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


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

Abstract

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

Status of This Memo

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

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

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

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








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Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture and Use Cases  . . . . . . . . . . . . . . . . .   5
     3.1.  Use Case: Cellular M2M  . . . . . . . . . . . . . . . . .   6
     3.2.  Use Case: Home and Building Automation  . . . . . . . . .   7
     3.3.  Use Case: Link Catalogues . . . . . . . . . . . . . . . .   7
   4.  Finding a Directory Server  . . . . . . . . . . . . . . . . .   8
     4.1.  Resource Directory Address Option (RDAO)  . . . . . . . .   9
   5.  Simple Registration . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Simple publishing to Resource Directory Server  . . . . .  11
     5.2.  Third-party registration  . . . . . . . . . . . . . . . .  12
   6.  Resource Directory Function Set . . . . . . . . . . . . . . .  12
     6.1.  Content Formats . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  15
     6.4.  Registration Update . . . . . . . . . . . . . . . . . . .  18
     6.5.  Registration Removal  . . . . . . . . . . . . . . . . . .  20
     6.6.  Read Endpoint Links . . . . . . . . . . . . . . . . . . .  21
     6.7.  Update Endpoint Links . . . . . . . . . . . . . . . . . .  22
   7.  Group Function Set  . . . . . . . . . . . . . . . . . . . . .  24
     7.1.  Register a Group  . . . . . . . . . . . . . . . . . . . .  24
     7.2.  Group Removal . . . . . . . . . . . . . . . . . . . . . .  26
   8.  RD Lookup Function Set  . . . . . . . . . . . . . . . . . . .  27
   9.  New Link-Format Attributes  . . . . . . . . . . . . . . . . .  32
     9.1.  Resource Instance attribute 'ins' . . . . . . . . . . . .  32
     9.2.  Export attribute 'exp'  . . . . . . . . . . . . . . . . .  33
   10. DNS-SD Mapping  . . . . . . . . . . . . . . . . . . . . . . .  33
     10.1.  DNS-based Service discovery  . . . . . . . . . . . . . .  33
     10.2.  mapping ins to <Instance>  . . . . . . . . . . . . . . .  34
     10.3.  Mapping rt to <ServiceType>  . . . . . . . . . . . . . .  35
     10.4.  Domain mapping . . . . . . . . . . . . . . . . . . . . .  35



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     10.5.  TXT Record key=value strings . . . . . . . . . . . . . .  35
     10.6.  Importing resource links into DNS-SD . . . . . . . . . .  36
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  37
     11.1.  Endpoint Identification and Authentication . . . . . . .  37
     11.2.  Access Control . . . . . . . . . . . . . . . . . . . . .  37
     11.3.  Denial of Service Attacks  . . . . . . . . . . . . . . .  37
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
     12.1.  Resource Types . . . . . . . . . . . . . . . . . . . . .  38
     12.2.  Link Extension . . . . . . . . . . . . . . . . . . . . .  38
     12.3.  IPv6 ND Resource Directory Address Option  . . . . . . .  38
     12.4.  RD Parameter Registry  . . . . . . . . . . . . . . . . .  38
   13. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  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.  Normative References . . . . . . . . . . . . . . . . . .  52
     16.2.  Informative References . . . . . . . . . . . . . . . . .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

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



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   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




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      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 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 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 to proactively register and maintain resource
   directory 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 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 Web
   Links from endpoints and add them as resource directory entries.  A
   lookup interface for discovering any of the Web Links held in the RD
   is provided using the CoRE Link Format.










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                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



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   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



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   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 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 are supplied by
   Resource Directories, which may be internally stored as triples, or
   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 Directory Server

   Endpoints that want to contact a directory server 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




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   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 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:



















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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length = 3   |       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 is
                           valid.  A value of all zero bits (0x0)
                           assumes a default value of 10,000
                           (~one week).

   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 a Resource Directory.  Instead, simple endpoints
   can implement the generic Simple Directory Discovery approach
   described in this section.  An RD implementing this specification
   MUST implement Simple Directory Discovery.  However, there may be



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   security reasons why this form of directory discovery would be
   disabled.

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

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

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

   The directory server integrates the information it received this way
   into its resource directory.  It MAY make 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 the scope of this document.

5.1.  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 directory server to perform GET
   requests at the requesting server's default discovery URI to obtain
   the link-format payload to register.

   The endpoint MAY include registration parameters in the POST request
   as per Section 6.3.

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


















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   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 for certain very constrained devices, in particular if the
   security requirements become too onerous.

   In a controlled environment (e.g. building control), the Resource
   Directory can be filled by a third device, called a commissioning
   tool.  The commissioning tool can fill the Resource Directory from a
   database or other means.  For that purpose the scheme, IP address and
   port of the registered device is indicated in the Context parameter
   of the registration described in Section 6.3.

6.  Resource Directory Function Set

   This section defines the REST interfaces between an RD and endpoints,
   which is called the Resource Directory Function Set. Although the
   examples throughout this section assume the use of CoAP [RFC7252],
   these REST interfaces can also be realized using HTTP [RFC7230].  In
   all definitions in this section, both CoAP response codes (with dot
   notation) and HTTP response codes (without dot notation) are shown.
   An RD implementing this specification MUST support the discovery,
   registration, update, lookup, and removal interfaces defined in this
   section.



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   Resource directory entries are designed to be easily exported to
   other discovery mechanisms such 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).

   Resource Directories implementing this specification MAY support
   additional content formats.

   Any additional content format supported by a Resource Directory
   implementing this specification MUST have an equivalent serialization
   in the application/link-format content format.

6.2.  Discovery

   Before an endpoint can make use of 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 for discovering the RD including assuming a
   default location (e.g. on an Edge Router in a LoWPAN), by assigning
   an anycast address to the RD, using DHCP, or by discovering the RD
   using the CoRE Link Format (see also Section 4).  This section
   defines discovery of the RD using the well-known interface of the
   CoRE Link Format [RFC6690] as the required mechanism.  It is however
   expected that RDs will also be discoverable via other methods
   depending on the deployment.

   Discovery of the RD function set is performed by sending either a
   multicast or unicast GET request to "/.well-known/core" and including
   a Resource Type (rt) parameter [RFC6690] with the value "core.rd" in
   the query string.  Likewise, a Resource Type parameter value of
   "core.rd-lookup" is used to discover the RD Lookup Function Set.
   Upon success, the response will contain a payload with a link format
   entry for each RD discovered, with the URL indicating the root
   resource of the RD.  When performing multicast discovery, the
   multicast IP address used will depend on the scope required and the
   multicast capabilities of the network.

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





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

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

   The discovery request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

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

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

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

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

   The following response codes are defined for this interface:

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

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

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

   Failure:  No error response to a multicast request.

   HTTP support :  YES (Unicast only)

   The following example shows an endpoint discovering an RD using this
   interface, thus learning that the base RD resource is, in this



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   example, at /rd and that the content_format delivered by the server
   hosting the resource is application.xml (ct=40).  Note that it is up
   to the RD to choose its base RD resource, although diagnostics and
   debugging is facilitated by 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>;rt="core.rd-lookup";ct=40,
   </rd-group>;rt="core.rd-group";ct=40

   The following example shows the way of indicating that a client may
   request alternate content-formats.  The Content-Format code attribute
   "ct" MAY include a space-separated sequence of Content-Format codes
   as specified in Section 7.2.1 of [RFC7252], indicating that multiple
   content-formats are available.  The example below shows the required
   Content-Format 40 (application/link-format) indicated as well as a
   more application-specific content format (picked as 65225 in this
   example; this is in the experimental space, not an assigned value).
   The base RD resource 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>;rt="core.rd-lookup";ct="40 65225",
   </rd-group>;rt="core.rd-group";ct="40 65225"

6.3.  Registration

   After discovering the location of an RD Function Set, an endpoint MAY
   register its resources using the registration interface.  This
   interface accepts a POST from an endpoint containing the list of
   resources to be added to the directory as the message payload in the
   CoRE Link Format [RFC6690], JSON CoRE Link Format (application/link-
   format+json), or CBOR CoRE Link Format (application/link-format+cbor)
   [I-D.ietf-core-links-json], along with query string parameters
   indicating the name of the endpoint, its domain and the lifetime of
   the registration.  All parameters except the endpoint name are
   optional.  It is expected that other specifications will define
   further parameters (see Section 12.4).  The RD then creates a new
   resource or updates an existing resource in the RD and returns its
   location.  An endpoint MUST use that location when refreshing
   registrations using this interface.  Endpoint resources in the RD are
   kept active for the period indicated by the lifetime parameter.  The



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   endpoint is responsible for refreshing the entry within this period
   using either the registration or update interface.  The registration
   interface MUST be implemented to be idempotent, so that registering
   twice with the same endpoint parameter does not create multiple RD
   entries.  A new registration may be created at any time to supersede
   an existing registration, replacing the registration parameters and
   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 the path of the
         RD Function Set, as obtained from discovery.  The value "rd" is
         recommended for this variable.

      ep :=  Endpoint name (mandatory).  The endpoint 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 to which this endpoint
         belongs.  The maximum length of this parameter is 63 bytes.
         When this parameter is elided, the RD MAY associate the
         endpoint with a configured default domain.  The domain value 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 of the 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
         when the directory is filled by a third party such as an
         commissioning tool.

   Content-Format:  application/link-format



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   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 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

   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







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6.4.  Registration Update

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

   An update MAY update the lifetime or context registration parameters
   "lt", "con" as in Section 6.3 ) if they have changed since the last
   registration or update.  Parameters that have not changed SHOULD NOT
   be included in an update.  Adding parameters that have not changed
   increases the size of the message but does not have any other
   implications.  Parameters MUST be included as query parameters in an
   update operation as in Section 6.3.

   Upon receiving an update request, an RD MUST reset the timeout for
   that endpoint and update 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" is
   included in the received update request, the RD MUST update the
   lifetime 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 in the payload of the update as a CoRE Link
   Format document.  A link is replaced only if both the target URI and
   relation type match.

   In addition to the use of POST, as described in this section, there
   is an alternate way to add, replace, and delete 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 :=  This is the Location path returned by the RD as a
         result of a successful earlier registration.

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



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

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

   HTTP support:  YES

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

   Req: POST /rd/4521

   Res: 2.04 Changed

   The following example shows an endpoint updating its registration
   with a new lifetime and context, changing an existing link, and
   adding a new link using this interface with the example location
   value /rd/4521.  With the initial registration the client set the
   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"



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   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 have soft state and will eventually timeout after
   their lifetime, an endpoint SHOULD explicitly remove its entry from
   the RD if it knows it will no longer be available (for example on
   shut-down).  This is accomplished using a removal interface on the RD
   by performing a DELETE on the endpoint resource.

   The removal request interface is specified as follows:

   Interaction:  EP -> 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 earlier registration.

   The following responses codes are defined for this interface:

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

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

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

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

   HTTP support: YES

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





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   Req: DELETE /rd/4521

   Res: 2.02 Deleted

6.6.  Read Endpoint Links

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

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

   The read request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

      location :=  This is the Location 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

   The following responses codes are defined for this interface:

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

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

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

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

   HTTP support: YES




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

   Req: GET /rd/4521

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

6.7.  Update Endpoint Links

   [This section will be removed before or as a result of a working-
   group last-call if the PATCH methods do not achieve the same level of
   consensus as the present document.]

   A PATCH update adds, removes or changes links for the endpoint by
   including link update information in the payload of 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 and values in the query
   parameters select one or more links on which to apply the PATCH
   operation.

   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 not present in any selected links,
   the attribute MUST be added to the links.

   The update request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  PATCH

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

   URI Template Variables:



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      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 codes are defined for this interface:

   Success:  2.04 "Changed" 0r 204 "No Content" in 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 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 endpoint adding </sensors/humid>,
   modifying </sensors/temp>, and removing </sensors/light> links in RD
   using the Update Endpoint Links function 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 /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 links at the example
   location /rd/4521 which are identified by href="/sensors/temp", from
   the initial link-value of </sensors/temp>;rt="temperature" to the new



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   link-value </sensors/temp>;rt="temperature-c";if="sensor" by changing
   the value of the link attribute "rt" and adding the link attribute
   if="sensor" using 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 an EP removing all links at the example 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 the creation of groups of
   endpoints for 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 of a list of
   endpoints and does not have a lifetime associated with it.  In order
   to make use of multicast requests with CoAP, a group MAY have a
   multicast address associated with it.

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
   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 endpoints that belong to
   that group.




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   All the endpoints in the group MUST be registered with the RD before
   registering a group.  If an endpoint is not yet registered to the RD
   before registering the group, the registration message returns an
   error.  The RD sends a blank target URI for every endpoint link when
   registering the group.

