Patch Method for Constrained Application Protocol (CoAP)
draft-vanderstok-core-patch-01

Abstract

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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 7, 2016.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 1.2. Terminology and Acronyms
• 2. Patch Method
• 2.1. A Simple PATCH Example
• 2.2. Response Codes
• 2.3. Option Numbers
• 3. Error Handling
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgements
• 7. Change log
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

This specification defines the new Constrained Application Protocol (CoAP) [RFC7252] method, PATCH, which is used to apply partial modifications to a resource.

PATCH is also specified for HTTP in [RFC5789]. Most of the motivation for PATCH described in [RFC5789] also applies here.

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. [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.

This document relies on knowledge of the PATCH specification for HTTP [RFC5789]. This document provides extracts from [RFC5789] to make independent reading possible.

1.1. Requirements Language

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 [RFC2119].

1.2. Terminology and Acronyms

This document uses terminology defined in [RFC5789] and [RFC7252].

2. Patch Method

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.

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 [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 extensively documented in [RFC5789].

PATCH is not safe but idempotent conformant to CoAP PUT specified in [RFC7252], Section 5.8.3.

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 [RFC7252].

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.

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:

• 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.
• 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. Servers are required to support a subset of the content formats as specified in sections 12.3 and 5.10.3 of [RFC7252]. Servers MUST ensure that a received PATCH payload is appropriate for the type of resource identified by the target resource of the request.

Clients MUST choose to use PATCH rather than PUT when the request affects partial updates of a given resource.

2.1. A Simple PATCH Example

The example is taken over from [RFC6902], which specifies a JSON notation for PATCH operations. A resource located at www.example.com/object contains a target JSON document.

	   
        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
             }
	

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.

2.2. Response Codes

PATCH for CoAP adopts the response codes as specified in sections 5.9 and 12.1.2 of [RFC7252].

2.3. Option Numbers

PATCH for CoAP adopts the option numbers as specified in sections 5.10 and 12.2 of [RFC7252].

3. Error Handling

A PATCH request may fail under certain known conditions. These situations should be dealt with as expressed below.

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.

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.

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:

• the server has insufficient computing resources to complete the request successfully -- 4.13 (Request Entity Too Large) CoAP Response Code,
• the resource specified in the request becomes invalid by applying the payload -- 4.06 (Not Acceptable) CoAP Response Code,

In case there are more specific errors that provide more insight into the problem, then those should be used.

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.

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.

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.

Conflicting modification:

In situations when a server detects possible conflicting modifications the server can return a 4.nr2 CoAP response code.

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.

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. Security Considerations

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 [RFC2616], section 11 of [RFC7252], and section 5 of [RFC5789] also apply.

The security considerations for PATCH are nearly identical to the security considerations for PUT ([RFC7252]). The mechanisms used for PUT can be used for PATCH as well.

PATCH is secured following the CoAP recommendations as specified in section 9 of [RFC7252]. When more appropriate security techniques are standardized for CoAP, PATCH can also be secured by those new techniques.

5. IANA Considerations

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 [RFC5226].

TODO, definition of CoAP response code 4.09 for addition to the sub-registry of CoAP response codes.

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" [RFC6902], and "application/merge-patch+json" [RFC7386].

6. Acknowledgements

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.

7. Change log

When published as a RFC, this section needs to be removed.

Version 0 to version 1:

• Changed patch motivation text.
• Removed sub-resource concept.
• Updated cache handling.
• Extended example.
• Update of error handling.

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses