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CoRE Working Group                                       P. van der Stok
Internet-Draft                                                consultant
Intended status: Informational                                 K. Hartke
Expires: August 30, 2018                         Universitaet Bremen TZI
                                                       February 26, 2018


  "Pending" Responses for the Constrained Application Protocol (CoAP)
                      draft-hartke-core-pending-02

Abstract

   This document proposes a new type of response for the Constrained
   Application Protocol (CoAP) called a "Pending" response.  A CoAP
   server can use a Pending response to indicate that it has accepted a
   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.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 30, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Pending Responses . . . . . . . . . . . . . . . . . . . . . .   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.  For example,
   a CoAP-over-UDP 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 the time 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
   actually 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 for this is the "possibly long" authorization
   request specified in EST-coaps [I-D.vanderstok-ace-coap-est].

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

      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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   Pending responses are not intended 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.  Pending Responses

   A Pending response is denoted by a response code in the 2.xx range
   and a Content-Format Option that is set to content-format ID TBD1.

   A 2.01 (Creation 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 retrieve the result, the client MAY poll or
   observe the resource at this location using the GET request method.

   A 2.02 (Deletion Pending) response in reply to a DELETE request
   indicates that the server has accepted the request but the target has
   not been fully deleted yet.

   A 2.04 (Change Pending) response in reply to a POST or PUT request
   indicates that the server has accepted the request but the result of
   processing the request is not available yet.

   A 2.05 (Content Pending) response in reply to GET request indicates
   that the target resource exists but a representation of the resource
   is not 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 [RFC7641] to get notified when
   the representation becomes available (see Section 2.1 for details).

   The payload of a Pending response MAY be a brief human-readable
   diagnostic message, explaining the situation, or MUST be absent.

   The cacheability of Pending responses is as specified for the
   respective response code.



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2.1.  Observing Resources

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

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

                    Figure 1: Sequence of Notifications

   Unless the server is unwilling to add the client to the list of
   observers, each 2.05 (Content 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 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 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.

   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 content-format used to signal Pending
   responses to the "CoAP Content-Formats" registry.

         +------------+----------------+------+-----------------+
         | Media Type | Content Coding | ID   | Reference       |
         +------------+----------------+------+-----------------+
         | -          | -              | TBD1 | [This Document] |
         +------------+----------------+------+-----------------+

                         New CoAP Content-Formats

   TBD1 is taken from the "First Come First Served" range of the "CoAP
   Content-Formats" registry.

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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7641>.

5.2.  Informative References

   [I-D.vanderstok-ace-coap-est]
              Stok, P., Kampanakis, P., Kumar, S., Richardson, M.,
              Furuhed, M., and S. Raza, "EST over secure CoAP (EST-
              coaps)", draft-vanderstok-ace-coap-est-04 (work in
              progress), January 2018.

   [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,
              <https://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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