CoRE                                                           Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                       December 10, 2010
Expires: June 13, 2011

                            CoRE Link Format


   This document defines Web Linking using a link format for use by
   constrained web servers to describe hosted resources, their
   attributes and other relationships between links.  Based on the HTTP
   Link Header format defined in RFC5988, 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
   links hosted by a server.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 13, 2011.

Copyright Notice

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

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   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Link Format  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Web Linking in CoRE  . . . . . . . . . . . . . . . . . . .  4
       2.1.1.  Target and context URIs  . . . . . . . . . . . . . . .  4
       2.1.2.  Link relations . . . . . . . . . . . . . . . . . . . .  5
       2.1.3.  Use of anchors . . . . . . . . . . . . . . . . . . . .  5
     2.2.  CoRE link extensions . . . . . . . . . . . . . . . . . . .  5
       2.2.1.  Resource name 'n' attribute  . . . . . . . . . . . . .  6
       2.2.2.  Interface description 'd' attribute  . . . . . . . . .  6
       2.2.3.  Content-type code 'ct' attribute . . . . . . . . . . .  6
       2.2.4.  Maximum size estimate 'sz' attribute . . . . . . . . .  6
       2.2.5.  Observable 'obs' attribute . . . . . . . . . . . . . .  7
   3.  Well-known Interface . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Query Filtering  . . . . . . . . . . . . . . . . . . . . .  8
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Well-known 'core' URI  . . . . . . . . . . . . . . . . . . 10
     6.2.  New 'hosts' relation type  . . . . . . . . . . . . . . . . 11
     6.3.  New link-format Internet media type  . . . . . . . . . . . 11
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14

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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

   The discovery of resources hosted 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 and the description of relations between
   resources is called Web Linking [RFC5988].  In this document we refer
   to the discovery of resources hosted by a constrained web server,
   their attributes and other resource relations as CoRE Resource

   The main function of such a discovery mechanism is to provide URIs
   (called links) for the resources hosted by the server, complemented
   by attributes about those resources and possible further link
   relations.  In CoRE this collection of links is carried as a resource
   of its own (as opposed to as HTTP headers delivered with a specific
   resource).  This document specifies a link format for use in CoRE
   Resource Discovery by extending the HTTP Link Header Format [RFC5988]
   to describe these link descriptions.  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 about resources hosted by a server, and
   thus performing CoRE Resource Discovery.

2.  Link Format

   The CoRE Link Format extends the HTTP Link Header format specified in
   [RFC5988], which is specified in Augmented Backus-Naur Form (ABNF)
   notation.  The format does not require special XML or binary parsing,
   is fairly compact, and is extensible - all important characteristics
   for CoRE.  It should be noted that this link format is just one
   serialization of typed links defined in [RFC5988], others include
   Atom or HTTP links.  It is expected that resources discovered in the
   CoRE Link Format may also be made available in alternative formats on
   the greater Internet.

   Section 5 of [RFC5988] did not require an Internet media type for the
   defined link format, as it was defined to be carried in an HTTP
   header.  This specification thus defines a Internet media type for

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   the CoRE Link Format (see Section 6.3).

   The CoRE link format uses the ABNF description and associated rules
   in Section 5 of [RFC5988].  In addition, the pchar rule is taken from
   [RFC3986].  The "Link:" text is omitted as that is part of the HTTP
   Link Header.  As in [RFC5988], multiple link descriptions are
   separated by commas.  Note that commas can also occur in quoted
   strings and URIs but do not end a description.  The CoRE link format
   MUST use UTF-8 encoding, which SHOULD be in NFC (Unicode
   Normalization Form C).  See Section 3 of [RFC5198], which explains
   why it useful to represent Unicode in a single unique form.

2.1.  Web Linking in CoRE

   This link format is used for a similar purpose to that described in
   [RFC5988], to describe relationships between resources via typed
   links, so-called "Web Linking".  However in this specification Web
   Linking is extended with specific constrained M2M attributes, links
   are carried as a message payload rather than in an HTTP Link Header,
   and a default interface is defined to discover resources hosted by a
   server.  This specification defines a new relation type "hosts",
   which indicates that the resource is hosted by the server from which
   the link document was requested.

