CoRE Resource Directory

3.1. Principles

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

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

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

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

3.2. Architecture

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

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

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

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

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

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

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

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

3.3. RD Content Model

The Entity-Relationship (ER) models shown in Figure 2 and Figure 3 model the contents of /.well-known/core and the 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
                               |      oooooooo
          oooooooooooo   0+    |
         o    target  o--------+
         o  attribute o        | 0+   oooooo
          oooooooooooo         +-----o rel  o
                               |      oooooo
                               |
                               | 1    ooooooooo
                               +-----o context o
                                      ooooooooo

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

• a set of links belonging to the hosting web server¶

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

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

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

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

• A link context URI: It defines the source of the relation, e.g. who "hosts" something.¶

  In link-format serialization, it is expressed in the "anchor" attribute. It defaults to that document's URI.¶

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

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

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

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

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

• 0 to n Registrations of endpoints,¶

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

• an endpoint name ("ep", a Unicode string) unique within a sector¶
• a Registration Base URI ("base", a URI typically describing the scheme://authority part)¶
• a lifetime ("lt"),¶
• a registration resource location inside the RD ("href"),¶
• optionally a sector ("d", a Unicode string)¶
• optional additional endpoint attributes (from Section 9.3)¶

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

Links are modelled as they are in Figure 2.¶

3.4. Link-local addresses and zone identifiers

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

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

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

3.5. Use Case: Cellular M2M

Over the last few years, mobile operators around the world have focused on development of M2M solutions in order to expand the business to the new type of users: machines. The machines are connected directly to a mobile network using an appropriate embedded wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing short and wide range wireless interfaces. 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 an appropriate set of link parameters, obtain information on how to contact them (URLs of the proxy server), and then initiate interaction to obtain information that is finally processed, displayed on the screen and usually stored in a database. Similarly, fleet management systems provide the appropriate link parameters to the RD to look up for EPs deployed on the vehicles the application is responsible for.¶

3.6. Use Case: Home and Building Automation

Home and commercial building automation systems can benefit from the use of 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.¶

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

3.7. Use Case: Link Catalogues

Resources may be shared through data brokers that have no knowledge beforehand of who is going to consume the data. 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 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] which may be internally stored as triples, or relation/attribute pairs providing metadata about resource links, need to be supported by Resource Directories . External catalogues 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 encoded in URNs [RFC8141], simple and lossless structural transforms should generally be sufficient to store external metadata in Resource Directories.¶

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

4.1. Finding a Resource Directory

A (re-)starting device may want to find one or more resource directories for discovery purposes. Dependent on the operational conditions, one or more of the techniques below apply.¶

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

1. It may be configured with a specific IP address for the RD. That IP address may also be an anycast address, allowing the network to forward RD requests to an RD that is topologically close; each target network environment in which some of these preconfigured nodes are to be brought up is then configured with a route for this anycast address that leads to an appropriate RD. (Instead of using an anycast address, a multicast address can also be preconfigured. The RD servers then need to configure one of their interfaces with this multicast address.)¶
2. It may be configured with a DNS name for the RD and use DNS to return the IP address of the RD; it can find a DNS server to perform the lookup using the usual mechanisms for finding DNS servers.¶
3. It may be configured to use a service discovery mechanism such as DNS-SD, as outlined in Section 4.1.2.¶

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

1. If the address configuration of the network is performed via SLAAC, this is provided by the RDAO option Section 4.1.1.¶
2. If the address configuration of the network is performed via DHCP, this could be provided via a DHCP option (no such option is defined at the time of writing).¶

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

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

1. In a 6LoWPAN, just assume the Border Router (6LBR) can act as a resource directory (using the ABRO option to find that [RFC6775]). Confirmation can be obtained by sending a Unicast to coap://[6LBR]/.well-known/core?rt=core.rd*.¶
2. In a network that supports multicast well, discovering the RD using a multicast query for /.well-known/core as specified in CoRE Link Format [RFC6690]: Sending a Multicast GET to coap://[MCD1]/.well-known/core?rt=core.rd*. RDs within the multicast scope will answer the query.¶

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

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

The following RD discovery mechanisms are recommended:¶

• In managed networks with border routers that need stand-alone operation, the RDAO option is recommended (e.g. operational phase described in Section 3.6).¶
• In managed networks without border router (no Internet services available), the use of a preconfigured anycast address is recommended (e.g. installation phase described in Section 3.6).¶
• In networks managed using DNS-SD, the use of DNS-SD for discovery as described in Section 4.1.2 is recommended.¶

The use of multicast discovery in mesh networks is NOT recommended.¶

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:                   TBD38

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.

