CoRE                                                           Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                        January 13, 30, 2012
Expires: July 16, August 2, 2012

                            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.

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   This Internet-Draft will expire on July 16, August 2, 2012.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Web Linking in CoRE  . . . . . . . . . . . . . . . . . . .  3
     1.2.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.2.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . .  4
       1.2.2.  Resource Collections . . . . . . . . . . . . . . . . .  5
       1.2.3.  Resource Directory . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Link Format  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Target and context URIs  . . . . . . . . . . . . . . . . .  7
     2.2.  Link relations . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Use of anchors . . . . . . . . . . . . . . . . . . . . . .  8
   3.  CoRE link extensions . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Resource type 'rt' attribute . . . . . . . . . . . . . . .  8
     3.2.  Interface description 'if' attribute . . . . . . . . . . .  9
     3.3.  Maximum size estimate 'sz' attribute . . . . . . . . . . .  9
   4.  Well-known Interface . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Query Filtering  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Well-known 'core' URI  . . . . . . . . . . . . . . . . . . 14
     7.2.  New 'hosts' relation type  . . . . . . . . . . . . . . . . 14
     7.3.  New link-format Internet media type  . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     10.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.  Introduction

   The Constrained RESTful Environments (CoRE) working group aims at
   realizing the Representational State Transfer (REST) architecture
   [REST] in a suitable form for the most constrained nodes (e.g. 8-bit
   microcontrollers with limited memory) and networks (e.g. 6LoWPAN
   [RFC4944]).  CoRE is aimed at Machine-to-Machine (M2M) applications
   such as smart energy and building automation.

   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 [RFC2616] Web Server is
   typically called Web Discovery and the description of relations
   between resources is called Web Linking [RFC5988].  In the present
   document we refer to the discovery of resources hosted by a
   constrained web server, their attributes and other resource relations
   as CoRE Resource Discovery.

   The main function of such a discovery mechanism is to provide
   Universal Resource Identifiers (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 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.  This specification is applicable for use with
   CoAP [I-D.ietf-core-coap], HTTP or any other suitable web transfer
   protocol.  The link format can also be saved in file format.

1.1.  Web Linking in CoRE

   What is the difference between the CoRE Link Format and [RFC5988]?
   Technically the CoRE Link Format is a serialization of a typed link
   as specified in [RFC5988], used to describe relationships between
   resources, so-called "Web Linking".  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 also defines a new relation type "hosts",
   which indicates that the resource is hosted by the server from which
   the link document was requested.

   Why not just use the HTTP Link Header?  In HTTP, the Link Header can
   be used to carry link information about a resource along with an HTTP
   response.  This works well for the typical use case for a web server
   and browser, where further information about a particular resource is
   useful after accessing it.  In CoRE the main use case for Web Linking
   is the discovery of which resources a server hosts in the first
   place.  Although some resources may have further links associated
   with them, this is expected to be an exception.  For that reason the
   CoRE Link Format serialization is carried as a resource
   representation of a well-known URI.  The CoRE Link Format does re-use
   the format of the HTTP Link Header serialization defined in

1.2.  Use Cases

   Typical use cases for Web Linking on today's web include e.g.
   describing the author of a web page, or describing relations between
   web pages (next chapter, previous chapter etc.).  Web Linking can
   also be applied to M2M applications, where typed links are used to
   assist a machine client in finding and understanding how to use
   resources on a server.  In this section a few use cases are described
   for how the CoRE Link Format could be used in M2M applications.  For
   further technical examples see Section 5.  As there are a large range
   of M2M applications, these use cases are purposely generic.  This
   document assumes that different deployments or application domains
   will define the appropriate REST Interface Descriptions along with
   Resource Types to make discovery meaningful.

1.2.1.  Discovery

   In M2M applications, for example home or building automation, there
   is a need for local clients and servers to find and interact with
   each other without human intervention.  The CoRE Link Format can be
   used by servers in such environments to enable Resource Discovery of
   the resources hosted by the server.

   Resource Discovery can be performed either unicast or multicast.
   When a server's IP address is already known, either a priori or
   resolved via the Domain Name System (DNS) [RFC1034][RFC1035], unicast
   discovery is performed in order to locate the entry point to the
   resource of interest.  This is performed using a GET to /.well-known/
   core on the server, which returns a payload in the CoRE Link Format.
   A client would then match the appropriate Resource Type, Interface
   Description and possible Content-Type [RFC2045] for its application.
   These attributes may also be included in the query string in order to
   filter the number of links returned in a response.

