CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Standards Track                               M. Koster
Expires: May 4, September 14, 2017                                  SmartThings
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                         P. van der Stok
                                                              consultant
                                                        October 31, 2016
                                                          March 13, 2017

                        CoRE Resource Directory
                 draft-ietf-core-resource-directory-09
                 draft-ietf-core-resource-directory-10

Abstract

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

Status of This Memo

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   This Internet-Draft will expire on May 4, September 14, 2017.

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   Copyright (c) 2016 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture and Use Cases  . . . . . . . . . . . . . . . . .   5
     3.1.  Use Case: Cellular M2M  . . . . . . . . . . . . . . . . .   6
     3.2.  Use Case: Home and Building Automation  . . . . . . . . .   7
     3.3.  Use Case: Link Catalogues . . . . . . . . . . . . . . . .   7
   4.  Finding a Resource Directory Server  .  . . . . . . . . . . . . . . . .   8
     4.1.  Resource Directory Address Option (RDAO)  . . . . . . . .   9
   5.  Simple Registration  Resource Directory  . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Simple publishing to Resource Directory Server  Content Formats . . . . .  11
     5.2.  Third-party registration . . . . . . . . . . . . . . . .  12
   6.  Resource Directory Function Set  11
     5.2.  Base URI Discovery  . . . . . . . . . . . . . . . .  12
     6.1.  Content Formats . . .  11
     5.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Discovery
       5.3.1.  Simple Registration . . . . . . . . . . . . . . . . .  16
       5.3.2.  Simple publishing to Resource Directory Server  . . .  17
       5.3.3.  Third-party registration  . . . .  13
     6.3.  Registration . . . . . . . . . .  17
       5.3.4.  Plurality of link references in a Registration  . . .  18
     5.4.  Operations on the Registration Resource . . . . . . . . .  15
     6.4.  18
       5.4.1.  Registration Update . . . . . . . . . . . . . . . . . . .  18
     6.5.  19
       5.4.2.  Registration Removal  . . . . . . . . . . . . . . . . . .  20
     6.6.  21
       5.4.3.  Read Endpoint Links . . . . . . . . . . . . . . . . . . .  21
     6.7.  22
       5.4.4.  Update Endpoint Links . . . . . . . . . . . . . . . .  23
   6.  RD Groups . .  22
   7.  Group Function Set . . . . . . . . . . . . . . . . . . . . .  24
     7.1. . . .  27
     6.1.  Register a Group  . . . . . . . . . . . . . . . . . . . .  24
     7.2.  27
     6.2.  Group Removal . . . . . . . . . . . . . . . . . . . . . .  26
   8.  29
   7.  RD Lookup Function Set  . . . . . . . . . . . . . . . . . . .  27
   9.  New Link-Format Attributes  . . . . . . . . .  30
   8.  Security Considerations . . . . . . . .  32
     9.1.  Resource Instance attribute 'ins' . . . . . . . . . . .  35
     8.1.  Endpoint Identification and Authentication  .  32
     9.2.  Export attribute 'exp' . . . . . .  35
     8.2.  Access Control  . . . . . . . . . . .  33
   10. DNS-SD Mapping . . . . . . . . . .  35
     8.3.  Denial of Service Attacks . . . . . . . . . . . . .  33
     10.1.  DNS-based Service discovery . . .  36
   9.  IANA Considerations . . . . . . . . . . .  33
     10.2.  mapping ins to <Instance> . . . . . . . . . .  36
     9.1.  Resource Types  . . . . .  34
     10.3.  Mapping rt to <ServiceType> . . . . . . . . . . . . . .  35
     10.4.  Domain mapping . .  36
     9.2.  IPv6 ND Resource Directory Address Option . . . . . . . .  36
     9.3.  RD Parameter Registry . . . . . . . . . . .  35
     10.5.  TXT Record key=value strings . . . . . . .  36
   10. Examples  . . . . . . .  35
     10.6.  Importing resource links into DNS-SD . . . . . . . . . .  36
   11. Security Considerations . . . . . . . . .  37
     10.1.  Lighting Installation  . . . . . . . . . .  37
     11.1.  Endpoint Identification and Authentication . . . . . . .  37
     11.2.  Access Control . .
       10.1.1.  Installation Characteristics . . . . . . . . . . . .  38
       10.1.2.  RD entries . . . . . . .  37
     11.3.  Denial of Service Attacks . . . . . . . . . . . . . .  39
     10.2.  OMA Lightweight M2M (LWM2M) Example  .  37
   12. IANA Considerations . . . . . . . . .  42
       10.2.1.  The LWM2M Object Model . . . . . . . . . . . .  38
     12.1.  Resource Types . . .  42
       10.2.2.  LWM2M Register Endpoint  . . . . . . . . . . . . . .  44
       10.2.3.  LWM2M Update Endpoint Registration . . . .  38
     12.2.  Link Extension . . . . .  45
       10.2.4.  LWM2M De-Register Endpoint . . . . . . . . . . . . .  46
   11. Acknowledgments . . .  38
     12.3.  IPv6 ND Resource Directory Address Option . . . . . . .  38
     12.4.  RD Parameter Registry . . . . . . . . . . . . .  46
   12. Changelog . . . .  38
   13. Examples . . . . . . . . . . . . . . . . . . . . . .  46
   13. References  . . . .  39
     13.1.  Lighting Installation . . . . . . . . . . . . . . . . .  39
       13.1.1.  Installation Characteristics . . . . . . . . . . . .  40
       13.1.2.  RD entries . . . . . . . . . . . . . . . . . . . . .  41
       13.1.3.  DNS entries  . . . . . . . . . . . . . . . . . . . .  44
     13.2.  OMA Lightweight M2M (LWM2M) Example  . . . . . . . . . .  44
       13.2.1.  The LWM2M Object Model . . . . . . . . . . . . . . .  45
       13.2.2.  LWM2M Register Endpoint  . . . . . . . . . . . . . .  46
       13.2.3.  Alternate Base URI . . . . . . . . . . . . . . . . .  48
       13.2.4.  LWM2M Update Endpoint Registration . . . . . . . . .  48
       13.2.5.  LWM2M De-Register Endpoint . . . . . . . . . . . . .  48
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  48
   15. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  49
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     16.1.  50
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  52
     16.2.  50
     13.2.  Informative References . . . . . . . . . . . . . . . . .  53  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54  51

1.  Introduction

   The work on Constrained RESTful Environments (CoRE) aims at realizing
   the REST architecture in a suitable form for the most constrained
   nodes (e.g., 8-bit microcontrollers with limited RAM and ROM) and
   networks (e.g. 6LoWPAN).  CoRE is aimed at machine-to-machine (M2M)
   applications such as smart energy and building automation.

   The discovery of resources offered by a constrained server is very
   important in machine-to-machine applications where there are no
   humans in the loop and static interfaces result in fragility.  The
   discovery of resources provided by an HTTP Web Server is typically
   called Web Linking [RFC5988].  The use of Web Linking for the
   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 requesting "/.well-known/core".  In many
   M2M scenarios, direct discovery of resources is not practical due to
   sleeping nodes, disperse networks, or networks where multicast
   traffic is inefficient.  These problems can be solved by employing an
   entity called a Resource Directory (RD), which hosts descriptions of
   resources held on other servers, allowing lookups to be performed for
   those resources.

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

2.  Terminology

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

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

   This specification makes use of the following additional terminology:

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

   Domain
      In the context of a Resource Directory, a domain is a logical
      grouping of endpoints.  This specification assumes that the list
      of Domains supported by an RD is pre-configured by that RD.  When
      a domain is exported to DNS, the domain value equates to the DNS
      domain name.

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

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

   Commissioning Tool
      Commissioning Tool (CT) is

   Context
      When registering links to a device that assists during Resource Directory, the
      installation of Context refers
      to the scheme, address, port, and base path for all the links
      registered on behalf of an endpoint, of the general form
      scheme://host:port/path/ where the client may explicitly set the
      scheme and host, and may supply the port and path as optional
      parameters.  When the context of a registration is explicitly set,
      the URI resolution rules in [RFC3986] MUST be applied.

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

   RDAO
      Resource Directory Address Option.

3.  Architecture and Use Cases

   The resource directory architecture is illustrated in Figure 1.  A
   Resource Directory (RD) is used as a repository for Web Links
   [RFC5988] about resources hosted on other web servers, which are
   called endpoints (EP).  An endpoint is a web server associated with a
   scheme, IP address and port (called Context), thus a physical node
   may host one or more endpoints.  The RD implements a set of REST
   interfaces for endpoints to register and maintain sets of Web Links
   (called resource directory registration entries), and for clients to
   lookup resources from the RD or maintain groups.  Endpoints
   themselves can also act as clients.  An RD can be logically segmented
   by the use of Domains.  The domain an endpoint is associated with can
   be defined by the RD or configured by an outside entity.  This
   information hierarchy is shown in Figure 2.

   Endpoints are assumed

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

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

   An endpoint is provided with interfaces to register, update and
   remove a resource directory registration entry.  Furthermore,
   a mechanism to discover an RD using the CoRE Link Format [RFC6690] is
   defined.  It is also possible
   for an RD to proactively discover fetch Web Links from endpoints and add them as resource
   directory entries.

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

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

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

              Figure 1: The resource directory architecture.

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

          Figure 2: The resource directory information hierarchy.

3.1.  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
   air interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing short
   and wide range wireless interfaces.  From the system design point of
   view, the ambition is to design horizontal solutions that can enable
   utilization of machines in different applications depending on their
   current availability and capabilities as well as application
   requirements, thus avoiding silo like solutions.  One of the crucial
   enablers of such design is the ability to discover resources
   (machines -- endpoints) capable of providing required information at
   a given time or acting on instructions from the end users.

   In a typical scenario, during a boot-up procedure (and periodically
   afterwards), the machines (endpoints) register with a Resource
   Directory (for example EPs installed on vehicles enabling tracking of
   their position for fleet management purposes and monitoring
   environment parameters) hosted by the mobile operator or somewhere
   else in the network, periodically a description of its own
   capabilities.  Due to the usual network configuration of mobile
   networks, the EPs attached to the mobile network may not always be
   efficiently reachable.  Therefore, a remote server is usually used to
   provide proxy access to the EPs.  The address of each (proxy)
   endpoint on this server is included in the resource description
   stored in the RD.  The users, for example mobile applications for
   environment monitoring, contact the RD, look up the endpoints capable
   of providing information about the environment using appropriate set
   of link parameters, obtain information on how to contact them (URLs
   of the proxy server) and then initiate interaction to obtain
   information that is finally processed, displayed on the screen and
   usually stored in a database.  Similarly, fleet management systems
   provide the appropriate link parameters to the RD to look up for EPs
   deployed on the vehicles the application is responsible for.

3.2.  Use Case: Home and Building Automation

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

   The exporting of resource information to other discovery systems is
   also important in these automation applications.  In home automation
   there is a need to interact with other consumer electronics, which
   may already support DNS-SD, and in building automation DNS-SD in
   combination with resource directories can cover multiple buildings.

3.3.  Use Case: Link Catalogues

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

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

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

   The additional features of Resource Directory allow domains to be
   defined to enable access to a particular set of resources from
   particular applications.  This provides isolation and protection of
   sensitive data when needed.  Resource groups may defined to allow
   batched reads from multiple resources.

4.  Finding a Resource Directory Server

   Several mechanisms can be employed for discovering the RD, including
   assuming a default location (e.g. on an Edge Router in a LoWPAN),
   assigning an anycast address to the RD, using DHCP, or discovering
   the RD using .well-known/core and hyperlinks as specified in CoRE
   Link Format [RFC6690].  Endpoints that want to contact a directory server Resource
   Directory can obtain candidate IP addresses for such servers in a
   number of ways.

