CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Standards Track                               M. Koster
Expires: April 26, June 23, 2019                                       SmartThings
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                         P. van der Stok
                                                              consultant
                                                         C. Amsuess, Ed.
                                                        October 23,
                                                       December 20, 2018

                        CoRE Resource Directory
                 draft-ietf-core-resource-directory-17
                 draft-ietf-core-resource-directory-18

Abstract

   In many M2M applications, direct discovery of resources is not
   practical due to sleeping nodes, disperse networks, or networks where
   multicast traffic is inefficient.  These problems can be solved by
   employing an entity called a Resource Directory (RD), which hosts
   registrations of contains
   information about resources held on other servers, allowing lookups
   to be performed for those resources.  The input to an RD is composed
   of links and the output is composed of links constructed from the
   information stored in the RD.  This document specifies the web
   interfaces that a Resource Directory supports for web servers to
   discover the RD and to register, maintain, lookup and remove resource
   descriptions.
   information on resources.  Furthermore, new link target attributes useful
   in conjunction with an RD are defined.

Status of This Memo

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   This Internet-Draft will expire on April 26, June 23, 2019.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture and Use Cases  . . . . . . . . . . . . . . . . .   6
     3.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  RD Content Model  . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Link-local addresses  . . . . . . . . . . . . . . . . . .  12
     3.5.  Use Case: Cellular M2M  . . . . . . . . . . . . . . . . .  12
     3.5.
     3.6.  Use Case: Home and Building Automation  . . . . . . . . .  13
     3.6.
     3.7.  Use Case: Link Catalogues . . . . . . . . . . . . . . . .  13
   4.  Finding a Resource Directory  . . . . . . . . . . . . . . . .  14
     4.1.  Resource Directory Address Option (RDAO)  . . . . . . . .  15  16
   5.  Resource Directory  . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Payload Content Formats . . . . . . . . . . . . . . . . .  17  18
     5.2.  URI Discovery . . . . . . . . . . . . . . . . . . . . . .  17  18
     5.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  20  21
       5.3.1.  Simple Registration . . . . . . . . . . . . . . . . .  25
       5.3.2.  Third-party registration  . . . . . . . . . . . . . .  27
       5.3.3.  RD-Groups  28
     5.4.  Operations on the Registration Resource . . . . . . . . .  28
       5.4.1.  Registration Update . . . . . . . . . . . . . . . .  28 .  29
       5.4.2.  Registration Removal  . . . . . . . . . . . . . . . .  32
       5.4.3.  Further operations  . . . . . . . . . . . . . . . . .  33
   6.  RD Lookup . . . . . . . . . . . . . . . . . . . . . . . . . .  29  33
     6.1.  Resource lookup . . . . . . . . . . . . . . . . . . . . .  29  34
     6.2.  Lookup filtering  . . . . . . . . . . . . . . . . . . . .  30  34
     6.3.  Resource lookup examples  . . . . . . . . . . . . . . . .  32  36
     6.4.  Endpoint lookup . . . . . . . . . . . . . . . . . . . . .  39
   7.  Security policies . . . . . . . . . . . . . . . . . . . . . .  35  40
     7.1.  Secure RD discovery . . . . . . . . . . . . . . . . . . .  36  41
     7.2.  Secure RD filtering . . . . . . . . . . . . . . . . . . .  37  41
     7.3.  Secure endpoint Name assignment . . . . . . . . . . . . .  37  42

   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  37  42
     8.1.  Endpoint Identification and Authentication  . . . . . . .  38  42
     8.2.  Access Control  . . . . . . . . . . . . . . . . . . . . .  38  43
     8.3.  Denial of Service Attacks . . . . . . . . . . . . . . . .  38  43
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39  43
     9.1.  Resource Types  . . . . . . . . . . . . . . . . . . . . .  39  43
     9.2.  IPv6 ND Resource Directory Address Option . . . . . . . .  39  44
     9.3.  RD Parameter Registry . . . . . . . . . . . . . . . . . .  39  44
       9.3.1.  Full description of the "Endpoint Type" Registration
               Parameter . . . . . . . . . . . . . . . . . . . . . .  42  46
     9.4.  "Endpoint Type" (et=) RD Parameter values . . . . . . . .  42  46
     9.5.  Multicast Address Registration  . . . . . . . . . . . . .  43  47
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  43  47
     10.1.  Lighting Installation  . . . . . . . . . . . . . . . . .  43  47
       10.1.1.  Installation Characteristics . . . . . . . . . . . .  43  47
       10.1.2.  RD entries . . . . . . . . . . . . . . . . . . . . .  44  49
     10.2.  OMA Lightweight M2M (LWM2M) Example  . . . . . . . . . .  47  51
       10.2.1.  The LWM2M Object Model . . . . . . . . . . . . . . .  48  52
       10.2.2.  LWM2M Register Endpoint  . . . . . . . . . . . . . .  49  53
       10.2.3.  LWM2M Update Endpoint Registration . . . . . . . . .  51  55
       10.2.4.  LWM2M De-Register Endpoint . . . . . . . . . . . . .  51  55
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  51  55
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  51  55
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  58  63
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  58  63
     13.2.  Informative References . . . . . . . . . . . . . . . . .  59  64
   Appendix A.  Groups Registration Management and Lookup . . . . . . . . . . .  66
   Appendix B.  Web links and the Resource Directory . . .  61
     A.1.  Registration Update . . . . .  68
     B.1.  A simple example  . . . . . . . . . . . . . .  62
     A.2.  Registration Removal . . . . . .  68
       B.1.1.  Resolving the URIs  . . . . . . . . . . . .  65
     A.3.  Read Endpoint Links . . . . .  68
       B.1.2.  Interpreting attributes and relations . . . . . . . .  69
     B.2.  A slightly more complex example . . . . . .  66
     A.4.  Update Endpoint Links . . . . . . .  69
     B.3.  Enter the Resource Directory  . . . . . . . . . . .  67
     A.5.  Endpoint lookup . . .  70
     B.4.  A note on differences between link-format and Link
           headers . . . . . . . . . . . . . . . . . .  67
   Appendix B.  Web links and the Resource Directory . . . . . . . .  68
     B.1.  A simple example  . . . . . . . . . . . . . . . . . . . .  68
       B.1.1.  Resolving the URIs  . . . . . . . . . . . . . . . . .  69
       B.1.2.  Interpreting attributes and relations . . . . . . . .  69
     B.2.  A slightly more complex example . . . . . . . . . . . . .  69
     B.3.  Enter the Resource Directory  . . . . . . . . . . . . . .  70
     B.4.  A note on differences between link-format and Link
           headers . . . . . . . . . . . . . . . . . . . . . . . . .  72  71
   Appendix C.  Syntax examples for Protocol Negotiation . . . . . .  73
   Appendix D.  Modernized  Limited Link Format parsing . . . . . . . . . . .  74
     D.1.  For endpoint developers . .  . . . . . . . . . . . . . . .  74
     D.2.  Examples of links with differing interpretations . . . .  75  72
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  75  73

1.  Introduction

   The work on Constrained RESTful Environments (CoRE) 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.

   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 Linking [RFC5988]. [RFC8288].  The use of Web Linking for the
   description and discovery of resources hosted by constrained web
   servers is specified by the CoRE Link Format [RFC6690].  However,
   [RFC6690] only describes how to discover resources from the web
   server that hosts them by querying "/.well-known/core".  In many M2M
   scenarios, direct discovery of resources is not practical due to
   sleeping nodes, disperse networks, or networks where multicast
   traffic is inefficient.  These problems can be solved by employing an
   entity called a Resource Directory (RD), which hosts registrations of contains information
   about resources held on other servers, allowing lookups to be
   performed for those resources.

   This document specifies the web interfaces that a Resource Directory
   supports for web servers to discover the RD and to register,
   maintain, lookup and remove resource descriptions. information on resources.  Furthermore,
   new
   link target attributes useful in conjunction with a Resource Directory
   are defined.  Although the examples in this document show the use of
   these interfaces with CoAP [RFC7252], they can be applied in an
   equivalent manner to HTTP [RFC7230].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].  The term "byte" is used in its now customary sense as a
   synonym for "octet".

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC3986], [RFC5988] [RFC8288] and
   [RFC6690].  Readers should also be familiar with the terms and
   concepts discussed in [RFC7252].  To describe the REST interfaces
   defined in this specification, the URI Template format is used
   [RFC6570].

   This specification makes use of the following additional terminology:

   resolve against
      The expression "a URI-reference is _resolved against_ a base URI"
      is used to describe the process of [RFC3986] Section 5.2.
      Noteworthy corner cases are that if the URI-reference is a (full)
      URI and resolved against any base URI, that gives the original
      full URI, and that resolving an empty URI reference gives the base
      URI without any fragment identifier.

   Resource Directory
      A web entity that stores information about web resources and
      implements the REST interfaces defined in this specification for
      registration and lookup of those resources.

   Sector
      In the context of a Resource Directory, a sector is a logical
      grouping of endpoints.

      The abbreviation "d=" is used for the sector in query parameters
      for compatibility with deployed implementations.

   Endpoint
      Endpoint (EP) is a term used to describe a web server or client in
      [RFC7252].  In the context of this specification an endpoint is
      used to describe a web server that registers resources to the
      Resource Directory.  An endpoint is identified by its endpoint
      name, which is included during registration, and has a unique name
      within the associated sector of the registration.

   Registration Base URI
      The Base URI of a Registration is a URI that typically gives
      scheme and authority information about an Endpoint.  The
      Registration Base URI is provided at registration time, and is
      used by the Resource Directory to resolve relative references of
      the registration into URIs.

   Target
      The target of a link is the destination address (URI) of the link.
      It is sometimes identified with "href=", or displayed as
      "<target>".  Relative targets need resolving with respect to the
      Base URI (section 5.2 of [RFC3986]).

      This use of the term Target is consistent with [RFC8288]'s use of
      the term.

   Context
      The context of a link is the source address (URI) of the link, and
      describes which resource is linked to the target.  A link's
      context is made explicit in serialized links as the "anchor="
      attribute.

      This use of the term Context is consistent with [RFC8288]'s use of
      the term.

   Directory Resource
      A resource in the Resource Directory (RD) containing registration
      resources.

   Registration Resource
      A resource in the RD that contains information about an Endpoint
      and its links.

   Commissioning Tool
      Commissioning Tool (CT) is a device that assists during the
      installation of the network by assigning values to parameters,
      naming endpoints and groups, or adapting the installation to the
      needs of the applications.

   Registrant-ep
      Registrant-ep is the endpoint that is registered into the RD.  The
      registrant-ep can register itself, or a CT registers the
      registrant-ep.

   RDAO
      Resource Directory Address Option.

   For several operations, interface descriptions templates are given in list form;
   those describe the operation participants, request codes, URIs,
   content formats and outcomes.  Those  Sections of those templates contain
   normative content in their about Interaction, Method, URI Template and URI
   Template Variables sections as well as the details of the Success condition.
   The additional sections on options like Content-Format and on Failure
   codes give typical cases that an implementation of the RD should deal
   with.  Those serve to illustrate the typical responses to readers who
   are not yet familiar with all the details of CoAP based interfaces;
   they do not limit what a server may respond under atypical
   circumstances.

3.  Architecture and Use Cases

3.1.  Principles

   The Resource Directory is primarily a tool to make discovery
   operations more efficient than querying /.well-known/core on all
   connected devices, or across boundaries that would be limiting those
   operations.

   It provides a cache (in the high-level sense, not as defined in
   [RFC7252]/[RFC2616]) of data information about resources hosted by other devices that
   could otherwise only be obtained by directly querying the /.well-known/core /.well-
   known/core resource on the target
   device, or these other devices, either by accessing those resources with a unicast
   request or a multicast request.

   Only information SHOULD be stored in the resource directory that is
   discoverable from can
   be obtained by querying the described device's /.well-known/core
   resource directly.

   Data in the resource directory can only be provided by the device
   which hosts those data or a dedicated Commissioning Tool (CT).  These
   CTs are thought to act on behalf of endpoints too constrained, or
   generally unable, to present that information themselves.  No other
   client can modify data in the resource directory.  Changes to the
   information in the Resource Directory do not propagate automatically
   back to the web
   server servers from where the links information originated.

3.2.  Architecture

   The resource directory architecture is illustrated in Figure 1.  A
   Resource Directory (RD) is used as a repository for Web Links
   [RFC5988] of registrations
   describing resources hosted on other web servers, also called
   endpoints (EP).  An endpoint is a web server associated with a
   scheme, IP address and port.  A physical node may host one or more
   endpoints.  The RD implements a set of REST interfaces for endpoints
   to register and maintain sets of Web Links (called resource directory
   registration entries), registrations, and for
   endpoints to lookup resources from the RD.  An RD can be logically
   segmented by the use of Sectors.  This
   information hierarchy is shown in Figure 2.

   A mechanism to discover an RD using CoRE Link Format [RFC6690] is
   defined.

   Registration entries

   Registrations in the RD are soft state and need to be periodically
   refreshed.

   An endpoint uses specific interfaces to register, update and remove a
   resource directory registration entry.
   registration.  It is also possible for an RD to fetch Web Links from
   endpoints and add them as their contents to resource directory
   registration entries. registrations.

   At the first registration of a set of entries, an endpoint, a "registration resource"
   is created, the location of which is returned to the registering
   endpoint.  The registering endpoint uses this registration resource
   to manage the contents of registration entries. registrations.

   A lookup interface for discovering any of the Web Links held stored in the
   RD is provided using the CoRE Link Format.

                Registration         Lookup
                 Interface         Interface
     +----+          |                 |
     | EP |----      |                 |
     +----+    ----  |                 |
                   --|-    +------+    |
     +----+          | ----|      |    |     +--------+
     | EP | ---------|-----|  RD  |----|-----| Client |
     +----+          | ----|      |    |     +--------+
                   --|-    +------+    |
     +----+    ----  |                 |
     | EP |----      |                 |
     +----+

              Figure 1: The resource directory architecture.

                  +------------+
                  |  Endpoint  |  <-- Name, Scheme, IP, Port
                  +------------+
                        |
                        |
                  +------------+
                  |  Resource  |  <-- Target, Parameters
                  +------------+

          Figure 2: The resource directory information hierarchy.

   A Registrant-EP MAY keep concurrent registrations to more than one RD
   at the same time if explicitly configured to do so, but that is not
   expected to be supported by typical EP implementations.  Any such
   registrations are independent of each other.  The usual expectation
   when multiple discovery mechanisms or addresses are configured is
   that they constitute a fallback fall-back path for a single registration.

3.3.  RD Content Model

   The Entity-Relationship (ER) models shown in Figure 3 2 and Figure 4 3
   model the contents of /.well-known/core and the resource directory
   respectively, with entity-relationship diagrams [ER].  Entities
   (rectangles) are used for concepts that exist independently.
   Attributes (ovals) are used for concepts that exist only in
   connection with a related entity.  Relations (diamonds) give a
   semantic meaning to the relation between entities.  Numbers specify
   the cardinality of the relations.

   Some of the attribute values are URIs.  Those values are always full
   URIs and never relative references in the information model.  They
   can, however, be expressed as relative references in serializations,
   and often are.

   These models provide an abstract view of the information expressed in
   link-format documents and a Resource Directory.  They cover the
   concepts, but not necessarily all details of an RD's operation; they
   are meant to give an overview, and not be a template for
   implementations.

                       +----------------------+
                       |   /.well-known/core  |
                       +----------------------+
                                  |
                                  | 1
                          ////////\\\\\\\
                         <    contains   >
                          \\\\\\\\///////
                                  |
                                  | 0+
                        +--------------------+
                        |      link          |
                        +--------------------+
                                  |
                                  |  1   oooooooo
                                  +-----o target o
                                  |      oooooooo
             oooooooooooo   0+    |
            o    target  o--------+
            o  attribute o        | 0+   oooooo
             oooooooooooo         +-----o rel  o
                                  |      oooooo
                                  |
                                  | 1    ooooooooo
                                  +-----o context o
                                         ooooooooo

          Figure 3: 2: E-R Model of the content of /.well-known/core

   The model shown in Figure 3 2 models the contents of /.well-known/core
   which contains:

   o  a set of links belonging to the hosting web server

   The web server is free to choose links it deems appropriate to be
   exposed in its ".well-known/core".  Typically, the links describe
   resources that are served by the host, but the set can also contain
   links to resources on other servers (see examples in [RFC6690] page
   14).  The set does not necessarily contain links to all resources
   served by the host.

   A link has the following attributes (see [RFC5988]): [RFC8288]):

   o  Zero or more link relations: They describe relations between the
      link context and the link target.

      In link-format serialization, they are expressed as space-
      separated values in the "rel" attribute, and default to "hosts".

   o  A link context URI: It defines the source of the relation, e.g.
      _who_ "hosts" something.

      In link-format serialization, it is expressed in the "anchor"
      attribute.  It defaults to that document's URI.

   o  A link target URI: It defines the destination of the relation
      (e.g. _what_ is hosted), and is the topic of all target
      attributes.

      In link-format serialization, it is expressed between angular
      brackets, and sometimes called the "href".

   o  Other target attributes (e.g. resource type (rt), interface (if),
      or content-type (ct)).  These provide additional information about
      the target URI.

                +----------------------+
                |  resource-directory  |
                +----------------------+
                           | 1
                           |
                           |
                           |
                           |
                      //////\\\\
                     < contains >
                      \\\\\/////
                           |
                        0+ |
    ooooooo     1  +---------------+
   o  base o-------|  registration |
    ooooooo        +---------------+
                       |       | 1
                       |       +--------------+
          oooooooo   1 |                      |
         o  href  o----+                 /////\\\\
          oooooooo     |                < contains >
                       |                 \\\\\/////
          oooooooo   1 |                      |
         o   ep   o----+                      | 0+
          oooooooo     |             +------------------+
                       |             |      link        |
          oooooooo 0-1 |             +------------------+
         o    d   o----+                      |
          oooooooo     |                      |  1   oooooooo
                       |                      +-----o target o
          oooooooo   1 |                      |      oooooooo
         o   lt   o----+     ooooooooooo   0+ |
          oooooooo     |    o  target   o-----+
                       |    o attribute o     | 0+   oooooo
       ooooooooooo 0+  |     ooooooooooo      +-----o rel  o
      o  endpoint o----+                      |      oooooo
      o attribute o                           |
       ooooooooooo                            | 1   ooooooooo
                                              +----o context o
                                                    ooooooooo

       Figure 4: 3: E-R Model of the content of the Resource Directory

   The model shown in Figure 4 3 models the contents of the resource
   directory which contains in addition to /.well-known/core:

   o  0 to n Registration (entries) Registrations of endpoints,
   A registration is associated with one endpoint.  A registration
   defines a set of links as defined for /.well-known/core.  A
   Registration has six types of attributes:

   o  a unique endpoint name ("ep") within a sector

   o  a Registration Base URI ("base", a URI typically describing the
      scheme://authority part)

   o  a lifetime ("lt"),

   o  a registration resource location inside the RD ("href"),

   o  optionally a sector ("d")

   o  optional additional endpoint attributes (from Section 9.3)

   The cardinality of "base" is currently 1; future documents are
   invited to extend the RD specification to support multiple values
   (e.g.  [I-D.silverajan-core-coap-protocol-negotiation]).  Its value
   is used as a Base URI when resolving URIs in the links contained in
   the endpoint.

   Links are modelled as they are in Figure 3. 2.

3.4.  Link-local addresses

   Registration requests to the RD may arrive from link-local IP
   addresses.  When building a Registration Base URI from that source IP
   address (which would become part of the resolved URIs in resource
   lookup), its link-local IP literal typically contains a zone
   identifier of the RD, and is not usable across hosts (see [RFC6874]
   Section 1).

   Therefore, RD servers SHOULD reject registrations which use of URIs
   containing link-local IP addresses.

3.5.  Use Case: Cellular M2M

   Over the last few years, mobile operators around the world have
   focused on development of M2M solutions in order to expand the
   business to the new type of users: machines.  The machines are
   connected directly to a mobile network using an appropriate embedded
   wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing
   short and wide range wireless interfaces.  From the system design
   point of view, the ambition is to design horizontal solutions that
   can enable utilization of machines in different applications
   depending on their current availability and capabilities as well as
   application requirements, thus avoiding silo like solutions.  One of
   the crucial enablers of such design is the ability to discover
   resources (machines -- endpoints) capable of providing required
   information at a given time or acting on instructions from the end
   users.

   Imagine a scenario where endpoints installed on vehicles enable
   tracking of the position of these vehicles for fleet management
   purposes and allow monitoring of environment parameters.  During the
   boot-up process endpoints register with a Resource Directory, which
   is hosted by the mobile operator or somewhere in the cloud.
   Periodically, these endpoints update their registration and may
   modify resources they offer.

   When endpoints are not always connected, for example because they
   enter a sleep mode, a remote server is usually used to provide proxy
   access to the endpoints.  Mobile apps or web applications for
   environment monitoring contact the RD, look up the endpoints capable
   of providing information about the environment using an appropriate
   set of link parameters, obtain information on how to contact them
   (URLs of the proxy server), and then initiate interaction to obtain
   information that is finally processed, displayed on the screen and
   usually stored in a database.  Similarly, fleet management systems
   provide the appropriate link parameters to the RD to look up for EPs
   deployed on the vehicles the application is responsible for.

3.5.

3.6.  Use Case: Home and Building Automation

   Home and commercial building automation systems can benefit from the
   use of M2M web services.  The discovery requirements of these
   applications are demanding.  Home automation usually relies on run-
   time discovery to commission the system, whereas in building
   automation a combination of professional commissioning and run-time
   discovery is used.  Both home and building automation involve peer-
   to-peer interactions between endpoints, and involve battery-powered
   sleeping devices.

3.6.