   Multicast resource discovery is useful when a client needs to locate
   a resource within a limited scope, and that scope supports IP
   multicast.  A GET request to the appropriate multicast address is
   made for /.well-known/core.  In order to limit the number and size or
   responses, a query string is recommended with the known attributes.
   Typically a resource would be discovered based on its Resource Type
   and/or Interface Description, along with possible application
   specific attributes.

1.2.2.  Resource Collections

   RESTful designs of M2M interfaces often make use of collections of
   resources.  For example an index of temperature sensors on a data
   collection node or a list of alarms on a home security controller.
   The CoRE Link Format can be used to make it possible to find the
   entry point to a collection and traverse its members.  The entry
   point of a collection would always be included in /.well-known/core
   to enable its discovery.  The members of the collection can be
   defined either through the Interface Description of the resource
   along with a parameter resource for the size of the collection, or by
   using the link format to describe each resource in the collection.
   These links could be located under /.well-known/core or hosted for
   example in the root resource of the collection.

1.2.3.  Resource Directory

   In many deployment scenarios, for example constrained networks with
   sleeping servers, or large M2M deployments with bandwidth limited
   access networks, it makes sense to deploy resource directory entities
   which store links to resources stored on other servers.  Think of
   this as a limited search engine for constrained M2M resources.

   The CoRE Link Format can be used by a server to register resources
   with a resource directory, or to allow a resource directory to poll
   for resources.  Resource polling uses the same process as unicast or
   multicast discovery, however usually without filtering.  Resource
   registration can be achieved by having each server POST their
   resources to /.well-known/core on the resource directory.  This in
   turn adds links to the resource directory under an appropriate
   resource.  These links can then be discovered by any client by a
   performing a GET on the resource directory using a query string

1.3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC5988].  This specification
   makes use of the following terminology:

   Web Linking
      A framework for indicating the relationships between web

      Also called "typed links" in RFC5988.  A link is a typed
      connection between two resources identified by URIs.  Made up of a
      context URI, a link relation type, a target URI, and optional
      target attributes.

   Link Format
      A particular serialization of typed links.

   CoRE Link Format
      A particular serialization of typed links based the HTTP Link
      Header serialization defined in Section 5 of RFC5988, but carried
      as a resource representation with a MIME type.

      Properly called "Target Attribute" in RFC5988.  A set of key/value
      pairs that describe the link or its target.

   CoRE Resource Discovery
      When a client discovers the list of resources hosted by a server,
      their attributes and other link relations by accessing /.well-

2.  Link Format

   The CoRE Link Format extends the HTTP Link Header format specified in
   [RFC5988].  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 HTML link, Atom feed links [RFC4287] or HTTP Link Headers.
   It is expected that resources discovered in the CoRE Link Format may
   also be made available in alternative formats on the greater
   Internet.  The CoRE Link Format is only expected to be supported in
   constrained networks and M2M systems.

   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 the Internet media type
   "application/link-format" for the CoRE Link Format (see Section 7.3).
   Whereas the HTTP Link Header format depends on [RFC2616] for its
   encoding, the CoRE Link Format is encoded as UTF-8 [RFC3629].  A
   decoder of the format is not expected to (but not prohibited from)
   validate UTF-8 encoding and doesn't need to perform any UTF-8
   normalization.  UTF-8 data can be compared bit-wise, which allows
   values to contain UTF-8 data without any added complexity for
   constrained nodes.

   The CoRE link format is the [RFC5988] production named "Link", and
   imports the ABNF description and associated rules in Section 5 of
   that document.  The "Link:" text is omitted as that is part of the
   HTTP Link Header.  Note that the ABNF in the present document is
   compliant with [RFC5234].  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.

2.1.  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.

2.2.  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 7.2).
   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, links can make use of any registered
   relation by including the relation parameter.  To simplify the
   constrained implementations, the value of a "rel" parameter in this
   link format SHOULD NOT contain more than one relation type.  There
   may be cases where multiple relation types cannot be avoided, for
   example when storing a RFC5988 Link header in this link format.  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 expressed.

