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Versions: (draft-shelby-core-resource-directory) 00 01 02 03 04 05 06 07 08 09 10 11

CoRE                                                           Z. Shelby
Internet-Draft                                                       ARM
Intended status: Standards Track                               M. Koster
Expires: January 4, 2018                                     SmartThings
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                         P. van der Stok
                                                              consultant
                                                         C. Amsuess, Ed.
                                             Energy Harvesting Solutions
                                                           July 03, 2017


                        CoRE Resource Directory
                 draft-ietf-core-resource-directory-11

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 January 4, 2018.






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Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture and Use Cases  . . . . . . . . . . . . . . . . .   5
     3.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Use Case: Cellular M2M  . . . . . . . . . . . . . . . . .   7
     3.4.  Use Case: Home and Building Automation  . . . . . . . . .   8
     3.5.  Use Case: Link Catalogues . . . . . . . . . . . . . . . .   8
   4.  Finding a Resource Directory  . . . . . . . . . . . . . . . .   9
     4.1.  Resource Directory Address Option (RDAO)  . . . . . . . .  10
   5.  Resource Directory  . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Content Formats . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  URI Discovery . . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  14
       5.3.1.  Simple Registration . . . . . . . . . . . . . . . . .  17
       5.3.2.  Simple publishing to Resource Directory Server  . . .  18
       5.3.3.  Third-party registration  . . . . . . . . . . . . . .  18
       5.3.4.  Plurality of link references in a Registration  . . .  19
     5.4.  Operations on the Registration Resource . . . . . . . . .  19
       5.4.1.  Registration Update . . . . . . . . . . . . . . . . .  20
       5.4.2.  Registration Removal  . . . . . . . . . . . . . . . .  22
       5.4.3.  Read Endpoint Links . . . . . . . . . . . . . . . . .  23
       5.4.4.  Update Endpoint Links . . . . . . . . . . . . . . . .  24
   6.  RD Groups . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     6.1.  Register a Group  . . . . . . . . . . . . . . . . . . . .  28
     6.2.  Group Removal . . . . . . . . . . . . . . . . . . . . . .  30
   7.  RD Lookup . . . . . . . . . . . . . . . . . . . . . . . . . .  31
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
     8.1.  Endpoint Identification and Authentication  . . . . . . .  36
     8.2.  Access Control  . . . . . . . . . . . . . . . . . . . . .  36
     8.3.  Denial of Service Attacks . . . . . . . . . . . . . . . .  37



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   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     9.1.  Resource Types  . . . . . . . . . . . . . . . . . . . . .  37
     9.2.  IPv6 ND Resource Directory Address Option . . . . . . . .  37
     9.3.  RD Parameter Registry . . . . . . . . . . . . . . . . . .  37
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  Lighting Installation  . . . . . . . . . . . . . . . . .  38
       10.1.1.  Installation Characteristics . . . . . . . . . . . .  39
       10.1.2.  RD entries . . . . . . . . . . . . . . . . . . . . .  40
     10.2.  OMA Lightweight M2M (LWM2M) Example  . . . . . . . . . .  43
       10.2.1.  The LWM2M Object Model . . . . . . . . . . . . . . .  43
       10.2.2.  LWM2M Register Endpoint  . . . . . . . . . . . . . .  45
       10.2.3.  LWM2M Update Endpoint Registration . . . . . . . . .  46
       10.2.4.  LWM2M De-Register Endpoint . . . . . . . . . . . . .  47
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  47
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  47
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     13.2.  Informative References . . . . . . . . . . . . . . . . .  52
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  53

1.  Introduction

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

   The discovery of resources offered by a constrained server is very
   important in machine-to-machine applications where there are no
   humans in the loop and static interfaces result in fragility.  The
   discovery of resources provided by an HTTP Web Server is typically
   called Web Linking [RFC5988].  The use of Web Linking for the
   description and discovery of resources hosted by constrained web
   servers is specified by the CoRE Link Format [RFC6690].  However,
   [RFC6690] only describes how to discover resources from the web
   server that hosts them by requesting "/.well-known/core".  In many
   M2M scenarios, direct discovery of resources is not practical due to
   sleeping nodes, disperse networks, or networks where multicast
   traffic is inefficient.  These problems can be solved by employing an
   entity called a Resource Directory (RD), which hosts descriptions of
   resources held on other servers, allowing lookups to be performed for
   those resources.

