CoRE Resource Directory
draft-ietf-core-resource-directory-12

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Last updated 2017-10-30
Replaces draft-shelby-core-resource-directory
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CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Standards Track                               M. Koster
Expires: May 3, 2018                                         SmartThings
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                         P. van der Stok
                                                              consultant
                                                         C. Amsuess, Ed.
                                             Energy Harvesting Solutions
                                                        October 30, 2017

                        CoRE Resource Directory
                 draft-ietf-core-resource-directory-12

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
   descriptions of resources held on other servers, allowing lookups to
   be performed for those resources.  This document specifies the web
   interfaces that a Resource Directory supports in order for web
   servers to discover the RD and to register, maintain, lookup and
   remove resource descriptions.  Furthermore, new link attributes
   useful in conjunction with an RD are defined.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.

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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture and Use Cases  . . . . . . . . . . . . . . . . .   5
     3.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Content model . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Use Case: Cellular M2M  . . . . . . . . . . . . . . . . .  11
     3.5.  Use Case: Home and Building Automation  . . . . . . . . .  12
     3.6.  Use Case: Link Catalogues . . . . . . . . . . . . . . . .  12
   4.  Finding a Resource Directory  . . . . . . . . . . . . . . . .  13
     4.1.  Resource Directory Address Option (RDAO)  . . . . . . . .  14
   5.  Resource Directory  . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Content Formats . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  URI Discovery . . . . . . . . . . . . . . . . . . . . . .  16
     5.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  18
       5.3.1.  Simple Registration . . . . . . . . . . . . . . . . .  22
       5.3.2.  Third-party registration  . . . . . . . . . . . . . .  23
     5.4.  Operations on the Registration Resource . . . . . . . . .  23
       5.4.1.  Registration Update . . . . . . . . . . . . . . . . .  24
       5.4.2.  Registration Removal  . . . . . . . . . . . . . . . .  26
       5.4.3.  Read Endpoint Links . . . . . . . . . . . . . . . . .  27
       5.4.4.  Update Endpoint Links . . . . . . . . . . . . . . . .  28
   6.  RD Groups . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     6.1.  Register a Group  . . . . . . . . . . . . . . . . . . . .  29
     6.2.  Group Removal . . . . . . . . . . . . . . . . . . . . . .  31
   7.  RD Lookup . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.1.  Resource lookup . . . . . . . . . . . . . . . . . . . . .  32
     7.2.  Endpoint and group lookup . . . . . . . . . . . . . . . .  33
     7.3.  Lookup filtering  . . . . . . . . . . . . . . . . . . . .  33
     7.4.  Lookup examples . . . . . . . . . . . . . . . . . . . . .  35
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  38

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     8.1.  Endpoint Identification and Authentication  . . . . . . .  38
     8.2.  Access Control  . . . . . . . . . . . . . . . . . . . . .  39
     8.3.  Denial of Service Attacks . . . . . . . . . . . . . . . .  39
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
     9.1.  Resource Types  . . . . . . . . . . . . . . . . . . . . .  40
     9.2.  IPv6 ND Resource Directory Address Option . . . . . . . .  40
     9.3.  RD Parameter Registry . . . . . . . . . . . . . . . . . .  40
       9.3.1.  Full description of the "Endpoint Type" Registration
               Parameter . . . . . . . . . . . . . . . . . . . . . .  42
     9.4.  "Endpoint Type" (et=) RD Parameter values . . . . . . . .  42
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  42
     10.1.  Lighting Installation  . . . . . . . . . . . . . . . . .  43
       10.1.1.  Installation Characteristics . . . . . . . . . . . .  43
       10.1.2.  RD entries . . . . . . . . . . . . . . . . . . . . .  44
     10.2.  OMA Lightweight M2M (LWM2M) Example  . . . . . . . . . .  47
       10.2.1.  The LWM2M Object Model . . . . . . . . . . . . . . .  47
       10.2.2.  LWM2M Register Endpoint  . . . . . . . . . . . . . .  49
       10.2.3.  LWM2M Update Endpoint Registration . . . . . . . . .  50
       10.2.4.  LWM2M De-Register Endpoint . . . . . . . . . . . . .  51
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  51
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  51
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  56
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  56
     13.2.  Informative References . . . . . . . . . . . . . . . . .  57
   Appendix A.  Web links and the Resource Directory . . . . . . . .  58
     A.1.  A simple example  . . . . . . . . . . . . . . . . . . . .  58
       A.1.1.  Resolving the URIs  . . . . . . . . . . . . . . . . .  59
       A.1.2.  Interpreting attributes and relations . . . . . . . .  59
     A.2.  A slightly more complex example . . . . . . . . . . . . .  59
     A.3.  Enter the Resource Directory  . . . . . . . . . . . . . .  60
     A.4.  A note on differences between link-format and Link
           headers . . . . . . . . . . . . . . . . . . . . . . . . .  62
   Appendix B.  Syntax examples for Protocol Negotiation . . . . . .  62
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63

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].  The use of Web Linking for the

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   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 descriptions of
   resources held on other servers, allowing lookups to be performed for
   those resources.

   This document specifies the web interfaces that a Resource Directory
   supports in order for web servers to discover the RD and to register,
   maintain, lookup and remove resource descriptions.  Furthermore, new
   link 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 [RFC5988] 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:

   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.

   Domain
      In the context of a Resource Directory, a domain is a logical
      grouping of endpoints.

   Group
      In the context of a Resource Directory, a group is a logical
      grouping of endpoints for the purpose of group communications.
      All groups within a domain have unique names.

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   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 domain of the registration.

   Context
      A Context is a base URL that gives scheme and (typically)
      authority information about an Endpoint.  The Context of an
      Endpoint is provided at registration time, and is used by the
      Resource Directory to resolve relative references inside the
      registration into absolute URIs.

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

   Group Resource
      A resource in the RD containing registration resources of the
      Endpoints that form a group.

   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.

   RDAO
      Resource Directory Address Option.

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 device, 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 that could otherwise only be obtained by

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   directly querying the /.well-known/core resource on the target
   device, or by accessing those resources with a multicast request.

   From that, it follows that no information should be stored in the
   resource directory that cannot be discovered from querying the
   described device's /.well-known/core resource directly.