4.2. Payload 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 SHOULD be able to express all the information expressible in link-format. It MAY be able to express information that is inexpressible in link-format, but those expressions SHOULD be avoided where possible.¶

4.3. URI Discovery

Before an endpoint can make use of an RD, it must first know the RD's address and port, and the URI path information for its REST APIs. This section defines discovery of the RD and its URIs using the well-known interface of the CoRE Link Format [RFC6690] after having discovered a host as described in Section 4.1.¶

Discovery of the RD registration URI 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, core.rd* is used to discover all URI paths for RD operations. Upon success, the response will contain a payload with a link format entry for each RD function discovered, indicating the URI of the RD function returned and the corresponding Resource Type. When performing multicast discovery, the multicast IP address used will depend on the scope required and the multicast capabilities of the network (see Section 9.5).¶

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

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

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

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

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

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

Interaction:

EP and Client -> RD¶

Method:

GET¶

URI Template:

/.well-known/core{?rt}¶

URI Template Variables:

rt :=

Resource Type. SHOULD contain one of the values "core.rd", "core.rd-lookup*", "core.rd-lookup-res", "core.rd-lookup-ep", or "core.rd*"¶

Accept:

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

The following response is expected on this interface:¶

Success:

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

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

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

Res: 2.05 Content
</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,

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

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

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

From a management and maintenance perspective, it is necessary to identify the components that constitute the RD server. The identification refers to information about for example client-server incompatibilities, supported features, required updates and other aspects. The URI discovery address, a described in section 4 of [RFC6690] can be used to find the identification.¶

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

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

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

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

5.1. Simple Registration

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

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

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

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

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

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

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

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

  Example request from the RD to the registrant-EP:¶

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

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

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

Res: 2.04 Changed

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

The simple registration request interface is specified as follows:¶

Interaction:

EP -> RD¶

Method:

POST¶

URI Template:

/.well-known/core{?ep,d,lt,extra-attrs*}¶

URI Template Variables are as they are for registration in Section 5. The base attribute is not accepted to keep the registration interface simple; that rules out registration over CoAP-over-TCP or HTTP that would need to specify one.¶

The following response is expected on this interface:¶

Success:

2.04 "Changed".¶

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

Interaction:

RD -> EP¶

Method:

GET¶

URI Template:

/.well-known/core¶

The following response is expected on this interface:¶

Success:

2.05 "Content".¶

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

5.2. Third-party registration

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

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

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

5.3. Operations on the Registration Resource

This section describes how the registering endpoint can maintain the registrations that it created. The registering endpoint can be the registrant-ep or the CT. An endpoint SHOULD NOT use this interface for registrations that it did not create. The registrations are resources of the RD.¶

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

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

These operations are described below.¶

Req: POST /rd/4521

Res: 2.04 Changed

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

Res: 2.05 Content
Payload:
<coap://local-proxy-old.example.com:5683/sensors/temp>;ct=41;
    rt="temperature-c";if="sensor";
    anchor="coap://local-proxy-old.example.com:5683/",
<http://www.example.com/sensors/temp>;
    anchor="coap://local-proxy-old.example.com:5683/sensors/temp";rel="describedby"

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

Res: 2.04 Changed

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

Res: 2.05 Content
Payload:
<coap://new.example.com:5684/sensors/temp>;ct=41;
    rt="temperature-c";if="sensor";
    anchor="coap://new.example.com:5684/",
<http://www.example.com/sensors/temp>;
    anchor="coap://new.example.com:5684/sensors/temp";rel="describedby"

Req: DELETE /rd/4521

Res: 2.02 Deleted

6.1. Resource lookup

Resource lookup results in links that are semantically equivalent to the links submitted to the RD. The links and link parameters returned by the lookup are equal to the submitted ones, except that the target and anchor references are fully resolved.¶

Links that did not have an anchor attribute are therefore returned with the base URI of the registration as the anchor. Links of which href or anchor was submitted as a (full) URI are returned with these attributes unmodified.¶

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

If the base URI of a registration contains a link-local address, the RD MUST NOT show its links unless the lookup was made from the same link. The RD MUST NOT include zone identifiers in the resolved URIs.¶

6.2. Lookup filtering

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

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. The Resource Directory MUST support matching with multiple search criteria.¶

A link matches a search criterion if it has an attribute of the same name and the same value, allowing for a trailing "*" wildcard operator as in Section 4.1 of [RFC6690]. Attributes that are defined as "link-type" match if the search value matches any of their values (see Section 4.1 of [RFC6690]; e.g. ?if=core.s matches ;if="abc core.s";). A resource link also matches a search criterion if its endpoint would match the criterion, and vice versa, an endpoint link matches a search criterion if any of its resource links matches it.¶

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

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

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

The lookup interface is specified as follows:¶

Interaction:

Client -> RD¶

Method:

GET¶

URI Template:

{+type-lookup-location}{?page,count,search*}¶

URI Template Variables:

type-lookup-location :=

RD Lookup URI for a given lookup type (mandatory). The address is discovered as described in Section 4.3.¶

search :=

Search criteria for limiting the number of results (optional).¶

page :=

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

count :=

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

Accept:

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

The following responses codes are defined for this interface:¶

Success:

2.05 "Content" or 200 "OK" with an application/link-format or other web link payload containing matching entries for the lookup. The payload can contain zero links (which is an empty payload in [RFC6690] link format, but could also be [] in JSON based formats), indicating that no entities matched the request.¶

6.3. Resource lookup examples

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

The following example shows a client performing a resource lookup with the example resource look-up locations discovered in Figure 5:¶

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

Res: 2.05 Content
<coap://[2001:db8:3::123]:61616/temp>;rt="temperature";
           anchor="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:
<coap://[2001:db8:3::124]/west>;rt="light";
    anchor="coap://[2001:db8:3::124]",
<coap://[2001:db8:3::124]/south>;rt="light";
    anchor="coap://[2001:db8:3::124]",
<coap://[2001:db8:3::124]/east>;rt="light";
    anchor="coap://[2001:db8:3::124]"

The following example shows a client performing a paginated resource lookup¶

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

Res: 2.05 Content
<coap://[2001:db8:3::123]:61616/res/0>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/1>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/2>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/3>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<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
<coap://[2001:db8:3::123]:61616/res/5>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/6>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/7>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/8>;rt=sensor;ct=60;
    anchor="coap://[2001:db8:3::123]:61616",
<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 of all endpoints of a given endpoint type. It assumes that two endpoints (with endpoint names sensor1 and sensor2) have previously registered with their respective addresses coap://sensor1.example.com and coap://sensor2.example.com, and posted the very payload of the 6th request of section 5 of [RFC6690].¶

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

Req: GET /rd-lookup/res?et=oic.d.sensor

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

6.4. Endpoint lookup

The endpoint lookup returns registration resources which can only be manipulated by the registering endpoint.¶

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

Links to endpoints SHOULD be presented in path-absolute form or, if required, as (full) URIs. (This avoids the RFC6690 ambiguities.)¶

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

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

A Resource Directory can report endpoints in lookup that are not hosted at the same address. Lookup clients MUST be prepared to see arbitrary URIs as registration resources in the results and treat them as opaque identifiers; the precise semantics of such links are left to future specifications.¶

The following example shows a client performing an endpoint type (et) lookup with the value oic.d.sensor (which is currently a registered rt value):¶

Req: GET /rd-lookup/ep?et=oic.d.sensor

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

7.1. Secure RD discovery

The Resource Server (RS) discussed in [I-D.ietf-ace-oauth-authz] is equated to the RD. The client (C) needs to discover the RD as discussed in Section 4.1. C can discover the related AS by sending a request to the RD. The RD denies the request by sending the address of the related AS, as discussed in section 5.1 of [I-D.ietf-ace-oauth-authz]. The client MUST send an authorization request to the AS. When appropriate, the AS returns a token that specifies the authorization permission which needs to be specified in a separate document.¶

7.2. Secure RD filtering

The authorized parameter values for the queries by a given endpoint must be registered by the AS. The AS communicates the parameter values in the token. A separate document needs to specify the parameter value combinations and their storage in the token. The RD decodes the token and checks the validity of the queries of the client.¶

7.3. Secure endpoint Name assignment

This section only considers the assignment of a name to the endpoint based on an automatic mechanism without use of AS. More elaborate protocols are out of scope. The registering endpoint is authorized by the AS to discover the RD and add registrations. A token is provided by the AS and communicated from registering endpoint to RD. It is assumed that DTLS is used to secure the channel between registering endpoint and RD, where the registering endpoint is the DTLS client. Assuming that the client is provided by a certificate at manufacturing time, the certificate is uniquely identified by the CN field and the serial number (see [RFC5280] Section 4.1.2.2). The RD can assign a unique endpoint name by using the certificate identifier as endpoint name. Proof of possession of the endpoint name by the registering endpoint is checked by encrypting the certificate identifier with the private key of the registering endpoint, which the RD can decrypt with the public key stored in the certificate. Even simpler, the authorized registering endpoint can generate a random number (or string) that identifies the endpoint. The RD can check for the improbable replication of the random value. The RD MUST check that registering endpoint uses only one random value for each authorized endpoint.¶

8.1. Endpoint Identification and Authentication

An Endpoint (name, sector) pair is unique within the et of endpoints registered by the RD. An Endpoint MUST NOT be identified by its protocol, port or IP address as these may change over the lifetime of an Endpoint.¶

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

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

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

Section 7.3 specifies an example that removes this threat for endpoints that have a certificate installed.¶

8.2. Access Control

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

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.¶

9.1. Resource Types

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

9.2. IPv6 ND Resource Directory Address Option

This document registers one new ND option type under the sub-registry "IPv6 Neighbor Discovery Option Formats":¶

