--- 1/draft-ietf-cbor-7049bis-12.txt 2020-03-08 14:13:44.590967186 -0700 +++ 2/draft-ietf-cbor-7049bis-13.txt 2020-03-08 14:13:44.782972061 -0700 @@ -1,19 +1,19 @@ Network Working Group C. Bormann Internet-Draft Universitaet Bremen TZI Obsoletes: 7049 (if approved) P. Hoffman Intended status: Standards Track ICANN -Expires: 20 June 2020 18 December 2019 +Expires: 9 September 2020 8 March 2020 Concise Binary Object Representation (CBOR) - draft-ietf-cbor-7049bis-12 + draft-ietf-cbor-7049bis-13 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. This document is a revised edition of RFC 7049, with editorial @@ -27,128 +27,132 @@ This document is being worked on in the CBOR Working Group. Please contribute on the mailing list there, or in the GitHub repository for this draft: https://github.com/cbor-wg/CBORbis The charter for the CBOR Working Group says that the WG will update RFC 7049 to fix verified errata. Security issues and clarifications may be addressed, but changes to this document will ensure backward compatibility for popular deployed codebases. This document will be targeted at becoming an Internet Standard. + [RFC editor: please remove this note.] + Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on 20 June 2020. + This Internet-Draft will expire on 9 September 2020. Copyright Notice - Copyright (c) 2019 IETF Trust and the persons identified as the + Copyright (c) 2020 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 2. CBOR Data Models . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Extended Generic Data Models . . . . . . . . . . . . . . 8 2.2. Specific Data Models . . . . . . . . . . . . . . . . . . 9 - 3. Specification of the CBOR Encoding . . . . . . . . . . . . . 9 + 3. Specification of the CBOR Encoding . . . . . . . . . . . . . 10 3.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Indefinite Lengths for Some Major Types . . . . . . . . . 13 3.2.1. The "break" Stop Code . . . . . . . . . . . . . . . . 13 3.2.2. Indefinite-Length Arrays and Maps . . . . . . . . . . 14 3.2.3. Indefinite-Length Byte Strings and Text Strings . . . 16 - 3.3. Floating-Point Numbers and Values with No Content . . . . 16 - 3.4. Tagging of Items . . . . . . . . . . . . . . . . . . . . 18 - 3.4.1. Standard Date/Time String . . . . . . . . . . . . . . 20 - 3.4.2. Epoch-based Date/Time . . . . . . . . . . . . . . . . 20 - 3.4.3. Bignums . . . . . . . . . . . . . . . . . . . . . . . 21 - 3.4.4. Decimal Fractions and Bigfloats . . . . . . . . . . . 22 - 3.4.5. Content Hints . . . . . . . . . . . . . . . . . . . . 23 - 3.4.5.1. Encoded CBOR Data Item . . . . . . . . . . . . . 23 + 3.2.4. Summary of indefinite-length use of major types . . . 17 + 3.3. Floating-Point Numbers and Values with No Content . . . . 17 + 3.4. Tagging of Items . . . . . . . . . . . . . . . . . . . . 19 + 3.4.1. Standard Date/Time String . . . . . . . . . . . . . . 22 + 3.4.2. Epoch-based Date/Time . . . . . . . . . . . . . . . . 22 + 3.4.3. Bignums . . . . . . . . . . . . . . . . . . . . . . . 23 + 3.4.4. Decimal Fractions and Bigfloats . . . . . . . . . . . 24 + 3.4.5. Content Hints . . . . . . . . . . . . . . . . . . . . 25 + 3.4.5.1. Encoded CBOR Data Item . . . . . . . . . . . . . 25 3.4.5.2. Expected Later Encoding for CBOR-to-JSON - Converters . . . . . . . . . . . . . . . . . . . . 24 - 3.4.5.3. Encoded Text . . . . . . . . . . . . . . . . . . 24 - 3.4.6. Self-Described CBOR . . . . . . . . . . . . . . . . . 25 - 4. Serialization Considerations . . . . . . . . . . . . . . . . 26 - 4.1. Preferred Serialization . . . . . . . . . . . . . . . . . 26 - 4.2. Deterministically Encoded CBOR . . . . . . . . . . . . . 27 - 4.2.1. Core Deterministic Encoding Requirements . . . . . . 27 - 4.2.2. Additional Deterministic Encoding Considerations . . 28 - 4.2.3. Length-first map key ordering . . . . . . . . . . . . 30 - 5. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 31 - 5.1. CBOR in Streaming Applications . . . . . . . . . . . . . 31 - 5.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 32 - 5.3. Validity of Items . . . . . . . . . . . . . . . . . . . . 32 - 5.3.1. Basic validity . . . . . . . . . . . . . . . . . . . 33 - 5.3.2. Tag validity . . . . . . . . . . . . . . . . . . . . 33 - 5.4. Validity and Evolution . . . . . . . . . . . . . . . . . 34 - 5.5. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 35 - 5.6. Specifying Keys for Maps . . . . . . . . . . . . . . . . 35 - 5.6.1. Equivalence of Keys . . . . . . . . . . . . . . . . . 36 - 5.7. Undefined Values . . . . . . . . . . . . . . . . . . . . 37 - 6. Converting Data between CBOR and JSON . . . . . . . . . . . . 38 - 6.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 38 - 6.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 39 - 7. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 40 - 7.1. Extension Points . . . . . . . . . . . . . . . . . . . . 41 - 7.2. Curating the Additional Information Space . . . . . . . . 41 - 8. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 42 - 8.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 43 - 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44 - 9.1. Simple Values Registry . . . . . . . . . . . . . . . . . 44 - 9.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 44 - 9.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 45 - 9.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 45 - 9.5. The +cbor Structured Syntax Suffix Registration . . . . . 46 - 10. Security Considerations . . . . . . . . . . . . . . . . . . . 47 - 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 - 11.1. Normative References . . . . . . . . . . . . . . . . . . 48 - 11.2. Informative References . . . . . . . . . . . . . . . . . 50 - Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 51 - Appendix B. Jump Table . . . . . . . . . . . . . . . . . . . . . 55 - Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 58 - Appendix D. Half-Precision . . . . . . . . . . . . . . . . . . . 61 + Converters . . . . . . . . . . . . . . . . . . . . 25 + 3.4.5.3. Encoded Text . . . . . . . . . . . . . . . . . . 26 + 3.4.6. Self-Described CBOR . . . . . . . . . . . . . . . . . 27 + + 4. Serialization Considerations . . . . . . . . . . . . . . . . 28 + 4.1. Preferred Serialization . . . . . . . . . . . . . . . . . 28 + 4.2. Deterministically Encoded CBOR . . . . . . . . . . . . . 29 + 4.2.1. Core Deterministic Encoding Requirements . . . . . . 29 + 4.2.2. Additional Deterministic Encoding Considerations . . 30 + 4.2.3. Length-first Map Key Ordering . . . . . . . . . . . . 32 + 5. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 33 + 5.1. CBOR in Streaming Applications . . . . . . . . . . . . . 33 + 5.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 34 + 5.3. Validity of Items . . . . . . . . . . . . . . . . . . . . 35 + 5.3.1. Basic validity . . . . . . . . . . . . . . . . . . . 35 + 5.3.2. Tag validity . . . . . . . . . . . . . . . . . . . . 35 + 5.4. Validity and Evolution . . . . . . . . . . . . . . . . . 36 + 5.5. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 37 + 5.6. Specifying Keys for Maps . . . . . . . . . . . . . . . . 38 + 5.6.1. Equivalence of Keys . . . . . . . . . . . . . . . . . 39 + 5.7. Undefined Values . . . . . . . . . . . . . . . . . . . . 40 + 6. Converting Data between CBOR and JSON . . . . . . . . . . . . 40 + 6.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 41 + 6.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 42 + 7. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 43 + 7.1. Extension Points . . . . . . . . . . . . . . . . . . . . 43 + 7.2. Curating the Additional Information Space . . . . . . . . 44 + 8. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 45 + 8.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 46 + 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 + 9.1. Simple Values Registry . . . . . . . . . . . . . . . . . 47 + 9.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 47 + 9.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 47 + 9.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 48 + 9.5. The +cbor Structured Syntax Suffix Registration . . . . . 49 + 10. Security Considerations . . . . . . . . . . . . . . . . . . . 50 + 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 52 + 11.1. Normative References . . . . . . . . . . . . . . . . . . 52 + 11.2. Informative References . . . . . . . . . . . . . . . . . 53 + Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 55 + Appendix B. Jump Table . . . . . . . . . . . . . . . . . . . . . 59 + Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 62 + Appendix D. Half-Precision . . . . . . . . . . . . . . . . . . . 65 Appendix E. Comparison of Other Binary Formats to CBOR's Design - Objectives . . . . . . . . . . . . . . . . . . . . . . . 62 - E.1. ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . . 63 - E.2. MessagePack . . . . . . . . . . . . . . . . . . . . . . . 63 - E.3. BSON . . . . . . . . . . . . . . . . . . . . . . . . . . 64 - E.4. MSDTP: RFC 713 . . . . . . . . . . . . . . . . . . . . . 64 - E.5. Conciseness on the Wire . . . . . . . . . . . . . . . . . 64 - Appendix F. Changes from RFC 7049 . . . . . . . . . . . . . . . 65 - Appendix G. Well-formedness errors and examples . . . . . . . . 65 - G.1. Examples for CBOR data items that are not well-formed . . 66 - Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 68 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 69 + Objectives . . . . . . . . . . . . . . . . . . . . . . . 66 + E.1. ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . . 67 + E.2. MessagePack . . . . . . . . . . . . . . . . . . . . . . . 67 + E.3. BSON . . . . . . . . . . . . . . . . . . . . . . . . . . 68 + E.4. MSDTP: RFC 713 . . . . . . . . . . . . . . . . . . . . . 68 + E.5. Conciseness on the Wire . . . . . . . . . . . . . . . . . 68 + Appendix F. Changes from RFC 7049 . . . . . . . . . . . . . . . 69 + Appendix G. Well-formedness errors and examples . . . . . . . . 70 + G.1. Examples for CBOR data items that are not well-formed . . 71 + Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 73 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 74 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 @@ -204,23 +208,24 @@ 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. + size, and by the implementation complexity limiting how much + effort can go into achieving that compactness. Using either + general compression schemes or extensive bit-fiddling violates + the complexity goals. 