   It also follows that data in the resource directory can only be
   provided by the device whose descriptions are cached or a dedicated
   Commissioning Tool (CT).  These CTs are thought to act on behalf
   agents too constrained, or generally unable, to present that
   information themselves.  No other client can modify data in the
   resource directory or even expect those changes to propagate back to
   its source.

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] about resources hosted on other web servers, which are
   called endpoints (EP).  An endpoint is a web server associated with a
   scheme, IP address and port, thus 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), and for clients to lookup resources
   from the RD or maintain groups.  Endpoints themselves can also act as
   clients.  An RD can be logically segmented by the use of Domains.
   The domain an endpoint is associated with can be defined by the RD or
   configured by an outside entity.  This information hierarchy is shown
   in Figure 2.

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

   Endpoints proactively register and maintain resource directory
   registration entries on the RD, which are soft state and need to be
   periodically refreshed.

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

   At the first registration of a set of entries, 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 the registration
   entry.

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

                Registration     Lookup, Group
                 Interface        Interfaces
     +----+          |                 |
     | EP |----      |                 |
     +----+    ----  |                 |
                   --|-    +------+    |
     +----+          | ----|      |    |     +--------+
     | EP | ---------|-----|  RD  |----|-----| Client |
     +----+          | ----|      |    |     +--------+
                   --|-    +------+    |
     +----+    ----  |                 |
     | EP |----      |                 |
     +----+

              Figure 1: The resource directory architecture.

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

          Figure 2: The resource directory information hierarchy.

3.3.  Content model

   The Entity-Relationship (ER) models shown in Figure 3 and Figure 4
   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

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   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
                               0+ |      oooooooo
             oooooooooooo         |
            o    target  o--------+
            o  attribute o        | 0+   oooooo
             oooooooooooo         +-----o rel  o
                                  |      oooooo
                                  |
                                  | 1    ooooooooo
                                  +-----o context o
                                         ooooooooo

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

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

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   o  a set of links belonging to the host

   The host 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:

   o  Zero or more link relations: They describe a 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, eg.
      _who_ "hosts" something.

      In link-format serialization, it is expressed in the "anchor"
      attribute.  There, it can be a relative reference, in which case
      it gets resolved against the URI of the ".well-known/core"
      document it was obtained from . It defaults to that document's
      URI.

      In the serialization, the context also serves as the Base URI for
      resolving the target reference.

   o  A link target URI: It defines the destination of the relation (eg.
      _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".  If it is a relative URI,
   it gets resolved against the link context URI.

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

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                +----------------------+
                |  resource-directory  |
                +----------------------+
                           |
                           |         oooooooooooo  0-1
                           |        o MC address o---+
                           |         oooooooooooo    |
                           |                         |
                      //////\\\\             0+  +--------+
                     < contains >----------------| group  |
                      \\\\\/////                 +--------+
                           |                         |
                       0-n |                         | 1+
    ooooooo     1  +---------------+          ///////\\\\\\
   o  con  o-------|  registration |---------< composed of >
    ooooooo        +---------------+          \\\\\\\//////
                       |       |
                       |       +--------------+
          oooooooo   1 |                      |
         o  loc   o----+                 /////\\\\
          oooooooo     |                < contains >
                       |                 \\\\\/////
          oooooooo   1 |                      |
         o   ep   o----+                      | 0+
          oooooooo     |             +------------------+
                       |             |      link        |
          oooooooo 0-1 |             +------------------+
         o    d   o----+                      |
          oooooooo     |                      |  1   oooooooo
                       |                      +-----o target o
          oooooooo 0-1 |                      |      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: E-R Model of the content of the Resource Directory

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

   o  0 to n Registration (entries),

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   o  0 or more Groups

   A Group has no or one Multicast address attribute and is composed of
   0 or more endpoints.  A registration is associated with one endpoint
   (ep).  An endpoint can be part of 0 or more Groups . A registration
   defines a set of links as defined for /.well-known/core.  A
   Registration has six attributes:

   o  one ep (endpoint with a unique name)

   o  one con (a string describing the scheme://authority part)

   o  one lt (lifetime),

   o  one loc (location in the RD)

   o  optional one d (domain for query filtering),

   o  optional additional endpoint attributes (from Section 9.3)

   The cardinality of con is currently 1.  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.

3.4.  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.

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   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 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.  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.  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 the 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] are supplied by Resource
   Directories, which may be internally stored as triples, or relation/
   attribute pairs providing metadata about resource links.  External

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   catalogs 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 domains 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.  Resource groups may defined to allow
   batched reads from multiple resources.

4.  Finding a Resource Directory

   A device coming up may want to find one or more resource directories
   to make itself known with.

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

   o  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 directory servers then need to configure
      one of their interfaces with this multicast address.)

   o  It may be configured with a DNS name for the RD and a resource-
      record type to look up under this name; it can find a DNS server
      to perform the lookup using the usual mechanisms for finding DNS
      servers.

   o  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.

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

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   o  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 offer 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:

   o  In a 6LoWPAN, just assume the Edge 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*".

   o  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://[ff02::1]/.well-known/core?rt=core.rd*".  RDs within the
      multicast scope will answer the query.

   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.

4.1.  Resource Directory Address Option (RDAO)

   The Resource Directory 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 link-local 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 lifetime 0x0 means that the RD address is invalid and to be
   removed.

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   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: 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

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   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.  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 the application/link-format content format.

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, and "core.rd-group" is used to discover the URI path for
   RD Group operations.  Upon success, the response will contain a
   payload with a link format entry for each RD function discovered,
   indicating the URI path 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.

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   A Resource Directory MAY provide hints about the content-formats it
   supports in the links it exposes or registers, using the "ct" link
   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.  Links to Resource Directories
   MAY be registered in other Resource Directories, and well-known entry
   points SHOULD be provided to enable the bootstrapping of unicast
   discovery.

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

   The discovery request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

      rt :=  Resource Type (optional).  MAY contain one of the values
         "core.rd", "core.rd-lookup*", "core.rd-lookup-res", "core.rd-
         lookup-ep", "core.rd-lookup-gp", "core.rd-group" or "core.rd*"

   Content-Format:  application/link-format (if any)

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

   Content-Format:  application/link-format+cbor (if any)

   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, or application/link-format+cbor
      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)

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   The following example shows an endpoint discovering an RD using this
   interface, thus learning that the directory resource is, in this
   example, at /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 resource paths.

