💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc8132.txt captured on 2023-06-14 at 15:06:25.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Internet Engineering Task Force (IETF) P. van der Stok
Request for Comments: 8132 Consultant
Category: Standards Track C. Bormann
ISSN: 2070-1721 Universitaet Bremen TZI
A. Sehgal
NAVOMI, Inc.
April 2017
PATCH and FETCH Methods for the Constrained Application Protocol (CoAP)
Abstract
The methods defined in RFC 7252 for the Constrained Application
Protocol (CoAP) only allow access to a complete resource, not to
parts of a resource. In case of resources with larger or complex
data, or in situations where resource continuity is required,
replacing or requesting the whole resource is undesirable. Several
applications using CoAP need to access parts of the resources.
This specification defines the new CoAP methods, FETCH, PATCH, and
iPATCH, which are used to access and update parts of a resource.
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
http://www.rfc-editor.org/info/rfc8132.
van der Stok, et al. Standards Track [Page 1]
RFC 8132 CoAP FETCH PATCH April 2017
Copyright Notice
Copyright (c) 2017 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
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. FETCH . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. PATCH and iPATCH . . . . . . . . . . . . . . . . . . . . 4
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.4. Terminology and Acronyms . . . . . . . . . . . . . . . . 5
2. FETCH Method . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Response Codes . . . . . . . . . . . . . . . . . . . . . 6
2.2. Error Handling . . . . . . . . . . . . . . . . . . . . . 6
2.3. Option Numbers . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. The Content-Format Option . . . . . . . . . . . . . . 7
2.3.2. The ETag Option . . . . . . . . . . . . . . . . . . . 8
2.4. Working with Observe . . . . . . . . . . . . . . . . . . 8
2.5. Working with Block . . . . . . . . . . . . . . . . . . . 8
2.6. Building FETCH Requests . . . . . . . . . . . . . . . . . 8
2.7. A Simple Example for FETCH . . . . . . . . . . . . . . . 8
3. PATCH and iPATCH Methods . . . . . . . . . . . . . . . . . . 9
3.1. Simple Examples for PATCH and iPATCH . . . . . . . . . . 12
3.2. Response Codes . . . . . . . . . . . . . . . . . . . . . 14
3.3. Option Numbers . . . . . . . . . . . . . . . . . . . . . 14
3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 15
4. The New Set of CoAP Methods . . . . . . . . . . . . . . . . . 16
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1. Normative References . . . . . . . . . . . . . . . . . . 19
7.2. Informative References . . . . . . . . . . . . . . . . . 19
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
van der Stok, et al. Standards Track [Page 2]
RFC 8132 CoAP FETCH PATCH April 2017
1. Introduction
Similar to HTTP, the GET method defined in [RFC7252] for the
Constrained Application Protocol (CoAP) only allows the specification
of a URI and request parameters in CoAP options, not the transfer of
a request payload detailing the request. This leads some
applications to use POST where a cacheable, idempotent, safe request
is actually desired.
Again, similar to the original specification of HTTP, the PUT method
defined in [RFC7252] only allows a complete resource to be replaced.
This also leads applications to use POST where a cacheable, possibly
idempotent request is actually desired.
The present specification adds new CoAP methods: FETCH, to perform
the equivalent of a GET with a request body; and the twin methods,
PATCH and iPATCH, to modify parts of a CoAP resource.
1.1. FETCH
The CoAP GET method [RFC7252] is used to obtain the representation of
a resource, where the resource is specified by a URI and additional
request parameters can also shape the representation. This has been
modeled after the HTTP GET operation and the REST model in general.
In HTTP, a resource is often used to search for information, and
existing systems varyingly use the HTTP GET and POST methods to
perform a search. Often, a POST method is used solely so that a
larger set of parameters to the search can be supplied in the request
body than can comfortably be transferred in the URI with a GET
request. [HTTP-SEARCH] proposes a SEARCH method that is similar to
GET in most properties but enables sending a request body, as is done
with POST. The FETCH method defined in the present specification is
inspired by [HTTP-SEARCH], which updates the definition and semantics
of the HTTP SEARCH request method previously defined by [RFC5323].