3.7.  Use Case: Link Catalogues

   Resources may be shared through data brokers that have no knowledge
   beforehand of who is going to consume the data.  Resource Directory
   can be used to hold links about resources and services hosted
   anywhere to make them discoverable by a general class of
   applications.

   For example, environmental and weather sensors that generate data for
   public consumption may provide data to an intermediary server, or
   broker.  Sensor data are published to the intermediary upon changes
   or at regular intervals.  Descriptions of the sensors that resolve to
   links to sensor data may be published to a Resource Directory.
   Applications wishing to consume the data can use RD Lookup to
   discover and resolve links to the desired resources and endpoints.
   The Resource Directory service need not be coupled with the data
   intermediary service.  Mapping of Resource Directories to data
   intermediaries may be many-to-many.

   Metadata in web link formats like [RFC6690] which may be internally
   stored as triples, or relation/attribute pairs providing metadata
   about resource links, need to be supported by Resource Directories .
   External catalogues that are represented in other formats may be
   converted to common web linking formats for storage and access by
   Resource Directories.  Since it is common practice for these to be
   URN encoded, simple and lossless structural transforms should
   generally be sufficient to store external metadata in Resource
   Directories.

   The additional features of Resource Directory allow sectors to be
   defined to enable access to a particular set of resources from
   particular applications.  This provides isolation and protection of
   sensitive data when needed.  Application groups with multicast
   addresses may be defined to support efficient data transport.

4.  Finding a Resource Directory

   A (re-)starting device may want to find one or more resource
   directories for discovery purposes.

   The device may be pre-configured to exercise specific mechanisms for
   finding the resource directory:

   1.  It may be configured with a specific IP address for the RD.  That
       IP address may also be an anycast address, allowing the network
       to forward RD requests to an RD that is topologically close; each
       target network environment in which some of these preconfigured
       nodes are to be brought up is then configured with a route for
       this anycast address that leads to an appropriate RD.  (Instead
       of using an anycast address, a multicast address can also be
       preconfigured.  The RD servers then need to configure one of
       their interfaces with this multicast address.)

   2.  It may be configured with a DNS name for the RD and use DNS to
       return the IP address of the RD; it can find a DNS server to
       perform the lookup using the usual mechanisms for finding DNS
       servers.

   3.  It may be configured to use a service discovery mechanism such as
       DNS-SD [RFC6763].  The present specification suggests configuring
       the service with name rd._sub._coap._udp, preferably within the
       domain of the querying nodes.

   For cases where the device is not specifically configured with a way
   to find a resource directory, the network may want to provide a
   suitable default.

   1.  If the address configuration of the network is performed via
       SLAAC, this is provided by the RDAO option Section 4.1.

   2.  If the address configuration of the network is performed via
       DHCP, this could be provided via a DHCP option (no such option is
       defined at the time of writing).

   Finally, if neither the device nor the network offers any specific
   configuration, the device may want to employ heuristics to find a
   suitable resource directory.

   The present specification does not fully define these heuristics, but
   suggests a number of candidates:

   1.  In a 6LoWPAN, just assume the Border Router (6LBR) can act as a
       resource directory (using the ABRO option to find that
       [RFC6775]).  Confirmation can be obtained by sending a Unicast to
       "coap://[6LBR]/.well-known/core?rt=core.rd*".

   2.  In a network that supports multicast well, discovering the RD
       using a multicast query for /.well-known/core as specified in
       CoRE Link Format [RFC6690]: Sending a Multicast GET to
       "coap://[MCD1]/.well-known/core?rt=core.rd*".  RDs within the
       multicast scope will answer the query.

   When answering a link-local multicast request, the RD SHOULD NOT
   respond with their link-local addresses but use a routable one;
   otherwise the registrant-ep would later need to pick an explicit base
   address to avoid the issue of Section 3.4.

   As some of the RD addresses obtained by the methods listed here are
   just (more or less educated) guesses, endpoints MUST make use of any
   error messages to very strictly rate-limit requests to candidate IP
   addresses that don't work out.  For example, an ICMP Destination
   Unreachable message (and, in particular, the port unreachable code
   for this message) may indicate the lack of a CoAP server on the
   candidate host, or a CoAP error response code such as 4.05 "Method
   Not Allowed" may indicate unwillingness of a CoAP server to act as a
   directory server.

   If multiple candidate addresses are discovered, the device may pick
   any of them initially, unless the discovery method indicates a more
   precise selection scheme.

4.1.  Resource Directory Address Option (RDAO)

   The Resource Directory Address Option (RDAO) using IPv6 Neighbor
   Discovery (ND) carries information about the address of the Resource
   Directory (RD).  This information is needed when endpoints cannot
   discover the Resource Directory with a link-local or realm-local
   scope multicast address because the endpoint and the RD are separated
   by a Border Router (6LBR).  In many circumstances the availability of
   DHCP cannot be guaranteed either during commissioning of the network.
   The presence and the use of the RD is essential during commissioning.

   It is possible to send multiple RDAO options in one message,
   indicating as many resource directory addresses.

   The RDAO format is:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length = 3   |       Valid Lifetime          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          RD Address                           +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:                   38

   Length:                 8-bit unsigned integer.  The length of
                           the option in units of 8 bytes.
                           Always 3.

   Valid Lifetime:         16-bit unsigned integer.  The length of
                           time in units of 60 seconds (relative to
                           the time the packet is received) that
                           this Resource Directory address is valid.
                           A value of all zero bits (0x0) indicates
                           that this Resource Directory address
                           is not valid anymore.

   Reserved:               This field is unused.  It MUST be
                           initialized to zero by the sender and
                           MUST be ignored by the receiver.

   RD Address:             IPv6 address of the RD.

                Figure 5: 4: Resource Directory Address Option

5.  Resource Directory

   This section defines the required set of REST interfaces between a
   Resource Directory (RD) and endpoints.  Although the examples
   throughout this section assume the use of CoAP [RFC7252], these REST
   interfaces can also be realized using HTTP [RFC7230].  In all
   definitions in this section, both CoAP response codes (with dot
   notation) and HTTP response codes (without dot notation) are shown.

   An RD implementing this specification MUST support the discovery,
   registration, update, lookup, and removal interfaces defined in this
   section.

   All operations on the contents of the Resource Directory MUST be
   atomic and idempotent.

   A resource directory MAY make the information submitted to it
   available to further directories, if it can ensure that a loop does
   not form.  The protocol used between directories to ensure loop-free
   operation is outside the scope of this document.

5.1.  Payload Content Formats

   Resource Directory implementations using this specification MUST
   support the application/link-format content format (ct=40).

   Resource Directories implementing this specification MAY support
   additional content formats.

   Any additional content format supported by a Resource Directory
   implementing this specification MUST have an equivalent serialization
   in SHOULD be able to express all the application/link-format content format.
   information expressible in link-format.  It MAY be able to express
   information that is inexpressible in link-format, but those
   expressions SHOULD be avoided where possible.

5.2.  URI Discovery

   Before an endpoint can make use of an RD, it must first know the RD's
   address and port, and the URI path information for its REST APIs.
   This section defines discovery of the RD and its URIs using the well-
   known interface of the CoRE Link Format [RFC6690].  A complete set of
   RD discovery methods is described in Section 4.

   Discovery of the RD registration URI path is performed by sending
   either a multicast or unicast GET request to "/.well-known/core" and
   including a Resource Type (rt) parameter [RFC6690] with the value
   "core.rd" in the query string.  Likewise, a Resource Type parameter
   value of "core.rd-lookup*" is used to discover the URIs for RD Lookup
   operations, core.rd* is used to discover all URI paths for RD
   operations.  Upon success, the response will contain a payload with a
   link format entry for each RD function discovered, indicating the URI
   of the RD function returned and the corresponding Resource Type.
   When performing multicast discovery, the multicast IP address used
   will depend on the scope required and the multicast capabilities of
   the network (see Section 9.5. 9.5).

   A Resource Directory MAY provide hints about the content-formats it
   supports in the links it exposes or registers, using the "ct" link target
   attribute, as shown in the example below.  Clients MAY use these
   hints to select alternate content-formats for interaction with the
   Resource Directory.

   HTTP does not support multicast and consequently only unicast
   discovery can be supported using HTTP.  The well-known entry points
   SHOULD be provided to enable unicast discovery.

   An implementation of this resource directory specification MUST
   support query filtering for the rt parameter as defined in [RFC6690].

   While the link targets in this discovery step are often expressed in
   path-absolute form, this is not a requirement.  Clients of the RD
   SHOULD therefore accept URIs of all schemes they support, both as
   URIs and relative references, and not limit the set of discovered
   URIs to those hosted at the address used for URI discovery.

   The URI Discovery operation can yield multiple URIs of a given
   resource type.  The client of the RD can use any of the discovered
   addresses initially.

   The discovery request interface is specified as follows (this is
   exactly the Well-Known Interface of [RFC6690] Section 4, with the
   additional requirement that the server MUST support query filtering):

   Interaction:  EP and Client -> RD

   Method:  GET

   URI Template:  /.well-known/core{?rt}

   URI Template Variables:

      rt :=  Resource Type.  SHOULD contain one of the values "core.rd",
         "core.rd-lookup*", "core.rd-lookup-res", "core.rd-lookup-ep",
         or "core.rd*"

   Content-Format:

   Accept:  absent, application/link-format (if any)

   Content-Format:  application/link-format+json (if any)
   Content-Format:  application/link-format+cbor (if any) or any other media type
      representing web links

   The following response codes are defined for this interface:

   Success:  2.05 "Content" or 200 "OK" with an application/link-format,
      application/link-format+json, application/link-format
      or application/link-format+cbor other web link payload containing one or more matching entries
      for the RD resource.

   Failure:  4.00 "Bad Request" or 400 "Bad Request" is returned in case
      of a malformed request for a unicast request.

   Failure:  No error response to a multicast request.

   HTTP support :  YES (Unicast only)

   The following example shows an endpoint discovering an RD using this
   interface, thus learning that the directory resource location, in
   this example, is /rd, and that the content-format delivered by the
   server hosting the resource is application/link-format (ct=40).  Note
   that it is up to the RD to choose its RD locations.

   Req: GET coap://[MCD1]/.well-known/core?rt=core.rd*

   Res: 2.05 Content
   </rd>;rt="core.rd";ct=40,
   </rd-lookup/ep>;rt="core.rd-lookup-ep";ct=40,
   </rd-lookup/res>;rt="core.rd-lookup-res";ct=40,

                   Figure 6: 5: Example discovery exchange

   The following example shows the way of indicating that a client may
   request alternate content-formats.  The Content-Format code attribute
   "ct" MAY include a space-separated sequence of Content-Format codes
   as specified in Section 7.2.1 of [RFC7252], indicating that multiple
   content-formats are available.  The example below shows the required
   Content-Format 40 (application/link-format) indicated as well as the a
   CBOR and JSON representation of link format.  The RD resource
   locations /rd, and /rd-lookup are example values.  The server in this
   example also from [I-D.ietf-core-links-json] (which
   have no numeric values assigned yet, so they are shown as TBD64 and
   TBD504 as in that draft).  The RD resource locations /rd, and /rd-
   lookup are example values.  The server in this example also indicates
   that it is capable of providing observation on resource lookups.

   [ The RFC editor is asked to replace these this and later occurrences of
   MCD1, TBD64 and TBD504
   MCD1 with the assigned IPv6 site-local address for "all CoRE Resource Directories" and the numeric ID values assigned by
   IANA to application/link-format+cbor and application/link-
   format+json, respectively, as they are defined in I-D.ietf-core-
   links-json.
   Directories". ]

   Req: GET coap://[MCD1]/.well-known/core?rt=core.rd*

   Res: 2.05 Content
   </rd>;rt="core.rd";ct="40 65225",
   </rd-lookup/res>;rt="core.rd-lookup-res";ct="40 TBD64 TBD504";obs,
   </rd-lookup/ep>;rt="core.rd-lookup-ep";ct="40 TBD64 TBD504",

   From a management and maintenance perspective, it is necessary to
   identify the components that constitute the RD server.  The
   identification refers to information about for example client-server
   incompatibilities, supported features, required updates and other
   aspects.  The URI discovery address, a described in section 4 of
   [RFC6690] can be used to find the identification.

   It would typically be stored in an implementation information link
   (as described in [I-D.bormann-t2trg-rel-impl]):

   Req: GET /.well-known/core?rel=impl-info

   Res: 2.05 Content
   <http://software.example.com/shiny-resource-directory/1.0beta1>;
       rel="impl-info"

   Note that depending on the particular server's architecture, such a
   link could be anchored at the RD server's root, at the discovery site
   (as in this example) or at individual RD components.  The latter is
   to be expected when different applications are run on the same
   server.

5.3.  Registration

   After discovering the location of an RD, a registrant-ep or CT MAY
   register the resources of the registrant-ep using the registration
   interface.  This interface accepts a POST from an endpoint containing
   the list of resources to be added to the directory as the message
   payload in the CoRE Link Format [RFC6690], JSON CoRE Link Format
   (application/link-format+json), [RFC6690] or CBOR CoRE Link Format
   (application/link-format+cbor) [I-D.ietf-core-links-json], other representations of
   web links, along with query parameters indicating the name of the
   endpoint, and optionally the sector, lifetime and base URI of the
   registration.  It is expected that other specifications will define
   further parameters (see Section 9.3).  The RD then creates a new
   registration resource in the RD and returns its location.  The
   receiving endpoint MUST use that location when refreshing
   registrations using this interface.  Registration resources in the RD
   are kept active for the period indicated by the lifetime parameter.
   The creating endpoint is responsible for refreshing the registration
   resource within this period using either the registration or update
   interface.  The registration interface MUST be implemented to be
   idempotent, so that registering twice with the same endpoint
   parameters ep and d (sector) does not create multiple registration
   resources.

   The following rules apply for an update identified by a registration request targeting a
   given (ep, d) value pair:

   o  when the parameter values of the Update generate the same
      attribute values as already present, the location of  When the already
      existing registration is returned.

   o  when for a given (ep, d) value pair of the update generates attribute
      values which are registration-request is
      different from the existing one, the any existing
      registration is removed and registration, a new registration with a new location is created.
      generated.

   o  when  When the (ep, d) value pair of the update registration-request is different from any equal
      to an existing registration, a new the content and parameters of the
      existing registration is generated. are replaced with the content of the
      registration request.

   The posted link-format document can (and typically does) contain
   relative references both in its link targets and in its anchors, or
   contain empty anchors.  The RD server needs to resolve these
   references in order to faithfully represent them in lookups.  They
   are resolved against the base URI of the registration, which is
   provided either explicitly in the "base" parameter or constructed
   implicitly from the requester's URI as constructed from its network
   address and scheme.

   Link format documents submitted

   For media types to the resource directory are
   interpreted as Modernized Link Format (see which Appendix D) by the RD.  A
   registrant-ep SHOULD NOT submit C applies (i.e. documents whose interpretations
   according to [RFC6690] and Appendix D differ to avoid the ambiguities
   described in Appendix B.4.

   In practice, most links (precisely listed
   application/link-format), the RD only needs to accept representations
   in Appendix D.1) can be
   submitted without consideration for those details. Limited Link Format as described there.  Its behavior with
   representations outside that subset is implementation defined.

   The registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  {+rd}{?ep,d,lt,base,extra-attrs*}

   URI Template Variables:

      rd :=  RD registration URI (mandatory).  This is the location of
         the RD, as obtained from discovery.

      ep :=  Endpoint name (mostly mandatory).  The endpoint name is an
         identifier that MUST be unique within a sector.  The maximum
         length of this parameter is 63 bytes.  If the RD is configured
         to recognize the endpoint (e.g. based on its security context),
         the endpoint sets no endpoint name, and the RD assigns one an endpoint name based on a set of configuration
         parameter values.

      d :=  Sector (optional).  The sector to which this endpoint
         belongs.  The maximum length of this parameter is 63 bytes.
         When this parameter is not present, the RD MAY associate the
         endpoint with a configured default sector or leave it empty.
         The endpoint name and sector name are not set when one or both
         are set in an accompanying authorization token.

      lt :=  Lifetime (optional).  Lifetime of the registration in
         seconds.  Range of 60-4294967295.  If no lifetime is included
         in the initial registration, a default value of 90000 (25
         hours) SHOULD be assumed.

      base :=  Base URI (optional).  This parameter sets the base URI of
         the registration, under which the relative links in the payload
         are to be interpreted.  The specified URI typically does not
         have a path component of its own, and MUST be suitable as a
         base URI to resolve any relative references given in the
         registration.  The parameter is therefore usually of the shape
         "scheme://authority" for HTTP and CoAP URIs.  The URI SHOULD
         NOT have a query or fragment component as any non-empty
         relative part in a reference would remove those parts from the
         resulting URI.

         In the absence of this parameter the scheme of the protocol,
         source address and source port of the registration request are
         assumed.  That  The Base URI is consecutively constructed by
         concatenating the used protcol's protocol's scheme with the characters
         "://", the requester's source address as an address literal and
         ":" followed by its port (if it was not the protocol's default
         one) in analogy to [RFC7252] Section 6.5.

         This parameter is mandatory when the directory is filled by a
         third party such as an commissioning tool.

         If the registrant-ep uses an ephemeral port to register with,
         it MUST include the base parameter in the registration to
         provide a valid network path.

         If the registrant-ep, located behind a NAT gateway, is
         registering with a Resource Directory which is on the network
         service side of the NAT gateway, the endpoint MUST use a
         persistent port for the outgoing registration in order to
         provide the NAT gateway with a valid network address for
         replies and incoming requests.

         Endpoints that register with a base that contains

         If the registrant-ep uses a path
         component link-local address to register, it
         MUST give an explicit routable base address unless configured
         otherwise as per Section 3.4 (or just register from that
         address in the first place).

         Endpoints that register with a base that contains a path
         component can not meaningfully use [RFC6690] Link Format due to
         its prevalence of the Origin concept in relative reference
         resolution; they can submit payloads for interpretation as
         Modernized Link Format.  Typically, links submitted by such an
         endpoint are
         resolution.  Those applications should use different
         representations of the "path-noscheme" (starts with a path links to which Appendix C is not
         preceded by a slash, precisely defined in [RFC3986]
         Section 3.3) form. applicable
         (e.g.  [I-D.hartke-t2trg-coral]).

      extra-attrs :=  Additional registration attributes (optional).
         The endpoint can pass any parameter registered at Section 9.3
         to the directory.  If the RD is aware of the parameter's
         specified semantics, it processes it accordingly.  Otherwise,
         it MUST store the unknown key and its value(s) as an endpoint
         attribute for further lookup.

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor or any other indicated media
      type representing web links

   The following response codes are defined for this interface:

   Success:  2.01 "Created" or 201 "Created".  The Location-Path option
      or Location header MUST be included in the response.  This
      location MUST be a stable identifier generated by the RD as it is
      used for all subsequent operations on this registration resource.
      The registration resource location thus returned is for the
      purpose of updating the lifetime of the registration and for
      maintaining the content of the registered links, including
      updating and deleting links.

      A registration with an already registered ep and d value pair
      responds with the same success code and location as the original
      registration; the set of links registered with the endpoint is
      replaced with the links from the payload.

      The location MUST NOT have a query or fragment component, as that
      could conflict with query parameters during the Registration
      Update operation.  Therefore, the Location-Query option MUST NOT
      be present in a successful response.

   Failure:  4.00 "Bad Request" or 400 "Bad Request".  Malformed
      request.

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the operation.

   HTTP support:  YES

   If the registration fails with a Service Unavailable response and a
   Max-Age option or Retry-After header, the registering endpoint SHOULD
   retry the operation after the time indicated.  If the registration
   fails in another way, including request timeouts, or if the Service
   Unavailable error persists after several retries, or indicates a
   longer time than the endpoint is willing to wait, it SHOULD pick
   another registration URI from the "URI Discovery" step and if there
   is only one or the list is exhausted, pick other choices from the
   "Finding a Resource Directory" step.  Care has to be taken to
   consider the freshness of results obtained earlier, e.g. of the
   result of a "/.well-known/core" response, the lifetime of an RDAO
   option and of DNS responses.  Any rate limits and persistent errors
   from the "Finding a Resource Directory" step must be considered for
   the whole registration time, not only for a single operation.

   The following example shows a registrant-ep with the name "node1"
   registering two resources to an RD using this interface.  The
   location "/rd" is an example RD location discovered in a request
   similar to Figure 6. 5.

   Req: POST coap://rd.example.com/rd?ep=node1
   Content-Format: 40
   Payload:
   </sensors/temp>;ct=41;rt="temperature-c";if="sensor";
         anchor="coap://spurious.example.com:5683",
   </sensors/light>;ct=41;rt="light-lux";if="sensor"

   Res: 2.01 Created
   Location-Path: /rd/4521

                  Figure 7: 6: Example registration payload

   A Resource Directory may optionally support HTTP.  Here is an example
   of almost the same registration operation above, when done using HTTP
   and the JSON Link Format.
   HTTP.

   Req: POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1
   Host: example.com
   Content-Type: application/link-format+json application/link-format
   Payload:
   [
   {"href": "/sensors/temp", "ct": "41", "rt": "temperature-c",
   "if": "sensor", "anchor": "coap://spurious.example.com:5683"},
   {"href": "/sensors/light", "ct": "41", "rt": "light-lux",
     "if": "sensor"}
   ]
   </sensors/temp>;ct=41;rt="temperature-c";if="sensor";
         anchor="coap://spurious.example.com:5683",
   </sensors/light>;ct=41;rt="light-lux";if="sensor"

   Res: 201 Created
   Location: /rd/4521

5.3.1.  Simple Registration

   Not all endpoints hosting resources are expected to know how to
   upload links to an RD as described in Section 5.3.  Instead, simple
   endpoints can implement the Simple Registration approach described in
   this section.  An RD implementing this specification MUST implement
   Simple Registration.  However, there may be security reasons why this
   form of directory discovery would be disabled.

