CoRE Resource Directory
draft-ietf-core-resource-directory-28

Document Type Active Internet-Draft (core WG)
Authors Christian Amsüss  , Zach Shelby  , Michael Koster  , Carsten Bormann  , Peter Van der Stok 
Last updated 2021-05-05 (latest revision 2021-03-07)
Replaces draft-shelby-core-resource-directory
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Details
CoRE                                                      C. Amsüss, Ed.
Internet-Draft                                                          
Intended status: Standards Track                               Z. Shelby
Expires: 8 September 2021                                            ARM
                                                               M. Koster
                                                             SmartThings
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                         P. van der Stok
                                                              consultant
                                                            7 March 2021

                        CoRE Resource Directory
                 draft-ietf-core-resource-directory-28

Abstract

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

Note to Readers

   Discussion of this document takes place on the CORE Working Group
   mailing list (core@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/core/
   (https://mailarchive.ietf.org/arch/browse/core/).

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/resource-directory (https://github.com/
   core-wg/resource-directory).

Status of This Memo

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

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   Internet-Drafts are working documents of the Internet Engineering
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   working documents as Internet-Drafts.  The list of current Internet-
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 September 2021.

Copyright Notice

   Copyright (c) 2021 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 (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture and Use Cases  . . . . . . . . . . . . . . . . .   6
     3.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  RD Content Model  . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Link-local addresses and zone identifiers . . . . . . . .  12
     3.5.  Use Case: Cellular M2M  . . . . . . . . . . . . . . . . .  12
     3.6.  Use Case: Home and Building Automation  . . . . . . . . .  13
     3.7.  Use Case: Link Catalogues . . . . . . . . . . . . . . . .  14
   4.  RD discovery and other interface-independent components . . .  14
     4.1.  Finding a Resource Directory  . . . . . . . . . . . . . .  15
       4.1.1.  Resource Directory Address Option (RDAO)  . . . . . .  17
       4.1.2.  Using DNS-SD to discover a Resource Directory . . . .  19
     4.2.  Payload Content Formats . . . . . . . . . . . . . . . . .  19
     4.3.  URI Discovery . . . . . . . . . . . . . . . . . . . . . .  19
   5.  Registration  . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.1.  Simple Registration . . . . . . . . . . . . . . . . . . .  27
     5.2.  Third-party registration  . . . . . . . . . . . . . . . .  29
     5.3.  Operations on the Registration Resource . . . . . . . . .  30

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       5.3.1.  Registration Update . . . . . . . . . . . . . . . . .  30
       5.3.2.  Registration Removal  . . . . . . . . . . . . . . . .  34
       5.3.3.  Further operations  . . . . . . . . . . . . . . . . .  34
       5.3.4.  Request freshness . . . . . . . . . . . . . . . . . .  35
   6.  RD Lookup . . . . . . . . . . . . . . . . . . . . . . . . . .  37
     6.1.  Resource lookup . . . . . . . . . . . . . . . . . . . . .  37
     6.2.  Lookup filtering  . . . . . . . . . . . . . . . . . . . .  38
     6.3.  Resource lookup examples  . . . . . . . . . . . . . . . .  40
     6.4.  Endpoint lookup . . . . . . . . . . . . . . . . . . . . .  42
   7.  Security policies . . . . . . . . . . . . . . . . . . . . . .  43
     7.1.  Endpoint name . . . . . . . . . . . . . . . . . . . . . .  44
       7.1.1.  Random endpoint names . . . . . . . . . . . . . . . .  44
     7.2.  Entered resources . . . . . . . . . . . . . . . . . . . .  44
     7.3.  Link confidentiality  . . . . . . . . . . . . . . . . . .  45
     7.4.  Segmentation  . . . . . . . . . . . . . . . . . . . . . .  46
     7.5.  First-Come-First-Remembered: A default policy . . . . . .  46
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
     8.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  48
     8.2.  Endpoint Identification and Authentication  . . . . . . .  48
     8.3.  Access Control  . . . . . . . . . . . . . . . . . . . . .  49
     8.4.  Denial of Service Attacks . . . . . . . . . . . . . . . .  49
     8.5.  Skipping freshness checks . . . . . . . . . . . . . . . .  50
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  50
     9.1.  Resource Types  . . . . . . . . . . . . . . . . . . . . .  50
     9.2.  IPv6 ND Resource Directory Address Option . . . . . . . .  51
     9.3.  RD Parameter Registry . . . . . . . . . . . . . . . . . .  51
       9.3.1.  Full description of the "Endpoint Type" RD
               Parameter . . . . . . . . . . . . . . . . . . . . . .  54
     9.4.  "Endpoint Type" (et=) RD Parameter values . . . . . . . .  54
     9.5.  Multicast Address Registration  . . . . . . . . . . . . .  55
     9.6.  Well-Known URIs . . . . . . . . . . . . . . . . . . . . .  55
     9.7.  Service Names and Transport Protocol Port Number
           Registry  . . . . . . . . . . . . . . . . . . . . . . . .  55
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  56
     10.1.  Lighting Installation  . . . . . . . . . . . . . . . . .  56
       10.1.1.  Installation Characteristics . . . . . . . . . . . .  56
       10.1.2.  RD entries . . . . . . . . . . . . . . . . . . . . .  57
     10.2.  OMA Lightweight M2M (LwM2M)  . . . . . . . . . . . . . .  60
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  61
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  61
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  76
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  76
     13.2.  Informative References . . . . . . . . . . . . . . . . .  77
   Appendix A.  Groups Registration and Lookup . . . . . . . . . . .  80
   Appendix B.  Web links and the Resource Directory . . . . . . . .  82
     B.1.  A simple example  . . . . . . . . . . . . . . . . . . . .  82
       B.1.1.  Resolving the URIs  . . . . . . . . . . . . . . . . .  82
       B.1.2.  Interpreting attributes and relations . . . . . . . .  83

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     B.2.  A slightly more complex example . . . . . . . . . . . . .  83
     B.3.  Enter the Resource Directory  . . . . . . . . . . . . . .  84
     B.4.  A note on differences between link-format and Link header
           fields  . . . . . . . . . . . . . . . . . . . . . . . . .  86
   Appendix C.  Limited Link Format  . . . . . . . . . . . . . . . .  86
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  87

1.  Introduction

   In the work on Constrained RESTful Environments (CoRE), a REST
   architecture suitable for constrained nodes (e.g. with limited RAM
   and ROM [RFC7228]) and networks (e.g. 6LoWPAN [RFC4944]) has been
   established and is used in Internet-of-Things (IoT) or 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 [RFC8288].  The use of Web Linking for the
   description and discovery of resources hosted by constrained web
   servers is specified by the CoRE Link Format [RFC6690].  However,
   [RFC6690] only describes how to discover resources from the web
   server that hosts them by querying "/.well-known/core".  In many
   constrained scenarios, direct discovery of resources is not practical
   due to sleeping nodes, or networks where multicast traffic is
   inefficient.  These problems can be solved by employing an entity
   called a Resource Directory (RD), which contains information about
   resources held on other servers, allowing lookups to be performed for
   those resources.

   This document specifies the web interfaces that an RD supports for
   web servers to discover the RD and to register, maintain, lookup and
   remove information on resources.  Furthermore, new target attributes
   useful in conjunction with an RD 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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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   The term "byte" is used in its now customary sense as a synonym for
   "octet".

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

   This specification makes use of the following additional terminology:

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

   Resource Directory (RD)
      A web entity that stores information about web resources and
      implements the REST interfaces defined in this specification for
      discovery, for the creation, maintenance and removal of
      registrations, and for lookup of the registered resources.

   Sector
      In the context of an RD, a sector is a logical grouping of
      endpoints.

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

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

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

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

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

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

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

   Directory Resource
      A resource in the RD containing registration resources.

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

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

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

   RDAO
      Resource Directory Address Option.  A new IPv6 Neighbor Discovery
      option defined for announcing an RD's address.

3.  Architecture and Use Cases

3.1.  Principles

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

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   It provides information about resources hosted by other devices that
   could otherwise only be obtained by directly querying the /.well-
   known/core resource on these other devices, either by a unicast
   request or a multicast request.

   Information SHOULD only be stored in the RD if it can be obtained by
   querying the described device's /.well-known/core resource directly.

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

3.2.  Architecture

   The RD architecture is illustrated in Figure 1.  An RD is used as a
   repository of registrations describing resources hosted on other web
   servers, also called endpoints (EP).  An endpoint is a web server
   associated with a scheme, IP address and port.  A physical node may
   host one or more endpoints.  The RD implements a set of REST
   interfaces for endpoints to register and maintain RD registrations,
   and for endpoints to lookup resources from the RD.  An RD can be
   logically segmented by the use of Sectors.

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

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

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

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

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

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

                       Figure 1: The RD architecture.

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

3.3.  RD Content Model

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

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

   These models provide an abstract view of the information expressed in
   link-format documents and an RD.  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.

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

           Figure 2: ER Model of the content of /.well-known/core

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

   *  a set of links belonging to the hosting web server

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

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

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

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

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

      In link-format serialization, it is expressed in the "anchor"
      attribute and defaults to the Origin of the target (practically:
      the target with its path and later components removed)

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

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

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

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

                Figure 3: ER Model of the content of the RD

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

   *  0 to n Registrations of endpoints,

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   A registration is associated with one endpoint.  A registration
   defines a set of links as defined for /.well-known/core.  A
   Registration has six types of attributes:

   *  an endpoint name ("ep", a Unicode string) unique within a sector

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

   *  a lifetime ("lt"),

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

   *  optionally a sector ("d", a Unicode string)

   *  optional additional endpoint attributes (from Section 9.3)

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

   Links are modelled as they are in Figure 2.

3.4.  Link-local addresses and zone identifiers

   Registration Base URIs can contain link-local IP addresses.  To be
   usable across hosts, those cannot be serialized to contain zone
   identifiers (see [RFC6874] Section 1).

   Link-local addresses can only be used on a single link (therefore RD
   servers cannot announce them when queried on a different link), and
   lookup clients using them need to keep track of which interface they
   got them from.

   Therefore, it is advisable in many scenarios to use addresses with
   larger scope if available.

3.5.  Use Case: Cellular M2M

   Over the last few years, mobile operators around the world have
   focused on development of M2M solutions in order to expand the
   business to the new type of users: machines.  The machines are
   connected directly to a mobile network using an appropriate embedded
   wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing
   short and wide range wireless interfaces.  The ambition in such
   systems is to build them from reusable components.  These speed up

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   development and deployment, and enable shared use of machines across
   different applications.  One crucial component of such systems is the
   discovery of resources (and thus the endpoints they are hosted on)
   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 an RD, which is hosted by the
   mobile operator or somewhere in the cloud.  Periodically, these
   endpoints update their registration and may modify resources they
   offer.

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

3.6.  Use Case: Home and Building Automation

   Home and commercial building automation systems can benefit from the
   use of IoT 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.  Both can use the common RD infrastructure to
   establish device interactions efficiently, but can pick security
   policies suitable for their needs.

