CoRE Z. Shelby
Internet-Draft ARM
Intended status: Standards Track M. Koster
Expires: May 3, 2018 SmartThings
C. Bormann
Universitaet Bremen TZI
P. van der Stok
consultant
C. Amsüss, Ed.
Energy Harvesting Solutions
October 30, 2017

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

Abstract

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

Status of This Memo

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

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

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

Copyright Notice

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

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Table of Contents

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 querying /.well-known/core. In many M2M scenarios, direct discovery of resources is not practical due to sleeping nodes, disperse networks, or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which hosts descriptions of resources held on other servers, allowing lookups to be performed for those resources.

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

2. Terminology

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

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

This specification makes use of the following additional terminology:

Resource Directory

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

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

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

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

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

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

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

A resource in the RD that contains information about an Endpoint and its links.
Commissioning Tool

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

Resource Directory Address Option.

3. Architecture and Use Cases

3.1. Principles

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

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

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

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

3.2. Architecture

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

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

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

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

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

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

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

Figure 1: The resource directory architecture.

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

Figure 2: The resource directory information hierarchy.

3.3. Content model

The Entity-Relationship (ER) models shown in Figure 3 and Figure 4 model the contents of /.well-known/core and the resource directory respectively, with entity-relationship diagrams [ER]. Entities (rectangles) are used for concepts that exist independently. Attributes (ovals) are used for concepts that exist only in connection with a related entity. Relations (diamonds) give a semantic meaning to the relation between entities. Numbers specify the cardinality of the relations.

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

These models provide an abstract view of the information expressed in link-format documents and a Resource Directory. They cover the concepts, but not necessarily all details of an RD’s operation; they are meant to give an overview, and not be a template for implementations.

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



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

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

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

A link has the following attributes:

In link-format serialization, it is expressed between angular brackets, and sometimes called the “href”. If it is a relative URI, it gets resolved against the link context URI.

             +----------------------+
             |  resource-directory  |
             +----------------------+
                        |
                        |         oooooooooooo  0-1
                        |        o MC address o---+
                        |         oooooooooooo    |
                        |                         |
                   //////\\\\             0+  +--------+
                  < contains >----------------| group  |
                   \\\\\/////                 +--------+
                        |                         |
                    0-n |                         | 1+
 ooooooo     1  +---------------+          ///////\\\\\\
o  con  o-------|  registration |---------< composed of >
 ooooooo        +---------------+          \\\\\\\//////
                    |       |
                    |       +--------------+
       oooooooo   1 |                      |
      o  loc   o----+                 /////\\\\
       oooooooo     |                < contains >
                    |                 \\\\\/////
       oooooooo   1 |                      |
      o   ep   o----+                      | 0+
       oooooooo     |             +------------------+
                    |             |      link        |
       oooooooo 0-1 |             +------------------+
      o    d   o----+                      |
       oooooooo     |                      |  1   oooooooo
                    |                      +-----o target o
       oooooooo 0-1 |                      |      oooooooo
      o   lt   o----+     ooooooooooo   0+ |
       oooooooo     |    o  target   o-----+
                    |    o attribute o     | 0+   oooooo
    ooooooooooo 0+  |     ooooooooooo      +-----o rel  o
   o  endpoint o----+                      |      oooooo
   o attribute o                           |
    ooooooooooo                            | 1   ooooooooo
                                           +----o context o
                                                 ooooooooo

Figure 4: E-R Model of the content of the Resource Directory

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

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

The cardinality of con is currently 1. Its value is used as a Base URI when resolving URIs in the links contained in the endpoint.

Links are modelled as they are in Figure 3.

3.4. Use Case: Cellular M2M

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

Imagine a scenario where endpoints installed on vehicles enable tracking of the position of these vehicles for fleet management purposes and allow monitoring of environment parameters. During the boot-up process endpoints register with a Resource Directory, which is hosted by the mobile operator or somewhere in the cloud. Periodically, these endpoints update their registration and may modify resources they offer.