   In a 6LoWPAN, good candidates can be taken from:

   o  specific static configuration (e.g., anycast addresses), if any,

   o  the ABRO option of 6LoWPAN-ND [RFC6775],

   o  other ND options that happen to point to servers (such as RDNSS),

   o  DHCPv6 options that might be defined later.

   o  The IPv6 Neighbor Discovery Resource Directory Address Option
      described in Section 4.1

   In networks with more inexpensive use of multicast, the candidate IP
   address may be a well-known multicast address, i.e. directory servers
   are found by simply sending GET requests to that well-known multicast
   address (see Section 6.2).

   As some of these sources are just (more or less 5.2).

   Constrained nodes configured in large batches may be configured for
   an anycast address for the RD.  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 RD
   that is appropriate for the environment.

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

4.1.  Resource Directory Address Option (RDAO)

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

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

   The lifetime 0x0 means that the RD address is invalid and to be
   removed.

   The RDAO format is:

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

   Fields:

   Type:                   38

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

   Valid Lifetime:         16-bit unsigned integer.  The length of
                           time in units of 60 seconds (relative to
                           the time the packet is received) that
                           this set of border router information Resource Directory address is valid.
                           A value of all zero bits (0x0)
                           assumes a default value of 10,000
                           (~one week). indicates
                           that this Resource Directory address
                           is not valid anymore.

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

   RD Address:             IPv6 address of the RD.

                Figure 3: Resource Directory Address Option

5.  Simple Registration

   Not all endpoints hosting resources are expected to know how to
   implement the  Resource Directory Function Set (see Section 6) hence
   cannot register with

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

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

5.1.  Content Formats

   Resource Directory Discovery.  However, there may be
   security reasons why implementations using this form of directory discovery would be
   disabled.

   This approach requires that specification MUST
   support the endpoint makes available application/link-format content format (ct=40).

   Resource Directories implementing this specification MAY support
   additional content formats.

   Any additional content format supported by a Resource Directory
   implementing this specification MUST have an equivalent serialization
   in the hosted
   resources that it wants to be discovered, as links on its "/.well-
   known/core" interface as specified in [RFC6690].

   The application/link-format content format.

5.2.  Base URI Discovery

   Before an endpoint then finds one or more addresses can make use of an RD, it must first know the directory server
   as described in Section 4.

   An endpoint can send (a selection of) hosted resources to a directory
   server for publication as described in Section 5.1.

   The directory server integrates RD's
   address and port, and the base URI information it received this way
   into for its resource directory.  It MAY make REST API.
   This section defines discovery of the information available
   to further directories, if it can ensure that a loop does not form.
   The protocol used between directories to ensure loop-free operation
   is outside RD and its base URI using the scope
   well-known interface of this document.

5.1.  Simple publishing to Resource Directory Server

   An endpoint the CoRE Link Format [RFC6690].  It is
   however expected that wants to make itself RDs will also be discoverable occasionally sends via other methods
   depending on the deployment.

   Discovery of the RD base URI is performed by sending either a POST
   multicast or unicast GET request to the "/.well-known/core" URI of any candidate
   directory server that it finds.  The body of and including
   a Resource Type (rt) parameter [RFC6690] with the POST request is
   empty, which triggers value "core.rd" in
   the resource directory server query string.  Likewise, a Resource Type parameter value of
   "core.rd-lookup*" is used to perform GET
   requests at discover the requesting server's default discovery base URI for RD Lookup
   operations, and "core.gp" is used to obtain discover the link-format payload to register.

   The endpoint MAY include registration parameters in base URI for RD
   Group operations.  Upon success, the POST request
   as per Section 6.3.

   The following example shows an endpoint using simple publishing, by
   simply sending an empty POST to response will contain a resource directory.

   Req:(to RD server from [ff02::1])
   POST coap://rd.example.com/.well-known/core?lt=6000

   Content-Format: 40

   payload:

   (empty payload)

   Res: 2.04 Changed

   (later)

   Req: (from RD server to [ff02::1])
   GET coap://[ff02::1]/.well-known/core

   Accept: 40

   Res: 2.05 Content

   payload:

   </sen/temp>

5.2.  Third-party registration

   For some applications, even Simple Directory Discovery may be too
   taxing payload
   with a link format entry for certain very constrained devices, in particular if each RD function discovered, indicating
   the
   security requirements become too onerous.

   In a controlled environment (e.g. building control), base URI of the Resource
   Directory can be filled by a third device, called a commissioning
   tool.  The commissioning tool can fill RD function returned and the corresponding
   Resource Directory from a
   database or other means.  For that purpose Type.  When performing multicast discovery, the scheme, multicast IP
   address and
   port of used will depend on the registered device is indicated in scope required and the Context parameter multicast
   capabilities of the registration described in Section 6.3.

6. network.

   A Resource Directory Function Set

   This section defines MAY provide hints about the REST interfaces between an RD and endpoints,
   which is called content-formats it
   supports in the Resource Directory Function Set. Although links it exposes or registers, using the
   examples throughout this section assume "ct" link
   attribute, as shown in the example below.  Clients MAY use of CoAP [RFC7252], these REST interfaces
   hints to select alternate content-formats for interaction with the
   Resource Directory.

   HTTP does not support multicast and consequently only unicast
   discovery can also be realized supported using HTTP [RFC7230].  In
   all definitions HTTP.  Links to Resource Directories
   MAY be registered in this section, both CoAP response codes (with dot
   notation) other Resource Directories, and HTTP response codes (without dot notation) are shown. well-known entry
   points SHOULD be provided to enable the bootstrapping of unicast
   discovery.

   An RD implementing implementation of this specification MUST support query
   filtering for the discovery,
   registration, update, lookup, and removal interfaces rt parameter as defined in this
   section.

   Resource directory entries are designed to be easily exported to
   other [RFC6690].

   The discovery mechanisms such request interface is specified as DNS-SD.  For that reason,
   parameters that would meaningfully be mapped to DNS SHOULD be limited
   to a maximum length of 63 bytes.

6.1.  Content Formats

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

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

      rt :=  Resource Directories implementing this specification Type (optional).  MAY support
   additional content formats.

   Any additional content format supported by a Resource Directory
   implementing this specification MUST have an equivalent serialization
   in contain one of the values
         "core.rd", "core.rd-lookup*", "core.rd-lookup-d", "core.rd-
         lookup-res", "core.rd-lookup-ep", "core.rd-lookup-gp",
         "core.rd-group" or "core.rd*"

   Content-Format:  application/link-format content format.

6.2.  Discovery

   Before an endpoint can make use of (if any)

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

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

   The following response codes are defined for this interface:

   Success:  2.05 "Content" with an RD, it must first know the RD's
   address and port, and the path of its RD Function Set. There can be
   several mechanisms application/link-format,
      application/link-format+json, or application/link-format+cbor
      payload containing one or more matching entries for discovering the RD including assuming
      resource.

   Failure:  4.04 "Not Found" is returned in case no matching entry is
      found for a
   default location (e.g. on an Edge Router unicast request.

   Failure:  4.00 "Bad Request" is returned in case of a LoWPAN), by assigning
   an anycast address malformed
      request for a unicast request.

   Failure:  No error response to the RD, using DHCP, or by a multicast request.

   HTTP support :  YES (Unicast only)

   The following example shows an endpoint discovering the an RD using this
   interface, thus learning that the CoRE Link Format (see also Section 4).  This section
   defines discovery of the base RD using resource is, in this
   example, at /rd and that the well-known interface of content-format delivered by the
   CoRE Link Format [RFC6690] as server
   hosting the required mechanism.  It resource is however
   expected application/link-format (ct=40).  Note that RDs will also be discoverable via other methods
   depending on the deployment.

   Discovery of
   it is up to the RD function set to choose its base RD resource, although
   diagnostics and debugging is performed facilitated by sending either a
   multicast or unicast using the base paths
   specified here where possible.

   Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*

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

   The following example shows the way of indicating that a client may
   request to "/.well-known/core" and including alternate content-formats.  The Content-Format code attribute
   "ct" MAY include a Resource Type (rt) parameter [RFC6690] with the value "core.rd" space-separated sequence of Content-Format codes
   as specified in
   the query string.  Likewise, a Resource Type parameter value Section 7.2.1 of
   "core.rd-lookup" is used to discover the RD Lookup Function Set.
   Upon success, [RFC7252], indicating that multiple
   content-formats are available.  The example below shows the response will contain a payload with required
   Content-Format 40 (application/link-format) indicated as well as a link
   more application-specific content format
   entry for each RD discovered, with the URL indicating (picked as 65225 in this
   example; this is in the root experimental space, not an assigned value).
   The base RD resource of the RD.  When performing multicast discovery, the
   multicast IP address used will depend on the scope required values /rd, /rd-lookup, and /rd-group are
   example values.

   Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*

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

5.3.  Registration

   After discovering the
   multicast capabilities location of the network.

   A Resource Directory an RD, an endpoint MAY provide hints about register its
   resources using the content-formats it
   supports in registration interface.  This interface accepts a
   POST from an endpoint containing the links it exposes or registers, using list of resources to be added to
   the "ct" link
   attribute, directory as shown the message payload in the example below.  Clients MAY use these
   hints to select alternate content-formats for interaction CoRE Link Format
   [RFC6690], JSON CoRE Link Format (application/link-format+json), or
   CBOR CoRE Link Format (application/link-format+cbor)
   [I-D.ietf-core-links-json], along with query parameters indicating
   the
   Resource Directory.

   HTTP does not support multicast name of the endpoint, and consequently only unicast
   discovery can be supported using HTTP.  Links to Resource Directories
   MAY be registered in other Resource Directories, optionally its domain and well-known entry
   points SHOULD be provided to enable the bootstrapping lifetime
   of unicast
   discovery.

   An the registration.  It is expected that other specifications will
   define further parameters (see Section 9.3).  The RD implementation of this specification then creates a
   new registration resource in the RD and returns its location.  An
   endpoint MUST support query
   filtering use that location when refreshing registrations using
   this interface.  Endpoint resources in the RD are kept active for the rt parameter as defined in [RFC6690].
   period indicated by the lifetime parameter.  The discovery request interface endpoint is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

      rt :=  Resource Type (optional).  MAY contain one or more of
   responsible for refreshing the
         values "core.rd", "core.rd-lookup", "core.rd-group" entry within this period using either
   the registration or
         "core.rd*"

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

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

   Content-Format:  application/link-format+cbor (if any) update interface.  The following response codes are defined for this interface:

   Success:  2.05 "Content" registration interface
   MUST be implemented to be idempotent, so that registering twice with
   the same endpoint parameters ep and d does not create multiple RD
   entries.  A new registration may be created at any time to supersede
   an application/link-format,
      application/link-format+json, or application/link-format+cbor
      payload containing one or more matching entries for existing registration, replacing the registration parameters and
   links.

   The registration request interface is specified as follows:

   Interaction:  EP -> RD
      resource.

   Failure:  4.04 "Not Found"

   Method:  POST

   URI Template:  /{+rd}{?ep,d,et,lt,con}

   URI Template Variables:

      rd :=  RD Base URI path (mandatory).  This is returned in case no matching entry the path of the RD,
         as obtained from discovery.  The value "rd" is
      found recommended for a unicast request.

   Failure:  4.00 "Bad Request"
         this variable.

      ep :=  Endpoint name (mandatory).  The endpoint name is returned in case of a malformed
      request for a unicast request.

   Failure:  No error response to an
         identifier that MUST be unique within a multicast request.

   HTTP support :  YES (Unicast only) domain.  The following example shows an maximum
         length of this parameter is 63 bytes.

      d :=  Domain (optional).  The domain to which this endpoint discovering an RD using
         belongs.  The maximum length of this
   interface, thus learning that parameter is 63 bytes.
         When this parameter is elided, the base RD resource is, in this
   example, at /rd and that MAY associate the content_format delivered by
         endpoint with a configured default domain.

      et :=  Endpoint Type (optional).  The semantic type of the server
   hosting
         endpoint.  This parameter SHOULD be less than 63 bytes.

      lt :=  Lifetime (optional).  Lifetime of the resource registration in
         seconds.  Range of 60-4294967295.  If no lifetime is application.xml (ct=40).  Note that it included
         in the initial registration, a default value of 86400 (24
         hours) SHOULD be assumed.  If the lt parameter is up
   to not included
         in a registration refresh or update operation, the RD to choose its base RD resource, although diagnostics most
         recently supplied value SHALL be re-used.

      con :=  Context (optional).  This parameter sets the scheme,
         address and
   debugging port at which this server is facilitated by using available in the base paths specified here where
   possible.

   Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*

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

   The following example shows form
         scheme://host:port/path.  In the way absence of indicating that a client may
   request alternate content-formats.  The Content-Format code attribute
   "ct" MAY include a space-separated sequence this parameter the
         scheme of Content-Format codes
   as specified in Section 7.2.1 the protocol, source address and source port of [RFC7252], indicating that multiple
   content-formats the
         register request are available.  The example below shows assumed.  This parameter is mandatory when
         the required
   Content-Format 40 (application/link-format) indicated as well as directory is filled by a
   more application-specific content format (picked third party such as 65225 in this
   example; this is in the experimental space, not an assigned value).
   The base RD resource values /rd, /rd-lookup,
         commissioning tool.  When con is used, scheme and /rd-group host are
   example values.

   Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*

   Res: 2.05 Content
   </rd>;rt="core.rd";ct="40 65225",
   </rd-lookup>;rt="core.rd-lookup";ct="40 65225",
   </rd-group>;rt="core.rd-group";ct="40 65225"

6.3.  Registration

   After discovering
         mandatory and port and path parameters are optional.  If the location of an RD Function Set, an
         endpoint MAY uses an ephemeral port to register its resources using with, it MUST
         include the con: parameter in the registration interface.  This
   interface accepts to provide a POST from an
         valid network path.  If the endpoint containing which is located behind a
         NAT gateway is registering with a Resource Directory which is
         on the list network service side of
   resources to be added to the directory as NAT gateway, the message payload endpoint
         MUST use a persistent port for the outgoing registration in
         order to provide the
   CoRE Link Format [RFC6690], JSON CoRE Link Format (application/link-
   format+json), or CBOR CoRE Link Format (application/link-format+cbor)
   [I-D.ietf-core-links-json], along NAT gateway with query string parameters
   indicating the name of the endpoint, its domain a valid network address
         for replies and the lifetime of
   the registration.  All parameters except the endpoint name incoming requests.

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor

   The following response codes are
   optional.  It is expected that other specifications will define
   further parameters (see Section 12.4). defined for this interface:

   Success:  2.01 "Created" or 201 "Created".  The RD then creates Location header
      option MUST be included in the response when a new registration
      resource or updates an existing resource in the RD and returns its
   location.  An endpoint is created.  This Location MUST use that location when refreshing
   registrations using this interface.  Endpoint resources in be a stable identifier
      generated by the RD are
   kept active as it is used for the period indicated by the lifetime parameter. all subsequent operations on
      this registration resource.  The
   endpoint registration resource location
      thus returned is responsible for refreshing the entry within this period
   using either purpose of updating the lifetime of the
      registration or update interface.  The registration
   interface MUST be implemented to be idempotent, so that registering
   twice with and for maintaining the same endpoint parameter does not create multiple RD
   entries.  A new registration may be created at any time to supersede
   an existing registration, replacing content of the registration parameters registered
      links, including updating and deleting links.

   The registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  /{+rd}{?ep,d,et,lt,con}

   URI Template Variables:

      rd :=  RD Function Set path (mandatory).  This is

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

   Failure:  4.09 "Conflict" or 409 "Conflict".  Attempt to update the path
      registration content with links resulting in plurality of
      references; see Section 5.3.4.

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the
         RD Function Set, as obtained from discovery.  The value "rd" is
         recommended for this variable.

      ep :=  Endpoint name (mandatory). operation.

   HTTP support:  YES

   The following example shows an endpoint with the name is an
         identifier that MUST be unique within a domain.  The maximum
         length of this parameter is 63 bytes.

      d :=  Domain (optional).  The domain "node1"
   registering two resources to which an RD using this endpoint
         belongs. interface.  The maximum length of this parameter is 63 bytes.
         When this parameter
   location "/rd" is elided, the RD MAY associate the
         endpoint with a configured default domain.  The domain an example value of an RD base location.

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

   Res: 2.01 Created
   Location: /rd/4521

   A Resource Directory may optionally support HTTP.  Here is
         needed to export the endpoint to DNS-SD (see Section 10).

      et :=  Endpoint Type (optional).  The semantic type of the
         endpoint.  This parameter SHOULD be less than 63 bytes.

      lt :=  Lifetime (optional).  Lifetime an example
   of the same registration in
         seconds.  Range of 60-4294967295.  If no lifetime is included,
         a default value of 86400 (24 hours) SHOULD be assumed.

      con :=  Context (optional).  This parameter sets the scheme,
         address and port at which this server is available in the form
         scheme://host:port.  In the absence of this parameter the
         scheme of the protocol, source IP address and source port of
         the register request are assumed.  This parameter is mandatory operation above, when the directory is filled by a third party such as an
         commissioning tool.

   Content-Format:  application/link-format
   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor

   The following response codes are defined for this interface:

   Success:  2.01 "Created" or 201 "Created".  The Location header MUST
      be included with the new resource entry for the endpoint.  This
      Location MUST be a stable identifier generated by the RD as it is
      used for all subsequent operations on this registration.  The
      resource returned in the Location 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.

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

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

   HTTP support:  YES

   The following example shows an endpoint with the name "node1"
   registering two resources to an RD done using this interface.  The
   location "/rd" is an example value of an RD base location.

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

   Res: 2.01 Created
   Location: /rd/4521 HTTP.

   Req: POST /rd?ep=node1 HTTP/1.1
   Host : example.com
   Content-Type: application/link-format
   Payload:
   </sensors/temp>;ct=41;rt="temperature-c";if="sensor",
   </sensors/light>;ct=41;rt="light-lux";if="sensor"

   Res: 201 Created
   Location: /rd/4521

6.4.

5.3.1.  Simple Registration Update

   The update interface is used by an

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

   This approach requires that the endpoint makes available the hosted
   resources that it wants to refresh be discovered, as links on its "/.well-
   known/core" interface as specified in [RFC6690].

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

   An endpoint can send (a selection of) hosted resources to a directory
   server for publication as described in Section 5.3.2.

   The directory server integrates the information it received this way
   into its
   registration with an RD.  To use resource directory.  It MAY make the interface, information available
   to further directories, if it can ensure that a loop does not form.
   The protocol used between directories to ensure loop-free operation
   is outside the scope of this document.

5.3.2.  Simple publishing to Resource Directory Server

   An endpoint that wants to make itself discoverable occasionally sends
   a POST request to the "/.well-known/core" URI of any candidate
   directory server that it finds.  The body of the POST request is
   empty, which triggers the resource returned in directory server to perform GET
   requests at the Location option in requesting server's default discovery URI to obtain
   the
   response link-format payload to register.

   The endpoint MUST include the first registration.

   An update endpoint name and MAY update include the lifetime or context
   registration parameters
   "lt", "con" as d, lt, and et, in Section 6.3 ) if they have changed since the last POST request as per
   Section 5.3.

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

   Req:(to RD server from [ff02::1])
   POST coap://rd.example.com/.well-known/core?lt=6000;ep=node1

   Content-Format: 40

   payload:

   (empty payload)

   Res: 2.04 Changed

   (later)

   Req: (from RD server to [ff02::1])
   GET coap://[ff02::1]/.well-known/core

   Accept: 40

   Res: 2.05 Content

   payload:

   </sen/temp>

5.3.3.  Third-party registration or update.  Parameters that have not changed SHOULD NOT

   For some applications, even Simple Directory Discovery may be included too
   taxing for certain very constrained devices, in an update.  Adding parameters that have not changed
   increases particular if the size of
   security requirements become too onerous.

   In a controlled environment (e.g. building control), the message but does not have any other
   implications.  Parameters MUST Resource
   Directory can be included as query parameters in an
   update operation as in Section 6.3.

   Upon receiving an update request, an RD MUST reset filled by a third device, called a commissioning
   tool.  The commissioning tool can fill the timeout for Resource Directory from a
   database or other means.  For that endpoint and update purpose the scheme, IP address and
   port of the
   endpoint, using the source address of the update, or the context
   ("con") parameter if present.  If the lifetime parameter "lt" registered device is
   included indicated in the received update request, the RD MUST update the
   lifetime Context parameter
   of the registration and set the timeout equal to the new
   lifetime.

   An update MAY optionally add or replace links for the endpoint by
   including those links described in the payload Section 5.3.

5.3.4.  Plurality of the update as link references in a CoRE Link
   Format document.  A Registration

   Plurality of link references within a Registration (registration
   resource) is replaced only if both the target URI an indication of some error condition and
   relation type match.

   In addition to the use should not be
   allowed.

   Plurality of POST, as described in this section, there
   is an alternate way to add, replace, link references exists if, and delete only if, two or more
   links using PATCH as
   described in Section 6.7.

   The update registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST

   URI Template:  /{+location}{?lt,con}

   URI Template Variables:

      location := a Registration contain identical context, target, and
   relation values.  This is the Location path returned by condition would be likely to arise if there
   were multiple co-ordinators or configuration tools, each with a
   different set of configuration values for the RD as same resource.

   A Resource Directory SHOULD reject a registration, or an operation on
   a registration, which would result of in a successful earlier registration.

      lt :=  Lifetime (optional).  Lifetime plurality of link references
   within the registration in
         seconds.  Range the context of 60-4294967295.  If no lifetime the registration.  There is included, no requirement
   in this document for a default value resource directory to check for plurality of 86400 (24 hours)
   reference between different registrations.  Resource Directory
   operations which are rejected due to reference plurality SHOULD be assumed.

      con :=  Context (optional).  This parameter sets
   returned the scheme,
         address and port at which this server "Conflict" code, indicating that there is available in someting wrong
   with the form
         scheme://host:port.  Optional.  In request.

5.4.  Operations on the absence of this
         parameter Registration Resource

   After the scheme initial registration, an endpoint should retain the
   returned location of the protocol, source IP address Registration Resource for further
   operations, including refreshing the registration in order to extend
   the lifetie and
         source port used "keep-alive" the registration.  If the lifetime of
   the registration expires, the RD SHOULD NOT respond to register are assumed.  This parameter is
         compulsory when discovery
   queries with information from the directory is filled by a third party such
         as endpoint.  The RD SHOULD continue
   to provide access to the Registration Resource after a commissioning tool.

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

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

   Content-Format:  application/link-format+cbor (optional) registration
   time-out occurs in order to enable the registering endpoint to
   eventually refresh the registration.  The following response codes are defined RD MAY eventually remove
   the registration resource for this interface:

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

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

   Failure:  4.04 "Not Found" or 404 "Not Found". purpose of resource recovery and
   garbage collection.  If the Registration does not
      exist (e.g. Resource is removed, the
   endpoint will need to re-register.

   The Registration Resource may have expired).

   Failure:  5.03 "Service Unavailable" also be used to inspect the
   registration resource using GET, update the registration link
   contents using PATCH, or 503 "Service Unavailable".
      Service could not perform cancel the operation.

   HTTP support:  YES

   The following example shows an endpoint updating its registration at
   an RD using DELETE.

   These operations are described in this interface section.

   In accordance with Section 5.3.4, operations which would result in
   plural link references within the example location value: /rd/4521.

   Req: POST /rd/4521

   Res: 2.04 Changed context of a registration resource
   SHOULD be rejected using the "Conflict" result code.

