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Versions: 00 draft-ietf-core-link-format

CoRE                                                           Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                      September 28, 2010
Expires: April 1, 2011


                            CoRE Link Format
                    draft-shelby-core-link-format-00

Abstract

   This document defines a link format for use by constrained CoAP web
   servers to describe URIs of resources offered along with other
   attributes.  Based on the HTTP Link Header format, the CoRE link
   format is carried as a payload and is assigned an Internet media
   type.  A well-known URI is defined as a default entry-point for
   requesting the list of links to resources hosted by a server.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
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   This Internet-Draft will expire on April 1, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Link Format  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Target and context IRIs  . . . . . . . . . . . . . . . . .  4
     2.2.  Link relation 'rel' usage  . . . . . . . . . . . . . . . .  4
     2.3.  Description 'd' usage  . . . . . . . . . . . . . . . . . .  5
     2.4.  Alternative URI 'sh' usage . . . . . . . . . . . . . . . .  5
     2.5.  Resource name 'n' usage  . . . . . . . . . . . . . . . . .  5
     2.6.  Content-type code 'ct' usage . . . . . . . . . . . . . . .  5
     2.7.  Resource identifier 'id' usage . . . . . . . . . . . . . .  6
     2.8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Well-known Interface . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Query Filtering  . . . . . . . . . . . . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Well-known 'core' URI  . . . . . . . . . . . . . . . . . .  8
     5.2.  New link-format Internet media type  . . . . . . . . . . .  8
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10


















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1.  Introduction

   The Constrained RESTful Environments (CoRE) working group aims at
   realizing the REST architecture in a suitable form for the most
   constrained nodes (e.g. 8-bit microcontrollers with limited RAM and
   ROM) and networks (e.g. 6LoWPAN).  CoRE is aimed at machine-to-
   machine (M2M) applications such as smart energy and building
   automation [I-D.shelby-core-coap-req].

   The discovery of resources offered by a constrained server is very
   important in machine-to-machine applications where there are no
   humans in the loop and static interfaces result in fragility.  The
   discovery of resources provided by an HTTP Web Server is typically
   called Web Discovery.  In this document we refer to the discovery of
   resources offered by a CoAP server as resource discovery.

   The core function of such a discovery mechanism is to provide URIs
   ("links") for the resources offered, complemented by information
   describing the relationship between the resource description and each
   resource as well as other attributes.  When such a collection of
   attributed resource references (links) is offered as a resource of
   its own (as opposed to as HTTP headers delivered with a different
   resource), we speak of its representation as a link-format.

   This document specifies a link-format for use in CoRE resource
   discovery by extending the HTTP Link Header Format
   [I-D.nottingham-http-link-header] to describe resources hosted by a
   constrained server.  The CoRE link-format is carried as a payload and
   is assigned an Internet media type.  A well-known URI "/.well-known/
   core" is defined as a default entry-point for requesting the list of
   links to resources hosted by a server.


2.  Link Format

   CoRE resource discovery extends the HTTP Link Header format specified
   in [I-D.nottingham-http-link-header] which is specified in Augmented
   Backus-Naur Form (ABNF) notation [RFC5234].  The format does not
   require special XML or binary parsing, and is extensible.

   This link format is used for a similar purpose to that described in
   [I-D.nottingham-http-link-header], to describe one or more
   relationships between resources.  However in this specification the
   link format is extended with specific constrained M2M link
   parameters, links are carried as a payload rather than in a message
   header, and a default interface is defined to discover resources
   described by these links.




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   [I-D.nottingham-http-link-header] did not require an Internet media
   type for this link format, as it assumes to be carried in an HTTP
   header.  This specification thus requests a Internet media type for
   this format (see Section 5.2).

   The CoRE link format uses the ABNF description and associated rules
   in Section 5 of [I-D.nottingham-http-link-header].  The "Link:" text
   is omitted as that is part of the HTTP Link Header.  Multiple link
   descriptions are separated by commas.  The CoRE link format MUST use
   the US-ASCII character set (support for RFC2231 encoding of non-ASCII
   content TBD).  The following CoRE specific link-extension parameters
   to the format are defined:


      link-extension    = ( "d" "=" URI )
      link-extension    = ( "sh" "=" URI-Reference )
      link-extension    = ( "n" "=" ( quoted-string | URI ) )
      link-extension    = ( "ct" "=" integer )
      link-extension    = ( "id" "=" ( quoted-string | URI ) )


2.1.  Target and context IRIs

   Each link description conveys one target URI as a URI-Reference
   inside angle brackets ("<>").  The context of a link conveyed in the
   description is by default the URI of the resource that returned the
   link-format representation (usually ./well-known/core).  Thus each
   link can be thought of as describing a target resource hosted by the
   server in the absence of further relation information.  This is an
   important difference to the way the HTTP Link Header format is used,
   as it is included in the header of an HTTP response for some URI
   (this URI is by default the context).  Thus the HTTP Link Header is
   by default relating the target URI to the URI that was requested.  In
   comparison, the CoRE link format includes one or more link entries,
   each describing a resource hosted by a server.

