not yet C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track P. Hoffman
Expires: December 18, 2013 VPN Consortium
June 16, 2013
Concise Binary Object Representation (CBOR)
draft-bormann-cbor-02
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 18, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Specification of the CBOR Encoding . . . . . . . . . . . . . 6
2.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Indefinite Lengths for Some Major Types . . . . . . . . . 8
2.3. Floating Point Numbers and Values with No Content . . . . 11
2.4. Optional Tagging of Items . . . . . . . . . . . . . . . . 12
2.4.1. Date and Time . . . . . . . . . . . . . . . . . . . . 14
2.4.2. Bignums . . . . . . . . . . . . . . . . . . . . . . . 14
2.4.3. Decimal Fractions and Bigfloats . . . . . . . . . . . 14
2.4.4. Content Hints . . . . . . . . . . . . . . . . . . . . 16
2.4.4.1. Encoded CBOR data item . . . . . . . . . . . . . 16
2.4.4.2. Expected Later Encoding for CBOR to JSON
Converters . . . . . . . . . . . . . . . . . . . 16
2.4.4.3. Encoded Text . . . . . . . . . . . . . . . . . . 17
3. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 17
3.1. CBOR in Streaming Applications . . . . . . . . . . . . . 18
3.2. Parsing Errors . . . . . . . . . . . . . . . . . . . . . 18
3.2.1. Enforcing Restrictions on the Value Following a Tag . 18
3.2.2. Handling Unknown Simple Values and Tags . . . . . . . 18
3.2.3. UTF-8 Strings . . . . . . . . . . . . . . . . . . . . 18
3.2.4. Incomplete CBOR data items . . . . . . . . . . . . . 19
3.2.5. Unknown Additional Information Values . . . . . . . . 19
3.3. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.4. Specifying Keys for Maps . . . . . . . . . . . . . . . . 20
3.5. Undefined Values . . . . . . . . . . . . . . . . . . . . 20
3.6. Canonical CBOR . . . . . . . . . . . . . . . . . . . . . 21
3.7. Generic Encoders and Parsers . . . . . . . . . . . . . . 22
4. Converting Data Between CBOR and JSON . . . . . . . . . . . . 22
4.1. Converting From CBOR to JSON . . . . . . . . . . . . . . 23
4.2. Converting From JSON to CBOR . . . . . . . . . . . . . . 24
5. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 24
5.1. Extension Points . . . . . . . . . . . . . . . . . . . . 25
5.2. Curating the Additional Information Space . . . . . . . . 26
6. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 26
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6.1. Encoding indicators . . . . . . . . . . . . . . . . . . . 27
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
7.1. Simple Values Registry . . . . . . . . . . . . . . . . . 28
7.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 28
7.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 28
8. Security Considerations . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 31
Appendix B. Jump Table . . . . . . . . . . . . . . . . . . . . . 36
Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 39
Appendix D. Half-precision . . . . . . . . . . . . . . . . . . . 41
Appendix E. Comparison of Other Binary Formats to CBOR's Design
Objectives . . . . . . . . . . . . . . . . . . . . . 42
E.1. ASN.1 DER and BER . . . . . . . . . . . . . . . . . . . . 42
E.2. MessagePack . . . . . . . . . . . . . . . . . . . . . . . 42
E.3. BSON . . . . . . . . . . . . . . . . . . . . . . . . . . 43
E.4. UBJSON . . . . . . . . . . . . . . . . . . . . . . . . . 43
E.5. MSDTP: RFC 713 . . . . . . . . . . . . . . . . . . . . . 44
E.6. Conciseness On The Wire . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
There are hundreds of standardized formats for binary representation
of structured data. 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 serialization is for 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 which starts
from JSON.
Appendix E lists some existing binary formats and discusses how well
they do or do not fit the design objectives of CBOR.
1.1. Objectives
The objectives of the Concise Binary Object Representation (CBOR),
roughly in decreasing order of importance, are:
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1. The representation must be able to unambiguously encode most
common data formats used in Internet standards.
* Representing 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 parser must be able to be compact in
order to support systems with very limited memory and processor
power and instruction sets.
* An encoder and a parser need to be implementable in a very
small amount of code, thus being applicable to class 1
constrained nodes as defined in [I-D.ietf-lwig-terminology].
* The format should use contemporary machine representations of
data (for example, not requiring binary-to-decimal
conversion).
3. Data must be able to be parsed without a schema description.
* Similar to JSON, encoded data should be self-describing so
that a generic parser can be written.
4. The serialization must be reasonably compact, but data
compactness is secondary to code compactness for the encoder and
parser.
* "Reasonable" here is bounded by JSON as an upper bound in
size, and by implementation complexity maintaining a lower
bound. Using either general compression schemes or extensive
bit-fiddling violates the complexity goals.
5. The format must be applicable to both constrained nodes and high-
volume applications.
* This means it must be reasonably frugal in CPU usage for both
encoding and parsing. This is relevant both for constrained
nodes and for potential usage in applications with a very high
volume of data.
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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 are 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, with the extended data being able
to be parsed by earlier parsers.
* The format is designed for decades of use.
* The format must support a form of extensibility that allows
fallback so that a parser that does not understand an
extension can still parse 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 parser.
Parser: A process that decodes a CBOR data item and makes it
available to an application. This is also sometimes called a
decoder.
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
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data items that make up a data stream are sometimes also referred
to as "top-level data items".
Stream parser: 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.
2. Specification of the CBOR Encoding
A CBOR encoded data item is structured and encoded as described in
this section. For the impatient reader, the encoding is summarized
in Table 4.
The initial byte of each data item contains both information about
the major type (the high-order 3 bits) 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 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 parser implementation can be based on a jump table with all
256 defined values for the initial byte (Table 4). A parser 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 like).
2.1. Major Types
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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
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: 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. 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
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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, the even-numbered ones serving
as keys and the following odd-numbered ones serving as values for
the key that comes immediately before it. 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.
Major type 6: optional semantic tagging of other major types. See
Section 2.4.
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 4).
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 (byte strings, text strings, arrays, and maps) can be
encoded with an indefinite length using additional information value
31 if the number of items inside the arrays or maps, or the total
length of the string is not known when the encoding of the item
starts; the application of this is often referred to as "streaming".
The indefinite length item is closed by encoding a "break" stop code.
"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
syntactical feature to close the indefinite length item.
For byte strings, indefinite length encoding means that the initial
indefinite length indicator is followed by zero or more byte strings,
then the "break". The parser combines the byte strings that followed
the item, in order, into a single byte string; this is sometimes
called "chunking". Every item between the byte string with the
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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 parsing, 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 items MUST be text strings.
Note that this implies that the bytes of a single UTF-8 character
cannot be spread between items: a new item can only be started at a
character boundary. Due to the properties of UTF-8, this is not a
very onerous requirement even on a streaming encoder.
Arrays and maps with indefinite lengths allow any number of items
(arrays) and key/value pairs (maps) to be given before the "break"
stop code. For example, assume an encoder wants to represent the
abstract array [1, [2, 3], [4, 5]]. The non-streaming 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
Streaming 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
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01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
9F -- Start indefinite length array
04 -- 4
05 -- 5
FF -- "break" (inner array)
FF -- "break" (outer array)
0x9f01820203820405ff
9F -- Start indefinite length array
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
82 -- Array of length 2
04 -- 4
05 -- 5
FF -- "break"
0x83018202039f0405ff
83 -- Array of length 3
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
9F -- Start indefinite length array
04 -- 4
05 -- 5
FF -- "break"
0x83019f0203ff820405
83 -- Array of length 3
01 -- 1
9F -- Start indefinite length array
02 -- 2
03 -- 3
FF -- "break"
82 -- Array of length 2
04 -- 4
05 -- 5
There is no restriction against nesting indefinite length array or
map items. A "break" only terminates a single item, so nested
streaming items need exactly as many "break" stop codes as there are
type bytes starting a streaming item.
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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.
+-------------+--------------------------------------------------+
| 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 | (unallocated) |
| | |
| 31 | "break" stop code for indefinite arrays and maps |
+-------------+--------------------------------------------------+
Table 1: Values for Additional Information in Major Type 7
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.)
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
allocated and available for simple types.
+---------+-----------------+
| value | semantics |
+---------+-----------------+
| 0..19 | (unallocated) |
| | |
| 20 | False |
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| | |
| 21 | True |
| | |
| 22 | Null |
| | |
| 23 | Undefined value |
| | |
| 24..31 | (reserved) |
| | |
| 32..255 | (unallocated) |
+---------+-----------------+
Table 2: Simple Values
2.4. Optional Tagging of Items
In CBOR, a data item can optionally be preceded by (enclosed 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. 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.