   This approach requires that the registrant-ep makes available the
   hosted resources that it wants to be discovered, as links on its
   "/.well-known/core" interface as specified in [RFC6690].  The links
   in that document are subject to the same limitations as the payload
   of a registration (with respect to Appendix D). C).

   The registrant-ep finds one or more addresses of the directory server
   as described in Section 4.

   The registrant-ep asks the selected directory server to probe its
   /.well-known/core and publish the links as follows:

   The registrant-ep sends (and regularly refreshes with) a POST request
   to the "/.well-known/core" URI of the directory server of choice.
   The body of the POST request is empty, and triggers the resource
   directory server to perform GET requests at the requesting
   registrant-ep's /.well-known/core to obtain the link-format payload
   to register.

   The registrant-ep includes the same registration parameters in the
   POST request as it would per Section 5.3.  The registration base URI
   of the registration is taken from the requesting server's URI. registrant-ep's network address
   (as is default with regular registrations).

   The Resource Directory MUST NOT needs to query the registrant-ep's data before
   sending the response; this is discovery
   resource to accommodate very limited endpoints.

   The determine the success condition only indicates that of the request was valid (i.e. operation.  It SHOULD keep a
   cache of the passed parameters are valid per se), discovery resource and not query it again as long as it
   is fresh.

   (This is to accomodate constrained registrant devices that can not
   process an incoming and outgoing request at the link data could same time.
   Registrants MUST be obtained or parsed or was successfully registered into able to serve a GET request to "/.well-known/
   core" after having requested registration.  Constrained devices MAY
   regard the RD. initial request as temporarily failed when they need RAM
   occupied by their own request to serve the RD's GET, and retry later
   when the RD already has a cached representation of their discovery
   resources.  Then, the RD can reply immediately and the registrant can
   receive the response.)

   The simple registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  /.well-known/core{?ep,d,lt,extra-attrs*}
   URI Template Variables are as they are for registration in
   Section 5.3.  The base attribute is not accepted to keep the
   registration interface simple; that rules out registration over CoAP-
   over-TCP or HTTP that would need to specify one.

   The following response codes are defined for this interface:

   Success:  2.04 "Changed".

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  5.03 "Service Unavailable".  Service could not perform the
      operation.

   HTTP support:  NO

   For the second interaction triggered by the above, the registrant-ep
   takes the role of server and the RD the role of client.  (Note that
   this is exactly the Well-Known Interface of [RFC6690] Section 4):

   Interaction:  RD -> EP

   Method:  GET

   URI Template:  /.well-known/core

   The following response codes are defined for this interface:

   Success:  2.05 "Content".

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  4.04 "Not Found". /.well-known/core does not exist or is
      empty. exist.

   Failure:  5.03 "Service Unavailable".  Service could not perform the
      operation.

   HTTP support:  NO

   The registration resources MUST be deleted after the expiration of
   their lifetime.  Additional operations on the registration resource
   cannot be executed because no registration location is returned.

   The following example shows a registrant-ep using Simple
   Registration, by simply sending an empty POST to a resource
   directory.

   Req:(to

   Req: (to RD server from [2001:db8:2::1])
   POST /.well-known/core?lt=6000&ep=node1
   No payload

   Res: 2.04 Changed

   (later)

   Req: (from RD server to [2001:db8:2::1])
   GET /.well-known/core
   Accept: 40

   Res: (to the RD from [2001:db8:2::1] ) 2.05 Content
   Content-Format: 40
   Payload:
   </sen/temp>

5.3.2.

   Res: (from the RD to [2001:db8:2::1]) 2.04 Changed

5.3.2.  Third-party registration

   For some applications, even Simple Registration may be too taxing for
   some very constrained devices, in particular if the security
   requirements become too onerous.

   In a controlled environment (e.g. building control), the Resource
   Directory can be filled by a third party device, called a
   Commissioning Tool (CT).  The commissioning tool can fill the
   Resource Directory from a database or other means.  For that purpose
   scheme, IP address and port of the URI of the registered device is
   the value of the "base" parameter of the registration described in
   Section 5.3.

   It should be noted that the value of the "base" parameter applies to
   all the links of the registration and has consequences for the anchor
   value of the individual links as exemplified in Appendix B.  An
   eventual (currently non-existing) "base" attribute of the link is not
   affected by the value of "base" parameter in the registration.

5.3.3.  RD-Groups

   The RD-Groups usage pattern allows announcing application groups
   inside a

5.4.  Operations on the Registration Resource Directory.

   Groups are represented by endpoint registrations.  Their base address
   is a multicast address, and they SHOULD be entered with

   This section describes how the registering endpoint
   type "core.rd-group". can maintain the
   registrations that it created.  The registering endpoint name can also be referred to as a
   group name in the
   registrant-ep or the CT.  An endpoint SHOULD NOT use this context. interface
   for registrations that it did not create.  The registration is inserted into registrations are
   resources of the RD.

   After the initial registration, the registering endpoint retains the
   returned location of the Registration Resource for further
   operations, including refreshing the RD by a Commissioning Tool,
   which might also be known as a group manager here.  It performs third
   party registration in order to extend
   the lifetime and registration updates.

   The links it registers SHOULD be available on all members that join "keep-alive" the group.  Depending on registration.  When the application, members that lack some
   resource MAY be permissible if requests lifetime of
   the registration has expired, the RD SHOULD NOT respond to them fail gracefully. discovery
   queries concerning this endpoint.  The following example shows RD SHOULD continue to provide
   access to the Registration Resource after a CT registration time-out
   occurs in order to enable the registering a group with endpoint to eventually
   refresh the name
   "lights" which provides two resources. registration.  The directory RD MAY eventually remove the
   registration resource path
   /rd for the purpose of garbage collection.  If the
   Registration Resource is an example RD location discovered in a request similar to
   Figure 6.

   Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
                                     &base=coap://[ff35:30:2001:db8::1]
   Content-Format: 40
   Payload:
   </light>;rt="light";if="core.a",
   </color-temperature>;if="core.p";u="K"

   Res: 2.01 Created
   Location-Path: /rd/12

   In this example, the group manager can easily permit devices that
   have no writable color-temperature to join, as they would still
   respond to brightness changing commands.  Had removed, the group instead
   contained a single resource that sets brightness and color
   temperature atomically, endpoints would corresponding endpoint will
   need to support both
   properties. be re-registered.

   The resources of a group can Registration Resource may also be looked up like any other resource,
   and used cancel the group registrations (along with any additional registration
   parameters) can be looked up
   using DELETE, and to perform further operations beyond the endpoint lookup interface.

6.  RD Lookup

   To discover the resources registered with the RD, a lookup interface
   must be provided.  This lookup scope of
   this specification.

   These operations are described below.

5.4.1.  Registration Update

   The update interface is defined as a default, and
   it is assumed that RDs may also support lookups used by the registering endpoint to return resource
   descriptions in alternative formats (e.g.  Atom or HTML Link) or
   using more advanced interfaces (e.g. supporting context refresh
   or semantic
   based lookup).

   RD Lookup allows lookups for endpoints and resources using attributes
   defined in this document and for use update its registration with an RD.  To use the CoRE Link Format.  The
   result of interface, the
   registering endpoint sends a lookup POST request is the list of links (if any)
   corresponding to the type of lookup.  Thus, an endpoint lookup MUST
   return a list of endpoints and a registration
   resource lookup MUST return a list
   of links to resources.

   The lookup type is selected returned by a URI endpoint, which is indicated by
   a Resource Type as per Table 1 below:

             +-------------+--------------------+-----------+
             | Lookup Type | Resource Type      | Mandatory |
             +-------------+--------------------+-----------+
             | Resource    | core.rd-lookup-res | Mandatory |
             | Endpoint    | core.rd-lookup-ep  | Mandatory |
             +-------------+--------------------+-----------+

                           Table 1: Lookup Types

6.1.  Resource lookup

   Resource lookup results in links that are semantically equivalent to
   the links submitted to the RD.  The links and link parameters
   returned by initial registration operation.

   An update MAY update the lookup are equal to lifetime- or the submitted ones, except context- registration
   parameters "lt", "base" as in Section 5.3.  Parameters that
   the target and anchor references are fully resolved.

   Links that did not have
   being changed SHOULD NOT be included in an anchor attribute are therefore returned
   with update.  Adding parameters
   that have not changed increases the base URI size of the registration message but does not
   have any other implications.  Parameters MUST be included as query
   parameters in an update operation as in Section 5.3.

   A registration update resets the anchor.  Links timeout of which
   href or anchor was submitted as a (full) URI are returned with these
   attributes unmodified.

   Above rules allow the client registration to interpret the response as links
   without any further knowledge
   (possibly updated) lifetime of the storage conventions registration, independent of
   whether a "lt" parameter was given.

   If the RD.
   The Resource Directory MAY replace context of the registration base URIs with a
   configured intermediate proxy, e.g. is changed in the case of an HTTP lookup
   interface for CoAP endpoints.

6.2.  Lookup filtering

   Using the Accept Option, update, relative
   references submitted in the requester can control whether original registration or later updates
   are resolved anew against the
   returned list is returned in CoRE Link Format ("application/link-
   format", default) or its alternate content-formats ("application/
   link-format+json" or "application/link-format+cbor"). new context.

   The page registration update operation only describes the use of POST with
   an empty payload.  Future standards might describe the semantics of
   using content formats and count parameters are used payloads with the POST method to obtain lookup results in
   specified increments using pagination, where count specifies how many update the
   links to return and page specifies which subset of links organized in
   sequential pages, each containing 'count' links, starting with link
   zero and page zero.  Thus, specifying count a registration (see Section 5.4.3).

   The update registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  {+location}{?lt,base,extra-attrs*}
   URI Template Variables:

      location :=  This is the Location returned by the RD as a result
         of 10 and page a successful earlier registration.

      lt :=  Lifetime (optional).  Lifetime of 0 will
   return the first 10 links registration in
         seconds.  Range of 60-4294967295.  If no lifetime is included,
         the result previous last lifetime set (links 0-9).  Count = 10
   and page = 1 will return on a previous update or the next 'page' containing links 10-19, and
   so on.

   Multiple search criteria MAY
         original registration (falling back to 90000) SHOULD be included used.

      base :=  Base URI (optional).  This parameter updates the Base URI
         established in a lookup.  All included
   criteria MUST match for a link the original registration to be returned.  The Resource
   Directory MUST support matching with multiple search criteria.

   A link matches a search criterion if it has new value.  If
         the parameter is set in an attribute of update, it is stored by the same
   name and the same value, allowing for a trailing "*" wildcard
   operator RD as
         the new Base URI under which to interpret the relative links
         present in Section 4.1 the payload of [RFC6690].  Attributes that are defined the original registration, following
         the same restrictions as "link-type" match if in the search registration.  If the parameter
         is not set in the request but was set before, the previous Base
         URI value matches any of their values
   (see Section 4.1 of [RFC6690]; e.g. "?if=core.s" matches ";if="abc
   core.s";").  A resource link also matches a search criterion if its
   endpoint would match is kept unmodified.  If the criterion, and vice versa, an endpoint link
   matches a search criterion if any of its resource links matches it.

   Note that "href" parameter is a valid search criterion not set in
         the request and matches target
   references.  Like all search criteria, on a resource lookup it can
   match was not set before either, the target reference source address
         and source port of the resource link itself, but also update request are stored as the Base
         URI.

      extra-attrs :=  Additional registration resource of attributes (optional).  As
         with the registration, the RD processes them if it knows their
         semantics.  Otherwise, unknown attributes are stored as
         endpoint that registered it.  Queries
   for resource link targets MUST be in URI form (i.e. not relative
   references) and attributes, overriding any previously stored endpoint
         attributes of the same key.

   Content-Format:  none (no payload)

   The following response codes are matched against a resolved link target.  Queries defined for endpoints SHOULD be expressed in path-absolute form this interface:

   Success:  2.04 "Changed" or 204 "No Content" if possible
   and MUST be expressed in URI form otherwise; the RD SHOULD recognize
   either.

   Endpoints that are interested in a lookup result repeatedly update was
      successfully processed.

   Failure:  4.00 "Bad Request" or
   continuously can use mechanisms like ETag caching, resource
   observation ([RFC7641]), 400 "Bad Request".  Malformed
      request.

   Failure:  4.04 "Not Found" or any future mechanism that might allow
   more efficient observations of collections.  These are advertised,
   detected and used according to their own specifications and can be
   used with 404 "Not Found".  Registration does not
      exist (e.g. may have been removed).

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the lookup interface as operation.

   HTTP support:  YES

   If the registration update fails with any other resource.

   When resource observation is used, every a "Service Unavailable"
   response and a Max-Age option or Retry-After header, the registering
   endpoint SHOULD retry the operation after the time indicated.  If the set of matching
   links changes,
   registration fails in another way, including request timeouts, or if
   the content of a matching link changes, time indicated exceeds the remaining lifetime, the registering
   endpoint SHOULD attempt registration again.

   The following example shows how the registering endpoint updates its
   registration resource at an RD
   sends a notification using this interface with the matching link set. example
   location value: /rd/4521.

   Req: POST /rd/4521

   Res: 2.04 Changed

   The notification
   contains the successful current response to following example shows the given request,
   especially registering endpoint updating its
   registration resource at an RD using this interface with respect to representing zero matching links (see
   "Success" item below). the example
   location value: /rd/4521.  The lookup interface is specified as follows:

   Interaction:  Client -> RD

   Method:  GET

   URI Template:  {+type-lookup-location}{?page,count,search*}

   URI Template Variables:

      type-lookup-location :=  RD Lookup initial registration by the
   registering endpoint set the following values:

   o  endpoint name (ep)=endpoint1

   o  lifetime (lt)=500

   o  Base URI for a given lookup type
         (mandatory). (base)=coap://local-proxy-old.example.com:5683

   o  payload of Figure 6

   The address is discovered as described in
         Section 5.2.

      search :=  Search criteria for limiting the number initial state of results
         (optional).

      page :=  Page (optional).  Parameter cannot be used without the
         count parameter.  Results are returned from result set in pages
         that contain 'count' links starting from index (page * count).
         Page numbering starts with zero.

      count :=  Count (optional).  Number of results Resource Directory is limited to this
         parameter value.  If reflected in the page parameter is also present,
   following request:

   Req: GET /rd-lookup/res?ep=endpoint1

   Res: 2.01 Content
   Payload:
   <coap://local-proxy-old.example.com:5683/sensors/temp>;ct=41;
    rt="temperature"; anchor="coap://spurious.example.com:5683",
   <coap://local-proxy-old.example.com:5683/sensors/light>;ct=41;
     rt="light-lux"; if="sensor";
     anchor="coap://local-proxy-old.example.com:5683"

   The following example shows the
         response MUST only include 'count' links starting with registering endpoint changing the
         (page * count) link in
   Base URI to "coaps://new.example.com:5684":

   Req: POST /rd/4521?base=coaps://new.example.com:5684

   Res: 2.04 Changed

   The consecutive query returns:

   Req: GET /rd-lookup/res?ep=endpoint1

   Res: 2.01 Content
   Payload:
   <coaps://new.example.com:5684/sensors/temp>;ct=41;rt="temperature";
       anchor="coap://spurious.example.com:5683",
   <coaps://new.example.com:5684/sensors/light>;ct=41;rt="light-lux";
       if="sensor"; anchor="coaps://new.example.com:5684",

5.4.2.  Registration Removal

   Although RD registrations have soft state and will eventually timeout
   after their lifetime, the result set registering endpoint SHOULD explicitly
   remove an entry from the query.  If the
         count parameter RD if it knows it will no longer be
   available (for example on shut-down).  This is not present, then accomplished using a
   removal interface on the response MUST return
         all matching links in RD by performing a DELETE on the result set.  Link numbering starts
         with zero.

      Content-Format:  application/link-format (optional)

      Content-Format:  application/link-format+json (optional)

      Content-Format:  application/link-format+cbor (optional) endpoint
   resource.

   The removal request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  DELETE

   URI Template:  {+location}

   URI Template Variables:

      location :=  This is the Location returned by the RD as a result
         of a successful earlier registration.

   The following responses response codes are defined for this interface:

   Success:  2.05 "Content" or 200 "OK" with an "application/link-
      format", "application/link-format+cbor", or "application/link-
      format+json" payload containing matching entries for the lookup.
      The payload can contain zero links (which is an empty payload,
      "80" (hex)  2.02 "Deleted" or "[]" in the respective content format), indicating
      that no entities matched the request.

   Failure:  No error response to a multicast request. 204 "No Content" upon successful deletion

   Failure:  4.00 "Bad Request" or 400 "Bad Request".  Malformed
      request.

   Failure:  4.04 "Not Found" or 404 "Not Found".  Registration does not
      exist (e.g. may already have been removed).

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the operation.

   HTTP support: YES

   The following examples shows successful removal of the endpoint lookup returns from
   the RD with example location value /rd/4521.

   Req: DELETE /rd/4521

   Res: 2.02 Deleted

5.4.3.  Further operations

   Additional operations on the registration resources which can only be
   manipulated by specified in future
   documents, for example:

   o  Send iPATCH (or PATCH) updates ([RFC8132]) to add, remove or
      change the registering endpoint.  Examples links of endpoint lookup
   belong a registration.

   o  Use GET to read the management aspects currently stored set of the RD and are shown in
   Appendix A.5.  The resource lookup examples are shown in this
   section.

6.3.  Resource lookup examples

   The examples links in this section assume the existence of CoAP hosts with a default CoAP port 61616.  HTTP hosts
      registration resource.

   Those operations are possible out of scope of this document, and do not change
   the nature will require
   media types suitable for modifying sets of links.

6.  RD Lookup

   To discover the examples.

   The following example shows a client performing a resource lookup resources registered with the example RD, a lookup interface
   must be provided.  This lookup interface is defined as a default, and
   it is assumed that RDs may also support lookups to return resource look-up locations
   descriptions in alternative formats (e.g.  JSON or CBOR link format
   [I-D.ietf-core-links-json]) or using more advanced interfaces (e.g.
   supporting context or semantic based lookup) on different resources
   that are discovered independently.

   RD Lookup allows lookups for endpoints and resources using attributes
   defined in Figure 6:

   Req: GET /rd-lookup/res?rt=temperature

   Res: 2.05 Content
   <coap://[2001:db8:3::123]:61616/temp>;rt="temperature";
              anchor="coap://[2001:db8:3::123]:61616" this document and for use with the CoRE Link Format.  The same
   result of a lookup using request is the CBOR Link Format media type:

   Req: GET /rd-lookup/res?rt=temperature
   Accept: TBD64

   Res: 2.05 Content
   Content-Format: TBD64
   Payload in Hex notation:
   81A3017823636F61703A2F2F5B323030313A6462383A333A3A3132335D3A363136313
   62F74656D7003781E636F61703A2F2F5B323030313A6462383A333A3A3132335D3A36
   31363136096B74656D7065726174757265
   Decoded payload:
   [{1: "coap://[2001:db8:3::123]:61616/temp", 9: "temperature",
   3: "coap://[2001:db8:3::123]:61616"}]
   A client that wants list of links (if any)
   corresponding to be notified the type of new resources as they show up
   can use observation:

   Req: GET /rd-lookup/res?rt=light
   Observe: 0

   Res: 2.05 Content
   Observe: 23
   Payload: empty

   (at a later point in time)

   Res: 2.05 Content
   Observe: 24
   Payload:
   <coap://[2001:db8:3::124]/west>;rt="light";
       anchor="coap://[2001:db8:3::124]",
   <coap://[2001:db8:3::124]/south>;rt="light";
       anchor="coap://[2001:db8:3::124]",
   <coap://[2001:db8:3::124]/east>;rt="light";
       anchor="coap://[2001:db8:3::124]"

   The following example shows lookup.  Thus, an endpoint lookup MUST
   return a client performing list of endpoints and a paginated resource lookup
   Req: GET /rd-lookup/res?page=0&count=5

   Res: 2.05 Content
   <coap://[2001:db8:3::123]:61616/res/0>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/1>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/2>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/3>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/4>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616"

   Req: GET /rd-lookup/res?page=1&count=5

   Res: 2.05 Content
   <coap://[2001:db8:3::123]:61616/res/5>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/6>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/7>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/8>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/9>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616"

   The following example shows a client performing MUST return a lookup of all
   resources from endpoints of all endpoints list
   of links to resources.

   The lookup type is selected by a given endpoint type.
   It assumes that two endpoints (with endpoint names "sensor1" and
   "sensor2") have previously registered with their respective addresses
   "coap://sensor1.example.com" and "coap://sensor2.example.com", and
   posted the very payload of the 6th request of section 5 of [RFC6690].