   Req: GET coap://[ff02::1]/.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,
   </rd-lookup/gp>;rt="core.rd-lookup-gp";ct=40,
   </rd-group>;rt="core.rd-group";ct=40

                   Figure 6: 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
   the CBOR and JSON representation of link format.  The RD resource
   paths /rd, /rd-lookup, and /rd-group are example values.

   [ The RFC editor is asked to replace these and later occurrences of
   TBD64 and TBD504 with 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. ]

   Req: GET coap://[ff02::1]/.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",
   </rd-lookup/ep>;rt="core.rd-lookup-ep";ct="40 TBD64 TBD504",
   </rd-lookup/gp>;rt="core.rd-lookup-gp";ct=40 TBD64 TBD504",
   </rd-group>;rt="core.rd-group";ct="40 TBD64 TBD504"

5.3.  Registration

   After discovering the location of an RD, an endpoint MAY register its
   resources 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), or
   CBOR CoRE Link Format (application/link-format+cbor)

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   [I-D.ietf-core-links-json], along with query parameters indicating
   the name of the endpoint, and optionally its domain and the lifetime
   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.  An
   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 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 does not
   create multiple registration resources.  A new registration resource
   may be created at any time to supersede an existing registration,
   replacing the registration parameters and links.

   An empty payload is considered a malformed 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.  The
   Base URI against which they are resolved is the context of the
   registration, which is provided either explicitly in the "con"
   parameter or constructed implicitly from the requester's network
   address.  When resolving relative target references, the server first
   resolves the context of that link, and then interprets the target as
   a reference relative to that context (see Appendix A.4).

   The registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  {+rd}{?ep,d,lt,con,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 domain.  The maximum
         length of this parameter is 63 bytes.  If the RD is configured
         to recognize the endpoint (eg. based on its security context),
         the endpoint can elide the endpoint name, and assign one based
         on the configuration.

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      d :=  Domain (optional).  The domain to which this endpoint
         belongs.  The maximum length of this parameter is 63 bytes.
         When this parameter is elided, the RD MAY associate the
         endpoint with a configured default domain.

      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 86400 (24
         hours) SHOULD be assumed.

      con :=  Context (optional).  This parameter sets the Default Base
         URI under which the request's links are to be interpreted.  The
         URI MUST NOT have a path component of its own, but MUST be
         suitable as a base URI to resolve any relative references given
         in the registration.  The parameter is therefore of the shape
         "scheme://authority" for HTTP and CoAP URIs.  In the absence of
         this parameter the scheme of the protocol, source address and
         source port of the registration request are assumed.  This
         parameter is mandatory when the directory is filled by a third
         party such as an commissioning tool.  If the endpoint uses an
         ephemeral port to register with, it MUST include the con
         parameter in the registration to provide a valid network path.
         If the endpoint which is 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.

      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

   The following response codes are defined for this interface:

   Success:  2.01 "Created" or 201 "Created".  The Location header
      option MUST be included in the response when a new registration
      resource is created.  This Location MUST be a stable identifier
      generated by the RD as it is used for all subsequent operations on

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      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.

   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

   The following example shows an endpoint 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.

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

   Res: 2.01 Created
   Location: /rd/4521

   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.

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

Res: 201 Created
Location: /rd/4521

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5.3.1.  Simple Registration

   Not all endpoints hosting resources are expected to know how to
   upload links to a 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 endpoint makes available the hosted
   resources that it wants to be discovered, as links on its "/.well-
   known/core" interface as specified in [RFC6690].

   The endpoint then finds one or more addresses of the directory server
   as described in Section 4.

   An endpoint finally asks the directory server to probe it for
   resources and publish them as follows:

   It 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, which triggers the resource directory
   server to perform GET requests at the requesting server's default
   discovery URI to obtain the link-format payload to register.

   The endpoint includes the same registration parameters in the POST
   request as it would per Section 5.3.  The context of the registration
   is taken from the requesting server's URI.

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

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

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   Req:(to RD server from [2001:db8:2::1])
   POST /.well-known/core?lt=6000&ep=node1
   Content-Format: 40
   No payload

   Res: 2.04 Changed

   (later)

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

   Res: 2.05 Content
   Payload:
   </sen/temp>

5.3.2.  Third-party registration

   For some applications, even Simple Registration may be too taxing for
   certain 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 device, called a commissioning
   tool.  The commissioning tool can fill the Resource Directory from a
   database or other means.  For that purpose the scheme, IP address and
   port of the registered device is indicated in the Context parameter
   of the registration described in Section 5.3.

5.4.  Operations on the Registration Resource

   After the initial registration, an endpoint should retain the
   returned location of the Registration Resource for further
   operations, including refreshing the registration in order to extend
   the lifetime and "keep-alive" the registration.  When the lifetime of
   the registration has expired, the RD SHOULD NOT respond to discovery
   queries concerning this endpoint.  The RD SHOULD continue to provide
   access to the Registration Resource after a registration time-out
   occurs in order to enable the registering endpoint to eventually
   refresh the registration.  The RD MAY eventually remove the
   registration resource for the purpose of resource recovery and
   garbage collection.  If the Registration Resource is removed, the
   endpoint will need to re-register.

   The Registration Resource may also be used to inspect the
   registration resource using GET, update the registration link
   contents, or cancel the registration using DELETE.

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   These operations are described in this section.

5.4.1.  Registration Update

   The update interface is used by an endpoint to refresh or update its
   registration with an RD.  To use the interface, the endpoint sends a
   POST request to the registration resource returned in the Location
   header option in the response returned from the initial registration
   operation.

   An update MAY update the lifetime- or the context- registration
   parameters "lt", "con" 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 query
   parameters in an update operation as in Section 5.3.

   A registration update resets the timeout of the registration to the
   (possibly updated) lifetime of the registration, independent of
   whether a "lt" parameter was given.

   If the context of the registration is changed in an update explicitly
   or implicitly, relative references submitted in the original
   registration or later updates are resolved anew against the new
   context (like in the original registration).