5. The format must be applicable to both constrained nodes and high- volume applications. * This means it must be reasonably frugal in CPU usage for both encoding and decoding. This is relevant both for constrained nodes and for potential usage in applications with a very high volume of data. 6. The format must support all JSON data types for conversion to and @@ -274,50 +279,62 @@ Data Stream: A sequence of zero or more data items, not further assembled into a larger containing data item. The independent data items that make up a data stream are sometimes also referred to as "top-level data items". Well-formed: A data item that follows the syntactic structure of CBOR. A well-formed data item uses the initial bytes and the byte strings and/or data items that are implied by their values as defined in CBOR and does not include following extraneous data. + CBOR decoders by definition only return contents from well-formed data items. Valid: A data item that is well-formed and also follows the semantic - restrictions that apply to CBOR data items. + restrictions that apply to CBOR data items (Section 5.3). Expected: Besides its normal English meaning, the term "expected" is used to describe requirements beyond CBOR validity that an application has on its input data. Well-formed (processable at all), valid (checked by a validity-checking generic decoder), and expected (checked by the application) form a hierarchy of layers of acceptability. Stream decoder: A process that decodes a data stream and makes each of the data items in the sequence available to an application as they are received. + Terms and concepts for floating-point values such as Infinity, NaN + (not a number), negative zero, and subnormal are defined in + [IEEE754]. + Where bit arithmetic or data types are explained, this document uses the notation familiar from the programming language C, except that "**" denotes exponentiation. Similar to the "0x" notation for hexadecimal numbers, numbers in binary notation are prefixed with "0b". Underscores can be added to a number solely for readability, so 0b00100001 (0x21) might be written 0b001_00001 to emphasize the desired interpretation of the bits in the byte; in this case, it is split into three bits and five bits. Encoded CBOR data items are sometimes given in the "0x" or "0b" notation; these values are first interpreted as numbers as in C and are then interpreted as byte strings in network byte order, including any leading zero bytes expressed in the notation. + Words may be _italicized_ for emphasis; in the plain text form of + this specification this is indicated by surrounding words with + underscore characters. Verbatim text (e.g., names from a programming + language) may be set in "monospace" type; in plain text this is + approximated somewhat ambiguously by surrounding the text in double + quotes (which also retain their usual meaning). + 2. CBOR Data Models CBOR is explicit about its generic data model, which defines the set of all data items that can be represented in CBOR. Its basic generic data model is extensible by the registration of simple type values and tags. Applications can then subset the resulting extended generic data model to build their specific data models. Within environments that can represent the data items in the generic data model, generic CBOR encoders and decoders can be implemented @@ -324,45 +341,46 @@ (which usually involves defining additional implementation data types for those data items that do not already have a natural representation in the environment). The ability to provide generic encoders and decoders is an explicit design goal of CBOR; however many applications will provide their own application-specific encoders and/or decoders. In the basic (un-extended) generic data model, a data item is one of: * an integer in the range -2**64..2**64-1 inclusive + * a simple value, identified by a number between 0 and 255, but - distinct from that number + distinct from that number itself * a floating-point value, distinct from an integer, out of the set representable by IEEE 754 binary64 (including non-finites) [IEEE754] * a sequence of zero or more bytes ("byte string") * a sequence of zero or more Unicode code points ("text string") * a sequence of zero or more data items ("array") * a mapping (mathematical function) from zero or more data items ("keys") each to a data item ("values"), ("map") * a tagged data item ("tag"), comprising a tag number (an integer in - the range 0..2**64-1) and a tagged value (a data item) + the range 0..2**64-1) and the tag content (a data item) Note that integer and floating-point values are distinct in this model, even if they have the same numeric value. Also note that serialization variants, such as the number of bytes of - the encoded floating value, or the choice of one of the ways in which - an integer, the length of a text or byte string, the number of + the encoded floating-point value, or the choice of one of the ways in + which an integer, the length of a text or byte string, the number of elements in an array or pairs in a map, or a tag number, (collectively "the argument", see Section 3) can be encoded, are not visible at the generic data model level. 2.1. Extended Generic Data Models This basic generic data model comes pre-extended by the registration of a number of simple values and tag numbers right in this document, such as: @@ -413,21 +431,21 @@ representations of integral values are equivalent, using both map keys "0" and "0.0" in a single map would be considered duplicates, even while encoded as different major types, and so invalid; and an encoder could encode integral-valued floats as integers or vice versa, perhaps to save encoded bytes. 3. Specification of the CBOR Encoding A CBOR data item (Section 2) is encoded to or decoded from a byte string carrying a well-formed encoded data item as described in this - section. The encoding is summarized in Table 6, indexed by the + section. The encoding is summarized in Table 7, indexed by the initial byte. An encoder MUST produce only well-formed encoded data items. A decoder MUST NOT return a decoded data item when it encounters input that is not a well-formed encoded CBOR data item (this does not detract from the usefulness of diagnostic and recovery tools that might make available some information from a damaged encoded CBOR data item). The initial byte of each encoded data item contains both information about the major type (the high-order 3 bits, described in Section 3.1) and additional information (the low-order 5 bits). With @@ -465,21 +483,21 @@ If the encoded sequence of bytes ends before the end of a data item, that item is not well-formed. If the encoded sequence of bytes still has bytes remaining after the outermost encoded item is decoded, that encoding is not a single well-formed CBOR item; depending on the application, the decoder may either treat the encoding as not well- formed or just identify the start of the remaining bytes to the application. A CBOR decoder implementation can be based on a jump table with all - 256 defined values for the initial byte (Table 6). A decoder in a + 256 defined values for the initial byte (Table 7). 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). 3.1. Major Types The following lists the major types and the additional information and other bytes associated with the type. Major type 0: an integer in the range 0..2**64-1 inclusive. The @@ -510,54 +528,56 @@ formed but invalid. This type is provided for systems that need to interpret or display human-readable text, and allows the differentiation between unstructured bytes and text that has a specified repertoire and encoding. In contrast to formats such as JSON, the Unicode characters in this type are never escaped. Thus, a newline character (U+000A) is always represented in a string as the byte 0x0a, and never as the bytes 0x5c6e (the characters "\" and "n") or as 0x5c7530303061 (the characters "\", "u", "0", "0", "0", and "a"). - Major type 4: an array of data items. Arrays are also called lists, - sequences, or tuples. The argument is the number of data items in - the array. Items in an array do not need to all be of the same - type. For example, an array that contains 10 items of any type - would have an initial byte of 0b100_01010 (major type of 4, - additional information of 10 for the length) followed by the 10 - remaining items. + Major type 4: an array of data items. In other formats, arrays are + also called lists, sequences, or tuples (a "CBOR sequence" is + something slightly different, though [RFC8742]). The argument is + the number of data items in the array. Items in an array do not + need to all be of the same type. For example, an array that + contains 10 items of any type would have an initial byte of + 0b100_01010 (major type of 4, additional information of 10 for the + length) followed by the 10 remaining items. Major type 5: a map of pairs of data items. Maps are also called tables, dictionaries, hashes, or objects (in JSON). A map is comprised of pairs of data items, each pair consisting of a key that is immediately followed by a value. The argument is the number of _pairs_ of data items in the map. For example, a map that contains 9 pairs would have an initial byte of 0b101_01001 (major type of 5, additional information of 9 for the number of pairs) followed by the 18 remaining items. The first item is the first key, the second item is the first value, the third item is the second key, and so on. Because items in a map come in pairs, their total number is always even: A map that contains an odd number of items (no value data present after the last key data item) is not well-formed. A map that has duplicate keys may be well-formed, but it is not valid, and thus it causes indeterminate decoding; see also Section 5.6. - Major type 6: a tagged data item ("tag") whose tag number is the - argument and whose enclosed data item ("tag content") is the - single encoded data item that follows the head. See Section 3.4. + Major type 6: a tagged data item ("tag") whose tag number, an + integer in the range 0..2**64-1 inclusive, is the argument and + whose enclosed data item ("tag content") is the single encoded + data item that follows the head. See Section 3.4. Major type 7: floating-point numbers and simple values, as well as the "break" stop code. See Section 3.3. These eight major types lead to a simple table showing which of the 256 possible values for the initial byte of a data item are used - (Table 6). + (Table 7). In major types 6 and 7, many of the possible values are reserved for future specification. See Section 9 for more information on these values. Table 1 summarizes the major types defined by CBOR, ignoring the next section for now. The number N in this table stands for the argument, mt for the major type. +----+-----------------------+---------------------------------+ @@ -573,31 +593,32 @@ +----+-----------------------+---------------------------------+ | 4 | array | N data items (elements) | +----+-----------------------+---------------------------------+ | 5 | map | 2N data items (key/value pairs) | +----+-----------------------+---------------------------------+ | 6 | tag of number N | 1 data item | +----+-----------------------+---------------------------------+ | 7 | simple/float | - | +----+-----------------------+---------------------------------+ - Table 1: Overview over CBOR major types (definite length - encoded) + Table 1: Overview over the definite-length use of CBOR major + types (mt = major type, N = argument) 3.2. Indefinite Lengths for Some Major Types Four CBOR items (arrays, maps, byte strings, and text strings) can be encoded with an indefinite length using additional information value 31. This is useful if the encoding of the item needs to begin before the number of items inside the array or map, or the total length of - the string, is known. (The application of this is often referred to - as "streaming" within a data item.) + the string, is known. (The ability to start sending a data item + before all of it is known is often referred to as "streaming" within + that data item.) Indefinite-length arrays and maps are dealt with differently than indefinite-length byte strings and text strings. 3.2.1. The "break" Stop Code The "break" stop code is encoded with major type 7 and additional information value 31 (0b111_11111). It is not itself a data item: it is just a syntactic feature to close an indefinite-length item. @@ -712,43 +733,73 @@ chunks, while not particularly useful, are permitted.) If any item between the indefinite-length string indicator (0b010_11111 or 0b011_11111) and the "break" stop code is not a definite-length string item of the same major type, the string is not well-formed. If any definite-length text string inside an indefinite-length text string is invalid, the indefinite-length text string is invalid. 