[core] Links, hosts, and resource directories

[Klaus Hartke <[e-mail]>]

draft-ietf-core-resource-directory-17 [1] says:

   [A Resource Directory (RD)] hosts registrations of
   resources held on other servers, allowing lookups
   to be performed for those resources.

Is this a lie?

I'm confused.

Example time!

In the following, I'm using URI references [2] in angle brackets --
like <coap://example.com/foo> -- to denote a reference to a resource
and a 3-tuple -- like (coap,example.com,5683) -- to denote a Web
origin [3]. Note that <coap://example.com/> references the resource at
the root in the hierarchy of the endpoint (coap,example.com,5683), not
the endpoint. Let `$origin(x)` be a function that returns the Web
origin for a given URI `x`.

Let's say we have an endpoint (coap,example.com,5683) that has three
resources: <coap://example.com/foo>, <coap://example.com/bar>, and
<coap://example.com/baz>. The endpoint wishes to enable resource
discovery [4], so it provides another resource
<coap://example.com/.well-known/core> with the following
representation in Link Format [5]:

    <coap://example.com/foo>;rt=a,<coap://example.com/bar>;rt=b,<coap://example.com/baz>;rt=a

This means, there are three links [6]:

   <coap://example.com/.well-known/core>  hosts
          <coap://example.com/foo>  which has type "a" .
   <coap://example.com/.well-known/core>  hosts
          <coap://example.com/bar>  which has type "b" .
   <coap://example.com/.well-known/core>  hosts
          <coap://example.com/baz>  which has type "a" .

The link context of each link is the represented resource. The link
relation type is implicitly the default value for "rel" in Link
Format, "hosts". The link target is the referenced resource.

Because the "hosts" link relation type is weird, these three links are
actually not links but the following statements:

   $origin(<coap://example.com/.well-known/core>)  has a resource
          <coap://example.com/foo>  which has type "a" .
   $origin(<coap://example.com/.well-known/core>)  has a resource
          <coap://example.com/bar>  which has type "b" .
   $origin(<coap://example.com/.well-known/core>)  has a resource
          <coap://example.com/baz>  which has type "a" .

or, in other words:

   (coap,example.com,5683)  has a resource  <coap://example.com/foo>
          which has type "a" .
   (coap,example.com,5683)  has a resource  <coap://example.com/bar>
          which has type "b" .
   (coap,example.com,5683)  has a resource  <coap://example.com/baz>
          which has type "a" .

This is totally redundant, because $origin(<coap://example.com/foo>)
is (coap,example.com,5683) as well. So the statements don't provide
any information that cannot already be derived from the resource
identifiers directly.

Ignoring this cognitive dissonance, if the endpoint now wishes to
enable resource discovery through a directory, the idea is that it can
upload the representation of its <coap://example.com/.well-known/core>
to this directory, allowing other nodes to perform lookups.

There seem to be two ways to model this: either as a directory of
resources or as a directory of links.

Asking a directory of resources at <coap://rd.example/> for resources
of type "a" should give a list of links from the directory to matching
resources:

    <coap://rd.example/>  contains a registration for
          <coap://example.com/foo>  which has type "a" .
    <coap://rd.example/>  contains a registration for
          <coap://example.com/baz>  which has type "a" .

or, in Link Format:

    <coap://example.com/foo>;rel=item;rt=a,<coap://example.com/baz>;rel=item;rt=a

Asking a directory of links at <coap://ld.example/> for links to
resources of type "a" should give a list of matching links:

   (coap,example.com,5683)  has a resource  <coap://example.com/foo>
          which has type "a" .
   (coap,example.com,5683)  has a resource  <coap://example.com/baz>
          which has type "a" .

or, in Link Format:

    <coap://example.com/foo>;rel=hosts;rt=a;anchor="coap://example.com/.well-known/core",<coap://example.com/baz>;rel=hosts;rt=a;anchor="coap://example.com/.well-known/core"

After skimming through draft-ietf-core-resource-directory [1] and the
interop spec [7] a few times, it's not clear to me whether the draft
describes a resource directory or a link directory, as none of the
examples seem to match my examples above.

Am I completely off?

Is it a lie that a resource directory allows lookups of resources; is
it for lookups of links? Isn't the interop spec fundamentally broken
then?

And why does the "hosts" link relation type have to be so weird?

Klaus

[1] https://tools.ietf.org/html/draft-ietf-core-resource-directory-17
[2] https://tools.ietf.org/html/rfc3986#section-4.1
[3] https://tools.ietf.org/html/rfc6454#section-4
[4] https://tools.ietf.org/html/rfc7252#section-7.2
[5] https://tools.ietf.org/html/rfc6690#section-2
[6] https://tools.ietf.org/html/rfc8288#section-2
[7] http://htmlpreview.github.io/?https://github.com/core-wg/resource-directory/blob/a978f7c2e89fa97daea967716765a798bbfda4e8/interop-spec.html