SenML Data Value Content-Format Indication
draft-ietf-core-senml-data-ct-07
Network Working Group A. Keränen
Internet-Draft Ericsson
Intended status: Standards Track C. Bormann
Expires: 24 April 2022 Universität Bremen TZI
21 October 2021
SenML Data Value Content-Format Indication
draft-ietf-core-senml-data-ct-07
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 facilitate processing of binary data
values, this document specifies a pair of new SenML fields for
indicating the content format of those binary data values, i.e.,
their Internet media type including parameters as well as any content
codings applied.
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 24 April 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Evolution . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SenML Content-Format ("ct") Field . . . . . . . . . . . . . . 5
4. SenML Base Content-Format ("bct") Field . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Sensor Measurement Lists (SenML) media types [RFC8428] can be
used to send various 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. The example is given in SenML JSON
representation, so the "vd" (data value) field is encoded as a
base64url string (without padding), as per Section 5 of [RFC8428].
[
{"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,
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especially if the pack is propagated over time and via multiple
entities. To facilitate automatic interpretation it is useful to be
able to indicate an Internet media type and, optionally, content
codings right in the SenML Record.
The CoAP Content-Format (Section 12.3 of [RFC7252]) provides this
information in the form of a single unsigned integer; enclosing a
Content-Format number (in this case number 60 as defined for content-
type application/cbor in [RFC8949]) in the Record is illustrated in
Figure 2. All registered CoAP Content-Format numbers are listed in
the COAP Content-Formats registry [IANA.core-parameters] as specified
by Section 12.3 of [RFC7252]. Note that, at the time of writing, the
structure of this registry only provides for zero or one content
codings; nothing in the present document needs to change if the
registry is extended to allow sequences of content codings.
{"n":"nfc-reader", "vd":"gmNmb28YKg", "ct":"60"}
Figure 2: SenML Record with binary data identified as CBOR
In this example SenML Record, the data value contains a string "foo"
and a number 42 encoded in a CBOR [RFC8949] array. Since the example
above uses the JSON format of SenML, the data value containing the
binary CBOR value is base64-encoded (Section 5 of [RFC4648]). 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
1.1. Evolution
As with SenML in general, there is no expectation that the creator of
a SenML pack knows (or has negotiated with) each consumer of that
pack, which may be very remote in space and particularly in time.
This means that the SenML creator in general has no way to know
whether the consumer knows:
* each specific media-type-name used
* each parameter and each parameter value used
* each content coding in use
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* each Content-Format number in use for a combination of these
What SenML, as well as the new fields defined here, guarantees is
that a recipient implementation _knows_ when it needs to be updated
to understand these field values and the values controlled by them;
registries are used to evolve these name spaces in a controlled way.
SenML packs can be processed by a consumer while not understanding
all the information in them, and information can generally be
preserved in this processing such that it is useful for further
consumers.
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.
Media Type: A registered label for representations (byte strings)
prepared for interchange, identified by a Media-Type-Name
[RFC1590], [RFC6838].
Media-Type-Name: A combination of a type-name and a subtype-name
registered in [IANA.media-types] as per [RFC6838], conventionally
identified by the two names separated by a slash.
Content-Type: A Media-Type-Name, optionally associated with
parameters (Section 5 of [RFC2045], separated from the Media-Type-
Name and from each other by a semicolon). In HTTP and many other
protocols, used in a Content-Type header field.
content coding: A name registered in the HTTP Content Coding
registry [IANA.http-parameters] as specified by Sections 16.6.1
and 18.6 of [I-D.ietf-httpbis-semantics], indicating an encoding
transformation with semantics further specified in Section 8.4.1
of [I-D.ietf-httpbis-semantics]. Confusingly, in HTTP, content
coding values are found in a header field called "Content-
Encoding", however "content coding" is the correct term for the
process and the registered values.
content format: the combination of a Content-Type and zero or more
content codings, identified by (1) a numeric identifier defined in
the COAP Content-Formats registry [IANA.core-parameters] as per
Section 12.3 of [RFC7252] (referred to as Content-Format number),
or (2) a Content-Format-String.
Content-Format-String: the string representation of the combination
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of a Content-Type and zero or more content codings.
Content-Format-Spec: the string representation of a content format;
either a Content-Format-String or the (decimal) string
representation of a Content-Format number.
Readers should also be familiar with the terms and concepts discussed
in [RFC8428].
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, using label "ct".
The value of this field is a Content-Format-Spec, i.e., one of:
* a CoAP Content-Format number in decimal form with no leading zeros
(except for the value "0" itself). This value represents an
unsigned integer in the range of 0-65535, similar to the "ct"
attribute defined in Section 7.2.1 of [RFC7252] for CoRE Link
Format [RFC6690]).
* or a Content-Format-String containing a Content-Type and zero or
more content codings (see below).
The syntax of this field is formally defined in Section 6.
The CoAP Content-Format number 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 numbers, using Content-Type and zero or more
content codings 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 number.
To indicate that one or more content codings are used with a Content-
Type, each of the content coding values is appended to the Content-
Type value (media type and parameters, if any), separated by a "@"
sign, in the order of the content codings were applied (the same
order as in Section 8.4 of [I-D.ietf-httpbis-semantics]). For
example (using a content coding value of "deflate" as defined in
Section 8.4.1.2 of [I-D.ietf-httpbis-semantics]):
text/plain; charset=utf-8@deflate
If no "@" sign is present after the media type and parameters, then
no content coding has been specified, and the "identity" content
coding is used -- no encoding transformation is employed.
