Network Working Group A. Keränen
Internet-Draft Ericsson
Intended status: Standards Track C. Bormann
Expires: 14 January 2021 Universität Bremen TZI
13 July 2020
SenML Data Value Content-Format Indication
draft-ietf-core-senml-data-ct-02
Abstract
The Sensor Measurement Lists (SenML) media type supports multiple
types of values, from numbers to text strings and arbitrary binary
data values. In order to simplify processing of the data values this
document proposes to specify a new SenML field for indicating the
Content-Format of the data.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SenML Content-Format ("ct") Field . . . . . . . . . . . . . . 3
4. SenML Base Content-Format ("bct") Field . . . . . . . . . . . 4
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The Sensor Measurement Lists (SenML) media type [RFC8428] can be used
to send various different kinds of data. In the example given in
Figure 1, a temperature value, an indication whether a lock is open,
and a data value (with SenML field "vd") read from an NFC reader is
sent in a single SenML pack.
[
{"bn":"urn:dev:ow:10e2073a01080063:","n":"temp","u":"Cel","v":7.1},
{"n":"open","vb":false},
{"n":"nfc-reader","vd":"aGkgCg"}
]
Figure 1: SenML pack with unidentified binary data
The receiver is expected to know how to interpret the data in the
"vd" field based on the context, e.g., name of the data source and
out-of-band knowledge of the application. However, this context may
not always be easily available to entities processing the SenML pack.
To facilitate automatic interpretation it is useful to be able to
indicate an Internet media type and content-coding right in the SenML
Record. The CoAP Content-Format (Section 12.3 in [RFC7252]) provides
just this information; enclosing a Content-Format number (in this
case number 60 as defined for content-type application/cbor in
[RFC7049]) in the Record is illustrated in Figure 2. All registered
CoAP Content-Formats are listed in the Content-Formats subregistry of
the CoRE Parameters registry [IANA.core-parameters].
{"n":"nfc-reader", "vd":"gmNmb28YKg", "ct":"60"}
Figure 2: SenML Record with binary data identified as CBOR
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In this example SenML Record the data value contains a string "foo"
and a number 42 encoded in a CBOR [RFC7049] array. Since the example
above uses the JSON format of SenML, the data value containing the
binary CBOR value is base64-encoded. The data value after base64
decoding is shown with CBOR diagnostic notation in Figure 3.
82 # array(2)
63 # text(3)
666F6F # "foo"
18 2A # unsigned(42)
Figure 3: Example Data Value in CBOR diagnostic notation
2. Terminology
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.
Readers should also be familiar with the terms and concepts discussed
in [RFC8428]. Awareness of terminology issues discussed in
[I-D.bormann-core-media-content-type-format] can also be very
helpful.
3. SenML Content-Format ("ct") Field
When a SenML Record contains a Data Value field ("vd"), the Record
MAY also include a Content-Format indication field. The Content-
Format indication uses label "ct" and a string value with either a
CoAP Content-Format identifier in decimal form with no leading zeros
except for the value "0" itself (representing an unsigned integer in
the range of 0-65535, similar to the CoRE Link Format [RFC6690] "ct"
attribute) or with a string containing a Content-Type and optionally
a Content-Coding (see below).
The CoAP Content-Format identifier provides a simple and efficient
way to indicate the type of the data. Since some Internet media
types and their content coding and parameter alternatives do not have
assigned CoAP Content-Format identifiers, using Content-Type and
Content-Coding is also allowed. Both methods use a string value in
the "ct" field to keep its data type consistent across uses. When
the "ct" field contains only digits, it is interpreted as a CoAP
Content-Format identifier.
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To indicate that a Content-Coding is used with a Content-Type, the
Content-Coding value (e.g., "deflate" [RFC7230]) is appended to the
Content Type (media type and parameters, if any), separated by a "@"
sign. For example: "text/plain; charset=utf-8@deflate". If Content-
Coding is not specified with a Content-Type (no "@" sign is present
outside any media type parameters), the identity (i.e., no)
transformation is used.
4. SenML Base Content-Format ("bct") Field
The Base Content-Format Field, label "bct", provides a default value
for the Content-Format Field (label "ct") within its range. The
range of the base field includes the Record containing it, up to (but
not including) the next Record containing a "bct" field, if any, or
up to the end of the pack otherwise. Resolution (Section 4.6 of
[RFC8428]) of this base field is performed by adding its value with
the label "ct" to all Records in this range that carry a "vd" field
but do not already contain a Content-Format ("ct") field.
5. Examples
The following examples are valid values for the "ct" and "bct" fields
(explanation/comments in parenthesis):
* "60" (CoAP Content-Format for "application/cbor")
* "0" (CoAP Content-Format for "text/plain" with parameter
"charset=utf-8")
* "application/json" (JSON Content-Type - equivalent to "50" CoAP
Content-Format identifier)
* "application/json@deflate" (JSON Content-Type with "deflate" as
Content-Coding - equivalent to "11050" CoAP Content-Format
identifier)
* "text/csv" (Comma-Separated Values (CSV) [RFC4180] Content-Type)
* "text/csv@gzip" (CSV with "gzip" as Content-Coding)
6. Security Considerations
The indication of a media type in the data does not exempt a
consuming application from properly checking its inputs. Also, the
ability for an attacker to supply crafted SenML data that specify
media types chosen by the attacker may expose vulnerabilities of
handlers for these media types to the attacker.
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7. IANA Considerations
(Note to RFC Editor: Please replace all occurrences of "RFC-AAAA"
with the RFC number of this specification and remove this note.)
IANA is requested to assign new labels in the "SenML Labels"
subregistry of the SenML registry [IANA.senml] (as defined in
[RFC8428]) for the Content-Format indication as per Table 1:
+=====================+=======+===========+==========+===========+
| Name | Label | JSON Type | XML Type | Reference |
+=====================+=======+===========+==========+===========+
| Base Content-Format | bct | String | string | RFC-AAAA |
+---------------------+-------+-----------+----------+-----------+
| Content-Format | ct | String | string | RFC-AAAA |
+---------------------+-------+-----------+----------+-----------+
Table 1: IANA Registration for new SenML Labels
8. References
8.1. Normative References
[IANA.senml]
IANA, "Sensor Measurement Lists (SenML)",
<http://www.iana.org/assignments/senml>.
[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>.
[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>.
[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>.
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[RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
DOI 10.17487/RFC8428, August 2018,
<https://www.rfc-editor.org/info/rfc8428>.
8.2. Informative References
[I-D.bormann-core-media-content-type-format]
Bormann, C., "On Media-Types, Content-Types, and related
terminology", Work in Progress, Internet-Draft, draft-
bormann-core-media-content-type-format-01, 8 July 2019,
<http://www.ietf.org/internet-drafts/draft-bormann-core-
media-content-type-format-01.txt>.
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<http://www.iana.org/assignments/core-parameters>.
[RFC4180] Shafranovich, Y., "Common Format and MIME Type for Comma-
Separated Values (CSV) Files", RFC 4180,
DOI 10.17487/RFC4180, October 2005,
<https://www.rfc-editor.org/info/rfc4180>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
Acknowledgements
The authors would like to thank Sergio Abreu for the discussions
leading to the design of this extension and Isaac Rivera for reviews
and feedback. Klaus Hartke suggested not burdening this draft with a
separate mandatory-to-implement version of the fields.
Authors' Addresses
Ari Keränen
Ericsson
FI-02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
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Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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