However, there is no intention to limit FETCH to search-type
operations, and the resulting properties may not be the same as those
of HTTP SEARCH.
van der Stok, et al. Standards Track [Page 3]
RFC 8132 CoAP FETCH PATCH April 2017
A major problem with GET is that the information that controls the
request needs to be bundled up in some unspecified way into the URI.
Using the request body for this information has a number of
advantages:
o The client can specify a media type (and a content coding) that
enables the server to unambiguously interpret the request
parameters in the context of that media type. Also, the request
body is not limited by the character set limitations of URIs,
which enables a more natural (and more efficient) representation
of certain domain-specific parameters.
o The request parameters are not limited by the maximum size of the
URI. In HTTP, that is a problem, as the practical limit for this
size varies. In CoAP, another problem is that the block-wise
transfer is not available for transferring large URI options in
multiple rounds.
As an alternative to using GET, many implementations make use of the
POST method to perform extended requests (even if they are
semantically idempotent, safe, and even cacheable) to be able to pass
along the input parameters within the request payload as opposed to
using the request URI.
The FETCH method provides a solution that spans the gap between the
use of GET and POST. As with POST, the input to the FETCH operation
is passed along within the payload of the request rather than as part
of the request URI. Unlike POST, however, the semantics of the FETCH
method are more specifically defined.
1.2. PATCH and iPATCH
PATCH is also specified for HTTP in [RFC5789]. Most of the
motivation for PATCH described in [RFC5789] also applies here. iPATCH
is the idempotent version of PATCH.
The PUT method exists to overwrite a resource with completely new
contents and cannot be used to perform partial changes. When using
PUT for partial changes, proxies and caches, and even clients and
servers, may get confused as to the result of the operation. PATCH
was not adopted in an early design stage of CoAP; however, it has
become necessary with the arrival of applications that require
partial updates to resources (e.g., [COAP-MGMNT]). Using PATCH
avoids transferring all data associated with a resource in case of
modifications, thereby not burdening the constrained communication
medium.
van der Stok, et al. Standards Track [Page 4]
RFC 8132 CoAP FETCH PATCH April 2017
This document relies on knowledge of the PATCH specification for HTTP
[RFC5789]. This document provides extracts from [RFC5789] to make
independent reading possible.
1.3. Requirements Language
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
[RFC2119].
1.4. Terminology and Acronyms
This document uses terminology defined in [RFC5789] and [RFC7252].
Specifically, it uses the terms "safe" and "idempotent" as defined in
Section 5.1 of [RFC7252]. (Further discussion of safe and idempotent
methods can now be found in Sections 4.2.1 and 4.2.2 of [RFC7231],
respectively; the implications of idempotence of methods on server
implementations are also discussed in Section 4.5 of [RFC7252].)
2. FETCH Method
The CoAP FETCH method is used to obtain a representation of a
resource, specified by a number of request parameters. Unlike the
CoAP GET method, which requests that a server return a representation
of the resource identified by the effective request URI (as defined
by [RFC7252]), the FETCH method is used by a client to ask the server
to produce a representation as described by the request parameters
(including the request options and the payload) based on the resource
specified by the effective request URI. The payload returned in
response to a FETCH cannot be assumed to be a complete representation
of the resource identified by the effective request URI, i.e., it
cannot be used by a cache as a payload to be returned by a GET
request.