CBOR tags are truly optional, and are probably of little value in
applications where the implementation creating a particular CBOR data
stream and the implementation parsing that stream know the semantic
meaning of each item in the stream. Their primary purpose in this
specification is to define common data types such as dates. A
secondary purpose it to allow optional tagging when the parser is a
generic CBOR parser that might be able to benefit from hints about
the content of items. Understanding the semantic tags is optional
for a parser; 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 enclosed 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.
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Applications may use specific tags defined in the following list and/
or defined by standard action or in the registry.
+-----------+-------------------+-----------------------------------+
| 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 | (unallocated) | (unallocated) |
| | | |
| 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 | (unallocated) | (unallocated) |
| | | |
| 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 |
| | | |
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| 36 | UTF-8 string | MIME message; see Section 2.4.4.3 |
| | | |
| 37+ | (unallocated) | (unallocated) |
+-----------+-------------------+-----------------------------------+
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. 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. Parsers 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
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.
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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:
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
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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 parsed 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 following this tag can be a byte string, an array, or a
map. In the latter two cases, the tag applies to all of the byte
strings in the data object.
These three tag types suggest conversions to three of the base data
encodings defined in [RFC4648]. Where the encoding allows the use of
padding ("="), this is not used. Later tags might be defined for
other data encodings of RFC 4648, or of other ways to encode binary
data in strings.
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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 parser. 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 PCRE/JavaScript syntax
[ECMA262].
o Tag 36 is for MIME messages, as defined in [RFC2045];
Note that tag 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 in the latter case.
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 parser can take a CBOR item
and parse it with no other knowledge.
Of course, in real-world implementations, the encoder and the parser
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, the second value is an integer,
followed by zero or more floating point numbers" or "a map whose keys
are byte strings that has to contain 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 parser 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 or
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 RFC 2119 sense.
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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 parser 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 parser; some parsers will buffer additional data
until a complete data item can be presented to the application.
Other parsers 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.
3.2. Parsing Errors
3.2.1. Enforcing Restrictions 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 followed by byte strings. A parser that
finds a data item of the wrong type after a tag might issue a
warning, might stop processing altogether, might handle the error and
make the incorrectly-typed value available to the application as
such, or take some other type of action.
3.2.2. Handling Unknown Simple Values and Tags
A parser 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 parser was deployed or a value that the parser 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, or take some other type of action.
A parser 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 parser was deployed or a tag that the parser 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, might ignore the tag
and simply present the contained data item only to the application,
or take some other type of action.
3.2.3. UTF-8 Strings
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A parser might or might not want to verify that the sequence of bytes
in an UTF-8 string (major type 3) is actually valid UTF-8. If a
parser attempts to validate the UTF-8 and fails, it might issue a
warning, might stop processing altogether, might handle the error and
present the invalid string to the application as such, or take some
other type of action.
3.2.4. 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 in the
input byte string, a parser may completely fail the decoding, or
substitute the missing data and data items using an decoder-specific
convention. A decoder may also implement incremental parsing, that
is, parse 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 are available.
For instance, if a parser is expecting a certain number of array or
map entries, but it instead encounters the end of the data, it should
probably issue an error and/or stop processing altogether, but it
might take some other action. The same is true if it is processing
what it expects to be the last pair in a map and it comes to the end
of the data. The same is also true for items that have indefinite
length and the end of data is reached before the "break" stop code is
reached.
Similarly, if a parser has just seen a tag and then encounters the
end of the data, it should probably issue an error and/or stop
processing altogether, but it might take some other action.
3.2.5. Unknown Additional Information Values
At the time this document is written, some additional information
values are undefined and reserved for future versions of this
document (see Section 5.2). A parser that sees an additional
information value that it does not understand should probably issue
an error and/or stop processing altogether, but it might take some
other action.
3.3. Numbers
For the purposes of this specification, all number representations
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,
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such as when the floating point value is more compact than a 64-bit
integer.
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 parse an integer of the
given size.
3.4. Specifying Keys for Maps
The encoding and parsing 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.
If multiple types of keys are to 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.
Applications for constrained devices that have maps with fewer than
24 known keys should consider using integers because the keys can
then be encoded in a single byte.
3.5. 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.
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3.6. Canonical CBOR
Some protocols may want encoders to only emit CBOR in a particular
canonical format; those protocols might also have the parsers check
that their input is canonical. Those protocols are free to define
what they mean by a canonical format and what encoders and parsers
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 data stream, the following three 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;
* 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.
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
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conversion to a 32-bit float; if the result is the same value, use
the shorter value and repeat the process with a test conversion to a
16-bit float. Also, there are many representations for NaN. If NaN
is an allowed value, it must always be represented as 0xf97e00.
CBOR tags make canonicalization more difficult. 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.
3.7. Generic Encoders and Parsers
A generic CBOR decoder can parse all well-formed CBOR data and
present them to an application. CBOR data are well-formed if the
structure of the initial bytes and the byte strings/data items
implied by their values is followed and no extraneous data follows
(Appendix C).
Even though CBOR attempts to minimize these cases, not all well-
formed CBOR data are 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 enclosing 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].
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.
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 consist of
bytes, not characters.
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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
suggestion 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.
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
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a JSON string. For tag value 3 (negative bignum), a "~" (ASCII
tilde) is inserted before the base-encoded value.
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.
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:
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
efficiently capable of 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 comes to mind is to
perform a JSON to CBOR encoding in place in a single buffer. This
strategy would need to consider the pathological case that some
strings represented with no or very few escapes and longer (or much
longer) than 255 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 in JSON.
5. Future Evolution of CBOR
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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.
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 code
point space that is not fully allocated from the outset, and the
protocol is designed to tolerate and embrace implementations that
start using more code points than initially allocated.
Sizing the code point 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. An IANA registry is appropriate
here.
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o the "tag" space (values in major type 6). Again, only a small
part of the code point 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 enclosed data item. An IANA registry is appropriate here.
o the "additional information" space. An implementation receiving
an unknown additional information has no way to continue parsing,
so allocating codepoints to this space is a major step. There are
also very few codepoints left.
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 code
point 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
code point 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.
The diagnostic notation is based on JSON as it is defined in RFC
4627. The notation borrows the JSON syntax for numbers (integer and
floating point), True, False, Null, UTF-8 strings, arrays and maps
(maps are called objects in JSON; the diagnostic notation extends
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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:
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 a indicator that a streaming 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
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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").
7. IANA Considerations
IANA will create two registries for new CBOR values. The registries
will follow the rules in [RFC5226]. IANA will also allocate a new
MIME media type.
7.1. Simple Values Registry
A registry called "CBOR Simple Values" will be created. The initial
values are shown in Table 2.
New entries in the range 0 to 19 will be allocated by Standards
Action, starting with the number 16. New entries in the range 32 to
255 will be allocated by Specification Required.
7.2. Tags Registry
A registry called "CBOR Tags" will be created. The initial values
are shown in Table 3.
New entries in the range 0 to 23 will be allocated by Standards
Action. New entries in the range 24 to 255 will be allocated by
Specification Required. New entries in the range 256 to
18446744073709551615 will be allocated by First Come First Served.
The template for First Come First Served will include point of
contact and an optional field for URL to a description of the
semantics of the tag; the latter can be something like an Internet-
Draft or a web page.
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
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Encoding considerations: none; CBOR is a binary format
Security considerations: Same as for the base 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 many 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:
Carsten Bormann
cabo@tzi.org
TBD: Maybe add application/mmmmm+cbor for specific protocols?
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.
9. Acknowledgements
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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
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 James Manger, Joe Hildebrand, Phillip Hallam-Baker, Tim Bray,
and Tony Finch.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[ASN.1] International Telecommunications 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", 2013, .
[ECMA262] European Computer Manufacturers Association, "ECMAScript
Language Specification 5.1 Edition", ECMA Standard
ECMA-262, June 2011, .
[I-D.ietf-lwig-terminology]
Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained Node Networks", draft-ietf-lwig-terminology-04
(work in progress), April 2013.
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[MessagePack]
FURUHASHI Sadayuki, "MessagePack", 2013,
.
[RFC0713] Haverty, J., "MSDTP-Message Services Data Transmission
Protocol", RFC 713, April 1976.
[RFC2045] Freed, N. and N.S. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[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.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[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.
[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, .
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).
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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.)
