Subject: [core] Link Format document

From: Peter Bigot <[email-masked]>
Date: Sun, 21 Nov 2010 10:55:10 -0600

Either at the IETF session or in some email I can't easily find, there was a
specific question raised about exactly how the link description is supposed
to be used in CoAP that started me down a path that led to the following
comment.  I've cc'd Mark Nottingham, author of RFC5988 "Web Linking", in
hopes he might have time to provide relevant information to guide us.

My understanding of RFC5988 is it proposes a way to provide, in an HTTP
response header, information about (target) resources that are related in
(rel) some way to the (context) resource identified in the original
request.  Other contexts can be specified through the anchor parameter, but
I expect this is unusual.

In CoAP's link format document at
http://tools.ietf.org/html/draft-ietf-core-link-format, we are rather
mis-using this to provide a list of resources that are hosted on a server.
Technically, the context of the list items is the resource which is the
server itself ("coap://authority"), though we provide access to it at
"coap://authority/.well-known/core".

The problem is that, because the document containing links lists the
resources available on the server, we provide additional information about
specific resources in link parameters, rather than as link descriptions.  I
now think that the roles served by the "d" and "sh" extensions, and some
uses of "n", should really be link descriptions whose context is the target,
target is the extension value, and rel is the extension name.  For example,
rather than:

<sensors/temperature>n="http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature
",sh="/l";

being one of the link descriptions at coap://authority/.well-known/core, use
consistent with RFC5988 would be to provide:

<http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature>rel="describedby";
</l>rel="alternate";

as the set of link descriptions for coap://authority/sensors/temperature.

Based on this perspective, and despite the fact that I've been added as an
author due to my contributions to disambiguating the syntax, I don't think
that CoRE Link Format describes a conceptually clean solution to this
problem that is compatible with what link descriptions mean in RFC5988.

I believe a conceptually clean and compatible solution would be to have
coap://authority/.well-known/core return a list of links like:

<sensors/temperature>rel="hosts";
<sensors/humidity>rel="hosts";

to indicate that these are resources hosted on the server.

Further, define that coap://authority/.well-known/core/<somepath> provides a
link description document for context coap://authority/<somepath>.  For
example, coap://authority/.well-known/core/sensors/temperature would return:

<http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature>rel="describedby";
</l>rel="alternate";

Thus providing in a single document the same sort of thing that would be
provided by a set of Link headers in an HTTP request to the context URI.
Note that what I called "further" is really the only case: returning the
list of hosted resources falls out as a consequence of applying this rule
with an empty <somepath>.  Very simple, very orthogonal, only difference
from HTTP is the URI used to obtain the link descriptions and their encoding
as a document rather than as header Link fields.

(I can't tell from RFC5988 what the "index" relation is intended to do, so
perhaps it could be used instead of the newly created "hosts" relation.)

I don't propose that CoAP actually implement this change, since I don't
think such a proposal would be accepted due to non-technical pressures.  But
I did feel it necessary to raise the possibility, and to ask for an outside
perspective so we don't end up sending IESG something that's completely at
odds with how the concept we're borrowing is understood and used by the rest
of the Internet community.

Peter

---

From: Mark Nottingham <[email-masked]>
Date: Mon, 22 Nov 2010 14:48:05 +1100

Peter,

I haven't dug into that part of CoAP, so please take my response with a grain of salt. 

Having said that, I've been uncomfortable with those extensions for a while, but didn't have the time to figure out why. If what you're saying is the case, I'd generally agree that there are more compatible ways to do this.

Regards,

On 22/11/2010, at 3:55 AM, Peter Bigot wrote:

> Either at the IETF session or in some email I can't easily find, there was a specific question raised about exactly how the link description is supposed to be used in CoAP that started me down a path that led to the following comment.  I've cc'd Mark Nottingham, author of RFC5988 "Web Linking", in hopes he might have time to provide relevant information to guide us.
> 
> My understanding of RFC5988 is it proposes a way to provide, in an HTTP response header, information about (target) resources that are related in (rel) some way to the (context) resource identified in the original request.  Other contexts can be specified through the anchor parameter, but I expect this is unusual.
> 
> In CoAP's link format document at http://tools.ietf.org/html/draft-ietf-core-link-format, we are rather mis-using this to provide a list of resources that are hosted on a server.  Technically, the context of the list items is the resource which is the server itself ("coap://authority"), though we provide access to it at "coap://authority/.well-known/core".
> 
> The problem is that, because the document containing links lists the resources available on the server, we provide additional information about specific resources in link parameters, rather than as link descriptions.  I now think that the roles served by the "d" and "sh" extensions, and some uses of "n", should really be link descriptions whose context is the target, target is the extension value, and rel is the extension name.  For example, rather than:
> 
> <sensors/temperature>n="http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature",sh="/l";
> 
> being one of the link descriptions at coap://authority/.well-known/core, use consistent with RFC5988 would be to provide:
> 
> Based on this perspective, and despite the fact that I've been added as an author due to my contributions to disambiguating the syntax, I don't think that CoRE Link Format describes a conceptually clean solution to this problem that is compatible with what link descriptions mean in RFC5988.
> 
> I believe a conceptually clean and compatible solution would be to have coap://authority/.well-known/core return a list of links like:
> 
> Further, define that coap://authority/.well-known/core/<somepath> provides a link description document for context coap://authority/<somepath>.  For example, coap://authority/.well-known/core/sensors/temperature would return:
> 
> Thus providing in a single document the same sort of thing that would be provided by a set of Link headers in an HTTP request to the context URI.  Note that what I called "further" is really the only case: returning the list of hosted resources falls out as a consequence of applying this rule with an empty <somepath>.  Very simple, very orthogonal, only difference from HTTP is the URI used to obtain the link descriptions and their encoding as a document rather than as header Link fields.
> 
> (I can't tell from RFC5988 what the "index" relation is intended to do, so perhaps it could be used instead of the newly created "hosts" relation.)
> 
> I don't propose that CoAP actually implement this change, since I don't think such a proposal would be accepted due to non-technical pressures.  But I did feel it necessary to raise the possibility, and to ask for an outside perspective so we don't end up sending IESG something that's completely at odds with how the concept we're borrowing is understood and used by the rest of the Internet community.
> 
> Peter

--
Mark Nottingham   http://www.mnot.net/

---

From: Zach Shelby <[email-masked]>
Date: Mon, 22 Nov 2010 14:21:24 +0200

Peter,

I also agree that we don't quite have the ideal use of the link relation concept yet, but are getting there. Consider that the first version of the CoRE link format didn't use the relation concept at all, in the last couple versions we have gotten much closer. But just because we are re-using the link format from RFC5988, does not mean we are bound to use exactly the same link relation semantics that were designed for hypertext HTTP servers (nor does that necessarily even make sense). However, I do find the link relation work done in RFC5988 to be useful also in our domain to some extent, so we should try to apply what we can to M2M. 

I wouldn't go quite as far as Peter's idea below though. For M2M these descriptions need to be as compact as possible, nor can we expect constrained device to traverse many links to find out simple information about a resource. I do think we should improve the way we use rel="" for example, and the use of link-extensions can use improvement too.

On a related note, I also received really helpful feedback from Martin Thomson on this same subject in Beijing. I will CC the list with some of that discussion pretty soon. He has good ideas for improving the use of rel= and how our link-extensions should be used. 

See some comments in-line:

> My understanding of RFC5988 is it proposes a way to provide, in an HTTP response header, information about (target) resources that are related in (rel) some way to the (context) resource identified…

Yes, this is the basic concept of RFC5988. I agree that anchor will be rare, as we point out in Section 2.1 of core-link-format-01.

> In CoAP's link format document at http://tools.ietf.org/html/draft-ietf-core-link-format, we are rather mis-using this to provide a list of resources that are hosted on a server.  Technically, the…

I wouldn't say we are misusing it, but that we are defining what the context is by default. Section 2.1 explains that the CoRE link entries are describing links to resources hosted on the server. The rel= relation is defined to be "service" by default in Section 2.2 to describe that the link is a service of the server. I do like your idea here of explaining that the context is the URI coap://authority. 

> The problem is that, because the document containing links lists the resources available on the server, we provide additional information about specific resources in link parameters, rather than as…

Link parameters are meant to provide further information about the link. This is the purpose of the link-extensions defined in the CoRE link format. In that way they are similar to the link-parameters defined in RFC5988.  