   As per Section 5.2 of [I-D.nottingham-http-link-header] a link
   description MAY include an "anchor" attribute, in which case the
   context is the URI included in that attribute.  This can be used to
   describe a relationship between two resources.  A consuming
   implementation can however choose to ignore such links.  It is not
   expected that most implementations will be able to derive useful from
   explicitly anchored links.

2.2.  Link relation 'rel' usage

   Link descriptions in CoRE are typically used to describe entry points
   to services hosted by the server, and thus in the absence of the rel



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   attribute the registered "service" relation type is assumed.  In the
   CoRE link format the service relation type indicates that the link is
   a service hosted by the server (in the absence of the anchor
   attribute).  A description can make use of any registered relation
   type or extension types in the form of a URI by including the rel
   attribute.

2.3.  Description 'd' usage

   The description "d" attribute can provide a URI to a specific
   interface definition used to access the target resource.  This could
   be for example a URI to the WADL definition of the target resource.
   Multiple description attributes MAY appear in a link description.

2.4.  Alternative URI 'sh' usage

   This attribute can be included to define an alternative short URI
   which can also be used to access the target resource.  Multiple
   alternative short URI attributes MAY appear in a link description.

2.5.  Resource name 'n' usage

   The resource name "n" attribute is used to assign eith a human
   readable or a semantically important name to a resource.  In the case
   of a temperature sensor resource the name could be something like
   "Temperature in Centigrade", a URI to an ontology like
   "http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature" or an
   application-specific semantic name like "TemperatureC".  Multiple
   name attributes MAY appear in a link description.

2.6.  Content-type code 'ct' usage

   The Content-type code "ct" attribute provides a hint about the
   Internet media type this resource returns.  The value is in the CoAP
   identifier code format as a decimal ASCII integer
   [I-D.ietf-core-coap].  For example application/xml would be indicated
   as "ct=41".  If no Content-type code attribute is present then text/
   plain is assumed.  The Content-type code attribute MUST NOT appear
   more than once in a link description.

   Alternatively, the "type" attribute MAY be used to indicate an
   Internet media type as a quoted-string.  It is not however expected
   that constrained implementations are able to parse quoted-string
   Content-type values.







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2.7.  Resource identifier 'id' usage

   The resource identifier "id" field is a unique identifier (e.g.
   UUID) for this resource for use in e.g. resource or search
   directories.  This attribute may be in quoted-string format (e.g. in
   the case of a UUID or XRI) or in URI format (e.g. in the case of a
   URN).  The resource identifier attribute MUST NOT appear more than
   once in a link description.

2.8.  Examples

   A few examples of typical link descriptions in this format follows.
   Multiple resource descriptions in a representation are separated by
   commas.  Commas can also occur in quoted strings and URIs but do not
   end a description.  Linefeeds never occur in the actual format, but
   are shown in the example for readability.

   This example includes link descriptions for an index to sensors
   hosted by a server, along with links two two different sensors.

   GET /.well-known/core

   </sensors>;rel="index";n="Sensor Index",
   </sensors/temp>;sh="/t";n="TemperatureC",
   </sensors/light>;sh="/l";ct=41;n="LightLux"

   This example arranges link descriptions hierarchically, with the
   entry point including a link description to a sub-resource containing
   link descriptions about the sensors.

   GET /.well-known/core

   </.well-known/core/sensors>;rel="section"
   ;type="application/link-format"

   GET /.well-known/core/sensors

   </sensors/temp>;sh="/t";n="TemperatureC",
   </sensors/light>;sh="/l";ct=41;n="LightLux"



3.  Well-known Interface

   Resource discovery in CoRE is accomplished through the use of a well-
   known resource URI which returns a list of links (resource
   descriptions) offered by that constrained server.  Well-known
   resources have a reserved base URI "/.well-known/" as specified in



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   [RFC5785].  This document defines a new well-known URI for CoRE
   discovery "/.well-known/core" Section 5.1.  A server implementing
   this specification MUST support this URI on the default port
   appropriate for the protocol, for the purpose of resource discovery.
   It is however up to the application which link descriptions are
   included and how they are organized.  In the absense of any links, a
   zero-length payload is returned.  The resource representation of this
   resource is described in Section 2.

