One document matched: draft-vanderstok-core-patch-01.xml

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<rfc category="info" docName="draft-vanderstok-core-patch-01"
     ipr="trust200902">
  <front>
    <title abbrev="CoAP Patch">
    Patch Method for Constrained Application Protocol (CoAP)
    </title>

    
    <author fullname="Peter van der Stok" initials="P." surname="van der Stok">
      <organization>Consultant</organization>

      <address>
        <email>consultancy@vanderstok.org</email>
      </address>
    </author>

    <author fullname="Anuj Sehgal" initials="A"
            surname="Sehgal">
      <organization>Consultant</organization>
      <address>
        <!--<postal>
          <street>Zum Rusch 1</street>
          <city>Bremen</city>
          <code>28717</code>
          <country>Germany</country>
        </postal>-->
        <email>anuj@iurs.org</email>
      </address>
    </author>



    <date/>

    <area>Applications</area>

    <workgroup>core</workgroup>

    <keyword>CoAP</keyword>

    <keyword>Patch</keyword>

    <abstract>
      <t>
	The existing Constrained Application Protocol (CoAP) PUT
	method only allows a complete replacement of a resource. This
	does not permit applications to perform partial resource
	modifications. In case of resources with larger or complex
	data, or in situations where a resource continuity is
	required, replacing a resource is not an option. Several
	applications using CoAP will need to perform partial resource
	modifications. This proposal adds a new CoAP method, PATCH, to
	modify an existing CoAP resource partially.
      </t>
    </abstract>
  </front>

  <middle>
    <section anchor="intro" title="Introduction">
      
      <t>
	This specification defines the new Constrained Application
	Protocol (CoAP) <xref target="RFC7252"/> method, PATCH, which
	is used to apply partial modifications to a resource.  
      </t>
      <t>
	PATCH is also specified for HTTP in <xref
	target="RFC5789"/>. Most of the motivation for PATCH described
	in <xref target="RFC5789"/> also applies here.
      </t>
      <t> 
	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. <xref target="I-D.vanderstok-core-comi"/>). Using PATCH
	avoids transferring all data associated with a resource in
	case of modifications, thereby not burdening the constrained
	communication medium.
      </t>
      <t> 
	This document relies on knowledge of the PATCH specification
	for HTTP <xref target="RFC5789"/>. This document provides
	extracts from <xref target="RFC5789"/> to make independent
	reading possible.
      </t>

      <section title="Requirements Language">
        <t>
	  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
	  "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
	  and "OPTIONAL" in this document are to be interpreted as
	  described in <xref target="RFC2119"/>.
	</t>
      </section>  <!-- Requirements language -->

      <section title="Terminology and Acronyms">
	<t>
	  This document uses terminology defined in <xref
	  target="RFC5789"/> and <xref target="RFC7252"/>.
	</t>
      </section>   <!-- terminology -->
      