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

   The registration request interface is specified as follows:

   Interaction:  CT -> RD

   Method:  POST

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

   URI Template Variables:

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

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

      d :=  Domain (optional).  The domain to which this group belongs.
         The maximum length of this parameter is 63 bytes.  Optional.
         When this parameter is elided, the RD MAY associate the
         endpoint with a configured default domain.  The domain value is
         needed to export the endpoint to DNS-SD (see Section 10)

      con :=  Context (optional).  This parameter is used to set the IP
         multicast address at which this server is available in the form
         scheme://multicast-address:port.  Optional.  In the absence of
         this parameter no multicast address is configured.  This
         parameter is compulsory when the directory is filled by a
         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:



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   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 the RD as it is used for delete
      operations on this registration.

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

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

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

   HTTP support:  YES

   The following example shows an EP registering a group with the name
   "lights" which has two endpoints to an RD using this interface.  The
   base location value /rd-group is 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

   Req: POST /rd-group?gp=lights HTTP/1.1
   Host: example.com
   Content-Type: application/link-format
   Payload:
   <>;ep="node1",
   <>;ep="node2"

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

7.2.  Group Removal

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

   The removal request interface is specified as follows:

   Interaction:  CT -> RD



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

   Req: DELETE /rd-group/12

   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).

   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 links (if any) corresponding to 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.




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   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 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 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 for a resource to be returned.  The
   character'*' MAY be included at the end of a parameter value as a
   wildcard operator.

   The rd-lookup interface MAY use any set of query parameters to match
   the registered attributes and 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 list
   of domains that contain the specified groups.  An endpoint lookup
   filtering on groups would return a list of endpoints that are in the
   specified groups.

   The lookup interface is specified as follows:

   Interaction:  Client -> RD



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   Method:  GET

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

   URI Template Variables:

      rd-lookup-base :=  RD Lookup Function Set 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.






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      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 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 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 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

   The examples in this section assume a CoAP host with IP address
   FDFD::123 and a default CoAP port 61616.  HTTP hosts are possible and
   do not change the nature of the examples.\

   The following example shows a client performing a resource lookup
   with the example 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:








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   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 a domain lookup:

   Req: GET /rd-lookup/d

   Res: 2.05 Content
   <>;d="domain1",
   <>;d="domain2"

   The following example shows a 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::123]: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










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   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 the CoRE Link Format to describe resources being
   discovered by or posted to a resource directory service, additional
   information about those resources is useful.  This specification
   defines the following new attributes for use in the CoRE Link Format
   [RFC6690]:

      link-extension    = ( "ins" "=" (ptoken | quoted-string) )
                          ; The token or string is max 63 bytes
      link-extension    = ( "exp" )

9.1.  Resource Instance attribute 'ins'

   The Resource Instance "ins" attribute is an identifier for this
   resource, which makes it possible to distinguish it from other
   similar resources.  This attribute is similar in use to the
   <Instance> portion of a DNS-SD record (see Section 10.1, and SHOULD
   be unique across resources with the same Resource Type attribute in
   the domain it 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.  This attribute is used by a Resource
   Directory to distinguish between multiple instances of the same
   resource type within the directory.

   This attribute MUST be no more than 63 bytes in length.  The resource
   identifier attribute MUST NOT appear more than once in a link
   description.  This attribute MAY be used as a query parameter in the
   RD Lookup Function Set defined in Section 7.




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9.2.  Export attribute 'exp'

   The Export "exp" attribute is used as a flag 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 it, determining the size
   of a resource, or traversing dynamic resource structures.  However,
   other links are very useful to be exported to other directories, for
   example the entry point resource to a functional service.  This
   attribute MAY be used as a query parameter in the RD Lookup Function
   Set 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 and service discovery are complementary in the case of large
   networks, where 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 of CoAP
   services by either method.

10.1.  DNS-based Service discovery

   DNS-Based Service Discovery (DNS-SD) defines a conventional method of
   configuring DNS PTR, SRV, and TXT resource records to facilitate
   discovery of services (such as CoAP servers in a subdomain) using the
   existing DNS infrastructure.  This section gives a brief overview of
   DNS-SD; see [RFC6763] for a detailed specification.

   DNS-SD service names are limited to 255 octets and are of the form:

   Service Name = <Instance>.<ServiceType>.<Domain>.

   The service name is the label of SRV/TXT resource records.  The SRV
   RR specifies the host and 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) part of the authority in URIs that identify servers or
   groups of servers.



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   The <ServiceType> part is composed of at least two labels.  The first
   label of the pair is the application protocol name [RFC6335] preceded
   by an underscore character.  The second label indicates the transport
   and is always "_udp" for UDP-based CoAP services.  In cases where
   narrowing the scope of the search may be useful, these labels may 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 of the service name may be set at the
   factory or during the commissioning process.  It SHOULD uniquely
   identify an instance of <ServiceType> within a <Domain>.  Taken
   together, these three elements comprise a unique name for an SRV/ TXT
   record pair within the DNS subdomain.