> <sensors/temperature>n="http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature",sh="/l";

I do see what you are getting at here, however in Beijing we basically decided to remove the Short URL "sh" parameter anyways. Furthermore, I don't consider the way you are thinking about the Name "n" is quite right. This is not meant to be a link (URL) to some document, but instead is meant to be a semantic name or URI used as a semantic name.

> <http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature>rel="describedby";

You could do this, however it would be both inefficient and complex. I think we can avoid this by using Name properly. Let's get back to that in a separate mail with Marin's suggestions. 

> to indicate that these are resources hosted on the server.

Now I like this idea. I tried to use "service" to indicate this, but would be happy with defining a new "hosts" relation for this purpose. What do others think?

That does not stop you from including the needed link-parameter meta-data with this link description. For example title="Title Name", n="SemanticName", ct=1, id=3 etc. 

> <http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature>rel="describedby";

You could do this with relations, however I don't think anybody really wants to do this.

So as I summary I do propose that we consider creating the new "hosts" relation and better explaining that our context is coap://authority. We should also better define how we should use our link-extensions. We might consider giving some example of more creative use of deeper sets of relations about a resource, but this is something that I definitely would not require from a constrained node. Peter, how about I make a ticket on this?

Zach

> _______________________________________________
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-- 
Zach Shelby, Chief Nerd, Sensinode Ltd.
http://zachshelby.org  - My blog "On the Internet of Things"
http://6lowpan.net - My book "6LoWPAN: The Wireless Embedded Internet"
Mobile: +358 40 7796297

---

From: Peter Bigot <[email-masked]>
Date: Mon, 22 Nov 2010 09:39:19 -0600

The CoRE working group doesn't really have consensus on the fundamental
principles by which a proposal should be judged, let alone how to resolve
conflicting goals.  In my opinion, this has diminished the conceptual
integrity of CoAP, resulting in ad hoc solutions prompted by specific cases
regardless of the holistic effect.

My own perspective is to strongly prefer existing practice in the web
services and REST communities, given their demonstrated success and the
requirement that CoAP bridge to HTTP.  (If my understanding of existing
practice is wrong, please help me correct it.)  In this case, though we may
not be bound to use the same link relation semantics as RFC5988 and HTTP, to
deviate from them without a clear rationale is inconsistent with my
philosophy.

As an author of an IETF document I'm willing to put some significant effort
into making it great, but I don't want to be an author of something I don't
believe in.  For CoRE Link Format to be acceptable to me as an author, my
starting position is that we need two things:

   - The document shall not define a link-param which expresses a relation
   to a resource that will be accessed.  That's what link descriptions are for,
   not link parameters.  I don't see any link parameters in RFC5988 where the
   value is a resource (URI), except for extensions where the value serves
   purely as an identifier.
   - The approach for CoAP shall define a mechanism by which a set of link
   relations can be obtained for a specific URI on a server.

I don't think either of these are onerous, and I believe they help preserve
consistency with what the concept means to the rest of the world. Let's
start with the first by reviewing the link extensions currently listed in
the draft:

   - d -- Should be removed.  I think this is what the "service" registered
   relation means.  I believe it will be used rarely anyway.
   - sh -- There's a proposal to remove this.  That's fine with me; when I
   want its function I'll use the "alternate" or "self" relations.
   - n -- I'm ok with describing this as a semantic identifier, and even
   allowing that identifier to be spelled the same way as a URL, as long as we
   make clear that the value is not intended to be used as a URL (a URN would
   be appropriate).  This is what's done in section 4.2 of RFC5988 which
   specifies that URIs in extension relations should not be accessed.  If a URI
   is to be used with the expectation that an implementation will attempt to
   access it, the "describedby" registered relation should be used.
   - ct -- Not a relation, fine as an extension.  It's just a different
   encoding of the media link parameter.
   - id -- Not a relation, fine as an extension (but see my response in the
   other thread).

So we end up with CoAP adding three link extensions: n, ct, and id, none of
which describe relations to resources.  Three potential relations to
resources survive (service, alternate, describedby) but none seem critical
for M2M, so there's no requirement for any specific implementation to
support them.

