CoRE Z. Shelby
Internet-Draft ARM
Intended status: Standards Track M. Koster
Expires: April 26, 2019 SmartThings
C. Bormann
Universitaet Bremen TZI
P. van der Stok
C. Amsüss, Ed.
October 23, 2018

CoRE Resource Directory


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 registrations 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 for web servers to discover the RD and to register, maintain, lookup and remove resource descriptions. Furthermore, new link attributes useful in conjunction with an RD are defined.

Status of This Memo

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This Internet-Draft will expire on April 26, 2019.

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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 registrations 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 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 [RFC3986], [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:

resolve against

The expression “a URI-reference is resolved against a base URI” is used to describe the process of [RFC3986] Section 5.2. Noteworthy corner cases are that if the URI-reference is a (full) URI and resolved against any base URI, that gives the original full URI, and that resolving an empty URI reference gives the base URI without any fragment identifier.
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.

In the context of a Resource Directory, a sector is a logical grouping of endpoints.
The abbreviation “d=” is used for the sector in query parameters for compatibility with deployed implementations.

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 sector of the registration.
Registration Base URI

The Base URI of a Registration is a URI that typically gives scheme and authority information about an Endpoint. The Registration Base URI is provided at registration time, and is used by the Resource Directory to resolve relative references of the registration into URIs.

The target of a link is the destination address (URI) of the link. It is sometimes identified with “href=”, or displayed as <target>. Relative targets need resolving with respect to the Base URI (section 5.2 of [RFC3986]).
This use of the term Target is consistent with [RFC8288]’s use of the term.

The context of a link is the source address (URI) of the link, and describes which resource is linked to the target. A link’s context is made explicit in serialized links as the “anchor=” attribute.
This use of the term Context is consistent with [RFC8288]’s use of the term.
Directory Resource

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

Registrant-ep is the endpoint that is registered into the RD. The registrant-ep can register itself, or a CT registers the registrant-ep.

Resource Directory Address Option.

For several operations, interface descriptions are given in list form; those describe the operation participants, request codes, URIs, content formats and outcomes. Those templates contain normative content in their Interaction, Method, URI Template and URI Template Variables sections as well as the details of the Success condition. The additional sections on options like Content-Format and on Failure codes give typical cases that an implementation of the RD should deal with. Those serve to illustrate the typical responses to readers who are not yet familiar with all the details of CoAP based interfaces; they do not limit what a server may respond under atypical circumstances.

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 devices, 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.

Only information SHOULD be stored in the resource directory that is discoverable from 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 in the Resource Directory do not propagate automatically back to the web server from where the links originated.

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] 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 sets of Web Links (called resource directory registration entries), and for endpoints to lookup resources from the RD. An RD can be logically segmented by the use of Sectors. This information hierarchy is shown in Figure 2.

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

Registration entries in the RD are soft state and need to be periodically refreshed.

An endpoint uses specific 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 registration entries.

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

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

Figure 1: The resource directory architecture.

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

Figure 2: The resource directory information hierarchy.

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 fallback path for a single registration.

3.3. RD 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
                               |      oooooooo
          oooooooooooo   0+    |
         o    target  o--------+
         o  attribute o        | 0+   oooooo
          oooooooooooo         +-----o rel  o
                               |      oooooo
                               | 1    ooooooooo
                               +-----o context o

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 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 [RFC5988]):

             |  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

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 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:

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 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 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.

3.5. 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 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 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 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. Finding a Resource Directory

A (re-)starting device may want to find one or more resource directories for discovery purposes.

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 [RFC6763]. The present specification suggests configuring the service with name rd._sub._coap._udp, preferably within the domain of the querying nodes.

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.
  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.

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.

If multiple candidate addresses are discovered, the device may pick any of them initially, unless the discovery method indicates a more precise selection scheme.

4.1. Resource Directory Address Option (RDAO)

The Resource Directory Address 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 a link-local or realm-local scope 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 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                           +
|                                                               |
+                                                               +
|                                                               |


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. 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 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, 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” 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. The well-known entry points SHOULD be provided to enable unicast discovery.