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

3.5. Use Case: Home and Building Automation

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

3.6. Use Case: Link Catalogues

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

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

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

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

4. Finding a Resource Directory

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

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

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

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

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

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

4.1. Resource Directory Address Option (RDAO)

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

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

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

The RDAO format is:

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

Fields:

Type:                   38

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

Valid Lifetime:         16-bit unsigned integer.  The length of
                        time in units of 60 seconds (relative to
                        the time the packet is received) that
                        this Resource Directory address is valid.
                        A value of all zero bits (0x0) indicates
                        that this Resource Directory address
                        is not valid anymore.

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

RD Address:             IPv6 address of the RD.

Figure 5: Resource Directory Address Option

5. Resource Directory

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

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

A resource directory MAY make the information submitted to it available to further directories, if it can ensure that a loop does not form. The protocol used between directories to ensure loop-free operation is outside the scope of this document.

5.1. Content Formats

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

Resource Directories implementing this specification MAY support additional content formats.

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

5.2. URI Discovery

Before an endpoint can make use of an RD, it must first know the RD’s address and port, and the URI path information for its REST APIs. This section defines discovery of the RD and its URIs using the well-known interface of the CoRE Link Format [RFC6690]. A complete set of RD discovery methods is described in Section 4.

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

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

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

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

The discovery request interface is specified as follows:

Interaction:
EP -> RD
Method:
GET
URI Template:
/.well-known/core{?rt}
URI Template Variables:
rt :=
Resource Type (optional). MAY contain one of the values “core.rd”, “core.rd-lookup*”, “core.rd-lookup-res”, “core.rd-lookup-ep”, “core.rd-lookup-gp”, “core.rd-group” or “core.rd*”

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

The following response codes are defined for this interface:

Success:
2.05 “Content” or 200 “OK” with an application/link-format, application/link-format+json, or application/link-format+cbor payload containing one or more matching entries for the RD resource.
Failure:
4.00 “Bad Request” or 400 “Bad Request” is returned in case of a malformed request for a unicast request.
Failure:
No error response to a multicast request.
HTTP support :
YES (Unicast only)

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

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

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

Figure 6: Example discovery exchange

The following example shows the way of indicating that a client may request alternate content-formats. The Content-Format code attribute “ct” MAY include a space-separated sequence of Content-Format codes as specified in Section 7.2.1 of [RFC7252], indicating that multiple content-formats are available. The example below shows the required Content-Format 40 (application/link-format) indicated as well as the the CBOR and JSON representation of link format. The RD resource paths /rd, /rd-lookup, and /rd-group are example values.

[ The RFC editor is asked to replace these and later occurrences of TBD64 and TBD504 with the numeric ID values assigned by IANA to application/link-format+cbor and application/link-format+json, respectively, as they are defined in I-D.ietf-core-links-json. ]

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

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

5.3. Registration

After discovering the location of an RD, an endpoint MAY register its resources using the registration interface. This interface accepts a POST from an endpoint containing the list of resources to be added to the directory as the message payload in the CoRE Link Format [RFC6690], JSON CoRE Link Format (application/link-format+json), or CBOR CoRE Link Format (application/link-format+cbor) [I-D.ietf-core-links-json], along with query parameters indicating the name of the endpoint, and optionally its domain and the lifetime of the registration. It is expected that other specifications will define further parameters (see Section 9.3). The RD then creates a new registration resource in the RD and returns its location. An endpoint MUST use that location when refreshing registrations using this interface. Registration resources in the RD are kept active for the period indicated by the lifetime parameter. The endpoint is responsible for refreshing the registration resource within this period using either the registration or update interface. The registration interface MUST be implemented to be idempotent, so that registering twice with the same endpoint parameters ep and d does not create multiple registration resources. A new registration resource may be created at any time to supersede an existing registration, replacing the registration parameters and links.

An empty payload is considered a malformed request.

The posted link-format document can (and typically does) contain relative references both in its link targets and in its anchors, or contain empty anchors. The RD server needs to resolve these references in order to faithfully represent them in lookups. The Base URI against which they are resolved is the context of the registration, which is provided either explicitly in the con parameter or constructed implicitly from the requester’s network address. When resolving relative target references, the server first resolves the context of that link, and then interprets the target as a reference relative to that context (see Appendix A.4).

The registration request interface is specified as follows:

Interaction:
EP -> RD
Method:
POST
URI Template:
{+rd}{?ep,d,lt,con,extra-attrs*}
URI Template Variables:
rd :=
RD registration URI (mandatory). This is the location of the RD, as obtained from discovery.
ep :=
Endpoint name (mostly mandatory). The endpoint name is an identifier that MUST be unique within a domain. The maximum length of this parameter is 63 bytes. If the RD is configured to recognize the endpoint (eg. based on its security context), the endpoint can elide the endpoint name, and assign one based on the configuration.
d :=
Domain (optional). The domain to which this endpoint belongs. The maximum length of this parameter is 63 bytes. When this parameter is elided, the RD MAY associate the endpoint with a configured default domain.
lt :=
Lifetime (optional). Lifetime of the registration in seconds. Range of 60-4294967295. If no lifetime is included in the initial registration, a default value of 86400 (24 hours) SHOULD be assumed.
con :=
Context (optional). This parameter sets the Default Base URI under which the request’s links are to be interpreted. The URI MUST NOT have a path component of its own, but MUST be suitable as a base URI to resolve any relative references given in the registration. The parameter is therefore of the shape “scheme://authority” for HTTP and CoAP URIs. In the absence of this parameter the scheme of the protocol, source address and source port of the registration request are assumed. This parameter is mandatory when the directory is filled by a third party such as an commissioning tool. If the endpoint uses an ephemeral port to register with, it MUST include the con parameter in the registration to provide a valid network path. If the endpoint which is located behind a NAT gateway is registering with a Resource Directory which is on the network service side of the NAT gateway, the endpoint MUST use a persistent port for the outgoing registration in order to provide the NAT gateway with a valid network address for replies and incoming requests.
extra-attrs :=
Additional registration attributes (optional). The endpoint can pass any parameter registered at Section 9.3 to the directory. If the RD is aware of the parameter’s specified semantics, it processes it accordingly. Otherwise, it MUST store the unknown key and its value(s) as an endpoint attribute for further lookup.

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

The following response codes are defined for this interface:

Success:
2.01 “Created” or 201 “Created”. The Location header option MUST be included in the response when a new registration resource is created. This Location MUST be a stable identifier generated by the RD as it is used for all subsequent operations on this registration resource. The registration resource location thus returned is for the purpose of updating the lifetime of the registration and for maintaining the content of the registered links, including updating and deleting links. A registration with an already registered ep and d value pair responds with the same success code and Location as the original registration; the set of links registered with the endpoint is replaced with the links from the payload.
Failure:
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:
YES

The following example shows an endpoint with the name “node1” registering two resources to an RD using this interface. The location “/rd” is an example RD location discovered in a request similar to Figure 6.

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

Res: 2.01 Created
Location: /rd/4521

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

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

Res: 201 Created
Location: /rd/4521

5.3.1. Simple Registration

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

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

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

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

It sends (and regularly refreshes with) a POST request to the /.well-known/core URI of the directory server of choice. The body of the POST request is empty, which triggers the resource directory server to perform GET requests at the requesting server’s default discovery URI to obtain the link-format payload to register.

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

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

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

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

Res: 2.04 Changed

(later)

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

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

5.3.2. Third-party registration

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

In a controlled environment (e.g. building control), the Resource Directory can be filled by a third device, called a commissioning tool. The commissioning tool can fill the Resource Directory from a database or other means. For that purpose the scheme, IP address and port of the registered device is indicated in the Context parameter of the registration described in Section 5.3.

5.4. Operations on the Registration Resource

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

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

These operations are described in this section.

5.4.1. Registration Update

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

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

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

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

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

The update registration request interface is specified as follows:

Interaction:
EP -> RD
Method:
POST
URI Template:
{+location}{?lt,con,extra-attrs*}
URI Template Variables:
location :=
This is the Location returned by the RD as a result of a successful earlier registration.
lt :=
Lifetime (optional). Lifetime of the registration in seconds. Range of 60-4294967295. If no lifetime is included, the previous last lifetime set on a previous update or the original registration (falling back to 86400) SHOULD be used.
con :=
Context (optional). This parameter updates the context established in the original registration to a new value. If the parameter is set in an update, it is stored by the RD as the new Base URI under which to interpret the links of the registration, following the same restrictions as in the registration. If the parameter is not set and was set explicitly before, the previous context value is kept unmodified. If the parameter is not set and was not set explicitly before either, the source address and source port of the update request are stored as the context.
extra-attrs :=
Additional registration attributes (optional). As with the registration, the RD processes them if it knows their semantics. Otherwise, unknown attributes are stored as endpoint attributes, overriding any previously stored endpoint attributes of the same key.