   This operation only describes the use of POST with an empty payload.
   As with modification of individual using iPATCH or PATCH as proposed
   in Section 5.4.4, future standards might describe the semantics of
   using content formats and payloads with the POST method to update the
   links of a registration.

   The update 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,

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         the previous last lifetime set on a previous update or the
         original registration (falling back to 86400) SHOULD be used.

      con :=  Context (optional).  This parameter updates the context
         established in the original registration to a new value.  If
         the parameter is set in an update, it is stored by the RD as
         the new Base URI under which to interpret the links of the
         registration, following the same restrictions as in the
         registration.  If the parameter is not set and was set
         explicitly before, the previous context value is kept
         unmodified.  If the parameter is not set and was not set
         explicitly before either, the source address and source port of
         the update request are stored as the context.

      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 of the same key.

   Content-Format:  none (no payload)

   The following response codes are defined 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

   The following example shows an endpoint updating its registration
   resource at an RD using this interface with the example location
   value: /rd/4521.

   Req: POST /rd/4521

   Res: 2.04 Changed

   The following example shows an endpoint updating its registration
   resource at an RD using this interface with the example location

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   value: /rd/4521.  The initial registration by the client set the
   following values:

   o  endpoint name (ep)=endpoint1

   o  lifetime (lt)=500

   o  context (con)=coap://local-proxy-old.example.com:5683

   The initial state of the Resource Directory is reflected in the
   following request:

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

Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature";anchor="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 an EP changing the context to
   "coaps://new.example.com:5684":

   Req: POST /rd/4521?con=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:
</sensors/temp>;ct=41;rt="temperature";anchor="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 entries have soft state and will eventually timeout after
   their lifetime, an endpoint SHOULD explicitly remove its entry from
   the RD if it knows it will no longer be available (for example on
   shut-down).  This is accomplished using a removal interface on the RD
   by performing a DELETE on the endpoint resource.

   Removed endpoints are implicitly removed from the groups to which
   they belong.

   The removal request interface is specified as follows:

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   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 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" or 503 "Service Unavailable".
      Service could not perform the operation.

   HTTP support: YES

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

   Req: DELETE /rd/4521

   Res: 2.02 Deleted

5.4.3.  Read Endpoint Links

   Some endpoints may wish to manage their links as a collection, and
   may need to read the current set of links stored in the registration
   resource, in order to determine link maintenance operations.

   One or more links MAY be selected by using query filtering as
   specified 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

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   Method:  GET

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

   URI Template Variables:

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

      href,rel,rt,if,ct := link relations and attributes specified in
      the query in order to select particular links based on their
      relations and attributes. "href" denotes the URI target of the
      link.  See [RFC6690] Sec. 4.1

   The following responses codes are defined 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.

   Req: GET /rd/4521

   Res: 2.01 Content
   Payload:
   </sensors/temp>;ct=41;rt="temperature-c";if="sensor",
   </sensors/light>;ct=41;rt="light-lux";if="sensor"

5.4.4.  Update Endpoint Links

   An iPATCH (or PATCH) update [RFC8132] adds, removes or changes links
   of a registration by including link update information in the payload
   of the update with a media type that still needs to be defined.

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6.  RD Groups

   This section defines the REST API for the creation, management, and
   lookup of endpoints for group operations.  Similar to endpoint
   registration entries in the RD, groups may be created or removed.
   However unlike an endpoint entry, a group entry consists of a list of
   endpoints and does not have a lifetime associated with it.  In order
   to make use of multicast requests with CoAP, a group MAY have a
   multicast address associated with it.

6.1.  Register a Group

   In order to create a group, a commissioning tool (CT) used to
   configure groups, makes a request to the RD indicating the name of
   the group to create (or update), optionally the domain the group
   belongs to, and optionally the multicast address of the group.  The
   registration message is a list of links to registration resources of
   the endpoints that belong to that group.

   The commissioning tool SHOULD not send any target attributes with the
   links to the registration resources, and the resource directory
   SHOULD ignore any attributes that are set.

   Configuration of the endpoints themselves is out of scope of this
   specification.  Such an interface for managing the group membership
   of an endpoint has been defined in [RFC7390].

   The registration request interface is specified as follows:

   Interaction:  CT -> RD

   Method:  POST

   URI Template:  {+rd-group}{?gp,d,con}

   URI Template Variables:

      rd-group :=  RD Group URI (mandatory).  This is the location of
         the RD Group REST API.

      gp :=  Group Name (mandatory).  The name of the group to be
         created or replaced, unique within that domain.  The maximum
         length of this parameter is 63 bytes.

      d :=  Domain (optional).  The domain to which this group belongs.
         The maximum length of this parameter is 63 bytes.  Optional.
         When this parameter is elided, the RD MAY associate the
         endpoint with a configured default domain.

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      con :=  Context (optional).  This parameter sets the scheme,
         address and port of the multicast address associated with the
         group.  When con is used, scheme and host are mandatory and
         port parameter is optional.

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor

   The following response codes are defined for this interface:

   Success:  2.01 "Created" or 201 "Created".  The Location header
      option MUST be returned in response to a successful group CREATE
      operation.  This Location MUST be a stable identifier generated by
      the RD as it is used for delete operations of the group resource.

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

   Failure:  4.04 "Not Found" or 404 "Not Found".  An Endpoint is not
      registered in the RD (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 example shows an EP registering a group with the name
   "lights" which has two endpoints.  The RD group path /rd-group is an
   example RD location discovered in a request similar to Figure 6.

   Req: POST coap://rd.example.com/rd-group?gp=lights
                                     &con=coap://[ff35:30:2001:db8::1]
   Content-Format: 40
   Payload:
   </rd/4521>,
   </rd/4522>

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

   The href value is the path to the registration resource of the
   Endpoint.

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6.2.  Group Removal

   A group can be removed simply by sending a removal message to the
   location of the group registration resource which was returned when
   initially registering the group.  Removing a group MUST NOT remove
   the endpoints of the group from the RD.

   The removal request interface is specified as follows:

   Interaction:  CT -> RD

   Method:  DELETE

   URI Template:  {+location}

   URI Template Variables:

      location :=  This is the path of the group resource returned by
         the RD as a result of a successful group registration.

   The following responses 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".  Group does not exist.