    </section>   <!-- Introduction -->

    <section anchor="patch" title="Patch Method">
      <t>
	The PATCH method requests that a set of changes described in
	the request payload is 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, etc.
	Creation of a new resource would result in a 2.01 (Created)
	Response Code dependent of the patch document type.
      </t>
      <t>
	Restrictions to a PATCH can be made by including the If-Match
	or If-None-Match options in the request (see Section 5.10.8.1
	and 5.10.8.2 of <xref target="RFC7252"/>).  If the resource
	could not be created or modified, then an appropriate Error
	Response Code SHOULD be sent.
      </t>
      <t> 
	The difference between the PUT and PATCH requests is
	extensively documented in <xref target="RFC5789"/>.
      </t>
      <t>
	PATCH is not safe but idempotent conformant to CoAP PUT
	specified in <xref target="RFC7252"/>, Section 5.8.3.
      </t>
      <t> 
	A PATCH request is idempotent to prevent bad outcomes from
	collisions between two PATCH requests on the same resource in
	a similar time frame. These collisions can be detected with
	the MessageId and the source end-point provided by the CoAP
	protocol (see section 4.5 of <xref target="RFC7252"/>.
      </t>
      <t>
	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 general,
	modifications MUST NOT be applied to the server state when an
	error occurs or only a partial execution is possible.  The
	atomicity requirement holds for all directly affected
	resources.
      </t>
      <t>
	A PATCH response can invalidate a cache conformant with the
	PUT response. Caching behaviour as function of the valid 2.xx
	response codes for PATCH are: </t>
      <t>
	<list style="empty">
	  <t>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.</t>
	  <t> A 2.04 (Changed) response invalidates any cache entry
	  for the target resource; the payload is a representation of
	  the action result.</t>
	</list>
      </t>
      <t>
	There is no guarantee that a resource can be modified with
	PATCH. Servers are required to support a subset of the content
	formats as specified in sections 12.3 and 5.10.3 of <xref
	target="RFC7252"/>.  Servers MUST ensure that a received PATCH
	payload is appropriate for the type of resource identified by
	the target resource of the request.  
      </t>
      <t> 
	Clients MUST choose to use PATCH rather than PUT when the
	request affects partial updates of a given resource.
      </t>

      <section anchor="example" title="A Simple PATCH Example">
	<t>
	  The example is taken over from <xref target="RFC6902"/>,
	  which specifies a JSON notation for PATCH operations. A
	  resource located at www.example.com/object contains a target
	  JSON document.
	</t>
	<figure><artwork align="left">
	  <![CDATA[ 
        JSON document orginal state
             { 
                "x-coord": 256,
                "y-coord": 45
             }

		   REQ: 
                  PATCH CoAP://www.example.com/object 
		   [
                 { "op":"replace","path":"x-coord","value":45}
              ]
		   RET:
		        CoAP 2.04 Changed
    
        JSON document final state
             { 
                "x-coord": 45,
                "y-coord": 45
             }
	]]></artwork></figure>

	<t>
	  This example illustrates use of a hypothetical PATCH on the
	   /object/x-coord of the existing resource "object".
	  The 2.04 (Changed) response code is conforms with the CoAP
	  PUT method.
	</t>
      </section>  <!-- patch example -->

      <section anchor="response" title= "Response Codes">
	<t>
	  PATCH for CoAP adopts the response codes as specified in
	  sections 5.9 and 12.1.2 of <xref target="RFC7252"/>.
	</t>
      </section>  <!-- Response codes -->


      <section anchor="option" title= "Option Numbers">
	<t>
	  PATCH for CoAP adopts the option numbers as specified in
	  sections 5.10 and 12.2 of <xref target="RFC7252"/>.
	</t>
      </section>  <!-- Option numbers -->

      <!--<section anchor="security" title=" Securing PATCH">
	<t>
	  PATCH is secured following the CoAP recommendations as
	  specified in section 9 of <xref target="RFC7252"/>. When
	  more appropriate security techniques are standardized for
	  CoAP, PATCH can also be secured by those new techniques.
	</t>
      </section>--> <!-- Securing patch  -->