   Two phases can be discerned for a network servicing the system: (1)
   installation and (2) operation.  During the operational phase, the
   network is connected to the Internet with a Border Router (e.g. a
   6LoWPAN Border Router (6LBR), see [RFC6775]) and the nodes connected
   to the network can use the Internet services that are provided by the
   Internet Provider or the network administrator.  During the
   installation phase, the network is completely stand-alone, no Border
   Router is connected, and the network only supports the IP
   communication between the connected nodes.  The installation phase is

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   usually followed by the operational phase.  As an RD's operations
   work without hard dependencies on names or addresses, it can be used
   for discovery across both phases.

3.7.  Use Case: Link Catalogues

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

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

   Metadata in web link formats like [RFC6690] which may be internally
   stored as triples, or relation/attribute pairs providing metadata
   about resource links, need to be supported by RDs.  External
   catalogues that are represented in other formats may be converted to
   common web linking formats for storage and access by RDs.  Since it
   is common practice for these to be encoded in URNs [RFC8141], simple
   and lossless structural transforms should generally be sufficient to
   store external metadata in RDs.

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

4.  RD discovery and other interface-independent components

   This and the following sections define the required set of REST
   interfaces between an RD, endpoints and lookup clients.  Although the
   examples throughout these sections assume the use of CoAP [RFC7252],
   these REST interfaces can also be realized using HTTP [RFC7230].  The
   multicast discovery and simple registration operations are exceptions
   to that, as they rely on mechanisms unavailable in HTTP.  In all
   definitions in these sections, 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.

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

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

   REST clients (registrant-EPs and CTs during registration and
   maintenance, lookup clients, RD servers during simple registrations)
   must be prepared to receive any unsuccessful code and act upon it
   according to its definition, options and/or payload to the best of
   their capabilities, falling back to failing the operation if recovery
   is not possible.  In particular, they SHOULD retry the request upon
   5.03 (Service Unavailable; 503 in HTTP) according to the Max-Age
   (Retry-After in HTTP) option, and SHOULD fall back to link-format
   when receiving 4.15 (Unsupported Content-Format; 415 in HTTP).

   An RD MAY make the information submitted to it available to further
   directories (subject to security policies on link confidentiality),
   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.

4.1.  Finding a Resource Directory

   A (re-)starting device may want to find one or more RDs before it can
   discover their URIs.  Dependent on the operational conditions, one or
   more of the techniques below apply.

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

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

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

   3.  It may be configured to use a service discovery mechanism such as
       DNS-SD, as outlined in Section 4.1.2.

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

   1.  The IPv6 Neighbor Discovery option RDAO Section 4.1.1 can do
       that.

   2.  When DHCP is in use, this could be provided via a DHCP option (no
       such option is defined at the time of writing).

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

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

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

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

   When answering a multicast request directed at a link-local group,
   the RD may want to respond from a routable address; this makes it
   easier for registrants to use one of their own routable addresses for

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   registration.  When [RFC6724] is used for source address selection,
   this can be achieved by applying the changes of its Section 10.4,
   picking public addresses in its Section 5 Rule 7, and superseding
   rule 8 with preferring the source address's precedence.

   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.

   The following RD discovery mechanisms are recommended:

   *  In managed networks with border routers that need stand-alone
      operation, the RDAO option is recommended (e.g. operational phase
      described in Section 3.6).

   *  In managed networks without border router (no Internet services
      available), the use of a preconfigured anycast address is
      recommended (e.g. installation phase described in Section 3.6).

   *  In networks managed using DNS-SD, the use of DNS-SD for discovery
      as described in Section 4.1.2 is recommended.

   The use of multicast discovery in mesh networks is NOT RECOMMENDED.

4.1.1.  Resource Directory Address Option (RDAO)

   The Resource Directory Address Option (RDAO) carries information
   about the address of the RD in RAs (Router Advertisements) of IPv6
   Neighbor Discovery (ND), similar to how RDNSS options [RFC8106] are
   sent.  This information is needed when endpoints cannot discover the
   RD with a link-local or realm-local scope multicast address, for
   instance 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 RD addresses.

   The RDAO format is:

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   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   |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Valid Lifetime                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                          RD Address                           +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:                   TBD38

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

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

   Valid Lifetime:         32-bit unsigned integer.  The length of
                           time in seconds (relative to
                           the time the packet is received) that
                           this RD address is valid.
                           A value of all zero bits (0x0) indicates
                           that this RD address
                           is not valid anymore.

   RD Address:             IPv6 address of the RD.

                Figure 4: Resource Directory Address Option

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4.1.2.  Using DNS-SD to discover a Resource Directory

   An RD can advertise its presence in DNS-SD [RFC6763] using the
   service name "_core-rd._udp" (for CoAP), "_core-rd-dtls._udp" (for
   CoAP over DTLS), "_core-rd._tcp" (for CoAP over TCP) or "_core-rd-
   tls._tcp" (for CoAP over TLS) defined in this document.  (For the
   WebSocket transports of CoAP, no service is defined as DNS-SD is
   typically unavailable in environments where CoAP over WebSockets is
   used).

   The selection of the service indicates the protocol used, and the SRV
   record points the client to a host name and port to use as a starting
   point for the URI discovery steps of Section 4.3.

   This section is a simplified concrete application of the more generic
   mechanism specified in [I-D.ietf-core-rd-dns-sd].

4.2.  Payload Content Formats

   RDs implementing this specification MUST support the application/
   link-format content format (ct=40).

   RDs implementing this specification MAY support additional content
   formats.

   Any additional content format supported by an RD implementing this
   specification SHOULD be able to express all the information
   expressible in link-format.  It MAY be able to express information
   that is inexpressible in link-format, but those expressions SHOULD be
   avoided where possible.

4.3.  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] after having
   discovered a host as described in Section 4.1.

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

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   performing multicast discovery, the multicast IP address used will
   depend on the scope required and the multicast capabilities of the
   network (see Section 9.5).

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

   HTTP does not support multicast and consequently only unicast
   discovery can be supported at the using the HTTP "/.well-known/core"
   resource.

   RDs implementing this specification MUST support query filtering for
   the rt parameter as defined in [RFC6690].

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

   With security policies where the client requires the RD to be
   authorized to act as an RD, that authorization may be limited to
   resources on which the authorized RD advertises the adequate resource
   types.  Clients that have obtained links they can not rely on yet can
   repeat the URI discovery step at the /.well-known/core resource of
   the indicated host to obtain the resource type information from an
   authorized source.

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

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

   Interaction:  EP, CT or Client -> RD

   Method:  GET

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

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

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

   The following response is expected on this interface:

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

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

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

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

                    Figure 5: Example discovery exchange

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

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

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

         Figure 6: Example discovery exchange indicating additional
                              content-formats

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   For maintenance, management and debugging, it can be useful to
   identify the components that constitute the RD server.  The
   identification can be used to find client-server incompatibilities,
   supported features, required updates and other aspects.  The Well-
   Known interface described in Section 4 of [RFC6690] can be used to
   find such data.

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

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

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

           Figure 7: Example exchange of obtaining implementation
       information, using the relation type currently proposed in the
                         work-in-progress document

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

5.  Registration

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

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

   *  When the (ep, d) value pair of the registration-request is
      different from any existing registration, a new registration is
      generated.

   *  When the (ep, d) value pair of the registration-request is equal
      to an existing registration, the content and parameters of the
      existing registration are replaced with the content of the
      registration request.  Like the later changes to registration
      resources, security policies (Section 7) usually require such
      requests to come from the same device.

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

   For media types to which Appendix C applies (i.e. documents in
   application/link-format), request bodies MUST be expressed in Limited
   Link Format.

   The registration request interface is specified as follows:

   Interaction:  EP or CT -> RD

   Method:  POST

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

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

                            ep :=  Endpoint name (mostly mandatory).
         The endpoint name is an identifier that MUST be unique within a
         sector.

         As the endpoint name is a Unicode string, it is encoded in
         UTF-8 (and possibly pct-encoded) during variable expansion (see
         [RFC6570] Section 3.2.1).  The endpoint name MUST NOT contain
         any character in the inclusive ranges 0-31 or 127-159.

         The maximum length of this parameter is 63 UTF-8 encoded bytes.

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         If the RD is configured to recognize the endpoint to be
         authorized to use exactly one endpoint name, the RD assigns
         that name.  In that case, giving the endpoint name becomes
         optional for the client; if the client gives any other endpoint
         name, it is not authorized to perform the registration.

                            d :=  Sector (optional).  The sector to
         which this endpoint belongs.  When this parameter is not
         present, the RD MAY associate the endpoint with a configured
         default sector (possibly based on the endpoint's authorization)
         or leave it empty.

         The sector is encoded like the ep parameter, and is limited to
         63 UTF-8 encoded bytes as well.

                            lt :=  Lifetime (optional).  Lifetime of the
         registration in seconds.  Range of 1-4294967295.  If no
         lifetime is included in the initial registration, a default
         value of 90000 (25 hours) SHOULD be assumed.

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

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

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

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

         A registrant that cannot be reached by potential lookup clients

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         at the address it registers from (e.g. because it is behind
         some form of Network Address Translation (NAT)) MUST provide a
         reachable base address with its registration.

         If the Base URI contains a link-local IP literal, it MUST NOT
         contain a Zone Identifier, and MUST be local to the link on
         which the registration request is received.

         Endpoints that register with a base that contains a path
         component cannot efficiently express their registrations in
         Limited Link Format (Appendix C).  Those applications should
         use different representations of links to which Appendix C is
         not applicable (e.g.  [I-D.hartke-t2trg-coral]).

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

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

   The following response is expected on this interface:

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

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

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

   If the registration fails, including request timeouts, or if delays
   from Service Unavailable responses with Max-Age or Retry-After
   accumulate to exceed the registrant's configured timeouts, it SHOULD

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   pick another registration URI from the "URI Discovery" step and if
   there is only one or the list is exhausted, pick other choices from
   the "Finding a Resource Directory" step.  Care has to be taken to
   consider the freshness of results obtained earlier, e.g. of the
   result of a "/.well-known/core" response, the lifetime of an RDAO
   option and of DNS responses.  Any rate limits and persistent errors
   from the "Finding a Resource Directory" step must be considered for
   the whole registration time, not only for a single operation.

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

   Req: POST coap://rd.example.com/rd?ep=node1
   Content-Format: 40
   Payload:
   </sensors/temp>;rt=temperature-c;if=sensor,
   <http://www.example.com/sensors/temp>;
     anchor="/sensors/temp";rel=describedby

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

                   Figure 8: Example registration payload

   An RD may optionally support HTTP.  Here is an example of almost the
   same registration operation above, when done using HTTP.

   Req:
   POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1
   Host: rd.example.com
   Content-Type: application/link-format

   </sensors/temp>;rt=temperature-c;if=sensor,
   <http://www.example.com/sensors/temp>;
     anchor="/sensors/temp";rel=describedby

   Res:
   HTTP/1.1 201 Created
   Location: /rd/4521

       Figure 9: Example registration payload as expressed using HTTP

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5.1.  Simple Registration

   Not all endpoints hosting resources are expected to know how to
   upload links to an RD as described in Section 5.  Instead, simple
   endpoints can implement the Simple Registration approach described in
   this section.  An RD implementing this specification MUST implement
   Simple Registration.  However, there may be security reasons why this
   form of directory discovery would be disabled.

