Internet DRAFT - draft-fossati-core-parametrized-cf
draft-fossati-core-parametrized-cf
Constrained RESTful Environments T. Fossati
Internet-Draft arm
Intended status: Standards Track H. Birkholz
Expires: 20 April 2023 Fraunhofer SIT
17 October 2022
Parametrized Content-Format for CoAP
draft-fossati-core-parametrized-cf-01
Abstract
This document specifies a "parametrized" CoAP Content-Format data
item that allows supplementing a Content-Format with additional media
type parameters.
This document also defines two new CoAP Options, Parmetrized-Content-
Format and Parametrized-Multi-Valued-Accept, that build upon the
"parametrized" Content-Format data item to work around some of the
limitations of the existing Accept and Content-Format Options.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://thomas-
fossati.github.io/draft-coap-parametrized-cf/draft-fossati-core-
parametrized-cf.html. Status information for this document may be
found at https://datatracker.ietf.org/doc/draft-fossati-core-
parametrized-cf/.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (mailto:core@ietf.org), which
is archived at https://mailarchive.ietf.org/arch/browse/core/.
Subscribe at https://www.ietf.org/mailman/listinfo/core/.
Source for this draft and an issue tracker can be found at
https://github.com/thomas-fossati/draft-coap-parametrized-cf.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Parametrized Content-Format . . . . . . . . . . . . . . . . . 3
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Parametrized Content-Format Option . . . . . . . . . . . . . 5
5. Parametrized Multi-Valued Accept Option . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
CoAP squashes the combination of a media type, media type parameters
and content coding into a single Content-Format number. (For an
example, see Table 2 in Section 2 of [STD96].) This number is
carried in the Content-Format and Accept Options.
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Such compression strategy is ideal in cases where the set of possible
parameters combinations is known upfront and has small cardinality.
However, it lacks the flexibility to deal smoothly with situations
where the number of combinations can grow unbounded.
An example is [I-D.lundblade-rats-eat-media-type], in which the
"profile" media type parameter can carry a number of different values
that are constantly minted through a loosely regulated process.
Another example is content negotiation of CoRAL [I-D.ietf-core-coral]
profiles.
To avoid the combinatorial explosion that derives from such premises,
this document defines the "parametrized" Content-Format data item
(Section 3) as a mechanism to enrich a given Content-Format with
additional media type parameters.
Two new CoAP Options that build upon such data item are also defined:
* Parametrized-Content-Format (Section 4)
* Parametrized-Multi-Valued-Accept (Section 5)
The latter also works around the limited content negotiation
capabilities of the CoAP Accept Option by allowing to accept more
than one Content-Format per request.
2. Conventions and Definitions
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 document, the structure of data is specified in CDDL
[RFC8610] [RFC9165].
The examples in Section 3.2 use CBOR diagnostic notation defined in
Section 8 of [STD94] and Appendix G of [RFC8610].
3. Parametrized Content-Format
The Parametrized Content-Format is a CBOR [STD94] data item defined
by the CDDL [RFC8610] in Figure 1.
The first element in the tuple is the Content-Format identifier,
followed by one or more name-value pairs representing the additional
media type parameters.
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parametrized-content-format = [
content-format,
+ [ parameter-name, parameter-value ]
]
content-format = 0..65535
parameter-name = textual / numeric
parameter-value = any
textual = text .abnf ("parameter-name" .det RFC6838-parameter-name)
numeric = int
RFC6838-parameter-name = '
parameter-name = restricted-name
restricted-name = restricted-name-first *126restricted-name-chars
restricted-name-first = ALPHA / DIGIT
restricted-name-chars = ALPHA / DIGIT / "!" / "#" /
"$" / "&" / "-" / "^" / "_"
restricted-name-chars =/ "." ; Characters before first dot always
; specify a facet name
restricted-name-chars =/ "+" ; Characters after last plus always
; specify a structured syntax suffix
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
'
Figure 1: CDDL for the Parametrized Content-Format
// TODO describe use of numeric identifiers for parameter name
// aliasing (requires a new registry).
3.1. Requirements
The list that follows details the semantic requirements that a
Parametrized Content-Format data item must satisfy:
* The intersection between the media parameters already encoded in
the Content-Format identifier and the set of parameters carried in
the name-value pairs of the Parametrized Content-Format MUST be
empty.
* Each name-value pair MUST be a registered parameter for the media
type.
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If any of the conditions listed above is not met, the entire data
item is considered invalid and MUST NOT be processed further.
