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Keywords: CoAP, IoT, data model

Internet Engineering Task Force (IETF) A. Keränen
Request for Comments: 8790 Ericsson
Category: Standards Track M. Mohajer
ISSN: 2070-1721 June 2020
FETCH and PATCH with Sensor Measurement Lists (SenML)
Abstract
The Sensor Measurement Lists (SenML) media type and data model can be
used to send collections of resources, such as batches of sensor data
or configuration parameters. The Constrained Application Protocol
(CoAP) FETCH, PATCH, and iPATCH methods enable accessing and updating
parts of a resource or multiple resources with one request. This
document defines new media types for the CoAP FETCH, PATCH, and
iPATCH methods for resources represented using the SenML data model.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8790.
Copyright Notice
Copyright (c) 2020 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
(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 and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Using FETCH and (i)PATCH with SenML
3.1. SenML FETCH
3.2. SenML (i)PATCH
4. Fragment Identification
5. Extensibility
6. Security Considerations
7. IANA Considerations
7.1. CoAP Content-Format Registration
7.2. senml-etch+json Media Type
7.3. senml-etch+cbor Media Type
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The Sensor Measurement Lists (SenML) media type [RFC8428] and data
model can be used to transmit collections of resources, such as
batches of sensor data or configuration parameters.
An example of a SenML collection is shown below:
[
{"bn":"2001:db8::2/3311/0/", "n":"5850", "vb":true},
{"n":"5851", "v":42},
{"n":"5750", "vs":"Ceiling light"}
]
Here, three resources, "3311/0/5850", "3311/0/5851", and
"3311/0/5750", of a dimmable light smart object [IPSO] are
represented using a single SenML Pack with three SenML Records. All
resources share the same base name "2001:db8::2/3311/0/"; hence, full
names for the resources are "2001:db8::2/3311/0/5850", etc.
The CoAP [RFC7252] FETCH, PATCH, and iPATCH methods [RFC8132] enable
accessing and updating parts of a resource or multiple resources with
one request.
This document defines two new media types, one using the JavaScript
Object Notation (JSON) [RFC8259] and one using the Concise Binary
Object Representation (CBOR) [RFC7049], which can be used with the
CoAP FETCH, PATCH, and iPATCH methods for resources represented using
the SenML data model (i.e., for both SenML and Sensor Streaming
Measurement Lists (SenSML) data). The rest of the document uses the
term "(i)PATCH" when referring to both methods as the semantics of
the new media types are the same for the CoAP PATCH and iPATCH
methods.
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 [RFC8132] and [RFC8428]. The following additional terms are used
in this document:
Fetch Record: One set of parameters that is used to match SenML
Record(s).
Fetch Pack: One or more Fetch Records in an array structure.
Patch Record: One set of parameters similar to Fetch Record but also
containing instructions on how to change existing SenML Pack(s).
Patch Pack: One or more Patch Records in an array structure.
Target Record: A Record in a SenML Pack that matches the selection
criteria of a Fetch or Patch Record and hence is a target for a
Fetch or Patch operation.
Target Pack: A SenML Pack that is a target for a Fetch or Patch
operation.
(i)PATCH: A term that refers to both CoAP "PATCH" and "iPATCH"
methods when there is no difference in this specification as to
which one is used.
3. Using FETCH and (i)PATCH with SenML
The FETCH/(i)PATCH media types for SenML are modeled as extensions to
the SenML media type to enable reuse of existing SenML parsers and
generators, in particular on constrained devices. Unless mentioned
otherwise, FETCH and PATCH Packs are constructed with the same rules
and constraints as SenML Packs.
The key differences from the SenML media type are allowing the use of
a "null" value for removing Records with the (i)PATCH method and the
lack of value fields in Fetch Records. Also, the Fetch and Patch
Records do not have a default time or base version when the fields
are omitted.
3.1. SenML FETCH
The FETCH method can be used to select and return a subset of
Records, in sequence, of one or more SenML Packs. The SenML Records
are selected by giving a set of names that, when resolved, match
resolved names in a Target SenML Pack. The names for a Fetch Pack
are given using the SenML "name" and/or "base name" fields. The
names are resolved by concatenating the base name with the name field
as defined in [RFC8428].
A Fetch Pack MUST contain at least one Fetch Record. A Fetch Record
MUST contain a name and/or base name field.