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

   *  The registrant-ep finds one or more addresses of the directory
      server as described in Section 4.1.

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

      The registrant-ep includes the same registration parameters in the
      POST request as it would with a regular registration per
      Section 5.  The registration base URI of the registration is taken
      from the registrant-ep's network address (as is default with
      regular registrations).

      Example request from registrant-EP to RD (unanswered until the
      next step):

   Req: POST /.well-known/rd?lt=6000&ep=node1
   (No payload)

      Figure 10: First half example exchange of a simple registration

   *  The RD queries the registrant-ep's discovery resource to determine
      the success of the operation.  It SHOULD keep a cache of the
      discovery resource and not query it again as long as it is fresh.

      Example request from the RD to the registrant-EP:

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   Req: GET /.well-known/core
   Accept: 40

   Res: 2.05 Content
   Content-Format: 40
   Payload:
   </sen/temp>

     Figure 11: Example exchange of the RD querying the simple endpoint

   With this response, the RD would answer the previous step's request:

   Res: 2.04 Changed

      Figure 12: Second half example exchange of a simple registration

   The sequence of fetching the registration content before sending a
   successful response was chosen to make responses reliable, and the
   point about caching was chosen to still allow very constrained
   registrants.  Registrants MUST be able to serve a GET request to
   "/.well-known/core" after having requested registration.  Constrained
   devices MAY regard the initial request as temporarily failed when
   they need RAM occupied by their own request to serve the RD's GET,
   and retry later when the RD already has a cached representation of
   their discovery resources.  Then, the RD can reply immediately and
   the registrant can receive the response.

   The simple registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  /.well-known/rd{?ep,d,lt,extra-attrs*}

   URI Template Variables are as they are for registration in Section 5.
   The base attribute is not accepted to keep the registration interface
   simple; that rules out registration over CoAP-over-TCP or HTTP that
   would need to specify one.  For some time during this document's
   development, the URI template "/.well-known/core{?ep,...}" has been
   in use instead.

   The following response is expected on this interface:

   Success:  2.04 "Changed".

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

   Interaction:  RD -> EP

   Method:  GET

   URI Template:  /.well-known/core

   The following response is expected on this interface:

   Success:  2.05 "Content".

   When the RD uses any authorization credentials to access the
   endpoint's discovery resource, or when it is deployed in a location
   where third parties might reach it but not the endpoint, it SHOULD
   verify that the apparent registrant-ep intends to register with the
   given registration parameters before revealing the obtained discovery
   information to lookup clients.  An easy way to do that is to verify
   the simple registration request's sender address using the Echo
   option as described in [I-D.ietf-core-echo-request-tag] Section 2.4.

   The RD MUST delete registrations created by simple registration after
   the expiration of their lifetime.  Additional operations on the
   registration resource cannot be executed because no registration
   location is returned.

5.2.  Third-party registration

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

   In a controlled environment (e.g. building control), the RD can be
   filled by a third party device, called a Commissioning Tool (CT).
   The commissioning tool can fill the RD from a database or other
   means.  For that purpose scheme, IP address and port of the URI of
   the registered device is the value of the "base" parameter of the
   registration described in Section 5.

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

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5.3.  Operations on the Registration Resource

   This section describes how the registering endpoint can maintain the
   registrations that it created.  The registering endpoint can be the
   registrant-ep or the CT.  The registrations are resources of the RD.

   An endpoint should not use this interface for registrations that it
   did not create.  This is usually enforced by security policies, which
   in general require equivalent credentials for creation of and
   operations on a registration.

   After the initial registration, the registering endpoint retains 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 garbage collection.  If the
   registration resource is removed, the corresponding endpoint will
   need to be re-registered.

   The registration resource may also be used cancel the registration
   using DELETE, and to perform further operations beyond the scope of
   this specification.

   Operations on the registration resource are sensitive to reordering;
   Section 5.3.4 describes how order is restored.

   The operations on the registration resource are described below.

5.3.1.  Registration Update

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

   An update MAY update registration parameters like lifetime, base URI
   or others.  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 are included as query parameters in an
   update operation as in Section 5.

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   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 base URI of the registration is changed in an update, relative
   references submitted in the original registration or later updates
   are resolved anew against the new base.

   The registration update operation only describes the use of POST with
   an empty payload.  Future standards might describe the semantics of
   using content formats and payloads with the POST method to update the
   links of a registration (see Section 5.3.3).

   The update registration request interface is specified as follows:

   Interaction:  EP or CT -> RD

   Method:  POST

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

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

                            lt :=  Lifetime (optional).  Lifetime of the
         registration in seconds.  Range of 1-4294967295.  If no
         lifetime is included, the previous last lifetime set on a
         previous update or the original registration (falling back to
         90000) SHOULD be used.

                            base :=  Base URI (optional).  This
         parameter updates the Base URI established in the original
         registration to a new value, and is subject to the same
         restrictions as in the registration.

         If the parameter is set in an update, it is stored by the RD as
         the new Base URI under which to interpret the relative links
         present in the payload of the original registration.

         If the parameter is not set in the request but was set before,
         the previous Base URI value is kept unmodified.

         If the parameter is not set in the request and was not set
         before either, the source address and source port of the update
         request are stored as the Base URI.

                            extra-attrs :=  Additional registration

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         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.

         Note that this default behavior does not allow removing an
         endpoint attribute in an update.  For attributes whose
         functionality depends on the endpoints' ability to remove them
         in an update, it can make sense to define a value whose
         presence is equivalent to the absence of a value.  As an
         alternative, an extension can define different updating rules
         for their attributes.  That necessitates either discovery of
         whether the RD is aware of that extension, or tolerating the
         default behavior.

   Content-Format:  none (no payload)

   The following responses are expected on this interface:

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

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

   If the registration update fails in any way, including "Not Found"
   and request timeouts, or if the time indicated in a Service
   Unavailable Max-Age/Retry-After exceeds the remaining lifetime, the
   registering endpoint SHOULD attempt registration again.

   The following example shows how the registering endpoint resets the
   timeout on its registration resource at an RD using this interface
   with the example location value: /rd/4521.

   Req: POST /rd/4521

   Res: 2.04 Changed

                Figure 13: Example update of a registration

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

   *  endpoint name (ep)=endpoint1

   *  lifetime (lt)=500

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   *  Base URI (base)=coap://local-proxy-old.example.com

   *  payload of Figure 8

   The initial state of the RD is reflected in the following request:

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

   Res: 2.05 Content
   Payload:
   <coap://local-proxy-old.example.com/sensors/temp>;
       rt=temperature-c;if=sensor,
   <http://www.example.com/sensors/temp>;
       anchor="coap://local-proxy-old.example.com/sensors/temp";
       rel=describedby

       Figure 14: Example lookup before a change to the base address

   The following example shows the registering endpoint changing the
   Base URI to "coaps://new.example.com:5684":

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

   Res: 2.04 Changed

    Figure 15: Example registration update that changes the base address

   The consecutive query returns:

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

   Res: 2.05 Content
   Payload:
   <coaps://new.example.com/sensors/temp>;
       rt=temperature-c;if=sensor,
   <http://www.example.com/sensors/temp>;
       anchor="coaps://new.example.com/sensors/temp";
       rel=describedby

        Figure 16: Example lookup after a change to the base address

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5.3.2.  Registration Removal

   Although RD registrations have soft state and will eventually timeout
   after their lifetime, the registering endpoint SHOULD explicitly
   remove an 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.

   The removal request interface is specified as follows:

   Interaction:  EP or CT -> 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 are expected on this interface:

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

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

   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

                Figure 17: Example of a registration removal

5.3.3.  Further operations

   Additional operations on the registration can be specified in future
   documents, for example:

   *  Send iPATCH (or PATCH) updates ([RFC8132]) to add, remove or
      change the links of a registration.

   *  Use GET to read the currently stored set of links in a
      registration resource.

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   Those operations are out of scope of this document, and will require
   media types suitable for modifying sets of links.

5.3.4.  Request freshness

   Some security mechanisms usable with an RD allow out of order request
   processing, or do not even mandate replay protection at all.  The RD
   needs to ensure that operations on the registration resource are
   executed in an order that does not distort the client's intentions.

   This ordering of operations is expressed in terms of freshness as
   defined in [I-D.ietf-core-echo-request-tag].  Requests that alter a
   resource's state need to be fresh relative to the latest request that
   altered that state in a conflicting way.

   An RD SHOULD determine a request's freshness, and MUST use the Echo
   option if it requires request freshness and can not determine the it
   in any other way.  An endpoint MUST support the use of the Echo
   option.  (One reason why an RD would not require freshness is when no
   relevant registration properties are covered by is security
   policies.)

5.3.4.1.  Efficient use of Echo by an RD

   To keep latency and traffic added by the freshness requirements to a
   minimum, RDs should avoid naive (sufficient but inefficient)
   freshness criteria.

   Some simple mechanisms the RD can employ are:

   *  State counter.  The RD can keep a monotonous counter that
      increments whenever a registration changes.  For every
      registration resource, it stores the post-increment value of that
      resource's last change.  Requests altering them need to have at
      least that value encoded in their Echo option, and are otherwise
      rejected with a 4.01 Unauthorized and the current counter value as
      the Echo value.  If other applications on the same server use Echo
      as well, that encoding may include a prefix indicating that it
      pertains to the RD's counter.

      The value associated with a resource needs to be kept across the
      removal of registrations if the same registration resource is to
      be reused.

      The counter can be reset (and the values of removed resources
      forgotten) when all previous security associations are reset.

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      This is the "Persistent Counter" method of
      [I-D.ietf-core-echo-request-tag] Appendix A.

   *  Preemptive Echo values.  The current state counter can be sent in
      an Echo option not only when requests are rejected with 4.01
      Unauthorized, but also with successful responses.  Thus, clients
      can be provided with Echo values sufficient for their next request
      on a regular basis.

      While endpoints may discard received Echo values at leisure
      between requests, they are encouraged to retain these values for
      the next request to avoid additional round trips.

   *  If the RD can ensure that only one security association has
      modifying access to any registration at any given time, and that
      security association provides order on the requests, that order is
      sufficient to show request freshness.

5.3.4.2.  Examples of Echo usage

   Figure 18 shows the interactions of an endpoint that has forgotten
   the server's latest Echo value and temporarily reduces its
   registration lifetime:

   Req: POST /rd/4521?lt=7200

   Res: 4.01 Unauthorized
   Echo: 0x0123

   (EP tries again immediately)

   Req: POST /rd/4521?lt=7200
   Echo: 0x0123

   Res: 2.04 Changed
   Echo: 0x0124

   (Later the EP regains its confidence in its long-term reachability)

   Req: POST /rd/4521?lt=90000
   Echo: 0x0124

   Res: 2.04 Changed
   Echo: 0x0247

                Figure 18: Example update of a registration

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   The other examples do not show Echo options for simplicity, and
   because they lack the context for any example values to have meaning.

6.  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.  JSON or CBOR link format
   [I-D.ietf-core-links-json]) or using more advanced interfaces (e.g.
   supporting context or semantic based lookup) on different resources
   that are discovered independently.