   It demonstrates how absolute link targets stay unmodified, while
   relative ones are resolved:

   Req: GET /rd-lookup/res?et=oic.d.sensor

   <coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index";
       anchor="coap://sensor1.example.com",
   <coap://sensor1.example.com/sensors/temp>;rt="temperature-c";
       if="sensor"; anchor="coap://sensor1.example.com",
   <coap://sensor1.example.com/sensors/light>;rt="light-lux";
       if="sensor"; anchor="coap://sensor1.example.com",
   <http://www.example.com/sensors/t123>;rel="describedby";
       anchor="coap://sensor1.example.com/sensors/temp",
   <coap://sensor1.example.com/t>;rel="alternate";
       anchor="coap://sensor1.example.com/sensors/temp",
   <coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index";
       anchor="coap://sensor2.example.com",
   <coap://sensor2.example.com/sensors/temp>;rt="temperature-c";
       if="sensor"; anchor="coap://sensor2.example.com",
   <coap://sensor2.example.com/sensors/light>;rt="light-lux";
       if="sensor"; anchor="coap://sensor2.example.com",
   <http://www.example.com/sensors/t123>;rel="describedby";
       anchor="coap://sensor2.example.com/sensors/temp",
   <coap://sensor2.example.com/t>;rel="alternate";
       anchor="coap://sensor2.example.com/sensors/temp"

   The following example shows a client performing URI endpoint, which is indicated by
   a Resource Type as per Table 1 below:

             +-------------+--------------------+-----------+
             | Lookup Type | Resource Type      | Mandatory |
             +-------------+--------------------+-----------+
             | Resource    | core.rd-lookup-res | Mandatory |
             | Endpoint    | core.rd-lookup-ep  | Mandatory |
             +-------------+--------------------+-----------+

                           Table 1: Lookup Types

6.1.  Resource lookup of all
   resources of all endpoints (groups) with et=core.rd-group.

   Req: GET /rd-lookup/res?et=core.rd-group

   <coap://[ff35:30:2001:db8::1]/light>;rt="light";if="core.a";
        et="core.rd-group";anchor="coap://[ff35:30:2001:db8::1]",
   <coap://[ff35:30:2001:db8::1]/color-temperature>;if="core.p";u="K";
        et="core.rd-group";
        anchor="coap://[ff35:30:2001:db8::1]"

7.  Security policies

   The

   Resource Directory (RD) provides assistance lookup results in links that are semantically equivalent to applications
   situated on a selection of nodes
   the links submitted to discover endpoints on connected
   nodes.  This section discusses different security aspects of
   accessing the RD.  The contents of links and link parameters
   returned by the RD lookup are inserted in two ways:

   1.  The node hosting equal to the discoverable endpoint fills submitted ones, except that
   the RD target and anchor references are fully resolved.

   Links that did not have an anchor attribute are therefore returned
   with the
       contents base URI of /.well-known/core by:

       *  Storing the contents directly into RD (see Section 5.3)
       *  Requesting the RD to load registration as the contents from /.well-known/core
          (see Section 5.3.1)

   2.  A Commissioning Tool (CT) fills the RD with endpoint information
       for a set anchor.  Links of discoverable nodes. (see Section 5.3 which
   href or anchor was submitted as a (full) URI are returned with
       base=authority parameter value)

   In both cases, the nodes filling these
   attributes unmodified.

   Above rules allow the RD should be authenticated and
   authorized client to change interpret the contents response as links
   without any further knowledge of the RD.  An Authorization Server
   (AS) is responsible to assign a token to storage conventions of the registering node to
   authorize RD.
   The Resource Directory MAY replace the node to discover or register endpoints in registration base URIs with a given RD
   [I-D.ietf-ace-oauth-authz].

   It can be imagined that an installation is divided
   configured intermediate proxy, e.g. in a set of
   security regions, each one with its own RD(s) to discover the
   endpoints that are part case of a given security region.  An endpoint that
   wants to discover an RD, responsible HTTP lookup
   interface for a given region, needs to be
   authorized to learn CoAP endpoints.

6.2.  Lookup filtering

   Using the contents of a given RD.  Within a region, for
   a given RD, a more fine-grained security division is possible based
   on Accept Option, the values of requester can control whether the endpoint registration parameters.  Authorization
   to discover endpoints with a given set of filter values
   returned list is
   recommended for those cases.

   When a node registers its endpoints, criteria returned in CoRE Link Format ("application/link-
   format", default) or in alternate content-formats (e.g. from
   [I-D.ietf-core-links-json]).

   The page and count parameters are needed to authorize
   the node used to enter them.  An important aspect is the uniqueness obtain lookup results in
   specified increments using pagination, where count specifies how many
   links to return and page specifies which subset of the
   (endpoint name, links organized in
   sequential pages, each containing 'count' links, starting with link
   zero and optional sector) pair within page zero.  Thus, specifying count of 10 and page of 0 will
   return the RD.  Consider first 10 links in the two cases separately: (1) CT registers endpoints, result set (links 0-9).  Count = 10
   and (2) page = 1 will return the
   registering node registers its own endpoint(s).  * A CT needs
   authorization to register a set of endpoints.  This authorization can next 'page' containing links 10-19, and
   so on.

   Multiple search criteria MAY be based on the region, i.e. included in a given CT is authorized to register any
   endpoint (endpoint name, sector) into lookup.  All included
   criteria MUST match for a given RD, or link to register an
   endpoint be returned.  The Resource
   Directory MUST support matching with (endpoint name, sector) value pairs assigned by multiple search criteria.

   A link matches a search criterion if it has an attribute of the AS,
   or can be more fine-grained, including same
   name and the same value, allowing for a subset trailing "*" wildcard
   operator as in Section 4.1 of registration
   parameter values.  * A given endpoint [RFC6690].  Attributes that registers itself, needs to
   proof its possession of its unique (endpoint name, sector) value
   pair.  Alternatively, the AS can authorize are defined
   as "link-type" match if the endpoint to register
   with an (endpoint name, sector) search value pair assigned by the AS.  * A
   separate document needs to specify these aspects to ensure
   interoperability between registering nodes matches any of their values
   (see Section 4.1 of [RFC6690]; e.g. "?if=core.s" matches ";if="abc
   core.s";").  A resource link also matches a search criterion if its
   endpoint would match the criterion, and RD.  The subsections
   below give some hints how to handle vice versa, an endpoint link
   matches a subset search criterion if any of the different
   aspects.

7.1.  Secure RD discovery

   The Resource Server (RS) discussed in [I-D.ietf-ace-oauth-authz] its resource links matches it.

   Note that "href" is
   equated to the RD.  The client (C) needs to discover the RD as
   discussed in Section 4.  C can discover the related AS by sending a
   request to the RD.  The RD denies the request by sending valid search criterion and matches target
   references.  Like all search criteria, on a resource lookup it can
   match the address target reference of the related AS, as discussed in section 5.1 of
   [I-D.ietf-ace-oauth-authz].  The client MUST send an authorization
   request to resource link itself, but also the AS.  When appropriate,
   registration resource of the AS returns a token endpoint that
   specifies the authorization permission which needs to registered it.  Queries
   for resource link targets MUST be specified in URI form (i.e. not relative
   references) and are matched against a separate document.

7.2.  Secure RD filtering

   The authorized parameter values resolved link target.  Queries
   for the queries by a given endpoint
   must endpoints SHOULD be registered by the AS.  The AS communicates the parameter
   values expressed in the token.  A separate document needs to specify the
   parameter value combinations path-absolute form if possible
   and their storage MUST be expressed in URI form otherwise; the token.  The RD
   decodes the token and checks the validity of the queries of the
   client.

7.3.  Secure endpoint Name assignment

   This section only considers the assignment of SHOULD recognize
   either.

   Endpoints that are interested in a name to the endpoint
   based on an automatic mechanism without lookup result repeatedly or
   continuously can use mechanisms like ETag caching, resource
   observation ([RFC7641]), or any future mechanism that might allow
   more efficient observations of AS.  More elaborate
   protocols collections.  These are out of scope.  The registering endpoint is authorized
   by the AS to discover the RD and add registrations.  A token is
   provided by the AS advertised,
   detected and communicated from registering endpoint to RD.
   It is assumed that DTLS is used according to secure the channel between
   registering endpoint and RD, where the registering endpoint is the
   DTLS client.  Assuming that the client is provided by a certificate
   at manufacturing time, the certificate is uniquely identified by the
   CN field their own specifications and the serial number.  The RD can assign a unique endpoint
   name by using be
   used with the certificate identifier lookup interface as endpoint name.  Proof of
   possession of the endpoint name by the registering endpoint is
   checked by encrypting the certificate identifier with any other resource.

   When resource observation is used, every time the private key set of matching
   links changes, or the registering endpoint, which content of a matching link changes, the RD can decrypt with the public
   key stored in the certificate.  Even simpler, the authorized
   registering endpoint can generate
   sends a random number (or string) that
   identifies notification with the endpoint. matching link set.  The RD can check for notification
   contains the improbable
   replication of successful current response to the random value. given request,
   especially with respect to representing zero matching links (see
   "Success" item below).

   The lookup interface is specified as follows:

   Interaction:  Client -> RD MUST check that registering
   endpoint uses only one random value

   Method:  GET

   URI Template:  {+type-lookup-location}{?page,count,search*}

   URI Template Variables:

      type-lookup-location :=  RD Lookup URI for each authorized endpoint.

8.  Security Considerations a given lookup type
         (mandatory).  The security considerations address is discovered as described in
         Section 7 of [RFC5988]
   and Section 6 5.2.

      search :=  Search criteria for limiting the number of [RFC6690] apply.  The "/.well-known/core" resource
   may be protected e.g. using DTLS when hosted on a CoAP server as
   described in [RFC7252].  DTLS or TLS based security SHOULD results
         (optional).

      page :=  Page (optional).  Parameter cannot be used on
   all resource directory interfaces defined without the
         count parameter.  Results are returned from result set in this document.

8.1.  Endpoint Identification and Authentication

   An Endpoint (name, sector) pair pages
         that contain 'count' links starting from index (page * count).
         Page numbering starts with zero.

      count :=  Count (optional).  Number of results is unique within limited to this
         parameter value.  If the et of endpoints
   regsitered by page parameter is also present, the RD.  An Endpoint
         response MUST NOT be identified by its
   protocol, port or IP address as these may change over only include 'count' links starting with the lifetime of
   an Endpoint.

   Every operation performed by an Endpoint on a resource directory
   SHOULD be mutually authenticated using Pre-Shared Key, Raw Public Key
   or Certificate based security.

   Consider
         (page * count) link in the following threat: two devices A and B are registered at
   a single server.  Both devices have unique, per-device credentials
   for use with DTLS to make sure that only parties with authorization
   to access A or B can do so.

   Now, imagine that a malicious device A wants to sabotage the device
   B.  It uses its credentials during the DTLS exchange.  Then, it
   specifies the endpoint name of device B as result set from the name of its own
   endpoint in device A. query.  If the server does
         count parameter is not check whether present, then the
   identifier provided response MUST return
         all matching links in the DTLS handshake matches the identifier used
   at the CoAP layer then it may be inclined to use the endpoint name result set.  Link numbering starts
         with zero.

      Accept:  absent, application/link-format or any other indicated
         media type representing web links

   The following responses codes are defined for looking up what information to provision to the malicious device.

   Section 7.3 specifies an example that removes this threat for
   endpoints that have a certificate installed.

8.2.  Access Control

   Access control SHOULD be performed separately for the RD registration
   and Lookup API paths, as different endpoints may be authorized to
   register interface:

   Success:  2.05 "Content" or 200 "OK" with an RD from those authorized to lookup endpoints from "application/link-
      format" or other web link payload containing matching entries for
      the RD.  Such access control SHOULD be performed lookup.  The payload can contain zero links (which is an empty
      payload in as fine-grained a
   level as possible.  For example access control for lookups [RFC6690] link format, but could also be
   performed either at "[]" in JSON
      based formats), indicating that no entities matched the sector, endpoint request.

   Failure:  No error response to a multicast request.

   Failure:  4.00 "Bad Request" or resource level.

8.3.  Denial of 400 "Bad Request".  Malformed
      request.

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service Attacks

   Services that run over UDP unprotected are vulnerable to unknowingly
   become part could not perform the operation.

   HTTP support:  YES

6.3.  Resource lookup examples

   The examples in this section assume the existence of CoAP hosts with
   a DDoS attack as UDP does default CoAP port 61616.  HTTP hosts are possible and do not require return
   routability check.  Therefore, an attacker can easily spoof change
   the
   source IP nature of the target entity and send requests to such examples.

   The following example shows a service
   which would then respond to the target entity.  This can be used for
   large-scale DDoS attacks on the target.  Especially, if the service
   returns client performing a response resource lookup
   with the example resource look-up locations discovered in Figure 5:

   Req: GET /rd-lookup/res?rt=temperature

   Res: 2.05 Content
   <coap://[2001:db8:3::123]:61616/temp>;rt="temperature";
              anchor="coap://[2001:db8:3::123]:61616"

   A client that is order wants to be notified of magnitudes larger than the
   request, the situation becomes even worse new resources as now the attack can be
   amplified.  DNS servers have been widely used for DDoS amplification
   attacks.  There is also a danger that NTP Servers could become
   implicated in denial-of-service (DoS) attacks since they run on
   unprotected UDP, there is no return routability check, and they show up
   can
   have use observation:

   Req: GET /rd-lookup/res?rt=light
   Observe: 0

   Res: 2.05 Content
   Observe: 23
   Payload: empty

   (at a large amplification factor.  The responses from the NTP server
   were found to be 19 times larger than the request.  A Resource
   Directory (RD) which responds to wild-card lookups is potentially
   vulnerable if run later point in time)

   Res: 2.05 Content
   Observe: 24
   Payload:
   <coap://[2001:db8:3::124]/west>;rt="light";
       anchor="coap://[2001:db8:3::124]",
   <coap://[2001:db8:3::124]/south>;rt="light";
       anchor="coap://[2001:db8:3::124]",
   <coap://[2001:db8:3::124]/east>;rt="light";
       anchor="coap://[2001:db8:3::124]"

   The following example shows a client performing a paginated resource
   lookup
   Req: GET /rd-lookup/res?page=0&count=5

   Res: 2.05 Content
   <coap://[2001:db8:3::123]:61616/res/0>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/1>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/2>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/3>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/4>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616"

   Req: GET /rd-lookup/res?page=1&count=5

   Res: 2.05 Content
   <coap://[2001:db8:3::123]:61616/res/5>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/6>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/7>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/8>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616",
   <coap://[2001:db8:3::123]:61616/res/9>;rt=sensor;ct=60;
       anchor="coap://[2001:db8:3::123]:61616"

   The following example shows a client performing a lookup of all
   resources from endpoints of all endpoints of a given endpoint type.
   It assumes that two endpoints (with endpoint names "sensor1" and
   "sensor2") have previously registered with CoAP over UDP.  Since there is no return
   routability check their respective addresses
   "coap://sensor1.example.com" and "coap://sensor2.example.com", and
   posted the responses can be significantly larger than
   requests, RDs can unknowingly become part very payload of a DDoS amplification
   attack.

9.  IANA Considerations

9.1.  Resource Types

   IANA is asked to enter the following values into the Resource Type
   (rt=) Link Target Attribute Values sub-registry 6th request of the Constrained
   Restful Environments (CoRE) Parameters registry defined in [RFC6690]:

   +--------------------+--------------------------+-------------------+
   | Value              | Description              | Reference         |
   +--------------------+--------------------------+-------------------+
   | core.rd            | Directory resource section 5 of an | RFCTHIS Section   |
   |                    | RD                       | 5.2               |
   | core.rd-lookup-res | Resource [RFC6690].

   It demonstrates how absolute link targets stay unmodified, while
   relative ones are resolved:

   Req: GET /rd-lookup/res?et=oic.d.sensor

   <coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index";
       anchor="coap://sensor1.example.com",
   <coap://sensor1.example.com/sensors/temp>;rt="temperature-c";
       if="sensor"; anchor="coap://sensor1.example.com",
   <coap://sensor1.example.com/sensors/light>;rt="light-lux";
       if="sensor"; anchor="coap://sensor1.example.com",
   <http://www.example.com/sensors/t123>;rel="describedby";
       anchor="coap://sensor1.example.com/sensors/temp",
   <coap://sensor1.example.com/t>;rel="alternate";
       anchor="coap://sensor1.example.com/sensors/temp",
   <coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index";
       anchor="coap://sensor2.example.com",
   <coap://sensor2.example.com/sensors/temp>;rt="temperature-c";
       if="sensor"; anchor="coap://sensor2.example.com",
   <coap://sensor2.example.com/sensors/light>;rt="light-lux";
       if="sensor"; anchor="coap://sensor2.example.com",
   <http://www.example.com/sensors/t123>;rel="describedby";
       anchor="coap://sensor2.example.com/sensors/temp",
   <coap://sensor2.example.com/t>;rel="alternate";
       anchor="coap://sensor2.example.com/sensors/temp"

6.4.  Endpoint lookup of an RD | RFCTHIS Section   |
   |                    |                          | 5.2               |
   | core.rd-lookup-ep  | Endpoint

   The endpoint lookup of an RD | RFCTHIS Section   |
   |                    |                          | 5.2               |
   | core.rd-ep         | Endpoint resource of an  | RFCTHIS Section 6 |
   |                    | RD                       |                   |
   +--------------------+--------------------------+-------------------+

9.2.  IPv6 ND Resource Directory Address Option

   This document registers one new ND option type under returns registration resources which can only be
   manipulated by the sub-registry
   "IPv6 Neighbor Discovery Option Formats":

   o  Resource Directory address Option (38)

9.3.  RD Parameter Registry

   This specification defines a new sub-registry for registering endpoint.

   Endpoint registration resources are annotated with their endpoint
   names (ep), sectors (d, if present) and
   lookup parameters called "RD Parameters" under "CoRE Parameters".
   Although this specification defines registration base URI (base;
   reports the registrant-ep's address if no explicit base was given) as
   well as a basic set of parameters, it constant resource type (rt="core.rd-ep"); the lifetime (lt)
   is
   expected that other standards that make use of this interface will
   define new ones.

   Each entry not reported.  Additional endpoint attributes are added as target
   attributes to their endpoint link unless their specification says
   otherwise.

   Links to endpoints SHOULD be presented in path-absolute form or, if
   required, as absolute references.  (This avoids the registry must include
   o  the human readable name of RFC6690
   ambiguities.)

   While Endpoint Lookup does expose the parameter,

   o registration resources, the short name as used in query parameters RD
   does not need to make them accessible to clients.  Clients SHOULD NOT
   attempt to dereference or link attributes,

   o  indication of whether it manipulate them.

   A Resource Directory can be passed as a query parameter at
      registration of endpoints, as a query parameter report endpoints in lookups, or lookup that are not
   hosted at the same address.  Lookup clients MUST be
      expressed prepared to see
   arbitrary URIs as a link attribute,

   o  validity requirements if any, registration resources in the results and

   o  a description. treat
   them as opaque identifiers; the precise semantics of such links are
   left to future specifications.

   The query parameter MUST be both following example shows a valid URI query key [RFC3986] and client performing an endpoint type (et)
   lookup with the value oic.d.sensor (which is currently a parmname as used in [RFC5988]. registered
   rt value):

   Req: GET /rd-lookup/ep?et=oic.d.sensor

   Res: 2.05 Content
   </rd/1234>;base="coap://[2001:db8:3::127]:61616";ep="node5";
   et="oic.d.sensor";ct="40";rt="core.rd-ep",
   </rd/4521>;base="coap://[2001:db8:3::129]:61616";ep="node7";
   et="oic.d.sensor";ct="40";d="floor-3";rt="core.rd-ep"

7.  Security policies

   The description must give details Resource Directory (RD) provides assistance to applications
   situated on whether the parameter can be
   updated, and how it is a selection of nodes to be processed in lookups. discover endpoints on connected
   nodes.  This section discusses different security aspects of
   accessing the RD.

   The mechanisms around new contents of the RD parameters should be designed are inserted in such a
   way that they tolerate two ways:

   1.  The node hosting the discoverable endpoint fills the RD implementations that are unaware with the
       contents of /.well-known/core by:

       *  Storing the
   parameter and expose any parameter passed at registration or updates
   on in endpoint lookups.  (For example, if a parameter used at
   registration were to be confidential, contents directly into RD (see Section 5.3)

       *  Requesting the registering endpoint should
   be instructed RD to only set that parameter if load the contents from /.well-known/core
          (see Section 5.3.1)

   2.  A Commissioning Tool (CT) fills the RD advertises support with endpoint information
       for keeping it confidential at a set of discoverable nodes. (see Section 5.3 with
       base=authority parameter value)

   In both cases, the discovery step.)

   Initial entries nodes filling the RD should be authenticated and
   authorized to change the contents of the RD.  An Authorization Server
   (AS) is responsible to assign a token to the registering node to
   authorize the node to discover or register endpoints in this sub-registry a given RD
   [I-D.ietf-ace-oauth-authz].

   It can be imagined that an installation is divided in a set of
   security regions, each one with its own RD(s) to discover the
   endpoints that are as follows:

   +--------------+-------+---------------+-----+----------------------+
   | Full name    | Short | Validity      | Use | Description          |
   +--------------+-------+---------------+-----+----------------------+
   | Endpoint     | ep    |               | RLA | Name part of a given security region.  An endpoint that
   wants to discover an RD, responsible for a given region, needs to be
   authorized to learn the contents of a given RD.  Within a region, for
   a given RD, a more fine-grained security division is possible based
   on the          |
   | Name         |       |               |     | endpoint, max 63     |
   |              |       |               |     | bytes                |
   | Lifetime     | lt    | 60-4294967295 | R   | Lifetime values of the      |
   |              |       |               |     | endpoint registration in      |
   |              |       |               |     | seconds              |
   | Sector       | d     |               | RLA | Sector parameters.  Authorization
   to which this |
   |              |       |               |     | endpoint belongs     |
   | Registration | base  | URI           | RLA | The scheme, address  |
   | Base URI     |       |               |     | and port and path at |
   |              |       |               |     | which this server is |
   |              |       |               |     | available            |
   | Page         | page  | Integer       | L   | Used for pagination  |
   | Count        | count | Integer       | L   | Used for pagination  |
   | Endpoint     | et    |               | RLA | Semantic name discover endpoints with a given set of the |
   | Type         |       |               |     | endpoint (see        |
   |              |       |               |     | Section 9.4)         |
   +--------------+-------+---------------+-----+----------------------+

                          Table 2: RD Parameters

   (Short: Short name used in query parameters or link attributes.  Use:
   R = used at registration, L = used at lookup, A = expressed in link
   attribute

   The descriptions filter values is
   recommended for the options defined in this document are only
   summarized here.  To which registrations they apply and when they those cases.

   When a node registers its endpoints, criteria are needed to be shown is described in authorize
   the respective sections of this document.