   The CoRE resource discovery interface supports the following
   interactions:

   o  Performing a GET on /.well-known/core to the default port returns
      a list of link descriptions available from a CoAP server (if any).

   o  Filtering may be performed on any of the link format attributes
      using a query string as specified in Section 3.1.  For example
      [GET /.well-known/core?n=TemperatureC] would request resources
      with the name TemperatureC.  A server is not however required to
      support filtering.

   o  More capable servers such as proxies could support a resource
      directory by requesting the resource descriptions of other end-
      points or accepting [POST /.well-known/core messages] from other
      servers.  This adds the resources of other end-points as a sub-
      resource in which absolute URIs are included for the link-values.
      The details of such resource directory functionality is however
      out of scope for this document.

   End-points with a large number of resources SHOULD include resource
   descriptions only for important services or collections and organize
   their resource descriptions into a hierarchy of link resources.  This
   is done by including links in the /.well-known/core list which point
   to other resource lists, e.g. </.well-known/core/sensors>.  Such a
   hierarchy SHOULD be under the /.well-known/core path but could be
   located elsewhere.

3.1.  Query Filtering

   A server implementing this document MAY support the query string
   /.well-known/core? with uri= corresponding to the link-value or any
   of the resource description attributes for the purpose of filtering a
   discovery.  It is not expected that simple implementations support
   filtering, but instead will just ignore the query string.  Wildcard *
   endings MAY be supported.  An exact match is performed on the query
   string, and a 200 OK response is returned with link descriptions that
   contains the matching entries (if any).  If resource descriptions are
   organized hierarchically, a query on the root resource /.well-known/



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   core SHOULD return all matching resource descriptions from the entire
   hierarchy.  An example query on the example link descriptions from
   Section 2 may look like:

   GET /.well-known/core?n=LightLux

   </sensors/light>;sh="/l";ct=41;n="LightLux"


4.  Security Considerations

   This document needs the same security considerations as described in
   Section 7 of [I-D.nottingham-http-link-header].  The /.well-known/
   core resource may be protected e.g. using DTLS when hosted on a CoAP
   server.


5.  IANA Considerations

5.1.  Well-known 'core' URI

   This memo registers the "core" well-known URI in the Well-Known URI
   Registry as defined by [RFC5785].

   URI suffix: core

   Change controller: IETF

   Specification document(s): [[ this document ]]

   Related information: None

5.2.  New link-format Internet media type

   This memo registers the a new Internet media type for the CoRE link
   format, application/link-format.

   Type name: application

   Subtype name: link-format

   Required parameters: None

   Optional parameters: The query string may contain uri= to match the
   URI, or any other attribute defined for the link format to match that
   attribute.

   Encoding considerations: US-ASCII



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   Security considerations: None

   Interoperability considerations:

   Published specification: [[ this document ]]

   Applications that use this media type: CoAP server and client
   implementations.

   Additional information:

   Magic number(s):

   File extension(s):

   Macintosh file type code(s):

   Intended usage: COMMON

   Restrictions on usage: None

   Author: CoRE WG

   Change controller: IETF


6.  Acknowledgments

   Special thanks to Mark Nottingham and Eran Hammer-Lahav for
   discussions and ideas that led to this draft, and to Carsten Bormann
   for extensive comments and contributions that improved the text.

   Thanks to Michael Stuber, Richard Kelsey, Cullen Jennings, Guido
   Moritz, Peter Van Der Stok, Adriano Pezzuto, Lisa Dussealt, Alexey
   Melnikov, Gilbert Clark, Salvatore Loreto, Petri Mutka, Szymon Sasin,
   Robert Quattlebaum, Robert Cragie, Angelo Castellani, Tom Herbst, Ed
   Beroset, Gilman Tolle, Robby Simpson, Peter Bigot, Colin O'Flynn and
   David Ryan for helpful comments and discussions that have shaped the
   document.


7.  Changelog


8.  References






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8.1.  Normative References

   [I-D.ietf-core-coap]
              Shelby, Z., Frank, B., and D. Sturek, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-02
              (work in progress), September 2010.

   [I-D.nottingham-http-link-header]
              Nottingham, M., "Web Linking",
              draft-nottingham-http-link-header-10 (work in progress),
              May 2010.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

8.2.  Informative References

   [I-D.shelby-core-coap-req]
              Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
              Kelsey, "CoAP Requirements and Features",
              draft-shelby-core-coap-req-01 (work in progress),
              April 2010.


Author's Address

   Zach Shelby
   Sensinode
   Kidekuja 2
   Vuokatti  88600
   FINLAND

   Phone: +358407796297
   Email: zach@sensinode.com













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