   RD Lookup allows lookups for endpoints and resources using attributes
   defined in 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, 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 |
             +-------------+--------------------+-----------+

                          Table 1: Lookup Types

6.1.  Resource lookup

   Resource lookup results in links that are semantically equivalent to
   the links submitted to the RD by the registrant.  The links and link
   parameters returned by the lookup are equal to the originally
   submitted ones, except that the target reference is fully resolved,
   and that the anchor reference is fully resolved if it is present in
   the lookup result at all.

   Links that did not have an anchor attribute in the registration are
   returned without an anchor attribute.  Links of which href or anchor
   was submitted as a (full) URI are returned with the respective
   attribute unmodified.

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   The above rules allow the client to interpret the response as links
   without any further knowledge of the storage conventions of the RD.
   The RD MAY replace the registration base URIs with a configured
   intermediate proxy, e.g. in the case of an HTTP lookup interface for
   CoAP endpoints.

   If the base URI of a registration contains a link-local address, the
   RD MUST NOT show its links unless the lookup was made from the link
   on which the registered endpoint can be reached.  The RD MUST NOT
   include zone identifiers in the resolved URIs.

6.2.  Lookup filtering

   Using the Accept Option, the requester can control whether the
   returned list is returned in CoRE Link Format ("application/link-
   format", default) or in alternate content-formats (e.g. from
   [I-D.ietf-core-links-json]).

   Multiple search criteria MAY be included in a lookup.  All included
   criteria MUST match for a link to be returned.  The RD MUST support
   matching with multiple search criteria.

   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 "relation-types" (in the link-format ABNF) match if the search
   value matches any of their values (see Section 4.1 of [RFC6690]; e.g.
   "?if=tag:example.net,2020:sensor" matches ";if="example.regname
   tag:example.net,2020:sensor";").  A resource link also matches a
   search criterion if its endpoint would match the criterion, and vice
   versa, an endpoint link matches a search criterion if any of its
   resource links matches it.

   Note that "href" is a valid search criterion and matches target
   references.  Like all search criteria, on a resource lookup it can
   match the target reference of the resource link itself, but also the
   registration resource of the endpoint that registered it.  Queries
   for resource link targets MUST be in URI form (i.e. not relative
   references) and are matched against a resolved link target.  Queries
   for endpoints SHOULD be expressed in path-absolute form if possible
   and MUST be expressed in URI form otherwise; the RD SHOULD recognize
   either.  The "anchor" attribute is usable for resource lookups, and,
   if queried, MUST be in URI form as well.

   Additional query parameters "page" and "count" 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,

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   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.  Unlike block-wise transfer of a compelte
   result set, these parameters ensure that each chunk of results can be
   interpreted on its own.  This simplifies the processing, but can
   result in duplicate or missed items when coinciding with changes from
   the registration interface.

   Endpoints 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.

   When resource observation is used, every time the set of matching
   links changes, or the content of a matching link changes, the RD
   sends a notification with the matching link set.  The notification
   contains the successful current response to the given request,
   especially with respect to representing zero matching links (see
   "Success" item below).

   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 4.3.

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

         The search criteria are an associative array, expressed in a
         form-style query as per the URI template (see [RFC6570]
         Sections 2.4.2 and 3.2.8)

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

                            count :=  Count (optional).  Number of

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         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.

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

   The following responses codes are defined for this interface:

   Success:  2.05 "Content" or 200 "OK" with an "application/link-
      format" or other web link payload containing matching entries for
      the lookup.

      The payload can contain zero links (which is an empty payload in
      [RFC6690] link format, but could also be "[]" in JSON based
      formats), indicating that no entities matched the request.

6.3.  Resource lookup examples

   The examples in this section assume the existence of 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 5:

   Req: GET /rd-lookup/res?rt=tag:example.org,2020:temperature

   Res: 2.05 Content
   Payload:
   <coap://[2001:db8:3::123]:61616/temp>;
       rt="tag:example.org,2020:temperature"

                    Figure 19: Example a resource lookup

   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=tag:example.org,2020:light
   Observe: 0

   Res: 2.05 Content
   Observe: 23
   Payload: empty

   (at a later point in time)

   Res: 2.05 Content
   Observe: 24
   Payload:
   <coap://[2001:db8:3::124]/west>;rt="tag:example.org,2020:light",
   <coap://[2001:db8:3::124]/south>;rt="tag:example.org,2020:light",
   <coap://[2001:db8:3::124]/east>;rt="tag:example.org,2020:light"

              Figure 20: Example an observing resource lookup

   The following example shows a client performing a paginated resource
   lookup

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

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

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

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

              Figure 21: Examples of paginated resource lookup

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   The following example shows a client performing a lookup of all
   resources of all 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 response of section 5 of
   [RFC6690].

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

   Req: GET /rd-lookup/res?et=tag:example.com,2020:platform

   Res: 2.05 Content
   Payload:
   <coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index",
   <coap://sensor1.example.com/sensors/temp>;rt=temperature-c;if=sensor,
   <coap://sensor1.example.com/sensors/light>;rt=light-lux;if=sensor,
   <http://www.example.com/sensors/t123>;rel=describedby;
       anchor="coap://sensor1.example.com/sensors/temp",
   <coap://sensor1.example.com/t>;rel=alternate;
       anchor="coap://sensor1.example.com/sensors/temp",
   <coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index",
   <coap://sensor2.example.com/sensors/temp>;rt=temperature-c;if=sensor,
   <coap://sensor2.example.com/sensors/light>;rt=light-lux;if=sensor,
   <http://www.example.com/sensors/t123>;rel=describedby;
       anchor="coap://sensor2.example.com/sensors/temp",
   <coap://sensor2.example.com/t>;rel=alternate;
       anchor="coap://sensor2.example.com/sensors/temp"

       Figure 22: Example of resource lookup from multiple endpoints

6.4.  Endpoint lookup

   The endpoint lookup returns links to and information about
   registration resources, which themselves can only be manipulated by
   the registering endpoint.

   Endpoint registration resources are annotated with their endpoint
   names (ep), sectors (d, if present) and registration base URI (base;
   reports the registrant-ep's address if no explicit base was given) as
   well as a constant resource type (rt="core.rd-ep"); the lifetime (lt)
   is not reported.  Additional endpoint attributes are added as target
   attributes to their endpoint link unless their specification says
   otherwise.

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   Links to endpoints SHOULD be presented in path-absolute form or, if
   required, as (full) URIs.  (This ensures that the output conforms to
   Limited Link Format as described in Appendix C.)

   Base addresses that contain link-local addresses MUST NOT include
   zone identifiers, and such registrations MUST NOT be shown unless the
   lookup was made from the same link from which the registration was
   made.

   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.

   An RD can report registrations in lookup whose URI scheme and
   authority differ from the lookup resource's.  Lookup clients MUST be
   prepared to see arbitrary URIs as registration resources in the
   results and treat them as opaque identifiers; the precise semantics
   of such links are left to future specifications.

   The following example shows a client performing an endpoint lookup
   limited to endpoints of endpoint type
   "tag:example.com,2020:platform":

   Req: GET /rd-lookup/ep?et=tag:example.com,2020:platform

   Res: 2.05 Content
   Payload:
   </rd/1234>;base="coap://[2001:db8:3::127]:61616";ep=node5;
       et="tag:example.com,2020:platform";ct=40;rt=core.rd-ep,
   </rd/4521>;base="coap://[2001:db8:3::129]:61616";ep=node7;
       et="tag:example.com,2020:platform";ct=40;d=floor-3;
       rt=core.rd-ep

                   Figure 23: Examples of endpoint lookup

7.  Security policies

   The security policies that are applicable to an RD strongly depend on
   the application, and are not set out normatively here.

   This section provides a list of aspects that applications should
   consider when describing their use of the RD, without claiming to
   cover all cases.  It is using terminology of
   [I-D.ietf-ace-oauth-authz], in which the RD acts as the Resource
   Server (RS), and both registrant-eps and lookup clients act as
   Clients (C) with support from an Authorization Server (AS), without
   the intention of ruling out other (e.g. certificate / public-key
   infrastructure (PKI) based) schemes.

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   Any, all or none of the below can apply to an application.  Which are
   relevant depends on its protection objectives.

   Security policies are set by configuration of the RD, or by choice of
   the implementation.  Lookup clients (and, where relevant, endpoints)
   can only trust an RD to uphold them if it is authenticated, and
   authorized to serve as an RD according to the application's
   requirements.

7.1.  Endpoint name

   Whenever an RD needs to provide trustworthy results to clients doing
   endpoint lookup, or resource lookup with filtering on the endpoint
   name, the RD must ensure that the registrant is authorized to use the
   given endpoint name.  This applies both to registration and later to
   operations on the registration resource.  It is immaterial whether
   the client is the registrant-ep itself or a CT is doing the
   registration: The RD cannot tell the difference, and CTs may use
   authorization credentials authorizing only operations on that
   particular endpoint name, or a wider range of endpoint names.

   It is up to the concrete security policy to describe how endpoint
   name and sector are transported when certificates are used.  For
   example, it may describe how SubjectAltName dNSName entries are
   mapped to endpoint and domain names.

7.1.1.  Random endpoint names

   Conversely, in applications where the RD does not check the endpoint
   name, the authorized registering endpoint can generate a random
   number (or string) that identifies the endpoint.  The RD should then
   remember unique properties of the registrant, associate them with the
   registration for as long as its registration resource is active
   (which may be longer than the registration's lifetime), and require
   the same properties for operations on the registration resource.

   Registrants that are prepared to pick a different identifier when
   their initial attempt (or attempts, in the unlikely case of two
   subsequent collisions) at registration is unauthorized should pick an
   identifier at least twice as long as the expected number of
   registrants; registrants without such a recovery options should pick
   significantly longer endpoint names (e.g. using UUID URNs [RFC4122]).

7.2.  Entered resources

   When lookup clients expect that certain types of links can only
   originate from certain endpoints, then the RD needs to apply
   filtering to the links an endpoint may register.

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   For example, if clients use an RD to find a server that provides
   firmware updates, then any registrant that wants to register (or
   update) links to firmware sources will need to provide suitable
   credentials to do so, independently of its endpoint name.

   Note that the impact of having undesirable links in the RD depends on
   the application: if the client requires the firmware server to
   present credentials as a firmware server, a fraudulent link's impact
   is limited to the client revealing its intention to obtain updates
   and slowing down the client until it finds a legitimate firmware
   server; if the client accepts any credentials from the server as long
   as they fit the provided URI, the impact is larger.

   An RD may also require that links are only registered if the
   registrant is authorized to publish information about the anchor (or
   even target) of the link.  One way to do this is to demand that the
   registrant present the same credentials as a client that they'd need
   to present if contacted as a server at the resources' URI, which may
   include using the address and port that are part of the URI.  Such a
   restriction places severe practical limitations on the links that can
   be registered.

   As above, the impact of undesirable links depends on the extent to
   which the lookup client relies on the RD.  To avoid the limitations,
   RD applications should consider prescribing that lookup clients only
   use the discovered information as hints, and describe which pieces of
   information need to be verified because they impact the application's
   security.  A straightforward way to verify such information is to
   request it again from an authorized server, typically the one that
   hosts the target resource.  That similar to what happens in
   Section 4.3 when the URI discovery step is repeated.