   The IANA policy for future additions node to the sub-registry is "Expert
   Review" as described in [RFC8126].  The evaluation should consider
   formal criteria, duplication of functionality (Is the new entry
   redundant with an existing one?), topical suitability (E.g. enter them.  An important aspect is the
   described property actually a property uniqueness of the endpoint
   (endpoint name, and not a
   property of a particular resource, in which case it should go into optional sector) pair within the payload of RD.  Consider
   the registration and need not be registered?), two cases separately: (1) CT registers endpoints, and (2) the
   potential for conflict with commonly used link attributes (For
   example, "if" could be used as a parameter for conditional
   registration if it were not
   registering node registers its own endpoint(s).

   o  A CT needs authorization to register a set of endpoints.  This
      authorization can be used in lookup or attributes, but
   would make based on the region, i.e. a bad parameter for lookup, because given CT is
      authorized to register any endpoint (endpoint name, sector) into a resource lookup with
      given RD, or to register an "if" query parameter could ambiguously filter endpoint with (endpoint name, sector)
      value pairs assigned by the registered
   endpoint property AS, or the [RFC6690] link attribute).  It is expected can be more fine-grained,
      including a subset of registration parameter values.

   o  A given endpoint that the registry will receive between 5 and 50 registrations in
   total over the next years.

9.3.1.  Full description registers itself, needs to proof its
      possession of its unique (endpoint name, sector) value pair.
      Alternatively, the "Endpoint Type" Registration Parameter

   An endpoint registering at an RD AS can describe itself with authorize the endpoint
   types, similar to how resources are described register with
      an (endpoint name, sector) value pair assigned by the AS.

   A separate document needs to specify these aspects to ensure
   interoperability between registering nodes and RD.  The subsections
   below give some hints how to handle a subset of the different
   aspects.

7.1.  Secure RD discovery

   The Resource Types Server (RS) discussed in
   [RFC6690].  An endpoint type [I-D.ietf-ace-oauth-authz] is expressed
   equated to the RD.  The client (C) needs to discover the RD as a string, which
   discussed in Section 4.  C can be
   either a URI or one of discover the values defined in related AS by sending a
   request to the Endpoint Type sub-
   registry.  Endpoint types can be passed in RD.  The RD denies the "et" query parameter
   as part of extra-attrs at request by sending the Registration step, are shown on
   endpoint lookups using address
   of the "et" target attribute, and can be filtered
   for using "et" as a search criterion in resource and endpoint lookup.
   Multiple endpoint types are given related AS, as separate query parameters or
   link attributes.

   Note that Endpoint Type differs from Resource Type in that it uses
   multiple attributes rather than space separated values.  As a result,
   Resource Directory implementations automatically support correct
   filtering discussed in section 5.1 of
   [I-D.ietf-ace-oauth-authz].  The client MUST send an authorization
   request to the lookup interfaces from AS.  When appropriate, the rules for unknown
   endpoint attributes.

9.4.  "Endpoint Type" (et=) RD Parameter values

   This specification establishes AS returns a new sub-registry under "CoRE
   Parameters" called '"Endpoint Type" (et=) RD Parameter values'.  The
   registry properties (required policy, requirements, template) are
   identical to those of token that
   specifies the Resource Type parameters in [RFC6690], authorization permission which needs to be specified in
   short:
   a separate document.

7.2.  Secure RD filtering

   The review policy is IETF Review for authorized parameter values starting with "core", and
   Specification Required for others.

   The requirements to the queries by a given endpoint
   must be enforced are:

   o registered by the AS.  The AS communicates the parameter
   values MUST be related in the token.  A separate document needs to specify the purpose described
   parameter value combinations and their storage in
      Section 9.3.1.

   o the token.  The registered values MUST conform to RD
   decodes the ABNF reg-rel-type
      definition of [RFC6690] token and MUST NOT be checks the validity of the queries of the
   client.

7.3.  Secure endpoint Name assignment

   This section only considers the assignment of a URI.

   o  It is recommended name to use the period "." character for
      segmentation.

   The registry initially contains one value:

   o  "core.rd-group": An application group as described in
      Section 5.3.3.

9.5.  Multicast Address Registration

   IANA has assigned the following multicast addresses for endpoint
   based on an automatic mechanism without use of AS.  More elaborate
   protocols are out of scope.  The registering endpoint is authorized
   by CoAP
   nodes:

   IPv4 - "all CoRE resource directories" address, from the "IPv4
   Multicast Address Space Registry" equal AS to "All CoAP Nodes",
   224.0.1.187.  As discover the address RD and add registrations.  A token is used for discovery that may span
   beyond a single network, it has come from
   provided by the Internetwork Control
   Block (224.0.1.x, RFC 5771).

   IPv6 - "all CoRE resource directories" address MCD1 (suggestions
   FF0X::FE), AS and communicated from registering endpoint to RD.
   It is assumed that DTLS is used to secure the "IPv6 Multicast Address Space Registry", in channel between
   registering endpoint and RD, where the
   "Variable Scope Multicast Addresses" space (RFC 3307).  Note that
   there registering endpoint is a distinct multicast address for each scope the
   DTLS client.  Assuming that interested
   CoAP nodes should listen to; CoAP needs the Link-Local and Site-Local
   scopes only.

10.  Examples

   Two examples are presented: client is provided by a Lighting Installation example in
   Section 10.1 certificate
   at manufacturing time, the certificate is uniquely identified by the
   CN field and the serial number.  The RD can assign a LWM2M example in Section 10.2.

10.1.  Lighting Installation

   This example shows a simplified lighting installation which makes use unique endpoint
   name by using the certificate identifier as endpoint name.  Proof of
   possession of the Resource Directory (RD) endpoint name by the registering endpoint is
   checked by encrypting the certificate identifier with a CoAP interface to facilitate the installation and start up private key
   of the application code in registering endpoint, which the lights
   and sensors.  In particular, RD can decrypt with the example leads to public
   key stored in the definition of certificate.  Even simpler, the authorized
   registering endpoint can generate a
   group and random number (or string) that
   identifies the enabling endpoint.  The RD can check for the improbable
   replication of the corresponding multicast address random value.  The RD MUST check that registering
   endpoint uses only one random value for each authorized endpoint.

8.  Security Considerations

   The security considerations as described in Section 5.3.3.  No conclusions must 5 of [RFC8288]
   and Section 6 of [RFC6690] apply.  The "/.well-known/core" resource
   may be drawn protected e.g. using DTLS when hosted on the
   realization of actual installation a CoAP server as
   described in [RFC7252].  DTLS or naming procedures, because TLS based security SHOULD be used on
   all resource directory interfaces defined in this document.

8.1.  Endpoint Identification and Authentication

   An Endpoint (name, sector) pair is unique within the
   example only "emphasizes" some et of endpoints
   registered by the issues that RD.  An Endpoint MUST NOT be identified by its
   protocol, port or IP address as these may influence change over the
   use lifetime of the RD and does not pretend to
   an Endpoint.

   Every operation performed by an Endpoint on a resource directory
   SHOULD be normative.

10.1.1.  Installation Characteristics

   The example assumes that mutually authenticated using Pre-Shared Key, Raw Public Key
   or Certificate based security.

   Consider the installation is managed.  That means following threat: two devices A and B are registered at
   a single server.  Both devices have unique, per-device credentials
   for use with DTLS to make sure that only parties with authorization
   to access A or B can do so.

   Now, imagine that a Commissioning Tool (CT) is used malicious device A wants to authorize sabotage the addition of
   nodes, device
   B.  It uses its credentials during the DTLS exchange.  Then, it
   specifies the endpoint name them, and of device B as the name their services.  The CT can be connected
   to of its own
   endpoint in device A.  If the installation server does not check whether the
   identifier provided in many ways: the CT can be part of DTLS handshake matches the
   installation network, connected by WiFi identifier used
   at the CoAP layer then it may be inclined to use the installation network,
   or connected via GPRS link, or other method.

   It is assumed that there are two naming authorities endpoint name
   for looking up what information to provision to the
   installation: (1) the network manager malicious device.

   Section 7.3 specifies an example that is responsible removes this threat for the
   correct operation of the network and the connected interfaces, and
   (2) the lighting manager
   endpoints that is responsible have a certificate installed.

8.2.  Access Control

   Access control SHOULD be performed separately for the correct
   functioning of networked lights RD registration
   and sensors.  The result is the
   existence of two naming schemes coming from the two managing
   entities.

   The example installation consists of one presence sensor, and two
   luminaries, luminary1 and luminary2, each Lookup API paths, as different endpoints may be authorized to
   register with their own wireless
   interface.  Each luminary contains three lamps: left, right and
   middle.  Each luminary is accessible through one endpoint.  For each
   lamp a resource exists an RD from those authorized to modify lookup endpoints from
   the settings of a lamp RD.  Such access control SHOULD be performed in as fine-grained a
   luminary.  The purpose of the installation is that the presence
   sensor notifies
   level as possible.  For example access control for lookups could be
   performed either at the presence sector, endpoint or resource level.

8.3.  Denial of persons Service Attacks

   Services that run over UDP unprotected are vulnerable to a group of lamps.  The
   group of lamps consists of: middle and left lamps of luminary1 and
   right lamp unknowingly
   become part of luminary2.

   Before commissioning by a DDoS attack as UDP does not require return
   routability check.  Therefore, an attacker can easily spoof the lighting manager,
   source IP of the network is
   installed target entity and access send requests to the interfaces is proven such a service
   which would then respond to work by the
   network manager.

   At target entity.  This can be used for
   large-scale DDoS attacks on the moment of installation, target.  Especially, if the network under installation service
   returns a response that is not
   necessarily connected to order of magnitudes larger than the
   request, the situation becomes even worse as now the attack can be
   amplified.  DNS infra structure.  Therefore, SLAAC
   IPv6 addresses are assigned to CT, RD, luminaries and sensor shown in
   Table 3 below:

                  +--------------------+----------------+
                  | Name               | IPv6 address   |
                  +--------------------+----------------+ servers have been widely used for DDoS amplification
   attacks.  There is also a danger that NTP Servers could become
   implicated in denial-of-service (DoS) attacks since they run on
   unprotected UDP, there is no return routability check, and they can
   have a large amplification factor.  The responses from the NTP server
   were found to be 19 times larger than the request.  A Resource
   Directory (RD) which responds to wild-card lookups is potentially
   vulnerable if run with CoAP over UDP.  Since there is no return
   routability check and the responses can be significantly larger than
   requests, RDs can unknowingly become part of a DDoS amplification
   attack.

9.  IANA Considerations

9.1.  Resource Types

   IANA is asked to enter the following values into the Resource Type
   (rt=) Link Target Attribute Values sub-registry of the Constrained
   Restful Environments (CoRE) Parameters registry defined in [RFC6690]:

   +--------------------+--------------------------+-------------------+
   | luminary1 Value              | 2001:db8:4::1 Description              | Reference         | luminary2
   +--------------------+--------------------------+-------------------+
   | 2001:db8:4::2 core.rd            | Directory resource of an | Presence sensor RFCTHIS Section   | 2001:db8:4::3
   |                    | Resource directory RD                       | 2001:db8:4::ff 5.2               |
                  +--------------------+----------------+

                    Table 3: interface SLAAC addresses

   In
   | core.rd-lookup-res | Resource lookup of an RD | RFCTHIS Section 10.1.2 the use   |
   |                    |                          | 5.2               |
   | core.rd-lookup-ep  | Endpoint lookup of resource directory during installation
   is presented.

10.1.2. an RD entries

   It is assumed that access to the DNS infrastructure is not always
   possible during installation.  Therefore, the SLAAC addresses are
   used in this section.

   For discovery, the | RFCTHIS Section   |
   |                    |                          | 5.2               |
   | core.rd-ep         | Endpoint resource types (rt) of the devices are important.
   The lamps in the luminaries have rt: light, and the presence sensor
   has rt: p-sensor.  The endpoints have names which are relevant to the
   light installation manager.  In this case luminary1, luminary2, and
   the presence sensor are located in room 2-4-015, where luminary1 is
   located at the window and luminary2 and the presence sensor are
   located at the door.  The endpoint names reflect this physical
   location.  The middle, left and right lamps are accessed via path
   /light/middle, /light/left, and /light/right respectively.  The
   identifiers relevant to the Resource Directory are shown in Table 4
   below:

   +----------------+------------------+---------------+---------------+
   | Name           | endpoint         | resource path | resource type |
   +----------------+------------------+---------------+---------------+
   | luminary1      | lm_R2-4-015_wndw | /light/left   | light         |
   | luminary1      | lm_R2-4-015_wndw | /light/middle | light         |
   | luminary1      | lm_R2-4-015_wndw | /light/right  | light         |
   | luminary2      | lm_R2-4-015_door | /light/left   | light         |
   | luminary2      | lm_R2-4-015_door | /light/middle | light         |
   | luminary2      | lm_R2-4-015_door | /light/right  | light         |
   | Presence       | ps_R2-4-015_door | /ps           | p-sensor an  | RFCTHIS Section 6 | sensor
   |                    | RD                       |                   |
   +----------------+------------------+---------------+---------------+

                  Table 4:
   +--------------------+--------------------------+-------------------+

9.2.  IPv6 ND Resource Directory identifiers

   It is assumed that the CT knows the RD's address, and has performed
   URI discovery on it that returned a response like the Address Option

   This document registers one in the
   Section 5.2 example.

   The CT inserts the endpoints of the luminaries and the sensor in new ND option type under the sub-registry
   "IPv6 Neighbor Discovery Option Formats":

   o  Resource Directory address Option (38)

9.3.  RD using the Parameter Registry

   This specification defines a new sub-registry for registration base URI parameter (base) to specify the
   interface address:

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015
   Payload:
   </light/left>;rt="light",
   </light/middle>;rt="light",
   </light/right>;rt="light"

   Res: 2.01 Created
   Location-Path: /rd/4521
   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015
   Payload:
   </light/left>;rt="light",
   </light/middle>;rt="light",
   </light/right>;rt="light"

   Res: 2.01 Created
   Location-Path: /rd/4522

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]d&d=R2-4-015
   Payload:
   </ps>;rt="p-sensor"

   Res: 2.01 Created
   Location-Path: /rd/4523

   The sector name d=R2-4-015 has been added for an efficient lookup
   because filtering on "ep" name is more awkward.  The same sector name
   is communicated to the two luminaries and the presence sensor by the
   CT.

   The group is specified in the RD.  The base parameter is
   lookup parameters called "RD Parameters" under "CoRE Parameters".
   Although this specification defines a basic set to the
   site-local multicast address allocated to the group.  In the POST of parameters, it is
   expected that other standards that make use of this interface will
   define new ones.

   Each entry in the example below, the resources supported by all group members are
   published.

   Req: POST coap://[2001:db8:4::ff]/rd
   ?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]
   Payload:
   </light/left>;rt="light",
   </light/middle>;rt="light",
   </light/right>;rt="light"

   Res: 2.01 Created
   Location-Path: /rd/501

   After registry must include

   o  the filling human readable name of the RD by the CT, parameter,

   o  the application short name as used in the
   luminaries query parameters or target attributes,

   o  indication of whether it can learn to which groups they belong, be passed as a query parameter at
      registration of endpoints, as a query parameter in lookups, or be
      expressed as a target attribute,

   o  validity requirements if any, and enable their
   interface for the multicast address.

   o  a description.

   The luminary, knowing its sector query parameter MUST be both a valid URI query key [RFC3986] and being configured to join any
   group containing lights, searches for candidate groups and joins
   them:

   Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
     ?d=R2-4-015&et=core.rd-group&rt=light

   Res: 2.05 Content
   </rd/501>;ep="grp_R2-4-015";et="core.rd-group";
                                base="coap://[ff05::1]"

   From
   a token as used in [RFC8288].

   The description must give details on whether the returned base parameter value, the luminary learns the
   multicast address of the multicast group.

   Alternatively, the CT can communicate the multicast address directly be
   updated, and how it is to the luminaries by using the "coap-group" resource specified be processed in
   [RFC7390].

   Req: POST coap://[2001:db8:4::1]/coap-group
   Content-Format: application/coap-group+json
   Payload:
   { "a": "[ff05::1]", "n": "grp_R2-4-015"}

   Res: 2.01 Created
   Location-Path: /coap-group/1

   Dependent on the situation, only lookups.

   The mechanisms around new RD parameters should be designed in such a
   way that they tolerate RD implementations that are unaware of the address, "a",
   parameter and expose any parameter passed at registration or the name, "n",
   is specified updates
   on in endpoint lookups.  (For example, if a parameter used at
   registration were to be confidential, the coap-group resource.

   The presence sensor can learn the presence of groups registering endpoint should
   be instructed to only set that parameter if the RD advertises support
   resources with rt=light in its own sector by sending
   for keeping it confidential at the same
   request, discovery step.)

   Initial entries in this sub-registry are as used by follows:

   +--------------+-------+---------------+-----+----------------------+
   | Full name    | Short | Validity      | Use | Description          |
   +--------------+-------+---------------+-----+----------------------+
   | Endpoint     | ep    |               | RLA | Name of the luminary.  The presence sensor learns          |
   | Name         |       |               |     | endpoint, max 63     |
   |              |       |               |     | bytes                |
   | Lifetime     | lt    | 60-4294967295 | R   | Lifetime of the
   multicast address to use for sending messages      |
   |              |       |               |     | registration in      |
   |              |       |               |     | seconds              |
   | Sector       | d     |               | RLA | Sector to the luminaries.

10.2.  OMA Lightweight M2M (LWM2M) Example

   This example shows how the OMA LWM2M specification makes use of
   Resource Directory (RD).

   OMA LWM2M is a profile for device services based on CoAP(OMA Name
   Authority).  LWM2M defines a simple object model and a number of
   abstract interfaces which this |
   |              |       |               |     | endpoint belongs     |
   | Registration | base  | URI           | RLA | The scheme, address  |
   | Base URI     |       |               |     | and operations for device management port and device
   service enablement.

   An LWM2M path at |
   |              |       |               |     | which this server is an instance of an LWM2M middleware service layer,
   containing a Resource Directory along with other LWM2M interfaces
   defined by the LWM2M specification.

   CoRE Resource Directory (RD) is used to provide the LWM2M
   Registration interface.

   LWM2M does not provide |
   |              |       |               |     | available            |
   | Page         | page  | Integer       | L   | Used for registration sectors and does not
   currently use the rd-lookup interface.

   The LWM2M specification describes a set of interfaces and a resource
   model used between a LWM2M device and an LWM2M server.  Other
   interfaces, proxies, and applications are currently out of scope pagination  |
   | Count        | count | Integer       | L   | Used for
   LWM2M.

   The location of the LWM2M Server and RD URI path is provided by the
   LWM2M Bootstrap process, so no dynamic discovery pagination  |
   | Endpoint     | et    |               | RLA | Semantic name of the |
   | Type         |       |               |     | endpoint (see        |
   |              |       |               |     | Section 9.4)         |
   +--------------+-------+---------------+-----+----------------------+

                          Table 2: RD is used.
   LWM2M Servers Parameters

   (Short: Short name used in query parameters or target attributes.
   Use: R = used at registration, L = used at lookup, A = expressed in
   target attribute

   The descriptions for the options defined in this document are only
   summarized here.  To which registrations they apply and endpoints when they are not required
   to implement be shown is described in the /.well-
   known/core resource.

10.2.1.  The LWM2M Object Model respective sections of this document.

   The OMA LWM2M object model IANA policy for future additions to the sub-registry is based on a simple 2 level class
   hierarchy consisting "Expert
   Review" as described in [RFC8126].  The evaluation should consider
   formal criteria, duplication of Objects and Resources.

   An LWM2M Resource functionality (Is the new entry
   redundant with an existing one?), topical suitability (E.g. is the
   described property actually a REST endpoint, allowed to be a single value or
   an array of values property of the same data type.

   An LWM2M Object is a resource template endpoint and container type that
   encapsulates not a set
   property of related resources.  An LWM2M Object represents a specific type particular resource, in which case it should go into
   the payload of information source; the registration and need not be registered?), and the
   potential for conflict with commonly used target attributes (For
   example, there is a LWM2M
   Device Management object that represents "if" could be used as a network connection,
   containing resources that represent individual properties like radio
   signal strength.

   Since there may potentially parameter for conditional
   registration if it were not to be more than one of used in lookup or attributes, but
   would make a given type object, bad parameter for example more than one network connection, LWM2M defines instances
   of objects lookup, because a resource lookup with
   an "if" query parameter could ambiguously filter by the registered
   endpoint property or the [RFC6690] target attribute).  It is expected
   that contain the registry will receive between 5 and 50 registrations in
   total over the next years.

9.3.1.  Full description of the "Endpoint Type" Registration Parameter

   An endpoint registering at an RD can describe itself with endpoint
   types, similar to how resources that represent are described with Resource Types in
   [RFC6690].  An endpoint type is expressed as a specific
   physical thing.

   The URI template for LWM2M consists of string, which can be
   either a base URI followed by Object,
   Instance, and Resource IDs:

   {/base-uri}{/object-id}{/object-instance}{/resource-id}{/resource-
   instance}

   The five variables given here are strings.  base-uri can also have or one of the special value "undefined" (sometimes called "null" values defined in RFC 6570).
   Each of the variables object-instance, resource-id, and resource-
   instance Endpoint Type sub-
   registry.  Endpoint types can be passed in the special value "undefined" only if "et" query parameter
   as part of extra-attrs at the values
   behind it in this sequence also Registration step, are "undefined".  As a special case,
   object-instance shown on
   endpoint lookups using the "et" target attribute, and can be "empty" (which is different from "undefined")
   if resource-id is not "undefined".

   base-uri := Base URI for LWM2M resources or "undefined" filtered
   for default
   (empty) base URI

   object-id := OMNA (OMA Name Authority) registered object ID (0-65535)

   object-instance := Object instance identifier (0-65535) or
   "undefined"/"empty" (see above)) to refer to all instances of an
   object ID

   resource-id := OMNA (OMA Name Authority) registered resource ID
   (0-65535) or "undefined" to refer to all resources within an instance

   resource-instance := Resource instance identifier or "undefined" to
   refer to single instance of using "et" as a search criterion in resource

   LWM2M IDs and endpoint lookup.
   Multiple endpoint types are 16 bit unsigned integers represented given as separate query parameters or
   link attributes.