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. + cannot be split up between chunks: a new chunk of a text string can + only be started at a character boundary. - For example, assume the sequence: + For example, assume an encoded data item consisting of the bytes: 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. +3.2.4. Summary of indefinite-length use of major types + + Table 2 summarizes the major types defined by CBOR as used for + indefinite length encoding (with additional information set to 31). + mt stands for the major type. + + +----+-------------------+----------------------------------+ + | mt | Meaning | enclosed up to "break" stop code | + +====+===================+==================================+ + | 0 | (not well-formed) | - | + +----+-------------------+----------------------------------+ + | 1 | (not well-formed) | - | + +----+-------------------+----------------------------------+ + | 2 | byte string | definite-length byte strings | + +----+-------------------+----------------------------------+ + | 3 | text string | definite-length text strings | + +----+-------------------+----------------------------------+ + | 4 | array | data items (elements) | + +----+-------------------+----------------------------------+ + | 5 | map | data items (key/value pairs) | + +----+-------------------+----------------------------------+ + | 6 | (not well-formed) | - | + +----+-------------------+----------------------------------+ + | 7 | "break" stop code | - | + +----+-------------------+----------------------------------+ + + Table 2: Overview over the indefinite-length use of CBOR + major types (mt = major type, additional information = + 31) + 3.3. Floating-Point Numbers and Values with No Content Major type 7 is for two types of data: floating-point numbers and "simple values" that do not need any content. Each value of the 5-bit additional information in the initial byte has its own separate - meaning, as defined in Table 2. Like the major types for integers, + meaning, as defined in Table 3. 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) | +-------------+---------------------------------------------------+ @@ -757,244 +808,280 @@ | 26 | IEEE 754 Single-Precision Float (32 bits follow) | +-------------+---------------------------------------------------+ | 27 | IEEE 754 Double-Precision Float (64 bits follow) | +-------------+---------------------------------------------------+ | 28-30 | Reserved, not well-formed in the present document | +-------------+---------------------------------------------------+ | 31 | "break" stop code for indefinite-length items | | | (Section 3.2.1) | +-------------+---------------------------------------------------+ - Table 2: Values for Additional Information in Major Type 7 + Table 3: 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 3 lists the values + additional information in the first byte. Table 4 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 3: Simple Values + Table 4: Simple Values An encoder MUST NOT issue two-byte sequences that start with 0xf8 (major type = 7, additional information = 24) and continue with a byte less than 0x20 (32 decimal). Such sequences are not well- formed. (This implies that an encoder cannot encode false, true, null, or undefined in two-byte sequences, only the one-byte variants of these are well-formed.) The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit IEEE 754 binary floating-point values [IEEE754]. These floating- point values are encoded in the additional bytes of the appropriate - size. (See Appendix D for some information about 16-bit floating - point.) + size. (See Appendix D for some information about 16-bit floating- + point numbers.) 3.4. Tagging of Items - In CBOR, a data item can be enclosed by a tag to give it additional - semantics while retaining its structure. The tag is major type 6, - and represents an unsigned integer as indicated by the tag's argument - (Section 3); the (sole) enclosed data item is carried as content - data. If a tag requires structured data, this structure is encoded - into the nested data item. The definition of a tag number usually - restricts what kinds of nested data item or items are valid for tags - using this tag number. + In CBOR, a data item can be enclosed by a tag to give it some + additional semantics, as uniquely identified by a "tag number". The + tag is major type 6, its argument (Section 3) indicates the tag + number, and it contains a single enclosed data item, the "tag + content". (If a tag requires further structure to its content, this + structure is provided by the enclosed data item.) We use the term + "tag" for the entire data item consisting of both a tag number and + the tag content: the tag content is the data item that is being + tagged. For example, assume that a byte string of length 12 is marked with a - tag of number 2 to indicate it is a positive bignum (Section 3.4.3). - This would be marked as 0b110_00010 (major type 6, additional - information 2 for the tag number) followed by 0b010_01100 (major type + tag of number 2 to indicate it is a positive "bignum" + (Section 3.4.3). The encoded data item would start with a byte + 0b110_00010 (major type 6, additional information 2 for the tag + number) followed by the encoded tag content: 0b010_01100 (major type 2, additional information of 12 for the length) followed by the 12 bytes of the bignum. + The definition of a tag number describes the additional semantics + conveyed for tags with this tag number in the extended generic data + model. These semantics may include equivalence of some tagged data + items with other data items, including some that can already be + represented in the basic generic data model. For instance, 0xc24101, + a bignum the tag content of which is the byte string with the single + byte 0x01, is equivalent to an integer 1, which could also be encoded + for instance as 0x01, 0x1801, or 0x190001. The tag definition may + include the definition of a preferred serialization (Section 4.1) + that is recommended for generic encoders; this may prefer basic + generic data model representations over ones that employ a tag. + + The tag definition usually restricts what kinds of nested data item + or items are valid for such tags. Tag definitions may restrict their + content to a very specific syntactic structure, as the tags defined + in this document do, or they may aim at a more semantically defined + definition of their content, as for instance tags 40 and 1040 do + [rfc8746]: These accept a number of different ways of representing + arrays. + + As a matter of convention, many tags do not accept null or undefined + values as tag content; instead, the expectation is that a null or + undefined value can be used in place of the entire tag; Section 3.4.2 + provides some further considerations for one specific tag about the + handling of this convention in application protocols and in mapping + to platform types. + Decoders do not need to understand tags of every tag number, and tags may be of little value in applications where the implementation creating a particular CBOR data item and the implementation decoding that stream know the semantic meaning of each item in the data flow. Their primary purpose in this specification is to define common data types such as dates. A secondary purpose is to provide conversion hints when it is foreseen that the CBOR data item needs to be translated into a different format, requiring hints about the content of items. Understanding the semantics of tags is optional for a - decoder; it can just jump over the initial bytes of the tag (that - encode the tag number) and interpret the tag content itself, - presenting both tag number and tag content to the application. + decoder; it can simply present both the tag number and the tag + content to the application, without interpreting the additional + semantics of the tag. - A tag applies semantics to the data item it encloses. Thus, if tag A - encloses tag B, which encloses data item C, tag A applies to the - result of applying tag B on data item C. That is, a tag is a data - item consisting of a tag number and an enclosed value. The content - of the tag (the enclosed data item) is the data item (the value) that - is being tagged. + A tag applies semantics to the data item it encloses. Tags can nest: + If tag A encloses tag B, which encloses data item C, tag A applies to + the result of applying tag B on data item C. IANA maintains a registry of tag numbers as described in Section 9.2. - Table 4 provides a list of tag numbers that were defined in + Table 5 provides a list of tag numbers that were defined in [RFC7049], with definitions in the rest of this section. Note that many other tag numbers have been defined since the publication of [RFC7049]; see the registry described at Section 9.2 for the complete list. +------------+-------------+----------------------------------+ | Tag Number | Data Item | Semantics | +============+=============+==================================+ | 0 | text string | Standard date/time string; see | | | | Section 3.4.1 | +------------+-------------+----------------------------------+ - | 1 | multiple | Epoch-based date/time; see | - | | | Section 3.4.2 | + | 1 | integer or | Epoch-based date/time; see | + | | float | Section 3.4.2 | +------------+-------------+----------------------------------+ | 2 | byte string | Positive bignum; see | | | | Section 3.4.3 | +------------+-------------+----------------------------------+ | 3 | byte string | Negative bignum; see | | | | Section 3.4.3 | +------------+-------------+----------------------------------+ | 4 | array | Decimal fraction; see | | | | Section 3.4.4 | +------------+-------------+----------------------------------+ | 5 | array | Bigfloat; see Section 3.4.4 | +------------+-------------+----------------------------------+ - | 21 | multiple | Expected conversion to base64url | + | 21 | (any) | Expected conversion to base64url | | | | encoding; see Section 3.4.5.2 | +------------+-------------+----------------------------------+ - | 22 | multiple | Expected conversion to base64 | + | 22 | (any) | Expected conversion to base64 | | | | encoding; see Section 3.4.5.2 | +------------+-------------+----------------------------------+ - | 23 | multiple | Expected conversion to base16 | + | 23 | (any) | Expected conversion to base16 | | | | encoding; see Section 3.4.5.2 | +------------+-------------+----------------------------------+ | 24 | byte string | Encoded CBOR data item; see | | | | Section 3.4.5.1 | +------------+-------------+----------------------------------+ | 32 | text string | URI; see Section 3.4.5.3 | +------------+-------------+----------------------------------+ | 33 | text string | base64url; see Section 3.4.5.3 | +------------+-------------+----------------------------------+ | 34 | text string | base64; see Section 3.4.5.3 | +------------+-------------+----------------------------------+ | 35 | text string | Regular expression; see | | | | Section 3.4.5.3 | +------------+-------------+----------------------------------+ | 36 | text string | MIME message; see | | | | Section 3.4.5.3 | +------------+-------------+----------------------------------+ - | 55799 | multiple | Self-described CBOR; see | + | 55799 | (any) | Self-described CBOR; see | | | | Section 3.4.6 | +------------+-------------+----------------------------------+ - Table 4: Tag numbers defined in RFC 7049 + Table 5: Tag numbers defined in RFC 7049 Conceptually, tags are interpreted in the generic data model, not at (de-)serialization time. A small number of tags (specifically, tag number 25 and tag number 29) have been registered with semantics that may require processing at (de-)serialization time: The decoder needs to be aware and the encoder needs to be in control of the exact sequence in which data items are encoded into the CBOR data stream. This means these tags cannot be implemented on top of every generic CBOR encoder/decoder (which might not reflect the serialization order for entries in a map at the data model level and vice versa); their implementation therefore typically needs to be integrated into the generic encoder/decoder. The definition of new tags with this property is NOT RECOMMENDED. Protocols using tag numbers 0 and 1 extend the generic data model (Section 2) with data items representing points in time; tag numbers 2 and 3, with arbitrarily sized integers; and tag numbers 4 and 5, - with floating point values of arbitrary size and precision. + with floating-point values of arbitrary size and precision. 