[core] More precision about CoAP Observe with FETCH

Simon Bernard <contact@simonbernard.eu> Tue, 29 June 2021 09:18 UTC

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From: Simon Bernard <contact@simonbernard.eu>
To: "core@ietf.org WG" <core@ietf.org>
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Date: Tue, 29 Jun 2021 11:18:36 +0200
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Subject: [core] More precision about CoAP Observe with FETCH
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Hi all,

  The PATCH and FETCH Methods for the Constrained Application Protocol (CoAP) (https://datatracker.ietf.org/doc/html/rfc8132" rel="nofollow">RFC8132) says that FETCH can be used with https://datatracker.ietf.org/doc/html/rfc7641" rel="nofollow">RFC7641 (Observing Resources in CoAP)

https://datatracker.ietf.org/doc/html/rfc8132#section-2.4" rel="nofollow">2.4.  Working with Observe

   The Observe option [https://datatracker.ietf.org/doc/html/rfc7641" title='"Observing Resources in the Constrained Application Protocol (CoAP)"' rel="nofollow">RFC7641] can be used with a FETCH request as it
   can be used with a GET request
  But "CoAP Observe" seems to be designed with a "GET without payload" in mind.
  E.g. it says :

https://datatracker.ietf.org/doc/html/rfc7641#section-3.1" rel="nofollow">3.1.  Request

   A client registers its interest in a resource by issuing a GET
   request with an Observe Option set to 0 (register).  If the server
   returns a 2.xx response that includes an Observe Option as well, the
   server has successfully added an entry with the client endpoint and
   request token to the list of observers of the target resource, and
   the client will be notified of changes to the resource state.

   Like a fresh response can be used to satisfy a request without
   contacting the server, the stream of updates resulting from one
   observation request can be used to satisfy another (observation or
   normal GET) request if the target resource is the same.  A client
   MUST aggregate such requests and MUST NOT register more than once for
   the same target resource.  The target resource is identified by all
   options in the request that are part of the Cache-Key. This includes,
   for example, the full request URI and the Accept Option.  

The target resource is not identified by the payload.  So 2 FETCH requests on same resource with 2 different payload seem to not be allowed ?

Is it intended or just an oversight ? if this is intended, is there any particular reason about this ?