For example, to select the resources "5850" and "5851" from the
example in Section 1, the following Fetch Pack can be used:
[
{"bn":"2001:db8::2/3311/0/", "n":"5850"},
{"n":"5851"}
]
The result of a FETCH request with the example above would be:
[
{"bn":"2001:db8::2/3311/0/", "n":"5850", "vb":true},
{"n":"5851", "v":42},
]
The SenML time and unit fields can be used in a Fetch Record to
further narrow the selection of matched SenML Records. When no time
or unit is given in a Fetch Record, all SenML Records with the given
name are matched (i.e., unlike with SenML Records, the lack of time
field in a Fetch Record does not imply a time value of zero). When
time is given in the Fetch Record, a Target Record is matched only
when its resolved time value and name are equal to those of the Fetch
Record. Similarly, when unit is given, a Target Record is matched
only when its resolved unit and name are equal to those of the Fetch
Record. If both the time and unit are given in the Fetch Record, a
Target Record is matched only when both are equal to those of the
Fetch Record. Each Target Record MUST be included in the response at
most once, even if multiple Fetch Records match with the same Target
Record.
For example, if the resource "5850" had multiple sensor readings
(SenML Records) with different time values, the following Fetch Pack
can be used to retrieve the Record with time "1.276020091e+09":
[
{"bn":"2001:db8::2/3311/0/", "n":"5850", "t":1.276020091e+09}
]
The resolved form of Records (Section 4.6 of [RFC8428]) is used when
comparing the names, times, and units of the Target and Fetch Records
to accommodate differences in the use of the base values. In the
resolved form, the SenML name in the example above becomes
"2001:db8::2/3311/0/5850". Since there is no base time in the Pack,
the time in resolved form is equal to the time in the example.
If no SenML Records match, an empty SenML Pack (i.e., array with no
elements) is returned as a response.
Fetch Records MUST NOT contain other fields than name, base name,
time, base time, unit, and base unit. Implementations MUST reject
and generate an error for a Fetch Pack with other fields. [RFC8132],
Section 2.2 provides guidance for FETCH request error handling, e.g.,
using the 4.22 (Unprocessable Entity) CoAP error response code.
3.2. SenML (i)PATCH
The (i)PATCH method can be used to change the fields of SenML
Records, to add new Records, and to remove existing Records. The
names, times, and units of the Patch Records are given and matched in
the same way as for the Fetch Records, except each Patch Record MUST
match at most one Target Record. A Patch Record matching more than
one Target Record is considered invalid (patching multiple Target
Records with one Patch Record would result in multiple copies of the
same Record). Patch Packs can also include new values and other
SenML fields for the Records. Application of Patch Packs is
idempotent; hence, the PATCH and iPATCH methods for SenML Packs are
equivalent.
When the name in a Patch Record matches with the name in an existing
Record, the resolved time values and units (if any) are compared. If
the time values and units either do not exist in both Records or are
equal, the Target Record is replaced with the contents of the Patch
Record. All Patch Records MUST contain at least a SenML Value or Sum
field.
If a Patch Record contains a name, or the combination of a time
value, unit, and name, that does not exist in any existing Record in
the Pack, the given Record, with all the fields it contains, is added
to the Pack.
If a Patch Record has a value ("v") field with a null value, it MUST
NOT be added, but the matched Record (if any) is removed from the
Target Pack.
The Patch Records MUST be applied in the same sequence as they are in
the Patch Pack. If multiple Patch Packs are being processed at the
same time, the result MUST be equivalent to applying them in one
sequence.
Implementations MUST reject and generate an error for Patch Packs
with invalid Records. If a Patch Pack is rejected, the state of the
Target Pack is not changed, i.e., either all or none of the Patch
Records are applied. [RFC8132], Section 3.4 provides guidance for
error handling with PATCH and iPATCH requests, e.g., using the 4.22
(Unprocessable Entity) and 4.09 (Conflict) CoAP error response codes.
For example, the following document could be given as an (i)PATCH
payload to change/set the values of two SenML Records for the example
in Section 1:
[
{"bn":"2001:db8::2/3311/0/", "n":"5850", "vb":false},
{"n":"5851", "v":10}
]
If the request is successful, the resulting representation of the
example SenML Pack would be as follows:
[
{"bn":"2001:db8::2/3311/0/", "n":"5850", "vb":false},
{"n":"5851", "v":10},
{"n":"5750", "vs":"Ceiling light"}
]
As another example, the following document could be given as an
(i)PATCH payload to remove the two SenML Records:
[
{"bn":"2001:db8::2/3311/0/", "n":"5850", "v":null},
{"n":"5851", "v":null}
]
4. Fragment Identification
Fragment identification for Records of Fetch and Patch Packs uses the
same mechanism as SenML JSON/CBOR fragment identification (see
Section 9 of [RFC8428]), i.e., the "rec" scheme followed by a comma-
separated list of Record positions or range(s) of Records. For
example, to select the 3rd and 5th Record of a Fetch or Patch Pack, a
fragment identifier "rec=3,5" can be used in the URI of the Fetch or
Patch Pack resource.
5. Extensibility
The SenML mandatory-to-understand field extensibility mechanism (see
Section 4.4 of [RFC8428]) does not apply to Patch Packs, i.e.,
unknown fields MUST NOT generate an error, but such fields are
treated like any other field (e.g., added to Patch target Records
where applicable).
This specification allows only a small subset of SenML fields in
Fetch Records, but future specifications may enable new fields for
Fetch Records and possibly also new fields for selecting targets for
Patch Records.
6. Security Considerations
The security and privacy considerations of SenML also apply to the
FETCH and (i)PATCH methods. CoAP's security mechanisms are used to
provide security for the FETCH and (i)PATCH methods.
In FETCH and (i)PATCH requests, the client can pass arbitrary names
to the target resource for manipulation. The resource implementer
must take care to only allow access to names that are actually part
of (or accessible through) the target resource. In particular, the
receiver needs to ensure that any input does not lead to uncontrolled
special interpretation by the system.
If the client is not allowed to do a GET or PUT on the full target
resource (and thus all the names accessible through it), access
control rules must be evaluated for each Record in the Pack.
7. IANA Considerations
This document registers two new media types and CoAP Content-Format
IDs for both media types.
7.1. CoAP Content-Format Registration
IANA has assigned CoAP Content-Format IDs for the SenML PATCH and
FETCH media types in the "CoAP Content-Formats" subregistry, within
the "Constrained RESTful Environments (CoRE) Parameters" registry
[RFC7252]. The assigned IDs are shown in Table 1.
+=============================+==========+=====+
| Media Type | Encoding | ID |
+=============================+==========+=====+
| application/senml-etch+json | - | 320 |
+-----------------------------+----------+-----+
| application/senml-etch+cbor | - | 322 |
+-----------------------------+----------+-----+
Table 1: CoAP Content-Format IDs
7.2. senml-etch+json Media Type
Type name: application
Subtype name: senml-etch+json
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See Section 6 of RFC 8790.
Interoperability considerations: N/A
Published specification: RFC 8790
Applications that use this media type: Applications that use the
SenML media type for resource representation.
Fragment identifier considerations: Fragment identification for
application/senml-etch+json is supported by using fragment
identifiers as specified by Section 4 of RFC 8790.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): senml-etchj
Windows Clipboard Name: "SenML FETCH/PATCH format"
Macintosh file type code(s): N/A
Macintosh Universal Type Identifier code:
org.ietf.senml-etch-json conforms to public.text
Person & email address to contact for further information:
Ari Keränen <ari.keranen@ericsson.com>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Ari Keränen <ari.keranen@ericsson.com>
Change controller: IESG
7.3. senml-etch+cbor Media Type
Type name: application
Subtype name: senml-etch+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See Section 6 of RFC 8790.
Interoperability considerations: N/A
Published specification: RFC 8790
Applications that use this media type: Applications that use the
SenML media type for resource representation.
Fragment identifier considerations: Fragment identification for
application/senml-etch+cbor is supported by using fragment
identifiers as specified by Section 4 of RFC 8790.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): senml-etchc
Macintosh file type code(s): N/A
Macintosh Universal Type Identifier code:
org.ietf.senml-etch-cbor conforms to public.data
Person & email address to contact for further information:
Ari Keränen <ari.keranen@ericsson.com>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Ari Keränen <ari.keranen@ericsson.com>
Change controller: IESG
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>.
[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>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
[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>.
[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>.
[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
[IPSO] IPSO, "IPSO Light Control Smart Object", 2019,
<http://www.openmobilealliance.org/tech/profiles/
lwm2m/3311.xml>.
Acknowledgements
The use of the FETCH and (i)PATCH methods with SenML was first
introduced by the OMA SpecWorks Lightweight Machine to Machine
(LwM2M) v1.1 specification. This document generalizes the use to any
SenML representation. The authors would like to thank Carsten
Bormann, Christian Amsüss, Jaime Jiménez, Klaus Hartke, Michael
Richardson, and other participants from the IETF CoRE and OMA
SpecWorks DMSE working groups who have contributed ideas and reviews.
Authors' Addresses
Ari Keränen
Ericsson
FI-02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
Mojan Mohajer
Email: mojanm@hotmail.com