   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 group from the
   RD with the example location value /rd-group/12.

   Req: DELETE /rd-group/12

   Res: 2.02 Deleted

7.  RD Lookup

   To discover the resources registered with the 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
   descriptions in alternative formats (e.g.  Atom or HTML Link) or

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   using more advanced interfaces (e.g. supporting context or semantic
   based lookup).

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

   The lookup type is selected 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 |
             | Group       | core.rd-lookup-gp  | Optional  |
             +-------------+--------------------+-----------+

                           Table 1: Lookup Types

7.1.  Resource lookup

   Resource lookup results in links that are semantically equivalent to
   the links submitted to the RD if they were accessed on the endpoint
   itself.  The links and link parameters returned are equal to the
   submitted ones except for anchor, which was resolved by the server
   against the endpoint's context.

   Links that did not have an anchor attribute are therefore returned
   with the (explicitly or implicitly set) context URI of the
   registration as the anchor.  Links whose anchor was submitted as an
   absolute URI are returned as they were registered.  The hrefs of
   links can always be served as they were submitted; the server MAY
   return relative references in absolute form in to resource lookups,
   but that results in needlessly verbose responses.

   Above rules allow the client to interpret the response as links
   without any further knowledge of what the RD does.  The Resource
   Directory MAY replace the contexts with a configured intermediate
   proxy, e.g. in the case of an HTTP lookup interface for CoAP
   endpoints.

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7.2.  Endpoint and group lookup

   Endpoint and group lookups result in links to registration resources
   and group resources, respectively.  Endpoint registration resources
   are annotated with their endpoint names (ep), domains (d, if
   present), context (con) and lifetime (lt, if present).  Additional
   endpoint attributes are added as link attributes to their endpoint
   link unless their specification says otherwise.  Group resources are
   annotated with their group names (gp), domain (d, if present) and
   multicast address (con, if present).

   While Endpoint Lookup does expose the registration resources, the RD
   does not need to make them accessible to clients.  Clients SHOULD NOT
   attempt to dereference or manipulate them.

7.3.  Lookup filtering

   Using the Accept Option, the requester can control whether this 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").

   The page and count parameters are used to obtain lookup results in
   specified increments using pagination, where count specifies how many
   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 of 10 and page of 0 will
   return the first 10 links in the result set (links 0-9).  Count = 10
   and page = 1 will return the next 'page' containing links 10-19, and
   so on.

   Multiple search criteria MAY be included in a lookup.  All included
   criteria MUST match for a link to be returned.

   A link matches a search criterion if it has an attribute of the same
   name and the same value, allowing for a trailing "*" wildcard
   operator as in Section 4.1 of [RFC6690].  Attributes that are defined
   as "link-type" match if the search value matches any of their values
   (see Section 4.1 of [RFC6690]; eg. "?if=core.s" matches ";if="abc
   core.s";").  A link also matches a search criterion if the link that
   would be produced for any of its containing entities would match the
   criterion: A search criterion matches an endpoint if it matches the
   endpoint itself or any of the groups it is contained in, and one on a
   resource if it matches the resource, the resource's endpoint, or any
   of the endpoint's groups.

   Note that "href" is also a valid search criterion and matches target
   references.  Like all search criteria, on a resource lookup it can

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   match the target reference of the resource link itself, but also the
   registration resource of the endpoint that registered it, or any
   group resource that endpoint is contained in.

   Clients that are interested in a 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 collections.  These are advertised,
   detected and used according to their own specifications and can be
   used with the lookup interface as with any other resource.

   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 URI for a given lookup type
         (mandatory).  The address is discovered as described in
         Section 5.2.

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

      page :=  Page (optional).  Parameter can not 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 is limited to this
         parameter value.  If the page parameter is also present, the
         response MUST only include 'count' links starting with the
         (page * count) link in the result set from the query.  If the
         count parameter is not present, then the response MUST return
         all matching links in 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)

   The following responses codes are defined for this interface:

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   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) or "[]" in the respective content format), indicating
      that no entities matched the request.

   Failure:  No error response to a multicast request.

   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

7.4.  Lookup examples

   The examples in this section assume CoAP hosts with a default CoAP
   port 61616.  HTTP hosts are possible and do not change the nature of
   the examples.

   The following example shows a client performing a resource lookup
   with the example resource look-up locations discovered in Figure 6:

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

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

   The same lookup using 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:
81A301652F74656D70096B74656D706572617475726503781E636F61703A2F2F5B323030
313A6462383A333A3A3132335D3A3631363136
Decoded payload:
[{1: "/temp", 9: "temperature", 3: "coap://[2001:db8:3::123]:61616"}]

   A client that wants to be notified of new resources as they show up
   can use observation:

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   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:
   </west>;rt="light";anchor="coap://[2001:db8:3::124]",
   </south>;rt="light";anchor="coap://[2001:db8:3::124]",
   </east>;rt="light";anchor="coap://[2001:db8:3::124]"

   The following example shows a client performing an endpoint type
   lookup:

   Req: GET /rd-lookup/ep?et=power-node

   Res: 2.05 Content
   </rd/1234>;con="coap://[2001:db8:3::127]:61616";ep="node5";
   et="power-node";ct="40";lt="600",
   </rd/4521>;con="coap://[2001:db8:3::129]:61616";ep="node7";
   et="power-node";ct="40";lt="600";d="floor-3"

   The following example shows a client performing a group lookup for
   all groups:

Req: GET /rd-lookup/gp

Res: 2.05 Content
</rd-group/1>;gp="lights1";d="example.com";con="coap://[ff35:30:2001:db8::1]",
</rd-group/2>;gp="lights2";d="example.com";con="coap://[ff35:30:2001:db8::2]"

   The following example shows a client performing a lookup for all
   endpoints in a particular group:

Req: GET /rd-lookup/ep?gp=lights1

Res: 2.05 Content
</rd/abcd>;con="coap://[2001:db8:3::123]:61616";ep="node1";et="power-node";ct="40";lt="600",
</rd/efgh>;con="coap://[2001:db8:3::124]:61616";ep="node2";et="power-node";ct="40";lt="600"

   The following example shows a client performing a lookup for all
   groups the endpoint "node1" belongs to:

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   Req: GET /rd-lookup/gp?ep=node1

   Res: 2.05 Content
   </rd-group/1>;gp="lights1"

   The following example shows a client performing a paginated resource
   lookup

   Req: GET /rd-lookup/res?page=0&count=5

   Res: 2.05 Content
   </res/0>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
   </res/1>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
   </res/2>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
   </res/3>;rt=sensor;ct=60;anchor="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
   </res/5>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
   </res/6>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
   </res/7>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
   </res/8>;rt=sensor;ct=60;anchor="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 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 the link targets stay unmodified, but the anchors
   get constructed by the resource directory:

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  Req: GET /rd-lookup/res?et=sensor-node

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

8.  Security Considerations

   The security considerations as described in Section 7 of [RFC5988]
   and Section 6 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 be used on
   all resource directory interfaces defined in this document.