3.2. Examples
[
65000,
[ "p1", "a-string-value" ],
[ "p2", 128 ]
]
Figure 2: Content-Format with two paramters
4. Parametrized Content-Format Option
+========+=+===+===+===+================+========+========+=========+
| Number |C| U | N | R | Name |Format | Length | Default |
+========+=+===+===+===+================+========+========+=========+
| TBD24 | | | | | Parametrized |See | | none |
| | | | | | Content-Format |Figure 3| | |
| | | | | | Option | | | |
+--------+-+---+---+---+----------------+--------+--------+---------+
Table 1: Parametrized Content-Format Option
The Parametrized Content-Format Option carries a CBOR-encoded
Parametrized Content-Format data item.
pcf-option-fmt = bytes .cbor parametrized-content-format
Figure 3: Parametrized Content-Format Option Format
The semantic is identical to the Content-Format Option described in
Section 5.10.3 of [RFC7252].
5. Parametrized Multi-Valued Accept Option
+========+===+===+===+===+==============+========+========+=========+
| Number | C | U | N | R |Name |Format | Length | Default |
+========+===+===+===+===+==============+========+========+=========+
| TBD13 | x | | | |Parametrized |See | | none |
| | | | | |Multi-Valued |Figure 4| | |
| | | | | |Accept Option | | | |
+--------+---+---+---+---+--------------+--------+--------+---------+
Table 2: Parametrized Multi-Valued Accept Option
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The Parametrized Multi-Valued Accept Option carries either a single
CBOR-encoded pa-content-format data item or two or more pa-content-
format items wrapped in a CBOR array. In turn, each pa-content-
format can be either a plain Content-Format or a Parametrized
Content-Format as described in Figure 4.
pa-content-format = content-format / parametrized-content-format
one-or-more<T> = T / [ 2* T ]
pmva-option-fmt = bytes .cbor one-or-more<pa-content-format>
Figure 4: Parametrized Multi-Valued Accept Option Format
The semantic is identical to the Accept Option described in
Section 5.10.4 of [RFC7252], except for the ability to list more than
one acceptable (parametrized) Content-Format, which is key to enable
finer-grained content negotiation.
The Content-Formats are listed in order of preference. If more than
one match is found, the entry with the lowest index in the array MUST
be selected.
6. Security Considerations
The security considerations in Section 11.1 of [RFC7252] related to
the parsing of protocol elements apply.
The security considerations in Section 11.3 of [RFC7252] related to
amplification risks apply.
TODO expand
7. IANA Considerations
// RFC Editor: please replace RFCthis with this RFC number and remove
// this note.
7.1. CoAP Option Numbers Registry
IANA is requested to add the entries from Table 3 to the CoAP Option
Numbers sub-registry of the Constrained RESTful Environments (CoRE)
Parameters [IANA.core-parameters] registry:
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+========+=========================================+============+
| Number | Name | Reference |
+========+=========================================+============+
| TBD13 | Parametrized Multi-Valued Accept Option | Section 5 |
| | | of RFCthis |
+--------+-----------------------------------------+------------+
| TBD24 | Parametrized Content-Format Option | Section 4 |
| | | of RFCthis |
+--------+-----------------------------------------+------------+
Table 3: New Options
This document suggests 13 (TBD13) and 24 (TBD24) as values to be
assigned for the new option numbers.
8. References
8.1. Normative References
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters", 8 June 2012,
<https://www.iana.org/assignments/core-parameters>.
[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/rfc/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/rfc/rfc7252>.
[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/rfc/rfc8174>.
[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/rfc/rfc8610>.
[RFC9165] Bormann, C., "Additional Control Operators for the Concise
Data Definition Language (CDDL)", RFC 9165,
DOI 10.17487/RFC9165, December 2021,
<https://www.rfc-editor.org/rfc/rfc9165>.
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[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
8.2. Informative References
[I-D.ietf-core-coral]
Amsüss, C. and T. Fossati, "The Constrained RESTful
Application Language (CoRAL)", Work in Progress, Internet-
Draft, draft-ietf-core-coral-05, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coral-05>.
[I-D.lundblade-rats-eat-media-type]
Lundblade, L., Birkholz, H., and T. Fossati, "EAT Media
Types", Work in Progress, Internet-Draft, draft-lundblade-
rats-eat-media-type-00, 26 May 2022,
<https://datatracker.ietf.org/doc/html/draft-lundblade-
rats-eat-media-type-00>.
[STD96] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
Acknowledgments
Thank you Carsten Bormann, Christian Amsüss, and Marco Tiloca for the
useful comments and suggestions.
Authors' Addresses
Thomas Fossati
arm
Email: thomas.fossati@arm.com
Henk Birkholz
Fraunhofer SIT
Email: henk.birkholz@sit.fraunhofer.de
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