   The granularity of a service name MAY be that of a host or group, or
   it could represent a particular resource within a CoAP server.  The
   SRV record contains the host name (AAAA record name) and port of the
   service while protocol is part of the service name.  In the case
   where a service name identifies a particular resource, the path part
   of 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 is indicated in the resource record header in the DNS
   response message.  The data consists of one 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>

   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 to match the DNS host
   name of a service, it SHOULD use the syntax defined in Section 3.5 of
   [RFC1034] and Section 2.1 of [RFC1123].

   The <Instance> part of the name of a service being offered on the
   network SHOULD be configurable by the user setting up the service, 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 be used.  A sensible choice of default name can allow the device
   or service to be accessed in many cases without any manual
   configuration at all.  The default name should be short and
   descriptive, and MAY include a collision-resistant substring such as
   the lower bits of the device's MAC address, serial number,



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   fingerprint, or other identifier in an attempt to make the name
   relatively unique.

   DNS labels are currently limited to 63 octets in length 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 a DNS-SD service name and SHOULD conform to the reg-rel-type
   production of the Link Format defined in Section 2 of [RFC6690].  The
   "rt" attribute MUST be composed of at least a single Net-Unicode text
   string, without underscore '_' or period '.' and limited to 15 octets
   in length, which represents the application protocol name.  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".

   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.  This string
   is mapped to 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 as in the previous paragraph.  For
   example, rt="dali.light" is mapped into "light._sub._dali._udp".

   The resulting string is used to form labels for DNS-SD records which
   are stored directly in the DNS.

10.4.  Domain mapping

   DNS domains may be derived from the "d" attribute.  The domain
   attribute may be suffixed with the zone name of the authoritative DNS
   server to generate the domain name.  The "ep" attribute is prefixed
   to the domain name to generate the FQDN to be stored into DNS 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 in the DNS-SD as key=value strings in a
   TXT record ([RFC6763], Section 6.3).

   The resource <URI> is exported as key/value pair "path=<URI>".

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




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   The DNS TXT record can be further populated by importing any other
   resource description attributes as they share the same key=value
   format specified in Section 6 of [RFC6763].

10.6.  Importing resource links into DNS-SD

   Assuming the ability to query a Resource Directory or multicast a GET
   (?exp) over the local link, CoAP resource discovery may be used to
   populate the DNS-SD database in an automated fashion.  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 with the Resource Type as
   the <ServiceType>, and registered in the correct <Domain>.  The agent
   responsible for exporting records to the DNS zone file SHOULD be
   authenticated to the DNS server.  The following example, using the
   example lookup location /rd-lookup, shows an agent discovering a
   resource to be exported:

      Req: GET /rd-lookup/res?exp

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


   The agent subsequently registers the following DNS-SD RRs, assuming 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.








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11.  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.  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.  Access Control

   Access control SHOULD be performed separately for the RD Function Set
   and the RD Lookup Function Set, 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.  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
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   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.  IANA Considerations

12.1.  Resource Types

   "core.rd", "core.rd-group" and "core.rd-lookup" 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.  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.  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:







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   +-----------+-------+---------------+-------------------------------+
   | 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: RD Parameters

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

13.  Examples

13.1.  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".






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13.1.1.  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 below:

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

                    Table 2: interface SLAAC addresses





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

13.1.2.  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
   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: 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:












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   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"
   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




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   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.







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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 on applications, not familiar with the existence of the RD,
   can use DNS to access the lighting group.

13.2.  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.




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   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 are currently
   out of scope for LWM2M.

   The location of the LWM2M Server and RD Function Set 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.  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]





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   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.  LWM2M Register Endpoint

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

   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.






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   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 :

   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: 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.



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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.  LWM2M Update Endpoint Registration

   An LWM2M Registration update proceeds as described in Section 6.4,
   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.  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.  Acknowledgments

   Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders
   Brandt, Matthieu Vial, Mohit Sethi, Sampo Ukkola and Linyi Tian 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.








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15.  Changelog

   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



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   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.




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   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:





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   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.  References

16.1.  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>.



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   [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.  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>.




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   [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












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   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

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


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

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


   Peter van der Stok
   consultant

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

























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