8.1.  Endpoint Identification and Authentication

   An Endpoint is determined to be unique within (the domain of) an RD
   by the Endpoint identifier parameter included during Registration,
   and any associated TLS or DTLS security bindings.  An Endpoint MUST
   NOT be identified by its protocol, port or IP address as these may
   change over the lifetime of an Endpoint.

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

   Consider te following threat: two devices A and B are managed by 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 TLS exchange.  Then, it puts

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   the endpoint name of device B.  If the server does not check whether
   the identifier provided in the DTLS handshake matches the identifier
   used at the CoAP layer then it may be inclined to use the endpoint
   name for looking up what information to provision to the malicious
   device.

   Therfore, Endpoints MUST include the Endpoint identifier in the
   message, and this identifier MUST be checked by a resource directory
   to match the Endpoint identifier included in the Registration
   message.

8.2.  Access Control

   Access control SHOULD be performed separately for the RD
   registration, Lookup, and group API paths, as different endpoints may
   be authorized to register with an RD from those authorized to lookup
   endpoints from the RD.  Such access control SHOULD be performed in as
   fine-grained a level as possible.  For example access control for
   lookups could be performed either at the domain, endpoint or resource
   level.

8.3.  Denial of Service Attacks

   Services that run over UDP unprotected are vulnerable to unknowingly
   become part of a DDoS attack as UDP does not require return
   routability check.  Therefore, an attacker can easily spoof the
   source IP of the target entity and send requests to such 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 a response that is order of magnitudes larger than the
   request, the situation becomes even worse 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 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

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9.1.  Resource Types

   "core.rd", "core.rd-group", "core.rd-lookup-ep", "core.rd-lookup-
   res", and "core.rd-lookup-gp" resource types need to be registered
   with the resource type registry defined by [RFC6690].

9.2.  IPv6 ND Resource Directory Address Option

   This document registers one new ND option type under the subregistry
   "IPv6 Neighbor Discovery Option Formats":

   o  Resource Directory address Option (38)

9.3.  RD Parameter Registry

   This specification defines a new sub-registry for registration and
   lookup parameters called "RD Parameters" under "CoRE Parameters".
   Although this specification defines a basic set of parameters, it is
   expected that other standards that make use of this interface will
   define new ones.

   Each entry in the registry must include * the human readable name of
   the parameter, * the short name as used in query parameters or link
   attributes, * indication of whether it can be passed as a query
   parameter at registration of endpoints or groups, as a query
   parameter in lookups, or be expressed as a link attribute, * validity
   requirements if any, and * a description.

   The query parameter MUST be both a valid URI query key [RFC3986] and
   a parmname as used in [RFC5988].

   The description must give details on which registrations they apply
   to (Endpoint, group registrations or both?  Can they be updated?),
   and how they are to be processed in lookups.

   The mechanisms around new RD parameters should be designed in such a
   way that they tolerate RD implementations that are unaware of 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, the registering endpoint should
   be instructed to only set that parameter if the RD advertises support
   for keeping it confidential at the discovery step.)

   Initial entries in this sub-registry are as follows:

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   +----------+-------+---------------+-----+--------------------------+
   | Full     | Short | Validity      | Use | Description              |
   | name     |       |               |     |                          |
   +----------+-------+---------------+-----+--------------------------+
   | Endpoint | ep    |               | RLA | Name of the endpoint,    |
   | Name     |       |               |     | max 63 bytes             |
   | Lifetime | lt    | 60-4294967295 | RLA | Lifetime of the          |
   |          |       |               |     | registration in seconds  |
   | Domain   | d     |               | RLA | Domain to which this     |
   |          |       |               |     | endpoint belongs         |
   | Context  | con   | URI           | RLA | The scheme, address and  |
   |          |       |               |     | port and path at which   |
   |          |       |               |     | this server is available |
   | Group    | gp    |               | RLA | Name of a group in the   |
   | Name     |       |               |     | RD                       |
   | Page     | page  | Integer       | L   | Used for pagination      |
   | Count    | count | Integer       | L   | Used for pagination      |
   | Endpoint | et    |               | RLA | Semantic name 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 for the options defined in this document are only
   summarized here.  To which registrations they apply and when they are
   to be shown is described in the respective sections of this document.

   The IANA policy for future additions 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 (Eg. is the
   described property actually a property of the endpoint and not a
   property of a particular resource, in which case it should go into
   the payload of the registration and need not be registered?), and 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 to be used in lookup or attributes, but
   would make a bad parameter for lookup, because a resource lookup with
   an "if" query parameter could ambiguously filter by the registered
   endpoint property or the [RFC6690] link attribute).  It is expected
   that the registry will receive between 5 and 50 registrations in
   total over the next years.

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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 are described with Resource Types in
   [RFC6690].  An endpoint type is expressed as a string, which can be
   either a URI or one of the values defined in the Endpoint Type
   subregistry.  Endpoint types can be passed in the "et" query
   parameter as part of extra-attrs at the Registration step, are shown
   on endpoint lookups using 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 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 in the lookup interfaces from the rules for unknown
   endpoint attributes.

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

   This specification establishes 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 the Resource Type parameters in [RFC6690], in
   short:

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

   The requirements to be enforced are:

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

   o  The registered values MUST conform to the ABNF reg-rel-type
      definition of [RFC6690] and MUST NOT be a URI.

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

   The registry is initially empty.

10.  Examples

   Two examples are presented: a Lighting Installation example in
   Section 10.1 and a LWM2M example in Section 10.2.

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10.1.  Lighting Installation

   This example shows a simplified lighting installation which makes use
   of the Resource Directory (RD) with a CoAP interface to facilitate
   the 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.  No
   conclusions must be drawn on the realization of actual installation
   or naming procedures, because the example only "emphasizes" some of
   the issues that may influence the use of the RD and does not pretend
   to be normative.

10.1.1.  Installation Characteristics

   The example assumes that the installation is 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 connected
   to the installation in many ways: the CT can be part of the
   installation network, connected by WiFi to the installation network,
   or connected via GPRS link, or other method.

   It is assumed that there are two naming authorities for the
   installation: (1) the network manager that is responsible for the
   correct operation of the network and the connected interfaces, and
   (2) the lighting manager that is responsible for the correct
   functioning of networked lights 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 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 to modify the settings of a lamp in a
   luminary.  The purpose of the installation is that the presence
   sensor notifies the presence of persons to a group of lamps.  The
   group of lamps consists of: middle and left lamps of luminary1 and
   right lamp of luminary2.

   Before commissioning by the lighting manager, the network is
   installed and access to the interfaces is proven to work by the
   network manager.

   At the moment of installation, the network under installation is not
   necessarily connected to the DNS infra structure.  Therefore, SLAAC
   IPv6 addresses are assigned to CT, RD, luminaries and sensor shown in
   Table 3 below:

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                  +--------------------+----------------+
                  | 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 the DNS infrastructure is not always
   possible during installation.  Therefore, the SLAAC addresses are
   used in this section.

   For discovery, the 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      |
   | sensor         |                  |               |               |
   +----------------+------------------+---------------+---------------+

                  Table 4: Resource Directory identifiers

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   It is assumed that the CT knows of the RD's address, and has
   performed URI discovery on it that gave a response like the one in
   the Section 5.2 example.

   The CT inserts the endpoints of the luminaries and the sensor in the
   RD using the Context parameter (con) to specify the interface
   address:

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_wndw&con=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: /rd/4521

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_door&con=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: /rd/4522

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

   Res: 2.01 Created
   Location: /rd/4523

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

   The group is specified in the RD.  The Context parameter is set to
   the site-local multicast address allocated to the group.  In the POST
   in the example below, these two endpoints and the endpoint of the
   presence sensor are registered as members of the group.

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   Req: POST coap://[2001:db8:4::ff]/rd-group
   ?gp=grp_R2-4-015&con=coap://[ff05::1]
   Payload:
   </rd/4521>,
   </rd/4522>,
   </rd/4523>

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

   After the filling of the RD by the CT, the application in the
   luminaries can learn to which groups they belong, and enable their
   interface for the multicast address.

   The luminary, knowing its domain, queries the RD for the endpoint
   with rt=light and d=R2-4-015.  The RD returns all endpoints in the
   domain.

   Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
     ?d=R2-4-015;rt=light

   Res: 2.05 Content
   </rd/4521>;con="coap://[2001:db8:4::1]",
     ep="lm_R2-4-015_wndw",
   </rd/4522>;con="coap://[2001:db8:4::2]",
      ep="lm_R2-4-015_door"

   Knowing its own IPv6 address, the luminary discovers its endpoint
   name.  With the endpoint name the luminary queries the RD for all
   groups to which the endpoint belongs.

   Req: GET coap://[2001:db8:4::ff]/rd-lookup/gp
     ?ep=lm_R2-4-015_wndw

   Res: 2.05 Content
   </rd-group/501>;gp="grp_R2-4-015";con="coap://[ff05::1]"

   From the context parameter value, the luminary learns the multicast
   address of the multicast group.

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

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   Req: POST //[2001:db8:4::1]/coap-group
             Content-Format: application/coap-group+json
          { "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 the coap-group resource.

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 and operations for device management and device
   service enablement.

   An LWM2M 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 for registration domains and does not
   currently use the rd-group or rd-lookup interfaces.

   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 for
   LWM2M.

   The location of the LWM2M Server and RD URI path is provided by the
   LWM2M Bootstrap process, so no dynamic discovery of the RD is used.
   LWM2M Servers and endpoints are not required to implement the /.well-
   known/core resource.

10.2.1.  The LWM2M Object Model

   The OMA LWM2M object model is based on a simple 2 level class
   hierarchy consisting of Objects and Resources.

   An LWM2M Resource is a REST endpoint, allowed to be a single value or
   an array of values of the same data type.

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   An LWM2M Object is a resource template and container type that
   encapsulates a set of related resources.  An LWM2M Object represents
   a specific type of information source; for example, there is a LWM2M
   Device Management object that represents a network connection,
   containing resources that represent individual properties like radio
   signal strength.

   Since there may potentially be more than one of a given type object,
   for example more than one network connection, LWM2M defines instances
   of objects that contain the resources that represent a specific
   physical thing.

   The URI template for LWM2M consists of 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
   the special value "undefined" (sometimes called "null" in RFC 6570).
   Each of the variables object-instance, resource-id, and resource-
   instance can be the special value "undefined" only if the values
   behind it in 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 decimal (no
   leading zeroes except for the value 0) by URI format strings.  For
   example, a LWM2M URI might be:

   /1/0/1

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   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 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 its resources are available for
   management and 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 LWM2M Version are mandatory parameters
   for the register operation, all other registration parameters are
   optional.

   Additional optional LWM2M registration parameters are defined:

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   +-----------+-------+-------------------------------+---------------+
   | 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
   con - Context

   The endpoint registration must include a payload containing links to
   all supported objects and 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 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 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 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

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   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 de-registration using the delete method on the
   returned location from the initial registration operation.  LWM2M de-
   registration proceeds as described in 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 Jan Newmarch have
   provided helpful comments, discussions and ideas to improve and 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 -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 appendix on the 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 same across all
      entities

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   o  removed all methods that modify links 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  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 "example use" of the base RD resource values /rd,
      /rd-lookup, and /rd-group.

   o  changed "ins" ABNF notation.

   o  various editorial improvements, including in examples

   o  clarifications for RDAO

   changes from -07 to -08

   o  removed link target value returned from domain and group lookup
      types

   o  Maximum length of domain parameter 63 bytes for consistency with
      group

   o  removed option for simple POST of link data, don't require a
      .well-known/core resource to accept POST data and handle it in a
      special way; we already have /rd for that

   o  add IPv6 ND Option for discovery of an RD

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   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 discovery section to allow content format hints
      to be exposed in the 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 PATCH section is contingent on the progress 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 OMA LWM2M

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   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 registration update to a POST with optional link
      format payload.  Removed the endpoint type update from the update.

   o  Additional examples section added for more complex use cases.

   o  New DNS-SD mapping section.

   o  Added text on endpoint identification and authentication.

   o  Error code 4.04 added to Registration Update and Delete requests.

   o  Made 63 bytes a SHOULD rather than a MUST for endpoint name 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.

