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Versions: 00 draft-vanderstok-core-etch

CoRE                                                          C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                        October 19, 2015
Expires: April 21, 2016

                           CoAP FETCH Method


   Similar to HTTP, the existing Constrained Application Protocol (CoAP)
   GET method 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 to some applications to using POST
   where actually a cacheable, idempotent, safe request is desired.

   The present proposal adds a new CoAP method, FETCH, to perform the
   equivalent of a GET with a request body.

   This specification is inspired by I-D.snell-search-method, 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.  For now, we
   therefore prefer to discuss the proposal under a different name, for
   which we have chosen the GET synonym FETCH.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.

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Copyright Notice

   Copyright (c) 2015 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  FETCH . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  The Content-Format Option . . . . . . . . . . . . . . . .   4
     2.2.  Working with Observe  . . . . . . . . . . . . . . . . . .   4
     2.3.  Working with Block  . . . . . . . . . . . . . . . . . . .   5
     2.4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   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 additionally shape the representation.  This
   has been modelled after the HTTP GET operation and the REST model in

   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 to enable
   supplying a larger set of parameters to the search than can be
   comfortably transferred in the URI with a GET request.
   [I-D.snell-search-method] proposes a SEARCH method that is similar to
   GET in most properties but enables sending a request body as with

   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.

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   Using the request body for this information has a number of

   o  The client can specify a media type (and a content encoding),
      enabling 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,
      enabling 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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in


   The CoAP FETCH method is used to obtain a representation of a
   resource, giving 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 based on the request parameters (described
   by the request options and 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.

   The body of the request defines the request parameters.
   Implementations MAY use a request body of any content type with the

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   FETCH method; it is outside the scope of this document how
   information about admissible content types is obtained by the client
   (although we can hint that form relations ([I-D.hartke-core-apps])
   might be the preferred way).

   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 expected to alter the state of the targeted resource.
   (However, while processing a search 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

   A successful response to a FETCH request is expected to provide some
   indication as to the final disposition of the requested operation.
   If the 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 payload is part of the
   cache key.  Specifically, 2.05 "Content" response codes, the
   responses for which are cacheable, are a usual way to respond to a
   FETCH request.  (Note that this aspect differs markedly from
   [I-D.snell-search-method].)  (Note also that caches that cannot use
   the request payload as part of the request 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.  [[This needs some
   additional text on what an ETag on a FETCH result means.]]

2.1.  The Content-Format Option

   A FETCH request MUST include a Content-Format option to specify the
   media type and content encoding of the request body.

2.2.  Working with Observe

   The Observe option [RFC7641] can be used with a FETCH request as it
   can be used with a GET request.

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2.3.  Working with Block

   The Block1 option [I-D.ietf-core-block] can be used with a FETCH
   request as it would be used with a POST request; the Block2 option
   can then be used as with GET or POST.

2.4.  Discussion

   One property of FETCH that may be non-obvious is that a FETCH request
   cannot be generated from a link alone, but also needs a way to
   generate the request payload.  Again, form relations
   ([I-D.hartke-core-apps]) may be able to fill parts of this gap.

3.  Security Considerations

   The FETCH method is subject to the same general security
   considerations as all CoAP methods as described in [RFC7252].

4.  IANA Considerations

   IANA is requested to add an entry to the sub-registry "CoAP Method

                      | Code | Name   | Reference |
                      | 0.07 | FETCH  | [RFCthis] |

   The FETCH method is idempotent and safe, and it returns the same
   response codes that GET can return, plus 4.15 "Unsupported Content-
   Format" with the same semantics as with POST.

5.  Acknowledgements

   Most of the text in this I-D was stolen, e.g. from
   [I-D.snell-search-method] or from [I-D.vanderstok-core-patch].  Thank

   Klaus Hartke found a number of problems while quickly checking an
   earlier version of this document.

6.  References

6.1.  Normative References

              Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",
              draft-ietf-core-block-18 (work in progress), September

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
              RFC7252, June 2014,

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641, DOI 10.17487/
              RFC7641, September 2015,

6.2.  Informative References

              Hartke, K., "CoRE Application Descriptions", draft-hartke-
              core-apps-02 (work in progress), August 2015.

              Reschke, J., Malhotra, A., and J. Snell, "HTTP SEARCH
              Method", draft-snell-search-method-00 (work in progress),
              April 2015.

              Stok, P. and A. Sehgal, "Patch Method for Constrained
              Application Protocol (CoAP)", draft-vanderstok-core-
              patch-02 (work in progress), October 2015.

Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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