3.4.1. Standard Date/Time String Tag number 0 contains a text string in the standard format described by the "date-time" production in [RFC3339], as refined by Section 3.3 of [RFC4287], representing the point in time described there. A nested item of another type or that doesn't match the [RFC4287] format is invalid. 3.4.2. Epoch-based Date/Time Tag number 1 contains a numerical value counting the number of seconds from 1970-01-01T00:00Z in UTC time to the represented point in civil time. - The enclosed item MUST be an unsigned or negative integer (major - types 0 and 1), or a floating-point number (major type 7 with - additional information 25, 26, or 27). Other contained types are - invalid. + The tag content MUST be an unsigned or negative integer (major types + 0 and 1), or a floating-point number (major type 7 with additional + information 25, 26, or 27). Other contained types are invalid. Non-negative values (major type 0 and non-negative floating-point numbers) stand for time values on or after 1970-01-01T00:00Z UTC and are interpreted according to POSIX [TIME_T]. (POSIX time is also known as UNIX Epoch time. Note that leap seconds are handled specially by POSIX time and this results in a 1 second discontinuity several times per decade.) Note that applications that require the expression of times beyond early 2106 cannot leave out support of - 64-bit integers for the enclosed value. + 64-bit integers for the tag content. Negative values (major type 1 and negative floating-point numbers) are interpreted as determined by the application requirements as there is no universal standard for UTC count-of-seconds time before 1970-01-01T00:00Z (this is particularly true for points in time that precede discontinuities in national calendars). The same applies to non-finite values. To indicate fractional seconds, floating-point values can be used within tag number 1 instead of integer values. Note that this generally requires binary64 support, as binary16 and binary32 provide non-zero fractions of seconds only for a short period of time around early 1970. An application that requires tag number 1 support may - restrict the enclosed value to be an integer (or a floating-point - value) only. + restrict the tag content to be an integer (or a floating-point value) + only. + + Note that platform types for date/time may include null or undefined + values, which may also be desirable at an application protocol level. + While emitting tag number 1 values with non-finite tag content values + (e.g., with NaN for undefined date/time values or with Infinite for + an expiry date that is not set) may seem an obvious way to handle + this, using untagged null or undefined is often a better solution. + Application protocol designers are encouraged to consider these cases + and include clear guidelines for handling them. 3.4.3. Bignums Protocols using tag numbers 2 and 3 extend the generic data model (Section 2) with "bignums" representing arbitrarily sized integers. - In the generic data model, bignum values are not equal to integers - from the basic data model, but specific data models can define that - equivalence, and preferred encoding never makes use of bignums that - also can be expressed as basic integers (see below). + In the basic generic data model, bignum values are not equal to + integers from the same model, but the extended generic data model + created by this tag definition defines equivalence based on numeric + value, and preferred serialization (Section 4.1) never makes use of + bignums that also can be expressed as basic integers (see below). Bignums are encoded as a byte string data item, which is interpreted as an unsigned integer n in network byte order. Contained items of other types are invalid. For tag number 2, the value of the bignum is n. For tag number 3, the value of the bignum is -1 - n. The - preferred encoding of the byte string is to leave out any leading - zeroes (note that this means the preferred encoding for n = 0 is the - empty byte string, but see below). Decoders that understand these - tags MUST be able to decode bignums that do have leading zeroes. The - preferred encoding of an integer that can be represented using major - type 0 or 1 is to encode it this way instead of as a bignum (which - means that the empty string never occurs in a bignum when using - preferred encoding). Note that this means the non-preferred choice - of a bignum representation instead of a basic integer for encoding a - number is not intended to have application semantics (just as the - choice of a longer basic integer representation than needed, such as - 0x1800 for 0x00 does not). + preferred serialization of the byte string is to leave out any + leading zeroes (note that this means the preferred serialization for + n = 0 is the empty byte string, but see below). Decoders that + understand these tags MUST be able to decode bignums that do have + leading zeroes. The preferred serialization of an integer that can + be represented using major type 0 or 1 is to encode it this way + instead of as a bignum (which means that the empty string never + occurs in a bignum when using preferred serialization). Note that + this means the non-preferred choice of a bignum representation + instead of a basic integer for encoding a number is not intended to + have application semantics (just as the choice of a longer basic + integer representation than needed, such as 0x1800 for 0x00 does + not). For example, the number 18446744073709551616 (2**64) is represented as 0b110_00010 (major type 6, tag number 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 49 -- Byte string of length 9 010000000000000000 -- Bytes content @@ -1003,37 +1090,37 @@ Protocols using tag number 4 extend the generic data model with data items representing arbitrary-length decimal fractions of the form m*(10**e). Protocols using tag number 5 extend the generic data model with data items representing arbitrary-length binary fractions of the form m*(2**e). As with bignums, values of different types are not equal in the generic data model. 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. + exact representation for many decimal fractions in binary floating- + point representations. - Bigfloats combine an integer mantissa with a base-2 scaling factor. + "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 3.3). 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 number 4) use base-10 exponents; the value of a decimal fraction data item is m*(10**e). Bigfloats (tag number 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 + of major type 0 or 1, while the mantissa can also be a bignum (Section 3.4.3). Contained items with other structures are invalid. 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 number 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 @@ -1054,26 +1141,21 @@ 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 decimal fraction or bigfloat, the IEEE 754 half-precision - representations from Section 3.3 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. + representations from Section 3.3 can be used. 3.4.5. Content Hints The tags in this section are for content hints that might be used by generic CBOR processors. These content hints do not extend the generic data model. 3.4.5.1. Encoded CBOR Data Item Sometimes it is beneficial to carry an embedded CBOR data item that @@ -1098,45 +1180,48 @@ 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 numbers suggest conversions to three of the base data - encodings defined in [RFC4648]. For base64url encoding (tag number - 21), padding is not used (see Section 3.2 of RFC 4648); that is, all - trailing equals signs ("=") are removed from the encoded string. For - base64 encoding (tag number 22), padding is used as defined in RFC - 4648. For both base64url and base64, padding bits are set to zero - (see Section 3.5 of RFC 4648), and encoding is performed without the - inclusion of any line breaks, whitespace, or other additional - characters. Note that, for all three tag numbers, the encoding of - the empty byte string is the empty text string. + encodings defined in [RFC4648]. Tag number 21 suggests conversion to + base64url encoding (Section 5 of RFC 4648), where padding is not used + (see Section 3.2 of RFC 4648); that is, all trailing equals signs + ("=") are removed from the encoded string. Tag number 22 suggests + conversion to classical base64 encoding (Section 4 of RFC 4648), with + padding as defined in RFC 4648. For both base64url and base64, + padding bits are set to zero (see Section 3.5 of RFC 4648), and + encoding is performed without the inclusion of any line breaks, + whitespace, or other additional characters. Tag number 23 suggests + conversion to base16 (hex) encoding, with uppercase alphabetics (see + Section 8 of RFC 4648). Note that, for all three tag numbers, the + encoding of the empty byte string is the empty text string. 3.4.5.3. 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. As with tag numbers 21 to 23, if these tags are applied to an item other than a text string, they apply to all text string data items it contains. * Tag number 32 is for URIs, as defined in [RFC3986]. If the text string doesn't match the "URI-reference" production, the string is invalid. * Tag numbers 33 and 34 are for base64url- and base64-encoded text - strings, as defined in [RFC4648]. If any of: + strings, respectively, as defined in [RFC4648]. If any of: - the encoded text string contains non-alphabet characters or only 1 character in the last block of 4, or - the padding bits in a 2- or 3-character block are not 0, or - the base64 encoding has the wrong number of padding characters, or - the base64url encoding has padding characters, @@ -1152,42 +1237,42 @@ itself, need to be conveyed.) Any contained string value is valid. * Tag number 36 is for MIME messages (including all headers), as defined in [RFC2045]. A text string that isn't a valid MIME message is invalid. (For this tag, validity checking may be particularly onerous for a generic decoder and might therefore not be offered. Note that many MIME messages are general binary data and can therefore not be represented in a text string; [IANA.cbor-tags] lists a registration for tag number 257 that is - similar to tag number 36 but is used with an enclosed byte - string.) + similar to tag number 36 but uses a byte string as its tag + content.) Note that tag numbers 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. 