   This document specifies the web interfaces that a Resource Directory
   supports in order for web servers to discover the RD and to register,
   maintain, lookup and remove resource descriptions.  Furthermore, new
   link attributes useful in conjunction with a Resource Directory are



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   defined.  Although the examples in this document show the use of
   these interfaces with CoAP [RFC7252], they can be applied in an
   equivalent manner to HTTP [RFC7230].

2.  Terminology

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

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

   This specification makes use of the following additional terminology:

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

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

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

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

   Context
      When registering links to a Resource Directory, the Context refers
      to the scheme, address, port, and base path for all the links



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      registered on behalf of an endpoint, of the general form
      scheme://host:port/path/ where the client may explicitly set the
      scheme and host, and may supply the port and path as optional
      parameters.  When the context of a registration is explicitly set,
      the URI resolution rules in [RFC3986] MUST be applied.

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

   RDAO
      Resource Directory Address Option.

3.  Architecture and Use Cases

3.1.  Principles

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

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

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

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

3.2.  Architecture

   The resource directory architecture is illustrated in Figure 1.  A
   Resource Directory (RD) is used as a repository for Web Links
   [RFC5988] about resources hosted on other web servers, which are
   called endpoints (EP).  An endpoint is a web server associated with a
   scheme, IP address and port (called Context), thus a physical node



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   may host one or more endpoints.  The RD implements a set of REST
   interfaces for endpoints to register and maintain sets of Web Links
   (called resource directory registration entries), and for clients to
   lookup resources from the RD or maintain groups.  Endpoints
   themselves can also act as clients.  An RD can be logically segmented
   by the use of Domains.  The domain an endpoint is associated with can
   be defined by the RD or configured by an outside entity.  This
   information hierarchy is shown in Figure 2.

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

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

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

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

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

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


              Figure 1: The resource directory architecture.





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


          Figure 2: The resource directory information hierarchy.

3.3.  Use Case: Cellular M2M

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

   In a typical scenario, during a boot-up procedure (and periodically
   afterwards), the machines (endpoints) register with a Resource
   Directory (for example EPs installed on vehicles enabling tracking of
   their position for fleet management purposes and monitoring
   environment parameters) hosted by the mobile operator or somewhere
   else in the network, periodically a description of its own
   capabilities.  Due to the usual network configuration of mobile
   networks, the EPs attached to the mobile network may not always be
   efficiently reachable.  Therefore, a remote server is usually used to
   provide proxy access to the EPs.  The address of each (proxy)
   endpoint on this server is included in the resource description
   stored in the RD.  The users, for example mobile applications for



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   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.

3.4.  Use Case: Home and Building Automation

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

3.5.  Use Case: Link Catalogues

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

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

   Metadata in web link formats like [RFC6690] are supplied by Resource
   Directories, which may be internally stored as triples, or relation/
   attribute pairs providing metadata about resource links.  External
   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.




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

4.  Finding a Resource Directory

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

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

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

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

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

   o  DHCPv6 options that might be defined later.

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

   In networks with more inexpensive use of multicast, the candidate IP
   address may be a well-known multicast address, i.e. directory servers
   are found by simply sending GET requests to that well-known multicast
   address (see Section 5.2).

   Constrained nodes configured in large batches may be configured for
   an anycast address for the RD.  Each target network environment in
   which some of these preconfigured nodes are to be brought up is then
   configured with a route for this anycast address that leads to an RD
   that is appropriate for the environment.

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



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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 Resource Directory address is valid.
                           A value of all zero bits (0x0) indicates
                           that this Resource Directory address
                           is not valid anymore.

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

   RD Address:             IPv6 address of the RD.

                Figure 3: Resource Directory Address Option

5.  Resource Directory

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



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

5.1.  Content Formats

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

   Resource Directories implementing this specification MAY support
   additional content formats.

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

5.2.  URI Discovery

   Before an endpoint can make use of an RD, it must first know the RD's
   address and port, and the URI path information for its REST APIs.
   This section defines discovery of the RD and its URIs using the well-
   known interface of the CoRE Link Format [RFC6690].  It is however
   expected that RDs will also be discoverable via other methods
   depending on the deployment.

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

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

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




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   points SHOULD be provided to enable the bootstrapping of unicast
   discovery.