For the second, we:

   1. Stipulate that the URI coap://authority/ refers to a resource which is
   the entire server.
   2. Define and register a new link relation "hosts" indicating a resource
   that can be located at the target URI.  ("hosts" is not necessarily the
   right name for this relation; we should discuss this after reviewing the
   existing relations.)  It is to these relations that the CoAP-added link
   parameters will be attached.
   3. Define that a GET on a URI
   coap://authority/.well-known/core/<somepath> provides a link description
   document for context coap://authority/<somepath>and default relation
   "hosts".

If this is done, then the existing practice of retrieving a list of
resources available on the server by retrieving
coap://authority/.well-known/core is preserved without change.  A system
with a need to provide relations between resources can do so by accepting
other URIs prefixed with coap://authority/.well-known/core, but nobody is
obliged to do so.

You could do this with relations, however I don't think anybody really wants
> to do this.

This is the sort of claim I don't find convincing.  First, there are already
situations where I do indeed want to do this: my own approach to observation
would benefit from being able to use the "monitor" and "monitor-group"
registered relations derived from RFC5989.  Second, if allowing this does
not impact implementations that choose not to use it, but ruling it out
prevents use of CoAP to solve somebody's problem (which I think would be the
case), I believe it should be allowed.

How does all that fly?

Peter

On Mon, Nov 22, 2010 at 6:21 AM, Zach Shelby <[email-masked]> wrote:

> Peter,
>
> I also agree that we don't quite have the ideal use of the link relation
> concept yet, but are getting there. Consider that the first version of the
> CoRE link format didn't use the relation concept at all, in the last couple
> versions we have gotten much closer. But just because we are re-using the
> link format from RFC5988, does not mean we are bound to use exactly the same
> link relation semantics that were designed for hypertext HTTP servers (nor
> does that necessarily even make sense). However, I do find the link relation
> work done in RFC5988 to be useful also in our domain to some extent, so we
> should try to apply what we can to M2M.
>
> I wouldn't go quite as far as Peter's idea below though. For M2M these
> descriptions need to be as compact as possible, nor can we expect
> constrained device to traverse many links to find out simple information
> about a resource. I do think we should improve the way we use rel="" for
> example, and the use of link-extensions can use improvement too.
>
> On a related note, I also received really helpful feedback from Martin
> Thomson on this same subject in Beijing. I will CC the list with some of
> that discussion pretty soon. He has good ideas for improving the use of rel=
> and how our link-extensions should be used.
>
> > Either at the IETF session or in some email I can't easily find, there
> was a specific question raised about exactly how the link description is
> supposed to be used in CoAP that started me down a path that led to the
> following comment.  I've cc'd Mark Nottingham, author of RFC5988 "Web
> Linking", in hopes he might have time to provide relevant information to
> guide us.
> >
> (rel) some way to the (context) resource identified in the original request.
>  Other contexts can be specified through the anchor parameter, but I expect
> this is unusual.
>
> Yes, this is the basic concept of RFC5988. I agree that anchor will be
> rare, as we point out in Section 2.1 of core-link-format-01.
>
> >
> I wouldn't say we are misusing it, but that we are defining what the
> context is by default. Section 2.1 explains that the CoRE link entries are
> describing links to resources hosted on the server. The rel= relation is
> defined to be "service" by default in Section 2.2 to describe that the link
> is a service of the server. I do like your idea here of explaining that the
> context is the URI coap://authority.
>
> Link parameters are meant to provide further information about the link.
> This is the purpose of the link-extensions defined in the CoRE link format.
> In that way they are similar to the link-parameters defined in RFC5988.
>
> > <sensors/temperature>n="
> http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature",sh="/l";
>
> I do see what you are getting at here, however in Beijing we basically
> decided to remove the Short URL "sh" parameter anyways. Furthermore, I don't
> consider the way you are thinking about the Name "n" is quite right. This is
> not meant to be a link (URL) to some document, but instead is meant to be a
> semantic name or URI used as a semantic name.
>
> > being one of the link descriptions at coap://authority/.well-known/core,
> use consistent with RFC5988 would be to provide:
> >
> You could do this, however it would be both inefficient and complex. I
> think we can avoid this by using Name properly. Let's get back to that in a
> separate mail with Marin's suggestions.
>
> >
> > Based on this perspective, and despite the fact that I've been added as
> an author due to my contributions to disambiguating the syntax, I don't
> think that CoRE Link Format describes a conceptually clean solution to this
> Now I like this idea. I tried to use "service" to indicate this, but would
> be happy with defining a new "hosts" relation for this purpose. What do
> others think?
>
> That does not stop you from including the needed link-parameter meta-data
> with this link description. For example title="Title Name",
> n="SemanticName", ct=1, id=3 etc.
>
> >
> > Further, define that coap://authority/.well-known/core/<somepath>
> provides a link description document for context
> coap://authority/<somepath>.  For example,
> coap://authority/.well-known/core/sensors/temperature would return:
> >
> So as I summary I do propose that we consider creating the new "hosts"
> relation and better explaining that our context is coap://authority. We
> should also better define how we should use our link-extensions. We might
> consider giving some example of more creative use of deeper sets of
> relations about a resource, but this is something that I definitely would
> not require from a constrained node. Peter, how about I make a ticket on
> this?
>
> Zach
>
> >
> > (I can't tell from RFC5988 what the "index" relation is intended to do,
> so perhaps it could be used instead of the newly created "hosts" relation.)
> >