3.4.6. Self-Described 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 that does not have disambiguating metadata. Here, it may help to have some distinguishing characteristics for the data itself. Tag number 55799 is defined for this purpose. It does not impart any special semantics on the data item that it encloses; that is, the - semantics of a data item enclosed in tag number 55799 is exactly - identical to the semantics of the data item itself. + semantics of the tag content enclosed in tag number 55799 is exactly + identical to the semantics of the tag content itself. The serialization of this tag's head is 0xd9d9f7, which does not appear to be in use as a distinguishing mark for any frequently used file types. In particular, 0xd9d9f7 is not a valid start of a Unicode text in any Unicode encoding if it is followed by a valid CBOR data item. For instance, a decoder might be able to decode 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 @@ -1199,49 +1284,58 @@ 4.1. Preferred Serialization For some values at the data model level, CBOR provides multiple serializations. For many applications, it is desirable that an encoder always chooses a preferred serialization (preferred encoding); however, the present specification does not put the burden of enforcing this preference on either encoder or decoder. Some constrained decoders may be limited in their ability to decode non-preferred serializations: For example, if only integers below - 1_000_000_000 are expected in an application, the decoder may leave - out the code that would be needed to decode 64-bit arguments in - integers. An encoder that always uses preferred serialization - ("preferred encoder") interoperates with this decoder for the numbers - that can occur in this application. More generally speaking, it - therefore can be said that a preferred encoder is more universally - interoperable (and also less wasteful) than one that, say, always - uses 64-bit integers. + 1_000_000_000 (one billion) are expected in an application, the + decoder may leave out the code that would be needed to decode 64-bit + arguments in integers. An encoder that always uses preferred + serialization ("preferred encoder") interoperates with this decoder + for the numbers that can occur in this application. More generally + speaking, it therefore can be said that a preferred encoder is more + universally interoperable (and also less wasteful) than one that, + say, always uses 64-bit integers. Similarly, a constrained encoder may be limited in the variety of representation variants it supports in such a way that it does not emit preferred serializations ("variant encoder"): Say, it could be designed to always use the 32-bit variant for an integer that it encodes even if a short representation is available (again, assuming that there is no application need for integers that can only be represented with the 64-bit variant). A decoder that does not rely on only ever receiving preferred serializations ("variation-tolerant decoder") can there be said to be more universally interoperable (it might very well optimize for the case of receiving preferred serializations, though). Full implementations of CBOR decoders are by definition variation-tolerant; the distinction is only relevant if a constrained implementation of a CBOR decoder meets a variant encoder. The preferred serialization always uses the shortest form of - representing the argument (Section 3)); it also uses the shortest - floating-point encoding that preserves the value being encoded (see - Section 5.5). Definite length encoding is preferred whenever the - length is known at the time the serialization of the item starts. + representing the argument (Section 3); it also uses the shortest + floating-point encoding that preserves the value being encoded. + + The preferred serialization for a floating-point value is the + shortest floating-point encoding that preserves its value, e.g., + 0xf94580 for the number 5.5, and 0xfa45ad9c00 for the number 5555.5. + For NaN values, a shorter encoding is preferred if zero-padding the + shorter significand towards the right reconstitutes the original NaN + value (for many applications, the single NaN encoding 0xf97e00 will + suffice). + + Definite length encoding is preferred whenever the length is known at + the time the serialization of the item starts. 4.2. Deterministically Encoded CBOR Some protocols may want encoders to only emit CBOR in a particular deterministic format; those protocols might also have the decoders check that their input is in that deterministic format. Those protocols are free to define what they mean by a "deterministic format" and what encoders and decoders are expected to do. This section defines a set of restrictions that can serve as the base of such a deterministic format. @@ -1261,23 +1355,28 @@ - 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. - Floating point values also MUST use the shortest form that + Floating-point values also MUST use the shortest form that preserves the value, e.g. 1.5 is encoded as 0xf93e00 and 1000000.5 - as 0xfa49742408. + as 0xfa49742408. (One implementation of this is 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 form and repeat the process with a test conversion to a + 16-bit float. This also works to select 16-bit float for positive + and negative Infinity as well.) * Indefinite-length items MUST NOT appear. They can be encoded as definite-length items instead. * The keys in every map MUST be sorted in the bytewise lexicographic order of their deterministic encodings. For example, the following keys are sorted correctly: 1. 10, encoded as 0x0a. @@ -1290,94 +1389,107 @@ 5. "aa", encoded as 0x626161. 6. [100], encoded as 0x811864. 7. [-1], encoded as 0x8120. 8. false, encoded as 0xf4. 4.2.2. Additional Deterministic Encoding Considerations - If a protocol allows for IEEE floats, then additional deterministic - encoding 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.) Although IEEE floats can represent both - positive and negative zero as distinct values, the application might - not distinguish these and might decide to represent all zero values - with a positive sign, disallowing negative zero. - CBOR tags present additional considerations for deterministic encoding. If a CBOR-based protocol were to provide the same semantics for the presence and absence of a specific tag (e.g., by allowing both tag 1 data items and raw numbers in a date/time position, treating the latter as if they were tagged), the - deterministic format would not allow them. In a protocol that - requires tags in certain places to obtain specific semantics, the tag - needs to appear in the deterministic format as well. Deterministic - encoding considerations also apply to the content of tags. + deterministic format would not allow the presence of the tag, based + on the "shortest form" principle. For example, a protocol might give + encoders the choice of representing a URL as either a text string or, + using Section 3.4.5.3, tag number 32 containing a text string. This + protocol's deterministic encoding needs to either require that the + tag is present or require that it is absent, not allow either one. - Protocols that include floating, big integer, or other complex values - need to define extra requirements on their deterministic encodings. - For example: + In a protocol that does require tags in certain places to obtain + specific semantics, the tag needs to appear in the deterministic + format as well. Deterministic encoding considerations also apply to + the content of tags. + + If a protocol includes a field that can express integers with an + absolute value of 2^64 or larger using tag numbers 2 or 3 + (Section 3.4.3), the protocol's deterministic encoding needs to + specify whether smaller integers are also expressed using these tags + or using major types 0 and 1. Preferred serialization uses the + latter choice, which is therefore recommended. + + Protocols that include floating-point values, whether represented + using basic floating-point values (Section 3.3) or using tags (or + both), may need to define extra requirements on their deterministic + encodings, such as: + + * Although IEEE floating-point values can represent both positive + and negative zero as distinct values, the application might not + distinguish these and might decide to represent all zero values + with a positive sign, disallowing negative zero. (The application + may also want to restrict the precision of floating point values + in such a way that there is never a need to represent 64-bit -- or + even 32-bit -- floating-point values.) * If a protocol includes a field that can express floating-point - values (Section 3.3), the protocol's deterministic encoding needs - to specify whether the integer 1.0 is encoded as 0x01, 0xf93c00, - 0xfa3f800000, or 0xfb3ff0000000000000. Three sensible rules for - this are: + values, with a specific data model that declares integer and + floating-point values to be interchangeable, the protocol's + deterministic encoding needs to specify whether the integer 1.0 is + encoded as 0x01, 0xf93c00, 0xfa3f800000, or 0xfb3ff0000000000000. + Example rules for this are: 1. Encode integral values that fit in 64 bits as values from - major types 0 and 1, and other values as the smallest of 16-, - 32-, or 64-bit floating point that accurately represents the - value, + major types 0 and 1, and other values as the preferred + (smallest of 16-, 32-, or 64-bit) floating-point + representation that accurately represents the value, - 2. Encode all values as the smallest of 16-, 32-, or 64-bit - floating point that accurately represents the value, even for + 2. Encode all values as the preferred floating-point + representation that accurately represents the value, even for integral values, or - 3. Encode all values as 64-bit floating point. + 3. Encode all values as 64-bit floating-point representations. - Rule 1 straddles the boundaries between integers and floating - point values, and Rule 3 does not use preferred encoding, so Rule - 2 may be a good choice in many cases. + Rule 1 straddles the boundaries between integers and floating- + point values, and Rule 3 does not use preferred serialization, so + Rule 2 may be a good choice in many cases. - If NaN is an allowed value and there is no intent to support NaN + * If NaN is an allowed value and there is no intent to support NaN payloads or signaling NaNs, the protocol needs to pick a single - representation, for example 0xf97e00. If that simple choice is - not possible, specific attention will be needed for NaN handling. + representation, typically 0xf97e00. If that simple choice is not + possible, specific attention will be needed for NaN handling. - Subnormal numbers (nonzero numbers with the lowest possible + * Subnormal numbers (nonzero numbers with the lowest possible exponent of a given IEEE 754 number format) may be flushed to zero - outputs or be treated as zero inputs in some floating point + outputs or be treated as zero inputs in some floating-point implementations. A protocol's deterministic encoding may want to - exclude them from interchange, interchanging zero instead. - - * If a protocol includes a field that can express integers with an - absolute value of 2^64 or larger using tag numbers 2 or 3 - (Section 3.4.3), the protocol's deterministic encoding needs to - specify whether small integers are expressed using the tag or - major types 0 and 1. + specifically accommodate such