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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. The process of resolving
(Section 4.6 of [RFC8428]) 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.
Figure 4 shows a variation of Figure 2 with multiple records, with
the "nfc-reader" records resolving to the base field value "60" and
the "iris-photo" record overriding this with the "image/png" media
type (actual data left out for brevity).
[
{"n":"nfc-reader", "vd":"gmNmb28YKg",
"bct":"60", "bt":1627430700},
{"n":"nfc-reader", "vd":"gmNiYXIYKw", "t":10},
{"n":"iris-photo", "vd":".....", "ct":"image/png", "t":10},
{"n":"nfc-reader", "vd":"gmNiYXoYLA", "t":20}
]
Figure 4: SenML pack with bct field
5. Examples
The following examples are valid values for the "ct" and "bct" fields
(explanation/comments in parentheses):
* "60" (CoAP Content-Format number for "application/cbor")
* "0" (CoAP Content-Format number for "text/plain" with parameter
"charset=utf-8")
* "application/json" (JSON Content-Type -- equivalent to "50" CoAP
Content-Format number)
* "application/json@deflate" (JSON Content-Type with "deflate" as
content coding -- equivalent to "11050" CoAP Content-Format
number)
* "application/json@deflate@aes128gcm" (JSON Content-Type with
"deflate" followed by "aes128gcm" as content codings)
* "text/csv" (Comma-Separated Values (CSV) [RFC4180] Content-Type)
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* "text/csv;header=present@gzip" (CSV with header row, using "gzip"
as content coding)
6. ABNF
This specification provides a formal definition of the syntax of
Content-Format-Spec strings using ABNF notation [RFC5234], which
contains three new rules and a number of rules collected and adapted
from various RFCs [I-D.ietf-httpbis-semantics] [RFC6838] [RFC5234]
[RFC8866].
; New in this document
Content-Format-Spec = Content-Format-Number / Content-Format-String
Content-Format-Number = "0" / (POS-DIGIT *DIGIT)
Content-Format-String = Content-Type *("@" Content-Coding)
; Cleaned up from [RFC-httpbis-semantics],
; leaving only SP as blank space,
; removing legacy 8-bit characters, and
; leaving the parameter as mandatory with each semicolon:
Content-Type = Media-Type-Name *( *SP ";" *SP parameter )
parameter = token "=" ( token / quoted-string )
token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA
quoted-string = %x22 *( qdtext / quoted-pair ) %x22
qdtext = SP / %x21 / %x23-5B / %x5D-7E
quoted-pair = "\" ( SP / VCHAR )
; Adapted from section 8.4.1 of [RFC-httpbis-semantics]
Content-Coding = token
; Adapted from various specs
Media-Type-Name = type-name "/" subtype-name
; RFC 6838
type-name = restricted-name
subtype-name = restricted-name
restricted-name = restricted-name-first *126restricted-name-chars
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restricted-name-first = ALPHA / DIGIT
restricted-name-chars = ALPHA / DIGIT / "!" / "#" /
"$" / "&" / "-" / "^" / "_"
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
; Boilerplate from RFC 5234 and RFC 8866
DIGIT = %x30-39 ; 0 – 9
POS-DIGIT = %x31-39 ; 1 – 9
ALPHA = %x41-5A / %x61-7A ; A – Z / a – z
SP = %x20
VCHAR = %x21-7E ; printable ASCII (no SP)
Figure 5: ABNF syntax of Content-Format-Spec
// RFC editor: Please replace [RFC-httpbis-semantics] by what gets
// published from [I-D.ietf-httpbis-semantics].
7. 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. This includes
"decompression bombs", compressed data that is crafted to decompress
to extremely large data items.
8. 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
Section 12.2 of [RFC8428]) for the Content-Format indication as per
Table 1:
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+=====================+=======+===========+==========+===========+
| 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
Note that as per Section 12.2 of [RFC8428], no CBOR labels or EXI
schemaId values (EXI ID column) are supplied.
9. References
9.1. Normative References
[I-D.ietf-httpbis-semantics]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://www.ietf.org/archive/id/draft-ietf-httpbis-
semantics-19.txt>.
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<https://www.iana.org/assignments/core-parameters>.
[IANA.http-parameters]
IANA, "Hypertext Transfer Protocol (HTTP) Parameters",
<https://www.iana.org/assignments/http-parameters>.
[IANA.media-types]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[IANA.senml]
IANA, "Sensor Measurement Lists (SenML)",
<https://www.iana.org/assignments/senml>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
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[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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[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>.
[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>.
9.2. Informative References
[RFC1590] Postel, J., "Media Type Registration Procedure", RFC 1590,
DOI 10.17487/RFC1590, March 1994,
<https://www.rfc-editor.org/info/rfc1590>.
[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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[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>.
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[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.
[RFC8949] 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/info/rfc8949>.
Acknowledgements
The authors would like to thank Sérgio 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. Alexey
Melnikov, Jim Schaad, and Thomas Fossati provided helpful comments at
Working-Group last call. Marco Tiloca asked for clarifying and using
the term Content-Format-Spec.
Authors' Addresses
Ari Keränen
Ericsson
FI-02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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