormann & Hoffman Standards Track [Page 47] RFC 7049 CBOR October 2013

 | 0xf8            | (simple value, one byte follows)                |
 |                 |                                                 |
 | 0xf9            | Half-Precision Float (two-byte IEEE 754)        |
 |                 |                                                 |
 | 0xfa            | Single-Precision Float (four-byte IEEE 754)     |
 |                 |                                                 |
 | 0xfb            | Double-Precision Float (eight-byte IEEE 754)    |
 |                 |                                                 |
 | 0xff            | "break" stop code                               |
 +-----------------+-------------------------------------------------+
                 Table 5: Jump Table for Initial Byte

Appendix C. Pseudocode

 The well-formedness of a CBOR item can be checked by the pseudocode
 in Figure 1.  The data is well-formed if and only if:
 o  the pseudocode does not "fail";
 o  after execution of the pseudocode, no bytes are left in the input
    (except in streaming applications)
 The pseudocode has the following prerequisites:
 o  take(n) reads n bytes from the input data and returns them as a
    byte string.  If n bytes are no longer available, take(n) fails.
 o  uint() converts a byte string into an unsigned integer by
    interpreting the byte string in network byte order.
 o  Arithmetic works as in C.
 o  All variables are unsigned integers of sufficient range.

Bormann & Hoffman Standards Track [Page 48] RFC 7049 CBOR October 2013

 well_formed (breakable = false) {
   // process initial bytes
   ib = uint(take(1));
   mt = ib >> 5;
   val = ai = ib & 0x1f;
   switch (ai) {
     case 24: val = uint(take(1)); break;
     case 25: val = uint(take(2)); break;
     case 26: val = uint(take(4)); break;
     case 27: val = uint(take(8)); break;
     case 28: case 29: case 30: fail();
     case 31:
       return well_formed_indefinite(mt, breakable);
   }
   // process content
   switch (mt) {
     // case 0, 1, 7 do not have content; just use val
     case 2: case 3: take(val); break; // bytes/UTF-8
     case 4: for (i = 0; i < val; i++) well_formed(); break;
     case 5: for (i = 0; i < val*2; i++) well_formed(); break;
     case 6: well_formed(); break;     // 1 embedded data item
   }
   return mt;                    // finite data item
 }
 well_formed_indefinite(mt, breakable) {
   switch (mt) {
     case 2: case 3:
       while ((it = well_formed(true)) != -1)
         if (it != mt)           // need finite embedded
           fail();               //    of same type
       break;
     case 4: while (well_formed(true) != -1); break;
     case 5: while (well_formed(true) != -1) well_formed(); break;
     case 7:
       if (breakable)
         return -1;              // signal break out
       else fail();              // no enclosing indefinite
     default: fail();            // wrong mt
   }
   return 0;                     // no break out
 }
            Figure 1: Pseudocode for Well-Formedness Check
 Note that the remaining complexity of a complete CBOR decoder is
 about presenting data that has been parsed to the application in an
 appropriate form.

Bormann & Hoffman Standards Track [Page 49] RFC 7049 CBOR October 2013

 Major types 0 and 1 are designed in such a way that they can be
 encoded in C from a signed integer without actually doing an if-then-
 else for positive/negative (Figure 2).  This uses the fact that
 (-1-n), the transformation for major type 1, is the same as ~n
 (bitwise complement) in C unsigned arithmetic; ~n can then be
 expressed as (-1)^n for the negative case, while 0^n leaves n
 unchanged for non-negative.  The sign of a number can be converted to
 -1 for negative and 0 for non-negative (0 or positive) by arithmetic-
 shifting the number by one bit less than the bit length of the number
 (for example, by 63 for 64-bit numbers).
 void encode_sint(int64_t n) {
   uint64t ui = n >> 63;    // extend sign to whole length
   mt = ui & 0x20;          // extract major type
   ui ^= n;                 // complement negatives
   if (ui < 24)
     *p++ = mt + ui;
   else if (ui < 256) {
     *p++ = mt + 24;
     *p++ = ui;
   } else
        ...
          Figure 2: Pseudocode for Encoding a Signed Integer

Appendix D. Half-Precision

 As half-precision floating-point numbers were only added to IEEE 754
 in 2008, today's programming platforms often still only have limited
 support for them.  It is very easy to include at least decoding
 support for them even without such support.  An example of a small
 decoder for half-precision floating-point numbers in the C language
 is shown in Figure 3.  A similar program for Python is in Figure 4;
 this code assumes that the 2-byte value has already been decoded as
 an (unsigned short) integer in network byte order (as would be done
 by the pseudocode in Appendix C).

Bormann & Hoffman Standards Track [Page 50] RFC 7049 CBOR October 2013

 #include <math.h>
 double decode_half(unsigned char *halfp) {
   int half = (halfp[0] << 8) + halfp[1];
   int exp = (half >> 10) & 0x1f;
   int mant = half & 0x3ff;
   double val;
   if (exp == 0) val = ldexp(mant, -24);
   else if (exp != 31) val = ldexp(mant + 1024, exp - 25);
   else val = mant == 0 ? INFINITY : NAN;
   return half & 0x8000 ? -val : val;
 }
             Figure 3: C Code for a Half-Precision Decoder
 import struct
 from math import ldexp
 def decode_single(single):
     return struct.unpack("!f", struct.pack("!I", single))[0]
 def decode_half(half):
     valu = (half & 0x7fff) << 13 | (half & 0x8000) << 16
     if ((half & 0x7c00) != 0x7c00):
         return ldexp(decode_single(valu), 112)
     return decode_single(valu | 0x7f800000)
          Figure 4: Python Code for a Half-Precision Decoder

Appendix E. Comparison of Other Binary Formats to CBOR's Design

           Objectives
 The proposal for CBOR follows a history of binary formats that is as
 long as the history of computers themselves.  Different formats have
 had different objectives.  In most cases, the objectives of the
 format were never stated, although they can sometimes be implied by
 the context where the format was first used.  Some formats were meant
 to be universally usable, although history has proven that no binary
 format meets the needs of all protocols and applications.
 CBOR differs from many of these formats due to it starting with a set
 of objectives and attempting to meet just those.  This section
 compares a few of the dozens of formats with CBOR's objectives in
 order to help the reader decide if they want to use CBOR or a
 different format for a particular protocol or application.

Bormann & Hoffman Standards Track [Page 51] RFC 7049 CBOR October 2013

 Note that the discussion here is not meant to be a criticism of any
 format: to the best of our knowledge, no format before CBOR was meant
 to cover CBOR's objectives in the priority we have assigned them.  A
 brief recap of the objectives from Section 1.1 is:
 1.  unambiguous encoding of most common data formats from Internet
     standards
 2.  code compactness for encoder or decoder
 3.  no schema description needed
 4.  reasonably compact serialization
 5.  applicability to constrained and unconstrained applications
 6.  good JSON conversion
 7.  extensibility

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

Bormann & Hoffman Standards Track [Page 52] RFC 7049 CBOR October 2013

 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,
 foregoing 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. UBJSON

 [UBJSON] has a design goal to make JSON faster and somewhat smaller,
 using a binary format that is limited to exactly the data model JSON
 uses.  Thus, there is expressly no intention to support, for example,
 binary data; however, there is a "high-precision number", expressed
 as a character string in JSON syntax.  UBJSON is not optimized for
 code compactness, and its type byte coding is optimized for human
 recognition and not for compact representation of native types such
 as small integers.  Although UBJSON is mostly counted, it provides a
 reserved "unknown-length" value to support streaming of arrays and
 maps (JSON objects).  Within these containers, UBJSON also has a
 "Noop" type for padding.

E.5. 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.6. 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 6 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.

Bormann & Hoffman Standards Track [Page 53] RFC 7049 CBOR October 2013

 +---------------+-------------------------+-------------------------+
 | Format        | [1, [2, 3]]             | [_ 1, [2, 3]]           |
 +---------------+-------------------------+-------------------------+
 | RFC 713       | c2 05 81 c2 02 82 83    |                         |
 |               |                         |                         |
 | ASN.1 BER     | 30 0b 02 01 01 30 06 02 | 30 80 02 01 01 30 06 02 |
 |               | 01 02 02 01 03          | 01 02 02 01 03 00 00    |
 |               |                         |                         |
 | MessagePack   | 92 01 92 02 03          |                         |
 |               |                         |                         |
 | BSON          | 22 00 00 00 10 30 00 01 |                         |
 |               | 00 00 00 04 31 00 13 00 |                         |
 |               | 00 00 10 30 00 02 00 00 |                         |
 |               | 00 10 31 00 03 00 00 00 |                         |
 |               | 00 00                   |                         |
 |               |                         |                         |
 | UBJSON        | 61 02 42 01 61 02 42 02 | 61 ff 42 01 61 02 42 02 |
 |               | 42 03                   | 42 03 45                |
 |               |                         |                         |
 | CBOR          | 82 01 82 02 03          | 9f 01 82 02 03 ff       |
 +---------------+-------------------------+-------------------------+
         Table 6: Examples for Different Levels of Conciseness

Authors' Addresses

 Carsten Bormann
 Universitaet Bremen TZI
 Postfach 330440
 D-28359 Bremen
 Germany
 Phone: +49-421-218-63921
 EMail: cabo@tzi.org
 Paul Hoffman
 VPN Consortium
 EMail: paul.hoffman@vpnc.org

Bormann & Hoffman Standards Track [Page 54]

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