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

   The discovery request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

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

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

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

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

   The following response codes are defined for this interface:

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

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

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

   Failure:  No error response to a multicast request.

   HTTP support :  YES (Unicast only)

   The following example shows an endpoint discovering an RD using this
   interface, thus learning that the RD registration resource is, in
   this example, at /rd, and that the content-format delivered by the




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   server hosting the resource is application/link-format (ct=40).  Note
   that it is up to the RD to choose its RD resource paths.

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

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

                   Figure 4: Example discovery exchange

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

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

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

5.3.  Registration

   After discovering the location of an RD, an endpoint MAY register its
   resources using the registration interface.  This interface accepts a
   POST from an endpoint containing the list of resources to be added to
   the directory as the message payload in the CoRE Link Format
   [RFC6690], JSON CoRE Link Format (application/link-format+json), or
   CBOR CoRE Link Format (application/link-format+cbor)
   [I-D.ietf-core-links-json], along with query parameters indicating
   the name of the endpoint, and optionally its domain and the lifetime
   of the registration.  It is expected that other specifications will
   define further parameters (see Section 9.3).  The RD then creates a
   new registration resource in the RD and returns its location.  An



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   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 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 parameters ep and d 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 registration URI (mandatory).  This is the location of
         the RD, as obtained from discovery.

      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.

      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
         in the initial registration, a default value of 86400 (24
         hours) SHOULD be assumed.  If the lt parameter is not included
         in a registration refresh or update operation, the most
         recently supplied value SHALL be re-used.

      con :=  Context (optional).  This parameter sets the scheme,
         address, port and path at which this server is available in the
         form scheme://host:port/path.  In the absence of this parameter
         the scheme of the protocol, source address and source port of
         the register request are assumed.  This parameter is mandatory



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         when the directory is filled by a third party such as an
         commissioning tool.  When con is used, scheme and host are
         mandatory and port and path parameters are optional.  If the
         endpoint uses an ephemeral port to register with, it MUST
         include the con: parameter in the registration to provide a
         valid network path.  If the endpoint which is located behind a
         NAT gateway is registering with a Resource Directory which is
         on the network service side of the NAT gateway, the endpoint
         MUST use a persistent port for the outgoing registration in
         order to provide the NAT gateway with a valid network address
         for replies and incoming requests.

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor

   The following response codes are defined for this interface:

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

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

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

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

   HTTP support:  YES

   The following example shows an endpoint with the name "node1"
   registering two resources to an RD using this interface.  The
   location "/rd" is an example RD location discovered in a request
   similar to Figure 4.






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

   Res: 2.01 Created
   Location: /rd/4521

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

   Req: POST /rd?ep=node1&con=http://[2001:db8::1:1] 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

5.3.1.  Simple Registration

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

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

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

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

   The directory server integrates the information it received this way
   into its 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.





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5.3.2.  Simple publishing to Resource Directory Server

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

   The endpoint MUST include the endpoint name and MAY include the
   registration parameters d, lt, and et, in the POST request as per
   Section 5.3.

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

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

   Content-Format: 40

   payload:

   (empty payload)

   Res: 2.04 Changed

   (later)

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

   Accept: 40

   Res: 2.05 Content

   payload:

   </sen/temp>

5.3.3.  Third-party registration

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

   In a controlled environment (e.g. building control), the Resource
   Directory can be filled by a third device, called a commissioning



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   tool.  The commissioning tool can fill the Resource Directory from a
   database or other means.  For that purpose the scheme, IP address and
   port of the registered device is indicated in the Context parameter
   of the registration described in Section 5.3.

5.3.4.  Plurality of link references in a Registration

   Plurality of link references within a Registration (registration
   resource) is an indication of some error condition and should not be
   allowed.

   Plurality of link references exists if, and only if, two or more
   links in a Registration contain identical context, target, and
   relation values.  This condition would be likely to arise if there
   were multiple co-ordinators or configuration tools, each with a
   different set of configuration values for the same resource.

   A Resource Directory SHOULD reject a registration, or an operation on
   a registration, which would result in a plurality of link references
   within the the context of the registration.  There is no requirement
   in this document for a resource directory to check for plurality of
   reference between different registrations.  Resource Directory
   operations which are rejected due to reference plurality SHOULD be
   returned the "Conflict" code, indicating that there is someting wrong
   with the request.