5.4.1.  Registration Update

   The following example shows update interface is used by an endpoint updating to refresh or update its
   registration with a new lifetime and context, changing an existing link, and
   adding a new link using this interface with RD.  To use the example location
   value /rd/4521.  With interface, the endpoint sends a
   POST request to the initial registration resource returned in the client set Location
   header option in the response returned from the intial registration
   operation.

   An update MAY update the
   following values:

   o lifetime (lt)=500

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

   o  resource= </sensors/temp>;ct=41;rt="foobar";if="sensor"
   Req: POST /rd/4521?lt=600&con="coap://local-proxy.example.com:5683"
   Content-Format: 40
   Payload:
   </sensors/temp>;ct=41;rt="temperature-f";if="sensor",
   </sensors/door>;ct=41;rt="door";if="sensor"

   Res: 2.04 Changed

6.5.  Registration Removal

   Although RD entries registration parameters
   "lt", "con" as in Section 5.3 ) if the previous settings are to be
   retained.  Parameters that are not being changed changed SHOULD NOT
   be included in an update.  Adding parameters that have soft state and will eventually timeout after
   their lifetime, not changed
   increases the size of the message but does not have any other
   implications.  Parameters MUST be included as query parameters in an
   update operation as in Section 5.3.

   Upon receiving an update request, an RD MUST reset the timeout for
   that endpoint SHOULD explicitly remove its entry from and update the RD scheme, IP address and port of the
   endpoint, using the source address of the update, or the context
   ("con") parameter if it knows it will no longer be available (for example on
   shut-down).  This present.  If the lifetime parameter "lt" is accomplished using a removal interface on
   included in the received update request, the RD
   by performing a DELETE on MUST update the endpoint resource.

   The removal request interface
   lifetime of the registration and set the timeout equal to the new
   lifetime.  If the lifetime parameter is specified as follows:

   Interaction:  EP -> RD

   Method:  DELETE

   URI Template:  /{+location}

   URI Template Variables:

      location :=  This not included in the
   registration update, the most recent setting is re-used for the Location path returned next
   registration time-out period.

   An update MAY optionally add or replace links for the endpoint by
   including those links in the RD as a
         result payload of the update as a successful earlier registration.

   The following responses codes are defined for this interface:

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

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

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

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

   HTTP support: YES

   The following examples shows successful removal CoRE Link
   Format document.  A link is replaced only if all of the endpoint from
   the RD with example location target URI
   and relation type (if present) and anchor value /rd/4521.

   Req: DELETE /rd/4521

   Res: 2.02 Deleted

6.6.  Read Endpoint Links

   Some endpoints may wish to manage their links (if present) match.

   If the link payload is included, it SHOULD be checked for reference
   plurality as a collection, described in Section 5.3.4 and
   may need rejected with a
   "Conflict" result if there are plural link references detected.

   In addition to read the current set use of links POST, as described in order this section, there
   is an alternate way to determine link
   maintenance operations.

   One or more add, replace, and delete links MAY be selected by using query filtering PATCH as
   specified
   described in [RFC6690] Section 4.1 5.4.4.

   The read update registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET  POST
   URI Template:  /{+location}{?href,rel,rt,if,ct}  /{+location}{?lt,con}

   URI Template Variables:

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

      href,rel,rt,if,ct

      lt := link relations and attributes specified in  Lifetime (optional).  Lifetime of the query registration in order to select particular links based on their
      relations and attributes. "href" denotes the URI target
         seconds.  Range of 60-4294967295.  If no lifetime is included,
         the
      link.  See [RFC6690] Sec. 4.1

   The following responses codes are defined for this interface:

   Success:  2.05 "Content" or 200 "OK" upon success with an
      "application/link-format", "application/link-format+cbor", previous last lifetime set on a previous update or
      "application/link-format+json" payload. the
         original registration (falling back to 86400) SHOULD be used.

      con :=  Context (optional).  This parameter sets the scheme,
         address and port at which this server is available in the form
         scheme://host:port/path.  In the absence of this parameter the
         scheme of the protocol, source address and source port of the
         register request are assumed.  This parameter is mandatory when
         the directory is filled by a third party such as an
         commissioning tool.  When con is used, scheme and host are
         mandatory and port and path parameters are optional.

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

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

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

   The following response codes are defined for this interface:

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

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

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

   Failure:  4.09 "Conflict" or 409 "Conflict".  Attempt to update the
      registration content with links resulting in plurality of
      references; see Section 5.3.4.

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

   HTTP support:  YES
   The following examples show successful read of the example shows an endpoint links
   from the RD, updating its registration at
   an RD using this interface with the example location value value: /rd/4521.

   Req: GET POST /rd/4521

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

6.7.  Update Endpoint Links

   [This section will be removed before or as 2.04 Changed

   The following example shows an endpoint updating its registration
   with a result of new lifetime and context, changing an existing link, and
   adding a working-
   group last-call if the PATCH methods do not achieve the same level of
   consensus as new link using this interface with the present document.]

   A PATCH update adds, removes or changes links for example location
   value /rd/4521.  With the endpoint by
   including link update information in initial registration the payload of client set the update as a
   merge-patch+json format [RFC7396] document.

   One or more links are selected for update by using query filtering as
   specified in [RFC6690] Section 4.1

   The query filter selects the links to be modified or deleted, by
   matching the query parameter values to the values of the link
   attributes.

   When the query parameters are not present in the request, the payload
   specifies links to be added to the target document.  When the query
   parameters are present, the attribute names
   following values:

   o  lifetime (lt)=500

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

   o  resource= </sensors/temp>;ct=41;rt="foobar";if="sensor"

   Req: POST /rd/4521?lt=600&con="coap://local-proxy.example.com:5683"
   Content-Format: 40
   Payload:
   </sensors/temp>;ct=41;rt="temperature-f";if="sensor",
   </sensors/door>;ct=41;rt="door";if="sensor"

   Res: 2.04 Changed

5.4.2.  Registration Removal

   Although RD entries have soft state and values in the query
   parameters select one or more links on which to apply the PATCH
   operation.

   If will eventually timeout after
   their lifetime, an attribute name specified in the PATCH document exists in any
   the set of selected links, all occurrences of the attribute value in
   the target document MUST be updated using the value endpoint SHOULD explicitly remove its entry from
   the PATCH
   payload.  If the attribute name RD if it knows it will no longer be available (for example on
   shut-down).  This is not present in any selected links, accomplished using a removal interface on the attribute MUST be added to RD
   by performing a DELETE on the links. endpoint resource.

   The update removal request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  PATCH  DELETE

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

   URI Template Variables:

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

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

   Content-Format:  application/merge-patch+json (mandatory)

   The following response responses codes are defined for this interface:

   Success:  2.04 "Changed" 0r  2.02 "Deleted" or 204 "No Content" in the update was
      successfully processed. upon successful deletion

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

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

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

   HTTP support: YES

   The following examples show an shows successful removal of the endpoint adding </sensors/humid>,
   modifying </sensors/temp>, and removing </sensors/light> links in RD
   using from
   the Update Endpoint Links function RD with the example location value /rd/4521.

   The following example shows an EP adding the link </sensors/
   humid>;ct=41;rt="humid-s";if="sensor" to the collection of links at
   the location /rd/4521.

   Req: PATCH DELETE /rd/4521

   Payload:
   [{"href":"/sensors/humid","ct": 41, "rt": "humid-s", "if": "sensor"}]

   Content-Format:
   application/merge-patch+json

   Res: 2.04 Changed

   The following example shows an EP modifying all 2.02 Deleted

5.4.3.  Read Endpoint Links

   Some endpoints may wish to manage their links at the example
   location /rd/4521 which are identified by href="/sensors/temp", from
   the initial link-value of </sensors/temp>;rt="temperature" as a collection, and
   may need to read the new
   link-value </sensors/temp>;rt="temperature-c";if="sensor" by changing
   the value current set of links stored in the registration
   resource, in order to determine link attribute "rt" and adding the link attribute
   if="sensor" maintenance operations.

   One or more links MAY be selected by using query filtering as
   specified in [RFC6690] Section 4.1

   If no links are selected, the PATCH operation with the supplied merge-
   patch+json document payload.

   Req: PATCH /rd/4521?href="/sensors/temp"

   Payload:
   {"rt": "temperature-c", "if": "sensor"},

   Content-Format:
   application/merge-patch+json

   Res: 2.04 Changed

   This example shows Resource Directory SHOULD return an
   empty payload.

   The read request interface is specified as follows:

   Interaction:  EP removing all links at the example -> RD

   Method:  GET

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

   URI Template Variables:

      location
   /rd/4521 which are identified by href="/sensors/light".

   Req: PATCH /rd/4521?href="/sensors/light"

   Payload:
   {null}

   Content-Format:
   application/merge-patch+json

   Res: 2.04 Changed

7.  Group Function Set :=  This section defines a function set for is the creation of groups of
   endpoints for Location path returned by the purpose of managing and looking up endpoints for
   group operations.  The group function set is similar to the resource
   directory function set, in that a group may be created or removed.
   However unlike an endpoint entry, a group entry consists RD as a
         result of a list of
   endpoints successful earlier registration.

      href,rel,rt,if,ct := link relations and does not have a lifetime associated with it.  In order
   to make use of multicast requests with CoAP, a group MAY have a
   multicast address associated with it.

7.1.  Register a Group

   In order to create a group, a commissioning tool (CT) used to
   configure groups, makes a request to the RD indicating the name of attributes specified in
      the group query in order to create (or update), optionally the domain the group
   belongs to, select particular links based on their
      relations and optionally attributes. "href" denotes the multicast address URI target of the group.
      link.  See [RFC6690] Sec. 4.1

   The
   registration message includes the list of endpoints that belong to
   that group.

   All the endpoints in the group MUST be registered following responses codes are defined for this interface:

   Success:  2.05 "Content" or 200 "OK" upon success with the RD before
   registering a group.  If an endpoint is
      "application/link-format", "application/link-format+cbor", or
      "application/link-format+json" payload.

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

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

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

   HTTP support: YES

   The following examples show successful read of the group, endpoint links
   from the registration message returns an
   error.  The RD sends a blank target URI RD, with example location value /rd/4521.

   Req: GET /rd/4521

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

5.4.4.  Update Endpoint Links

   A PATCH update adds, removes or changes links for every the endpoint by
   including link when
   registering update information in the group.

   Configuration payload of the endpoints themselves is out of scope of this
   specification.  Such an interface update as a
   merge-patch+json format [RFC7396] document.

   Other PATCH document formats may be used as appropriate for managing patching
   the group membership array of an endpoint has been defined in [RFC7390].

   The registration request interface is specified as follows:

   Interaction:  CT -> RD

   Method:  POST

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

   URI Template Variables:

      rd-group :=  RD Group Function Set path (mandatory).  This is the
         path objects format of a Registration Resource.  In
   particular, a select-merge patch document format could combine the RD Group Function Set. The value "rd-group" is
         recommended
   function of link selection query and link attribute replacement
   values.

   One or more links are selected for this variable.

      gp :=  Group Name (mandatory). update by using query filtering as
   specified in [RFC6690] Section 4.1
   The name of query filter selects the group links to be
         created modified or replaced, unique within that domain.  The maximum
         length of this deleted, by
   matching the query parameter is 63 bytes.

      d :=  Domain (optional).  The domain values to which this group belongs.
         The maximum length the values of this parameter is 63 bytes.  Optional. the link
   attributes.

   When this parameter is elided, the RD MAY associate query parameters are not present in the
         endpoint with a configured default domain.  The domain value is
         needed to export request, the endpoint payload
   specifies links to DNS-SD (see Section 10)

      con :=  Context (optional).  This parameter is used be added to set the IP
         multicast address at target document.  When the query
   parameters are present, the attribute names and values in the query
   parameters select one or more links on which this server to apply the PATCH
   operation.

