Accept-Any option for CoAP

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Last updated 2019-03-25
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CoRE                                                          C. Amsuess
Internet-Draft                                            March 25, 2019
Updates: 7641 (if approved)
Intended status: Standards Track
Expires: September 26, 2019

                       Accept-Any option for CoAP


   This memoy defines the Accept-Any option, which provides a more
   flexible content negotiation than the one originally specified for
   the Constrained Application Protocol (CoAP) in [RFC7252].  As this is
   particularly useful with but ruled out in CoAP observation
   ([RFC7641]), that is updated to allow it.

Status of This Memo

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   This Internet-Draft will expire on September 26, 2019.

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   the Trust Legal Provisions and are provided without warranty as
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1.  Introduction

   [ This document is being developed in git at [1]. ]

   When CoAP content format defined in [RFC7252], the choice was made to
   have the initial content negotiation allow the client to only pick
   zero or one content format.  This is a good choice for many
   situations and ensures that proxies can cache as much as possible
   without added complexity.  A client that does not know a usable
   content format would either leave the request field absent,
   indicating it would accept any response, or try several acceptble
   values in a series of requests.

   In line with that choice, observation ([RFC7641]) required
   notification responses to carry the same content format in the
   notification as in the original response.

   For applications that expect a representation to change its
   representability to change during an observation's lifetime,
   [I-D.ietf-core-multipart-ct] introduces a way of wrapping responses
   in an application/multipart-core response.  That approach is
   convenient for bags of representations, but lacks actual content
   negotiation and produces representations that are not directly usable
   but need to be processed from inside the multipart representation.

   This document introduces an additional way for the client to indicate
   its set of acceptable content formats, and removes the same-content-
   format limitation during observation.

2.  The Accept-Any option

   A new option Accept-Any is defined.  It is critical [ I don't fully
   see why but follow Accept here ], safe-to-foward and part of the
   cache key.  Its format is uint up to 2 long (indicating content
   types), it is Class E in OSCORE, usable in requests only, and

   Repeatability is the only aspect in which it differs from Accept in
   terms of option properties.

   Its values indicate a list of acceptable representations in order of
   decreasing preference.  A server MUST answer with the first format it
   can represent the requested state in, or 4.06 (Not Acceptable) if it

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   could answer successfully but the response would not match any of the
   option values.

   The Accept-Any option MUST NOT be used exactly once; that request's
   meaning would be identical to that of a single Accept option.
   Instead, a single Accept option is used.

2.1.  Proxy behavior

   A proxy MAY ignore this option per its properties (and serve a cached
   response if the cache key matches), but can implement additional
   behavior to enhance its cache.

   A proxy is allowed to serve a cached representation to a request with
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