Content-Format:
none (no payload)

The following response codes are defined for this interface:

Success:
2.04 “Changed” or 204 “No Content” if the update was successfully processed.
Failure:
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
Failure:
4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:
YES

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

Req: POST /rd/4521

Res: 2.04 Changed

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

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

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

Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature";anchor="coap://local-proxy-old.example.com:5683",
</sensors/light>;ct=41;rt="light-lux";if="sensor";anchor="coap://local-proxy-old.example.com:5683"

The following example shows an EP changing the context to coaps://new.example.com:5684:

Req: POST /rd/4521?con=coaps://new.example.com:5684

Res: 2.04 Changed

The consecutive query returns:

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

Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature";anchor="coaps://new.example.com:5684",
</sensors/light>;ct=41;rt="light-lux";if="sensor";anchor="coaps://new.example.com:5684",

5.4.2. Registration Removal

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

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

The removal request interface is specified as follows:

Interaction:
EP -> RD
Method:
DELETE
URI Template:
{+location}
URI Template Variables:
location :=
This is the Location returned by the RD as a result of a successful earlier registration.

The following responses codes are defined for this interface:

Success:
2.02 “Deleted” or 204 “No Content” upon successful deletion
Failure:
4.00 “Bad Request” or 400 “Bad request”. Malformed request.
Failure:
4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support: YES

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

Req: DELETE /rd/4521

Res: 2.02 Deleted

5.4.3. Read Endpoint Links

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

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

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

The read request interface is specified as follows:

Interaction:
EP -> RD
Method:
GET
URI Template:
{+location}{?href,rel,rt,if,ct}
URI Template Variables:
location :=
This is the Location returned by the RD as a result of a successful earlier registration.

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

The following responses codes are defined for this interface:

Success:
2.05 “Content” or 200 “OK” upon success with an application/link-format, application/link-format+cbor, or application/link-format+json payload.
Failure:
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
Failure:
4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.

HTTP support: YES

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

Req: GET /rd/4521

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

5.4.4. Update Endpoint Links

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

6. RD Groups

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

6.1. Register a Group

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

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

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

The registration request interface is specified as follows:

Interaction:
CT -> RD
Method:
POST
URI Template:
{+rd-group}{?gp,d,con}
URI Template Variables:
rd-group :=
RD Group URI (mandatory). This is the location of the RD Group REST API.
gp :=
Group Name (mandatory). The name of the group to be created or replaced, unique within that domain. The maximum length of this parameter is 63 bytes.
d :=
Domain (optional). The domain to which this group belongs. The maximum length of this parameter is 63 bytes. Optional. When this parameter is elided, the RD MAY associate the endpoint with a configured default domain.
con :=
Context (optional). This parameter sets the scheme, address and port of the multicast address associated with the group. When con is used, scheme and host are mandatory and port parameter is optional.

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

The following response codes are defined for this interface:

Success:
2.01 “Created” or 201 “Created”. The Location header option MUST be returned in response to a successful group CREATE operation. This Location MUST be a stable identifier generated by the RD as it is used for delete operations of the group resource.
Failure:
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
Failure:
4.04 “Not Found” or 404 “Not Found”. An Endpoint is not registered in the RD (e.g. may have expired).
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:
YES

The following example shows an EP registering a group with the name “lights” which has two endpoints. The RD group path /rd-group is an example RD location discovered in a request similar to Figure 6.

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

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

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

6.2. Group Removal

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

The removal request interface is specified as follows:

Interaction:
CT -> RD
Method:
DELETE
URI Template:
{+location}
URI Template Variables:
location :=
This is the path of the group resource returned by the RD as a result of a successful group registration.

