| Internet-Draft | FETCH & PATCH with SenML | March 2020 |
| Keranen & Mohajer | Expires 10 September 2020 | [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 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 with the SenML data model.¶
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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 an IPSO 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 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 with the SenML data model (i.e., for both SenML and Sensor Streaming Measurement Lists (SenSML) data). The rest of the document uses 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.¶
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:¶
The FETCH/(i)PATCH media types for SenML are modeled as extensions to the SenML media type to enable re-use 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 to the SenML media type are allowing the use of a "null" value for removing records with the (i)PATCH method and lack of value fields in Fetch Records. Also the Fetch and Patch Records do not have default time or base version when the fields are omitted.¶
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 a base name field.¶
For example, to select the IPSO 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 to 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, lack of time field in a Fetch Record does not imply time value zero). When time is given in the Fetch Record, only the SenML Records (if any) with equal resolved time value and name are matched. Similarly, when unit is given, only the SenML Records with equal resolved unit and name are matched. If both time and unit are given in the Fetch Record, both MUST to match for the SenML Record to match. Each Target Record MUST be included in the response only once, even if multiple Fetch Records match with the same Target Record.¶
For example, if the IPSO resource "5850" would have 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 for differences in use of the base values. In 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, 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.¶
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 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 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 combination of a time value, unit, and a name, that do 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 value null, 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 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 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}
]
¶
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., "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.¶
The SenML mandatory to understand fields extensibility mechanism (see section 4.4 in [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.¶
The security and privacy considerations of SenML apply also with 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.¶
This document registers two new media types and CoAP Content-Format IDs for both media types.¶
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with the RFC number of this document.¶
IANA is requested to assign CoAP Content-Format IDs for the SenML PATCH and FETCH media types in the "CoAP Content-Formats" sub-registry, within the "CoRE Parameters" registry [RFC7252]. The assigned IDs are shown in Table 1.¶
| Media type | Encoding | ID |
|---|---|---|
| application/senml-etch+json | - | TBD-320 |
| application/senml-etch+cbor | - | TBD-322 |
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-AAAA.¶
Interoperability considerations: N/A¶
Published specification: RFC-AAAA¶
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 RFC AAAA.¶
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 Keranen ari.keranen@ericsson.com¶
Intended usage: COMMON¶
Restrictions on usage: N/A¶
Author: Ari Keranen ari.keranen@ericsson.com¶
Change controller: IESG¶
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-AAAA.¶
Interoperability considerations: N/A¶
Published specification: RFC-AAAA¶
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 RFC AAAA.¶
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 Keranen ari.keranen@ericsson.com¶
Intended usage: COMMON¶
Restrictions on usage: N/A¶
Author: Ari Keranen ari.keranen@ericsson.com¶
Change controller: IESG¶
The use of FETCH and (i)PATCH methods with SenML was first introduced by the OMA SpecWorks LwM2M v1.1 specification. This document generalizes the use to any SenML representation. The authors would like to thank Carsten Bormann, Christian Amsuess, Jaime Jimenez, Klaus Hartke, Michael Richardson, and other participants from the IETF CoRE and OMA SpecWorks DMSE working groups who have contributed ideas and reviews.¶