implementations while creating an + onus on other implementations, by excluding subnormal numbers from + interchange, interchanging zero instead. - * A protocol might give encoders the choice of representing a URL as - either a text string or, using Section 3.4.5.3, tag number 32 - containing a text string. This protocol's deterministic encoding - needs to either require that the tag is present or require that - it's absent, not allow either one. + * The same number can be represented by different decimal fractions, + by different bigfloats, and by different forms under other tags + that may be defined to express numeric values. Depending on the + implementation, it may not always be practical to determine + whether any of these forms (or forms in the basic generic data + model) are equivalent. An application protocol that presents + choices of this kind for the representation format of numbers + needs to be explicit in how the formats are to be chosen for + deterministic encoding. -4.2.3. Length-first map key ordering +4.2.3. Length-first Map Key Ordering - The core deterministic encoding requirements sort map keys in a - different order from the one suggested by Section 3.9 of [RFC7049] - (called "Canonical CBOR" there). Protocols that need to be + The core deterministic encoding requirements (Section 4.2.1) sort map + keys in a different order from the one suggested by Section 3.9 of + [RFC7049] (called "Canonical CBOR" there). Protocols that need to be compatible with [RFC7049]'s order can instead be specified in terms of this specification's "length-first core deterministic encoding requirements": A CBOR encoding satisfies the "length-first core deterministic encoding requirements" if it satisfies the core deterministic encoding requirements except that the keys in every map MUST be sorted such that: 1. If two keys have different lengths, the shorter one sorts @@ -1432,26 +1544,26 @@ and other unexpected data. CBOR-based protocols MAY specify that they treat arbitrary valid data as unexpected. Encoders for CBOR- based protocols MUST produce only valid items, that is, the protocol cannot be designed to make use of invalid items. 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. With few exceptions, it is advisory only and explicitly - excludes any language from BCP 14 other than words that could be - interpreted as "MAY" in the sense of BCP 14. The exceptions aim at - facilitating interoperability of CBOR-based protocols while making - use of a wide variety of both generic and application-specific + The rest of this section discusses some considerations in creating + CBOR-based protocols. With few exceptions, it is advisory only and + explicitly excludes any language from BCP 14 other than words that + could be interpreted as "MAY" in the sense of BCP 14. The exceptions + aim at facilitating interoperability of CBOR-based protocols while + making use of a wide variety of both generic and application-specific encoders and decoders. 5.1. CBOR in Streaming Applications In a streaming application, a data stream may be composed of a sequence of CBOR data items concatenated back-to-back. In such an environment, the decoder immediately begins decoding a new data item if data is found after the end of a previous data item. Not all of the bytes making up a data item may be immediately @@ -1535,29 +1647,29 @@ 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. 5.3.2. Tag validity Two additional kinds of validity errors are introduced by adding tags to the basic generic data model: - Inadmissible type for tag content: Tags (Section 3.4) specify what - type of data item is supposed to be enclosed by 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. + Inadmissible type for tag content: Tag numbers (Section 3.4) specify + what type of data item is supposed to be used as their tag + content; for example, the tag numbers 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. Inadmissible value for tag content: The type of data item may be admissible for a tag's content, but the specific value may not be; e.g., a value of "yesterday" is not acceptable for the content of tag 0, even though it properly is a text string. A decoder that normally ingests such tags into equivalent platform types might present this tag to the application in a similar way to how it would present a tag with an unknown tag number (Section 5.4). 5.4. Validity and Evolution @@ -1584,75 +1696,73 @@ with an indication that the decoder did not recognize that tag number or simple value. The latter approach, which is also appropriate for decoders that do not support validity checking, provides forward compatibility with newly registered tags and simple values without the requirement to update the encoder at the same time as the calling application. (For this, the API for the decoder needs to have a way to mark unknown items so that the calling application can handle them in a manner appropriate for the program.) - Since some of the processing needed for validity checking may have an appreciable cost (in particular with duplicate detection for maps), support of validity checking is not a requirement placed on all CBOR decoders. Some encoders will rely on their applications to provide input data in such a way that valid CBOR results from the encoder. A generic - encoder also may want to provide a validity-checking mode where it + encoder may also want to provide a validity-checking mode where it reliably limits its output to valid CBOR, independent of whether or not its application is indeed providing API-conformant data. 5.5. Numbers CBOR-based protocols should take into account that different language environments pose different restrictions on the range and precision - of numbers that are representable. For example, the JavaScript - number system treats all numbers as floating point, which may result - in silent loss of precision in decoding integers with more than 53 - significant bits. A protocol that uses numbers should define its - expectations on the handling of non-trivial numbers in decoders and - receiving applications. + of numbers that are representable. For example, the basic JavaScript + number system treats all numbers as floating-point values, which may + result in silent loss of precision in decoding integers with more + than 53 significant bits. A protocol that uses numbers should define + its expectations on the handling of non-trivial numbers in decoders + and receiving applications. A CBOR-based protocol that includes floating-point numbers can restrict which of the three formats (half-precision, single- precision, and double-precision) are to be supported. For an integer-only application, a protocol may want to completely exclude the use of floating-point values. A CBOR-based protocol designed for compactness may want to exclude specific integer encodings that are longer than necessary for the application, such as to save the need to implement 64-bit integers. There is an expectation that encoders will use the most compact integer representation that can represent a given value. However, a - compact application should accept values that use a longer-than- - needed encoding (such as encoding "0" as 0b000_11001 followed by two - bytes of 0x00) as long as the application can decode an integer of - the given size. + compact application that does not require deterministic encoding + should accept values that use a longer-than-needed encoding (such as + encoding "0" as 0b000_11001 followed by two bytes of 0x00) as long as + the application can decode an integer of the given size. Similar + considerations apply to floating-point values; decoding both + preferred serializations and longer-than-needed ones is recommended. - The preferred encoding for a floating-point value is the shortest - floating-point encoding that preserves its value, e.g., 0xf94580 for - the number 5.5, and 0xfa45ad9c00 for the number 5555.5, unless the - CBOR-based protocol specifically excludes the use of the shorter - floating-point encodings. For NaN values, a shorter encoding is - preferred if zero-padding the shorter significand towards the right - reconstitutes the original NaN value (for many applications, the - single NaN encoding 0xf97e00 will suffice). + CBOR-based protocols for constrained applications that provide 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), might express a quality-of-implementation expectation + that the integer representation is used directly. 5.6. Specifying Keys for Maps The encoding and decoding applications need to agree on what types of keys are going to be used in maps. In applications that need to - interwork with JSON-based applications, keys probably should be - limited to UTF-8 strings only; otherwise, there has to be a specified - mapping from the other CBOR types to Unicode characters, and this + interwork with JSON-based applications, conversion is simplified by + limiting keys to text strings only; otherwise, there has to be a + specified mapping from the other CBOR types to text strings, and this often leads to implementation errors. In applications where keys are numeric in nature and numeric ordering of keys is important to the application, directly using the numbers for the keys is useful. If multiple types of keys are to be used, consideration should be given to how these types would be represented in the specific programming environments that are to be used. For example, in JavaScript Maps [ECMA262], a key of integer 1 cannot be distinguished from a key of floating-point 1.0. This means that, if integer keys are used, the protocol needs to avoid use of floating-point keys the @@ -1665,32 +1775,61 @@ the enclosing data item is completely available ("streaming encoder") may want to reduce its overhead significantly by relying on its data source to maintain uniqueness. A CBOR-based protocol MUST define what to do when a receiving application does see multiple identical keys in a map. The resulting rule in the protocol MUST respect the CBOR data model: it cannot prescribe a specific handling of the entries with the identical keys, except that it might have a rule that having identical keys in a map indicates a malformed map and that the decoder has to stop with an - error. Duplicate keys are also prohibited by CBOR decoders that - enforce validity (Section 5.4). + error. When processing maps that exhibit entries with duplicate + keys, a generic decoder might do one of the following: + + * Not accept maps duplicate keys (that is, enforce validity for + maps, see also Section 5.4). These generic decoders are + universally useful. An application may still need to do perform + its own duplicate checking based on application rules (for + instance if the application equates integers and floating point + values in map key positions for specific maps). + + * Pass all map entries to the application, including ones with + duplicate keys. This requires the application to handle (check + against) duplicate keys, even if the application rules are + identical to the generic data model rules. + + * Lose some entries with duplicate keys, e.g. by only delivering the + final (or first) entry out of the entries with the same key. With + such a generic decoder, applications may get different results for + a specific key on different runs and with different generic + decoders as which value is returned is based on generic decoder + implementation and the actual order of keys in the map. In + particular, applications cannot validate key uniqueness on their + own as they do not necessarily see all entries; they may not be + able to use such a generic decoder if they do need to validate key + uniqueness. These generic decoders can only be used in situations + where the data source and transfer can be relied upon to always + provide valid maps; this is not possible if the data source and + transfer can be attacked. + + Generic decoders need to document which of these three approaches + they implement. The CBOR data model for maps does not allow ascribing semantics to the order of the key/value pairs in the map representation. Thus, a CBOR-based protocol MUST NOT specify that changing the key/value pair order in a map would change the semantics, except to specify that - some, orders are disallowed, for example where they would not meet - the requirements of a deterministic encoding (Section 4.2). (Any + some orders are disallowed, for example where they would not meet the + requirements of a deterministic encoding (Section 4.2). (Any secondary effects of map ordering such as on timing, cache usage, and other potential side channels are not considered part of the - semantics but may be enough reason on its own for a protocol to + semantics but may be enough reason on their own for a protocol to require a deterministic encoding format.) Applications for constrained devices that have maps where a small number of frequently used keys can be identified should consider using small integers as keys; for instance, a set of 24 or fewer frequent keys can be encoded in a single byte as unsigned integers, up to 48 if negative integers are also used. Less frequently occurring keys can then use integers with longer encodings. 