2.1.1.  Target and context URIs

   Each link conveys one target URI as a URI-reference inside angle
   brackets ("<>").  The context URI of a link (also called base URI in
   [RFC3986]) conveyed in the CoRE Link Format is by default built from
   the scheme and authority parts of the target URI.  In the absence of
   this information in the target URI, the context URI is built from the
   scheme and authority that was used for referencing the resource
   returning the set of links, replacing the path with an empty path.
   Thus by default links can be thought of as describing a target
   resource hosted by the server.  Other relations can be expressed by
   including an anchor parameter (which defines the context URI) along
   with an explicit relation parameter.  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 URI).  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 links, each describing a
   resource hosted by a server by default.  Other relations can be
   expressed by using the anchor parameter.  See Section 5 of [RFC3986]
   for a description of how URIs are constructed from URI references.

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2.1.2.  Link relations

   Since links in the CoRE Link Format are typically used to describe
   resources hosted by a server, and thus in the absence of the relation
   parameter the new relation type "hosts" is assumed (see Section 6.2).
   The "hosts" relation type indicates that the target URI is a resource
   hosted by the server given by the base URI, or, if present, the
   anchor parameter.

   To express other relations a links can make use of any registered
   relation parameter or target attributes by including the relation
   parameter.  The context of a relation can be defined using the anchor
   parameter.  In this way, relations between resources hosted on a
   server, or between hosted resources and external resources can be

2.1.3.  Use of anchors

   As per Section 5.2 of [RFC5988] a link description MAY include an
   "anchor" attribute, in which case the context is the URI included in
   that attribute.  This is used to describe a relationship between two
   resources.  A consuming implementation can however choose to ignore
   such links.  It is not expected that all implementations will be able
   to derive useful information from explicitly anchored links.

2.2.  CoRE link extensions

   The following CoRE specific target attributes are defined.  These
   attributes describe information useful in accessing the target link
   of the relation, and in some cases may be URIs.  These URIs MUST be
   treated as indicators, and are not meant to be actually retrieved
   like a URL.  When attributes are compared, they MUST be compared as
   strings.  Relationships to resources that are meant to be retrieved
   should be expressed as separate links using the anchor attribute and
   the appropriate relation type.

      link-extension    = ( "n" "=" quoted-string )
      link-extension    = ( "d" "=" <"> URI-reference <">)
      link-extension    = ( "ct" "=" integer )
      link-extension    = ( "sz" "=" integer )
      link-extension    = ( "obs" )
      integer           = 1*DIGIT

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2.2.1.  Resource name 'n' attribute

   The resource name "n" attribute is used to assign a semantically
   important name to a resource.  In the case of a temperature sensor
   resource this could be an application-specific semantic name like
   "TemperatureC", a URN like "urn:temperature:centigrade" or a URI
   referencing a specific concept in an ontology like
   "".  Multiple name
   attributes MAY appear in a link.

   The name attribute is not meant to used to assign a human readable
   name to a resource.  The "title" attribute defined in [RFC5988] is
   meant for that purpose.

2.2.2.  Interface description 'd' attribute

   The interface description "d" attribute is used to provide a URI or
   URN indicating a specific interface definition used to interact with
   the target resource.  This could be for example the URI of a WADL
   definition of the target resource
   "", or a URN indicating the type of
   interface to the resource "urn:sensor".  Multiple description
   attributes MAY appear in a link.

2.2.3.  Content-type code 'ct' attribute

   The Content-type code "ct" attribute provides a hint about the
   Internet media type this resource returns.  Note that this is only a
   hint, and does not override the Content-type Option of a CoAP
   response obtained by actually following the link.  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
   nothing about the type can be assumed.  The Content-type code
   attribute MUST NOT appear more than once in a link.

   Alternatively, the "type" attribute MAY be used to indicate an
   Internet media type as a quoted-string [RFC5988].  It is not however
   expected that constrained implementations are able to parse quoted-
   string Content-type values.  A link MAY include either a ct attribute
   or a type attribute, but MUST NOT include both.