Together with the request options, the body of the request (which may
be constructed from multiple payloads using the block protocol
[RFC7959]) defines the request parameters. With the FETCH method,
implementations may submit a request body of any media type that is
defined with the semantics of selecting information from a resource
in such a FETCH request; it is outside the scope of this document how
information about media types admissible for the specific resource is
obtained by the client (although we can hint that form relations
[CORE-APP] might be a preferred way). It is RECOMMENDED that any
discovery method that allows a client to find out that the server
supports FETCH also provides information regarding what FETCH payload
media types are applicable.
van der Stok, et al. Standards Track [Page 5]
RFC 8132 CoAP FETCH PATCH April 2017
FETCH requests are both safe and idempotent with regards to the
resource identified by the request URI. That is, the performance of
a FETCH is not intended to alter the state of the targeted resource.
(However, while processing a FETCH request, a server can be expected
to allocate computing and memory resources or even create additional
server resources through which the response to the search can be
retrieved.)
A successful response to a FETCH request is expected to provide some
indication as to the final disposition of the requested operation.
If a successful response includes a body payload, the payload is
expected to describe the results of the FETCH operation.
Depending on the response code as defined by [RFC7252], the response
to a FETCH request is cacheable; the request body is part of the
cache key. Specifically, 2.05 (Content) response codes (the
responses for which are cacheable) are a typical way to respond to a
FETCH request. (Note that this aspect differs markedly from
[HTTP-SEARCH] and also that caches that cannot use the request
payload as part of the cache key will not be able to cache responses
to FETCH requests at all.) The Max-Age option in the response has
equivalent semantics to its use in a GET.
The semantics of the FETCH method change to a "conditional FETCH" if
the request message includes an If-Match or If-None-Match option
[RFC7252]. A conditional FETCH requests that the query be performed
only under the circumstances described by the conditional option(s).
It is important to note, however, that such conditions are evaluated
against the state of the target resource itself as opposed to the
results of the FETCH operation.
2.1. Response Codes
FETCH for CoAP adopts the response codes as specified in Sections 5.9
and 12.1.2 of [RFC7252] as well as the additional response codes
mentioned in Section 2.2.
2.2. Error Handling
A FETCH request may fail under certain known conditions. Beyond the
conditions already defined in [RFC7252] for GET, noteworthy ones are:
Malformed FETCH payload: If a server determines that the payload
provided with a FETCH request is not properly formatted, it can
return a 4.00 (Bad Request) CoAP error. The definition of a
malformed payload depends upon the CoAP Content-Format specified
with the request.
van der Stok, et al. Standards Track [Page 6]
RFC 8132 CoAP FETCH PATCH April 2017
Unsupported FETCH payload: In case a client sends a payload that is
inappropriate for the resource identified by the Request-URI, the
server can return a 4.15 (Unsupported Content-Format) CoAP error.
The server can determine if the payload is supported by checking
the CoAP Content-Format specified with the request.
Unprocessable request: This situation occurs when the payload of a
FETCH request is determined to be valid (i.e., well-formed and
supported) but the server is unable to or is incapable of
processing the request. The server can return a 4.22
(Unprocessable Entity) CoAP error. In situations when the server
has insufficient computing resources to complete the request
successfully, it can return a 4.13 (Request Entity Too Large) CoAP
error (see also below). If there are more specific errors that
provide additional insight into the problem, then those should be
used.
Request too large: If the payload of the FETCH request is larger
than a CoAP server can process, then it can return the 4.13
(Request Entity Too Large) CoAP error.
It is possible that other error situations not mentioned here are
encountered by a CoAP server while processing the FETCH request. In
these situations, other appropriate CoAP response codes can also be
returned.
2.3. Option Numbers
FETCH for CoAP adopts the option numbers as specified in Sections
5.10 and 12.2 of [RFC7252].
Generally, options defined for GET act in an analogous way for FETCH.
Two specific cases are called out in the rest of this section.