5.6.1. Equivalence of Keys @@ -1710,21 +1849,21 @@ purpose of map key equivalence, NaN (not a number) values are equivalent if they have the same significand after zero-extending both significands at the right to 64 bits. (Byte and text) strings are compared byte by byte, arrays element by element, and are equal if they have the same number of bytes/elements and the same values at the same positions. Two maps are equal if they have the same set of pairs regardless of their order; pairs are equal if both the key and value are equal. - Tagged values are equal if both the tag number and the enclosed item + Tagged values are equal if both the tag number and the tag content are equal. (Note that a generic decoder that provides processing for a specific tag may not be able to distinguish some semantically equivalent values, e.g. if leading zeroes occur in the content of tag 2/3 (Section 3.4.3).) Simple values are equal if they simply have the same value. Nothing else is equal in the generic data model, a simple value 2 is not equivalent to an integer 2 and an array is never equivalent to a map. As discussed in Section 2.2, specific data models can make values equivalent for the purpose of comparing map keys that are distinct in @@ -1805,23 +1944,22 @@ * A bignum (major type 6, tag number 2 or 3) is represented by encoding its byte string in base64url without padding and becomes a JSON string. For tag number 3 (negative bignum), a "~" (ASCII tilde) is inserted before the base-encoded value. (The conversion to a binary blob instead of a number is to prevent a likely numeric overflow for the JSON decoder.) * A byte string with an encoding hint (major type 6, tag number 21 through 23) is encoded as described and becomes a JSON string. - * For all other tags (major type 6, any other tag number), the - enclosed CBOR item is represented as a JSON value; the tag number - is ignored. + * For all other tags (major type 6, any other tag number), the tag + content is represented as a JSON value; the tag number is ignored. * Indefinite-length items are made definite before conversion. 6.2. Converting from JSON to CBOR All JSON values, once decoded, directly map into one or more CBOR values. As with any kind of CBOR generation, decisions have to be made with respect to number representation. In a suggested conversion: @@ -1909,69 +2047,69 @@ been allocated. Implementations receiving an unknown simple data item may be able to process it as such, given that the structure of the value is indeed simple. The IANA registry in Section 9.1 is the appropriate way to address the extensibility of this codepoint space. * the "tag" space (values in major type 6). Again, only a small part of the codepoint space has been allocated, and the space is abundant (although the early numbers are more efficient than the later ones). Implementations receiving an unknown tag number can - choose to simply ignore it or to process it as an unknown tag - number wrapping the enclosed data item. The IANA registry in - Section 9.2 is the appropriate way to address the extensibility of - this codepoint space. + choose to simply ignore it (process just the enclosed tag content) + or to process it as an unknown tag number wrapping the tag + content. The IANA registry in Section 9.2 is the appropriate way + to address the extensibility of this codepoint space. * the "additional information" space. An implementation receiving an unknown additional information value has no way to continue decoding, so allocating codepoints to this space is a major step. - There are also very few codepoints left. + There are also very few codepoints left. See also Section 7.2. 7.2. Curating the Additional Information Space The human mind is sometimes drawn to filling in little perceived gaps to make something neat. We expect the remaining gaps in the codepoint space for the additional information values to be an attractor for new ideas, just because they are there. The present specification does not manage the additional information codepoint space by an IANA registry. Instead, allocations out of this space can only be done by updating this specification. For an additional information value of n >= 24, the size of the additional data typically is 2**(n-24) bytes. Therefore, additional information values 28 and 29 should be viewed as candidates for 128-bit and 256-bit quantities, in case a need arises to add them to the protocol. Additional information value 30 is then the only additional information value available for general allocation, and there should be a very good reason for allocating it before assigning - it through an update of this protocol. + it through an update of the present specification. 8. 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 8259, extending it where needed. The notation borrows the JSON syntax for numbers (integer and - floating point), True (>true<), False (>false<), Null (>null<), UTF-8 + 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 tag is written as an integer number for the tag number, followed by the tag content in parentheses; for instance, an RFC 3339 (ISO 8601) date could be notated as: @@ -1985,81 +2123,85 @@ 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. + A number of useful extensions to the diagnostic notation defined here + are provided in Appendix G of [RFC8610], "Extended Diagnostic + Notation" (EDN). + 8.1. Encoding Indicators Sometimes it is useful to indicate in the diagnostic notation which of several alternative representations were actually used; for example, a data item written >1.5< by a diagnostic decoder might have been encoded as a half-, single-, or double-precision float. The convention for encoding indicators is that anything starting with an underscore and all following characters that are alphanumeric or underscore, is an encoding indicator, and can be ignored by anyone - not interested in this information. Encoding indicators are always - optional. + not interested in this information. For example, "_" or "_3". + 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"). + Byte and text strings of indefinite length can be notated in the form + (_ h'0123', h'4567') and (_ "foo", "bar"). 9. 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 [RFC8126]. IANA has also assigned a new MIME media type and an associated Constrained Application Protocol (CoAP) Content-Format entry. [To be removed by RFC editor:] IANA is requested to update these registries to point to the present document instead of RFC 7049. 9.1. Simple Values Registry IANA has created the "Concise Binary Object Representation (CBOR) Simple Values" registry at [IANA.cbor-simple-values]. The initial - values are shown in Table 3. + values are shown in Table 4. New entries in the range 0 to 19 are assigned by Standards Action. It is suggested that these Standards Actions allocate values starting with the number 16 in order to reserve the lower numbers for contiguous blocks (if any). New entries in the range 32 to 255 are assigned by Specification Required. 9.2. Tags Registry IANA has created the "Concise Binary Object Representation (CBOR) Tags" registry at [IANA.cbor-tags]. The tags that were defined in - [RFC7049] are described in detail in Section 3.4, but other tags have + [RFC7049] are described in detail in Section 3.4, and other tags have already been defined. New entries in the range 0 to 23 are assigned by Standards Action. New entries in the range 24 to 255 are assigned by Specification Required. New entries in the range 256 to 18446744073709551615 are assigned by First Come First Served. The template for registration requests is: * Data item @@ -2068,28 +2210,27 @@ In addition, First Come First Served requests should include: * Point of contact * Description of semantics (URL) - This description is optional; the URL can point to something like an Internet-Draft or a web page. 9.3. Media Type ("MIME Type") The Internet media type [RFC6838] for a single encoded CBOR data item - is application/cbor. + is application/cbor, as defined in [IANA.media-types]: Type name: application Subtype name: cbor Required parameters: n/a - Optional parameters: n/a Encoding considerations: binary Security considerations: See Section 10 of this document Interoperability considerations: n/a Published specification: This document @@ -2089,87 +2230,105 @@ Security considerations: See Section 10 of this document Interoperability considerations: n/a Published specification: This document Applications that use this media type: None yet, but it is expected that this format will be deployed in protocols and applications. - Additional information: - Magic number(s): n/a - File extension(s): .cbor - Macintosh file type code(s): n/a + Additional information: * Magic number(s): n/a - Person & email address to contact for further information: - Carsten Bormann - cabo@tzi.org + * File extension(s): .cbor + + * Macintosh file type code(s): n/a + + Person & email address to contact for further information: IETF CBOR + Working Group cbor@ietf.org (mailto:cbor@ietf.org) or IETF + Applications and Real-Time Area art@ietf.org (mailto:art@ietf.org) Intended usage: COMMON Restrictions on usage: none - Author: - Carsten Bormann + Author: IETF CBOR Working Group cbor@ietf.org (mailto:cbor@ietf.org) - Change controller: - The IESG + Change controller: The IESG iesg@ietf.org (mailto:iesg@ietf.org) 9.4. CoAP Content-Format + The CoAP Content-Format for CBOR is defined in + [IANA.core-parameters]: + Media Type: application/cbor Encoding: - + Id: 60 Reference: [RFCthis] 9.5. The +cbor Structured Syntax Suffix Registration + The Structured Syntax Suffix [RFC6838] for media types based on a + single encoded CBOR data item is +cbor, as defined in + [IANA.media-type-structured-suffix]: + Name: Concise Binary Object Representation (CBOR) +suffix: +cbor References: [RFCthis] Encoding Considerations: CBOR is a binary format. Interoperability Considerations: n/a - Fragment Identifier Considerations: - The syntax and semantics of fragment identifiers specified for - +cbor SHOULD be as specified for "application/cbor". (At - publication of this document, there is no fragment identification - syntax defined for "application/cbor".) + 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 + resolves per the +cbor rules, then process as specified in + +cbor. - For cases defined in +cbor, where the fragment identifier does + * 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 + * For cases not defined in +cbor, then process as specified in "xxx/yyy+cbor". Security Considerations: See Section 10 of this document - Contact: - Apps Area Working Group (apps-discuss@ietf.org) + Contact: IETF CBOR Working Group cbor@ietf.org + (mailto:cbor@ietf.org) or IETF Applications and Real-Time Area + art@ietf.org (mailto:art@ietf.org) - Author/Change Controller: - The Apps Area Working Group. - The IESG has change control over this registration. + Author/Change Controller: The IESG iesg@ietf.org + (mailto:iesg@ietf.org) + // Editors' note: RFC 6838 has a template + field Author/Change + // controller, the descriptive text of + which makes clear that this is + // the change controller, not the author. + Go figure. There is no + // separate author entry as in the media + types registry. (RFC + // editor: Please remove this note before + publication.) 