2.2.4.  Maximum size estimate 'sz' attribute

   The maximum size estimate attribute "sz" gives an indication of the
   maximum size of the link indicated by the target URI.  This attribute
   is not expected to be included for small resources that can
   comfortably by carried in a single MTU, but SHOULD be included for

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   resources larger than that.  The maximum size estimate attribute MUST
   NOT appear more than once in a link.

2.2.5.  Observable 'obs' attribute

   The observable attribute "obs" is used to indicate that this resource
   supports the observation feature defined in [I-D.ietf-core-observe].
   This attribute is used as a flag, and thus it has no value component.
   The observable attribute MUST NOT appear more than once in a link.

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 about resources
   hosted by that server and other link relations.  Well-known resources
   have a path component that begins with "/.well-known/" as specified
   in [RFC5785].  This document defines a new well-known resource for
   CoRE Resource Discovery "/.well-known/core".  A server implementing
   this specification MUST support this resource on the default port
   appropriate for the protocol for the purpose of resource discovery.
   It is however up to the application which links are included and how
   they are organized.  In the absence of any links, a zero-length
   payload is returned.  The resource representation of this resource
   uses the CoRE Link Format described in Section 2.  URIs under the
   path /.well-known/core are reserved by this specification for use by
   servers in organizing links into multiple resources, each with their
   own URI.

   The CoRE resource discovery interface supports the following

   o  Performing a GET on /.well-known/core to the default port returns
      a set of links available from the CoAP server (if any) in the CoRE
      Link Format.  These links might describe resources hosted on that
      server, on other servers, or express other kinds of link relations
      as described in Section 2.

   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 allowing servers to POST requests to /.well-known/core.
      The details of such resource directory functionality is however

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      out of scope for this document, and is expected to be specified

   End-points with a large number of resources SHOULD include resource
   descriptions only for important links and MAY 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 using the "section" relation type, e.g. </.well-known/
   core/sensors>.  Such a hierarchy MUST be under the /.well-known/core

3.1.  Query Filtering

   A server implementing this document MAY recognize the query part of a
   resource-discovery URI as a filter on the resources to be returned.
   The query part should conform to the following syntax.  Note that
   this only defines querying for a single parameter at a time.

     filter-query = resource-param "=" query-pattern
     resource-param = "uri" | parmname
     query-pattern = 1*pchar [ "*" ]

   The resource-param "uri" refers to the URI-reference between the "<"
   and ">" characters of a link.  Other resource-param values refer to
   the link attribute they name.  Filtering is performed by comparing
   the query-pattern against the value of the attribute identified by
   the resource-param for each link-value in the collection of resources
   identified by the URI path.

   If the decoded query-pattern does not end with "*", a link value
   matches the query only if the value of the attribute or URI-reference
   denoted by the resource-param is bytewise identical to the query-
   pattern.  If the decoded query-pattern ends with "*", it is
   sufficient that the remainder of the query-pattern be a prefix of the
   value denoted by the resource-param.

   It is not expected that very constrained nodes support filtering.
   Implementations not supporting filtering MUST simply ignore the query
   string and return the whole resource for unicast requests.  An
   implementation supporting filtering MUST return only those entries
   for which the value of the link attribute identified by the resource-
   param contains the query-pattern as a substring.  If resource
   descriptions are organized hierarchically, a query on the root
   resource /.well-known/core MUST return all matching resource
   descriptions from the entire hierarchy.

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   When using a transfer protocol like CoAP that supports multicast
   requests, special care is taken.  A multicast request with a query
   string MUST not be responded to if filtering is not supported (to
   avoid a needless response storm).

4.  Examples

   A few examples of typical link descriptions in this format follows.
   Multiple resource descriptions in a representation are separated by
   commas.  Linefeeds never occur in the actual format, but are shown in
   the example for readability.

   This example includes links to two different sensors.

   REQ: GET /.well-known/core

   RES: 200 OK

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

   REQ: GET /.well-known/core

   RES: 200 OK

   REQ: GET /.well-known/core/sensors

   RES: 200 OK

   An example query filter may look like:

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

   RES: 200 OK

   This example shows the use of an anchor attribute to relate the
   temperature sensor resource to an external description and to an
   alternative URL.