2.3.1. The Content-Format Option
A FETCH request MUST include a Content-Format option (see
Section 5.10.3 of [RFC7252]) to specify the media type and content
coding of the request body. (Typically, the media type will have
been specifically designed to specify details for a selection or a
search on a resource.)
van der Stok, et al. Standards Track [Page 7]
RFC 8132 CoAP FETCH PATCH April 2017
2.3.2. The ETag Option
The ETag option on a FETCH result has the same semantics as defined
in Section 5.10.6 of [RFC7252]. In particular, its use as a response
option describes the "tagged representation", which for FETCH is the
same as the "selected representation". The FETCH payload is input to
that selection process and therefore needs to be part of the cache
key. Similarly, the use of ETag as a request option can elicit a
2.03 (Valid) response if the representation associated with the ETag
would still be selected by the FETCH request (including its payload).
2.4. Working with Observe
The Observe option [RFC7641] can be used with a FETCH request as it
can be used with a GET request.
2.5. Working with Block
The Block1 option [RFC7959] can be used with a FETCH request as it
would be used with a POST request; the Block2 option can then be used
as it would with GET or POST.
2.6. Building FETCH Requests
One property of FETCH that may be non-obvious is that a FETCH request
cannot be generated from a link alone; the client also needs a way to
generate the request payload. Again, form relations [CORE-APP] may
be able to fill parts of this gap.
2.7. A Simple Example for FETCH
The FETCH method needs a media type for its payload (as expressed by
the Content-Format request option) that specifies the search query in
similar detail as is shown for the PATCH payload in the PATCH example
in Section 3.1. ([HTTP-SEARCH] invents a "text/query" format based
on some hypothetical SQL dialect for its examples.)
The example below illustrates retrieval of a subset of a JSON
[RFC7159] object (the same object as used in Section 3.1). Using a
hypothetical media type "application/example-map-keys+json" (with a
Content-Format ID of NNN, which is not defined as this is just an
example), the client specifies the items in the object that it wants:
it supplies a JSON array that gives the map keys for these items. A
resource located at <coap://www.example.com/object> can be
represented by a JSON document that we will consider as the target of
the FETCH. The client wants to learn the contents of the single map
key "foo" within this target:
van der Stok, et al. Standards Track [Page 8]
RFC 8132 CoAP FETCH PATCH April 2017
{
"x-coord": 256,
"y-coord": 45,
"foo": ["bar","baz"]
}
FETCH Example: JSON Document Returned by GET
The example FETCH request specifies a single top-level member desired
by giving its map key as the sole element of the "example-map-keys"
payload:
FETCH CoAP://www.example.com/object
Content-Format: NNN (application/example-map-keys+json)
Accept: application/json
[
"foo"
]
FETCH Example: Request
The server returns a subset document with just the selected member:
2.05 Content
Content-Format: 50 (application/json)
{
"foo": ["bar","baz"]
}
FETCH Example: Response with Subset JSON Document
By the logic of this example, the requester could have entered more
than one map key into the request payload array and would have
received a more complete subset of the top-level JSON object that is
representing the resource.
3. PATCH and iPATCH Methods
The PATCH and iPATCH methods request that a set of changes described
in the request payload be applied to the target resource of the
request. The set of changes is represented in a format identified by
a media type. If the Request-URI does not point to an existing
resource, the server MAY create a new resource with that URI,
depending on the PATCH document type (whether it can logically modify
a null resource) and permissions, as well as other conditions such as
the degree of control the server gives clients in creating new
van der Stok, et al. Standards Track [Page 9]
RFC 8132 CoAP FETCH PATCH April 2017
entries in its URI space (see also Section 3.4). Creation of a new
resource would result in a 2.01 (Created) response code dependent on
the PATCH document type.
Restrictions to a PATCH or iPATCH request can be made by including
the If-Match or If-None-Match options in the request (see Sections
5.10.8.1 and 5.10.8.2 of [RFC7252]). If the resource could not be
created or modified, then an appropriate error response code SHOULD
be sent.
The difference between the PUT and PATCH requests is documented in
[RFC5789]. When a request is intended to effect a partial update of
a given resource, clients cannot use PUT while supplying just the
update, but they might be able to use PATCH or iPATCH.