7.3.  Link confidentiality

   When registrants publish information in the RD that is not available
   to any client that would query the registrant's /.well-known/core
   interface, or when lookups to that interface are subject so stricter
   firewalling than lookups to the RD, the RD may need to limit which
   lookup clients may access the information.

   In this case, the endpoint (and not the lookup clients) needs to be
   careful to check the RD's authorization.  The RD needs to check any
   lookup client's authorization before revealing information directly
   (in resource lookup) or indirectly (when using it to satisfy a
   resource lookup search criterion).

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7.4.  Segmentation

   Within a single RD, different security policies can apply.

   One example of this are multi-tenant deployments separated by the
   sector (d) parameter.  Some sectors might apply limitations on the
   endpoint names available, while others use a random identifier
   approach to endpoint names and place limits on the entered links
   based on their attributes instead.

   Care must be taken in such setups to determine the applicable access
   control measures to each operation.  One easy way to do that is to
   mandate the use of the sector parameter on all operations, as no
   credentials are suitable for operations across sector borders anyway.

7.5.  First-Come-First-Remembered: A default policy

   The First-Come-First-Remembered policy is provided both as a
   reference example for a security policy definition, and as a policy
   that implementations may choose to use as default policy in absence
   of other configuration.  It is designed to enable efficient discovery
   operations even in ad-hoc settings.

   Under this policy, the RD accepts registrations for any endpoint name
   that is not assigned to an active registration resource, and only
   accepts registration updates from the same endpoint.  The policy is
   minimal in that towards lookup clients it does not make any of the
   claims of Section 7.2 and Section 7.3, and its claims on Section 7.1
   are limited to the lifetime of that endpoint's registration.  It
   does, however, guarantee towards any endpoint that for the duration
   of its registration, its links will be discoverable on the RD.

   When a registration or operation is attempted, the RD MUST determine
   the client's subject name or public key:

   *  If the client's credentials indicate any subject name that is
      certified by any authority which the RD recognizes (which may be
      the system's trust anchor store), all such subject names are
      stored.  With CWT or JWT based credentials (as common with ACE),
      the Subject (sub) claim is stored as a single name, if it exists.
      With X.509 certificates, the Common Name (CN) and the complete
      list of SubjectAltName entries are stored.  In both cases, the
      authority that certified the claim is stored along with the
      subject, as the latter may only be locally unique.

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   *  Otherwise, if the client proves possession of a private key, the
      matching public key is stored.  This applies both to raw public
      keys and to the public keys indicated in certificates that failed
      the above authority check.

   *  If neither is present, a reference to the security session itself
      is stored.  With (D)TLS, that is the connection itself, or the
      session resumption information if available.  With OSCORE, that is
      the security context.

   As part of the registration operation, that information is stored
   along with the registration resource.

   The RD MUST accept all registrations whose registration resource is
   not already active, as long as they are made using a security layer
   supported by the RD.

   Any operation on a registration resource, including registrations
   that lead to an existing registration resource, MUST be rejected by
   the RD unless all the stored information is found in the new
   request's credentials.

   Note that even though subject names are compared in this policy, they
   are never directly compared to endpoint names, and an endpoint can
   not expect to "own" any particular endpoint name outside of an active
   registration -- even if a certificate says so.  It is an accepted
   shortcoming of this approach that the endpoint has no indication of
   whether the RD remembers it by its subject name or public key;
   recognition by subject happens on a best-effort base (given the RD
   may not recognize any authority).  Clients MUST be prepared to pick a
   different endpoint name when rejected by the RD initially or after a
   change in their credentials; picking an endpoint name as per
   Section 7.1.1 is an easy option for that.

   For this policy to be usable without configuration, clients should
   not set a sector name in their registrations.  An RD can set a
   default sector name for registrations accepted under this policy,
   which is useful especially in a segmented setup where different
   policies apply to different sectors.  The configuration of such a
   behavior, as well as any other configuration applicable to such an RD
   (i.e. the set of recognized authorities) is out of scope for this
   document.

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8.  Security Considerations

   The security considerations as described in Section 5 of [RFC8288]
   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].

   Access that is limited or affects sensitive data SHOULD be protected,
   e.g. using (D)TLS or OSCORE ([RFC8613]; which aspects of the RD this
   affects depends on the security policies of the application (see
   Section 7).

8.1.  Discovery

   Most steps in discovery of the RD, and possibly its resources, are
   not covered by CoAP's security mechanisms.  This will not endanger
   the security properties of the registrations and lookup itself (where
   the client requires authorization of the RD if it expects any
   security properties of the operation), but may leak the client's
   intention to third parties, and allow them to slow down the process.

   To mitigate that, clients can retain the RD's address, use secure
   discovery options like configured addresses, and send queries for RDs
   in a very general form ("?rt=core.rd*" rather than "?rt=core.rd-
   lookup-ep").

8.2.  Endpoint Identification and Authentication

   An Endpoint (name, sector) pair is unique within the set of endpoints
   registered by the RD.  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 on an RD SHOULD be mutually
   authenticated using Pre-Shared Key, Raw Public Key or Certificate
   based security.

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

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   Now, imagine that a malicious device A wants to sabotage the device
   B.  It uses its credentials during the DTLS exchange.  Then, it
   specifies the endpoint name of device B as the name of its own
   endpoint in device A.  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.

   Endpoint authorization needs to be checked on registration and
   registration resource operations independently of whether there are
   configured requirements on the credentials for a given endpoint name
   (and sector; Section 7.1) or whether arbitrary names are accepted
   (Section 7.1.1).

   Simple registration could be used to circumvent address-based access
   control: An attacker would send a simple registration request with
   the victim's address as source address, and later look up the
   victim's /.well-known/core content in the RD.  Mitigation for this is
   recommended in Section 5.1.

   The registration resource path is visible to any client that is
   allowed endpoint lookup, and can be extracted by resource lookup
   clients as well.  The same goes for registration attributes that are
   shown as target attributes or lookup attributes.  The RD needs to
   consider this in the choice of registration resource paths, and
   administrators or endpoint in their choice of attributes.

8.3.  Access Control

   Access control SHOULD be performed separately for the RD registration
   and Lookup 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 sector, endpoint or resource level.

   The precise access controls necessary (and the consequences of
   failure to enforce them) depend on the protection objectives of the
   application and the security policies (Section 7) derived from them.

8.4.  Denial of Service Attacks

   Services that run over UDP unprotected are vulnerable to unknowingly
   amplify and distribute a DoS attack as UDP does not require return
   routability check.  Since RD lookup responses can be significantly
   larger than requests, RDs are prone to this.

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   [RFC7252] describes this at length in its Section 11.3, including
   some mitigation by using small block sizes in responses.  The
   upcoming [I-D.ietf-core-echo-request-tag] updates that by describing
   a source address verification mechanism using the Echo option.

   [ If this document is published together with or after I-D.ietf-core-
   echo-request-tag, the above paragraph is replaced with the following:

   [RFC7252] describes this at length in its Section 11.3, and
   [I-D.ietf-core-echo-request-tag] (which updates the former)
   recommends using the Echo option to verify the request's source
   address.

   ]

8.5.  Skipping freshness checks

   When RD based applications are built in which request freshness
   checks are not performed, these concerns need to be balanced:

   *  When alterations to registration attributes are reordered, an
      attacker may create any combination of attributes ever set, with
      the attack difficulty determined by the security layer's replay
      properties.

      For example, if Figure 18 were conducted without freshness
      assurances, an attacker could later reset the lifetime back to
      7200.  Thus, the device is made unreachable to lookup clients.

   *  When registration updates without query parameters (which just
      serve to restart the lifetime) can be reordered, an attacker can
      use intercepted messages to give the appearance of the device
      being alive to the RD.

      This is unacceptable when when the RD's security policy promises
      reachability of endpoints (e.g. when disappearing devices would
      trigger further investigation), but may be acceptable with other
      policies.

9.  IANA Considerations

9.1.  Resource Types

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

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    +====================+=============================+=============+
    | Value              | Description                 | Reference   |
    +====================+=============================+=============+
    | core.rd            | Directory resource of an RD | RFCTHIS     |
    |                    |                             | Section 4.3 |
    +--------------------+-----------------------------+-------------+
    | core.rd-lookup-res | Resource lookup of an RD    | RFCTHIS     |
    |                    |                             | Section 4.3 |
    +--------------------+-----------------------------+-------------+
    | core.rd-lookup-ep  | Endpoint lookup of an RD    | RFCTHIS     |
    |                    |                             | Section 4.3 |
    +--------------------+-----------------------------+-------------+
    | core.rd-ep         | Endpoint resource of an RD  | RFCTHIS     |
    |                    |                             | Section 6   |
    +--------------------+-----------------------------+-------------+

                                 Table 2

9.2.  IPv6 ND Resource Directory Address Option

   This document registers one new ND option type under the sub-registry
   "IPv6 Neighbor Discovery Option Formats" of the "Internet Control
   Message Protocol version 6 (ICMPv6) Parameters" registry:

   *  Resource Directory Address Option (TBD38)

   [ The RFC editor is asked to replace TBD38 with the assigned number
   in the document; the value 38 is suggested. ]

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 target attributes,

   *  indication of whether it can be passed as a query parameter at
      registration of endpoints, as a query parameter in lookups, or be
      expressed as a target attribute,

   *  syntax and validity requirements if any,

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   *  a description,

   *  and a link to reference documentation.

   The query parameter MUST be both a valid URI query key [RFC3986] and
   a token as used in [RFC8288].

   The description must give details on whether the parameter can be
   updated, and how it is 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 name    | Short | Validity     | Use | Description         |
    +==============+=======+==============+=====+=====================+
    | Endpoint     | ep    | Unicode*     | RLA | Name of the         |
    | Name         |       |              |     | endpoint            |
    +--------------+-------+--------------+-----+---------------------+
    | Lifetime     | lt    | 1-4294967295 | R   | Lifetime of the     |
    |              |       |              |     | registration in     |
    |              |       |              |     | seconds             |
    +--------------+-------+--------------+-----+---------------------+
    | Sector       | d     | Unicode*     | RLA | Sector to which     |
    |              |       |              |     | this endpoint       |
    |              |       |              |     | belongs             |
    +--------------+-------+--------------+-----+---------------------+
    | Registration | base  | URI          | RLA | The scheme, address |
    | Base URI     |       |              |     | and port and path   |
    |              |       |              |     | at which this       |
    |              |       |              |     | server is available |
    +--------------+-------+--------------+-----+---------------------+
    | Page         | page  | Integer      | L   | Used for pagination |
    +--------------+-------+--------------+-----+---------------------+
    | Count        | count | Integer      | L   | Used for pagination |
    +--------------+-------+--------------+-----+---------------------+
    | Endpoint     | et    | Section      | RLA | Semantic type of    |
    | Type         |       | 9.3.1        |     | the endpoint (see   |
    |              |       |              |     | Section 9.4)        |
    +--------------+-------+--------------+-----+---------------------+

                           Table 3: RD Parameters

   (Short: Short name used in query parameters or target attributes.
   Validity: Unicode* = 63 Bytes of UTF-8 encoded Unicode, with no
   control characters as per Section 5.  Use: R = used at registration,
   L = used at lookup, A = expressed in target attribute.)