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   REQ: GET /.well-known/core

   RES: 200 OK
   </sensors>;rel="index";n="Sensor Index",

   If a client is interested to find relations about a particular
   resource, it can perform a query on the anchor parameter:

   REQ: GET /.well-known/core?anchor=/sensors/temp

   RES: 200 OK

5.  Security Considerations

   This document needs the same security considerations as described in
   Section 7 of [RFC5988].  The /.well-known/core resource may be
   protected e.g. using DTLS when hosted on a CoAP server as per
   [I-D.ietf-core-coap] Section 10.2.

   Multicast requests using CoAP for the well-known link-format
   resources could be used to perform denial of service on a constrained
   network.  A multicast request SHOULD only be accepted if the request
   is sufficiently authenticated and secured.

   CoRE link format parsers should be aware that a link description may
   be cyclical, i.e., contain a link to itself.  These cyclical links
   could be direct or indirect (i.e., through referenced link
   resources).  Care should be taken when parsing link descriptions and
   accessing cyclical links.

6.  IANA Considerations

6.1.  Well-known 'core' URI

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

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   URI suffix: core

   Change controller: IETF

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

   Related information: None

6.2.  New 'hosts' relation type

   This memo registers the new "hosts" Web Linking relation type as per

   Relation Name: hosts

   Description: Refers to a resource hosted by the server indicated by
   the link context.

   Reference: [[ this document ]]

   Notes: This relation is used in CoRE where links are retrieved as a
   /.well-known/core resource representation, and by default the context
   of the links is the server at coap://authority from which /.well-
   known/core was requested.

   Application Data: None

6.3.  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

   Encoding considerations: UTF-8 (NFC)

   Security considerations: None

   Interoperability considerations:

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   Published specification: [[ this document ]]

   Applications that use this media type: CoAP server and client
   implementations for resource discovery and HTTP applications that use
   the link-format as a payload.

   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

7.  Acknowledgments

   Special thanks to Peter Bigot, who has made a considerable number
   reviews and text contributions that greatly improved the document.
   In particular, Peter is responsible for the ABNF descriptions and the
   idea for a new "hosts" relation type.

   Thanks to Mark Nottingham and Eran Hammer-Lahav for discussions and
   ideas that led to this draft, and to Carsten Bormann and Martin
   Thomson for extensive comments and contributions that improved the

   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, Colin O'Flynn and David Ryan
   for helpful comments and discussions that have shaped the document.

8.  Changelog

   Changes from ietf-01 to ietf-02:

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      o Added references to RFC5988 (#41).

      o Removed sh and id link-extensions (#42).

      o Defined the use of UTF-8 (#84).

      o Changed query filter definition for any parameter (#70).

      o Added more example, now as a separate section (#43).

      o Mentioned cyclical links in the security section (#57).

      o Removed the sh and id attributes, added obs and sz attributes

      o Improved the context and relation description wrt RFC5988 and
      requested a new "hosts" default relation type (#85).

   Changes from ietf-00 to ietf-01:

      o Editorial changes to correct references.

      o Formal definition for filter query string.

      o Removed URI-reference option from "n" and "id".

      o Added security text about multicast requests.

   Changes from shelby-00 to ietf-00:

      o Fixed the ABNF link-extension definitions (quotes around URIs,
      integer definition).

      o Clarified that filtering is optional, and the query string is to
      be ignored if not supported (and the URL path processed as

      o Required support of wildcard * processing if filtering is

      o Removed the aussumption of a default content-type assumption.

9.  References

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9.1.  Normative References

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

9.2.  Informative References

              Shelby, Z., Frank, B., and D. Sturek, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-03
              (work in progress), October 2010.

              Hartke, K. and Z. Shelby, "Observing Resources in CoAP",
              draft-ietf-core-observe-00 (work in progress),
              October 2010.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

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

Author's Address

   Zach Shelby
   Kidekuja 2
   Vuokatti  88600

   Phone: +358407796297

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