The PATCH method is "not safe" and "not idempotent", as is the HTTP
PATCH method specified in [RFC5789].
The iPATCH method is not safe but idempotent, as with the CoAP PUT
method specified in Section 5.8.3 of [RFC7252].
A client can mark a request as idempotent by using the iPATCH method
instead of the PATCH method. This is the only difference between the
two. The indication of idempotence may enable the server to keep
less state about the interaction; some constrained servers may only
implement the iPATCH variant for this reason.
PATCH and iPATCH are both atomic. The server MUST apply the entire
set of changes atomically and never provide a partially modified
representation to a concurrently executed GET request. Given the
constrained nature of the servers, most servers will only execute
CoAP requests consecutively, thus preventing a concurrent partial
overlapping of request modifications. In other words, modifications
MUST NOT be applied to the server state when an error occurs or when
only a partial execution is possible on the resources present in the
server.
The atomicity applies to a single server. When a PATCH or iPATCH
request is multicast to a set of servers, each server can either
execute all required modifications or not. It is not required that
all servers execute all modifications or none. An Atomic Commit
protocol that provides multiple server atomicity is out of scope.
van der Stok, et al. Standards Track [Page 10]
RFC 8132 CoAP FETCH PATCH April 2017
A PATCH or iPATCH response can invalidate a cache in a similar manner
to the PUT response. For the successful (2.xx) response codes, PATCH
or iPATCH have the following caching behavior:
o A 2.01 (Created) response invalidates any cache entry for the
resource indicated by the Location-* options; the payload is a
representation of the action result.
o A 2.04 (Changed) response invalidates any cache entry for the
target resource; the payload is a representation of the action
result.
There is no guarantee that a resource can be modified with PATCH or
iPATCH. Servers MUST ensure that a received PATCH body is
appropriate for the type of resource identified by the target
resource of the request.
It is RECOMMENDED that any discovery method that allows a client to
find out that the server supports one of PATCH and iPATCH also
provide information regarding what PATCH payload media types are
applicable and which of the two methods are implemented by the server
for each of these media types.
Servers that do not rely on the idempotence of iPATCH can easily
support both PATCH and iPATCH, and it is RECOMMENDED they do so.
This is inexpensive to do, as, for iPATCH, there is no requirement on
the server to check that the client's intention that the request be
idempotent is fulfilled (although there is diagnostic value in that
check, so a less-constrained implementation may want to perform it).
van der Stok, et al. Standards Track [Page 11]
RFC 8132 CoAP FETCH PATCH April 2017
3.1. Simple Examples for PATCH and iPATCH
The example is taken over from [RFC6902], which specifies a JSON
notation for PATCH operations. A resource located at
<coap://www.example.com/object> contains a target JSON document.
JSON document original state:
{
"x-coord": 256,
"y-coord": 45,
"foo": ["bar","baz"]
}
REQ: iPATCH CoAP://www.example.com/object
Content-Format: 51 (application/json-patch+json)
[
{ "op":"replace", "path":"x-coord", "value":45}
]
RET: CoAP 2.04 Changed
JSON document final state:
{
"x-coord": 45,
"y-coord": 45,
"foo": ["bar","baz"]
}
This example illustrates use of an idempotent modification to the
x-coord member of the existing resource "object". The 2.04 (Changed)
response code conforms with the CoAP PUT method.
van der Stok, et al. Standards Track [Page 12]
RFC 8132 CoAP FETCH PATCH April 2017
The same example using the Content-Format application/merge-
patch+json from [RFC7396] looks like the following:
JSON document original state:
{
"x-coord": 256,
"y-coord": 45,
"foo": ["bar","baz"]
}
REQ: iPATCH CoAP://www.example.com/object
Content-Format: 52 (application/merge-patch+json)
{ "x-coord":45}
RET: CoAP 2.04 Changed
JSON document final state:
{
"x-coord": 45,
"y-coord": 45,
"foo": ["bar","baz"]
}
The examples show the use of the iPATCH method, but the use of the
PATCH method would have led to the same result. Below, a non-
idempotent modification is shown. Because the action is non-
idempotent, iPATCH returns an error, while PATCH executes the action.