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):

EP and Client -> RD
URI Template:
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*”

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

The following response codes are defined for this interface:

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.
4.00 “Bad Request” or 400 “Bad Request” is returned in case of a malformed request for a unicast request.
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 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

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 CBOR and JSON representation of link format. 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.

[ The RFC editor is asked to replace these and later occurrences of MCD1, TBD64 and TBD504 with the assigned IPv6 site-local address for “all CoRE Resource Directories” and 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://[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

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.

5.3. Registration

After discovering the location of an RD, a registrant-ep or CT MAY register the resources of the registrant-ep 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 the sector, lifetime and base URI 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. The receiving 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 creating 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 (sector) does not create multiple registration resources.

The following rules apply for an update identified by a given (ep, d) value pair:

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. They are resolved against the base URI of the registration, which is provided either explicitly in the base parameter or constructed implicitly from the requester’s URI as constructed from its network address and scheme.

Link format documents submitted to the resource directory are interpreted as Modernized Link Format (see Appendix D) by the RD. A registrant-ep SHOULD NOT submit documents whose interpretations according to [RFC6690] and Appendix D differ to avoid the ambiguities described in Appendix B.4.

In practice, most links (precisely listed in Appendix D.1) can be submitted without consideration for those details.

The registration request interface is specified as follows:

EP -> RD
URI Template:
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 sector. The maximum length of this parameter is 63 bytes. If the RD is configured to recognize the endpoint (e.g. based on its security context), the endpoint sets no endpoint name, and the RD assigns one based on a set of configuration parameter values.
d :=
Sector (optional). The sector to which this endpoint belongs. The maximum length of this parameter is 63 bytes. When this parameter is not present, the RD MAY associate the endpoint with a configured default sector or leave it empty. The endpoint name and sector name are not set when one or both are set in an accompanying authorization token.
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 90000 (25 hours) SHOULD be assumed.
base :=
Base URI (optional). This parameter sets the base URI of the registration, under which the relative links in the payload are to be interpreted. The specified URI typically does not have a path component of its own, and MUST be suitable as a base URI to resolve any relative references given in the registration. The parameter is therefore usually of the shape “scheme://authority” for HTTP and CoAP URIs. The URI SHOULD NOT have a query or fragment component as any non-empty relative part in a reference would remove those parts from the resulting URI.
In the absence of this parameter the scheme of the protocol, source address and source port of the registration request are assumed. That Base URI is constructed by concatenating the used protcol’s scheme with the characters “://”, the requester’s source address as an address literal and “:” followed by its port (if it was not the protocol’s default one) in analogy to [RFC7252] Section 6.5.
This parameter is mandatory when the directory is filled by a third party such as an commissioning tool.
If the registrant-ep uses an ephemeral port to register with, it MUST include the base parameter in the registration to provide a valid network path.
If the registrant-ep, 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.
Endpoints that register with a base that contains a path component can not meaningfully use [RFC6690] Link Format due to its prevalence of the Origin concept in relative reference resolution; they can submit payloads for interpretation as Modernized Link Format. Typically, links submitted by such an endpoint are of the path-noscheme (starts with a path not preceded by a slash, precisely defined in [RFC3986] Section 3.3) form.
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.


The following response codes are defined for this interface:

2.01 “Created” or 201 “Created”. The Location-Path option or Location header MUST be included in the response. 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.
The location MUST NOT have a query or fragment component, as that could conflict with query parameters during the Registration Update operation. Therefore, the Location-Query option MUST NOT be present in a successful response.
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:

If the registration fails with a Service Unavailable response and a Max-Age option or Retry-After header, the registering endpoint SHOULD retry the operation after the time indicated. If the registration fails in another way, including request timeouts, or if the Service Unavailable error persists after several retries, or indicates a longer time than the endpoint is willing to wait, it SHOULD pick another registration URI from the “URI Discovery” step and if there is only one or the list is exhausted, pick other choices from the “Finding a Resource Directory” step. Care has to be taken to consider the freshness of results obtained earlier, e.g. of the result of a /.well-known/core response, the lifetime of an RDAO option and of DNS responses. Any rate limits and persistent errors from the “Finding a Resource Directory” step must be considered for the whole registration time, not only for a single operation.

