Internet DRAFT - draft-bormann-cbor-sequence
draft-bormann-cbor-sequence
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track June 23, 2019
Expires: December 25, 2019
Concise Binary Object Representation (CBOR) Sequences
draft-bormann-cbor-sequence-01
Abstract
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
Sequences.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 25, 2019.
Copyright Notice
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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. Optimizing CBOR Sequences for Skipping Elements . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6.1. Media Type . . . . . . . . . . . . . . . . . . . . . . . 5
6.2. CoAP Content-Format Registration . . . . . . . . . . . . 6
6.3. Structured Syntax Suffix . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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. Media type structured syntax
suffixes [RFC6838] were introduced as a way for a media type to
signal that it is based on another media type as its foundation.
CBOR [RFC7049] defines the "+cbor" structured syntax suffix. This
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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.
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
representation.)
This specification makes use of the fact that CBOR data items are
self-delimiting (there is no delimiter used between items, as there
is in JSON Text Sequences [RFC7464]).
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.
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 decoding the rest of the bytes as a
CBOR sequence. (A streaming decoder would therefore simply
deliver a sequence of CBOR data model values, each of which as
soon as the bytes making it up are available.)
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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
specification.)
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
received.
3. The "+cbor-seq" Structured Syntax Suffix
The use case for the "+cbor-seq" structured syntax suffix is the same
as 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:
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:
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; 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
specification.)
4.2. 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:
; 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.
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.media-types]. IANA is requested to register the MIME media
type for CBOR Sequence, application/cbor-seq, as follows:
Type name: application
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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.
Applications that use this media type: Data serialization and
deserialization.
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:
cbor@ietf.org
Intended usage: COMMON
Author: Carsten Bormann (cabo@tzi.org)
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.
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+----------------------+----------+-------+-----------+
| 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.media-type-structured-suffix]. IANA is requested to register
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
"application/cbor-seq".)
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.
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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
Contact: CBOR WG mailing list (cbor@ietf.org), or any IESG-
designated successor.
Author/Change controller: IETF
7. References
7.1. Normative References
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<http://www.iana.org/assignments/core-parameters>.
[IANA.media-type-structured-suffix]
IANA, "Structured Syntax Suffix Registry",
<http://www.iana.org/assignments/
media-type-structured-suffix>.
[IANA.media-types]
IANA, "Media Types",
<http://www.iana.org/assignments/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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7464] Williams, N., "JavaScript Object Notation (JSON) Text
Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,
<https://www.rfc-editor.org/info/rfc7464>.
[RFC8091] Wilde, E., "A Media Type Structured Syntax Suffix for JSON
Text Sequences", RFC 8091, DOI 10.17487/RFC8091, February
2017, <https://www.rfc-editor.org/info/rfc8091>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[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, <https://www.rfc-editor.org/info/rfc8610>.
Acknowledgements
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.
Author's Address
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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