---

From: Mark Nottingham <[email-masked]>
Date: Tue, 23 Nov 2010 10:54:08 +1100

It's very important to note that RFC5988 is *not* just about putting link headers into HTTP; rather, it's a framework that's designed to allow links to be serialised in various ways with equivalent semantics. 

As such, syntactic compatibility is a nice-to-have, but not critical; the only thing that I'd say is that inventing a syntax that's almost-but-not-quite-like another probably isn't productive.

OTOH semantic compatibility is very important; if you change the meaning of links, or how they operate, it's probably not helpful to use the framework in 5988.

Regards,

---

From: Zach Shelby <[email-masked]>
Date: Wed, 24 Nov 2010 14:51:53 +0200

On Nov 22, 2010, at 5:39 PM, Peter Bigot wrote:

> 	• The document shall not define a link-param which expresses a relation to a resource that will be accessed.  That's what link descriptions are for, not link parameters.  I don't see any link parameters in RFC5988 where the value is a resource (URI), except for extensions where the value serves purely as an identifier.

None of the link-params were ever meant to contain a URL that is accessed (with the exception of sh, but we are removing that). I think you misunderstood what is meant by URI in d and n, those are purely identifiers in the examples. I will clarify that point in link-format-02.  

In general, the philosophy in the CoRE Link Format draft should be that all the metadata needed to access a hosted resource is available in its link entry, without requiring a client to traverse other links or make further requests to find out. In other words, the "hosts" relation must be self describing. I agree that it should be possible to include other relations about a resource or the server.  

> 	• The approach for CoAP shall define a mechanism by which a set of link relations can be obtained for a specific URI on a server.
> I don't think either of these are onerous, and I believe they help preserve consistency with what the concept means to the rest of the world. Let's start with the first by reviewing the link extensions currently listed in the draft:

I have updated ticket #42 on this. I'll address your comments below as well. 

> 	• d -- Should be removed.  I think this is what the "service" registered relation means.  I believe it will be used rarely anyway.

I think you misunderstood what this is for. In REST interfaces between machines, it is often useful to be able to identify what kind of REST interface this is, in order to be able to use the resource. This is usually an identifier (URI) or URN, and it is pretty critical for REST to work properly in an M2M environment. 

> 	• sh -- There's a proposal to remove this.  That's fine with me; when I want its function I'll use the "alternate" or "self" relations.

I agree, we should recommend "alternate" if someone want to do this. So far it seems URLs are short enough from interop events. 

> 	• n -- I'm ok with describing this as a semantic identifier, and even allowing that identifier to be spelled the same way as a URL, as long as we make clear that the value is not intended to be used as a URL (a URN would be appropriate).  This is what's done in section 4.2 of RFC5988 which specifies that URIs in extension relations should not be accessed.  If a URI is to be used with the expectation that an implementation will attempt to access it, the "describedby" registered relation should be used.