The following example shows a registrant-ep 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://
Content-Format: 40

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

Figure 7: Example registration payload

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&base=http://[2001:db8:1::1] HTTP/1.1
Content-Type: application/link-format+json
{"href": "/sensors/temp", "ct": "41", "rt": "temperature-c",
"if": "sensor", "anchor": "coap://"},
{"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 an 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 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 D).

The registrant-ep finds one or more addresses of the directory server as described in Section 4.

The registrant-ep asks the selected directory server to probe its /.well-known/core and publish the links as follows:

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 5.3. The registration base URI of the registration is taken from the requesting server’s URI.

The Resource Directory MUST NOT query the registrant-ep’s data before sending the response; this is to accommodate very limited endpoints. The success condition only indicates that the request was valid (i.e. the passed parameters are valid per se), not that the link data could be obtained or parsed or was successfully registered into the RD.

The simple registration request interface is specified as follows:

EP -> RD
URI Template:

URI Template Variables are as they are for registration in Section 5.3. 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 codes are defined for this interface:

2.04 “Changed”.
4.00 “Bad Request”. Malformed request.
5.03 “Service Unavailable”. Service could not perform the operation.
HTTP support:

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):

RD -> EP
URI Template:

The following response codes are defined for this interface:

2.05 “Content”.
4.00 “Bad Request”. Malformed request.
4.04 “Not Found”. /.well-known/core does not exist or is empty.
5.03 “Service Unavailable”. Service could not perform the operation.
HTTP support:

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

The following example shows a registrant-ep 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
No payload

Res: 2.04 Changed


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

Res: 2.05 Content
Content-Format: 40

5.3.2. 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 5.3.

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.

5.3.3. RD-Groups

The RD-Groups usage pattern allows announcing application groups inside a Resource Directory.

Groups are represented by endpoint registrations. Their base address is a multicast address, and they SHOULD be entered with the endpoint type core.rd-group. The endpoint name can also be referred to as a group name in this context.

The registration is inserted into the RD by a Commissioning Tool, which might also be known as a group manager here. It performs third party registration and registration updates.

The links it registers SHOULD be available on all members that join the group. Depending on the application, members that lack some resource MAY be permissible if requests to them fail gracefully.

The following example shows a CT registering a group with the name “lights” which provides two resources. The directory resource path /rd is an example RD location discovered in a request similar to Figure 6.

Req: POST coap://
Content-Format: 40

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

In this example, the group manager can easily permit devices that have no writable color-temperature to join, as they would still respond to brightness changing commands. Had the group instead contained a single resource that sets brightness and color temperature atomically, endpoints would need to support both properties.

The resources of a group can be looked up like any other resource, and the group registrations (along with any additional registration parameters) can be looked up using the endpoint lookup interface.

6. 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 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, 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

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.

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 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. 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.

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:

Client -> RD
URI Template:
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 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.
application/link-format (optional)
application/link-format+json (optional)
application/link-format+cbor (optional)

The following responses codes are defined for this interface:

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.
No error response to a multicast request.
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
5.03 “Service Unavailable” or 503 “Service Unavailable”. Service could not perform the operation.
HTTP support:

The endpoint lookup returns registration resources which can only be manipulated by the registering endpoint. Examples of endpoint lookup belong to the management aspects of the RD and are shown in Appendix A.5. The resource lookup examples are shown in this section.

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 6:

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

Res: 2.05 Content

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:
Decoded payload:
[{1: "coap://[2001:db8:3::123]:61616/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

The following example shows a client performing a paginated resource lookup

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

Res: 2.05 Content

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

Res: 2.05 Content

The following example shows a client performing a lookup of all resources from endpoints 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:// and coap://, 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://>;ct=40;title="Sensor Index";
    if="sensor"; anchor="coap://",
    if="sensor"; anchor="coap://",
<coap://>;ct=40;title="Sensor Index";
    if="sensor"; anchor="coap://",
    if="sensor"; anchor="coap://",

The following example shows a client performing a lookup of all resources of all endpoints (groups) with et=core.rd-group.