The following responses codes are defined for this interface:

Success:
2.02 “Deleted” or 204 “No Content” upon successful deletion
Failure:
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
Failure:
4.04 “Not Found” or 404 “Not Found”. Group does not exist.
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:
YES

The following examples shows successful removal of the group from the RD with the example location value /rd-group/12.

Req: DELETE /rd-group/12

Res: 2.02 Deleted

7. RD Lookup

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

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

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

Lookup Types
Lookup Type Resource Type Mandatory
Resource core.rd-lookup-res Mandatory
Endpoint core.rd-lookup-ep Mandatory
Group core.rd-lookup-gp Optional

7.1. Resource lookup

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

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

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

7.2. Endpoint and group lookup

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

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

7.3. Lookup filtering

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

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

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

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

Note that href is also a valid search criterion and matches target references. Like all search criteria, on a resource lookup it can match the target reference of the resource link itself, but also the registration resource of the endpoint that registered it, or any group resource that endpoint is contained in.

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

The lookup interface is specified as follows:

Interaction:
Client -> RD
Method:
GET
URI Template:
{+type-lookup-location}{?page,count,search*}
URI Template Variables:
type-lookup-location :=
RD Lookup URI for a given lookup type (mandatory). The address is discovered as described in Section 5.2.
search :=
Search criteria for limiting the number of results (optional).
page :=
Page (optional). Parameter can not be used without the count parameter. Results are returned from result set in pages that contain ‘count’ links starting from index (page * count). Page numbering starts with zero.
count :=
Count (optional). Number of results is limited to this parameter value. If the page parameter is also present, the response MUST only include ‘count’ links starting with the (page * count) link in the result set from the query. If the count parameter is not present, then the response MUST return all matching links in the result set. Link numbering starts with zero.
Content-Format:
application/link-format (optional)
Content-Format:
application/link-format+json (optional)
Content-Format:
application/link-format+cbor (optional)

The following responses codes are defined for this interface:

Success:
2.05 “Content” or 200 “OK” with an application/link-format, application/link-format+cbor, or application/link-format+json payload containing matching entries for the lookup. The payload can contain zero links (which is an empty payload, 80 (hex) or [] in the respective content format), indicating that no entities matched the request.
Failure:
No error response to a multicast request.
Failure:
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
Failure:
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:
YES

7.4. Lookup examples

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

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

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

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

The same lookup using the CBOR Link Format media type:

Req: GET /rd-lookup/res?rt=temperature
Accept: TBD64

Res: 2.05 Content
Content-Format: TBD64
Payload in Hex notation:
81A301652F74656D70096B74656D706572617475726503781E636F61703A2F2F5B323030
313A6462383A333A3A3132335D3A3631363136
Decoded payload:
[{1: "/temp", 9: "temperature", 3: "coap://[2001:db8:3::123]:61616"}]

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

Req: GET /rd-lookup/res?rt=light
Observe: 0

Res: 2.05 Content
Observe: 23
Payload: empty

(at a later point in time)

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

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

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

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

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

Req: GET /rd-lookup/gp

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

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

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

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

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

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

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

The following example shows a client performing a paginated resource lookup

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

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

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

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

The following example shows a client performing a lookup of all resources from endpoints of a given endpoint type. It assumes that two endpoints (with endpoint names sensor1 and sensor2) have previously registered with their respective addresses coap://sensor1.example.com and coap://sensor2.example.com, and posted the very payload of the 6th request of section 5 of [RFC6690].

It demonstrates how the link targets stay unmodified, but the anchors get constructed by the resource directory:

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

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

8. Security Considerations

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

8.1. Endpoint Identification and Authentication

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

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

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

Now, imagine that a malicious device A wants to sabotage the device B. It uses its credentials during the TLS exchange. Then, it puts the endpoint name of device B. If the server does not check whether the identifier provided in the DTLS handshake matches the identifier used at the CoAP layer then it may be inclined to use the endpoint name for looking up what information to provision to the malicious device.

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

8.2. Access Control

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

8.3. Denial of Service Attacks

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

9. IANA Considerations

9.1. Resource Types

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

9.2. IPv6 ND Resource Directory Address Option

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

9.3. RD Parameter Registry

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

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

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

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

The mechanisms around new RD parameters should be designed in such a way that they tolerate RD implementations that are unaware of the parameter and expose any parameter passed at registration or updates on in endpoint lookups. (For example, if a parameter used at registration were to be confidential, the registering endpoint should be instructed to only set that parameter if the RD advertises support for keeping it confidential at the discovery step.)

Initial entries in this sub-registry are as follows:

RD Parameters
Full name Short Validity Use Description
Endpoint Name ep RLA Name of the endpoint, max 63 bytes
Lifetime lt 60-4294967295 RLA Lifetime of the registration in seconds
Domain d RLA Domain to which this endpoint belongs
Context con URI RLA The scheme, address and port and path at which this server is available
Group Name gp RLA Name of a group in the RD
Page page Integer L Used for pagination
Count count Integer L Used for pagination
Endpoint Type et RLA Semantic name of the endpoint (see Section 9.4)

(Short: Short name used in query parameters or link attributes. Use: R = used at registration, L = used at lookup, A = expressed in link attribute

The descriptions for the options defined in this document are only summarized here. To which registrations they apply and when they are to be shown is described in the respective sections of this document.

The IANA policy for future additions to the sub-registry is “Expert Review” as described in [RFC8126]. The evaluation should consider formal criteria, duplication of functionality (Is the new entry redundant with an existing one?), topical suitability (Eg. is the described property actually a property of the endpoint and not a property of a particular resource, in which case it should go into the payload of the registration and need not be registered?), and the potential for conflict with commonly used link attributes (For example, if could be used as a parameter for conditional registration if it were not to be used in lookup or attributes, but would make a bad parameter for lookup, because a resource lookup with an if query parameter could ambiguously filter by the registered endpoint property or the [RFC6690] link attribute). It is expected that the registry will receive between 5 and 50 registrations in total over the next years.

9.3.1. Full description of the “Endpoint Type” Registration Parameter

An endpoint registering at an RD can describe itself with endpoint types, similar to how resources are described with Resource Types in [RFC6690]. An endpoint type is expressed as a string, which can be either a URI or one of the values defined in the Endpoint Type subregistry. Endpoint types can be passed in the et query parameter as part of extra-attrs at the Registration step, are shown on endpoint lookups using the et target attribute, and can be filtered for using et as a search criterion in resource and endpoint lookup. Multiple endpoint types are given as separate query parameters or link attributes.

Note that Endpoint Type differs from Resource Type in that it uses multiple attributes rather than space separated values. As a result, Resource Directory implementations automatically support correct filtering in the lookup interfaces from the rules for unknown endpoint attributes.

9.4. “Endpoint Type” (et=) RD Parameter values

This specification establishes a new sub-registry under “CoRE Parameters” called ‘“Endpoint Type” (et=) RD Parameter values’. The registry properties (required policy, requirements, template) are identical to those of the Resource Type parameters in [RFC6690], in short:

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

The requirements to be enforced are:

The registry is initially empty.

10. Examples

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

10.1. Lighting Installation

This example shows a simplified lighting installation which makes use of the Resource Directory (RD) with a CoAP interface to facilitate the installation and start up of the application code in the lights and sensors. In particular, the example leads to the definition of a group and the enabling of the corresponding multicast address. No conclusions must be drawn on the realization of actual installation or naming procedures, because the example only “emphasizes” some of the issues that may influence the use of the RD and does not pretend to be normative.

10.1.1. Installation Characteristics

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

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

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

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

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

interface SLAAC addresses
Name IPv6 address
luminary1 2001:db8:4::1
luminary2 2001:db8:4::2
Presence sensor 2001:db8:4::3
Resource directory 2001:db8:4::ff

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:

Resource Directory identifiers
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 sensor ps_R2-4-015_door /ps p-sensor

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

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

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

Res: 2.01 Created
Location: /rd/4521
Req: POST coap://[2001:db8:4::ff]/rd
  ?ep=lm_R2-4-015_door&con=coap://[2001:db8:4::2]&d=R2-4-015
Payload:
</light/left>;rt="light",
</light/middle>;rt="light",
</light/right>;rt="light"

Res: 2.01 Created
Location: /rd/4522
Req: POST coap://[2001:db8:4::ff]/rd
  ?ep=ps_R2-4-015_door&con=coap://[2001:db8:4::3]d&d=R2-4-015
Payload:
</ps>;rt="p-sensor"

Res: 2.01 Created
Location: /rd/4523

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

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

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

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

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

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

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

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

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

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

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

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

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

Req: POST //[2001:db8:4::1]/coap-group
          Content-Format: application/coap-group+json
       { "a": "[ff05::1]",
          "n": "grp_R2-4-015"}

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

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

10.2. OMA Lightweight M2M (LWM2M) Example

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

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

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

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

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

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

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

10.2.1. The LWM2M Object Model

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

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

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

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

10.2.2. LWM2M Register Endpoint

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

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

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

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

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

ep - Endpoint Name
lt - registration lifetime

Endpoint Name, Lifetime, and LWM2M Version are mandatory parameters for the register operation, all other registration parameters are optional.

Additional optional LWM2M registration parameters are defined:

LWM2M Additional Registration Parameters
Name Query Validity Description
Binding Mode b {“U”,UQ”,”S”,”SQ”,”US”,”UQS”} Available Protocols
LWM2M Version ver 1.0 Spec Version
SMS Number sms MSISDN

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

et - Endpoint Type
con - Context

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

Here is an example LWM2M registration payload:

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

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

10.2.3. LWM2M Update Endpoint Registration

The LwM2M update is really very similar to the registration update as described in Section 5.4.1, with the only difference that there are more parameters defined and available. All the parameters listed in that section are also available with the initial registration but are all optional:

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

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

10.2.4. LWM2M De-Register Endpoint

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

11. Acknowledgments

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

12. Changelog

changes from -11 to -12

changes from -09 to -10

changes from -08 to -09

changes from -07 to -08

changes from -06 to -07

Changes from -05 to -06

changes from -04 to -05

Changes from -03 to -04:

Changes from -02 to -03:

Changes from -01 to -02:

Changes from -00 to -01:

Changes from -05 to WG Document -00:

Changes from -04 to -05:

Changes from -03 to -04:

Changes from -02 to -03:

Changes from -01 to -02:

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", Internet-Draft draft-ietf-core-links-json-09, July 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M. and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, March 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013.
[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017.
[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.

13.2. Informative References

[ER] Chen, P., "The entity-relationship model---toward a unified view of data", ACM Transactions on Database Systems Vol. 1, pp. 9-36, DOI 10.1145/320434.320440, March 1976.
[I-D.arkko-core-dev-urn] Arkko, J., Jennings, C. and Z. Shelby, "Uniform Resource Names for Device Identifiers", Internet-Draft draft-arkko-core-dev-urn-04, July 2017.
[I-D.nottingham-rfc5988bis] Nottingham, M., "Web Linking", Internet-Draft draft-nottingham-rfc5988bis-08, August 2017.
[I-D.silverajan-core-coap-protocol-negotiation] Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation", Internet-Draft draft-silverajan-core-coap-protocol-negotiation-07, October 2017.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/RFC2616, June 1999.
[RFC6775] Shelby, Z., 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.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC7390] Rahman, A. and E. Dijk, "Group Communication for the Constrained Application Protocol (CoAP)", RFC 7390, DOI 10.17487/RFC7390, October 2014.
[RFC7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015.

Appendix A. Web links and the Resource Directory

Understanding the semantics of a link-format document and its URI references is a journey through different documents ([RFC3986] defining URIs, [RFC6690] defining link-format documents based on [RFC5988] which defines link headers, and [RFC7252] providing the transport). This appendix summarizes the mechanisms and semantics at play from an entry in .well-known/core to a resource lookup.

This text is primarily aimed at people entering the field of Constrained Restful Environments from applications that previously did not use web mechanisms.

A.1. A simple example

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

The client sends a link-local multicast:

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

RES 2.05 Content
</temp>;rt=temperature;ct=0

where the response is sent by the server, [2001:db8:f0::1]:5683.