Yep, it was always the intention of a URI in n to be an identifier. We can reference section 4.2 of RFC5988 to clarify that.  

> 	• ct -- Not a relation, fine as an extension.  It's just a different encoding of the media link parameter.
> 	• id -- Not a relation, fine as an extension (but see my response in the other thread).

The ticket is considering removing this if we can't find enough interest in using it. 

> So we end up with CoAP adding three link extensions: n, ct, and id, none of which describe relations to resources.  Three potential relations to resources survive (service, alternate, describedby) but none seem critical for M2M, so there's no requirement for any specific implementation to support them.

http://trac.tools.ietf.org/wg/core/trac/ticket/42 proposes keeping d, n and ct. There is a question about id. In addition in Beijing we decided to add "obs" and "sz". None of these are relations, so I think we are in good shape here. Mostly editorial work needed to make their use clear.

> For the second, we:
> 	• Stipulate that the URI coap://authority/ refers to a resource which is the entire server.

Agreed, http://trac.tools.ietf.org/wg/core/trac/ticket/85

> 	• Define and register a new link relation "hosts" indicating a resource that can be located at the target URI.  ("hosts" is not necessarily the right name for this relation; we should discuss this after reviewing the existing relations.)  It is to these relations that the CoAP-added link parameters will be attached.

Agreed, http://trac.tools.ietf.org/wg/core/trac/ticket/85

> 	• Define that a GET on a URI coap://authority/.well-known/core/<somepath> provides a link description document for context coap://authority/<somepath>and default relation "hosts".
> If this is done, then the existing practice of retrieving a list of resources available on the server by retrieving coap://authority/.well-known/core is preserved without change.  A system with a need to provide relations between resources can do so by accepting other URIs prefixed with coap://authority/.well-known/core, but nobody is obliged to do so.

This needs more thought and discussion first. I do agree that it should be possible to include further link relations about hosted resources (or about the server itself!) as this is what RFC5988 is for. I also agree that we can not require constrained nodes to provide or understand such relations. 

However, there are multiple ways to so this. In addition, we would need to think carefully before reserving the whole /.well-known/core/<somepath> for this purpose. 

- Currently link-format specifies that  /.well-known/core/<somepath> may be used for organizing link entries as the server sees fit. It gives an example of using the "section" relation to do that. With your proposal this would not be possible. Is it an issue to have to include all links about coap://authority in /.well-known/core? This might become quite long, although filtering helps. You and others have also suggested other uses of the /.well-known/core/<somepath> path. 

- How does a client know that /.well-known/core/<somepath> is available in the first place? That is a pretty big space, do you just do it by trial and error? Obviously not all hosted resources would have further link relations (or only in homogeneous applications). 

- Use of the "anchor=" link-parameter would be another way to achieve this. This way you immediately know the further link relations without having to make more requests, and you don't need to define what /.well-known/core/<somepath> has to be used for. This is actually what link-format-01 assumes people would do.

</sensors/temperature>; rel="hosts",
</l>; rel="alternate"; anchor="/sensors/temperature"

Another useful relation would be to use "describedby" in order to point to a resource that describes the server itself. This is very easy in /.well-known/core and doesn't require anything new to link-format-01.

</serverinfo>; rel="describedby"

> This is the sort of claim I don't find convincing.  

What I meant is that nobody in M2M would want to require a client to make multiple requests in order to find out basic metadata about a hosted resource such as the interface description identifier or semantic name. I didn't disagree that people should be able to make further relations, but we can't require that. Now I think we both agree about that. 

> First, there are already situations where I do indeed want to do this: my own approach to observation would benefit from being able to use the "monitor" and "monitor-group" registered relations derived from RFC5989.  Second, if allowing this does not impact implementations that choose not to use it, but ruling it out prevents use of CoAP to solve somebody's problem (which I think would be the case), I believe it should be allowed.

Not sure I understand what you want to do there. Could it be done using the "anchor" proposal above just as well? 

In general I think we need to collect more real M2M examples of how these further relations can be used. So far we have:

- Providing an alternate URL to access a resource.
- Pointing to a resource that describes the server itself.
- You monitoring case, a more complete example would be helpful.
- Others?

Zach