Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                      September 25, 2019
Expires: March 28, 2020

         Concise Binary Object Representation (CBOR) Sequences


   This document describes the Concise Binary Object Representation
   (CBOR) Sequence format and associated media type "application/cbor-
   seq".  A CBOR Sequence consists of any number of encoded CBOR data
   items, simply concatenated in sequence.

   Structured syntax suffixes for media types allow other media types to
   build on them and make it explicit that they are built on an existing
   media type as their foundation.  This specification defines and
   registers "+cbor-seq" as a structured syntax suffix for CBOR

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 28, 2020.

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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
   2.  CBOR Sequence Format  . . . . . . . . . . . . . . . . . . . .   3
   3.  The "+cbor-seq" Structured Syntax Suffix  . . . . . . . . . .   4
   4.  Practical Considerations  . . . . . . . . . . . . . . . . . .   4
     4.1.  Specifying CBOR Sequences in CDDL . . . . . . . . . . . .   4
     4.2.  Diagnostic Notation . . . . . . . . . . . . . . . . . . .   5
     4.3.  Optimizing CBOR Sequences for Skipping Elements . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Media Type  . . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  CoAP Content-Format Registration  . . . . . . . . . . . .   7
     6.3.  Structured Syntax Suffix  . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Concise Binary Object Representation (CBOR) [RFC7049] can be used
   for serialization of data in the JSON [RFC8259] data model or in its
   own, somewhat expanded data model.  When serializing a sequence of
   such values, it is sometimes convenient to have a format where these
   sequences can simply be concatenated to obtain a serialization of the
   concatenated sequence of values, or to encode a sequence of values
   that might grow at the end by just appending further CBOR data items.

   This document describes the concept and format of "CBOR Sequences",
   which are composed of zero or more encoded CBOR data items.  CBOR
   Sequences can be consumed (and produced) incrementally without
   requiring a streaming CBOR parser that is able to deliver
   substructures of a data item incrementally (or a streaming encoder
   able to encode from substructures incrementally).

   This document defines and registers the "application/cbor-seq" media
   type in the media type registry, along with a CoAP Content-Format
   identifier.  Media type structured syntax suffixes [RFC6838] were
   introduced as a way for a media type to signal that it is based on

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   another media type as its foundation.  CBOR [RFC7049] defines the
   "+cbor" structured syntax suffix.  This document defines and
   registers the "+cbor-seq" structured syntax suffix in the "Structured
   Syntax Suffix Registry".

1.1.  Conventions Used in This Document

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

   In this specification, the term "byte" is used in its now-customary
   sense as a synonym for "octet".

2.  CBOR Sequence Format

   Formally, a CBOR Sequence is a sequence of bytes that is recursively
   defined as either

   o  an empty (zero-length) sequence of bytes

   o  the sequence of bytes making up an encoded CBOR data item
      [RFC7049], followed by a CBOR Sequence.

   In short, concatenating zero or more encoded CBOR data items
   generates a CBOR Sequence.  (Consequently, concatenating zero or more
   CBOR Sequences also results in a CBOR Sequence.)

   There is no end of sequence indicator.  (If one is desired, CBOR-
   encoding an array of the CBOR data model values being encoded --
   employing either a definite or an indefinite length encoding -- as a
   single CBOR data item may actually be the more appropriate

   CBOR Sequences, unlike JSON Text Sequences [RFC7464], do not use a
   marker between items.  This is possible because CBOR encoded data
   items are self-delimiting and the end can always be calculated.
   (Note that, while the early object/array-only form of JSON was self-
   delimiting as well, this stopped being the case when simple values
   such as single numbers were made valid JSON documents.)

   Decoding a CBOR Sequence works as follows:

   o  If the CBOR Sequence is an empty sequence of bytes, the result is
      an empty sequence of CBOR data model values.

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   o  Otherwise, decode a single CBOR data item from the bytes of the
      CBOR sequence, and insert the resulting CBOR data model value at
      the start of the result of repeating this decoding process
      recursively with the remaining bytes.  (A streaming decoder would
      therefore simply deliver zero or more CBOR data model values, each
      as soon as the bytes making it up are available.)

   This means that if any data item in the sequence is not well-formed,
   it is not possible to reliably decode the rest of the sequence.  (An
   implementation may be able to recover from some errors in a sequence
   of bytes that is almost, but not entirely a well-formed encoded CBOR
   data item.  Handling malformed data is outside the scope of this

   This also means that the CBOR Sequence format can reliably detect
   truncation of the bytes making up the last CBOR data item in the
   sequence, but not entirely missing CBOR data items at the end.  A
   CBOR Sequence decoder that is used for consuming streaming CBOR
   Sequence data may simply pause for more data (e.g., by suspending and
   later resuming decoding) in case a truncated final item is being

3.  The "+cbor-seq" Structured Syntax Suffix

   The use case for the "+cbor-seq" structured syntax suffix is
   analogous to that for "+cbor": It SHOULD be used by a media type when
   parsing the bytes of the media type object as a CBOR Sequence leads
   to a meaningful result that is at least sometimes not just a single
   CBOR data item.  (Without the qualification at the end, this sentence
   is trivially true for any +cbor media type, which of course should
   continue to use the "+cbor" structured syntax suffix.)

   Applications encountering a "+cbor-seq" media type can then either
   simply use generic processing if all they need is a generic view of
   the CBOR Sequence, or they can use generic CBOR Sequence tools for
   initial parsing and then implement their own specific processing on
   top of that generic parsing tool.

4.  Practical Considerations

4.1.  Specifying CBOR Sequences in CDDL

   In CDDL [RFC8610], CBOR sequences are already supported as contents
   of byte strings using the ".cborseq" control operator (Section 3.8.4
   of [RFC8610]), by employing an array as the controller type:

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   my-embedded-cbor-seq = bytes .cborseq my-array
   my-array = [* my-element]
   my-element = my-foo / my-bar

   CDDL currently does not provide for unadorned CBOR sequences as a
   top-level subject of a specification.  For now, the suggestion is to
   use an array, as for the ".cborseq" control operator, for the top-
   level rule and add English text that explains that the specification
   is really about a CBOR sequence with the elements of the array:

   ; This defines an array, the elements of which are to be used
   ; in a CBOR sequence:
   my-sequence = [* my-element]
   my-element = my-foo / my-bar

   (Future versions of CDDL may provide a notation for top-level CBOR
   sequences, e.g. by using a group as the top-level rule in a CDDL

4.2.  Diagnostic Notation

   CBOR diagnostic notation (see Section 6 of [RFC7049]) or extended
   diagnostic notation (Appendix G of [RFC8610]) also does not provide
   for unadorned CBOR Sequences at this time (the latter does provide
   for CBOR Sequences embedded in a byte string in Appendix G.3 of

   In a similar spirit to the recommendation for CDDL above, this
   specification recommends enclosing the CBOR data items in an array.
   In a more informal setting, where the boundaries within which the
   notation is used are obvious, it is also possible to leave off the
   outer brackets for this array, as shown in these two examples:

   [1, 2, 3]

   1, 2, 3

   Note that it is somewhat difficult to discuss zero-length CBOR
   Sequences in the latter form.

4.3.  Optimizing CBOR Sequences for Skipping Elements

   In certain applications, being able to efficiently skip an element
   without the need for decoding its substructure, or efficiently
   fanning out elements to multi-threaded decoding processes, is of the
   utmost importance.  For these applications, byte strings (which carry
   length information in bytes) containing embedded CBOR can be used as
   the elements of a CBOR sequence:

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   ; This defines an array of CBOR byte strings, the elements of which
   ; are to be used in a CBOR sequence:
   my-sequence = [* my-element]
   my-element = bytes .cbor my-element-structure
   my-element-structure = my-foo / my-bar

   Within limits, this may also enable recovering from elements that
   internally are not well-formed -- the limitation is that the sequence
   of byte strings does need to be well-formed as such.

5.  Security Considerations

   The security considerations of CBOR [RFC7049] apply.  This format
   provides no cryptographic integrity protection of any kind, but can
   be combined with security specifications such as COSE [RFC8152] to do
   so.  (COSE protections can be applied to an entire CBOR sequence or
   to each of the elements of the sequence independently; in the latter
   case, additional effort may be required if there is a need to protect
   the relationship of the elements in the sequence.)

   As usual, decoders must operate on input that is assumed to be
   untrusted.  This means that decoders MUST fail gracefully in the face
   of malicious inputs.

6.  IANA Considerations

6.1.  Media Type

   Media types are registered in the media types registry
   [].  IANA is requested to register the MIME media
   type for CBOR Sequence, application/cbor-seq, as follows:

   Type name: application

   Subtype name: cbor-seq

   Required parameters: N/A

   Optional parameters: N/A

   Encoding considerations: binary

   Security considerations: See RFCthis, Section 5.

   Interoperability considerations: Described herein.

   Published specification: RFCthis.

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   Applications that use this media type: Data serialization and

   Fragment identifier considerations: N/A

   Additional information:

   o  Deprecated alias names for this type: N/A

   o  Magic number(s): N/A

   o  File extension(s): N/A

   o  Macintosh file type code(s): N/A

   Person & email address to contact for further information:

   Intended usage: COMMON

   Author: Carsten Bormann (

   Change controller: IETF

6.2.  CoAP Content-Format Registration

   IANA is requested to assign a CoAP Content-Format ID for the media
   type "application/cbor-seq", in the CoAP Content-Formats subregistry
   of the core-parameter registry [IANA.core-parameters], from the
   "Expert Review" (0-255) range.  The assigned ID is shown in Table 1.

          | Media type           | Encoding | ID    | Reference |
          | application/cbor-seq | -        | TBD63 | RFCthis   |

                      Table 1: CoAP Content-Format ID

   RFC editor: Please replace TBD63 by the number actually assigned and
   delete this paragraph.

6.3.  Structured Syntax Suffix

   Structured Syntax Suffixes are registered within the "Structured
   Syntax Suffix Registry" maintained at
   [].  IANA is requested to register

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   the "+cbor-seq" structured syntax suffix in accordance with
   [RFC6838], as follows:

      Name: CBOR Sequence

      +suffix: +cbor-seq

      References: RFCthis

      Encoding considerations: binary

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

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

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

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

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

      Interoperability considerations: n/a

      Security considerations: See RFCthis, Section 5

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      Contact: CBOR WG mailing list (, or any IESG-
      designated successor.

      Author/Change controller: IETF

7.  References

7.1.  Normative References

              IANA, "Constrained RESTful Environments (CoRE)

              IANA, "Structured Syntax Suffix Registry",

              IANA, "Media Types",

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

7.2.  Informative References

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

   [RFC7464]  Williams, N., "JavaScript Object Notation (JSON) Text
              Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,

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   [RFC8091]  Wilde, E., "A Media Type Structured Syntax Suffix for JSON
              Text Sequences", RFC 8091, DOI 10.17487/RFC8091, February
              2017, <>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,

   [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, <>.


   This draft has mostly been generated from [RFC7464] by Nico Williams
   and [RFC8091] by Erik Wilde, which do a similar, but slightly more
   complicated exercise for JSON [RFC8259].  Laurence Lundblade raised
   an issue on the CBOR mailing list that pointed out the need for this
   document.  Jim Schaad and John Mattsson provided helpful comments.

Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921

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