While the client – on the practical or implementation side – can just go ahead and create a new request to [2001:db8:f0::1]:5683 with Uri-Path: temp, the full resolution steps without any shortcuts are:

A.1.1. Resolving the URIs

The client parses the single returned record. The link’s target (sometimes called “href”) is “/temp”, which is a relative URI that needs resolving. The Base URI to resolve that against is, in absence of an “anchor” parameter, the URI of the requested resource as described in [RFC6690] Section 2.1.

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

The record’s target is resolved by replacing the path “/.well-known/core” from the Base URI (section 5.2 [RFC3986]) with the relative target URI “/temp” into “coap://[2001:db8:f0::1]/temp”.

A.1.2. Interpreting attributes and relations

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

A relation in a web link is a three-part statement that the context resource has a named relation to the target resource, like “This page has its table of contents at /toc.html”. In [RFC6690] link-format documents, there is an implicit “host relation” specified with default parameter: rel=”hosts”.

In our example, the context of the link is the URI of the requested document itself. A full English expression of the “host relation” is:

coap://[2001:db8:f0::1]/.well-known/core is hosting the resource coap://[2001:db8:f0::1]/temp, which is of the resource type “temperature” and can be accessed using the text/plain content format.’

A.2. A slightly more complex example

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

</temp>;rt=temperature;ct=0,
</light>;rt=light-lux;ct=0,
</t>;anchor="/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;anchor="/sensors/temp";
    rel=describedby,
<t123.pdf>;rel=alternate;ct=65001;
    anchor="http://www.example.com/sensors/t123"

Parsing the third record, the client encounters the “anchor” parameter. It is a URI relative to the document’s Base URI and is thus resolved to “coap://[2001:db8:f0::1]/sensors/temp”. That is the context resource of the link, so the “rel” statement is not about the target and the document Base URI any more, but about the target and that address.

Thus, the third record could be read as “coap://[2001:db8:f0::1]/sensors/temp has an alternate representation at coap://[2001:db8:f0::1]/t”.

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

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

A.3. Enter the Resource Directory

The resource directory tries to carry the semantics obtainable by classical CoAP discovery over to the resource lookup interface as faithfully as possible.

For the following queries, we will assume that the simple host has used Simple Registration to register at the resource directory that was announced to it, sending this request from its UDP port [2001:db8:f0::1]:6553:

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

The resource directory would have accepted the registration, and queried the simple host’s .well-known/core by itself. As a result, the host is registered as an endpoint in the RD with the name “simple-host1”. The registration is active for 86400 seconds, and the endpoint registration Base URI is “coap://[2001:db8:f0::1]/” because that is the address the registration was sent from (and no explicit con= was given).

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

    </temp>;rt=temperature;ct=0;anchor="coap://[2001:db8:f0::1]"

This is not literally the same response that it would have received from a multicast request, but it would contain the (almost) same statement:

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

(The difference is whether / or /.well-known/core hosts the resources, which is subject of ongoing discussion about RFC6690).

To complete the examples, the client could also query all resources hosted at the endpoint with the known endpoint name “simple-host1”. A request to coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1 would return

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

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

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

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

and analogous records.

A.4. A note on differences between link-format and Link headers

While link-format and Link headers look very similar and are based on the same model of typed links, there are some differences between [RFC6690] and [RFC5988] that should be kept in mind when using or implementing a Resource Directory:

Appendix B. Syntax examples for Protocol Negotiation

[ This appendix should not show up in a published version of this document. ]

The protocol negotiation that is being worked on in [I-D.silverajan-core-coap-protocol-negotiation] makes use of the Resource Directory.

Until that document is update to use the latest resource-directory specification, here are some examples of protocol negotiation with the current Resource Directory:

An endpoint could register as follows:

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

Res: 2.01 Created
Location: /rd/1234

A UDP client would then query:

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

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

while a TCP capable client could say:

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

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

Authors' Addresses

Zach Shelby ARM 150 Rose Orchard San Jose, 95134 USA Phone: +1-408-203-9434 EMail: zach.shelby@arm.com
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 Amsüss (editor) Energy Harvesting Solutions Hollandstr. 12/4 1020 Austria Phone: +43-664-9790639 EMail: c.amsuess@energyharvesting.at