Req: GET /rd-lookup/res?et=core.rd-group


7. Security policies

The Resource Directory (RD) provides assistance to applications situated on a selection of nodes to discover endpoints on connected nodes. This section discusses different security aspects of accessing the RD.

The contents of the RD are inserted in two ways:

  1. The node hosting the discoverable endpoint fills the RD with the contents of /.well-known/core by:
  2. A Commissioning Tool (CT) fills the RD with endpoint information for a set of discoverable nodes. (see Section 5.3 with base=authority parameter value)

In both cases, the nodes filling the RD should be authenticated and authorized to change the contents of the RD. An Authorization Server (AS) is responsible to assign a token to the registering node to authorize the node to discover or register endpoints in a given RD [I-D.ietf-ace-oauth-authz].

It can be imagined that an installation is divided in a set of security regions, each one with its own RD(s) to discover the endpoints that are part of a given security region. An endpoint that wants to discover an RD, responsible for a given region, needs to be authorized to learn the contents of a given RD. Within a region, for a given RD, a more fine-grained security division is possible based on the values of the endpoint registration parameters. Authorization to discover endpoints with a given set of filter values is recommended for those cases.

When a node registers its endpoints, criteria are needed to authorize the node to enter them. An important aspect is the uniqueness of the (endpoint name, and optional sector) pair within the RD. Consider the two cases separately: (1) CT registers endpoints, and (2) the registering node registers its own endpoint(s). * A CT needs authorization to register a set of endpoints. This authorization can be based on the region, i.e. a given CT is authorized to register any endpoint (endpoint name, sector) into a given RD, or to register an endpoint with (endpoint name, sector) value pairs assigned by the AS, or can be more fine-grained, including a subset of registration parameter values. * A given endpoint that registers itself, needs to proof its possession of its unique (endpoint name, sector) value pair. Alternatively, the AS can authorize the endpoint to register with an (endpoint name, sector) value pair assigned by the AS. * A separate document needs to specify these aspects to ensure interoperability between registering nodes and RD. The subsections below give some hints how to handle a subset of the different aspects.

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. 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. 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. 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 (name, sector) pair is unique within the et of endpoints regsitered 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. IANA Considerations

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]:

Value Description Reference
core.rd Directory resource of an RD RFCTHIS Section 5.2
core.rd-lookup-res Resource lookup of an RD RFCTHIS Section 5.2
core.rd-lookup-ep Endpoint lookup of an RD RFCTHIS Section 5.2
core.rd-ep Endpoint resource of an RD RFCTHIS Section 6

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”:

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

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:

RD Parameters
Full name Short Validity Use Description
Endpoint Name ep RLA Name of the endpoint, max 63 bytes
Lifetime lt 60-4294967295 R Lifetime of the registration in seconds
Sector d RLA Sector to which this endpoint belongs
Registration Base URI base URI RLA The scheme, address and port and path at which this server is available
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 (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 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 sub-registry. 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 initially contains one value:

9.5. Multicast Address Registration

IANA has assigned the following multicast addresses for use by CoAP nodes:

IPv4 – “all CoRE resource directories” address, from the “IPv4 Multicast Address Space Registry” equal to “All CoAP Nodes”, 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.

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 as described in Section 5.3.3. 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 the RD’s address, and has performed URI discovery on it that returned 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 registration base URI parameter (base) to specify the interface address:

Req: POST coap://[2001:db8:4::ff]/rd

Res: 2.01 Created
Location-Path: /rd/4521
Req: POST coap://[2001:db8:4::ff]/rd

Res: 2.01 Created
Location-Path: /rd/4522
Req: POST coap://[2001:db8:4::ff]/rd

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

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

The group is specified in the RD. The base parameter is set to the site-local multicast address allocated to the group. In the POST in the example below, the resources supported by all group members are published.

Req: POST coap://[2001:db8:4::ff]/rd

Res: 2.01 Created
Location-Path: /rd/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 sector and being configured to join any group containing lights, searches for candidate groups and joins them:

Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep

Res: 2.05 Content

From the returned base 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 coap://[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.

The presence sensor can learn the presence of groups that support resources with rt=light in its own sector by sending the same request, as used by the luminary. The presence sensor learns the multicast address to use for sending messages to the luminaries.

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.

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:


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:


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
base - Registration Base URI

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:


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 Appendix A.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 Appendix A.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 -16 to -17

(Note that -17 is published as a direct follow-up to -16, containing a single change to be discussed at IETF103)

changes from -15 to -16

changes from -14 to -15

changes from -13 to -14

changes from -12 to -13

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-10, February 2018.
[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.

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-05, October 2017.
[I-D.bormann-t2trg-rel-impl] Bormann, C., "impl-info: A link relation type for disclosing implementation information", Internet-Draft draft-bormann-t2trg-rel-impl-00, January 2018.
[I-D.ietf-ace-oauth-authz] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S. and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)", Internet-Draft draft-ietf-ace-oauth-authz-16, October 2018.
[I-D.ietf-anima-bootstrapping-keyinfra] Pritikin, M., Richardson, M., Behringer, M., Bjarnason, S. and K. Watsen, "Bootstrapping Remote Secure Key Infrastructures (BRSKI)", Internet-Draft draft-ietf-anima-bootstrapping-keyinfra-16, June 2018.
[I-D.silverajan-core-coap-protocol-negotiation] Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation", Internet-Draft draft-silverajan-core-coap-protocol-negotiation-09, July 2018.
[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.
[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.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S. and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, May 2018.

Appendix A. Registration Management

This section describes how the registering endpoint can maintain the registries that it created. The registering endpoint can be the registrant-ep or the CT. An endpoint SHOULD NOT use this interface for registries that it did not create. The registries 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 to inspect the registration resource using GET, update the registration, cancel the registration using DELETE, or do an endpoint lookup.

These operations are described below.

A.1. Registration Update

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

An update MAY update the lifetime- or the context- registration parameters “lt”, “base” 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, relative references submitted in the original registration or later updates are resolved anew against the new context.

The registration update operation only describes the use of POST with an empty payload. Future standards might describe the semantics of using content formats and payloads with the POST method to update the links of a registration (see Appendix A.4).

The update registration request interface is specified as follows:

EP -> RD
URI Template:
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 90000) SHOULD be used.
base :=
Base URI (optional). This parameter updates the Base URI 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 relative links present in the payload of the original registration, following the same restrictions as in the registration. If the parameter is not set in the request but was set before, the previous Base URI value is kept unmodified. If the parameter is not set in the request and was not set before either, the source address and source port of the update request are stored as the Base URI.
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.

none (no payload)

The following response codes are defined for this interface:

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

If the registration update fails with a “Service Unavailable” response and a Max-Age option or Retry-After header, the registering endpoint SHOULD retry the operation after the time indicated. If the registration fails in another way, including request timeouts, or if the time indicated exceeds the remaining lifetime, the registering endpoint SHOULD attempt registration again.

The following example shows how the registering endpoint updates 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 the registering endpoint updating its registration resource at an RD using this interface with the example location value: /rd/4521. The initial registration by the registering endpoint 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
 rt="temperature"; anchor="coap://",
  rt="light-lux"; if="sensor";

The following example shows the registering endpoint changing the Base URI to coaps://

Req: POST /rd/4521?base=coaps://

Res: 2.04 Changed

The consecutive query returns:

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

Res: 2.01 Content
    if="sensor"; anchor="coaps://",

The following example shows a client performing and enpoint lookup for all groups.

Req: GET /rd-lookup/ep?et=core.rd-group

Res: 2.01 Content

A.2. Registration Removal

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

The removal request interface is specified as follows:

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

The following response codes are defined for this interface:

2.02 “Deleted” or 204 “No Content” upon successful deletion
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).
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

A.3. Read Endpoint Links

Some registering 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:

EP -> RD
URI Template:
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 response codes are defined for this interface:

2.05 “Content” or 200 “OK” upon success with an application/link-format, application/link-format+cbor, or application/link-format+json payload.
4.00 “Bad Request” or 400 “Bad Request”. Malformed request.
4.04 “Not Found” or 404 “Not Found”. Registration does not exist (e.g. may have expired).
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 and example registration payload of Figure 7.