• Resource Directory Address Option (TBD38)¶

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

9.3. RD Parameter Registry

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

Each entry in the registry must include¶

• the human readable name of the parameter,¶
• the short name as used in query parameters or target attributes,¶
• indication of whether it can be passed as a query parameter at registration of endpoints, as a query parameter in lookups, or be expressed as a target attribute,¶
• validity requirements if any,¶
• a description,¶
• and a link to reference documentation.¶

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

The description must give details on whether the parameter can be updated, and how it is to be processed in lookups.¶

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

Initial entries in this sub-registry are as follows:¶

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

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

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

9.4. "Endpoint Type" (et=) RD Parameter values

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

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

The requirements to be enforced are:¶

• The values MUST be related to the purpose described in Section 9.3.1.¶
• The registered values MUST conform to the ABNF reg-rel-type definition of [RFC6690] and MUST NOT be a URI.¶
• It is recommended to use the period "." character for segmentation.¶

The registry initially contains one value:¶

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

9.5. Multicast Address Registration

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

IPv4 - "all CoRE resource directories" address MCD2 (suggestion: 224.0.1.189), from the "IPv4 Multicast Address Space Registry". As the address is used for discovery that may span beyond a single network, it has come from the Internetwork Control Block (224.0.1.x, RFC 5771).¶

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

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

9.6. Well-Kown URIs

IANA is asked to extend the reference for the "core" URI suffix in the "Well-Known URIs" registry to reference this document next to [RFC6690], as this defines the resource's behavior for POST requests.¶

9.7. Service Names and Transport Protocol Port Number Registry

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

• Service name: "core-rd", Protocol: "udp", Description: "Resource Directory accessed using CoAP"¶
• Service name "core-rd-dtls", Protocol: "udp", Description: "Resource Directory accedded using CoAP over DTLS"¶
• Service name: "core-rd", Protocol: "tcp", Description: "Resource Directory accessed using CoAP over TCP"¶
• Service name "core-rd-tls", Protocol: "tcp", Description: "Resource Directory accedded using CoAP over TLS"¶

All in common have this document as their reference.¶

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 as described in Appendix A. No conclusions must be drawn on the realization of actual installation or naming procedures, because the example only "emphasizes" some of the issues that may influence the use of the RD and does not pretend to be normative.¶

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

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

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

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

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

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

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

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

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

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

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

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

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 sectors and does not currently use the rd-lookup interface.¶

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.¶

/1/0/1

ep - Endpoint Name
lt - registration lifetime

et - Endpoint Type
base - Registration Base URI

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

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

B.1. A simple example

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

The client sends a link-local multicast:¶

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 for insertion into and retrieval from the RD without any shortcuts are:¶

B.2. A slightly more complex example

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

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

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

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

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

B.3. Enter the Resource Directory

The 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 90000 seconds, and the endpoint registration Base URI is "coap://[2001:db8:f0::1]" following the resolution steps described in Appendix B.1.1. It should be remarked that the Base URI constructed that way always yields a URI of the form: scheme://authority without path suffix.¶

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

<coap://[2001:db8:f0::1]/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 contains the equivalent 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 does not matter in this application; if it did, the endpoint would have been more explicit. Actually, /.well-known/core does NOT host the resource but stores a URI reference to the resource.)¶

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¶

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

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

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

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

and analogous records.¶

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

While link-format and Link header fields look very similar and are based on the same model of typed links, there are some differences between [RFC6690] and [RFC8288], which are dealt with differently:¶

• "Resolving the target against the anchor": [RFC6690] Section 2.1 states that the anchor of a link is used as the Base URI against which the term inside the angle brackets (the target) is resolved, falling back to the resource's URI with paths stripped off (its "Origin"). In contrast to that, [RFC8288] Section B.2 describes that the anchor is immaterial to the resolution of the target reference.¶

  RFC6690, in the same section, also states that absent anchors set the context of the link to the target's URI with its path stripped off, while according to [RFC8288] Section 3.2, the context is the resource's base URI.¶

  The rules introduced in Appendix C ensure that an RD does not need to deal with those differences when processing input data. Lookup results are required to be absolute references for the same reason.¶

• There is no percent encoding in link-format documents.¶

  A link-format document is a UTF-8 encoded string of Unicode characters and does not have percent encoding, while Link header fields are practically ASCII strings that use percent encoding for non-ASCII characters, stating the encoding explicitly when required.¶

  For example, while a Link header field in a page about a Swedish city might read¶

  Link: </temperature/Malm%C3%B6>;rel="live-environment-data"
  

  ¶

  a link-format document from the same source might describe the link as¶

  </temperature/Malmö>;rel="live-environment-data"
  

  ¶

  Parsers and producers of link-format and header fields need to be aware of this difference.¶