2.3.  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.

3.  CoRE link extensions

   The following CoRE specific target attributes are defined in addition
   to the ABNF rules in Section 5 of [RFC5988].  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 non resolvable identifiers (they are not meant to be retrieved).
   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    = <Defined in RFC5988>
      link-extension    =/ ( "rt=" quoted-string )
      link-extension    =/ ( "if=" quoted-string )
      link-extension    =/ ( "sz=" cardinal )
      cardinal          = "0" / %x31-39 *DIGIT

3.1.  Resource type 'rt' attribute

   The resource type "rt" attribute is an opaque string used to assign a
   semantically important type to a resource.  One can think of this as
   a noun describing the resource.  In the case of a temperature
   resource this could be e.g. an application-specific semantic type
   like "OutdoorTemperature", a Universal Resource Name (URN) like
   "urn:temperature:outdoor" or a URI referencing a specific concept in
   an ontology like
   "".  Multiple
   resource type attributes MAY appear in a link.

   The resource type 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.

3.2.  Interface description 'if' attribute

   The Interface Description "if" attribute is an opaque string used to
   provide a name, URI or URN indicating a specific interface definition
   used to interact with the target resource.  One can think of this as
   describing verbs usable on a resource.  The Interface Description
   attribute is meant to describe the generic REST interface to interact
   with a resource or a set of resources.  It is expected that an
   Interface Description will be re-used by different resource types.
   For example the resource types "OutdoorTemperature", "DewPoint" and
   "RelHumidity" could all be accessible using the Interface Description

   The Interface Description could be for example the URI of a Web
   Application Description Language (WADL) [WADL] definition of the
   target resource "", a URN
   indicating the type of interface to the resource "urn:myapp:sensor",
   or an application-specific name "Sensor".  Multiple Interface
   Description attributes MAY appear in a link.

3.3.  Maximum size estimate 'sz' attribute

   The maximum size estimate attribute "sz" gives an indication of the
   maximum size of the resource indicated by the target URI.  This
   attribute is not expected to be included for small resources that can
   comfortably by carried in a single Maximum Transmission Unit (MTU),
   but SHOULD be included for resources larger than that.  The maximum
   size estimate attribute MUST NOT appear more than once in a link.

   Note that there is no defined upper limit to the value of the sz
   attributes.  Implementations MUST be prepared to accept large values.
   One implementation strategy is to convert any value larger than a
   reasonable size limit for this implementation to a special value
   "Big", which in further processing would indicate that a size value
   was given that was so big that it cannot be processed by this

4.  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.  The resource /.well-known/
   core is meant to be used to return links to the entry points of
   resource interfaces on a server.  More sophisticated link
   organization can be achieved by including links to CoRE Link Format
   resources located elsewhere on the server, for example to achieve an
   index.  In the absence of any links, a zero-length payload is
   returned.  The resource representation of this resource MUST be the
   CoRE Link Format described in Section 2.

   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 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 4.1.  For example
      [GET /.well-known/core?rt=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
      out of scope for this document, and is expected to be specified

4.1.  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. syntax (parmname is as
   defined in [RFC5988], pct-encoded and unreserved are defined in
   [RFC3986]).  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 = search-token [ "*" ]
          search-token = *search-char
          search-char = unreserved / pct-encoded
                      / ":" / "@"   ; from pchar
                      / "/" / "?"   ; from query
                      / "!" / "$" / "'" / "(" / ")"
                      / "+" / "," / ";" / "="  ; from sub-delims

   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 byte-wise 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.  A query-pattern of "*" matches
   to an empty string value as well as to any other non-empty string.
   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.

   When using a transfer protocol like the Constrained Application
   Protocol (CoAP) that supports multicast requests, special care needs
   to be taken.  A multicast request with a query string SHOULD NOT be
   responded to if filtering is not supported or if the filter does not
   match (to avoid a needless response storm).  The exception is in
   cases where the IP stack interface is not able to indicate that the
   destination address was multicast.

5.  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
   these examples for readability.  Although the following examples use
   CoAP response codes, the examples are applicable to HTTP as well (the
   corresponding response code would be 200 OK).

   This example includes links to two different sensors sharing the same
   Interface Description.

