The 'Pending' Response Code for the Constrained Application Protocol (CoAP)
draft-hartke-core-pending-00

CoRE Working Group                                       P. van der Stok
Internet-Draft                                                consultant
Intended status: Informational                                 K. Hartke
Expires: August 31, 2017                         Universitaet Bremen TZI
                                                       February 27, 2017

  The 'Pending' Response Code for the Constrained Application Protocol
                                 (CoAP)
                      draft-hartke-core-pending-00

Abstract

   This document proposes a new CoAP response code, 2.06 Pending.  A
   CoAP server can use this response code to signal that it has accepted
   the request but has not yet started processing it or that processing
   the request will take longer than a client is typically willing to
   wait for a response.  A 2.06 response can include status information
   and indicate a location where the result will become available.

Status of This Memo

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   This Internet-Draft will expire on August 31, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  2.06 Pending  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Observing Resources . . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a request/
   response protocol not unlike HTTP.  CoAP defines no upper bound for
   the time between a request and the resulting response.  E.g., in CoAP
   over UDP, a server is expected to return an empty Acknowledgement to
   the client if it cannot provide a response right away, but there is
   no limit on when the server should return the Separate Response.

   In particular in the case of requests with long processing times, a
   CoAP client faces the problem that it cannot easily determine how
   long it should wait for the response and whether the CoAP server is
   even still processing the request.  Long processing times occur, for
   example, when requests need manual intervention to authorize their
   processing, or when they perform a long sequence of remote actions.
   An example is provided by the "possibly long" authorization request
   specified in EST-coaps [I-D.vanderstok-ace-coap-est].

   This document proposes a new CoAP response code, 2.06 Pending.  The
   semantics of this response code are modelled after the HTTP [RFC7231]
   202 (Accepted) status code:

      The 202 (Accepted) status code indicates that the request has been
      accepted for processing, but the processing has not been
      completed.  The request might or might not eventually be acted
      upon, as it might be disallowed when processing actually takes
      place. [...] The representation sent with this response ought to
      describe the request's current status and point to (or embed) a
      status monitor that can provide the user with an estimate of when
      the request will be fulfilled.

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   The 2.06 (Pending) response code is not meant for overload cases,
   which are better handled by the 5.03 (Service Unavailable) response
   code.

1.1.  Terminology

   Readers are expected to be familiar with the terms and concepts
   described in [RFC7252] and [RFC7641].

   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].

2.  2.06 Pending

   A 2.06 (Pending) response in reply to a GET request indicates that
   the target resource exists but no representation of the resource is
   available yet.  The Max-Age Option indicates after what time a client
   should retry its GET request to retrieve the representation.  The
   client MAY observe the resource (see Section 2.1) to be notified when
   the representation becomes available.

   A 2.06 (Pending) response in reply to a POST request indicates that
   the result of processing the request is not available yet, for
   example, because the server needs more time to process the request
   than a client is typically willing to wait for a response.  The
   server MAY specify a location using the Location-* options where the
   result will become available.  If the server does not specify a
   location, the result will become available at the target resource of
   the POST request.  To receive the result, the client MAY poll or
   observe the resource at the specified location using a GET request.
   The Max-Age Option indicates how long the client should wait before
   making the GET request.

   A 2.06 (Pending) response MAY contain a payload that represents the
   progress of processing the original request or any other status
   information.  The content format of this representation is specified
   by the Content-Format Option.

   A 2.06 (Pending) response is cacheable, but cannot be validated.  If
   it contains Location-* options, it invalidates any cached response
   for the resource at the specified location; otherwise, it invalidates
   any cached response for the target resource of the request.

   As a consequence of being cacheable, a 2.06 (Pending) response in
   reply to a POST request makes the POST method temporarily idempotent:
   until Max-Age expires, any POST request with the same cache-key -- be

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   it from the same client or any another client -- can yield the same
   2.06 (Pending) response.  (This is the same behavior as for 4.xx and
   5.xx error responses in reply to POST requests.)

2.1.  Observing Resources

   When a client registers to observe [RFC7641] a resource for which no
   representation is available yet, the server MAY send one or more 2.06
   (Pending) notifications before sending the first 2.05 (Content) or
   2.03 (Valid) notification.  The possible resulting sequence of
   notifications is shown in Figure 1.

                  __________       __________       __________
                 |          |     |          |     |          |
            ---->|   2.06   |---->|  2.05 /  |---->|  4.xx /  |
                 | Pending  |     |   2.03   |     |   5.xx   |
                 |__________|     |__________|     |__________|
                    ^   \ \          ^    \           ^
                     \__/  \          \___/          /
                            \_______________________/

                    Figure 1: Sequence of Notifications

   Unless the server is not willing to add the client to the list of
   observers, each 2.06 (Pending) notification MUST include an Observe
   Option with a sequence number as specified in [RFC7641].  Otherwise,
   the registration request falls back to a normal GET request.

3.  Security Considerations

   This section analyses the possible threats related to 2.06 (Pending)
   responses.  It is meant to inform protocol and application developers
   about the security limitations of the response code as described in
   this document.

   A 2.06 (Pending) response is subject to the same general security
   considerations as all CoAP responses as described in Section 11 of
   [RFC7252].  Specifically, the security considerations for the
   response code are closest to those of the Observe Option as stated in
   Section 7 of [RFC7641], because the server stores additional state
   over an extended period.

   2.06 (Pending) responses are secured following the recommendations
   for the existing CoAP response codes as specified in Section 9 of
   [RFC7252].  When additional security techniques are standardized for
   CoAP (e.g., based on object security), these are then also available
   for securing the responses.

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4.  IANA Considerations

   This document adds the 2.06 (Pending) response code to the "CoAP
   Response Codes" registry.

                    +------+-------------+-----------+
                    | Code | Description | Reference |
                    +------+-------------+-----------+
                    | 2.06 | Pending     | [RFCXXXX] |
                    +------+-------------+-----------+

                     Table 1: New CoAP Response Codes

5.  References

5.1.  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>.

   [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>.

5.2.  Informative References

   [I-D.vanderstok-ace-coap-est]
              Kumar, S. and P. Stok, "EST based on DTLS secured CoAP
              (EST-coaps)", draft-vanderstok-ace-coap-est-00 (work in
              progress), December 2016.

   [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>.

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Authors' Addresses

   Peter van der Stok
   consultant

   Phone: +31-492474673 (Netherlands), +33-966015248 (France)
   Email: consultancy@vanderstok.org
   URI:   www.vanderstok.org

   Klaus Hartke
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63905
   Email: hartke@tzi.org

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