5.4.  Operations on the Registration Resource

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

   The Registration Resource may also be used to inspect the
   registration resource using GET, update the registration link
   contents using PATCH (as introduced in [RFC8132]), or cancel the
   registration using DELETE.

   These operations are described in this section.




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   In accordance with Section 5.3.4, operations which would result in
   plural link references within the context of a registration resource
   SHOULD be rejected using the "Conflict" result code.

5.4.1.  Registration Update

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

   An update MAY update the lifetime or context registration parameters
   "lt", "con" as in Section 5.3 ) if the previous settings are to be
   retained.  Parameters that are not being changed changed SHOULD NOT
   be included in an update.  Adding parameters that have not changed
   increases the size of the message but does not have any other
   implications.  Parameters MUST be included as query parameters in an
   update operation as in Section 5.3.

   Upon receiving an update request, an RD MUST reset the timeout for
   that endpoint 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.  If the lifetime parameter is not included in the
   registration update, the most recent setting is re-used for the next
   registration time-out period.

   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 all of the target URI
   and relation type (if present) and anchor value (if present) match.

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

   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 5.4.4.

   The update registration request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  POST



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

   URI Template Variables:

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

      lt :=  Lifetime (optional).  Lifetime of the registration in
         seconds.  Range of 60-4294967295.  If no lifetime is included,
         the previous last lifetime set on a previous update or the
         original registration (falling back to 86400) SHOULD be used.

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

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

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

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

   The following response codes are defined for this interface:

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

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

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

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

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

   HTTP support:  YES





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   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"

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

   Res: 2.04 Changed

5.4.2.  Registration Removal

   Although RD entries have soft state and 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 returned by the RD as a result
         of a successful earlier registration.




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   The following responses codes are defined for this interface:

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

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

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

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

   HTTP support: YES

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

   Req: DELETE /rd/4521

   Res: 2.02 Deleted

5.4.3.  Read Endpoint Links

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

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

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

   The read request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  GET

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

   URI Template Variables:

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





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

   The following responses codes are defined for this interface:

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

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

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

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

   HTTP support: YES

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

   Req: GET /rd/4521

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

5.4.4.  Update Endpoint Links

   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.

   Other PATCH document formats may be used as appropriate for patching
   the array of objects format of a Registration Resource.  In
   particular, a select-merge patch document format could combine the
   function of link selection query and link attribute replacement
   values.

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





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

   If an attribute name specified in the PATCH document exists in any
   the set of selected links, all occurrences of the attribute value in
   the target document MUST be updated using the value from the PATCH
   payload.  If the attribute name is not present in any selected links,
   the attribute MUST be added to the links.

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

   If the PATCH payload results in the modification of link target,
   context, or relation values, that is "href", "rel", or "anchor", the
   request SHOULD be rejected with a "Conflict" result code.

   The update request interface is specified as follows:

   Interaction:  EP -> RD

   Method:  PATCH

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

   URI Template Variables:

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

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

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




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

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

   HTTP support: YES

   The following examples show 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 Registration Resource initial state is:

   Req: GET /rd/4521

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

   The following example 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




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   Req: GET /rd/4521

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

   The following example shows an EP modifying all links at the example
   location /rd/4521 which are identified by href="/sensors/temp", from
   the initial link-value of </sensors/temp>;rt="temperature" to the new
   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

   Req: GET /rd/4521

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

   This 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:
   {}

   Content-Format:
   application/merge-patch+json

   Res: 2.04 Changed





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   Req: GET /rd/4521

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

6.  RD Groups

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

6.1.  Register a Group

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

   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 URI (mandatory).  This is the location of
         the RD Group REST API.



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      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.

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

   Content-Format:  application/link-format

   Content-Format:  application/link-format+json

   Content-Format:  application/link-format+cbor

   The following response codes are defined for this interface:

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

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

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

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

   HTTP support:  YES

   The following example shows an EP registering a group with the name
   "lights" which has two endpoints to an RD using this interface.  The
   RD group path /rd-group is an example RD location discovered in a
   request similar to Figure 4.




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

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

6.2.  Group Removal

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

   The removal request interface is specified as follows:

   Interaction:  CT -> RD

   Method:  DELETE

   URI Template:  {+location}

   URI Template Variables:

      location :=  This is the Location 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.