van der Stok, et al. Standards Track [Page 13]
RFC 8132 CoAP FETCH PATCH April 2017
JSON document original state:
{
"x-coord": 256,
"y-coord": 45,
"foo": ["bar","baz"]
}
REQ: iPATCH CoAP://www.example.com/object
Content-Format: 51 (application/json-patch+json)
[
{ "op":"add","path":"foo/1","value":"bar"}
]
RET: CoAP 4.00 Bad Request
Diagnostic payload: Patch format not idempotent
JSON document final state is unchanged
REQ: PATCH CoAP://www.example.com/object
Content-Format: 51 (application/json-patch+json)
[
{ "op":"add","path":"foo/1","value":"bar"}
]
RET: CoAP 2.04 Changed
JSON document final state:
{
"x-coord": 45,
"y-coord": 45,
"foo": ["bar","bar","baz"]
}
3.2. Response Codes
PATCH and iPATCH for CoAP adopt the response codes as specified in
Sections 5.9 and 12.1.2 of [RFC7252] and add 4.09 (Conflict) and 4.22
(Unprocessable Entity) with the semantics specified in Section 3.4 of
the present specification.
3.3. Option Numbers
PATCH and iPATCH for CoAP adopt the option numbers as specified in
Sections 5.10 and 12.2 of [RFC7252].
van der Stok, et al. Standards Track [Page 14]
RFC 8132 CoAP FETCH PATCH April 2017
3.4. Error Handling
A PATCH or iPATCH request may fail under certain known conditions.
These situations should be dealt with as expressed below.
Malformed PATCH or iPATCH payload: If a server determines that the
payload provided with a PATCH or iPATCH request is not properly
formatted, it can return a 4.00 (Bad Request) CoAP error. The
definition of a malformed payload depends upon the CoAP Content-
Format specified with the request.
Unsupported PATCH or iPATCH payload: In case a client sends a
payload that is inappropriate for the resource identified by the
Request-URI, the server can return a 4.15 (Unsupported Content-
Format) CoAP error. The server can determine if the payload is
supported by checking the CoAP Content-Format specified with the
request.
Unprocessable request: This situation occurs when the payload of a
PATCH request is determined to be valid (i.e., well-formed and
supported) but the server is unable to or is incapable of
processing the request. The server can return a 4.22
(Unprocessable Entity) CoAP error. More specific scenarios might
include situations such as:
* the server has insufficient computing resources to complete the
request successfully -- 4.13 (Request Entity Too Large) CoAP
response code (see below); or
* the resource specified in the request becomes invalid by
applying the payload -- 4.09 (Conflict) CoAP response code (see
"Conflicting state" below)).
In case there are more specific errors that provide additional
insight into the problem, then those should be used.
Resource not found: The 4.04 (Not Found) error should be returned if
the payload of a PATCH request cannot be applied to a non-existent
resource.
Failed precondition: In case the client uses the conditional
If-Match or If-None-Match option to define a precondition for the
PATCH request, and that precondition fails, then the server can
return the 4.12 (Precondition Failed) CoAP error.
Request too large: If the payload of the PATCH request is larger
than a CoAP server can process, then it can return the 4.13
(Request Entity Too Large) CoAP error.
van der Stok, et al. Standards Track [Page 15]
RFC 8132 CoAP FETCH PATCH April 2017
Conflicting state: If the modification specified by a PATCH or
iPATCH request causes the resource to enter an inconsistent state
that the server cannot resolve, the server can return the 4.09
(Conflict) CoAP response. The server SHOULD generate a payload
that includes enough information for a user to recognize the
source of the conflict. The server MAY return the actual resource
state to provide the client with the means to create a new
consistent resource state. Such a situation might be encountered
when a structural modification is applied to a configuration data
store but the structures being modified do not exist.