    </section>  <!-- Patch method -->

    <section anchor="errors" title="Error Handling">
      <t>
	A PATCH request may fail under certain known conditions. These
	situations should be dealt with as expressed below.
      </t>
      <t>
	<list style="hanging">
	  <t hangText="Malformed PATCH payload:">If a server
	  determines that the payload provided with a PATCH 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.</t>
	  <t hangText="Unsupported PATCH payload:">In case a client
	  sends 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.</t>
	  <t hangText="Unprocessable request:">This situation occurs
	  when the payload of a PATCH request is determined as valid,
	  i.e. well-formed and supported, however, the server is
	  unable to or incapable of processing the request. The server
	  can return a 4.22 (Unprocessable Entity) CoAP error. More
	  specific scenarios might include situations when:
	  <list style="symbols">
	    <t> the server has insufficient computing resources to
	    complete the request successfully -- 4.13 (Request Entity
	    Too Large) CoAP Response Code,</t>
	    <t> the resource specified in the request becomes invalid
	    by applying the payload -- 4.06 (Not Acceptable) CoAP
	    Response Code,</t>
            <!--<t> 4.nr1 CoAP response code: the modification specified
            by a PATCH request cannot be applied to a resource in its
            current state, or causes the resource to enter an
            inconsistent state</t>-->
	  </list></t>
	  <t>In case there are more specific errors that provide more
	  insight into the problem, then those should be used.</t>
	  <t hangText="Resource not found:">The 4.04 (Not Found) error
	  should be returned in case the payload of a PATCH request
	  cannot be applied to a non-existent resource.</t>
	  <!--<t hangText="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.</t>-->
	  <t hangText="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.</t>
	  <t hangText="Conflicting state:">If the modification
	  specified by a PATCH request cannot be applied to a resource
	  in its current state, or causes the resource to enter an
	  inconsistent state the server can return the 4.09 (Conflict)
	  CoAP response. 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
	  or lead the device into an inconsistent state if the
	  modifications are made.</t>	    
	  <t hangText="Conflicting modification:">In situations when a
	  server detects possible conflicting modifications <!--and no
	  precondition is defined in the requests,--> the server can
	  return a 4.nr2 CoAP response code.</t>
	  <t hangText="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 4.09 (Conflict) CoAP response code.</t>
	</list>
      </t>
      <t>
	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.
      </t>
    </section>

    <section title="Security Considerations">
      <t>
	This section analyses the possible threats to the CoAP PATCH
	protocol.  It is meant to inform protocol and application
	developers about the security limitations of CoAP PATCH as
	described in this document.  The security consideration of
	section 15 of <xref target="RFC2616"/>, section 11 of <xref
	target="RFC7252"/>, and section 5 of <xref target="RFC5789"/>
	also apply.
      </t>
      <t>
	The security considerations for PATCH are nearly identical to
	the security considerations for PUT (<xref
	target="RFC7252"/>).  The mechanisms used for PUT can be used
	for PATCH as well.
      </t>
      <t>
	PATCH is secured following the CoAP recommendations as
	specified in section 9 of <xref target="RFC7252"/>. When more
	appropriate security techniques are standardized for CoAP,
	PATCH can also be secured by those new techniques.
      </t>
    </section>  <!-- security consideration  -->

    <section title="IANA Considerations">
      <t> 
	The entry with name PATCH in the sub-registry, "CoAP Method
	Codes", is 0.05. the addition will follow the "IETF Review or
	IESG Approval" procedure as described in <xref
	target="RFC5226"/>.
      </t>
      <t> TODO, definition of CoAP response code 4.09 for addition to
      the sub-registry of CoAP response codes.</t>
<t>
Additions to the sub-registry "CoAP
Content-Formats", within the "CoRE Parameters" registry are needed for the following media type formats: "application/json-patch+json" <xref target="RFC6902"/>, and "application/merge-patch+json" <xref target="RFC7386"/>.
</t>
    </section>   <!-- IANA considerations  -->

    <section title="Acknowledgements">
      <t>
Klaus Hartke has pointed out some essential differences between CoAP and HTTP. We are grateful for discussions with Carsten Bormann, Kovatsch Matthias, and Thomas Watteyne.
      </t>
    </section>  <!-- Acknowledgements -->

    <section title="Change log">
      <t>
	When published as a RFC, this section needs to be removed.	
      </t><t>
Version 0 to version 1:
<list style="symbols">
<t> Changed patch motivation text.</t>
<t> Removed sub-resource concept. </t>
<t> Updated cache handling.</t>
<t> Extended example. </t>
<t> Update of error handling.</t>
</list>
</t>
    </section>  <!-- Change log -->

  </middle>

  <back>
    <references title="Normative References">
	&RFC2119;
	&RFC2616;
	&RFC5226;
	&RFC5789;
     &RFC6902;
	&RFC7252;
      &RFC7386;    
    </references>

    <references title="Informative References">
      &I-D.vanderstok-core-comi;
    </references>   
  </back>
</rfc>