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   Req: DELETE /rd-group/12

   Res: 2.02 Deleted

7.  RD Lookup

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

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

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

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

             +-------------+--------------------+-----------+
             | Lookup Type | Resource Type      | Mandatory |
             +-------------+--------------------+-----------+
             | Resource    | core.rd-lookup-res | Mandatory |
             | Endpoint    | core.rd-lookup-ep  | Mandatory |
             | Domain      | core.rd-lookup-d   | Optional  |
             | Group       | core.rd-lookup-gp  | Optional  |
             +-------------+--------------------+-----------+

                           Table 1: Lookup Types

   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.




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   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.

   RD Lookup requests 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.

   Clients that are interested in a lookup result repeatedly or
   continuously can use mechanisms like ETag caching, resource
   observation ([RFC7641]), or any future mechanism that might allow
   more efficient observations of collections.  These are advertised,
   detected and used according to their own specifications and can be
   used with the lookup interface as with any other resource.

   The lookup interface is specified as follows:

   Interaction:  Client -> RD

   Method:  GET



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

   URI Template Variables:

      type-lookup-location :=  RD Lookup URI for a given lookup type
         (mandatory).  The address is discovered as described in
         Section 5.2.

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

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

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

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

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

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

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

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

      resource-param :=  Link attribute parameters (optional).  Any link
         target 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)




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      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 CoAP hosts with a default CoAP
   port 61616.  HTTP hosts are possible and do not change the nature of
   the examples.

   The following example shows a client performing a resource lookup
   with the example resource look-up locations discovered in Figure 4:

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

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

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

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

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

   The following example shows a client performing a domain lookup:








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   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::124]:61616>;ep="node2"

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

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

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

   The following example shows a client performing a paginated lookup


















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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

8.  Security Considerations

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

8.1.  Endpoint Identification and Authentication

   An Endpoint is determined to be unique 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.

8.2.  Access Control

   Access control SHOULD be performed separately for the RD
   registration, Lookup, and group API paths, as different endpoints may
   be authorized to register with an RD from those authorized to lookup
   endpoints from the RD.  Such access control SHOULD be performed in as



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   fine-grained a level as possible.  For example access control for
   lookups could be performed either at the domain, endpoint or resource
   level.

8.3.  Denial of Service Attacks

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

9.  IANA Considerations

9.1.  Resource Types

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

9.2.  IPv6 ND Resource Directory Address Option

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

   o  Resource Directory address Option (38)

9.3.  RD Parameter Registry

   This specification defines a new sub-registry for registration and
   lookup parameters called "RD Parameters" under "CoRE Parameters".
   Although this specification defines a basic set of parameters, it is




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   expected that other standards that make use of this interface will
   define new ones.

   Each entry in the registry must include the human readable name of
   the parameter, the query parameter, validity requirements if any and
   a description.  The query parameter MUST be a valid URI query key
   [RFC3986].

   Initial entries in this sub-registry are as follows:

   +----------+-------+---------------+--------------------------------+
   | Name     | Query | Validity      | Description                    |
   +----------+-------+---------------+--------------------------------+
   | Endpoint | ep    |               | Name of the endpoint, max 63   |
   | Name     |       |               | bytes                          |
   | Lifetime | lt    | 60-4294967295 | Lifetime of the registration   |
   |          |       |               | in seconds                     |
   | Domain   | d     |               | Domain to which this endpoint  |
   |          |       |               | belongs                        |
   | Endpoint | et    |               | Semantic name of the endpoint  |
   | Type     |       |               |                                |
   | Context  | con   | URI           | The scheme, address and port   |
   |          |       |               | and path 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            |
   +----------+-------+---------------+--------------------------------+

                          Table 2: RD Parameters

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

10.  Examples

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

10.1.  Lighting Installation

   This example shows a simplified lighting installation which makes use
   of the 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



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   group and the enabling of the corresponding multicast address.  No
   conclusions must be drawn on the realization of actual installation
   or naming procedures, because the example only "emphasizes" some of
   the issues that may influence the use of the RD and does not pretend
   to be normative.

10.1.1.  Installation Characteristics

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

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

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

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

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










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

                    Table 3: interface SLAAC addresses

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

10.1.2.  RD entries

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

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

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

                  Table 4: Resource Directory identifiers





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

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

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

   Res: 2.01 Created
   Location: /rd/4521

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

   Res: 2.01 Created
   Location: /rd/4522

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

   Res: 2.01 Created
   Location: /rd/4523

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

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






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

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

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

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

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

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

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

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

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

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

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








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   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.