Concurrent modification: Resource-constrained devices might need to
process requests in the order they are received. In case requests
are received concurrently to modify the same resource but they
cannot be queued, the server can return a 5.03 (Service
Unavailable) CoAP response code.
Conflict handling failure: If the modification implies the
reservation of resources or the wait time for conditions to become
true leads to a too-long request execution time, the server can
return a 5.03 (Service Unavailable) response code.
It is possible that other error situations not mentioned here are
encountered by a CoAP server while processing the PATCH request. In
these situations, other appropriate CoAP status codes can also be
returned.
4. The New Set of CoAP Methods
Adding three new methods to CoAP's existing four may seem like a
major change. However, FETCH and the two PATCH variants fit well
into the REST paradigm and have been anticipated on the HTTP side.
Adding both a non-idempotent and an idempotent PATCH variant allows
interoperability with HTTP's PATCH method to be kept and allows the
use/indication of an idempotent PATCH when that is possible, which
saves significant effort on the server side.
van der Stok, et al. Standards Track [Page 16]
RFC 8132 CoAP FETCH PATCH April 2017
Interestingly, the three new methods fit into the old table of
methods with a surprising similarity in the idempotence and safety
attributes:
+------+--------+------+--------+------+------------+
| Code | Name | Code | Name | safe | idempotent |
+------+--------+------+--------+------+------------+
| 0.01 | GET | 0.05 | FETCH | yes | yes |
| 0.02 | POST | 0.06 | PATCH | no | no |
| 0.03 | PUT | 0.07 | iPATCH | no | yes |
| 0.04 | DELETE | | | no | yes |
+------+--------+------+--------+------+------------+
5. Security Considerations
This section analyzes the possible threats to the CoAP FETCH and
PATCH or iPATCH methods. It is meant to inform protocol and
application developers about the security limitations of CoAP FETCH
and PATCH or iPATCH as described in this document.
The FETCH method is subject to the same general security
considerations as all CoAP methods as described in Section 11 of
[RFC7252]. Specifically, the security considerations for FETCH are
closest to those of GET, except that the FETCH request carries a
payload that may need additional protection. The payload of a FETCH
request may reveal more detailed information about the specific
portions of a resource of interest to the requester than a GET
request for the entire resource would; this may mean that
confidentiality protection of the request by Datagram Transport Layer
Security (DTLS) or other means is needed for FETCH where it wouldn't
be needed for GET.
The PATCH and iPATCH methods are subject to the same general security
considerations as all CoAP methods as described in Section 11 of
[RFC7252]. The specific security considerations for PATCH or iPATCH
are nearly identical to the security considerations for PUT
[RFC7252]; the security considerations of Section 5 of [RFC5789] also
apply to PATCH and iPATCH. Specifically, there is likely to be a
need for authorizing requests (possibly through access control and/or
authentication) and for ensuring that data is not corrupted through
transport errors or through accidental overwrites. The mechanisms
used for PUT can be used for PATCH or iPATCH as well.
The new methods defined in the present specification are secured
following the CoAP recommendations for the existing methods as
specified in Section 9 of [RFC7252]. When additional security
techniques are standardized for CoAP (e.g., Object Security), these
techniques are then also available for securing the new methods.
van der Stok, et al. Standards Track [Page 17]
RFC 8132 CoAP FETCH PATCH April 2017
6. IANA Considerations
IANA has added the following entries to the subregistry "CoAP Method
Codes":
+------+--------+-----------+
| Code | Name | Reference |
+------+--------+-----------+
| 0.05 | FETCH | RFC 8132 |
| 0.06 | PATCH | RFC 8132 |
| 0.07 | iPATCH | RFC 8132 |
+------+--------+-----------+
The FETCH method is idempotent and safe, and it returns the same
response codes that GET can return, plus 4.13 (Request Entity Too
Large), 4.15 (Unsupported Content-Format), and 4.22 (Unprocessable
Entity) with the semantics specified in Section 2.2.
The PATCH method is neither idempotent nor safe. It returns the same
response codes that POST can return, plus 4.09 (Conflict) and 4.22
(Unprocessable Entity) with the semantics specified in Section 3.4.
The iPATCH method is identical to the PATCH method, except that it is
idempotent.
IANA has added the following code to the subregistry "CoAP Response
Codes":
+------+----------------------+-----------+
| Code | Name | Reference |
+------+----------------------+-----------+
| 4.09 | Conflict | RFC 8132 |
| 4.22 | Unprocessable Entity | RFC 8132 |
+------+----------------------+-----------+
IANA has added entries to the subregistry "CoAP Content-Formats":
+------------------------------+----------------+----+-----------+
| Media Type | Content Coding | ID | Reference |
+------------------------------+----------------+----+-----------+
| application/json-patch+json | identity | 51 | [RFC6902] |
| application/merge-patch+json | identity | 52 | [RFC7396] |
+------------------------------+----------------+----+-----------+
van der Stok, et al. Standards Track [Page 18]
RFC 8132 CoAP FETCH PATCH April 2017
7. References
7.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
<http://www.rfc-editor.org/info/rfc5789>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<http://www.rfc-editor.org/info/rfc7959>.
7.2. Informative References
[RFC5323] Reschke, J., Ed., Reddy, S., Davis, J., and A. Babich,
"Web Distributed Authoring and Versioning (WebDAV)
SEARCH", RFC 5323, DOI 10.17487/RFC5323, November 2008,
<http://www.rfc-editor.org/info/rfc5323>.
[RFC6902] Bryan, P., Ed. and M. Nottingham, Ed., "JavaScript Object
Notation (JSON) Patch", RFC 6902, DOI 10.17487/RFC6902,
April 2013, <http://www.rfc-editor.org/info/rfc6902>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
van der Stok, et al. Standards Track [Page 19]
RFC 8132 CoAP FETCH PATCH April 2017
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
<http://www.rfc-editor.org/info/rfc7396>.
[COAP-MGMNT]
Stok, P., Bierman, A., Veillette, M., and A. Pelov, "CoAP
Management Interface", Work in Progress,
draft-ietf-core-comi-00, January 2017.
[CORE-APP] Hartke, K., "CoRE Application Descriptions", Work in
Progress, draft-hartke-core-apps-07, February 2017.
[HTTP-SEARCH]
Reschke, J., Malhotra, A., and J. Snell, "HTTP SEARCH
Method", Work in Progress, draft-snell-search-method-00,
April 2015.
Acknowledgements
Klaus Hartke has pointed out some essential differences between CoAP
and HTTP concerning PATCH and found a number of problems in an
earlier draft version of Section 2. We are grateful for discussions
with Christian Amsuss, Andy Bierman, Timothy Carey, Paul Duffy,
Matthias Kovatsch, Michel Veillette, Michael Verschoor, Thomas
Watteyne, and Gengyu Wei. Christian Groves provided detailed
comments during the Working Group Last Call, and Christer Holmberg's
Gen-ART review provided some further editorial improvement. Further
Last Call reviews were provided by Sheng Jiang and Phillip Hallam-
Baker. As usual, the IESG had some very good reviews, and we would
like to specifically call out those by Alexey Melnikov (responsible
AD) and Alissa Cooper.
van der Stok, et al. Standards Track [Page 20]
RFC 8132 CoAP FETCH PATCH April 2017
Authors' Addresses
Peter van der Stok
Consultant
Email: consultancy@vanderstok.org
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Anuj Sehgal
NAVOMI, Inc.
Email: anuj.sehgal@navomi.com
van der Stok, et al. Standards Track [Page 21]