   If no links are selected by the query parameters, the PATCH operation
   SHOULD NOT update the state of any resource, and SHOULD return a
   reply of "Changed".

   If an attribute name specified in the PATCH document exists in any
   the set of selected links, all occurrences of the attribute value in
   the target document MUST be updated using the value from the PATCH
   payload.  If the attribute name is available not present in any selected links,
   the form
         scheme://multicast-address:port.  Optional.  In attribute MUST be added to the absence links.

   If the PATCH payload contains plural link references, or processing
   the PATCH payload would result in plural link references, the request
   SHOULD be rejected with a "Conflict" result.

   If the PATCH payload results in the modification of
         this parameter no multicast address link target,
   context, or relation values, that is "href", "rel", or "anchor", the
   request SHOULD be rejected with a "Conflict" result code.

   The update request interface is configured. specified as follows:

   Interaction:  EP -> RD

   Method:  PATCH

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

   URI Template Variables:

      location :=  This
         parameter is compulsory when the directory is filled Location path returned by the RD as a
         commissioning tool.

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json
         result of a successful earlier registration.

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

   Content-Format:  application/link-format+cbor  application/merge-patch+json (mandatory)
   The following response codes are defined for this interface:

   Success:  2.01 "Created" or 201 "Created".  The Location header MUST
      be included with the new group entry.  This Location MUST be a
      stable identifier generated by  2.04 "Changed" 0r 204 "No Content" in the RD as it is used for delete
      operations on this registration. update was
      successfully processed.

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

   Failure:  4.04 "Not Found" or 404 "Not Found".  An Endpoint is  Registration resource
      does not
      registered in the RD exist (e.g. may have expired).

   Failure:  4.09 "Conflict" or 409 "Conflict".  Attempt to update the
      registration content with links resulting in plurality of
      references; see Section 5.3.4.

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

   HTTP support: YES

   The following example shows an EP registering a group with the name
   "lights" which has two endpoints to examples show an endpoint adding </sensors/humid>,
   modifying </sensors/temp>, and removing </sensors/light> links in RD
   using this interface.  The
   base the Update Endpoint Links function with the example location
   value /rd-group is an /rd/4521.

   The Registration Resource initial state is:

   Req: GET /rd/4521

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

   The following example of shows an RD base location. EP adding the link </sensors/
   humid>;ct=41;rt="humid-s";if="sensor" to the collection of links at
   the location /rd/4521.

   Req: POST coap://rd.example.com/rd-group?gp=lights
   Content-Format: 40 PATCH /rd/4521

   Payload:
   <>;ep="node1",
   <>;ep="node2"
   [{"href":"/sensors/humid","ct": 41, "rt": "humid-s", "if": "sensor"}]

   Content-Format:
   application/merge-patch+json

   Res: 2.01 Created
   Location: /rd-group/12 2.04 Changed
   Req: POST /rd-group?gp=lights HTTP/1.1
   Host: example.com
   Content-Type: application/link-format
   Payload:
   <>;ep="node1",
   <>;ep="node2" GET /rd/4521

   Res: 201 Created
   Location: /rd-group/12

7.2.  Group Removal

   A group can be removed simply by sending a removal message to 2.01 Content
   Payload:
   </sensors/temp>;ct=41;rt="temperature",
   </sensors/light>;ct=41;rt="light-lux";if="sensor",
   </sensors/humid>;ct=41;rt="humid-s";if="sensor"

   The following example shows an EP modifying all links at the example
   location returned when registering /rd/4521 which are identified by href="/sensors/temp", from
   the group.  Removing a group MUST
   NOT remove initial link-value of </sensors/temp>;rt="temperature" to the endpoints new
   link-value </sensors/temp>;rt="temperature-c";if="sensor" by changing
   the value of the group from link attribute "rt" and adding the RD.

   The removal request interface is specified as follows:

   Interaction:  CT -> RD
   Method:  DELETE

   URI Template:  /{+location}

   URI Template Variables:

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

   The following responses codes are defined for this interface:

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

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

   Failure:  4.04 "Not Found" or 404 "Not Found".  Group does not exist.

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

   HTTP support:  YES

   The following examples shows successful removal of the group from link attribute
   if="sensor" using the
   RD PATCH operation with the example location value /rd-group/12. supplied merge-
   patch+json document payload.

   Req: DELETE /rd-group/12 PATCH /rd/4521?href=/sensors/temp

   Payload:
   {"rt": "temperature-c", "if": "sensor"},

   Content-Format:
   application/merge-patch+json

   Res: 2.02 Deleted

8.  RD Lookup Function Set

   In order for an RD to be used for discovering resources registered
   with it, a lookup interface can be provided using this function set.
   This lookup interface is defined as a default, and it is assumed that
   RDs may also support lookups to return resource descriptions in
   alternative formats (e.g.  Atom or HTML Link) or using more advanced
   interfaces (e.g. supporting context or semantic based lookup). 2.04 Changed

   Req: GET /rd/4521

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

   This function set allows lookups for domains, groups, endpoints and
   resources using attributes defined in the RD Function Set and for use
   with the CoRE Link Format.  The result of a lookup request is the
   list of example shows an EP removing all links (if any) corresponding to at the type of lookup.  Thus, a
   domain lookup MUST return a list of domains, a group lookup MUST
   return a list of groups, an endpoint lookup MUST return a list of
   endpoints and a resource lookup MUST return a list of links to
   resources.

   Each endpoint and resource lookup result returns respectively the
   scheme (IP address and port) followed by the path part of the URI of
   every endpoint and resource inside angle brackets ("<>") and followed
   by the other parameters.

   The target of these links SHOULD be the actual example location of the
   domain, endpoint or resource, but MAY be an intermediate proxy e.g.
   in the case of an HTTP lookup interface for CoAP endpoints.

   The domain lookup returns every lookup domain with a base RD resource
   value (e.g. "/rd") encapsulated within angle brackets.

   In case that a group does not implement any multicast address, the
   group lookup returns every group lookup with a group base resource
   value encapsulated within angle brackets (e.g. "/rd/look-up").
   Otherwise, the group lookup returns the multicast address of the
   group inside angle brackets.

   Using the Accept Option, the requester can control whether this list
   is returned in CoRE Link Format ("application/link-format", default)
   or its alternate content-formats ("application/link-format+json" or
   "application/link-format+cbor").

   The page and count parameters are used to obtain lookup results in
   specified increments using pagination, where count specifies how many
   links to return and page specifies
   /rd/4521 which subset of links organized in
   sequential pages, each containing 'count' links, starting with link
   zero and page zero.  Thus, specifying count of 10 and page of 0 will
   return the first 10 links in the result set (links 0-9).  Count = 10
   and page = 1 will return the next 'page' containing links 10-19, and
   so on.

   Multiple query parameters MAY be included in a lookup, all included
   parameters MUST match are identified by href="/sensors/light".

   Req: PATCH /rd/4521?href=/sensors/light

   Payload:
   {}

   Content-Format:
   application/merge-patch+json

   Res: 2.04 Changed
   Req: GET /rd/4521

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

6.  RD Groups

   This section defines the REST API for a resource to be returned.  The
   character'*' MAY be included at the end creation, management, and
   lookup of endpoints for group operations.  Similar to endpoint
   registration entries in the RD, groups may be created or removed.
   However unlike an endpoint entry, a parameter value as group entry consists of a
   wildcard operator.

   The rd-lookup interface MAY use any set list of query parameters to match
   the registered attributes
   endpoints and relations. does not have a lifetime associated with it.  In addition, this interface order
   to make use of multicast requests with CoAP, a group MAY be used have a
   multicast address associated with queries that specify domains, endpoints, and groups.
   For example, it.

6.1.  Register a domain lookup filtering on groups would return Group

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

   All the specified groups.  An endpoints in the group MUST be registered with the RD before
   registering a group.  If an endpoint lookup
   filtering on groups would return is not yet registered to the RD
   before registering the group, the registration message returns an
   error.  The RD sends a list blank target URI for every endpoint link when
   registering the group.

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

   The lookup registration request interface is specified as follows:

   Interaction:  Client  CT -> RD

   Method:  GET  POST

   URI Template:  /{+rd-lookup-base}/{lookup-
      type}{?d,ep,gp,et,rt,page,count,resource-param}  /{+rd-group}{?gp,d,con}

   URI Template Variables:

      rd-lookup-base

      rd-group :=  RD Lookup Function Set Group Base URI path (mandatory).  This is the path of the RD Lookup Function Set. The recommended
         value for this variable is: "rd-lookup".

      lookup-type :=  ("d", "ep", "res", "gp") (mandatory) This variable
         is used to select the kind of lookup to perform (domain,
         endpoint, resource, or group).

      ep :=  Endpoint name (optional).  Used for endpoint, group and
         resource lookups.

      d :=  Domain (optional).  Used for domain, group, endpoint and
         resource lookups.

      res :=  resource (optional).  Used for domain, group, endpoint and
         resource lookups.

      gp := Group name (optional).  Used for endpoint, group and
      resource lookups.

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

      count :=  Count (optional).  Number of results is limited to this
         parameter value.  If the page parameter is also present, the
         response MUST only include 'count' links starting with the
         (page * count) link in the result set from the query.  If the
         count parameter is not present, then the response MUST return
         all matching links in the result set.  Link numbering starts
         with zero.

      rt :=  Resource type (optional).  Used for group, endpoint and
         resource lookups.

      et :=  Endpoint type (optional).  Used for group, endpoint and
         resource lookups.

      resource-param :=  Link attribute parameters (optional).  Any link
         attribute as defined in Section 4.1 of [RFC6690], used for
         resource lookups.

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

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

      Content-Format:  application/link-format+cbor (optional) the path
         of the RD Group REST API.  The following responses codes are defined value "rd-group" is recommended
         for this interface:

   Success:  2.05 "Content" or 200 "OK" with an "application/link-
      format", "application/link-format+cbor", or "application/link-
      format+json" payload containing matching entries for variable.

      gp :=  Group Name (mandatory).  The name of the lookup.

   Failure:  4.04 "Not Found" group to be
         created or 404 "Not Found" in case no matching
      entry replaced, unique within that domain.  The maximum
         length of this parameter is found for a unicast request.

   Failure:  No error response 63 bytes.

      d :=  Domain (optional).  The domain to a multicast request.

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

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

   HTTP support:  YES which this group belongs.
         The examples in maximum length of this section assume a CoAP host parameter is 63 bytes.  Optional.
         When this parameter is elided, the RD MAY associate the
         endpoint with IP address
   FDFD::123 and a configured default CoAP port 61616.  HTTP hosts are possible domain.

      con :=  Context (optional).  This parameter sets the scheme,
         address and
   do not change port at which this server is available in the nature form
         scheme://host:port/path.  In the absence of this parameter the examples.\

   The following example shows a client performing a resource lookup
   with
         scheme of the example look-up location /rd-lookup/:

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616/temp>;rt="temperature"

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

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616>;ep="node5",
   <coap://[FDFD::123]:61616>;ep="node7"

   The following example shows a client performing protocol, source address and source port of the
         register request are assumed.  This parameter is mandatory when
         the directory is filled by a domain lookup:

   Req: GET /rd-lookup/d

   Res: 2.05 Content
   <>;d="domain1",
   <>;d="domain2" third party such as an
         commissioning tool.  When con is used, scheme and host are
         mandatory and port and path parameters are optional.

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor

   The following example shows a client performing a group lookup response codes are defined for
   all groups:

   Req: GET /rd-lookup/gp

   Res: 2.05 Content
   <>;gp="lights1";d="example.com"
   <>;gp="lights2";d="ecample.com" this interface:

   Success:  2.01 "Created" or 201 "Created".  The following example shows a client performing a lookup for all
   endpoints Location header
      option MUST be returned in response to a particular group:

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616>;ep="node1",
   <coap://[FDFD::123]:61616>;ep="node2"

   The following example shows a client performing successful group CREATE
      operation.  This Location MUST be a lookup stable identifier generated by
      the RD as it is used for all
   groups an endpoint belongs to:

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

   Res: 2.05 Content
   <>;gp="lights1"

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

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

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616/res/5>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/6>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/7>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/8>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/9>;rt=sensor;ct=60

9.  New Link-Format Attributes

   When using delete operations of the CoRE Link Format to describe resources being
   discovered by group
      registration resource.