   Note that Endpoint Type differs from Resource Type in decimal (no
   leading zeroes except for the value 0) by URI format strings.  For
   example, that it uses
   multiple attributes rather than space separated values.  As a LWM2M URI might be:

   /1/0/1

   The base uri is empty, the Object ID is 1, the instance ID is 0, result,
   Resource Directory implementations automatically support correct
   filtering in the
   resource ID is 1, and lookup interfaces from the resource instance is "undefined". rules for unknown
   endpoint attributes.

9.4.  "Endpoint Type" (et=) RD Parameter values

   This
   example URI points to internal resource 1, which represents the
   registration lifetime configured, in instance 0 of a type 1 object
   (LWM2M Server Object).

10.2.2.  LWM2M Register Endpoint

   LWM2M defines specification establishes a registration interface based on the REST API,
   described in Section 5.  The new sub-registry under "CoRE
   Parameters" called '"Endpoint Type" (et=) RD registration URI path Parameter values'.  The
   registry properties (required policy, requirements, template) are
   identical to those of the LWM2M Resource Directory Type parameters in [RFC6690], in
   short:

   The review policy is specified to be "/rd".

   LWM2M endpoints register object IDs, IETF Review for example </1>, to indicate
   that a particular object type is supported, values starting with "core", and register object
   instances,
   Specification Required for example </1/0>, others.

   The requirements to indicate that a particular instance
   of that object type exists.

   Resources within the LWM2M object instance are not registered with
   the RD, but may be discovered by reading enforced are:

   o  The values MUST be related to the resource links from purpose described in
      Section 9.3.1.

   o  The registered values MUST conform to the
   object instance using GET with a CoAP Content-Format ABNF reg-rel-type
      definition of application/
   link-format.  Resources may also [RFC6690] and MUST NOT be read as a structured object by
   performing a GET to the object instance with a Content-Format of
   senml+json.

   When an LWM2M object or instance URI.

   o  It is registered, this indicates recommended to the
   LWM2M server that the object and its resources are available for
   management and service enablement (REST API) operations.

   LWM2M endpoints may use the following RD registration parameters period "." character for
      segmentation.

   The registry initially contains one value:

   o  "core.rd-group": An application group as
   defined described in Table 2 :

   ep - Endpoint Name
   lt Appendix A.

9.5.  Multicast Address Registration

   IANA has assigned the following multicast addresses for use by CoAP
   nodes:

   IPv4 - registration lifetime

   Endpoint Name, Lifetime, and LWM2M Version are mandatory parameters "all CoRE resource directories" address, from the "IPv4
   Multicast Address Space Registry" equal to "All CoAP Nodes",
   224.0.1.187.  As the address is used for discovery that may span
   beyond a single network, it has come from the register operation, all other registration parameters are
   optional.

   Additional optional LWM2M registration parameters are defined:

   +-----------+-------+-------------------------------+---------------+
   | Name      | Query | Validity                      | Description   |
   +-----------+-------+-------------------------------+---------------+
   | Binding   | b     | {"U",UQ","S","SQ","US","UQS"} | Available     |
   | Mode      |       |                               | Protocols     |
   |           |       |                               |               |
   | LWM2M     | ver   | 1.0                           | Spec Version  |
   | Version   |       |                               |               |
   |           |       |                               |               |
   | SMS       | sms   |                               | MSISDN        |
   | Number    |       |                               |               |
   +-----------+-------+-------------------------------+---------------+

             Table 5: LWM2M Additional Registration Parameters

   The following RD registration parameters are not currently specified
   for use in LWM2M:

   et - Endpoint Type
   base Internetwork Control
   Block (224.0.1.x, RFC 5771).

   IPv6 - Registration Base URI

   The endpoint registration must include a payload containing links to
   all supported objects and existing object instances, optionally
   including "all CoRE resource directories" address MCD1 (suggestions
   FF0X::FE), from the appropriate link-format relations.

   Here "IPv6 Multicast Address Space Registry", in the
   "Variable Scope Multicast Addresses" space (RFC 3307).  Note that
   there is an a distinct multicast address for each scope that interested
   CoAP nodes should listen to; CoAP needs the Link-Local and Site-Local
   scopes only.

10.  Examples

   Two examples are presented: a Lighting Installation example in
   Section 10.1 and a LWM2M registration payload:

   </1>,</1/0>,</3/0>,</5> example in Section 10.2.

10.1.  Lighting Installation

   This link format payload indicates that object ID 1 (LWM2M Server
   Object) is supported, with example shows a single instance 0 existing, object ID 3
   (LWM2M Device object) is supported, simplified lighting installation which makes use
   of the Resource Directory (RD) with a single instance 0
   existing, and object 5 (LWM2M Firmware Object) is supported, with no
   existing instances.

10.2.3.  LWM2M Update Endpoint Registration

   The LwM2M update is really very similar CoAP interface to facilitate
   the registration update installation and start-up of the application code in the lights
   and sensors.  In particular, the example leads to the definition of a
   group and the enabling of the corresponding multicast address as
   described in Appendix A.1, with A.  No conclusions must be drawn on the
   realization of actual installation or naming procedures, because the
   example only difference "emphasizes" some of the issues that there are
   more parameters defined and available.  All may influence the parameters listed in
   use of the RD and does not pretend to be normative.

10.1.1.  Installation Characteristics

   The example assumes that section are also available with the initial registration but are
   all optional:

   lt - Registration Lifetime
   b - Protocol Binding
   sms - MSISDN
   link payload - new or modified links

   A Registration update installation is also specified managed.  That means
   that a Commissioning Tool (CT) is used to authorize the addition of
   nodes, name them, and name their services.  The CT can be used connected
   to update the
   LWM2M server whenever installation in many ways: the endpoint's UDP port CT can be part of the
   installation network, connected by WiFi to the installation network,
   or IP address connected via GPRS link, or other method.

   It is assumed that there are
   changed.

10.2.4.  LWM2M De-Register Endpoint

   LWM2M allows two naming authorities for de-registration using the delete method on
   installation: (1) the
   returned location from network manager that is responsible for the initial registration operation.  LWM2M de-
   registration proceeds as described in Appendix A.2.

11.  Acknowledgments

   Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders
   Brandt, Matthieu Vial, Jim Schaad, Mohit Sethi, Hauke Petersen,
   Hannes Tschofenig, Sampo Ukkola, Linyi Tian, and Jan Newmarch have
   provided helpful comments, discussions and ideas to improve
   correct operation of the network and shape
   this document.  Zach would also like to thank his colleagues from the
   EU FP7 SENSEI project, where many of connected interfaces, and
   (2) the resource directory concepts
   were originally developed.

12.  Changelog

   changes from -16 to -17

   (Note lighting manager that -17 is published as a direct follow-up to -16, containing
   a single change to be discussed at IETF103)

   o  Removed groups that are enumerations responsible for the correct
   functioning of registrations networked lights and have
      dedicated mechanism

   o  Add groups that are enumerations sensors.  The result is the
   existence of shared resources two naming schemes coming from the two managing
   entities.

   The example installation consists of one presence sensor, and are two
   luminaries, luminary1 and luminary2, each with their own wireless
   interface.  Each luminary contains three lamps: left, right and
   middle.  Each luminary is accessible through one endpoint.  For each
   lamp a
      special case of endpoint registrations

   changes from -15 resource exists to -16
   o  Recommend modify the settings of a common set lamp in a
   luminary.  The purpose of resources for members the installation is that the presence
   sensor notifies the presence of persons to a group

   o  Clarified use of multicast lamps.  The
   group in lighting example

   o  Add note on concurrent registrations from one EP being possible
      but not expected

   o  Refresh web examples appendix to reflect current use of Modernized
      Link Format

   o  Add examples lamps consists of: middle and left lamps of URIs where Modernized Link Format matters

   o  Editorial changes

   changes from -14 to -15

   o  Rewrite luminary1 and
   right lamp of section "Security policies"

   o  Clarify that luminary2.

   Before commissioning by the "base" parameter text applies both to relative
      references both in anchor lighting manager, the network is
   installed and href

   o  Renamed "Registree-EP" access to Registrant-EP"

   o  Talk of "relative references" and "URIs" rather than "relative"
      and "absolute" URIs.  (The concept of "absolute URIs" the interfaces is proven to work by the
   network manager.

   At the moment of [RFC3986] installation, the network under installation is not needed in RD).

   o  Fixed examples

   o  Editorial changes

   changes from -13 to -14

   o  Rename "registration context"
   necessarily connected to "registration base URI" (and
      "con" the DNS infra structure.  Therefore, SLAAC
   IPv6 addresses are assigned to "base") CT, RD, luminaries and "domain" sensor shown in
   Table 3 below:

                  +--------------------+----------------+
                  | Name               | IPv6 address   |
                  +--------------------+----------------+
                  | luminary1          | 2001:db8:4::1  |
                  | luminary2          | 2001:db8:4::2  |
                  | Presence sensor    | 2001:db8:4::3  |
                  | Resource directory | 2001:db8:4::ff |
                  +--------------------+----------------+

                    Table 3: interface SLAAC addresses

   In Section 10.1.2 the use of resource directory during installation
   is presented.

10.1.2.  RD entries

   It is assumed that access to "sector" (where the abbreviation
      "d" stays for compatibility reasons)

   o  Introduced DNS infrastructure is not always
   possible during installation.  Therefore, the SLAAC addresses are
   used in this section.

   For discovery, the resource types core.rd-ep (rt) of the devices are important.
   The lamps in the luminaries have rt: light, and core.rd-gp

   o  Registration management moved the presence sensor
   has rt: p-sensor.  The endpoints have names which are relevant to appendix A, including endpoint the
   light installation manager.  In this case luminary1, luminary2, and group lookup

   o  Minor editorial changes

      *  PATCH/iPATCH is clearly deferred to another document

      *  Recommend against query / fragment identifier in con=
      *  Interface description lists
   the presence sensor are described as illustrative

      *  Rewording of Simple Registration

   o  Simple registration carries no error information located in room 2-4-015, where luminary1 is
   located at the window and succeeds
      immediately (previously, sequence was unspecified)

   o  Lookup: href luminary2 and the presence sensor are matched against resolved values (previously,
   located at the door.  The endpoint names reflect this
      was unspecified)

   o  Lookup: lt are not exposed any more

   o  con/base: Paths physical
   location.  The middle, left and right lamps are allowed

   o  Registration resource locations can not have query or fragment
      parts

   o  Default life time extended to 25 hours

   o  clarified registration update rules

   o  lt-value semantics for lookup clarified.

   o  added template for simple registration

   changes from -12 to -13

   o  Added "all resource directory" nodes MC address

   o  Clarified observation behavior

   o  version identification

   o  example rt= accessed via path
   /light/middle, /light/left, and et= values

   o  domain from figure 2

   o  more explanatory text

   o  endpoints of a groups hosted by different RD

   o  resolve RFC6690-vs-8288 resolution ambiguities:

      *  require registered links not /light/right respectively.  The
   identifiers relevant to be relative when using anchor

      *  return absolute URIs the Resource Directory are shown in Table 4
   below:

   +----------------+------------------+---------------+---------------+
   | Name           | endpoint         | resource lookup

   changes from -11 to -12
   o  added Content Model section, including ER diagram

   o  removed domain lookup interface; domains are now plain attributes
      of groups and endpoints

   o  updated chapter "Finding a path | resource type |
   +----------------+------------------+---------------+---------------+
   | luminary1      | lm_R2-4-015_wndw | /light/left   | light         |
   | luminary1      | lm_R2-4-015_wndw | /light/middle | light         |
   | luminary1      | lm_R2-4-015_wndw | /light/right  | light         |
   | luminary2      | lm_R2-4-015_door | /light/left   | light         |
   | luminary2      | lm_R2-4-015_door | /light/middle | light         |
   | luminary2      | lm_R2-4-015_door | /light/right  | light         |
   | Presence       | ps_R2-4-015_door | /ps           | p-sensor      |
   | sensor         |                  |               |               |
   +----------------+------------------+---------------+---------------+

                  Table 4: Resource Directory"; now distinguishes
      configuration-provided, network-provided and heuristic sources

   o  improved text on: atomicity, idempotency, lookup with multiple
      parameters, endpoint removal, simple registration

   o  updated LWM2M description

   o  clarified where relative references are resolved, and how context Directory identifiers

   It is assumed that the CT knows the RD's address, and anchor interact

   o  new appendix has performed
   URI discovery on it that returned a response like the interaction with RFCs 6690, 5988 and 3986

   o  lookup interface: group and endpoint lookup return group one in the
   Section 5.2 example.

   The CT inserts the endpoints of the luminaries and
      registration resources as link targets

   o  lookup interface: search parameters work the same across all
      entities

   o  removed all methods that modify links sensor in an existing the
   RD using the registration
      (POST with payload, PATCH and iPATCH)

   o  removed plurality definition (was only needed for link
      modification)

   o  enhanced IANA registry text

   o  state that lookup resources can be observable

   o  More examples and improved text

   changes from -09 to -10

   o  removed "ins" base URI parameter (base) to specify the
   interface address:

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015
   Payload:
   </light/left>;rt="light",
   </light/middle>;rt="light",
   </light/right>;rt="light"

   Res: 2.01 Created
   Location-Path: /rd/4521

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015
   Payload:
   </light/left>;rt="light",
   </light/middle>;rt="light",
   </light/right>;rt="light"

   Res: 2.01 Created
   Location-Path: /rd/4522

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]d&d=R2-4-015
   Payload:
   </ps>;rt="p-sensor"

   Res: 2.01 Created
   Location-Path: /rd/4523

   The sector name d=R2-4-015 has been added for an efficient lookup
   because filtering on "ep" name is more awkward.  The same sector name
   is communicated to the two luminaries and "exp" link-format extensions.

   o  removed all text concerning DNS-SD.

   o  removed inconsistency the presence sensor by the
   CT.

   The group is specified in RDAO text.

   o  suggestions taken over from various sources

   o  replaced "Function Set" with "REST API", "base URI", "base path"
   o  moved simple registration the RD.  The base parameter is set to registration section

   changes from -08 the
   site-local multicast address allocated to -09

   o  clarified the "example use" group.  In the POST in
   the example below, the resources supported by all group members are
   published.

   Req: POST coap://[2001:db8:4::ff]/rd
   ?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]
   Payload:
   </light/left>;rt="light",
   </light/middle>;rt="light",
   </light/right>;rt="light"

   Res: 2.01 Created
   Location-Path: /rd/501
   After the filling of the base RD resource values /rd,
      /rd-lookup, and /rd-group.

   o  changed "ins" ABNF notation.

   o  various editorial improvements, including by the CT, the application in examples

   o  clarifications for RDAO

   changes from -07 the
   luminaries can learn to -08

   o  removed link target value returned from domain which groups they belong, and group lookup
      types

   o  Maximum length of domain parameter 63 bytes enable their
   interface for consistency with the multicast address.

   The luminary, knowing its sector and being configured to join any
   group

   o  removed option containing lights, searches for simple POST candidate groups and joins
   them:

   Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
     ?d=R2-4-015&et=core.rd-group&rt=light

   Res: 2.05 Content
   </rd/501>;ep="grp_R2-4-015";et="core.rd-group";
             base="coap://[ff05::1]";rt="core.rd-ep"

   From the returned base parameter value, the luminary learns the
   multicast address of link data, don't require a
      .well-known/core resource the multicast group.

   Alternatively, the CT can communicate the multicast address directly
   to accept the luminaries by using the "coap-group" resource specified in
   [RFC7390].

   Req: POST data and handle it coap://[2001:db8:4::1]/coap-group
   Content-Format: application/coap-group+json
   Payload:
   { "a": "[ff05::1]", "n": "grp_R2-4-015"}

   Res: 2.01 Created
   Location-Path: /coap-group/1

   Dependent on the situation, only the address, "a", or the name, "n",
   is specified in a
      special way; we already have /rd for that

   o  add IPv6 ND Option for discovery the coap-group resource.

   The presence sensor can learn the presence of an RD

   o  clarify group configuration section 6.1 groups that endpoints must be
      registered before including them in a group

   o  removed all superfluous client-server diagrams

   o  simplified lighting example

   o  introduced Commissioning Tool

   o  RD-Look-up text is extended.

   changes from -06 to -07

   o  added text support
   resources with rt=light in its own sector by sending the discovery section to allow content format hints
      to be exposed in same
   request, as used by the discovery link attributes

   o  editorial updates luminary.  The presence sensor learns the
   multicast address to section 9

   o  update author information

   o  minor text corrections
   Changes from -05 use for sending messages to -06

   o  added note that the PATCH section is contingent on luminaries.

10.2.  OMA Lightweight M2M (LWM2M) Example

   This example shows how the progress OMA LWM2M specification makes use of
      the PATCH method

   changes from -04 to -05

   o  added Update Endpoint Links using PATCH

   o  http access made explicit in interface specification

   o  Added http examples

   Changes from -03 to -04:

   o  Added http response codes

   o  Clarified endpoint name usage

   o  Add application/link-format+cbor content-format

   Changes from -02 to -03:

   o  Added an example for lighting and DNS integration

   o  Added an example for RD use in
   Resource Directory (RD).

   OMA LWM2M

   o  Added Read Links operation is a profile for link inspection by endpoints

   o  Expanded DNS-SD section

   o  Added draft authors Peter van der Stok device services based on CoAP(OMA Name
   Authority).  LWM2M defines a simple object model and Michael Koster

   Changes from -01 to -02:

   o  Added a catalogue use case.

   o  Changed the registration update to number of
   abstract interfaces and operations for device management and device
   service enablement.

   An LWM2M server is an instance of an LWM2M middleware service layer,
   containing a POST Resource Directory along with optional link
      format payload.  Removed other LWM2M interfaces
   defined by the endpoint type update from LWM2M specification.

   CoRE Resource Directory (RD) is used to provide the update.

   o  Additional examples section added LWM2M
   Registration interface.

   LWM2M does not provide for more complex use cases.

   o  New DNS-SD mapping section.

   o  Added text on endpoint identification registration sectors and authentication.

   o  Error code 4.04 added to Registration Update does not
   currently use the rd-lookup interface.

   The LWM2M specification describes a set of interfaces and Delete requests.

   o  Made 63 bytes a SHOULD rather than resource
   model used between a MUST for endpoint name LWM2M device and
      resource type parameters.

   Changes from -00 to -01:

   o  Removed the ETag validation feature.

   o  Place holder for the DNS-SD mapping section.

   o  Explicitly disabled GET or POST on returned Location.

   o  New registry for RD parameters.

   o  Added support for the JSON Link Format.

   o  Added reference to the Groupcomm WG draft.

   Changes from -05 to WG Document -00:

   o  Updated the version and date.

   Changes from -04 to -05:

   o  Restricted Update to parameter updates.

   o  Added pagination support for the Lookup interface.

   o  Minor editing, bug fixes an LWM2M server.  Other
   interfaces, proxies, and reference updates.

   o  Added group support.

   o  Changed rt to et applications are currently out of scope for
   LWM2M.

   The location of the registration LWM2M Server and update interface.

   Changes from -03 to -04:

   o  Added the ins= parameter back for the DNS-SD mapping.

   o  Integrated the Simple Directory Discovery from Carsten.

   o  Editorial improvements.

   o  Fixed RD URI path is provided by the use
   LWM2M Bootstrap process, so no dynamic discovery of ETags.

   o  Fixed tickets 383 the RD is used.
   LWM2M Servers and 372

   Changes from -02 endpoints are not required to -03:

   o  Changed implement the endpoint name back to /.well-
   known/core resource.

10.2.1.  The LWM2M Object Model

   The OMA LWM2M object model is based on a single registration parameter
      ep= and removed the h= simple 2 level class
   hierarchy consisting of Objects and ins= parameters.

   o  Updated Resources.

   An LWM2M Resource is a REST interface descriptions endpoint, allowed to use RFC6570 URI Template
      format.

   o  Introduced be a single value or
   an improved RD Lookup design as its own function set.

   o  Improved array of values of the security considerations section.

   o  Made the POST registration interface idempotent by requiring the
      ep= parameter to be present.

   Changes from -01 to -02:

   o  Added same data type.

   An LWM2M Object is a terminology section.

   o  Changed the inclusion of an ETag in registration or update to resource template and container type that
   encapsulates a
      MAY.

   o  Added the concept set of an RD Domain and related resources.  An LWM2M Object represents
   a registration parameter specific type of information source; for
      it.

   o  Recommended the Location returned from example, there is a registration to LWM2M
   Device Management object that represents a network connection,
   containing resources that represent individual properties like radio
   signal strength.

   Since there may potentially be
      stable, allowing more than one of a given type object,
   for endpoint and Domain information to be changed
      during updates.

   o  Changed the lookup interface to accept endpoint and Domain as
      query string parameters to control example more than one network connection, LWM2M defines instances
   of objects that contain the scope resources that represent a specific
   physical thing.

   The URI template for LWM2M consists of a lookup.

13.  References

13.1.  Normative References

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., base URI followed by Object,
   Instance, and C. Bormann, "Representing
              Constrained RESTful Environments (CoRE) Link Format Resource IDs:

   {/base-uri}{/object-id}{/object-instance}{/resource-id}{/resource-
   instance}

   The five variables given here are strings.  base-uri can also have
   the special value "undefined" (sometimes called "null" in
              JSON RFC 6570).

   Each of the variables object-instance, resource-id, and CBOR", draft-ietf-core-links-json-10 (work resource-
   instance can be the special value "undefined" only if the values
   behind it in
              progress), February 2018.

   [RFC2119]  Bradner, S., "Key words for use this sequence also are "undefined".  As a special case,
   object-instance can be "empty" (which is different from "undefined")
   if resource-id is not "undefined".

   base-uri := Base URI for LWM2M resources or "undefined" for default
   (empty) base URI

   object-id := OMNA (OMA Name Authority) registered object ID (0-65535)

   object-instance := Object instance identifier (0-65535) or
   "undefined"/"empty" (see above)) to refer to all instances of an
   object ID

   resource-id := OMNA (OMA Name Authority) registered resource ID
   (0-65535) or "undefined" to refer to all resources within an instance

   resource-instance := Resource instance identifier or "undefined" to
   refer to single instance of a resource

   LWM2M IDs are 16 bit unsigned integers represented in RFCs decimal (no
   leading zeroes except for the value 0) by URI format strings.  For
   example, a LWM2M URI might be:

   /1/0/1

   The base uri is empty, the Object ID is 1, the instance ID is 0, the
   resource ID is 1, and the resource instance is "undefined".  This
   example URI points to internal resource 1, which represents the
   registration lifetime configured, in instance 0 of a type 1 object
   (LWM2M Server Object).

10.2.2.  LWM2M Register Endpoint

   LWM2M defines a registration interface based on the REST API,
   described in Section 5.  The RD registration URI path of the LWM2M
   Resource Directory is specified to be "/rd".

   LWM2M endpoints register object IDs, for example </1>, to indicate
   that a particular object type is supported, and register object
   instances, for example </1/0>, to indicate that a particular instance
   of that object type exists.

   Resources within the LWM2M object instance are not registered with
   the RD, but may be discovered by reading the resource links from the
   object instance using GET with a CoAP Content-Format of application/
   link-format.  Resources may also be read as a structured object by
   performing a GET to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <https://www.rfc-editor.org/info/rfc5988>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., the object instance with a Content-Format of
   senml+json.

   When an LWM2M object or instance is registered, this indicates to the
   LWM2M server that the object and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC6763]  Cheshire, S. its resources are available for
   management and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC8126]  Cotton, M., Leiba, B., service enablement (REST API) operations.

   LWM2M endpoints may use the following RD registration parameters as
   defined in Table 2 :

   ep - Endpoint Name
   lt - registration lifetime

   Endpoint Name, Lifetime, and T. Narten, "Guidelines LWM2M Version are mandatory parameters
   for the register operation, all other registration parameters are
   optional.

   Additional optional LWM2M registration parameters are defined:

   +-----------+-------+-------------------------------+---------------+
   | Name      | Query | Validity                      | Description   |
   +-----------+-------+-------------------------------+---------------+
   | Binding   | b     | {"U",UQ","S","SQ","US","UQS"} | Available     |
   | Mode      |       |                               | Protocols     |
   |           |       |                               |               |
   | LWM2M     | ver   | 1.0                           | Spec Version  |
   | Version   |       |                               |               |
   |           |       |                               |               |
   | SMS       | sms   |                               | MSISDN        |
   | Number    |       |                               |               |
   +-----------+-------+-------------------------------+---------------+

             Table 5: LWM2M Additional Registration Parameters

   The following RD registration parameters are not currently specified
   for
              Writing an IANA Considerations Section use in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

13.2.  Informative References

   [ER]       Chen, P., "The entity-relationship model---toward LWM2M:

   et - Endpoint Type
   base - Registration Base URI

   The endpoint registration must include a
              unified view of data", ACM Transactions on Database
              Systems Vol. 1, pp. 9-36, DOI 10.1145/320434.320440, March
              1976.

   [I-D.arkko-core-dev-urn]
              Arkko, J., Jennings, C., payload containing links to
   all supported objects and Z. Shelby, "Uniform Resource
              Names for Device Identifiers", draft-arkko-core-dev-urn-05
              (work in progress), October 2017.

   [I-D.bormann-t2trg-rel-impl]
              Bormann, C., "impl-info: A existing object instances, optionally
   including the appropriate link-format relations.

   Here is an example LWM2M registration payload:

   </1>,</1/0>,</3/0>,</5>
   This link relation type for
              disclosing implementation information", draft-bormann-
              t2trg-rel-impl-00 (work in progress), January 2018.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., format payload indicates that object ID 1 (LWM2M Server
   Object) is supported, with a single instance 0 existing, object ID 3
   (LWM2M Device object) is supported, with a single instance 0
   existing, and
              H. Tschofenig, "Authentication object 5 (LWM2M Firmware Object) is supported, with no
   existing instances.

10.2.3.  LWM2M Update Endpoint Registration

   The LwM2M update is really very similar to the registration update as
   described in Section 5.4.1, with the only difference that there are
   more parameters defined and Authorization available.  All the parameters listed in
   that section are also available with the initial registration but are
   all optional:

   lt - Registration Lifetime
   b - Protocol Binding
   sms - MSISDN
   link payload - new or modified links

   A Registration update is also specified to be used to update the
   LWM2M server whenever the endpoint's UDP port or IP address are
   changed.

10.2.4.  LWM2M De-Register Endpoint

   LWM2M allows for
              Constrained Environments (ACE) de-registration using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-16
              (work in progress), October 2018.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-16 (work delete method on the
   returned location from the initial registration operation.  LWM2M de-
   registration proceeds as described in progress), June 2018.

   [I-D.silverajan-core-coap-protocol-negotiation]
              Silverajan, B. Section 5.4.2.

11.  Acknowledgments

   Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders
   Brandt, Matthieu Vial, Jim Schaad, Mohit Sethi, Hauke Petersen,
   Hannes Tschofenig, Sampo Ukkola, Linyi Tian, and M. Ocak, "CoAP Protocol Negotiation",
              draft-silverajan-core-coap-protocol-negotiation-09 (work
              in progress), July 2018.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Jan Newmarch have
   provided helpful comments, discussions and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., ideas to improve and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7230]  Fielding, R., Ed. shape
   this document.  Zach would also like to thank his colleagues from the
   EU FP7 SENSEI project, where many of the resource directory concepts
   were originally developed.

12.  Changelog

   changes from -17 to -18

   o  Rather than re-specifying link format (Modernized Link Format),
      describe a Limited Link Format that's the uncontested subset of
      Link Format

   o  Acknowledging the -17 version as part of the draft
   o  Move "Read endpoint links" operation to future specification like
      PATCH

   o  Demote links-json to an informative reference, and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax removed them
      from exchange examples

   o  Add note on unusability of link-local IP addresses, and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7252]  Shelby, Z., Hartke, K., describe
      mitigation.

   o  Reshuffling of sections: Move additional operations and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7390]  Rahman, A., Ed. endpoint
      lookup back from appendix, and E. Dijk, Ed., "Group Communication groups into one

   o  Lookup interface tightened to not imply applicability for non
      link-format lookups (as those can have vastly different views on
      link cardinality)

   o  Simple registration: Change sequence of GET and POST-response,
      ensuring unsuccessful registrations are reported as such, and
      suggest how devices that would have required the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

   [RFC7641]  Hartke, K., "Observing Resources inverse behavior
      can still cope with it.

   o  Abstract and introduction reworded to avoid the impression that
      resources are stored in full in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC8132]  van der Stok, P., Bormann, C., RD

   o  Simplify the rules governing when a registration resource can or
      must be changed.

   o  Drop a figure that has become useless due to the changes of and A. Sehgal, "PATCH
      -13 and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., -17

   o  Wording consistency fixes: Use "Registrations" and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

Appendix A.  Registration Management

   This section describes how "target
      attributes"

   o  Fix incorrect use of content negotiation in discovery interface
      description (Content-Format -> Accept)

   o  State that the registering base attribute value is part of endpoint can maintain lookup
      even when implicit in the
   registries registration

   o  Update references from RFC5988 to its update RFC8288

   o  Remove appendix on protocol-negotiation (which had a note to be
      removed before publication)

   changes from -16 to -17

   (Note that it created.  The registering endpoint can -17 is published as a direct follow-up to -16, containing
   a single change to be the
   registrant-ep or the CT.  An endpoint SHOULD NOT use this interface
   for registries discussed at IETF103)
   o  Removed groups that are enumerations of registrations and have
      dedicated mechanism

   o  Add groups that it did not create.  The registries are enumerations of shared resources and are a
      special case of the RD.

   After the initial registration, the registering endpoint retains the
   returned location registrations

   changes from -15 to -16

   o  Recommend a common set of the Registration Resource resources for further
   operations, including refreshing the registration members of a group

   o  Clarified use of multicast group in order lighting example

   o  Add note on concurrent registrations from one EP being possible
      but not expected

   o  Refresh web examples appendix to extend
   the lifetime and "keep-alive" the registration.  When the lifetime reflect current use of
   the registration has expired, the RD SHOULD NOT respond to discovery
   queries concerning this endpoint.  The RD SHOULD continue to provide
   access Modernized
      Link Format

   o  Add examples of URIs where Modernized Link Format matters

   o  Editorial changes

   changes from -14 to -15

   o  Rewrite of section "Security policies"

   o  Clarify that the Registration Resource after a registration time-out
   occurs in order "base" parameter text applies both to enable the registering endpoint relative
      references both in anchor and href

   o  Renamed "Registree-EP" to eventually
   refresh the registration.  The RD MAY eventually remove the
   registration resource for the purpose Registrant-EP"

   o  Talk of garbage collection.  If the
   Registration Resource "relative references" and "URIs" rather than "relative"
      and "absolute" URIs.  (The concept of "absolute URIs" of [RFC3986]
      is removed, the corresponding endpoint will
   need not needed in RD).

   o  Fixed examples

   o  Editorial changes

   changes from -13 to be re-registered.

   The Registration Resource may also be used -14

   o  Rename "registration context" to "registration base URI" (and
      "con" to inspect "base") and "domain" to "sector" (where the
   registration abbreviation
      "d" stays for compatibility reasons)

   o  Introduced resource using GET, update the registration, cancel the
   registration using DELETE, or do an endpoint lookup.

   These operations are described below.

A.1. types core.rd-ep and core.rd-gp
   o  Registration Update

   The update interface is used by the registering endpoint management moved to refresh
   or update its registration with an RD.  To use the interface, the
   registering appendix A, including endpoint sends a POST request
      and group lookup

   o  Minor editorial changes

      *  PATCH/iPATCH is clearly deferred to the registration
   resource returned by the initial registration operation.

   An update MAY update the lifetime- or the context- registration
   parameters "lt", "base" as in Section 5.3.  Parameters that are not
   being changed SHOULD NOT be included in an update.  Adding parameters
   that have not changed increases the size of the message but does not
   have any other implications.  Parameters MUST be included as another document

      *  Recommend against query
   parameters in an update operation as / fragment identifier in Section 5.3.

   A registration update resets the timeout con=

      *  Interface description lists are described as illustrative

      *  Rewording of the Simple Registration

   o  Simple registration to the
   (possibly updated) lifetime of the registration, independent of
   whether a "lt" parameter carries no error information and succeeds
      immediately (previously, sequence was given.

   If the context of the registration is changed in an update, relative
   references submitted in the original registration or later updates unspecified)

   o  Lookup: href are resolved anew matched against the new context.

   The registration update operation only describes the use of POST with
   an empty payload.  Future standards might describe the semantics of
   using content formats and payloads with the POST method resolved values (previously, this
      was unspecified)

   o  Lookup: lt are not exposed any more

   o  con/base: Paths are allowed

   o  Registration resource locations can not have query or fragment
      parts

   o  Default life time extended to update the
   links of a 25 hours

   o  clarified registration (see Appendix A.4).

   The update rules

   o  lt-value semantics for lookup clarified.

   o  added template for simple registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  {+location}{?lt,con,extra-attrs*}

   URI Template Variables:

      location :=  This is the Location returned by the RD as a result
         of a successful earlier registration.

      lt :=  Lifetime (optional).  Lifetime of the registration in
         seconds.  Range of 60-4294967295.  If no lifetime is included,
         the previous last lifetime set on

   changes from -12 to -13

   o  Added "all resource directory" nodes MC address

   o  Clarified observation behavior

   o  version identification

   o  example rt= and et= values

   o  domain from figure 2

   o  more explanatory text
   o  endpoints of a previous update or the
         original registration (falling back groups hosted by different RD

   o  resolve RFC6690-vs-8288 resolution ambiguities:

      *  require registered links not to 90000) SHOULD be used.

      base :=  Base URI (optional).  This parameter updates the Base URI
         established relative when using anchor

      *  return absolute URIs in the original registration resource lookup

   changes from -11 to -12

   o  added Content Model section, including ER diagram

   o  removed domain lookup interface; domains are now plain attributes
      of groups and endpoints

   o  updated chapter "Finding a Resource Directory"; now distinguishes
      configuration-provided, network-provided and heuristic sources

   o  improved text on: atomicity, idempotency, lookup with multiple
      parameters, endpoint removal, simple registration

   o  updated LWM2M description

   o  clarified where relative references are resolved, and how context
      and anchor interact

   o  new value.  If
         the parameter is set in an update, it is stored by appendix on the RD interaction with RFCs 6690, 5988 and 3986

   o  lookup interface: group and endpoint lookup return group and
      registration resources as link targets

   o  lookup interface: search parameters work the new Base URI under which to interpret the relative same across all
      entities

   o  removed all methods that modify links
         present in an existing registration
      (POST with payload, PATCH and iPATCH)

   o  removed plurality definition (was only needed for link
      modification)

   o  enhanced IANA registry text

   o  state that lookup resources can be observable

   o  More examples and improved text

   changes from -09 to -10
   o  removed "ins" and "exp" link-format extensions.

   o  removed all text concerning DNS-SD.

   o  removed inconsistency in RDAO text.

   o  suggestions taken over from various sources

   o  replaced "Function Set" with "REST API", "base URI", "base path"

   o  moved simple registration to registration section

   changes from -08 to -09

   o  clarified the payload "example use" of the original registration, following
         the same restrictions as in the registration.  If the parameter
         is not set base RD resource values /rd,
      /rd-lookup, and /rd-group.

   o  changed "ins" ABNF notation.

   o  various editorial improvements, including in the request but was set before, the previous Base
         URI examples

   o  clarifications for RDAO

   changes from -07 to -08

   o  removed link target value is kept unmodified.  If the parameter is not set in
         the request and was not set before either, the source address returned from domain and source port of the update request are stored as the Base
         URI.

      extra-attrs :=  Additional registration attributes (optional).  As
         with the registration, the RD processes them if it knows their
         semantics.  Otherwise, unknown attributes are stored as
         endpoint attributes, overriding any previously stored endpoint
         attributes group lookup
      types

   o  Maximum length of the same key.

   Content-Format:  none (no payload)

   The following response codes are defined domain parameter 63 bytes for this interface:

   Success:  2.04 "Changed" or 204 "No Content" if the update was
      successfully processed.

   Failure:  4.00 "Bad Request" or 400 "Bad Request".  Malformed
      request.

   Failure:  4.04 "Not Found" or 404 "Not Found".  Registration does not
      exist (e.g. may have expired).

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the operation.

   HTTP support:  YES

   If the registration update fails consistency with
      group

   o  removed option for simple POST of link data, don't require a "Service Unavailable"
   response
      .well-known/core resource to accept POST data and handle it in a Max-Age option or Retry-After header, the registering
   endpoint SHOULD retry the operation after the time indicated.  If
      special way; we already have /rd for that

   o  add IPv6 ND Option for discovery of an RD

   o  clarify group configuration section 6.1 that endpoints must be
      registered before including them in a group

   o  removed all superfluous client-server diagrams

   o  simplified lighting example

   o  introduced Commissioning Tool

   o  RD-Look-up text is extended.

   changes from -06 to -07

   o  added text in the
   registration fails discovery section to allow content format hints
      to be exposed in another way, including request timeouts, or if the time indicated exceeds discovery link attributes

   o  editorial updates to section 9

   o  update author information

   o  minor text corrections

   Changes from -05 to -06

   o  added note that the remaining lifetime, PATCH section is contingent on the registering
   endpoint SHOULD attempt registration again.

   The following example shows how progress of
      the registering endpoint updates its
   registration resource at an RD PATCH method

   changes from -04 to -05

   o  added Update Endpoint Links using this PATCH

   o  http access made explicit in interface with the example
   location value: /rd/4521.

   Req: POST /rd/4521

   Res: 2.04 Changed
   The following example shows the registering specification

   o  Added http examples

   Changes from -03 to -04:

   o  Added http response codes

   o  Clarified endpoint updating its
   registration resource at name usage

   o  Add application/link-format+cbor content-format

   Changes from -02 to -03:

   o  Added an example for lighting and DNS integration

   o  Added an example for RD using this interface with use in OMA LWM2M

   o  Added Read Links operation for link inspection by endpoints

   o  Expanded DNS-SD section

   o  Added draft authors Peter van der Stok and Michael Koster

   Changes from -01 to -02:

   o  Added a catalogue use case.

   o  Changed the example
   location value: /rd/4521.  The initial registration by update to a POST with optional link
      format payload.  Removed the
   registering endpoint set type update from the following values: update.

   o  Additional examples section added for more complex use cases.

   o  New DNS-SD mapping section.

   o  Added text on endpoint name (ep)=endpoint1 identification and authentication.

   o  lifetime (lt)=500  Error code 4.04 added to Registration Update and Delete requests.

   o  Base URI (base)=coap://local-proxy-old.example.com:5683  Made 63 bytes a SHOULD rather than a MUST for endpoint name and
      resource type parameters.

   Changes from -00 to -01:

   o  payload of Figure 7

   The initial state of  Removed the Resource Directory is reflected in ETag validation feature.

   o  Place holder for the
   following request:

   Req: DNS-SD mapping section.

   o  Explicitly disabled GET /rd-lookup/res?ep=endpoint1

   Res: 2.01 Content
   Payload:
   <coap://local-proxy-old.example.com:5683/sensors/temp>;ct=41;
    rt="temperature"; anchor="coap://spurious.example.com:5683",
   <coap://local-proxy-old.example.com:5683/sensors/light>;ct=41;
     rt="light-lux"; if="sensor";
     anchor="coap://local-proxy-old.example.com:5683"

   The following example shows or POST on returned Location.

   o  New registry for RD parameters.

   o  Added support for the registering endpoint changing JSON Link Format.

   o  Added reference to the
   Base URI Groupcomm WG draft.

   Changes from -05 to "coaps://new.example.com:5684":

   Req: POST /rd/4521?base=coaps://new.example.com:5684

   Res: 2.04 Changed

   The consecutive query returns:

   Req: GET /rd-lookup/res?ep=endpoint1

   Res: 2.01 Content
   Payload:
   <coaps://new.example.com:5684/sensors/temp>;ct=41;rt="temperature";
       anchor="coap://spurious.example.com:5683",
   <coaps://new.example.com:5684/sensors/light>;ct=41;rt="light-lux";
       if="sensor"; anchor="coaps://new.example.com:5684",

   The following example shows a client performing WG Document -00:

   o  Updated the version and date.

   Changes from -04 to -05:

   o  Restricted Update to parameter updates.

   o  Added pagination support for the Lookup interface.

   o  Minor editing, bug fixes and enpoint lookup reference updates.

   o  Added group support.

   o  Changed rt to et for all groups.

   Req: GET /rd-lookup/ep?et=core.rd-group

   Res: 2.01 Content
   Payload:
   </rd/501>;ep="GRP_R2-4-015";et="core.rd-group";
                                      base="coap://[ff05:;1]",
   <rd/12>;ep=lights&et=core.rd-group;
                            base="coap://[ff35:30:2001:db8::1]"

A.2.  Registration Removal

   Although RD entries have soft state the registration and will eventually timeout after
   their lifetime, update interface.

   Changes from -03 to -04:

   o  Added the registering endpoint SHOULD explicitly remove an
   entry ins= parameter back for the DNS-SD mapping.

   o  Integrated the Simple Directory Discovery from Carsten.

   o  Editorial improvements.

   o  Fixed the RD if it knows it will no longer be available (for
   example on shut-down).  This is accomplished using a removal
   interface on use of ETags.

   o  Fixed tickets 383 and 372

   Changes from -02 to -03:

   o  Changed the RD by performing endpoint name back to a DELETE on single registration parameter
      ep= and removed the endpoint resource.

   The removal request h= and ins= parameters.

   o  Updated REST interface is specified as follows:

   Interaction:  EP -> RD

   Method:  DELETE

   URI Template:  {+location} descriptions to use RFC6570 URI Template Variables:

      location :=  This is the Location returned by the
      format.

   o  Introduced an improved RD Lookup design as a result
         of a successful earlier registration.

   The following response codes are defined for this interface:

   Success:  2.02 "Deleted" or 204 "No Content" upon successful deletion

   Failure:  4.00 "Bad Request" or 400 "Bad Request".  Malformed
      request.

   Failure:  4.04 "Not Found" or 404 "Not Found".  Registration does not
      exist (e.g. may have expired).

   Failure:  5.03 "Service Unavailable" its own function set.

   o  Improved the security considerations section.

   o  Made the POST registration interface idempotent by requiring the
      ep= parameter to be present.

   Changes from -01 to -02:

   o  Added a terminology section.

   o  Changed the inclusion of an ETag in registration or 503 "Service Unavailable".
      Service could not perform update to a
      MAY.

   o  Added the operation.

   HTTP support: YES

   The following examples shows successful removal concept of an RD Domain and a registration parameter for
      it.

   o  Recommended the endpoint Location returned from a registration to be
      stable, allowing for endpoint and Domain information to be changed
      during updates.

   o  Changed the RD with example location value /rd/4521.

   Req: DELETE /rd/4521

   Res: 2.02 Deleted

A.3.  Read Endpoint Links

   Some registering endpoints may wish lookup interface to manage their links accept endpoint and Domain as
      query string parameters to control the scope of a lookup.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

13.2.  Informative References

   [ER]       Chen, P., "The entity-relationship model---toward a
   collection, and may need to read the current set
              unified view of links stored in
   the registration resource, data", ACM Transactions on Database
              Systems Vol. 1, pp. 9-36, DOI 10.1145/320434.320440, March
              1976.

   [I-D.arkko-core-dev-urn]
              Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
              Names for Device Identifiers", draft-arkko-core-dev-urn-05
              (work in order to determine progress), October 2017.

   [I-D.bormann-t2trg-rel-impl]
              Bormann, C., "impl-info: A link maintenance
   operations.

   One or more links MAY be selected by using query filtering as
   specified relation type for
              disclosing implementation information", draft-bormann-
              t2trg-rel-impl-00 (work in [RFC6690] Section 4.1

   If no links are selected, the Resource Directory SHOULD return an
   empty payload.

   The read request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

   URI Template:  {+location}{?href,rel,rt,if,ct}

   URI Template Variables:

      location :=  This is the Location returned by progress), January 2018.

   [I-D.hartke-t2trg-coral]
              Hartke, K., "The Constrained RESTful Application Language
              (CoRAL)", draft-hartke-t2trg-coral-06 (work in progress),
              October 2018.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the RD as a result
         of a successful earlier registration.

      href,rel,rt,if,ct := link relations OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-17
              (work in progress), November 2018.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-17 (work in progress), November 2018.

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., and C. Bormann, "Representing
              Constrained RESTful Environments (CoRE) Link Format in
              JSON and CBOR", draft-ietf-core-links-json-10 (work in
              progress), February 2018.

   [I-D.silverajan-core-coap-protocol-negotiation]
              Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
              draft-silverajan-core-coap-protocol-negotiation-09 (work
              in progress), July 2018.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and attributes specified in
      the query C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in order to select particular links based on their
      relations Address Literals and attributes. "href" denotes the URI target of the
      link.  See [RFC6690] Sec. 4.1

   The following response codes are defined Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <https://www.rfc-editor.org/info/rfc6874>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for this interface:

   Success:  2.05 "Content" or 200 "OK" upon success with an
      "application/link-format", "application/link-format+cbor", or
      "application/link-format+json" payload.

   Failure:  4.00 "Bad Request" or 400 "Bad Request".  Malformed
      request.

   Failure:  4.04 "Not Found" or 404 "Not Found".  Registration does not
      exist (e.g. may have expired).

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the operation.

   HTTP support: YES

   The following examples show successful read of
              the endpoint links
   from the RD, with example location value /rd/4521 and example
   registration payload of Figure 7.

   Req: GET /rd/4521

   Res: 2.01 Content
   Payload:
   </sensors/temp>;ct=41;rt="temperature-c";if="sensor";
   anchor="coap://spurious.example.com:5683",
   </sensors/light>;ct=41;rt="light-lux";if="sensor"

A.4.  Update Endpoint Links

   An iPATCH (or PATCH) update ([RFC8132]) can add, remove or change Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

   [RFC7641]  Hartke, K., "Observing Resources in the
   links of a registration.

   Those operations are out of scope of this document, Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC8132]  van der Stok, P., Bormann, C., and will require
   media types suitable A. Sehgal, "PATCH and
              FETCH Methods for modifying sets of links.

A.5.  Endpoint lookup

   Endpoint lookups result in links to registration resources.  Endpoint
   registration resources the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

Appendix A.  Groups Registration and Lookup

   The RD-Groups usage pattern allows announcing application groups
   inside a Resource Directory.

   Groups are annotated with their represented by endpoint names (ep),
   sectors (d, if present) and registration registrations.  Their base URI (base) as well as address
   is a
   constant resource type (rt="core.rd-ep"); multicast address, and they SHOULD be entered with the lifetime (lt) is not
   reported.  Additional endpoint attributes are added as link
   attributes to their
   type "core.rd-group".  The endpoint link unless their specification says
   otherwise.

   Serializations derived from Link Format, SHOULD present links name can also be referred to
   endpoints in path-absolute form or, if required, as absolute
   references.  (This approach avoids the RFC6690 ambiguities.)

   While Endpoint Lookup does expose the a
   group name in this context.

   The registration resources, is inserted into the RD
   does not need by a Commissioning Tool,
   which might also be known as a group manager here.  It performs third
   party registration and registration updates.

   The links it registers SHOULD be available on all members that join
   the group.  Depending on the application, members that lack some
   resource MAY be permissible if requests to make them accessible to clients.  Clients SHOULD NOT
   attempt fail gracefully.

   The following example shows a CT registering a group with the name
   "lights" which provides two resources.  The directory resource path
   /rd is an example RD location discovered in a request similar to dereference or manipulate them.

   A Resource Directory
   Figure 5.

   Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
                                     &base=coap://[ff35:30:2001:db8::1]
   Content-Format: 40
   Payload:
   </light>;rt="light";if="core.a",
   </color-temperature>;if="core.p";u="K"

   Res: 2.01 Created
   Location-Path: /rd/12

   In this example, the group manager can report endpoints in lookup easily permit devices that are not
   hosted at the same address.  Lookup clients MUST be prepared
   have no writable color-temperature to see
   arbitrary URIs join, as registration resources in they would still
   respond to brightness changing commands.  Had the results group instead
   contained a single resource that sets brightness and treat
   them as opaque identifiers; the precise semantics of such links are
   left color
   temperature atomically, endpoints would need to future specifications. support both
   properties.

   The resources of a group can be looked up like any other resource,
   and the group registrations (along with any additional registration
   parameters) can be looked up using the endpoint lookup interface.

   The following example shows a client performing an and endpoint type (et) lookup with the value oic.d.sensor (which is currently a registered
   rt value):
   for all groups.

   Req: GET /rd-lookup/ep?et=oic.d.sensor /rd-lookup/ep?et=core.rd-group

   Res: 2.05 2.01 Content
   </rd/1234>;base="coap://[2001:db8:3::127]:61616";ep="node5";
   et="oic.d.sensor";ct="40",
   </rd/4521>;base="coap://[2001:db8:3::129]:61616";ep="node7";
   et="oic.d.sensor";ct="40";d="floor-3"
   Payload:
   </rd/501>;ep="GRP_R2-4-015";et="core.rd-group";
                                      base="coap://[ff05::1]",
   </rd/12>;ep=lights&et=core.rd-group;
            base="coap://[ff35:30:2001:db8::1]";rt="core.rd-ep"

   The following example shows a client performing a lookup of all
   resources of all endpoints (groups) with et=core.rd-group.

   Req: GET /rd-lookup/res?et=core.rd-group

   <coap://[ff35:30:2001:db8::1]/light>;rt="light";if="core.a";
        et="core.rd-group";anchor="coap://[ff35:30:2001:db8::1]",
   <coap://[ff35:30:2001:db8::1]/color-temperature>;if="core.p";u="K";
        et="core.rd-group";
        anchor="coap://[ff35:30:2001:db8::1]"

Appendix B.  Web links and the Resource Directory

   Understanding the semantics of a link-format document and its URI
   references is a journey through different documents ([RFC3986]
   defining URIs, [RFC6690] defining link-format documents based on
   [RFC8288] which defines link headers, and [RFC7252] providing the
   transport).  This appendix summarizes the mechanisms and semantics at
   play from an entry in ".well-known/core" to a resource lookup.

   This text is primarily aimed at people entering the field of
   Constrained Restful Environments from applications that previously
   did not use web mechanisms.

   At all examples in this section give compatible results for both
   Modernized and RFC6690 Link Format; the

   The explanation of the steps
   follow Modernized makes some shortcuts in the more
   confusing details of [RFC6690], which are justified as all examples
   being in Limited Link Format.

B.1.  A simple example

   Let's start this example with a very simple host, "2001:db8:f0::1".
   A client that follows classical CoAP Discovery ([RFC7252] Section 7),
   sends the following multicast request to learn about neighbours
   supporting resources with resource-type "temperature".

   The client sends a link-local multicast:

   GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature

   RES 2.05 Content
   </temp>;rt=temperature;ct=0

   where the response is sent by the server, "[2001:db8:f0::1]:5683".

   While the client - on the practical or implementation side - can just
   go ahead and create a new request to "[2001:db8:f0::1]:5683" with
   Uri-Path: "temp", the full resolution steps for insertion into and
   retrieval from the RD without any shortcuts are:

B.1.1.  Resolving the URIs

   The client parses the single returned record.  The link's target
   (sometimes called "href") is ""/temp"", which is a relative URI that
   needs resolving.  The base URI <coap://[ff02::fd]:5683/.well-known/
   core> is used to resolve the reference /temp against.

   The Base URI of the requested resource can be composed from the
   header options of the CoAP GET request by following the steps of

   [RFC7252] section 6.5 (with an addition at the end of 8.2) into
   ""coap://[2001:db8:f0::1]/.well-known/core"".

   Because ""/temp"" starts with a single slash, the record's target is
   resolved by replacing the path ""/.well-known/core"" from the Base
   URI (section 5.2 [RFC3986]) with the relative target URI ""/temp""
   into ""coap://[2001:db8:f0::1]/temp"".

B.1.2.  Interpreting attributes and relations

   Some more information but the record's target can be obtained from
   the payload: the resource type of the target is "temperature", and
   its content type is text/plain (ct=0).

   A relation in a web link is a three-part statement that specifies a
   named relation between the so-called "context resource" and the
   target resource, like "_This page_ has _its table of contents_ at _/
   toc.html_".  In [RFC6690] and modernized link-format link format documents, there is an implicit "host
   relation" specified with default parameter: rel="hosts".

   In our example, the context resource of the link is the URI specified
   in the GET request "coap:://[2001:db8:f0::1]/.well-known/core".  A
   full English expression of the "host relation" is:

   '"coap://[2001:db8:f0::1]/.well-known/core" is hosting the resource
   "coap://[2001:db8:f0::1]/temp", which is of the resource type
   "temperature" and can be accessed using the text/plain content
   format.'

B.2.  A slightly more complex example

   Omitting the "rt=temperature" filter, the discovery query would have
   given some more records in the payload:

   GET coap://[ff02::fd]:5683/.well-known/core

   RES 2.05 Content
   </temp>;rt=temperature;ct=0,
   </light>;rt=light-lux;ct=0,
   </t>;anchor="/sensors/temp";rel=alternate,
   <http://www.example.com/sensors/t123>;anchor="/sensors/temp";
       rel="describedby"

   Parsing the third record, the client encounters the "anchor"
   parameter.  It is a URI relative to the Base URI of the request and
   is thus resolved to ""coap://[2001:db8:f0::1]/sensors/temp"".  That
   is the context resource of the link, so the "rel" statement is not
   about the target and the Base URI any more, but about the target and
   the resolved URI.  Thus, the third record could be read as
   ""coap://[2001:db8:f0::1]/sensors/temp" has an alternate
   representation at "coap://[2001:db8:f0::1]/t"".

   Following the same resolution steps, the fourth record can be read as
   ""coap://[2001:db8:f0::1]/sensors/temp" is described by
   "http://www.example.com/sensors/t123"".

B.3.  Enter the Resource Directory

   The resource directory tries to carry the semantics obtainable by
   classical CoAP discovery over to the resource lookup interface as
   faithfully as possible.

   For the following queries, we will assume that the simple host has
   used Simple Registration to register at the resource directory that
   was announced to it, sending this request from its UDP port
   "[2001:db8:f0::1]:6553":

   POST coap://[2001:db8:f01::ff]/.well-known/core?ep=simple-host1

   The resource directory would have accepted the registration, and
   queried the simple host's ".well-known/core" by itself.  As a result,
   the host is registered as an endpoint in the RD with the name
   "simple-host1".  The registration is active for 90000 seconds, and
   the endpoint registration Base URI is ""coap://[2001:db8:f0::1]""
   following the resolution steps described in Appendix B.1.1.  It
   should be remarked that the Base URI constructed that way always
   yields a URI of the form: scheme://authority without path suffix.

   If the client now queries the RD as it would previously have issued a
   multicast request, it would go through the RD discovery steps by
   fetching "coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-
   lookup-res", obtain "coap://[2001:db8:f0::ff]/rd-lookup/res" as the
   resource lookup endpoint, and issue a request to
   "coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature" to receive
   the following data:

   <coap://[2001:db8:f0::1]/temp>;rt=temperature;ct=0;
       anchor="coap://[2001:db8:f0::1]"

   This is not _literally_ the same response that it would have received
   from a multicast request, but it contains the equivalent statement:

   '"coap://[2001:db8:f0::1]" is hosting the resource
   "coap://[2001:db8:f0::1]/temp", which is of the resource type
   "temperature" and can be accessed using the text/plain content
   format.'
   (The difference is whether "/" or "/.well-known/core" hosts the
   resources, which is one of does not matter in this application; if it did, the often misunderstood subtleties
   Modernized Link Format addresses.
   endpoint would have been more explicit.  Actually, /.well-known/core
   does NOT host the resource but stores a URI reference to the
   resource.)

   To complete the examples, the client could also query all resources
   hosted at the endpoint with the known endpoint name "simple-host1".
   A request to "coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1"
   would return

   <coap://[2001:db8:f0::1]/temp>;rt=temperature;ct=0;
       anchor="coap://[2001:db8:f0::1]",
   <coap://[2001:db8:f0::1]/light>;rt=light-lux;ct=0;
       anchor="coap://[2001:db8:f0::1]",
   <coap://[2001:db8:f0::1]/t>;
       anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,
   <http://www.example.com/sensors/t123>;
       anchor="coap://[2001:db8:f0::1]/sensors/temp";rel="describedby"

   All the target and anchor references are already in absolute form
   there, which don't need to be resolved any further.

   Had the simple host done an equivalent full registration with a base=
   parameter (e.g.  "?ep=simple-host1&base=coap+tcp://simple-
   host1.example.com"), that context would have been used to resolve the
   relative anchor values instead, giving

   <coap+tcp://simple-host1.example.com/temp>;rt=temperature;ct=0;
       anchor="coap+tcp://simple-host1.example.com"

   and analogous records.

B.4.  A note on differences between link-format and Link headers

   While link-format and Link headers look very similar and are based on
   the same model of typed links, there are some differences between
   [RFC6690] and [RFC5988], [RFC8288], which are dealt with differently:

   o  "Resolving the target against the anchor": [RFC6690] Section 2.1
      states that the anchor of a link is used as the Base URI against
      which the term inside the angle brackets (the target) is resolved,
      falling back to the resource's URI with paths stripped off (its
      "Origin").  In contrast to that, [RFC8288] Section B.2 describes
      that the anchor is immaterial to the resolution of the target
      reference.

      RFC6690, in the same section, also states that absent anchors set
      the context of the link to the target's URI with its path stripped
      off, while according to [RFC8288] Section 3.2, the context is the
      resource's base URI.

      In the context of a Resource Directory, the authors decided to not
      let this become an issue by recommending that links in the
      Resource Directory be _deserializable_ by either rule set to give
      the same results.  Note that all examples of [RFC6690], [RFC8288]
      and this document comply with that rule.

      The Modernized Link Format is rules introduced in Appendix D C ensure that an RD does not need
      to
      formalize what it means deal with those differences when processing input data.  Lookup
      results are required to apply be absolute references for the ruleset of RFC8288 to Link
      Format documents. same
      reason.

   o  There is no percent encoding in link-format documents.

      A link-format document is a UTF-8 encoded string of Unicode
      characters and does not have percent encoding, while Link headers
      are practically ASCII strings that use percent encoding for non-
      ASCII characters, stating the encoding explicitly when required.

      For example, while a Link header in a page about a Swedish city
      might read

      "Link: </temperature/Malm%C3%B6>;rel="live-environment-data""

      a link-format document from the same source might describe the
      link as

      "</temperature/Malmoe>;rel="live-environment-data""

      Parsers and producers of link-format and header data need to be
      aware of this difference.

Appendix C.  Syntax examples for Protocol Negotiation

   [ This appendix should not show up in a published version of this
   document. ]

   The protocol negotiation that is being worked on in
   [I-D.silverajan-core-coap-protocol-negotiation] makes use of the
   Resource Directory.

   Until that document is update to use the latest resource-directory
   specification, here are some examples of protocol negotiation with
   the current Resource Directory:

   An endpoint could register as follows from its address
   [2001:db8:f1::2]:5683:

   Req: POST coap://rd.example.com/rd?ep=node1
       &at=coap+tcp://[2001:db8:f1::2]
   Content-Format: 40
   Payload:
   </temperature>;ct=0;rt="temperature";if="core.s"

   Res: 2.01 Created
   Location-Path: /rd/1234

   An endpoint lookup would just reflect the registered attributes:

   Req: GET /rd-lookup/ep

   Res: 2.05 Content
   </rd/1234>;ep="node1";base="coap://[2001:db8:f1::2]:5683";
       at="coap+tcp://[2001:db8:f1::2]"

   A UDP client would then see the following in a resource lookup:

   Req: GET /rd-lookup/res?rt=temperature

   Res: 2.05 Content
   <coap://[2001:db8:f1::2]/temperature>;ct=0;rt="temperature";
       if="core.s"; anchor="coap://[2001:db8:f1::2]"

   while
      might read

      "Link: </temperature/Malm%C3%B6>;rel="live-environment-data""

      a TCP capable client could say:

   Req: GET /rd-lookup/res?rt=temperature&tt=tcp

   Res: 2.05 Content
   <coap+tcp://[2001:db8:f1::2]/temperature>;ct=0;rt="temperature";
       if="core.s";anchor="coap+tcp://[2001:db8:f1::2]" link-format document from the same source might describe the
      link as

      "</temperature/Malmoe>;rel="live-environment-data""

      Parsers and producers of link-format and header data need to be
      aware of this difference.

Appendix D.  Modernized C.  Limited Link Format parsing

   The CoRE Link Format as described in [RFC6690] is unsuitable for has been interpreted
   differently by implementers, and a strict implementation rules out
   some use cases of the a Resource Directory, and their resolution scheme is
   often misunderstood by developers familiar with [RFC8288].

   For the correct application of Directory (e.g. base URIs, we describe the
   interpretation of a Link Format document as a Modernized Link Format.
   In Modernized Link Format, the document is processed as in Link
   Format, values with the exception of Section 2.1 of [RFC6690]:

   o  The URI-reference inside angle brackets ("<>") path
   components).

   This appendix describes the
      target URI of the link.

   o  The context of the link is expressed by the "anchor" parameter.
      If the anchor attribute is absent, it defaults to the empty
      reference ("").

   o  Both these references are resolved according to Section 5 a subset of
      [RFC3986].

   Content formats derived from [RFC6690] which inherit its resolution
   rules, like JSON and CBOR link format of [I-D.ietf-core-links-json],
   can be interpreted documents called
   Limited Link Format.  The rules herein are not very limiting in analogy to that.

   For where the Resource Directory is concerned, all common forms of
   links (e.g.
   practice - all the examples of RFC6690) yield identical results.
   When interpreting data read from ".well-known/core", differences in
   interpretation only affect links where the absent anchor attribute
   means "coap://host/" according to RFC6690 RFC6690, and "coap://host/.well-
   known/core" according all deployments the authors
   are aware of already stick to Modernized Link format; those typically only
   occur in conjunction with them - but ease the vaguely defined implicit "hosts"
   relationship.

D.1.  For endpoint developers

   When developing endpoints, i.e. when generating documents that will
   be submitted implementation of
   resource directory servers.

   It is applicable to a Resource Directory, representations in the differences between
   Modernized Link Format and RFC6690 can be ignored as long as

   o  all relative references start with a slash, application/link-format
   media type, and any of other media types that inherit [RFC6690]
   Section 2.1.

   A link format representation is in Limited Link format if, for each
   link in it, the following applies:

   o  There is no anchor attribute, and the context of  All URI references either follow the link does not
      matter to URI or the application.

      Example: "</sensors>;ct=40"

   o  The anchor is a relative reference.

      Example: "</t>;anchor="/sensors/temp";rel="alternate""

   o  The target is an absolute reference.

      Example: "<http://www.example.com/sensors/t123>;anchor="/sensors/
      temp";rel="describedby""

D.2.  Examples of links with differing interpretations

   Examples path-absolute ABNF
      rule of links with different interpretations from RFC3986 (i.e. target and anchor each either applying
   RFC6690 start with a
      scheme or Modernized Link Format are shown here.  The example is
   assumed to be obtained from with a </device/index> document. single slash),

   o  "<sensors>": The  if the anchor reference starts with a scheme, the target is "/sensors" in RFC6690 and "/device/
      sensors" reference
      starts with a scheme as well (i.e. relative references in Modernized Link Format (whereas "</sensors>" would be
      unambiguous).

   o  "<?which=these>": The target
      cannot be used when the anchor is "/?which=these" in RFC6690 a full URI), and
      "/device/index?which=these" in Modernized Link Format.

   o  "<sensors>;anchor="http://example.com/calib-
      proto/1234";rel="topic"" is about "http://example.com/sensors" in
      RFC6690 and about "/device/sensors" in Modernized Link Format.

      This link can  the application does not be expressed in RFC6690 link format care whether links without the
      server an explicitly expressing most of its own URI (which is
      problematic in reverse proxy scenarios or when the Uri-Host option
      is not sent).

   o  "</i>;rel="alternate";anchor=""": According to RFC6690, this
      states that
      given anchor have the origin's "/" or "/.well-known/core" resource has an alternative representation at
      "/i", whereas Modernized Link Format says that "/devices/index"
      has an alternative representation at "/i".

      The "anchor" attribute is usually left out; the
      as their link
      "</i>;rel="alternate"" is equivalent to the above and results in
      the same interpretations. context.

Authors' Addresses

   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   USA

   Phone: +1-408-203-9434
   Email: zach.shelby@arm.com

   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

   Phone: +1-707-502-5136
   Email: Michael.Koster@smartthings.com

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org
   Peter van der Stok
   consultant

   Phone: +31-492474673 (Netherlands), +33-966015248 (France)
   Email: consultancy@vanderstok.org
   URI:   www.vanderstok.org

   Christian Amsuess (editor)
   Hollandstr. 12/4
   1020
   Austria

   Phone: +43-664-9790639
   Email: christian@amsuess.com