+----------------------+--------------------------------------------+
| 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 |
| | |
| -1844674407370955161 | 0x3bffffffffffffffff |
| 6 | |
| | |
| -1844674407370955161 | 0xc349010000000000000000 |
| 7 | |
| | |
| -1 | 0x20 |
| | |
| -10 | 0x29 |
| | |
| -100 | 0x3863 |
| | |
| -1000 | 0x3903e7 |
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| | |
| 0.0 | 0xf90000 |
| | |
| -0.0 | 0xf98000 |
| | |
| 1.0 | 0xf93c00 |
| | |
| 1.1 | 0xfb3ff199999999999a |
| | |
| 1.5 | 0xf93e00 |
| | |
| 65504.0 | 0xf97bff |
| | |
| 100000.0 | 0xfa47c35000 |
| | |
| 3.4028234663852886e+ | 0xfa7f7fffff |
| 38 | |
| | |
| 1.0e+300 | 0xfb7e37e43c8800759c |
| | |
| 5.960464477539063e-0 | 0xf90001 |
| 8 | |
| | |
| 6.103515625e-05 | 0xf90400 |
| | |
| -4.0 | 0xf9c400 |
| | |
| -4.1 | 0xfbc010666666666666 |
| | |
| Infinity | 0xf97c00 |
| | |
| NaN | 0xf97e00 |
| | |
| -Infinity | 0xf9fc00 |
| | |
| Infinity | 0xfa7f800000 |
| | |
| NaN | 0xfa7fc00000 |
| | |
| -Infinity | 0xfaff800000 |
| | |
| Infinity | 0xfb7ff0000000000000 |
| | |
| NaN | 0xfb7ff8000000000000 |
| | |
| -Infinity | 0xfbfff0000000000000 |
| | |
| false | 0xf4 |
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| | |
| true | 0xf5 |
| | |
| nil | 0xf6 |
| | |
| undefined | 0xf7 |
| | |
| simple(16) | 0xf0 |
| | |
| simple(24) | 0xf818 |
| | |
| simple(255) | 0xf8ff |
| | |
| 0("2013-03-21T20:04: | 0xc074323031332d30332d32315432303a30343a30 |
| 00Z") | 305a |
| | |
| 1(1363896240) | 0xc11a514b67b0 |
| | |
| 1(1363896240.5) | 0xc1fb41d452d9ec200000 |
| | |
| 23(h'01020304') | 0xd74401020304 |
| | |
| 24(h'6449455446') | 0xd818456449455446 |
| | |
| 32("http://www.examp | 0xd82076687474703a2f2f7777772e6578616d706c |
| le.com") | 652e636f6d |
| | |
| h'' | 0x40 |
| | |
| h'01020304' | 0x4401020304 |
| | |
| "" | 0x60 |
| | |
| "a" | 0x6161 |
| | |
| "IETF" | 0x6449455446 |
| | |
| "\"\\" | 0x62225c |
| | |
| "\u00fc" | 0x62c3bc |
| | |
| "\u6c34" | 0x63e6b0b4 |
| | |
| "\ud800\udd51" | 0x64f0908591 |
| | |
| [] | 0x80 |
| | |
| [1, 2, 3] | 0x83010203 |
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| | |
| [1, [2, 3], [4, 5]] | 0x8301820203820405 |
| | |
| [1, 2, 3, 4, 5, 6, | 0x98190102030405060708090a0b0c0d0e0f101112 |
| 7, 8, 9, 10, 11, 12, | 131415161718181819 |
| 13, 14, 15, 16, 17, | |
| 18, 19, 20, 21, 22, | |
| 23, 24, 25] | |
| | |
| {} | 0xa0 |
| | |
| {1: 2, 3: 4} | 0xa201020304 |
| | |
| {"a": 1, "b": [2, | 0xa26161016162820203 |
| 3]} | |
| | |
| ["a", {"b": "c"}] | 0x826161a161626163 |
| | |
| {"a": "A", "b": "B", | 0xa561616141616261426163614361646144616561 |
| "c": "C", "d": "D", | 45 |
| "e": "E"} | |
| | |
| (_ h'0102', | 0x5f42010243030405ff |
| h'030405') | |
| | |
| (_ "strea", "ming") | 0x7f657374726561646d696e67ff |
| | |
| [_ ] | 0x9fff |
| | |
| [_ 1, [2, 3], [_ 4, | 0x9f018202039f0405ffff |
| 5]] | |
| | |
| [_ 1, [2, 3], [4, | 0x9f01820203820405ff |
| 5]] | |
| | |
| [1, [2, 3], [_ 4, | 0x83018202039f0405ff |
| 5]] | |
| | |
| [1, [_ 2, 3], [4, | 0x83019f0203ff820405 |
| 5]] | |
| | |
| [_ 1, 2, 3, 4, 5, 6, | 0x9f0102030405060708090a0b0c0d0e0f10111213 |
| 7, 8, 9, 10, 11, 12, | 1415161718181819ff |
| 13, 14, 15, 16, 17, | |
| 18, 19, 20, 21, 22, | |
| 23, 24, 25] | |
| | |
| {_ "a": 1, "b": [_ | 0xbf61610161629f0203ffff |
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| 2, 3]} | |
| | |
| ["a", {_ "b": "c"}] | 0x826161bf61626163ff |
+----------------------+--------------------------------------------+
TBD: add more examples?
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.)
TBD: check again that we have all the single-byte tags represented in
the table
+-----------------+-------------------------------------------------+
| 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 |
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| | bytes follow) |
| | |
| 0x59 | byte string (two-byte uint16_t for n, and then |
| | n bytes follow) |
| | |
| 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) |
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| | |
| 0xb8 | map (one-byte uint8_t for n, and then n pairs |
| | of data items follow) |
| | |
| 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) |
| | |
| 0xd5..0xd7 | Expected Conversion (data item follows, see |
| | Section 2.4.4.2) |
| | |
| 0xd8 | (more tagged items, one byte and then a data |
| | item follow) |
| | |
| 0xf4 | False |
| | |
| 0xf5 | True |
| | |
| 0xf6 | Null |
| | |
| 0xf7 | Undefined |
| | |
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| 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 4: Jump Table for Initial Byte
Appendix C. Pseudocode
The well-formedness of a CBOR item can be checked by the pseudo-code
in Figure 1. The data is well-formed if and only if:
o the pseudo-code does not "fail";
o after execution of the pseudo-code, no bytes are left in the input
(except in streaming applications)
The pseudo-code 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.
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
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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: Pseudo-Code 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.
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
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ui ^= n; // complement negatives
if (ui < 24)
*p++ = mt + ui;
else if (ui < 256) {
*p++ = mt + 24;
*p++ = ui;
} else
...
Figure 2: Pseudo-code 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. This code assumes that the 2-byte value has
already been parsed as an unsigned integer in network byte order (as
would be done by the pseudocode in Appendix C). A similar program
for Python is in Figure 4.
#include
double decode_half(int half) {
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)
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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.
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. unambiguously encode common data formats from Internet standards
2. code compactness for encoder or parser
3. no schema description needed
4. reasonably compact serialization
5. applicable to constrained and unconstrained applications
6. good JSON conversion
7. extensibility
E.1. ASN.1 DER and BER
[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 parse numeric items can be complex
on a constrained device.
E.2. MessagePack
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[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 RPC applications
and for long-term storage of data.
MessagePack has been essentially stable since it was first published
around 2011; it has not yet had a transition. The evolution of
MessagePack is impeded by an imperative to maintain complete
backwards compatibility with existing stored data, while only few
bytecodes are still available for extension. Repeated requests over
the years from the MessagePack user community to separate out binary
and text strings in the encoding recently have led to an extension
proposal that would leave MessagePack's "raw" data ambiguous between
its usages for binary and text data. The extension mechanism for
MessagePack remains unclear.
E.3. BSON
[BSON] is a data format that was developed for the storage of JSON-
like maps (JSON objects) in the MongoDB database. Its major
distinguishing feature is the capability for in-place update,
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.
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E.5. MSDTP: RFC 713
A very early example of a compact message format is described in
[RFC0713], defined 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 higher than its objective of conciseness on the wire,
many people focus on the wire size. Table 5 shows some encoding
examples for the simple nested array [1, [2, 3]]; where streaming is
supported by the encoding, [_ 1, [2, 3]] (indefinite length on the
outer array) is also shown.
(Entries marked with an asterisk have not been checked against an
implementation and might be applying some liberty in translating the
CBOR data model to that format. Corrections are appreciated.)
+---------------+-------------------------+-------------------------+
| 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 5: Examples for different levels of conciseness
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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
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