   Failure:  4.00 "Bad Request" or posted to a resource directory service, additional
   information about those resources 400 "Bad Request".  Malformed
      request.

   Failure:  4.04 "Not Found" or 404 "Not Found".  An Endpoint is useful.  This specification
   defines the following new attributes for use not
      registered in the CoRE Link Format
   [RFC6690]:

      link-extension    = ( "ins" "=" (ptoken | quoted-string) )
                          ; The token RD (e.g. may have expired).

   Failure:  5.03 "Service Unavailable" or string is max 63 bytes
      link-extension    = ( "exp" )

9.1.  Resource Instance attribute 'ins' 503 "Service Unavailable".
      Service could not perform the operation.

   HTTP support:  YES
   The Resource Instance "ins" attribute is following example shows an identifier for this
   resource, EP registering a group with the name
   "lights" which makes it possible has two endpoints to distinguish it from other
   similar resources.  This attribute an RD using this interface.  The
   base location value /rd-group is similar in use an example of an RD base location.

   Req: POST coap://rd.example.com/rd-group?gp=lights
   Content-Format: 40
   Payload:
   <>;ep="node1",
   <>;ep="node2"

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

6.2.  Group Removal

   A group can be removed simply by sending a removal message to the
   <Instance> portion
   location of the group registration resource which was returned when
   intially registering the group.  Removing a DNS-SD record (see Section 10.1, and SHOULD
   be unique across resources with group MUST NOT remove the same Resource Type attribute in
   endpoints of the domain it group from the RD.

   The removal request interface is used.  A Resource Instance might be a descriptive
   string like "Ceiling Light, Room 3", a short ID like "AF39" or a
   unique UUID or iNumber. specified as follows:

   Interaction:  CT -> RD

   Method:  DELETE

   URI Template:  /{+location}

   URI Template Variables:

      location :=  This attribute is used the Location path returned by the RD as a Resource
   Directory to distinguish between multiple instances
         result of a successful group registration.

   The following responses codes are defined for this interface:

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

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

   Failure:  4.04 "Not Found" or 404 "Not Found".  Group does not exist.

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the same
   resource type within the directory.

   This attribute MUST be no more than 63 bytes in length. operation.

   HTTP support:  YES
   The resource
   identifier attribute MUST NOT appear more than once in a link
   description.  This attribute MAY be used as a query parameter in following examples shows successful removal of the group from the
   RD Lookup Function Set defined in Section with the example location value /rd-group/12.

   Req: DELETE /rd-group/12

   Res: 2.02 Deleted

7.

9.2.  Export attribute 'exp'

   The Export "exp" attribute is used as a flag  RD Lookup

   In order for an RD to indicate that a link
   description MAY be exported by a resource directory to external
   directories.

   The CoRE Link Format is used for many purposes between CoAP
   endpoints.  Some are useful mainly locally, for example checking the
   observability of a resource before accessing discovering resources registered
   with it, determining the size
   of a resource, or traversing dynamic resource structures.  However,
   other links are very useful to an optional lookup interface may be exported to other directories, for
   example the entry point resource to a functional service. provided.  This
   attribute MAY be used lookup
   interface is defined as a query parameter default, and it is assumed that RDs may
   also support lookups to return resource descriptions in the alternative
   formats (e.g.  Atom or HTML Link) or using more advanced interfaces
   (e.g. supporting context or semantic based lookup).

   RD Lookup Function
   Set allows lookups for domains, groups, endpoints and resources
   using attributes defined in Section 7.

10.  DNS-SD Mapping

   CoRE Resource Discovery is intended to support fine-grained discovery
   of hosted resources, their attributes, and possibly other resource
   relations [RFC6690].  In contrast, service discovery generally refers
   to a coarse-grained resolution of an endpoint's IP address, port
   number, and protocol.

   Resource this document and service discovery are complementary in the case of large
   networks, where for use with the latter can facilitate scaling.  This document
   defines a mapping between CoRE
   Link Format attributes and DNS-Based
   Service Discovery [RFC6763] fields that permits discovery Format.  The result of CoAP
   services by either method.

10.1.  DNS-based Service discovery

   DNS-Based Service Discovery (DNS-SD) defines a conventional method lookup request is the list of
   configuring DNS PTR, SRV, and TXT resource records links (if
   any) corresponding to facilitate
   discovery the type of services (such as CoAP servers in lookup.  Thus, a subdomain) using the
   existing DNS infrastructure.  This section gives domain lookup MUST
   return a brief overview list of
   DNS-SD; see [RFC6763] for domains, a detailed specification.

   DNS-SD service names are limited to 255 octets group lookup MUST return a list of
   groups, an endpoint lookup MUST return a list of endpoints and are a
   resource lookup MUST return a list of the form:

   Service Name = <Instance>.<ServiceType>.<Domain>. links to resources.

   RD Lookup does not expose registration resources directly, but
   returns link content from registration resource entries which satisfy
   RD Lookup queries.

   The service name lookup type is the label of SRV/TXT 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 |
             | Domain      | core.rd-lookup-d   | Optional  |
             | Group       | core.rd-lookup-gp  | Optional  |
             +-------------+--------------------+-----------+

                           Table 1: Lookup Types

   Each endpoint and resource records.  The SRV
   RR specifies lookup result returns respectively the host
   scheme (IP address and port) followed by the port of the endpoint.  The TXT RR
   provides additional information in the form of key/value pairs.

   The <Domain> part of the service name is identical to the global (DNS
   subdomain) path part of the authority in URIs that identify servers or
   groups of servers.

   The <ServiceType> part is composed of at least two labels.  The first
   label URI of the pair is the application protocol name [RFC6335] preceded
   by an underscore character.  The second label indicates the transport
   every endpoint and is always "_udp" for UDP-based CoAP services.  In cases where
   narrowing resource inside angle brackets ("<>") and followed
   by the scope other parameters.

   The target of the search may be useful, these labels may links SHOULD be
   optionally preceded by a subtype name followed by the "_sub" label.
   An example of this more specific <ServiceType> is
   "light._sub._dali._udp".

   A default <Instance> part actual location of the service name may be set at the
   factory
   domain, endpoint or during the commissioning process.  It SHOULD uniquely
   identify resource, but MAY be an instance intermediate proxy e.g.
   in the case of <ServiceType> within a <Domain>.  Taken
   together, these three elements comprise a unique name for an SRV/ TXT
   record pair within the DNS subdomain. HTTP lookup interface for CoAP endpoints.

   The granularity of domain lookup returns every lookup domain with a service name MAY be base RD resource
   value (e.g. "/rd") encapsulated within angle brackets.

   In case that of a host or group, or
   it could represent group does not implement any multicast address, the
   group lookup returns every group lookup with a particular group base resource
   value encapsulated within a CoAP server.  The
   SRV record contains angle brackets (e.g. "/rd/look-up").
   Otherwise, the host name (AAAA record name) and port of group lookup returns the
   service while protocol is part multicast address of the service name.  In the case
   where a service name identifies a particular resource,
   group inside angle brackets.

   Using the path part
   of Accept Option, the URI must be carried in a corresponding TXT record.

   A DNS TXT record is in practice limited to a few hundred octets in
   length, which requester can control whether this list
   is indicated in the resource record header returned in the DNS
   response message.  The data consists of one CoRE Link Format ("application/link-format", default)
   or more strings
   comprising a key=value pair.  By convention, the first pair is
   txtver=<number> (to support different versions of a service
   description).

10.2.  mapping ins to <Instance> its alternate content-formats ("application/link-format+json" or
   "application/link-format+cbor").

   The Resource Instance "ins" attribute maps to the <Instance> part of
   a DNS-SD service name.  It is stored directly in the DNS as a single
   DNS label of canonical precomposed UTF-8 [RFC3629] "Net-Unicode"
   (Unicode Normalization Form C) [RFC5198] text.  However, to the
   extent that the "ins" attribute may be chosen page and count parameters are used to match the DNS host
   name of a service, it SHOULD use the syntax defined obtain lookup results in Section 3.5 of
   [RFC1034]
   specified increments using pagination, where count specifies how many
   links to return and Section 2.1 page specifies which subset of [RFC1123].

   The <Instance> part links organized in
   sequential pages, each containing 'count' links, starting with link
   zero and page zero.  Thus, specifying count of the name 10 and page of a service being offered on 0 will
   return the
   network SHOULD be configurable by first 10 links in the user setting up result set (links 0-9).  Count = 10
   and page = 1 will return the service, next 'page' containing links 10-19, and
   so
   that he or she may give it an informative name.  However, the device
   or service SHOULD NOT require the user to configure a name before it
   can on.

   Multiple query parameters MAY be used.  A sensible choice of default name can allow the device
   or service included in a lookup, all included
   parameters MUST match for a resource to be accessed in many cases without any manual
   configuration at all. returned.  The default name should be short and
   descriptive, and
   character'*' MAY include be included at the end of a collision-resistant substring such parameter value as
   the lower bits a
   wildcard operator.

   RD Lookup requests MAY use any set of the device's MAC address, serial number,
   fingerprint, or other identifier in an attempt query parameters to make match the name
   relatively unique.

   DNS labels are currently limited to 63 octets in length
   registered attributes and the
   entire service name may not exceed 255 octets.

10.3.  Mapping rt to <ServiceType>

   The resource type "rt" attribute is mapped into the <ServiceType>
   part of relations.  In addition, this interface MAY
   be used with queries that specify domains, endpoints, and groups.
   For example, a domain lookup filtering on groups would return a DNS-SD service name and SHOULD conform to the reg-rel-type
   production list
   of domains that contain the Link Format defined in Section 2 of [RFC6690].  The
   "rt" attribute MUST be composed of at least specified groups.  An endpoint lookup
   filtering on groups would return a single Net-Unicode text
   string, without underscore '_' or period '.' and limited to 15 octets list of endpoints that are in length, which represents the application protocol name.
   specified groups.

   The lookup interface is specified as follows:

   Interaction:  Client -> RD

   Method:  GET

   URI Template:  /{+rd-lookup-base}/{lookup-
      type}{?d,res,ep,gp,et,rt,page,count,resource-param}

   URI Template Variables:

      rd-lookup-base :=  RD Lookup Base URI path (mandatory).  This
   string is mapped to the DNS-SD <ServiceType> by prepending an
   underscore and appending a period followed by
         the "_udp" label.  For
   example, rt="dali" is mapped into "_dali._udp". base path for RD Lookup requests.  The application protocol name may be optionally followed by a period
   and a service subtype name consisting of a Net-Unicode text string,
   without underscore or period and limited to 63 octets. recommended value
         for this variable is: "rd-lookup".

      lookup-type :=  ("ep", "res") (mandatory), ("d","gp") (optional)
         This string variable is mapped used to select the DNS-SD <ServiceType> by appending a period followed
   by the "_sub" label and then appending a period followed by the
   service type label pair derived of lookup to perform
         (endpoint, resource, domain, or group).  The values are
         recommended defaults and MAY use other values as needed.  The
         supported lookup-types SHOULD be listed in .well-known/core
         using the previous paragraph.  For
   example, rt="dali.light" is mapped into "light._sub._dali._udp".

   The resulting string is used to form labels specified resource types.

      ep :=  Endpoint name (optional).  Used for DNS-SD records which endpoint, group and
         resource lookups.

      d :=  Domain (optional).  Used for domain, group, endpoint and
         resource lookups.

      res :=  resource (optional).  Used for domain, group, endpoint and
         resource lookups.

      gp := Group name (optional).  Used for endpoint, group and
      resource lookups.

      page :=  Page (optional).  Parameter can not be used without the
         count parameter.  Results are stored directly returned from result set in the DNS.

10.4.  Domain mapping

   DNS domains may be derived pages
         that contain 'count' links starting from the "d" attribute.  The domain
   attribute may be suffixed index (page * count).
         Page numbering starts with the zone name zero.

      count :=  Count (optional).  Number of the authoritative DNS
   server to generate the domain name.  The "ep" attribute results is prefixed limited to this
         parameter value.  If the domain name to generate page parameter is also present, the FQDN to be stored into DNS
         response MUST only include 'count' links starting with an
   AAAA RR.

10.5.  TXT Record key=value strings

   A number of [RFC6763] key/value pairs are derived from link-format
   information, to be exported the
         (page * count) link in the DNS-SD as key=value strings result set from the query.  If the
         count parameter is not present, then the response MUST return
         all matching links in a
   TXT record ([RFC6763], Section 6.3).

   The the result set.  Link numbering starts
         with zero.

      rt :=  Resource type (optional).  Used for group, endpoint and
         resource <URI> is exported as key/value pair "path=<URI>".

   The Interface Description "if" attribute is exported as key/value
   pair "if=<Interface Description>".

   The DNS TXT record can be further populated by importing any other lookups.

      et :=  Endpoint type (optional).  Used for group, endpoint and
         resource description attributes lookups.

      resource-param :=  Link attribute parameters (optional).  Any link
         target attribute as they share the same key=value
   format specified defined in Section 6 4.1 of [RFC6763].

10.6.  Importing [RFC6690], used
         for resource links into DNS-SD

   Assuming lookups.

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

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

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

   The following responses codes are defined for this interface:

   Success:  2.05 "Content" or 200 "OK" with an "application/link-
      format", "application/link-format+cbor", or "application/link-
      format+json" payload containing matching entries for the ability lookup.

   Failure:  4.04 "Not Found" or 404 "Not Found" in case no matching
      entry is found for a unicast request.

   Failure:  No error response to query a Resource Directory or multicast a GET
   (?exp) over the local link, CoAP resource discovery may be used to
   populate request.

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

   Failure:  5.03 "Service Unavailable" or 503 "Service Unavailable".
      Service could not perform the DNS-SD database operation.

   HTTP support:  YES

   The examples in an automated fashion. this section assume CoAP resource
   descriptions (links) can be exported to DNS-SD for exposure to
   service discovery by using the Resource Instance attribute as the
   basis for a unique service name, composed hosts with the Resource Type as
   the <ServiceType>, a default CoAP
   port 61616.  HTTP hosts are possible and registered in the correct <Domain>.  The agent
   responsible for exporting records to do not change the DNS zone file SHOULD be
   authenticated to nature of
   the DNS server. examples.

   The following example, using the example shows a client performing a resource lookup
   with the example look-up location /rd-lookup, /rd-lookup/:

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616/temp>;rt="temperature"

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

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

   Res: 2.05 Content
   <coap://[FDFD::127]:61616>;ep="node5",
   <coap://[FDFD::129]:61616>;ep="node7"

   The following example shows a
   resource to be exported: client performing a domain lookup:

   Req: GET /rd-lookup/res?exp /rd-lookup/d

   Res: 2.05 Content
      <coap://[FDFD::1234]:5683/light/1>;
        exp;rt="dali.light";ins="Spot";
                  d="office";ep="node1"
   <>;d="domain1",
   <>;d="domain2"

   The agent subsequently registers the following DNS-SD RRs, assuming example shows a
   zone name "example.com" prefixed with "office":

   node1.office.example.com.          IN AAAA        FDFD::1234
   _dali._udp.office.example.com      IN PTR
                             Spot._dali._udp.office.example.com
   light._sub._dali._udp.example.com  IN PTR
                             Spot._dali._udp.office.example.com
   Spot._dali._udp.office.example.com IN SRV  0 0 5683
                             node1.office.example.com.
   Spot._dali._udp.office.example.com IN TXT
                             txtver=1;path=/light/1

   In the above figure the Service Name is chosen as
   Spot._dali._udp.office.example.com without the light._sub service
   prefix.  An alternative Service Name would be:
   Spot.light._sub._dali._udp.office.example.com.

11. client performing a group lookup for
   all groups:

   Req: GET /rd-lookup/gp

   Res: 2.05 Content
   <>;gp="lights1";d="example.com"
   <>;gp="lights2";d="ecample.com"

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

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616>;ep="node1",
   <coap://[FDFD::124]:61616>;ep="node2"

   The following example shows a client performing a lookup for all
   groups an endpoint belongs to:

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

   Res: 2.05 Content
   <>;gp="lights1"

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

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

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

   Res: 2.05 Content
   <coap://[FDFD::123]:61616/res/5>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/6>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/7>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/8>;rt=sensor;ct=60
   <coap://[FDFD::123]:61616/res/9>;rt=sensor;ct=60

8.  Security Considerations

   The security considerations as described in Section 7 of [RFC5988]
   and Section 6 of [RFC6690] apply.  The "/.well-known/core" resource
   may be protected e.g. using DTLS when hosted on a CoAP server as
   described in [RFC7252].  DTLS or TLS based security SHOULD be used on
   all resource directory interfaces defined in this document.

11.1.

8.1.  Endpoint Identification and Authentication

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

   Every operation performed by an Endpoint or Client on a resource
   directory SHOULD be mutually authenticated using Pre-Shared Key, Raw
   Public Key or Certificate based security.  Endpoints using a
   Certificate MUST include the Endpoint identifier as the Subject of
   the Certificate, and this identifier MUST be checked by a resource
   directory to match the Endpoint identifier included in the
   Registration message.

11.2.

8.2.  Access Control

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

11.3.

8.3.  Denial of Service Attacks

   Services that run over UDP unprotected are vulnerable to unknowingly
   become part of a DDoS attack as UDP does not require return
   routability check.  Therefore, an attacker can easily spoof the
   source IP of the target entity and send requests to such a service
   which would then respond to the target entity.  This can be used for
   large-scale DDoS attacks on the target.  Especially, if the service
   returns a response that is order of magnitudes larger than the
   request, the situation becomes even worse as now the attack can be
   amplified.  DNS servers have been widely used for DDoS amplification
   attacks.  There is also a danger that NTP Servers could become
   implicated in denial-of-service (DoS) attacks since they run on
   unprotected UDP, there is no return routability check, and they can
   have a large amplification factor.  The responses from the NTP server
   were found to be 19 times larger than the request.  A Resource
   Directory (RD) which responds to wild-card lookups is potentially
   vulnerable if run with CoAP over UDP.  Since there is no return
   routability check and the responses can be significantly larger than
   requests, RDs can unknowingly become part of a DDoS amplification
   attack.  Therefore, it is RECOMMENDED that implementations ensure
   return routability.  This can be done, for example by responding to
   wild card lookups only over DTLS or TLS or TCP.

12.

9.  IANA Considerations

12.1.

9.1.  Resource Types

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

12.2.  Link Extension

   The "exp" and "ins" attributes need to be registered when a future
   Web Linking link-extension registry is created (e.g. in RFC5988bis).

12.3.

9.2.  IPv6 ND Resource Directory Address Option

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

   o  Resource Directory address Option (38)

12.4.

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 query parameter, validity requirements if any and
   a description.  The query parameter MUST be a valid URI query key
   [RFC3986].

   Initial entries in this sub-registry are as follows:

   +-----------+-------+---------------+-------------------------------+

   +----------+-------+---------------+--------------------------------+
   | Name     | Query | Validity      | Description                    |
   +-----------+-------+---------------+-------------------------------+
   +----------+-------+---------------+--------------------------------+
   | Endpoint | ep    |               | Name of the endpoint, max 63   |
   | Name     |       |               | bytes                          |
   | Lifetime | lt    | 60-4294967295 | Lifetime of the registration   |
   |          |       |               | in seconds                     |
   | Domain   | d     |               | Domain to which this endpoint  |
   |          |       |               | belongs                        |
   | Endpoint | et    |               | Semantic name of the endpoint  |
   | Type     |       |               |                                |
   | Context  | con   | URI           | The scheme, address and port   |
   |          |       |               | at which this server is        |
   |          |       |               | available                      |
   | Resource | res   |               | Name of the resource           |
   | Name     |       |               |                                |
   | Group    | gp    |               | Name of a group in the RD      |
   | Name     |       |               |                                |
   | Page     | page  | Integer       | Used for pagination            |
   | Count    | count | Integer       | Used for pagination            |
   | Resource  | ins   |               | Instance Identifier           |
   | Instance  |       |               |                               |
   | Export    | exp   |               | A link MAY be exported to     |
   |           |       |               | another Resource Directory    |
   +-----------+-------+---------------+-------------------------------+
   +----------+-------+---------------+--------------------------------+

                          Table 1: 2: RD Parameters

   The IANA policy for future additions to the sub-registry is "Expert
   Review" as described in [RFC5226].

13.

10.  Examples

13.1.

   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 Resource Directory (RD) with a CoAP interface to facilitate
   the installation and start up of the application code in the lights
   and sensors.  In particular, the example leads to the definition of a
   group and the enabling of the corresponding multicast address.  No
   conclusions must be drawn on the realization of actual installation
   or naming procedures, because the example only "emphasizes" some of
   the issues that may influence the use of the RD and does not pretend
   to be normative.  The example uses the recommended values for the
   base resources: "/rd", "/rd-lookup", and "/rd-group".

13.1.1.

10.1.1.  Installation Characteristics

   The example assumes that the installation is managed.  That means
   that a Commissioning Tool (CT) is used to authorize the addition of
   nodes, name them, and name their services.  The CT can be connected
   to the installation in many ways: the CT can be part of the
   installation network, connected by WiFi to the installation network,
   or connected via GPRS link, or other method.

   It is assumed that there are two naming authorities for the
   installation: (1) the network manager that is responsible for the
   correct operation of the network and the connected interfaces, and
   (2) the lighting manager that is responsible for the correct
   functioning of networked lights and sensors.  The result is the
   existence of two naming schemes coming from the two managing
   entities.

   The example installation consists of one presence sensor, and two
   luminaries, luminary1 and luminary2, each with their own wireless
   interface.  Each luminary contains three lamps: left, right and
   middle.  Each luminary is accessible through one endpoint.  For each
   lamp a resource exists to modify the settings of a lamp in a
   luminary.  The purpose of the installation is that the presence
   sensor notifies the presence of persons to a group of lamps.  The
   group of lamps consists of: middle and left lamps of luminary1 and
   right lamp of luminary2.

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

   At the moment of installation, the network under installation is not
   necessarily connected to the DNS infra structure.  Therefore, SLAAC
   IPv6 addresses are assigned to CT, RD, luminaries and sensor shown in
   Table 2 3 below:

                   +--------------------+--------------+
                   | Name               | IPv6 address |
                   +--------------------+--------------+
                   | luminary1          | FDFD::ABCD:1 |
                   | luminary2          | FDFD::ABCD:2 |
                   | Presence sensor    | FDFD::ABCD:3 |
                   | Resource directory | FDFD::ABCD:0 |
                   +--------------------+--------------+

                    Table 2: 3: interface SLAAC addresses

   In Section 13.1.2 10.1.2 the use of resource directory during installation
   is presented.  In Section 13.1.3 the connection to DNS is discussed.

13.1.2.

10.1.2.  RD entries

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

   For discovery, the resource types (rt) of the devices are important.
   The lamps in the luminaries have rt: light, and the presence sensor
   has rt: p-sensor.  The endpoints have names which are relevant to the
   light installation manager.  In this case luminary1, luminary2, and
   the presence sensor are located in room 2-4-015, where luminary1 is
   located at the window and luminary2 and the presence sensor are
   located at the door.  The endpoint names reflect this physical
   location.  The middle, left and right lamps are accessed via path
   /light/middle, /light/left, and /light/right respectively.  The
   identifiers relevant to the Resource Directory are shown in Table 3 4
   below:

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

                  Table 3: 4: Resource Directory identifiers

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

   Req: POST coap://[FDFD::ABCD:0]/rd
     ?ep=lm_R2-4-015_wndw&con=coap://[FDFD::ABCD:1]
   Payload:
   </light/left>;rt="light"; d="R2-4-015",
   </light/middle>;rt="light"; d="R2-4-015",
   </light/right>;rt="light";d="R2-4-015"

   Res: 2.01 Created
   Location: /rd/4521

   Req: POST coap://[FDFD::ABCD:0]/rd
     ?ep=lm_R2-4-015_door&con=coap://[FDFD::ABCD:2]
   Payload:
   </light/left>;rt="light"; d="R2-4-015",
   </light/middle>;rt="light"; d="R2-4-015",
   </light/right>;rt="light"; d="R2-4-015"

   Res: 2.01 Created
   Location: /rd/4522

   Req: POST coap://[FDFD::ABCD:0]/rd
     ?ep=ps_R2-4-015_door&con=coap://[FDFD::ABCD:3]
   Payload:
   </ps>;rt="p-sensor"; d="R2-4-015"

   Res: 2.01 Created
   Location: /rd/4523

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

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

   Req: POST coap://[FDFD::ABCD:0]/rd-group
   ?gp=grp_R2-4-015;con="coap//[FF05::1]";exp;ins="grp1234"
   ?gp=grp_R2-4-015;con="coap//[FF05::1]"
   Payload:
   <>ep=lm_R2-4-015_wndw,
   <>ep=lm_R2-4-015_door,
   <>ep=ps_R2-4-015_door

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

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

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

   Req: GET coap://[FDFD::ABCD:0]/rd-lookup/ep
     ?d=R2-4-015;rt=light

   Res: 2.05 Content
   <coap://[FDFD::ABCD:1]>;
     ep="lm_R2-4-015_wndw",
   <coap://[FDFD::ABCD:2]>;
      ep="lm_R2-4-015_door"

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

   Req: GET coap://[FDFD::ABCD:0]/rd-lookup/gp
     ?ep=lm_R2-4-015_wndw

   Res: 2.05 Content
   <coap://[FF05::1]>;gp="grp_R2-4-015"

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

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

   Req: POST //[FDFD::ABCD:1]/coap-group
             Content-Format: application/coap-group+json
          { "a": "[FF05::1]",
             "n": "grp_R2-4-015"}

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

   Dependent on the situation, only the address, "a", or the name, "n",
   is specified in the coap-group resource.

13.1.3.  DNS entries

   It may be profitable to discover the light groups for applications,
   which are unaware ot the existence of the RD.  An agent needs to
   query the RD to return all groups which are exported to be inserted
   into DNS.

      Req: GET /rd-lookup/gp?exp

      Res: 2.05 Content
      <coap://[FF05::1]/>;exp;gp="grp_R2-4-015;ins="grp1234";
   ep="lm_R2-4-015_wndw";
   ep="lm_R2-4-015_door

   The group with FQDN grp_R2-4-015.bc.example.com can be entered into
   the DNS by the agent.  The accompanying instance name is grp1234.
   The <ServiceType> is chosen to be _group._udp.  The agent enters the
   following RRs into the DNS.

   grp_R2-4-015.bc.example.com.        IN AAAA            FF05::1
   _group._udp.bc.example.com          IN PTR
                               grp1234._group._udp.bc.example.com
   grp1234._group._udp.bc.example.com  IN SRV  0 0 5683
                                grp_R2-4-015_door.bc.example.com.
   grp1234._group._udp.bc.example.com  IN TXT
                                        txtver=1;path=/light/grp1

   From then using the "coap-group" resource specified in
   [RFC7390].

   Req: POST //[FDFD::ABCD:1]/coap-group
             Content-Format: application/coap-group+json
          { "a": "[FF05::1]",
             "n": "grp_R2-4-015"}

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

   Dependent on applications, not familiar with the existence of situation, only the RD,
   can use DNS to access address, "a", or the lighting group.

13.2. name, "n",
   is specified in the coap-group resource.

10.2.  OMA Lightweight M2M (LWM2M) Example

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

   OMA LWM2M is a profile for device services based on CoAP(OMA Name
   Authority).  LWM2M defines a simple object model and a number of
   abstract interfaces and operations for device management and device
   service enablement.

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

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

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

   The LWM2M specification describes a set of interfaces and a resource
   model used between a LWM2M device and an LWM2M server.  Other
   interfaces, proxies, applications, and function sets applications are currently out of scope for
   LWM2M.

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

13.2.1.

10.2.1.  The LWM2M Object Model

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

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

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

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

   The URI template for LWM2M consists of a base URI followed by Object,
   Instance, and Resource IDs:

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

   The five variables given here are strings.  base-uri can also have
   the special value "undefined" (sometimes called "null" in RFC 6570).
   Each of the variables object-instance, resource-id, and resource-
   instance can be the special value "undefined" only if the values
   behind it in this sequence also are "undefined".  As a special case,
   object-instance can be "empty" (which is different from "undefined")
   if resource-id is not "undefined".  [_TEMPLATE_TODO]

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

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

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

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

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

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

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

13.2.2.

10.2.2.  LWM2M Register Endpoint

   LWM2M defines a registration interface based on the Resource
   Directory Function Set, REST API,
   described in Section 6. 5.  The base URI path of the LWM2M Resource
   Directory function set is specified to be "/rd" as recommended in Section 6.3. 5.3.

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

   Resources within the LWM2M object instance are not registered with
   the RD, but may be discovered by reading the resource links from the
   object instance using GET with a CoAP Content-Format of application/
   link-format.  Resources may also be read as a structured object by
   performing a GET to the object instance with a Content-Format of
   senml+json.

   When an LWM2M object or instance is registered, this indicates to the
   LWM2M server that the object and its resources are available for
   management and service enablement (REST API) operations.

   LWM2M endpoints may use the following RD registration parameters as
   defined in Table 1 2 :

   ep - Endpoint Name
   lt - registration lifetime

   Endpoint Name is mandatory, all other registration parameters are
   optional.

   Additional optional LWM2M registration parameters are defined:

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

             Table 4: 5: LWM2M Additional Registration Parameters

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

   et - Endpoint Type
   con - Context

   The endpoint registration must include a payload containing links to
   all supported objects and existing object instances, optionally
   including the appropriate link-format relations.

   Here is an example LWM2M registration payload:

   </1>,</1/0>,</3/0>,</5>

   This link format payload indicates that object ID 1 (LWM2M Server
   Object) is supported, with a single instance 0 existing, object ID 3
   (LWM2M Device object) is supported, with a single instance 0
   existing, and object 5 (LWM2M Firmware Object) is supported, with no
   existing instances.

13.2.3.  Alternate Base URI

   If the LWM2M endpoint exposes objects at a base URI other than the
   default empty base path, the endpoint must register the base URI
   using rt="oma.lwm2m".  An example link payload using alternate base
   URI would be:

   </my_lwm2m>;rt="oma.lwm2m",</my_lwm2m/1>,<my_lwm2m/1/0>,<my_lwm2m/5>

   This link payload indicates that the lwm2m objects will be placed
   under the base URI "/my_lwm2m" and that object ID 1 (server) is
   supported, with a single instance 0 existing, and object 5 (firmware
   update) is supported.

13.2.4.

10.2.3.  LWM2M Update Endpoint Registration

   An LWM2M Registration update proceeds as described in Section 6.4, 5.4.1,
   and adds some optional parameter updates:

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

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

13.2.5.

10.2.4.  LWM2M De-Register Endpoint

   LWM2M allows for de-registration using the delete method on the
   returned location from the initial registration operation.  LWM2M de-
   registration proceeds as described in Section 6.5.

14. 5.4.2.

11.  Acknowledgments

   Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders
   Brandt, Matthieu Vial, Mohit Sethi, Sampo Ukkola and Ukkola, Linyi Tian Tian,
   Chistian Amsuss, and Jan Newmarch have provided helpful comments,
   discussions and ideas to improve and shape this document.  Section 9 is based on an earlier draft by Kerry Lynn.  Zach would
   also like to thank his colleagues from the EU FP7 SENSEI project,
   where many of the resource directory concepts were originally
   developed.

15.

12.  Changelog

   changes from -09 to -10

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

   o  removed all text concerning DNS-SD.

   o  removed inconsistency in RDAO text.

   o  suggestions taken over from various sources

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

   o  moved simple registration to registration section

   changes from -08 to -09

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

   o  changed "ins" ABNF notation.

   o  various editorial improvements, including in examples

   o  clarifications for RDAO

   changes from -07 to -08

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

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

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

   o  add IPv6 ND Option for discovery of an RD

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

   o  removed all superfluous client-server diagrams

   o  simplified lighting example

   o  introduced Commissioning Tool

   o  RD-Look-up text is extended.

   changes from -06 to -07

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

   o  editorial updates to section 9

   o  update author information

   o  minor text corrections

   Changes from -05 to -06

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

   changes from -04 to -05

   o  added Update Endpoint Links using PATCH

   o  http access made explicit in interface specification

   o  Added http examples

   Changes from -03 to -04:

   o  Added http response codes
   o  Clarified endpoint name usage

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

   Changes from -02 to -03:

   o  Added an example for lighting and DNS integration

   o  Added an example for RD use in OMA LWM2M

   o  Added Read Links operation for link inspection by endpoints

   o  Expanded DNS-SD section

   o  Added draft authors Peter van der Stok and Michael Koster

   Changes from -01 to -02:

   o  Added a catalogue use case.

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

   o  Additional examples section added for more complex use cases.

   o  New DNS-SD mapping section.

   o  Added text on endpoint identification and authentication.

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

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

   Changes from -00 to -01:

   o  Removed the ETag validation feature.

   o  Place holder for the DNS-SD mapping section.

   o  Explicitly disabled GET or POST on returned Location.

   o  New registry for RD parameters.

   o  Added support for the JSON Link Format.

   o  Added reference to the Groupcomm WG draft.

   Changes from -05 to WG Document -00:

   o  Updated the version and date.

   Changes from -04 to -05:

   o  Restricted Update to parameter updates.

   o  Added pagination support for the Lookup interface.

   o  Minor editing, bug fixes and reference updates.

   o  Added group support.

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

   Changes from -03 to -04:

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

   o  Integrated the Simple Directory Discovery from Carsten.

   o  Editorial improvements.

   o  Fixed the use of ETags.

   o  Fixed tickets 383 and 372

   Changes from -02 to -03:

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

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

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

   o  Improved the security considerations section.

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

   Changes from -01 to -02:

   o  Added a terminology section.

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

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

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

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

16.

13.  References

16.1.

13.1.  Normative References

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., and C. Bormann, "Representing CoRE
              Formats in JSON and CBOR", draft-ietf-core-links-json-06
              (work in progress), July 2016.

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

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

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

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <http://www.rfc-editor.org/info/rfc6335>.

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

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

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

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 2014,
              <http://www.rfc-editor.org/info/rfc7396>.

16.2.

13.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <http://www.rfc-editor.org/info/rfc1123>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <http://www.rfc-editor.org/info/rfc5198>.

   [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,
              <http://www.rfc-editor.org/info/rfc6775>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <http://www.rfc-editor.org/info/rfc7390>.

Editorial Comments

[_TEMPLATE_TODO] This text needs some help from an RFC 6570 expert.

Authors' Addresses

   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   USA

   Phone: +1-408-203-9434
   Email: zach.shelby@arm.com
   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

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

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

   Peter van der Stok
   consultant

   Phone: +31-492474673 (Netherlands), +33-966015248 (France)
   Email: consultancy@vanderstok.org
   URI:   www.vanderstok.org