| RFC 8790 | FETCH and PATCH with SenML | June 2020 |
| Keränen & Mohajer | Standards Track | [Page] |
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.¶
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 athttps://www.rfc-editor.org/info/rfc8790.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The Sensor Measurement Lists (SenML) media type[RFC8428] and datamodel can be used to transmit collections of resources, such asbatches 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 usinga single SenML Pack with three SenML Records. All resources share thesame base name "2001:db8::2/3311/0/"; hence, full names for the resourcesare "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 resourceswith one request.¶
This document defines two new media types, one using the JavaScriptObject Notation (JSON)[RFC8259] and one using the Concise BinaryObject Representation (CBOR)[RFC7049], which can be used with theCoAP FETCH, PATCH, and iPATCH methods for resources represented using theSenML data model (i.e., for both SenML and Sensor Streaming MeasurementLists (SenSML) data). The rest of the document uses the term "(i)PATCH" whenreferring to both methods as the semantics of the new media types are thesame for the CoAP PATCH and iPATCH methods.¶
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 discussedin[RFC8132] and[RFC8428]. The following additional terms are used inthis document:¶
The FETCH/(i)PATCH media types for SenML are modeled as extensions to theSenML media type to enable reuse of existing SenML parsers andgenerators, in particular on constrained devices. Unless mentionedotherwise, FETCH and PATCH Packs are constructed with the same rules andconstraints 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 ofvalue fields in Fetch Records. Also, the Fetch and Patch Records do nothave a default time or base version when the fields are omitted.¶
The FETCH method can be used to select and return a subset of Records, insequence, of one or more SenML Packs. The SenML Records are selected bygiving a set of names that, when resolved, match resolved names in aTarget SenML Pack. The names for a Fetch Pack are given using the SenML"name" and/or "base name" fields. The names are resolved by concatenatingthe base name with the name field as defined in[RFC8428].¶
A Fetch PackMUST contain at least one Fetch Record. A Fetch RecordMUSTcontain a name and/or base name field.¶
For example, to select the resources "5850" and "5851" from theexample inSection 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 furthernarrow the selection of matched SenML Records. When no time or unit isgiven in a Fetch Record, all SenML Records with the given name arematched (i.e., unlike with SenML Records, the lack of time field in a FetchRecord does not imply a time value of zero). When time is given in the FetchRecord, a Target Record is matched only when its resolved time value and nameare equal to those of the Fetch Record. Similarly, when unit is given, aTarget Record is matched only when its resolved unit and name are equal tothose of the Fetch Record. If both the time and unit are given in the FetchRecord, a Target Record is matched only when both are equal to those of theFetch Record. Each Target RecordMUST be included in the response atmost once, even if multiple Fetch Records match with the same TargetRecord.¶
For example, if the resource "5850" had multiple sensor readings (SenML Records) with different time values, the following FetchPack 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 whencomparing the names, times, and units of the Target and Fetch Records toaccommodate 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 isequal to the time in the example.¶
If no SenML Records match, an empty SenML Pack (i.e., array with noelements) is returned as a response.¶
Fetch RecordsMUST NOT contain other fields than name, base name, time,base time, unit, and base unit. ImplementationsMUST reject and generatean 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.¶
The (i)PATCH method can be used to change the fields of SenML Records, toadd new Records, and to remove existing Records. The names, times, andunits of the Patch Records are given and matched in the same way as for theFetch Records, except each Patch RecordMUST match at most one TargetRecord. A Patch Record matching more than one Target Record is consideredinvalid (patching multiple Target Records with one Patch Record wouldresult in multiple copies of the same Record). Patch Packs can alsoinclude new values and other SenML fields for the Records. Application ofPatch Packs is idempotent; hence, the PATCH and iPATCH methods for SenML Packsare equivalent.¶
When the name in a Patch Record matches with the name in an existingRecord, the resolved time values and units (if any) are compared. If thetime 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. AllPatch RecordsMUST 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, thegiven 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, itMUST NOT beadded, but the matched Record (if any) is removed from the Target Pack.¶
The Patch RecordsMUST be applied in the same sequence as they are in thePatch Pack. If multiple Patch Packs are being processed at the same time,the resultMUST be equivalent to applying them in one sequence.¶
ImplementationsMUST reject and generate an error for Patch Packs withinvalid Records. If a Patch Pack is rejected, the state of the TargetPack is not changed, i.e., either all or none of the Patch Records areapplied.[RFC8132],Section 3.4 provides guidance for error handlingwith PATCH and iPATCH requests, e.g., using the 4.22 (UnprocessableEntity) and 4.09 (Conflict) CoAP error response codes.¶
For example, the following document could be given as an (i)PATCH payloadto change/set the values of two SenML Records for the example inSection 1:¶
[ {"bn":"2001:db8::2/3311/0/", "n":"5850", "vb":false}, {"n":"5851", "v":10}]¶If the request is successful, the resulting representation of theexample 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)PATCHpayload to remove the two SenML Records:¶
[ {"bn":"2001:db8::2/3311/0/", "n":"5850", "v":null}, {"n":"5851", "v":null}]¶Fragment identification for Records of Fetch and Patch Packs uses thesame mechanism as SenML JSON/CBOR fragment identification (seeSection 9 of [RFC8428]), i.e., the "rec" scheme followed by a comma-separated list ofRecord positions or range(s) of Records. For example, to select the 3rdand 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.¶
The SenML mandatory-to-understand field extensibility mechanism (seeSection 4.4 of [RFC8428]) does not apply to Patch Packs, i.e., unknownfieldsMUST NOT generate an error, but such fields are treated like anyother field (e.g., added to Patch target Records where applicable).¶
This specification allows only a small subset of SenML fields in FetchRecords, but future specifications may enable new fields for Fetch Recordsand possibly also new fields for selecting targets for Patch Records.¶
The security and privacy considerations of SenML also apply to theFETCH and (i)PATCH methods. CoAP's security mechanisms are used toprovide security for the FETCH and (i)PATCH methods.¶
In FETCH and (i)PATCH requests, the client can pass arbitrary names tothe target resource for manipulation. The resource implementer must takecare to only allow access to names that are actually part of (oraccessible through) the target resource. In particular, the receiver needsto ensure that any input does not lead to uncontrolled specialinterpretation by the system.¶
If the client is not allowed to do a GET or PUT on the full targetresource (and thus all the names accessible through it), accesscontrol rules must be evaluated for each Record in the Pack.¶
This document registers two new media types and CoAP Content-Format IDsfor both media types.¶
IANA has assigned CoAP Content-Format IDs for the SenML PATCHand FETCH media types in the "CoAP Content-Formats" subregistry, withinthe "Constrained RESTful Environments (CoRE) Parameters" registry[RFC7252]. The assigned IDs are shown inTable 1.¶
| Media Type | Encoding | ID |
|---|---|---|
| application/senml-etch+json | - | 320 |
| application/senml-etch+cbor | - | 322 |
Ari Keränen <ari.keranen@ericsson.com>¶
Ari Keränen <ari.keranen@ericsson.com>¶
Ari Keränen <ari.keranen@ericsson.com>¶
Ari Keränen <ari.keranen@ericsson.com>¶
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 thankCarsten 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.¶