   The descriptions for the options defined in this document are only
   summarized here.  To which registrations they apply and when they are
   to be shown is described in the respective sections of this document.
   All their reference documentation entries point to 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 (E.g. 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 target attributes (For

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   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] target attribute).

9.3.1.  Full description of the "Endpoint Type" RD 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 sub-
   registry.  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,
   RDs implementing this specification 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:

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

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

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

   The registry initially contains one value:

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   *  "core.rd-group": An application group as described in Appendix A.

9.5.  Multicast Address Registration

   IANA is asked to assign the following multicast addresses for use by
   CoAP nodes:

   IPv4 -- "all CoRE Resource Directories" address MCD2 (suggestion:
   224.0.1.189), from the "IPv4 Multicast Address Space Registry".  As
   the address is used for discovery that may span beyond a single
   network, it has come from the Internetwork Control Block (224.0.1.x)
   [RFC5771].

   IPv6 -- "all CoRE Resource Directories" address MCD1 (suggestions
   FF0X::FE), from the "IPv6 Multicast Address Space Registry", in the
   "Variable Scope Multicast Addresses" space (RFC 3307).  Note that
   there is a distinct multicast address for each scope that interested
   CoAP nodes should listen to; CoAP needs the Link-Local and Site-Local
   scopes only.

   [ The RFC editor is asked to replace MCD1 and MCD2 with the assigned
   addresses throughout the document. ]

9.6.  Well-Known URIs

   IANA is asked to permanently register the URI suffix "rd" in the
   "Well-Known URIs" registry.  The change controller is the IETF, this
   document is the reference.

9.7.  Service Names and Transport Protocol Port Number Registry

   IANA is asked to enter four new items into the Service Names and
   Transport Protocol Port Number Registry:

   *  Service name: "core-rd", Protocol: "udp", Description: "Resource
      Directory accessed using CoAP"

   *  Service name "core-rd-dtls", Protocol: "udp", Description:
      "Resource Directory accessed using CoAP over DTLS"

   *  Service name: "core-rd", Protocol: "tcp", Description: "Resource
      Directory accessed using CoAP over TCP"

   *  Service name "core-rd-tls", Protocol: "tcp", Description:
      "Resource Directory accessed using CoAP over TLS"

   All in common have this document as their reference.

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10.  Examples

   Two examples are presented: a Lighting Installation example in
   Section 10.1 and a LwM2M example in Section 10.2.

10.1.  Lighting Installation

   This example shows a simplified lighting installation which makes use
   of the 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 as described in
   Appendix A.  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.

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   At the moment of installation, the network under installation is not
   necessarily connected to the DNS infrastructure.  Therefore, SLAAC
   IPv6 addresses are assigned to CT, RD, luminaries and the sensor.
   The addresses shown in Table 4 below stand in for these in the
   following examples.

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

                      Table 4: Addresses used in the
                                 examples

   In Section 10.1.2 the use of RD 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=tag:example.com,2020:light, and
   the presence sensor has rt=tag:example.com,2020: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 RD are
   shown in Table 5 below:

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   +=========+================+========+===============================+
   |Name     |endpoint        |resource| resource type                 |
   |         |                |path    |                               |
   +=========+================+========+===============================+
   |luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light    |
   |         |                |left    |                               |
   +---------+----------------+--------+-------------------------------+
   |luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light    |
   |         |                |middle  |                               |
   +---------+----------------+--------+-------------------------------+
   |luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light    |
   |         |                |right   |                               |
   +---------+----------------+--------+-------------------------------+
   |luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light    |
   |         |                |left    |                               |
   +---------+----------------+--------+-------------------------------+
   |luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light    |
   |         |                |middle  |                               |
   +---------+----------------+--------+-------------------------------+
   |luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light    |
   |         |                |right   |                               |
   +---------+----------------+--------+-------------------------------+
   |Presence |ps_R2-4-015_door|/ps     | tag:example.com,2020:p-sensor |
   |sensor   |                |        |                               |
   +---------+----------------+--------+-------------------------------+

                          Table 5: RD identifiers

   It is assumed that the CT has performed RD discovery and has received
   a response like the one in the Section 4.3 example.

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

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   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015
   Payload:
   </light/left>;rt="tag:example.com,2020:light",
   </light/middle>;rt="tag:example.com,2020:light",
   </light/right>;rt="tag:example.com,2020:light"

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

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015
   Payload:
   </light/left>;rt="tag:example.com,2020:light",
   </light/middle>;rt="tag:example.com,2020:light",
   </light/right>;rt="tag:example.com,2020:light"

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

   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]&d=R2-4-015
   Payload:
   </ps>;rt="tag:example.com,2020:p-sensor"

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

         Figure 24: Example of registrations a CT enters into an RD

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

   The group is specified in the RD.  The base parameter is set to the
   site-local multicast address allocated to the group.  In the POST in
   the example below, the resources supported by all group members are
   published.

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   Req: POST coap://[2001:db8:4::ff]/rd
     ?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]
   Payload:
   </light/left>;rt="tag:example.com,2020:light",
   </light/middle>;rt="tag:example.com,2020:light",
   </light/right>;rt="tag:example.com,2020:light"

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

       Figure 25: Example of a multicast group a CT enters into an RD

   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 sector and being configured to join any
   group containing lights, searches for candidate groups and joins
   them:

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

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

          Figure 26: Example of a lookup exchange to find suitable
                            multicast addresses

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

   The presence sensor can learn the presence of groups that support
   resources with rt=tag:example.com,2020:light in its own sector by
   sending the same request, as used by the luminary.  The presence
   sensor learns the multicast address to use for sending messages to
   the luminaries.

10.2.  OMA Lightweight M2M (LwM2M)

   OMA LwM2M is a profile for device services based on CoAP, providing
   interfaces and operations for device management and device service
   enablement.

   An LwM2M server is an instance of an LwM2M middleware service layer,
   containing an RD ([LwM2M] page 36f).

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   That RD only implements the registration interface, and no lookup is
   implemented.  Instead, the LwM2M server provides access to the
   registered resources, in a similar way to a reverse proxy.

   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.

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, Jan Newmarch, Matthias
   Kovatsch, Jaime Jimenez and Ted Lemon 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 RD concepts were originally developed.

12.  Changelog

   changes from -27 to -28

   *  Security policies / link confidentiality: Point out the RD's
      obligations that follow from such a policy.

   *  Simple registration: clarify term "regular registration" by
      introducing it along with the reference to Section 5

   *  Wording fix in first-come-first-remembered

   *  Wording fixes in RD definition

   *  Capitalization: Consistently using "registration resource"

   changes from -26 to -27

   *  In general, this addresses the points that were pointed out in
      https://mailarchive.ietf.org/arch/msg/core/xWLomwwhovkU-
      CPGNxnvs40BhaM/ as having "evolved from the review comments being
      discussed in the interim meetings", and the review comments from
      Esko Dijk that were largely entangled in these points.

   *  Relaxation of the serialization rules for link-format

      The interpretation of RFC6690 used in Appendix B.4 was shown to be
      faulty.  Along with a correction, the common implementations of
      link-format were surveyed again and it was found that the only one

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      that employed the faulty interpretation can still safely be
      upgraded.  These were removed from the set considered for Limited
      Link Format, making the set of valid Limited Link Format documents
      larger.

      As a consequence, the prescribed serialization of RD output can be
      roughly halved in bytes.

      There might be additional usage patterns that are possible with
      the new set of constraints, but there is insufficient
      implementation and deployment experience with them to warrant a
      change changes on that front at this point.  The specification can
      later be extended compatibly to allow these cases and drop the
      requirement of Limited Link Format.

   *  Add Request freshness subsection

      It is now recommended (with security considerations on
      consequences of not doing it) to require ordering of RD
      operations.

      The Echo mechanism (previously suggested in various places but
      never exclusively) is the one prescribed way of getting this
      ordering, making the echo-request-tag reference normative.

   *  Improved expression about when an RD needs to verify simple
      registration.

      The simple wording missed the authorization part, and did not
      emphasize that this is a per-deployment property.

   *  Point out the non-atomic properties of paginated access.

   *  Clarification around impl-info reference.

   *  Inconsistencies and extraneous quotings removed from examples.

   changes from -25 to -26

   *  Security policies:

      -  The First-Come-First-Remembered policy is added as an example
         and a potential default behavior.

      -  Clarify that the mapping between endpoint names and subject
         fields is up to a policy that defines reliance on names, and
         give an example.

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      -  Random EP names: Point that multiple collisions are possible
         but unlikely.

      -  Add pointers to policies:

         o  RD replication: Point out that policies may limit that.

         o  Registration: Reword (ep, d) mapping to a previous
            registration's resource that could have been read as another
            endpoint taking over an existing registration.

      -  Clarify that the security policy is a property of the RD the
         any client may need to verify by checking the RD's
         authorization.

      -  Clarify how information from an untrusted RD can be verified

      -  Remove speculation about how in detail ACE scopes are obtained.

   *  Security considerations:

      -  Generalize to all current options for security layers usable
         with CoAP (OSCORE was missing as the text predated RFC8613)

      -  Relax the previous SHOULD on secure access to SHOULD where
         protection is indicated by security policies (bringing the text
         in line with the -25 changes)

      -  Point out that failure to follow the security considerations
         has implications depending on the protection objective
         described with the security policies

      -  Shorten amplification mitigation

      -  Add note about information in Registration Resource path.

      -  Acknowledge that most host discovery operations are not
         secured; mention consequences and mitigation.

   *  Abstract, introduction: removed "or disperse networks"

   *  RD discovery:

      -  Drop the previously stated assumption that RDAO and any DHCP
         options would only be used together with SLAAC and DHCP for
         address configuration, respectivly.

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      -  Give concrete guidance for address selection based on RFC6724
         when responding to multicasts

      -  RDAO:

         o  Clarify that it is an option for RAs and not other ND
            messages.

         o  Change Lifetime from 16-bit minutes to 32-bit seconds and
            swap it with Reserved (aligning it with RDNSS which it
            shares other properties as well).

      -  Point out that clients may need to check RD authorization
         already in last discovery step

   *  Registration:

      -  Wording around "mostly mandatory" has been improved, conflicts
         clarified and sector default selection adjusted.

   *  Simple registration: Rather than coopting POSTs to /.well-known/
      core, a new resource /.well-known/rd is registered.  A historical
      note in the text documents the change.

   *  Examples:

      -  Use example URIs rather than unclear reg names (unless it's
         RFC6690 examples, which were kept for continuity)

      -  The LwM2M example was reduced from an outdated explanation of
         the complete LwM2M model to a summary of how RD is used in
         there, with a reference to the current specification.

      -  Luminary example: Explain example addresses

      -  Luminary example: Drop reference to coap-group mechanism that's
         becoming obsolete, and thus also to RFC7390

      -  Multicast addresses in the examples were changed from
         ff35:30:2001:db8::x to ff35:30:2001:db8:f1::8000:x; the 8000 is
         to follow RFC 3307, and the f1 is for consistency with all the
         other example addresses where 2001:db8::/32 is subnetted to
         2001:db8:x::/48 by groups of internally consistent examples.

   *  Use case text enhancements

      -  Home and building automation: Tie in with RD

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      -  M2M: Move system design paragraph towards the topic of
         reusability.

   *  Various editorial fixes in response to Gen-ART and IESG reviews.

   *  Rename 'Full description of the "Endpoint Type" Registration
      Parameter' section to '... RD Parameter'

   *  Error handling: Place a SHOULD around the likely cases, and make
      the previous "MUST to the best of their capabilities" a "must".

   *  impl-info: Add note about the type being WIP

   *  Interaction tables: list CTs as possible initiators where
      applicable

   *  Registration update: Relax requirement to not send parameters
      needlessly

   *  Terminology: Clarify that the CTs' installation events can occur
      multiple times.

   *  Promote RFCs 7252, 7230 and 8288 to normative references

   *  Moved Christian Amsuess to first author

   changes from -24 to -25

   *  Large rework of section 7 (Security policies)

      Rather than prescribing which data in the RD _is_ authenticated
      (and how), it now describes what applications built on an RD _can_
      choose to authenticate, show possibilities on how to do it and
      outline what it means for clients.

      This addresses Russ' Genart review points on details in the text
      in a rather broad fashion.  That is because the discussion on the
      topic inside the WG showed that that text on security has been
      driven more review-by-review than by an architectural plan of the
      authors and WG.

   *  Add concrete suggestions (twice as long as registrant number with
      retries, or UUIDs without) for random endpoint names

   *  Point out that simple registration can have faked origins,
      RECOMMEND mitigation when applicable and suggest the Echo
      mechanism to implement it.

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   *  Reference existing and upcoming specifications for DDOS mitigation
      in CoAP.

   *  Explain the provenance of the example's multicast address.

   *  Make "SHOULD" of not manipulating foreign registrations a "should"
      and explain how it is enforced

   *  Clarify application of RFC6570 to search parameters

   *  Syntactic fixes in examples

   *  IANA:

      -  Don't announce expected number of registrations (goes to write-
         up)

      -  Include syntax as part of a field's validity in entry
         requirements

   *  Editorial changes

      -  Align wording between abstract and introduction

      -  Abbreviation normalization: "ER model", "RD"

      -  RFC8174 boilerplate update

      -  Minor clarity fixes

      -  Markup and layouting

   changes from -23 to -24

   *  Discovery using DNS-SD added again

   *  Minimum lifetime (lt) reduced from 60 to 1

   *  References added

   *  IANA considerations

      -  added about .well-known/core resource

      -  added DNS-SD service names

      -  made RDAO option number a suggestion

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      -  added "reference" field to endpoint type registry

   *  Lookup: mention that anchor is a legitimate lookup attribute

   *  Terminology and example fixes

   *  Layout fixes, esp. the use of non-ASCII characters in figures

   changes from -22 to -23

   *  Explain that updates can not remove attributes

   *  Typo fixes

   changes from -21 to -22

   *  Request a dedicated IPv4 address from IANA (rather than sharing
      with All CoAP nodes)

   *  Fix erroneous examples

   *  Editorial changes

      -  Add figure numbers to examples

      -  Update RD parameters table to reflect changes of earlier
         versions in the text

      -  Typos and minor wording

   changes from -20 to -21

   (Processing comments during WGLC)

   *  Defer outdated description of using DNS-SD to find an RD to the
      defining document

   *  Describe operational conditions in automation example

   *  Recommend particular discovery mechanisms for some managed network
      scenarios

   changes from -19 to -20

   (Processing comments from the WG chair review)

   *  Define the permissible characters in endpoint and sector names

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   *  Express requirements on NAT situations in more abstract terms

   *  Shifted heading levels to have the interfaces on the same level

   *  Group instructions for error handling into general section

   *  Simple Registration: process reflowed into items list

   *  Updated introduction to reflect state of CoRE in general,
      reference RFC7228 (defining "constrained") and use "IoT" term in
      addition to "M2M"

   *  Update acknowledgements

   *  Assorted editorial changes

      -  Unify examples style

      -  Terminology: RDAO defined and not only expanded

      -  Add CT to Figure 1

      -  Consistency in the use of the term "Content Format"

   changes from -18 to -19

   *  link-local addresses: allow but prescribe split-horizon fashion
      when used, disallow zone identifiers

   *  Remove informative references to documents not mentioned any more

   changes from -17 to -18

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

   *  Acknowledging the -17 version as part of the draft

   *  Move "Read endpoint links" operation to future specification like
      PATCH

   *  Demote links-json to an informative reference, and removed them
      from exchange examples

   *  Add note on unusability of link-local IP addresses, and describe
      mitigation.

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   *  Reshuffling of sections: Move additional operations and endpoint
      lookup back from appendix, and groups into one

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

   *  Simple registration: Change sequence of GET and POST-response,
      ensuring unsuccessful registrations are reported as such, and
      suggest how devices that would have required the inverse behavior
      can still cope with it.

   *  Abstract and introduction reworded to avoid the impression that
      resources are stored in full in the RD

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

   *  Drop a figure that has become useless due to the changes of and
      -13 and -17

   *  Wording consistency fixes: Use "Registrations" and "target
      attributes"

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

   *  State that the base attribute value is part of endpoint lookup
      even when implicit in the registration

   *  Update references from RFC5988 to its update RFC8288

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

   changes from -16 to -17

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

   *  Removed groups that are enumerations of registrations and have
      dedicated mechanism

   *  Add groups that are enumerations of shared resources and are a
      special case of endpoint registrations

   changes from -15 to -16

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   *  Recommend a common set of resources for members of a group

   *  Clarified use of multicast group in lighting example

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

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

   *  Add examples of URIs where Modernized Link Format matters

   *  Editorial changes

   changes from -14 to -15

   *  Rewrite of section "Security policies"

   *  Clarify that the "base" parameter text applies both to relative
      references both in anchor and href

   *  Renamed "Registree-EP" to Registrant-EP"

   *  Talk of "relative references" and "URIs" rather than "relative"
      and "absolute" URIs.  (The concept of "absolute URIs" of [RFC3986]
      is not needed in RD).

   *  Fixed examples

   *  Editorial changes

   changes from -13 to -14

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

   *  Introduced resource types core.rd-ep and core.rd-gp

   *  Registration management moved to appendix A, including endpoint
      and group lookup

   *  Minor editorial changes

      -  PATCH/iPATCH is clearly deferred to another document

      -  Recommend against query / fragment identifier in con=

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      -  Interface description lists are described as illustrative

      -  Rewording of Simple Registration

   *  Simple registration carries no error information and succeeds
      immediately (previously, sequence was unspecified)

   *  Lookup: href are matched against resolved values (previously, this
      was unspecified)

   *  Lookup: lt are not exposed any more

   *  con/base: Paths are allowed

   *  Registration resource locations can not have query or fragment
      parts

   *  Default life time extended to 25 hours

   *  clarified registration update rules

   *  lt-value semantics for lookup clarified.

   *  added template for simple registration

   changes from -12 to -13

   *  Added "all resource directory" nodes MC address

   *  Clarified observation behavior

   *  version identification

   *  example rt= and et= values

   *  domain from figure 2

   *  more explanatory text

   *  endpoints of a groups hosted by different RD

   *  resolve RFC6690-vs-8288 resolution ambiguities:

      -  require registered links not to be relative when using anchor

      -  return absolute URIs in resource lookup

   changes from -11 to -12

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   *  added Content Model section, including ER diagram

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

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

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

   *  updated LWM2M description

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

   *  new appendix on the interaction with RFCs 6690, 5988 and 3986

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

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

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

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

   *  enhanced IANA registry text

   *  state that lookup resources can be observable

   *  More examples and improved text

   changes from -09 to -10

   *  removed "ins" and "exp" link-format extensions.

   *  removed all text concerning DNS-SD.

   *  removed inconsistency in RDAO text.

   *  suggestions taken over from various sources

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

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   *  moved simple registration to registration section

   changes from -08 to -09

   *  clarified the "example use" of the base RD resource values /rd,
      /rd-lookup, and /rd-group.

   *  changed "ins" ABNF notation.

   *  various editorial improvements, including in examples

   *  clarifications for RDAO

   changes from -07 to -08

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

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

   *  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

   *  add IPv6 ND Option for discovery of an RD

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

   *  removed all superfluous client-server diagrams

   *  simplified lighting example

   *  introduced Commissioning Tool

   *  RD-Look-up text is extended.

   changes from -06 to -07

   *  added text in the discovery section to allow content format hints
      to be exposed in the discovery link attributes

   *  editorial updates to section 9

   *  update author information

   *  minor text corrections

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   Changes from -05 to -06

   *  added note that the PATCH section is contingent on the progress of
      the PATCH method

   changes from -04 to -05

   *  added Update Endpoint Links using PATCH

   *  http access made explicit in interface specification

   *  Added http examples

   Changes from -03 to -04:

   *  Added http response codes

   *  Clarified endpoint name usage

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

   Changes from -02 to -03:

   *  Added an example for lighting and DNS integration

   *  Added an example for RD use in OMA LWM2M

   *  Added Read Links operation for link inspection by endpoints

   *  Expanded DNS-SD section

   *  Added draft authors Peter van der Stok and Michael Koster

   Changes from -01 to -02:

   *  Added a catalogue use case.

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

   *  Additional examples section added for more complex use cases.

   *  New DNS-SD mapping section.

   *  Added text on endpoint identification and authentication.

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

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   *  Made 63 bytes a SHOULD rather than a MUST for endpoint name and
      resource type parameters.

   Changes from -00 to -01:

   *  Removed the ETag validation feature.

   *  Place holder for the DNS-SD mapping section.

   *  Explicitly disabled GET or POST on returned Location.

   *  New registry for RD parameters.

   *  Added support for the JSON Link Format.

   *  Added reference to the Groupcomm WG draft.

   Changes from -05 to WG Document -00:

   *  Updated the version and date.

   Changes from -04 to -05:

   *  Restricted Update to parameter updates.

   *  Added pagination support for the Lookup interface.

   *  Minor editing, bug fixes and reference updates.

   *  Added group support.

   *  Changed rt to et for the registration and update interface.

   Changes from -03 to -04:

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

   *  Integrated the Simple Directory Discovery from Carsten.

   *  Editorial improvements.

   *  Fixed the use of ETags.

   *  Fixed tickets 383 and 372

   Changes from -02 to -03:

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

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

   *  Introduced an improved RD Lookup design as its own function set.

   *  Improved the security considerations section.

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

   Changes from -01 to -02:

   *  Added a terminology section.

   *  Changed the inclusion of an ETag in registration or update to a
      MAY.

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

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

   *  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-echo-request-tag]
              Amsüss, C., Mattsson, J. P., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", Work in Progress,
              Internet-Draft, draft-ietf-core-echo-request-tag-12, 1
              February 2021, <https://www.ietf.org/archive/id/draft-
              ietf-core-echo-request-tag-12.txt>.

   [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>.

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

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

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

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

   [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>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

13.2.  Informative References

   [ER]       Chen, P., "The entity-relationship model--toward a unified
              view of data", DOI 10.1145/320434.320440, ACM Transactions
              on Database Systems Vol. 1, pp. 9-36, March 1976,
              <https://doi.org/10.1145/320434.320440>.