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   o  Minor editing, bug fixes and reference updates.

   o  Added group support.

   o  Changed rt to et for the registration 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 the use of ETags.

   o  Fixed tickets 383 and 372

   Changes from -02 to -03:

   o  Changed the endpoint name back to a single registration parameter
      ep= and removed the h= and ins= parameters.

   o  Updated REST interface descriptions to use RFC6570 URI Template
      format.

   o  Introduced an improved RD Lookup design as 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 update to a
      MAY.

   o  Added the concept of an RD Domain and a registration parameter for
      it.

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

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   o  Changed the lookup interface to accept endpoint and Domain as
      query string parameters to control the scope of a lookup.

13.  References

13.1.  Normative References

   [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-09 (work in
              progress), July 2017.

   [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>.

   [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.,
              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>.

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   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

13.2.  Informative References

   [ER]       Chen, P., "The entity-relationship model---toward 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., and Z. Shelby, "Uniform Resource
              Names for Device Identifiers", draft-arkko-core-dev-urn-04
              (work in progress), July 2017.

   [I-D.nottingham-rfc5988bis]
              Nottingham, M., "Web Linking", draft-nottingham-
              rfc5988bis-08 (work in progress), August 2017.

   [I-D.silverajan-core-coap-protocol-negotiation]
              Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
              draft-silverajan-core-coap-protocol-negotiation-07 (work
              in progress), October 2017.

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

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., 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. 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>.

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   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              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 in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015, <https://www.rfc-
              editor.org/info/rfc7641>.

Appendix A.  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
   [RFC5988] 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.

A.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 without any shortcuts
   are:

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A.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 to resolve that against is, in absence
   of an "anchor" parameter, the URI of the requested resource as
   described in [RFC6690] Section 2.1.

   The URI of the requested resource can be composed 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"".

   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"".

A.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 the context
   resource has a named relation to the target resource, like "_This
   page_ has _its table of contents_ at _/toc.html_".  In [RFC6690]
   link-format documents, there is an implicit "host relation" specified
   with default parameter: rel="hosts".

   In our example, the context of the link is the URI of the requested
   document itself.  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.'

A.2.  A slightly more complex example

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

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   </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,
   <t123.pdf>;rel=alternate;ct=65001;
       anchor="http://www.example.com/sensors/t123"

   Parsing the third record, the client encounters the "anchor"
   parameter.  It is a URI relative to the document's Base URI 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 document Base URI any more, but about the target
   and that address.

   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"".

   The fourth record can be read as ""coap://[2001:db8:f0::1]/sensors/
   temp" is described by "http://www.example.com/sensors/t123""

   In the last example the anchor is absolute, where a ""t123.pdf"" is
   resolved relative to ""http://www.example.com/sensors/t123"", which
   gives a statement that ""http://www.example.com/sensors/t123/
   t123.pdf" is an alternate representation to
   ""http://www.example.com/sensors/t123" of which the content type is
   PDF".

A.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 86400 seconds, and
   the endpoint registration Base URI is ""coap://[2001:db8:f0::1]/""

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   because that is the address the registration was sent from (and no
   explicit "con=" was given).

   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:

       </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 would contain the (almost) same
   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 subject of ongoing discussion about RFC6690).

   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

   </temp>;rt=temperature;ct=0;anchor="coap://[2001:db8:f0::1]",
   </light>;rt=light-lux;ct=0;anchor="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,
   <t123.pdf>;rel=alternate;ct=65001;
       anchor="http://www.example.com/sensors/t123"

   Note that the last link was not modified at all because its anchor
   was already an absolute reference.

   Had the simple host registered with an explicit context (eg.
   "?ep=simple-host1&con=coap+tcp://simple-host1.example.com"), that
   context would have been used to resolve the relative anchor values
   instead, giving

</temp>;rt=temperature;ct=0;anchor="coap+tcp://simple-host1.example.com"

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   and analogous records.

A.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] that should be kept in mind when using or
   implementing a Resource Directory:

   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 explictly 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""

   o  In a link-format document, if the anchor attribute is present, the
      link target reference is resolved by using the the (resolved)
      anchor value as Base URI for that link, while in Link headers, it
      is resolved against the URI of the requested document.

      This is explicit in [RFC6690] section 2.1 for link-format, and
      spelled out in section B.2 of [I-D.nottingham-rfc5988bis] , which
      obsoletes the older [RFC5988].  [RFC6690] is based on [RFC5988]
      and has not been updated with clarifications from
      [I-D.nottingham-rfc5988bis].

Appendix B.  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.

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   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:

   Req: POST coap://rd.example.com/rd?ep=node1
       &at=coap+tcp://[2001:db8:f1::2]
       &at=coap://[2001:db8:f1::2]
   Content-Format: 40
   Payload:
   </temperature>;ct=0;rt="temperature";if="core.s"

   Res: 2.01 Created
   Location: /rd/1234

   A UDP client would then query:

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

   Res: 2.05 Content
   </temperature>;ct=0;rt="temperature";if="core.s";
       anchor="coap://[2001:db8:f1::2]"

   while a TCP capable client could say:

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

   Res: 2.05 Content
   </temperature>;ct=0;rt="temperature";if="core.s";
       anchor="coap+tcp://[2001:db8:f1::2]"

Authors' Addresses

   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   USA

   Phone: +1-408-203-9434
   Email: zach.shelby@arm.com

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   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)
   Energy Harvesting Solutions
   Hollandstr. 12/4
   1020
   Austria

   Phone: +43-664-9790639
   Email: c.amsuess@energyharvesting.at

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