10. 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. @@ -2316,35 +2475,43 @@ [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, . - [I-D.ietf-cbor-sequence] - Bormann, C., "Concise Binary Object Representation (CBOR) - Sequences", Work in Progress, Internet-Draft, draft-ietf- - cbor-sequence-02, 25 September 2019, . - [IANA.cbor-simple-values] IANA, "Concise Binary Object Representation (CBOR) Simple Values", . [IANA.cbor-tags] IANA, "Concise Binary Object Representation (CBOR) Tags", . + [IANA.core-parameters] + IANA, "Constrained RESTful Environments (CoRE) + Parameters", + . + + [IANA.media-type-structured-suffix] + IANA, "Structured Syntax Suffix Registry", + . + + [IANA.media-types] + IANA, "Media Types", + . + [MessagePack] Furuhashi, S., "MessagePack", 2013, . [PCRE] Ho, A., "PCRE - Perl Compatible Regular Expressions", 2018, . [RFC0713] Haverty, J., "MSDTP-Message Services Data Transmission Protocol", RFC 713, DOI 10.17487/RFC0713, April 1976, . @@ -2359,30 +2526,54 @@ [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014, . [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, March 2015, . + [RFC7991] Hoffman, P., "The "xml2rfc" Version 3 Vocabulary", + RFC 7991, DOI 10.17487/RFC7991, December 2016, + . + [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, . + [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data + Definition Language (CDDL): A Notational Convention to + Express Concise Binary Object Representation (CBOR) and + JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, + June 2019, . + [RFC8618] Dickinson, J., Hague, J., Dickinson, S., Manderson, T., and J. Bond, "Compacted-DNS (C-DNS): A Format for DNS Packet Capture", RFC 8618, DOI 10.17487/RFC8618, September 2019, . + [RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR) + Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020, + . + + [RFC8746] Bormann, C., Ed., "Concise Binary Object Representation + (CBOR) Tags for Typed Arrays", RFC 8746, + DOI 10.17487/RFC8746, February 2020, + . + + [rfc8746] Bormann, C., Ed., "Concise Binary Object Representation + (CBOR) Tags for Typed Arrays", RFC 8746, + DOI 10.17487/RFC8746, February 2020, + . + [SIPHASH] Aumasson, J. and D. Bernstein, "SipHash: A Fast Short- Input PRF", DOI 10.1007/978-3-642-34931-7_28, Lecture Notes in Computer Science pp. 489-508, 2012, . [YAML] Ben-Kiki, O., Evans, C., and I.d. Net, "YAML Ain't Markup Language (YAML[TM]) Version 1.2", 3rd Edition, October 2009, . Appendix A. Examples @@ -2572,21 +2763,21 @@ | 17, 18, 19, 20, 21, 22, 23, | | | 24, 25] | | +------------------------------+------------------------------------+ | {_ "a": 1, "b": [_ 2, 3]} | 0xbf61610161629f0203ffff | +------------------------------+------------------------------------+ | ["a", {_ "b": "c"}] | 0x826161bf61626163ff | +------------------------------+------------------------------------+ | {_ "Fun": true, "Amt": -2} | 0xbf6346756ef563416d7421ff | +------------------------------+------------------------------------+ - Table 5: Examples of Encoded CBOR Data Items + Table 6: 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 | @@ -2723,28 +2914,29 @@ +------------+------------------------------------------------+ | 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 6: Jump Table for Initial Byte + Table 7: 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: * the pseudocode does not "fail"; + * after execution of the pseudocode, no bytes are left in the input (except in streaming applications) The pseudocode has the following prerequisites: * 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. * uint() converts a byte string into an unsigned integer by interpreting the byte string in network byte order. @@ -2965,21 +3157,21 @@ Message Services Data Transmission (MSDTP) is a very early example of a compact message format; it is described in [RFC0713], written in 1976. It is included here for its historical value, not because it was ever widely used. E.5. Conciseness on the Wire While CBOR's design objective of code compactness for encoders and decoders is a higher priority than its objective of conciseness on - the wire, many people focus on the wire size. Table 7 shows some + the wire, many people focus on the wire size. Table 8 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. +-------------+----------------------------+----------------+ | 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 01 | 30 80 02 01 01 | @@ -2990,61 +3182,111 @@ +-------------+----------------------------+----------------+ | 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 | | +-------------+----------------------------+----------------+ | CBOR | 82 01 82 02 03 | 9f 01 82 02 03 | | | | ff | +-------------+----------------------------+----------------+ - Table 7: Examples for Different Levels of Conciseness + Table 8: Examples for Different Levels of Conciseness Appendix F. Changes from RFC 7049 The following is a list of known changes from RFC 7049. This list is non-authoritative. It is meant to help reviewers see the significant differences. - * Updated reference for [RFC4627] to [RFC8259] in many places + * Made some use of new RFCXML functionality [RFC7991] - * Updated reference for [CNN-TERMS] to [RFC7228] + * Updated references, e.g. for [RFC4627] to [RFC8259] in many + places, for [CNN-TERMS] to [RFC7228]; added missing reference to + [IEEE754] and updated to [ECMA262] - * Added a comment to the last example in Section 2.2.1 (added + * Fixed errata: in the example in Section 2.4.2 ("29" -> "49"), and + in the last paragraph of Section 3.6 ("0b000_11101" -> + "0b000_11001") + + * Added a comment to the last example in Section 3.2.2 (added "Second value") - * Fixed a bug in the example in Section 2.4.2 ("29" -> "49") + * Applied numerous small editorial changes - * Fixed a bug in the last paragraph of Section 3.6 ("0b000_11101" -> - "0b000_11001") + * Added a few tables for illustration + + * More stringently used terminology for well-formed and valid data, + avoiding less well-defined alternative terms such as "syntax + error", "decoding error" and "strict mode" outside examples + + * Streamlined terminology to talk about tags, tag numbers, and tag + content + + * Clarified the restrictions on tag content, in general and + specifically for tag 1 + + * Added text about the CBOR data model and its small variations + (basic generic, extended generic, specific) + + * More clearly separated integers from floating-point values; + provided a suggestion (based on I-JSON [RFC7493]) for handling + these types when converting JSON to CBOR + + * Added term "preferred serialization" and defined it for various + kinds of data items + + * Added comment about tags with semantics that depend on + serialization order + + * Defined "deterministic encoding", making use of "preferred + serialization", and simplified the suggested map ordering for the + "Core Deterministic Encoding Requirements", easing implementation, + while keeping RFC 7049 map ordering as an alternative "length- + first map key ordering"; now avoiding the terms "canonical" and + "canonicalization" + + * Clarified map validity (handling of duplicate keys) and explained + the domain of applicability of certain implementation choices + + * Updated IANA considerations + + * Added security considerations + + * Clarified handling of non-well-formed simple values in text and + pseudocode + + * Added Appendix G, well-formedness errors and examples + + * Removed UBJSON from Appendix E, as that format has completely + changed since RFC 7049; added reference to [RFC8618] Appendix G. Well-formedness errors and examples There are three basic kinds of well-formedness errors that can occur in decoding a CBOR data item: * Too much data: There are input bytes left that were not consumed. This is only an error if the application assumed that the input bytes would span exactly one data item. Where the application uses the self-delimiting nature of CBOR encoding to permit additional data after the data item, as is for example done in - CBOR sequences [I-D.ietf-cbor-sequence], the CBOR decoder can - simply indicate what part of the input has not been consumed. + CBOR sequences [RFC8742], the CBOR decoder can simply indicate + what part of the input has not been consumed. * Too little data: The input data available would need additional bytes added at their end for a complete CBOR data item. This may indicate the input is truncated; it is also a common error when trying to decode random data as CBOR. For some applications - however, this may not be actually be an error, as the application - may not be certain it has all the data yet and can obtain or wait - for additional input bytes. Some of these applications may have - an upper limit for how much additional data can show up; here the + however, this may not actually be an error, as the application may + not be certain it has all the data yet and can obtain or wait for + additional input bytes. Some of these applications may have an + upper limit for how much additional data can show up; here the decoder may be able to indicate that the encoded CBOR data item cannot be completed within this limit. * Syntax error: The input data are not consistent with the requirements of the CBOR encoding, and this cannot be remedied by adding (or removing) data at the end. In Appendix C, errors of the first kind are addressed in the first paragraph/bullet list (requiring "no bytes are left"), and errors of the second kind are addressed in the second paragraph/bullet list @@ -3091,20 +3333,22 @@ 00 00, fb 00 00 00 * Definite length strings with short data: 41, 61, 5a ff ff ff ff 00, 5b ff ff ff ff ff ff ff ff 01 02 03, 7a ff ff ff ff 00, 7b 7f ff ff ff ff ff ff ff 01 02 03 * Definite length maps and arrays not closed with enough items: 81, 81 81 81 81 81 81 81 81 81, 82 00, a1, a2 01 02, a1 00, a2 00 00 00 + * Tag number not followed by tag content: c0 + * Indefinite length strings not closed by a break stop code: 5f 41 00, 7f 61 00 * Indefinite length maps and arrays not closed by a break stop code: 9f, 9f 01 02, bf, bf 01 02 01 02, 81 9f, 9f 80 00, 9f 9f 9f 9f 9f ff ff ff ff, 9f 81 9f 81 9f 9f ff ff ff A few examples for the five subkinds of well-formedness error kind 3 (syntax error) are shown below.