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   [I-D.bormann-t2trg-rel-impl]
              Bormann, C., "impl-info: A link relation type for
              disclosing implementation information", Work in Progress,
              Internet-Draft, draft-bormann-t2trg-rel-impl-02, 27
              September 2020, <https://www.ietf.org/archive/id/draft-
              bormann-t2trg-rel-impl-02.txt>.

   [I-D.hartke-t2trg-coral]
              Hartke, K., "The Constrained RESTful Application Language
              (CoRAL)", Work in Progress, Internet-Draft, draft-hartke-
              t2trg-coral-09, 8 July 2019,
              <https://www.ietf.org/archive/id/draft-hartke-t2trg-coral-
              09.txt>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-37, 4 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-37.txt>.

   [I-D.ietf-core-links-json]
              LI, K., Rahman, A., and C. Bormann, "Representing
              Constrained RESTful Environments (CoRE) Link Format in
              JSON and CBOR", Work in Progress, Internet-Draft, draft-
              ietf-core-links-json-10, 26 February 2018,
              <https://www.ietf.org/archive/id/draft-ietf-core-links-
              json-10.txt>.

   [I-D.ietf-core-rd-dns-sd]
              Stok, P. V. D., Koster, M., and C. Amsüss, "CoRE Resource
              Directory: DNS-SD mapping", Work in Progress, Internet-
              Draft, draft-ietf-core-rd-dns-sd-05, 7 July 2019,
              <https://www.ietf.org/archive/id/draft-ietf-core-rd-dns-
              sd-05.txt>.

   [I-D.silverajan-core-coap-protocol-negotiation]
              Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
              Work in Progress, Internet-Draft, draft-silverajan-core-
              coap-protocol-negotiation-09, 2 July 2018,
              <https://www.ietf.org/archive/id/draft-silverajan-core-
              coap-protocol-negotiation-09.txt>.

   [LwM2M]    Open Mobile Alliance, "Lightweight Machine to Machine
              Technical Specification: Transport Bindings (Candidate
              Version 1.1)", 12 June 2018,

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              <https://openmobilealliance.org/RELEASE/LightweightM2M/
              V1_1-20180612-C/OMA-TS-LightweightM2M_Transport-
              V1_1-20180612-C.pdf>.

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306,
              August 2002, <https://www.rfc-editor.org/info/rfc3306>.

   [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
              Reserved for Documentation", RFC 3849,
              DOI 10.17487/RFC3849, July 2004,
              <https://www.rfc-editor.org/info/rfc3849>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              DOI 10.17487/RFC5771, March 2010,
              <https://www.rfc-editor.org/info/rfc5771>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [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>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <https://www.rfc-editor.org/info/rfc6874>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

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   [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>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.

   [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>.

   [RFC8141]  Saint-Andre, P. and J. Klensin, "Uniform Resource Names
              (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
              <https://www.rfc-editor.org/info/rfc8141>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

Appendix A.  Groups Registration and Lookup

   The RD-Groups usage pattern allows announcing application groups
   inside an RD.

   Groups are represented by endpoint registrations.  Their base address
   is a multicast address, and they SHOULD be entered with the endpoint
   type "core.rd-group".  The endpoint name can also be referred to as a
   group name in this context.

   The registration is inserted into the RD by a Commissioning Tool,
   which might also be known as a group manager here.  It performs third
   party registration and registration updates.

   The links it registers SHOULD be available on all members that join
   the group.  Depending on the application, members that lack some
   resource MAY be permissible if requests to them fail gracefully.

   The following example shows a CT registering a group with the name
   "lights" which provides two resources.  The directory resource path
   /rd is an example RD location discovered in a request similar to
   Figure 5.  The group address in the example is constructed from
   [RFC3849]'s reserved 2001:db8:: prefix as a unicast-prefix based
   site-local address (see [RFC3306].

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   Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
                              &base=coap://[ff35:30:2001:db8:f1::8000:1]
   Content-Format: 40
   Payload:
   </light>;rt="tag:example.com,2020:light";
        if="tag:example.net,2020:actuator",
   </color-temperature>;if="tag:example.net,2020:parameter";u=K

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

                 Figure 27: Example registration of a group

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

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

   The following example shows a client performing an endpoint lookup
   for all groups.

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

   Res: 2.05 Content
   Payload:
   </rd/12>;ep=lights&et=core.rd-group;
            base="coap://[ff35:30:2001:f1:db8::8000:1]";rt=core.rd-ep

                    Figure 28: Example lookup of groups

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

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

   Res: 2.05 Content
   Payload:
   <coap://[ff35:30:2001:db8:f1::8000:1]/light>;
        rt="tag:example.com,2020:light";
        if="tag:example.net,2020:actuator",
   <coap://[ff35:30:2001:db8:f1::8000:1]/color-temperature>;
        if="tag:example.net,2020:parameter";u=K,

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            Figure 29: Example lookup of resources inside groups

Appendix B.  Web links and the Resource Directory

   Understanding the semantics of a link-format document and its URI
   references is a journey through different documents ([RFC3986]
   defining URIs, [RFC6690] defining link-format documents based on
   [RFC8288] which defines Link header fields, 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.

B.1.  A simple example

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

   The client sends a link-local multicast:

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

   Res: 2.05 Content
   Payload:
   </sensors/temp>;rt=temperature;ct=0

              Figure 30: Example of direct resource discovery

   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: "sensors" and "temp", the full resolution steps for
   insertion into and retrieval from the RD without any shortcuts are:

B.1.1.  Resolving the URIs

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

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   The Base URI of the requested resource can be composed from the
   options of the CoAP GET request by following the steps of [RFC7252]
   section 6.5 (with an addition at the end of 8.2) into
   ""coap://[2001:db8:f0::1]/.well-known/core"".

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

B.1.2.  Interpreting attributes and relations

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

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

   In our example, the context resource of the link is implied to be
   "coap:://[2001:db8:f0::1]" by the default value of the anchor (see
   Appendix B.4).  A full English expression of the "host relation" is:

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

B.2.  A slightly more complex example

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

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

   Res: 2.05 Content
   Payload:
   </sensors/temp>;rt=temperature;ct=0,
   </sensors/light>;rt=light-lux;ct=0,
   </t>;anchor="/sensors/temp";rel=alternate,
   <http://www.example.com/sensors/t123>;anchor="/sensors/temp";
       rel=describedby

          Figure 31: Extended example of direct resource discovery

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   Parsing the third record, the client encounters the "anchor"
   parameter.  It is a URI relative to the Base URI of the request and
   is thus resolved to ""coap://[2001:db8:f0::1]/sensors/temp"".  That
   is the context resource of the link, so the "rel" statement is not
   about the target and the Base URI any more, but about the target and
   the resolved URI.  Thus, the third record could be read as
   ""coap://[2001:db8:f0::1]/sensors/temp" has an alternate
   representation at "coap://[2001:db8:f0::1]/t"".

   Following the same resolution steps, the fourth record can be read as
   ""coap://[2001:db8:f0::1]/sensors/temp" is described by
   "http://www.example.com/sensors/t123"".

B.3.  Enter the Resource Directory

   The RD 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 RD that was announced to
   it, sending this request from its UDP port "[2001:db8:f0::1]:6553":

   Req: POST coap://[2001:db8:f01::ff]/.well-known/rd?ep=simple-host1

   Res: 2.04 Changed

                Figure 32: Example of a simple registration

   The RD would have accepted the registration, and queried the simple
   host's "/.well-known/core" by itself.  As a result, the host is
   registered as an endpoint in the RD with the name "simple-host1".
   The registration is active for 90000 seconds, and the endpoint
   registration Base URI is ""coap://[2001:db8:f0::1]"" following the
   resolution steps described in Appendix B.1.1.  It should be remarked
   that the Base URI constructed that way always yields a URI of the
   form: scheme://authority without path suffix.

   If the client now queries the RD as it would previously have issued a
   multicast request, it would go through the RD discovery steps by
   fetching "coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-
   lookup-res", obtain "coap://[2001:db8:f0::ff]/rd-lookup/res" as the
   resource lookup endpoint, and ask it for all temperature resources:

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   Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature

   Res: 2.05 Content
   Payload:
   <coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0

           Figure 33: Example exchange performing resource lookup

   This is not _literally_ the same response that it would have received
   from a multicast request, but it contains the equivalent statement:

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

   To complete the examples, the client could also query all resources
   hosted at the endpoint with the known endpoint name "simple-host1":

   Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1

   Res: 2.05 Content
   Payload:
   <coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0,
   <coap://[2001:db8:f0::1]/sensors/light>;rt=light-lux;ct=0,
   <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

      Figure 34: Extended example exchange performing resource lookup

   All the target and anchor references are already in absolute form
   there, which don't need to be resolved any further.

   Had the simple host done an equivalent full registration with a base=
   parameter (e.g. "?ep=simple-host1&base=coap+tcp://simple-
   host1.example.com"), that context would have been used to resolve the
   relative anchor values instead, giving

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

      Figure 35: Example payload of a response to a resource lookup
                        with a dedicated base URI

   and analogous records.

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B.4.  A note on differences between link-format and Link header fields

   While link-format and Link header fields look very similar and are
   based on the same model of typed links, there are some differences
   between [RFC6690] and [RFC8288].  When implementing an RD or
   interacting with an RD, care must be taken to follow the [RFC6690]
   behavior whenever application/link-format representations are used.

   *  "Default value of anchor": Both under [RFC6690] and [RFC8288],
      relative references in the term inside the angle brackets (the
      target) and the anchor attribute are resolved against the relevant
      base URI (which usually is the URI used to retrieve the entity),
      and independent of each other.

      When, in an [RFC8288] Link header, the anchor attribute is absent,
      the link's context is the URI of the selected representation (and
      usually equal to the base URI).

      In [RFC6690] links, if the anchor attribute is absent, the default
      value is the Origin of (for all relevant cases: the URI reference
      "/" resolved against) the link's target.

   *  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 header
      fields are practically ASCII strings that use percent encoding for
      non-ASCII characters, stating the encoding explicitly when
      required.

      For example, while a Link header field 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/Malmö>;rel=live-environment-data

Appendix C.  Limited Link Format

   The CoRE Link Format as described in [RFC6690] has been interpreted
   differently by implementers, and a strict implementation rules out
   some use cases of an RD (e.g. base values with path components in
   combination with absent anchors).

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   This appendix describes a subset of link format documents called
   Limited Link Format.  The one rule herein is not very limiting in
   practice -- all examples in RFC6690, and all deployments the authors
   are aware of already stick to them -- but ease the implementation of
   RD servers.

   It is applicable to representations in the application/link-format
   media type, and any other media types that inherit [RFC6690]
   Section 2.1.

   A link format representation is in Limited Link format if, for each
   link in it, the following applies:

   All URI references either follow the URI or the path-absolute ABNF
   rule of RFC3986 (i.e. target and anchor each either start with a
   scheme or with a single slash).

Authors' Addresses

   Christian Amsüss (editor)
   Hollandstr. 12/4
   1020
   Austria

   Phone: +43-664-9790639
   Email: christian@amsuess.com

   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose,  95134
   United States of America

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

   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View,  94043
   United States of America

   Phone: +1-707-502-5136
   Email: Michael.Koster@smartthings.com

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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen
   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

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