10.2.  OMA Lightweight M2M (LWM2M) Example

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

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

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

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

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

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

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

10.2.1.  The LWM2M Object Model

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

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



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

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

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

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

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

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

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

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

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

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

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

   /1/0/1





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

10.2.2.  LWM2M Register Endpoint

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

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

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

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

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

   ep - Endpoint Name
   lt - registration lifetime

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

   Additional optional LWM2M registration parameters are defined:













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   +------------+-------+-------------------------------+--------------+
   | 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 5: LWM2M Additional Registration Parameters

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

   et - Endpoint Type
   con - Context

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

   Here is an example LWM2M registration payload:

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

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

10.2.3.  LWM2M Update Endpoint Registration

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

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

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




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10.2.4.  LWM2M De-Register Endpoint

   LWM2M allows for de-registration using the delete method on the
   returned location from the initial registration operation.  LWM2M de-
   registration proceeds as described in Section 5.4.2.

11.  Acknowledgments

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

12.  Changelog

   changes from -09 to -10

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

   o  removed all text concerning DNS-SD.

   o  removed inconsistency in RDAO text.

   o  suggestions taken over from various sources

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

   o  moved simple registration to registration section

   changes from -08 to -09

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

   o  changed "ins" ABNF notation.

   o  various editorial improvements, including in examples

   o  clarifications for RDAO

   changes from -07 to -08

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




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

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

   o  add IPv6 ND Option for discovery of an RD

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

   o  removed all superfluous client-server diagrams

   o  simplified lighting example

   o  introduced Commissioning Tool

   o  RD-Look-up text is extended.

   changes from -06 to -07

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

   o  editorial updates to section 9

   o  update author information

   o  minor text corrections

   Changes from -05 to -06

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

   changes from -04 to -05

   o  added Update Endpoint Links using PATCH

   o  http access made explicit in interface specification

   o  Added http examples

   Changes from -03 to -04:

   o  Added http response codes




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   o  Clarified endpoint name usage

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

   Changes from -02 to -03:

   o  Added an example for lighting and DNS integration

   o  Added an example for RD use in OMA LWM2M

   o  Added Read Links operation for link inspection by endpoints

   o  Expanded DNS-SD section

   o  Added draft authors Peter van der Stok and Michael Koster

   Changes from -01 to -02:

   o  Added a catalogue use case.

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

   o  Additional examples section added for more complex use cases.

   o  New DNS-SD mapping section.

   o  Added text on endpoint identification and authentication.

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

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

   Changes from -00 to -01:

   o  Removed the ETag validation feature.

   o  Place holder for the DNS-SD mapping section.

   o  Explicitly disabled GET or POST on returned Location.

   o  New registry for RD parameters.

   o  Added support for the JSON Link Format.

   o  Added reference to the Groupcomm WG draft.




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   Changes from -05 to WG Document -00:

   o  Updated the version and date.

   Changes from -04 to -05:

   o  Restricted Update to parameter updates.

   o  Added pagination support for the Lookup interface.

   o  Minor editing, bug fixes and reference updates.

   o  Added group support.

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

   Changes from -03 to -04:

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

   o  Integrated the Simple Directory Discovery from Carsten.

   o  Editorial improvements.

   o  Fixed the use of ETags.

   o  Fixed tickets 383 and 372

   Changes from -02 to -03:

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

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

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

   o  Improved the security considerations section.

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

   Changes from -01 to -02:

   o  Added a terminology section.





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   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.

13.  References

13.1.  Normative References

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., and C. Bormann, "Representing
              Constrained RESTful Environments (CoRE) Link Format in
              JSON and CBOR", draft-ietf-core-links-json-08 (work in
              progress), April 2017.

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

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", 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>.

   [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>.





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

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

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

13.2.  Informative References

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

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <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>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://www.rfc-editor.org/info/rfc7641>.





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Authors' Addresses

   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   USA

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


   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

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


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

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


   Peter van der Stok
   consultant

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


   Christian Amsuess (editor)
   Energy Harvesting Solutions
   Hollandstr. 12/4
   1020
   Austria

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



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