CoRE Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational March 18, 2018
Expires: September 19, 2018
The application/maybe media type
draft-bormann-core-maybe-00
Abstract
Many media types may be used in situations where it may beneficial to
indicate that the object represented in this media type is not yet
(or no longer) present.
The Observe option introduced in Observing Resources in the
Constrained Application Protocol (CoAP) (RFC7641) requires sequences
of responses (notifications) to carry the same Content-Format.
The application/maybe media type provides a way to use a single media
type (and thus Content-Format) to express presence or absence of
information in a specific media type.
Status of This Memo
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This Internet-Draft will expire on September 19, 2018.
Copyright Notice
Copyright (c) 2018 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. The application/maybe media type . . . . . . . . . . . . . . 2
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Implementation hints . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
(See Abstract.)
1.1. Terminology
This memo uses terms from [RFC7252], [RFC7641] and [RFC7049].
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. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
The term "byte", abbreviated by "B", is used in its now customary
sense as a synonym for "octet".
2. The application/maybe media type
An application/maybe object either indicates the absence of the
underlying media type or its presence together with an object of that
media type.
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This is represented by a CBOR [RFC7049] object structured as follows
(illustrated in CDDL [I-D.ietf-cbor-cddl]):
maybe = [
? (
content-format: uint,
content: bstr
)
]
3. Discussion
The position for indicating a content-format could also provide an
alternative for indicating a media type represented as a string.
maybe1 = [
? (
(content-format: uint // media-type: tstr),
content: bstr
)
]
This would make the application/maybe media type easier to use with
media types that do not have a content-format registered yet.
4. Implementation hints
This section describes the serialization for readers that may be new
to CBOR. It does not contain any new information.
An absent object is represented by an empty CBOR array, which is
serialized as a single byte with the value 0x80.
A present object is represented by a two-element CBOR array, which is
serialized as 0x82 followed by the two elements. The first element
is an unsigned integer for the Content-Format value, which is
represented as described in Table 1. The second element is the
object as a byte string, which is represented as a length as
described in Table 2 followed by the bytes of the object.
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+----------------+------------+
| Serialization | Value |
+----------------+------------+
| 0x00..0x17 | 0..23 |
| | |
| 0x18 0xnn | 24..255 |
| | |
| 0x19 0xnn 0xnn | 256..66535 |
+----------------+------------+
Table 1: Serialization of content-format
+-----------------------------+-------------------+
| Serialization | Length |
+-----------------------------+-------------------+
| 0x40..0x57 | 0..23 |
| | |
| 0x58 0xnn | 24..255 |
| | |
| 0x59 0xnn 0xnn | 256..66535 |
| | |
| 0x5a 0xnn 0xnn 0xnn 0xnn | 66536..4294967295 |
| | |
| 0x5b 0xnn .. 0xnn (8 bytes) | 4294967296.. |
+-----------------------------+-------------------+
Table 2: Serialization of object length
For example, a present text/plain object (content-format 0) of value
"Hello World" (11 characters) would be serialized as
0x82 0x00 0x4b H e l l o 0x20 w o r l d
In effect, the serialization is done by prefixing the object with
information about its content-format.
5. IANA Considerations
TBD (add in the obvious template information).
6. Security Considerations
TBD
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7. Acknowledgements
The potential need for an application/maybe media type was suggested
by Klaus Hartke.
8. References
8.1. Normative References
[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>.
[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>.
8.2. Informative References
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-02
(work in progress), February 2018.
[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/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
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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