   REQ: GET /.well-known/core

   RES: 2.05 "Content"

   Without the linefeeds included inserted here for readability, the format
   actually looks as follows.


   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: 2.05 "Content"

   REQ: GET /sensors

   RES: 2.05 "Content"

   An example query filter may look like:

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

   RES: 2.05 "Content"

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

   REQ: GET /.well-known/core

   RES: 2.05 "Content"
   </sensors>;rt="index";title="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: 2.05 "Content"

   The following example shows a large firmware resource with a size
   attribute.  The consumer of this link would use the sz attribute to
   determine if the resource representation is too large and if block
   transfer would be required to request it.  In this case a client with
   only a 64 KiB flash might only support a 16-bit integer for storing
   the sz attribute.  Thus a special flag or value should be used to
   indicate "Big" (larger than 64 KiB).

   REQ: GET /.well-known/core?rt=firmware

   RES: 2.05 "Content"

6.  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 using e.g.  IPsec or an
   appropriate object security mechanism.

   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.

7.  IANA Considerations

7.1.  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

7.2.  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

7.3.  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: None

   Encoding considerations: Binary data

   Security considerations:

   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 using e.g.  IPsec or an
   appropriate object security mechanism.

   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.

   Interoperability considerations:

   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): *.wlnk

   Macintosh file type code(s):

   Intended usage: COMMON

   Restrictions on usage: None

   Author: CoRE WG

   Change controller: IETF

8.  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 the discussions
   and ideas that led to this draft, and to Carsten Bormann, Martin
   Thomson, Alexey Melnikov and Peter Saint-Andre 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, Colin O'Flynn and David Ryan
   for helpful comments and discussions that have shaped the document.

9.  Changelog

   Changes from ietf-10 to ietf-11:

      o Fixed editorial nits.

   Changes from ietf-09 to ietf-10:

      o Changed to SHOULD NOT for multiple relation types (#178).

      o Changed to SHOULD NOT for multicast response repression (#179).

      o Updated ABNF for queries (#179).

      o Editorial improvements from WGLC comments.

   Changes from ietf-08 to ietf-09:

      o Corrected ABNF and editorial nits.

      o Elided empty responses to multicast request.

   Changes from ietf-07 to ietf-08:

      o IESG submission nits.

   Changes from ietf-06 to ietf-07:

      o Moved the Content-type attribute (ct=) to the base CoAP

   Changes from ietf-05 to ietf-06:

      o Added improved text about the encoding of the format as UTF-8,
      but treating it as binary data without normalization.

   Changes from ietf-04 to ietf-05:

      o Removed mention of UTF-8 as this is already defined by RFC5988

      o Changed encoding considerations to "Binary data" (#157)

      o Updated ABNF to disallow leading zeros in integers (#159)

      o Updated examples and reference for coap-06 (#152)

      o Removed the application/link-format CoAP code registration, now
      included in the CoAP specification directly (#160)

   Changes from ietf-03 to ietf-04:

      o Removed the attribute registry (#145).

      o Requested a CoAP media type for application/link-format (#144).

      o Editorial and reference improvements from AD review (#146).

      o Added a range limitation for ct attribute.

      o Added security considerations and file extension for
      application/link-format registration.

   Changes from ietf-02 to ietf-03:

      o Removed 'obs' attribute definition, now defined in the CoAP
      Observation spec (#99).

      o Changed Resource name (n=) to Resource type (rt=) and d= to if=

      o Hierarchical organization of links under /.well-known/core
      removed (#95).

      o Bug in Section 3.1 on byte-wise query matching fixed (#91).

      o Explanatory text added about alternative Web link formats (#92).

      o Fixed a bug in Section 2.2.4 (#93).

      o Added use case examples (#89).

      o Clarified how the CoRE link format is used and how it differs
      from RFC5988 (#90, #98).

      o Changed the Interface definition format to quoted-string to
      match the resource type.

      o Added an IANA registry for CoRE Link Format attributes (#100).

   Changes from ietf-01 to ietf-02:

      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 assumption of a default content-type.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

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

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

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

10.2.  Informative References

              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", 2000, <http://

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

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

   [WADL]     Hadley, M., "Web Application Description Language (WADL)",
              2009, <

Author's Address

   Zach Shelby
   Kidekuja 2
   Vuokatti  88600

   Phone: +358407796297