Req: GET /rd/4521

Res: 2.01 Content

A.4. Update Endpoint Links

An iPATCH (or PATCH) update ([RFC8132]) can add, remove or change the links of a registration.

Those operations are out of scope of this document, and will require media types suitable for modifying sets of links.

A.5. Endpoint lookup

Endpoint lookups result in links to registration resources. Endpoint registration resources are annotated with their endpoint names (ep), sectors (d, if present) and registration base URI (base) as well as a constant resource type (rt=”core.rd-ep”); the lifetime (lt) is not reported. Additional endpoint attributes are added as link attributes to their endpoint link unless their specification says otherwise.

Serializations derived from Link Format, SHOULD present links to endpoints in path-absolute form or, if required, as absolute references. (This approach avoids the RFC6690 ambiguities.)

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

Appendix B. 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 [RFC8288] 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.

At all examples in this section give compatible results for both Modernized and RFC6690 Link Format; the explanation of the steps follow Modernized Link Format.

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

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.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 <coap://[ff02::fd]:5683/.well-known/core> is used to resolve the reference /temp against.

The Base URI of the requested resource can be composed from the header options of the CoAP GET request 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”.

Because “/temp” starts with a single slash, 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”.

B.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 specifies a named relation between the so-called “context resource” and the target resource, like “This page has its table of contents at /toc.html”. In [RFC6690] and modernized link-format documents, there is an implicit “host relation” specified with default parameter: rel=”hosts”.

In our example, the context resource of the link is the URI specified in the GET request “coap:://[2001:db8:f0::1]/.well-known/core”. 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.’

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

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”.

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:


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 is one of the often misunderstood subtleties Modernized Link Format addresses. 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


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://, that context would have been used to resolve the relative anchor values instead, giving


and analogous records.

B.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], which are dealt with differently:

Appendix C. 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 from its address [2001:db8:f1::2]:5683:

Req: POST coap://
Content-Format: 40

Res: 2.01 Created
Location-Path: /rd/1234

An endpoint lookup would just reflect the registered attributes:

Req: GET /rd-lookup/ep

Res: 2.05 Content

A UDP client would then see the following in a resource lookup:

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

Res: 2.05 Content
    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

Appendix D. Modernized Link Format parsing

The CoRE Link Format as described in [RFC6690] is unsuitable for some use cases of the Resource Directory, and their resolution scheme is often misunderstood by developers familiar with [RFC8288].

For the correct application of base URIs, we describe the interpretation of a Link Format document as a Modernized Link Format. In Modernized Link Format, the document is processed as in Link Format, with the exception of Section 2.1 of [RFC6690]:

Content formats derived from [RFC6690] which inherit its resolution rules, like JSON and CBOR link format of [I-D.ietf-core-links-json], can be interpreted in analogy to that.

For where the Resource Directory is concerned, all common forms of links (e.g. all the examples of RFC6690) yield identical results. When interpreting data read from .well-known/core, differences in interpretation only affect links where the absent anchor attribute means coap://host/ according to RFC6690 and coap://host/.well-known/core according to Modernized Link format; those typically only occur in conjunction with the vaguely defined implicit “hosts” relationship.

D.1. For endpoint developers

When developing endpoints, i.e. when generating documents that will be submitted to a Resource Directory, the differences between Modernized Link Format and RFC6690 can be ignored as long as

and any of the following applies:

D.2. Examples of links with differing interpretations

Examples of links with different interpretations from either applying RFC6690 or Modernized Link Format are shown here. The example is assumed to be obtained from a </device/index> document.

Authors' Addresses

Zach Shelby ARM 150 Rose Orchard San Jose, 95134 USA Phone: +1-408-203-9434 EMail:
Michael Koster SmartThings 665 Clyde Avenue Mountain View, 94043 USA Phone: +1-707-502-5136 EMail:
Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63921 EMail:
Peter van der Stok consultant Phone: +31-492474673 (Netherlands), +33-966015248 (France) EMail: URI:
Christian Amsüss (editor) Hollandstr. 12/4 1020 Austria Phone: +43-664-9790639 EMail: