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IETF RFC 9176
Last modified on Monday, April 25th, 2022
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Internet Engineering Task Force (IETF) C. Amsüss, Ed.
Request for Comments: 9176
Category: Standards Track Z. Shelby
ISSN: 2070-1721 Edge Impulse
M. Koster
PassiveLogic
C. Bormann
Universität Bremen TZI
P. van der Stok
vanderstok consultancy
April 2022
Constrained RESTful Environments (CoRE) Resource Directory
 Abstract
In many Internet of Things (IoT) applications, direct discovery of
resources is not practical due to sleeping nodes or networks where
multicast traffic is inefficient. These problems can be solved by
employing an entity called a Resource Directory (RD), which contains
information about resources held on other servers, allowing lookups
to be performed for those resources. The input to an RD is composed
of links, and the output is composed of links constructed from the
information stored in the RD. This document specifies the web
interfaces that an RD supports for web servers to discover the RD and
to register, maintain, look up, and remove information on resources.
Furthermore, new target attributes useful in conjunction with an RD
are defined.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9176.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Architecture and Use Cases
3.1. Principles
3.2. Architecture
3.3. RD Content Model
3.4. Link-Local Addresses and Zone Identifiers
3.5. Use Case: Cellular M2M
3.6. Use Case: Home and Building Automation
3.7. Use Case: Link Catalogues
4. RD Discovery and Other Interface-Independent Components
4.1. Finding a Resource Directory
4.1.1. Resource Directory Address Option (RDAO)
4.1.2. Using DNS-SD to Discover a Resource Directory
4.2. Payload Content Formats
4.3. URI Discovery
5. Registration
5.1. Simple Registration
5.2. Third-Party Registration
5.3. Operations on the Registration Resource
5.3.1. Registration Update
5.3.2. Registration Removal
5.3.3. Further Operations
5.3.4. Request Freshness
6. RD Lookup
6.1. Resource Lookup
6.2. Lookup Filtering
6.3. Resource Lookup Examples
6.4. Endpoint Lookup
7. Security Policies
7.1. Endpoint Name
7.1.1. Random Endpoint Names
7.2. Entered Links
7.3. Link Confidentiality
7.4. Segmentation
7.5. "First Come First Remembered": A Default Policy
8. Security Considerations
8.1. Discovery
8.2. Endpoint Identification and Authentication
8.3. Access Control
8.4. Denial-of-Service Attacks
8.5. Skipping Freshness Checks
9. IANA Considerations
9.1. Resource Types
9.2. IPv6 ND Resource Directory Address Option
9.3. RD Parameters Registry
9.3.1. Full Description of the "Endpoint Type" RD Parameter
9.4. Endpoint Type (et=) RD Parameter Values
9.5. Multicast Address Registration
9.6. Well-Known URIs
9.7. Service Name and Transport Protocol Port Number Registry
10. Examples
10.1. Lighting Installation
10.1.1. Installation Characteristics
10.1.2. RD Entries
10.2. OMA Lightweight M2M (LwM2M)
11. References
11.1. Normative References
11.2. Informative References
Appendix A. Groups Registration and Lookup
Appendix B. Web Links and the Resource Directory
B.1. A Simple Example
B.1.1. Resolving the URIs
B.1.2. Interpreting Attributes and Relations
B.2. A Slightly More Complex Example
B.3. Enter the Resource Directory
B.4. A Note on Differences between Link-Format and Link Header
Fields
Appendix C. Limited Link Format
Acknowledgments
Authors' Addresses
1. Introduction
In the work on Constrained RESTful Environments (CoRE), a
Representational State Transfer (REST) architecture suitable for
constrained nodes (e.g., with limited RAM and ROM [RFC 7228]) and
networks (e.g., IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) [RFC 4944]) has been established and is used in Internet of
Things (IoT) or 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 [RFC 8288]. The use of web linking for the
description and discovery of resources hosted by constrained web
servers is specified by the CoRE Link Format [RFC 6690]. However,
[RFC 6690] only describes how to discover resources from the web
server that hosts them by querying /.well-known/core. In many
constrained scenarios, direct discovery of resources is not practical
due to sleeping nodes or networks where multicast traffic is
inefficient. These problems can be solved by employing an entity
called a Resource Directory (RD), which contains information about
resources held on other servers, allowing lookups to be performed for
those resources.
This document specifies the web interfaces that an RD supports for
web servers to discover the RD and to register, maintain, look up,
and remove information on resources. Furthermore, new target
attributes useful in conjunction with an RD are defined. Although
the examples in this document show the use of these interfaces with
the Constrained Application Protocol (CoAP) [RFC 7252], they can be
applied in an equivalent manner to HTTP [RFC 7230].
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 BCP
14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
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 [RFC 3986], [RFC 8288], and
[RFC 6690]. Readers should also be familiar with the terms and
concepts discussed in [RFC 7252]. To describe the REST interfaces
defined in this specification, the URI Template format is used
[RFC 6570].
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 [RFC 3986], Section 5.2.
Noteworthy corner cases include the following: (1) if the URI
reference is a (full) URI, resolving against any base URI gives
the original full URI and (2) resolving an empty URI reference
gives the base URI without any fragment identifier.
Resource Directory (RD)
A web entity that stores information about web resources and
implements the REST interfaces defined in this specification for
discovery, for the creation, maintenance, and removal of
registrations, and for lookup of the registered resources.
Sector
In the context of an RD, 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)
Endpoint (EP) is a term used to describe a web server or client in
[RFC 7252]. In the context of this specification, an endpoint is
used to describe a web server that registers resources to the RD.
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 RD to resolve relative references of the registration into
URIs.
Target
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 [RFC 3986]).
This use of the term "target" is consistent with the use in
[RFC 8288].
Context
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 the use in
[RFC 8288].
Directory Resource
A directory resource is a resource in the RD containing
registration resources.
Registration Resource
A registration resource is a resource in the RD that contains
information about an endpoint and its links.
Commissioning Tool (CT)
A Commissioning Tool (CT) is a device that assists during
installation events by assigning values to parameters, naming
endpoints and groups, or adapting the installation to the needs of
the applications.
Registrant-EP
A 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 (RDAO)
A Resource Directory Address Option (RDAO) is a new IPv6 Neighbor
Discovery option defined for announcing an RD's address.
3. Architecture and Use Cases
3.1. Principles
The RD is primarily a tool to make discovery operations more
efficient than querying /.well-known/core on all connected devices or
across boundaries that would limit those operations.
It provides information about resources hosted by other devices that
could otherwise only be obtained by directly querying the /.well-
known/core resource on these other devices, either by a unicast
request or a multicast request.
Information SHOULD only be stored in the RD if it can be obtained by
querying the described device's /.well-known/core resource directly.
Data in the RD can only be provided by the device that hosts the data
or a dedicated Commissioning Tool (CT). These CTs act on behalf of
endpoints too constrained, or generally unable, to present that
information themselves. No other client can modify data in the RD.
Changes to the information in the RD do not propagate automatically
back to the web servers from where the information originated.
3.2. Architecture
The RD architecture is illustrated in Figure 1. An RD is used as a
repository of registrations describing resources hosted on other web
servers, also called endpoints (EPs). 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 RD registrations
and for endpoints to look up resources from the RD. An RD can be
logically segmented by the use of sectors.
A mechanism to discover an RD using CoRE Link Format [RFC 6690] is
defined.
Registrations in the RD are soft state and need to be periodically
refreshed.
An endpoint uses specific interfaces to register, update, and remove
a registration. It is also possible for an RD to fetch web links
from endpoints and add their contents to its registrations.
At the first registration of an endpoint, a "registration resource"
is created, the location of which is returned to the registering
endpoint. The registering endpoint uses this registration resource
to manage the contents of registrations.
A lookup interface for discovering any of the web links stored in the
RD is provided using the CoRE Link Format.
Registration Lookup
Interface Interface
+----+ | |
| EP |---- | |
+----+ ---- | |
--|- +------+ |
+----+ | ----| | | +--------+
| EP | ---------|-----| RD |----|-----| Client |
+----+ | ----| | | +--------+
--|- +------+ |
+----+ ---- | |
| CT |---- | |
+----+
Figure 1: The RD Architecture
A registrant-EP MAY keep concurrent registrations to more than one RD
at the same time if explicitly configured to do so, but that is not
expected to be supported by typical EP implementations. Any such
registrations are independent of each other. The usual expectation
when multiple discovery mechanisms or addresses are configured is
that they constitute a fall-back path for a single registration.
3.3. RD Content Model
The Entity-Relationship (ER) models shown in Figures 2 and 3 model
the contents of /.well-known/core and the RD 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.
However, they can be expressed as relative references in
serializations, and they often are.
These models provide an abstract view of the information expressed in
link-format documents and an RD. They cover the concepts but not
necessarily all details of an RD's operation; they are meant to give
an overview and not be a template for implementations.
+----------------------+
| /.well-known/core |
+----------------------+
|
| 1
////////\\\\\\\
< contains >
\\\\\\\\///////
|
| 0+
+--------------------+
| link |
+--------------------+
|
| 1 oooooooo
+-----o target o
| oooooooo
oooooooooooo 0+ |
o target o--------+
o attribute o | 0+ oooooo
oooooooooooo +-----o rel o
| oooooo
|
| 1 ooooooooo
+-----o context o
ooooooooo
Figure 2: ER Model of the Content of /.well-known/core
Figure 2 models the contents of /.well-known/core, which contains a
set of links belonging to the hosting web server.
The web server is free to choose links it deems appropriate to be
exposed in its /.well-known/core. Typically, the links describe
resources that are served by the host, but the set can also contain
links to resources on other servers (see examples in Section 5 of
[RFC 6690]). The set does not necessarily contain links to all
resources served by the host.
A link has the following attributes (see Section 5 of [RFC 8288]):
* Zero or more link relations: They describe relations between the
link context and the link target.
In link-format serialization, they are expressed as space-
separated values in the "rel" attribute and default to "hosts".
* A link context URI: It defines the source of the relation, e.g.,
_who_ "hosts" something.
In link-format serialization, it is expressed in the "anchor"
attribute and defaults to the Origin of the target (practically,
the target with its path and later components removed).
* A link target URI: It defines the destination of the relation
(e.g., _what_ is hosted) and is the topic of all target
attributes.
In link-format serialization, it is expressed between angular
brackets and sometimes called the "href".
* Other target attributes (e.g., resource type (rt), interface (if),
or content format (ct)): These provide additional information
about the target URI.
+--------------+
+ RD +
+--------------+
| 1
|
|
|
|
//////\\\\
< contains >
\\\\\/////
|
0+ |
ooooooo 1 +---------------+
o base o-------| registration |
ooooooo +---------------+
| | 1
| +--------------+
oooooooo 1 | |
o href o----+ /////\\\\
oooooooo | < contains >
| \\\\\/////
oooooooo 1 | |
o ep o----+ | 0+
oooooooo | +------------------+
| | link |
oooooooo 0-1 | +------------------+
o d o----+ |
oooooooo | | 1 oooooooo
| +-----o target o
oooooooo 1 | | oooooooo
o lt o----+ ooooooooooo 0+ |
oooooooo | o target o-----+
| o attribute o | 0+ oooooo
ooooooooooo 0+ | ooooooooooo +-----o rel o
o endpoint o----+ | oooooo
o attribute o |
ooooooooooo | 1 ooooooooo
+----o context o
ooooooooo
Figure 3: ER Model of the Content of the RD
Figure 3 models the contents of the RD, which contains, in addition
to /.well-known/core, 0 to n registrations of endpoints.
A registration is associated with one endpoint. A registration
defines a set of links, as defined for /.well-known/core. A
registration has six types of attributes:
* an endpoint name ("ep", a Unicode string) unique within a sector
* a registration base URI ("base", a URI typically describing the
scheme://authority part)
* a lifetime ("lt")
* a registration resource location inside the RD ("href")
* optionally, a sector ("d", a Unicode string)
* optional additional endpoint attributes (from Section 9.3)
The cardinality of "base" is currently 1; future documents are
invited to extend the RD specification to support multiple values
(e.g., [COAP-PROT-NEG]). Its value is used as a base URI when
resolving URIs in the links contained in the endpoint.
Links are modeled as they are in Figure 2.
3.4. Link-Local Addresses and Zone Identifiers
Registration base URIs can contain link-local IP addresses. To be
usable across hosts, those cannot be serialized to contain zone
identifiers (see [RFC 6874], Section 1).
Link-local addresses can only be used on a single link (therefore, RD
servers cannot announce them when queried on a different link), and
lookup clients using them need to keep track of which interface they
got them from.
Therefore, it is advisable in many scenarios to use addresses with
larger scopes, if available.
3.5. Use Case: Cellular M2M
Over the last few years, mobile operators around the world have
focused on development of M2M solutions in order to expand the
business to the new type of users: machines. The machines are
connected directly to a mobile network using an appropriate embedded
wireless interface (GSM/General Packet Radio Service (GPRS), Wideband
Code Division Multiple Access (W-CDMA), LTE, etc.) or via a gateway
providing short- and wide-range wireless interfaces. The ambition in
such systems is to build them from reusable components. These speed
up development and deployment and enable shared use of machines
across different applications. One crucial component of such systems
is the discovery of resources (and thus the endpoints they are hosted
on) 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 an RD, which is hosted by
the mobile operator or somewhere in the cloud. Periodically, these
endpoints update their registration and may modify resources they
offer.
When endpoints are not always connected, for example, because they
enter a sleep mode, a remote server is usually used to provide proxy
access to the endpoints. Mobile apps or web applications for
environment monitoring contact the RD, look up the endpoints capable
of providing information about the environment using an appropriate
set of link parameters, obtain information on how to contact them
(URLs of the proxy server), and then initiate interaction to obtain
information that is finally processed, displayed on the screen, and
usually stored in a database. Similarly, fleet management systems
provide the appropriate link parameters to the RD to look up for EPs
deployed on the vehicles the application is responsible for.
3.6. Use Case: Home and Building Automation
Home and commercial building automation systems can benefit from the
use of IoT 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. Both can use the common RD infrastructure to
establish device interactions efficiently but can pick security
policies suitable for their needs.
Two phases can be discerned for a network servicing the system: (1)
installation and (2) operation. During the operational phase, the
network is connected to the Internet with a border router (e.g., a
6LoWPAN Border Router (6LBR) [RFC 6775]), and the nodes connected to
the network can use the Internet services that are provided by the IP
or network administrator. During the installation phase, the network
is completely stand-alone, no border router is connected, and the
network only supports the IP communication between the connected
nodes. The installation phase is usually followed by the operational
phase. As an RD's operations work without hard dependencies on names
or addresses, it can be used for discovery across both phases.
3.7. Use Case: Link Catalogues
Resources may be shared through data brokers that have no knowledge
beforehand of who is going to consume the data. An RD 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 an RD. Applications wishing
to consume the data can use RD lookup to discover and resolve links
to the desired resources and endpoints. The RD service need not be
coupled with the data intermediary service. Mapping of RDs to data
intermediaries may be many-to-many.
Metadata in web link formats, such as the one defined in [RFC 6690],
which may be internally stored as triples or relation/attribute pairs
providing metadata about resource links, need to be supported by RDs.
External catalogues that are represented in other formats may be
converted to common web linking formats for storage and access by
RDs. Since it is common practice for these to be encoded in URNs
[RFC 8141], simple and lossless structural transforms should generally
be sufficient to store external metadata in RDs.
The additional features of an RD 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. RD Discovery and Other Interface-Independent Components
This and the following sections define the required set of REST
interfaces between an RD, endpoints, and lookup clients. Although
the examples throughout these sections assume the use of CoAP
[RFC 7252], these REST interfaces can also be realized using HTTP
[RFC 7230]. The multicast discovery and simple registration
operations are exceptions to that, as they rely on mechanisms
unavailable in HTTP. In all definitions in these sections, 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.
All operations on the contents of the RD MUST be atomic and
idempotent.
For several operations, interface templates are given in list form;
those describe the operation participants, request codes, URIs,
content formats, and outcomes. Sections of those templates contain
normative content about Interaction, Method, URI Template, and URI
Template Variables, as well as the details of the Success condition.
The additional sections for options (such as Content-Format) and for
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 how a server may respond under atypical
circumstances.
REST clients (registrant-EPs and CTs during registration and
maintenance, lookup clients, and RD servers during simple
registrations) must be prepared to receive any unsuccessful code and
act upon it according to its definition, options, and/or payload to
the best of their capabilities, falling back to failing the operation
if recovery is not possible. In particular, they SHOULD retry the
request upon 5.03 (Service Unavailable; 503 in HTTP) according to the
Max-Age (Retry-After in HTTP) option and SHOULD fall back to link
format when receiving 4.15 (Unsupported Content-Format; 415 in HTTP).
An RD MAY make the information submitted to it available to further
directories (subject to security policies on link confidentiality) 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.
4.1. Finding a Resource Directory
A (re)starting device may want to find one or more RDs before it can
discover their URIs. Dependent on the operational conditions, one or
more of the techniques below apply.
The device may be preconfigured to exercise specific mechanisms for
finding the RD:
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-based Service Discovery (DNS-SD), as outlined in
Section 4.1.2.
For cases where the device is not specifically configured with a way
to find an RD, the network may want to provide a suitable default.
1. The IPv6 Neighbor Discovery option RDAO (Section 4.1.1) can do
that.
2. When DHCP is in use, 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 RD.
The present specification does not fully define these heuristics but
suggests a number of candidates:
1. In a 6LoWPAN, just assume the 6LBR can act as an RD (using the
Authoritative Border Router Option (ABRO) to find that
[RFC 6775]). Confirmation can be obtained by sending a unicast
GET to coap://[6LBR]/.well-known/core?rt=core.rd*.
2. In a network that supports multicast well, discover the RD using
a multicast query for /.well-known/core, as specified in CoRE
Link Format [RFC 6690], and send a Multicast GET to
coap://[ff0x::fe]/.well-known/core?rt=core.rd*. RDs within the
multicast scope will answer the query.
When answering a multicast request directed at a link-local group,
the RD may want to respond from a routable address; this makes it
easier for registrants to use one of their own routable addresses for
registration. When source addresses are selected using the mechanism
described in [RFC 6724], this can be achieved by applying the changes
of its Section 10.4, picking public addresses in Rule 7 of its
Section 5, and superseding Rule 8 with preferring the source
address's precedence.
As some of the RD addresses obtained by the methods listed here are
just (more or less educated) guesses, endpoints MUST make use of any
error messages to very strictly rate-limit requests to candidate IP
addresses that don't work out. For example, an ICMP Destination
Unreachable message (and, in particular, the port unreachable code
for this message) may indicate the lack of a CoAP server on the
candidate host, or a CoAP error response code, such as 4.05 (Method
Not Allowed), may indicate unwillingness of a CoAP server to act as a
directory server.
The following RD discovery mechanisms are recommended:
* In managed networks with border routers that need stand-alone
operation, the RDAO is recommended (e.g., the operational phase
described in Section 3.6).
* In managed networks without border routers (no Internet services
available), the use of a preconfigured anycast address is
recommended (e.g., the installation phase described in
Section 3.6).
* In networks managed using DNS-SD, the use of DNS-SD for discovery,
as described in Section 4.1.2, is recommended.
The use of multicast discovery in mesh networks is NOT RECOMMENDED.
4.1.1. Resource Directory Address Option (RDAO)
The Resource Directory Address Option (RDAO) carries information
about the address of the RD in RAs (Router Advertisements) of IPv6
Neighbor Discovery (ND), similar to how Recursive DNS Server (RDNSS)
options [RFC 8106] are sent. This information is needed when
endpoints cannot discover the RD with a link-local or realm-local
scope multicast address, for instance, because the endpoint and the
RD are separated by a 6LBR. In many circumstances, the availability
of DHCP cannot be guaranteed during commissioning of the network
either. The presence and the use of the RD is essential during
commissioning.
It is possible to send multiple RDAOs in one message, indicating as
many RD 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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ RD Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Resource Directory Address Option
Fields:
Type: 41
Length: 8-bit unsigned integer. The length of the option
in units of 8 bytes. Always 3.
Reserved: This field is unused. It MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
Valid Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is
received) that this RD address is valid. A value
of all zero bits (0x0) indicates that this RD
address is not valid anymore.
RD Address: IPv6 address of the RD.
4.1.2. Using DNS-SD to Discover a Resource Directory
An RD can advertise its presence in DNS-SD [RFC 6763] using the
service names defined in this document: _core-rd._udp (for CoAP),
_core-rd-dtls._udp (for CoAP over DTLS), _core-rd._tcp (for CoAP over
TCP), or _core-rd-tls._tcp (for CoAP over TLS). (For the WebSocket
transports of CoAP, no service is defined, as DNS-SD is typically
unavailable in environments where CoAP over WebSockets is used.)
The selection of the service indicates the protocol used, and the SRV
record points the client to a host name and port to use as a starting
point for the "URI discovery" steps of Section 4.3.
This section is a simplified, concrete application of the more
generic mechanism specified in [CORE-RD-DNS-SD].
4.2. Payload Content Formats
RDs implementing this specification MUST support the application/
link-format content format (ct=40).
RDs implementing this specification MAY support additional content
formats.
Any additional content format supported by an RD implementing this
specification SHOULD be able to express all the information
expressible in link format. It MAY be able to express information
that is inexpressible in link format, but those expressions SHOULD be
avoided where possible.
4.3. URI Discovery
Before an endpoint can make use of an RD, it must first know the RD's
address and port and the URI path information for its REST APIs.
This section defines discovery of the RD and its URIs using the well-
known interface of the CoRE Link Format [RFC 6690] after having
discovered a host, as described in Section 4.1.
Discovery of the RD registration URI is performed by sending either a
multicast or unicast GET request to /.well-known/core and including a
resource type (rt) parameter [RFC 6690] with the value "core.rd" in
the query string. Likewise, a resource type parameter value of
"core.rd-lookup*" is used to discover the URIs for RD lookup
operations, and "core.rd*" is used to discover all URIs 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).
An RD MAY provide hints about the content formats it supports in the
links it exposes or registers, using the "ct" target attribute, as
shown in the example below. Clients MAY use these hints to select
alternate content formats for interaction with the RD.
HTTP does not support multicast, and, consequently, only unicast
discovery can be supported using the HTTP /.well-known/core resource.
RDs implementing this specification MUST support query filtering for
the rt parameter, as defined in [RFC 6690].
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.
With security policies where the client requires the RD to be
authorized to act as an RD, that authorization may be limited to
resources on which the authorized RD advertises the adequate resource
types. Clients that have obtained links they cannot rely on yet can
repeat the "URI discovery" step at the /.well-known/core resource of
the indicated host to obtain the resource type information from an
authorized source.
The URI discovery operation can yield multiple URIs of a given
resource type. The client of the RD can try out any of the
discovered addresses.
The discovery request interface is specified as follows (this is
exactly the well-known interface of [RFC 6690], Section 4, with the
additional requirement that the server MUST support query filtering):
Interaction: EP, CT, or Client -> RD
Method: GET
URI Template: /.well-known/core{?rt}
URI Template Variables:
rt := Resource Type. SHOULD contain one of the values "core.rd",
"core.rd-lookup*", "core.rd-lookup-res", "core.rd-lookup-ep",
or "core.rd*"
Accept: absent, application/link-format, or any other media type
representing web links
The following response is expected on this interface:
Success: 2.05 (Content) or 200 (OK) with an application/link-format
or other web link payload containing one or more matching entries
for the RD resource.
The following example shows an endpoint discovering an RD using this
interface, thus learning that the directory resource location in this
example is /rd and that the content format delivered by the server
hosting the resource is application/link-format (ct=40). Note that
it is up to the RD to choose its RD locations.
Req: GET coap://[ff02::fe]/.well-known/core?rt=core.rd*
Res: 2.05 Content
Payload:
</rd>;rt=core.rd;ct=40,
</rd-lookup/ep>;rt=core.rd-lookup-ep;ct=40,
</rd-lookup/res>;rt=core.rd-lookup-res;ct=40
Figure 5: 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 [RFC 7252], indicating that multiple
content formats are available. The example below shows the required
Content-Format 40 (application/link-format) indicated, as well as
Concise Binary Object Representation (CBOR) and JSON representations
in the style of [CORE-LINKS-JSON] (for which the experimental values
65060 and 65050 are used in this example). The RD resource locations
/rd and /rd-lookup are example values. The server in this example
also indicates that it is capable of providing observation on
resource lookups.
Req: GET coap://[ff02::fe]/.well-known/core?rt=core.rd*
Res: 2.05 Content
Payload:
</rd>;rt=core.rd;ct=40,
</rd-lookup/res>;rt=core.rd-lookup-res;ct="40 65060 65050";obs,
</rd-lookup/ep>;rt=core.rd-lookup-ep;ct="40 65060 65050"
Figure 6: Example Discovery Exchange Indicating Additional
Content-Formats
For maintenance, management, and debugging, it can be useful to
identify the components that constitute the RD server. The
identification can be used to find client-server incompatibilities,
supported features, required updates, and other aspects. The well-
known interface described in Section 4 of [RFC 6690] can be used to
find such data.
It would typically be stored in an implementation information link
(as described in [T2TRG-REL-IMPL]).
Req: GET /.well-known/core?rel=impl-info
Res: 2.05 Content
Payload:
<http://software.example.com/shiny-resource-directory/1.0beta1>;
rel=impl-info
Figure 7: Example Exchange of Obtaining Implementation
Information Using the Relation Type Currently Proposed in
[T2TRG-REL-IMPL]
Note that, depending on the particular server's architecture, such a
link could be anchored at the RD server's root (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. 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 [RFC 6690] or other representations of
web links, 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 a registration request targeting a
given (ep, d) value pair:
* When the (ep, d) value pair of the registration request is
different from any existing registration, a new registration is
generated.
* When the (ep, d) value pair of the registration request is equal
to an existing registration, the content and parameters of the
existing registration are replaced with the content of the
registration request. As with changes to registration resources,
security policies (Section 7) usually require such requests to
come from the same device.
The posted link-format document can (and typically does) contain
relative references both in its link targets and in its anchors; it
can also 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.
For media types to which Appendix C applies (i.e., documents in
application/link-format), request bodies MUST be expressed in Limited
Link Format.
The registration request interface is specified as follows:
Interaction: EP or CT -> RD
Method: POST
URI Template: {+rd}{?ep,d,lt,base,extra-attrs*}
URI Template Variables:
rd := RD registration URI (mandatory). This is the location of
the RD, as obtained from discovery.
ep := Endpoint name (mostly mandatory). The endpoint name is an
identifier that MUST be unique within a sector.
As the endpoint name is a Unicode string, it is encoded in
UTF-8 (and possibly percent encoded) during variable expansion
(see [RFC 6570], Section 3.2.1). The endpoint name MUST NOT
contain any character in the inclusive ranges 0-31 or 127-159.
The maximum length of this parameter is 63 bytes encoded in
UTF-8.
If the RD is configured to recognize the endpoint that is to be
authorized to use exactly one endpoint name, the RD assigns
that name. In that case, giving the endpoint name becomes
optional for the client; if the client gives any other endpoint
name, it is not authorized to perform the registration.
d := Sector (optional). This is the sector to which this
endpoint belongs. When this parameter is not present, the RD
MAY associate the endpoint with a configured default sector
(possibly based on the endpoint's authorization) or leave it
empty.
The sector is encoded like the ep parameter and is limited to
63 bytes encoded in UTF-8 as well.
lt := Lifetime (optional). This is the lifetime of the
registration in seconds, with a range of 1-4294967295. 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,
the source address, and the source port of the registration
request are assumed. The base URI is consecutively constructed
by concatenating the used protocol'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). This is analogous to the process described in
[RFC 7252], Section 6.5.
This parameter is mandatory when the directory is filled by a
third party, such as a 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.
A registrant that cannot be reached by potential lookup clients
at the address it registers from (e.g., because it is behind
some form of Network Address Translation (NAT)) MUST provide a
reachable base address with its registration.
If the base URI contains a link-local IP literal, it MUST NOT
contain a Zone Identifier and MUST be local to the link on
which the registration request is received.
Endpoints that register with a base that contains a path
component cannot efficiently express their registrations in
Limited Link Format (Appendix C). Those applications should
use different representations of links to which Appendix C is
not applicable (e.g., [CORE-CORAL]).
extra-attrs := Additional registration attributes (optional).
The endpoint can pass any parameter registered in Section 9.3
to the directory. If the RD is aware of the parameter's
specified semantics, it processes the parameter accordingly.
Otherwise, it MUST store the unknown key and its value(s) as an
endpoint attribute for further lookup.
Content-Format: application/link-format or any other indicated media
type representing web links
The following response is expected on this interface:
Success: 2.01 (Created) or 201 (Created). The Location-Path option
or Location header field 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.
If the registration fails, including request timeouts, or if delays
from Service Unavailable responses with Max-Age or Retry-After
accumulate to exceed the registrant's configured timeouts, it SHOULD
pick another registration URI from the "URI discovery" step of
Section 4.3, and, if there is only one or the list is exhausted, pick
other choices from the "finding a resource directory" step of
Section 4.1. Care has to be taken to consider the freshness of
results obtained earlier, e.g., the result of a /.well-known/core
response, the lifetime of an RDAO, and 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 5.
Req: POST coap://rd.example.com/rd?ep=node1
Content-Format: 40
Payload:
</sensors/temp>;rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="/sensors/temp";rel=describedby
Res: 2.01 Created
Location-Path: /rd/4521
Figure 8: Example Registration Payload
An RD may optionally support HTTP. Here is an example of almost the
same registration operation above when done using HTTP.
Req:
POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1
Host: rd.example.com
Content-Type: application/link-format
</sensors/temp>;rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="/sensors/temp";rel=describedby
Res:
HTTP/1.1 201 Created
Location: /rd/4521
Figure 9: Example Registration Payload as Expressed Using HTTP
5.1. Simple Registration
Not all endpoints hosting resources are expected to know how to
upload links to an RD, as described in Section 5. Instead, simple
endpoints can implement the simple registration approach described in
this section. An RD implementing this specification MUST implement
simple registration. However, there may be security reasons why this
form of directory discovery would be disabled.
This approach requires that the registrant-EP makes available the
hosted resources that it wants to be discovered as links on its
/.well-known/core interface, as specified in [RFC 6690]. The links in
that document are subject to the same limitations as the payload of a
registration (with respect to Appendix C).
* The registrant-EP finds one or more addresses of the directory
server, as described in Section 4.1.
* The registrant-EP sends (and regularly refreshes with) a POST
request to the /.well-known/rd 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 (redone before
lifetime expiry) 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 with a regular registration, per
Section 5. The registration base URI of the registration is taken
from the registrant-EP's network address (as is default with
regular registrations).
The following is an example request from the registrant-EP to the
RD (unanswered until the next step):
Req: POST /.well-known/rd?lt=6000&ep=node1
(No payload)
Figure 10: First-Half Example Exchange of a Simple Registration
* The RD queries the registrant-EP's discovery resource to determine
the success of the operation. It SHOULD keep a cache of the
discovery resource and not query it again as long as it is fresh.
The following is an example request from the RD to the registrant-
EP:
Req: GET /.well-known/core
Accept: 40
Res: 2.05 Content
Content-Format: 40
Payload:
</sen/temp>
Figure 11: Example Exchange of the RD Querying the Simple Endpoint
With this response, the RD would answer the previous step's request:
Res: 2.04 Changed
Figure 12: Second-Half Example Exchange of a Simple Registration
The sequence of fetching the registration content before sending a
successful response was chosen to make responses reliable, and the
point about caching was chosen to still allow very constrained
registrants. Registrants MUST be able to serve a GET request to
/.well-known/core after having requested registration. Constrained
devices MAY regard the initial request as temporarily failed when
they need RAM occupied by their own request to serve the RD's GET and
retry later when the RD already has a cached representation of their
discovery resources. Then, the RD can reply immediately, and the
registrant can receive the response.
The simple registration request interface is specified as follows:
Interaction: EP -> RD
Method: POST
URI Template: /.well-known/rd{?ep,d,lt,extra-attrs*}
URI Template Variables are the same as for registration in Section 5.
The base attribute is not accepted to keep the registration interface
simple; that rules out registration over CoAP-over-TCP or HTTP that
would need to specify one. For some time during this document's
development, the URI Template /.well-known/core{?ep,...} was in use
instead.
The following response is expected on this interface:
Success: 2.04 (Changed)
For the second interaction triggered by the above, the registrant-EP
takes the role of server and the RD takes the role of client. (Note
that this is exactly the well-known interface of [RFC 6690],
Section 4):
Interaction: RD -> EP
Method: GET
URI Template: /.well-known/core
The following response is expected on this interface:
Success: 2.05 (Content)
When the RD uses any authorization credentials to access the
endpoint's discovery resource or when it is deployed in a location
where third parties might reach it but not the endpoint, it SHOULD
verify that the apparent registrant-EP intends to register with the
given registration parameters before revealing the obtained discovery
information to lookup clients. An easy way to do that is to verify
the simple registration request's sender address using the Echo
option, as described in [RFC 9175], Section 2.4.
The RD MUST delete registrations created by simple registration after
the expiration of their lifetime. Additional operations on the
registration resource cannot be executed because no registration
location is returned.
5.2. Third-Party Registration
For some applications, even simple registration may be too taxing for
some very constrained devices, in particular, if the security
requirements become too onerous.
In a controlled environment (e.g., building control), the RD can be
filled by a third-party device, called a Commissioning Tool (CT).
The CT can fill the RD from a database or other means. For that
purpose scheme, the IP address and port of the URI of the registered
device is the value of the "base" parameter of the registration
described in Section 5.
It should be noted that the value of the "base" parameter applies to
all the links of the registration and has consequences for the anchor
value of the individual links, as exemplified in Appendix B. A
potential (currently nonexistent) "base" attribute of the link is not
affected by the value of "base" parameter in the registration.
5.3. Operations on the Registration Resource
This section describes how the registering endpoint can maintain the
registrations that it created. The registering endpoint can be the
registrant-EP or the CT. The registrations are resources of the RD.
An endpoint should not use this interface for registrations that it
did not create. This is usually enforced by security policies,
which, in general, require equivalent credentials for creation of and
operations on a registration.
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 timeout
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 reregistered.
The registration resource may also be used to cancel the registration
using DELETE and to perform further operations beyond the scope of
this specification.
Operations on the registration resource are sensitive to reordering;
Section 5.3.4 describes how order is restored.
The operations on the registration resource are described below.
5.3.1. 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 registration parameters, such as lifetime, base
URI, or others. 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 are included as query parameters in an
update operation, as in Section 5.
A registration update resets the timeout of the registration to the
(possibly updated) lifetime of the registration, independent of
whether an lt parameter was given.
If the base URI of the registration is changed in an update, relative
references submitted in the original registration or later updates
are resolved anew against the new base.
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 Section 5.3.3).
The update registration request interface is specified as follows:
Interaction: EP or CT -> RD
Method: POST
URI Template: {+location}{?lt,base,extra-attrs*}
URI Template Variables:
location := This is the location returned by the RD as a result
of a successful earlier registration.
lt := Lifetime (optional). This is the lifetime of the
registration in seconds, with a range of 1-4294967295. 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 and is
subject to the same restrictions as in the registration.
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.
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.
Note that this default behavior does not allow removing an
endpoint attribute in an update. For attributes whose
functionality depends on the endpoints' ability to remove them
in an update, it can make sense to define a value whose
presence is equivalent to the absence of a value. As an
alternative, an extension can define different updating rules
for their attributes. That necessitates either discovering
whether the RD is aware of that extension or tolerating the
default behavior.
Content-Format: none (no payload)
The following responses are expected on this interface:
Success: 2.04 (Changed) or 204 (No Content) if the update was
successfully processed.
Failure: 4.04 (Not Found) or 404 (Not Found). Registration does not
exist (e.g., may have been removed).
If the registration update fails in any way, including "Not Found"
and request timeouts, or if the time indicated in a Service
Unavailable Max-Age/Retry-After exceeds the remaining lifetime, the
registering endpoint SHOULD attempt registration again.
The following example shows how the registering endpoint resets the
timeout on its registration resource at an RD using this interface
with the example location value /rd/4521:
Req: POST /rd/4521
Res: 2.04 Changed
Figure 13: Example Update of a Registration
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:
* endpoint name (ep)=endpoint1
* lifetime (lt)=500
* base URI (base)=coap://local-proxy-old.example.com
* payload of Figure 8
The initial state of the RD is reflected in the following request:
Req: GET /rd-lookup/res?ep=endpoint1
Res: 2.05 Content
Payload:
<coap://local-proxy-old.example.com/sensors/temp>;
rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="coap://local-proxy-old.example.com/sensors/temp";
rel=describedby
Figure 14: Example Lookup Before a Change to the Base Address
The following example shows the registering endpoint changing the
base URI to coaps://new.example.com:5684:
Req: POST /rd/4521?base=coaps://new.example.com
Res: 2.04 Changed
Figure 15: Example Registration Update that Changes the Base Address
The consecutive query returns:
Req: GET /rd-lookup/res?ep=endpoint1
Res: 2.05 Content
Payload:
<coaps://new.example.com/sensors/temp>;
rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="coaps://new.example.com/sensors/temp";
rel=describedby
Figure 16: Example Lookup After a Change to the Base Address
5.3.2. Registration Removal
Although RD registrations have soft state and will eventually time
out 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 shutdown). This is accomplished using a
removal interface on the RD by performing a DELETE on the endpoint
resource.
The removal request interface is specified as follows:
Interaction: EP or CT -> RD
Method: DELETE
URI Template: {+location}
URI Template Variables:
location := This is the location returned by the RD as a result
of a successful earlier registration.
The following responses are expected on this interface:
Success: 2.02 (Deleted) or 204 (No Content) upon successful
deletion.
Failure: 4.04 (Not Found) or 404 (Not Found). Registration does not
exist (e.g., may already have been removed).
The following example shows successful removal of the endpoint from
the RD with example location value /rd/4521:
Req: DELETE /rd/4521
Res: 2.02 Deleted
Figure 17: Example of a Registration Removal
5.3.3. Further Operations
Additional operations on the registration can be specified in future
documents, for example:
* Send iPATCH (or PATCH) updates [RFC 8132] to add, remove, or change
the links of a registration.
* Use GET to read the currently stored set of links in a
registration resource.
Those operations are out of scope of this document and will require
media types suitable for modifying sets of links.
5.3.4. Request Freshness
Some security mechanisms usable with an RD allow out-of-order request
processing or do not even mandate replay protection at all. The RD
needs to ensure that operations on the registration resource are
executed in an order that does not distort the client's intentions.
This ordering of operations is expressed in terms of freshness, as
defined in [RFC 9175]. Requests that alter a resource's state need to
be fresh relative to the latest request that altered that state in a
conflicting way.
An RD SHOULD determine a request's freshness and MUST use the Echo
option if it requires request freshness and cannot determine it in
any other way. An endpoint MUST support the use of the Echo option.
(One reason why an RD would not require freshness is when no relevant
registration properties are covered by its security policies.)
5.3.4.1. Efficient Use of Echo by an RD
To keep latency and traffic added by the freshness requirements to a
minimum, RDs should avoid naive (sufficient but inefficient)
freshness criteria.
Some simple mechanisms the RD can employ are:
* State counter. The RD can keep a monotonous counter that
increments whenever a registration changes. For every
registration resource, it stores the post-increment value of that
resource's last change. Requests altering them need to have at
least that value encoded in their Echo option and are otherwise
rejected with a 4.01 (Unauthorized) and the current counter value
as the Echo value. If other applications on the same server use
Echo as well, that encoding may include a prefix indicating that
it pertains to the RD's counter.
The value associated with a resource needs to be kept across the
removal of registrations if the same registration resource is to
be reused.
The counter can be reset (and the values of removed resources
forgotten) when all previous security associations are reset.
This is the "Persistent Counter" method of [RFC 9175], Appendix A.
* Preemptive Echo values. The current state counter can be sent in
an Echo option not only when requests are rejected with 4.01
(Unauthorized) but also with successful responses. Thus, clients
can be provided with Echo values sufficient for their next request
on a regular basis. This is also described in Section 2.3 of
[RFC 9175]
While endpoints may discard received Echo values at leisure
between requests, they are encouraged to retain these values for
the next request to avoid additional round trips.
* If the RD can ensure that only one security association has
modifying access to any registration at any given time and that
security association provides order on the requests, that order is
sufficient to show request freshness.
5.3.4.2. Examples of Echo Usage
Figure 18 shows the interactions of an endpoint that has forgotten
the server's latest Echo value and temporarily reduces its
registration lifetime:
Req: POST /rd/4521?lt=7200
Res: 4.01 Unauthorized
Echo: 0x0123
(EP tries again immediately.)
Req: POST /rd/4521?lt=7200
Echo: 0x0123
Res: 2.04 Changed
Echo: 0x0124
(Later, the EP regains its confidence in its long-term reachability.)
Req: POST /rd/4521?lt=90000
Echo: 0x0124
Res: 2.04 Changed
Echo: 0x0247
Figure 18: Example Update of a Registration
The other examples do not show Echo options for two reasons: (1) for
simplicity and (2) because they lack the context for any example
values to have meaning.
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., JSON or CBOR link format
[CORE-LINKS-JSON]) or use more advanced interfaces (e.g., supporting
context- or semantic-based lookup) on different resources that are
discovered independently.
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 implemented by a lookup resource is indicated by a
resource type, as per Table 1:
+=============+====================+===========+
| Lookup Type | Resource Type | Mandatory |
+=============+====================+===========+
| Resource | core.rd-lookup-res | Mandatory |
+-------------+--------------------+-----------+
| Endpoint | core.rd-lookup-ep | Mandatory |
+-------------+--------------------+-----------+
Table 1: Lookup Types
6.1. Resource Lookup
Resource lookup results in links that are semantically equivalent to
the links submitted to the RD by the registrant. The links and link
parameters returned by the lookup are equal to the originally
submitted ones, except that the target reference is fully resolved
and that the anchor reference is fully resolved if it is present in
the lookup result at all.
Links that did not have an anchor attribute in the registration are
returned without an anchor attribute. Links of which href or anchor
was submitted as a (full) URI are returned with the respective
attribute unmodified.
The above rules allow the client to interpret the response as links
without any further knowledge of the storage conventions of the RD.
The RD MAY replace the registration base URIs with a configured
intermediate proxy, e.g., in the case of an HTTP lookup interface for
CoAP endpoints.
If the base URI of a registration contains a link-local address, the
RD MUST NOT show its links unless the lookup was made from the link
on which the registered endpoint can be reached. The RD MUST NOT
include zone identifiers in the resolved URIs.
6.2. Lookup Filtering
Using the Accept option, the requester can control whether the
returned list is returned in CoRE Link Format (application/link-
format, default) or in alternate content formats (e.g., from
[CORE-LINKS-JSON]).
Multiple search criteria MAY be included in a lookup. All included
criteria MUST match for a link to be returned. The RD 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 [RFC 6690]. Attributes that are
defined as relation-types (in the link-format ABNF) match if the
search value matches any of their values (see Section 4.1 of
[RFC 6690]; for example, ?if=tag:example.net,2020:sensor matches
;if="example.regname tag:example.net,2020:sensor";. A resource link
also matches a search criterion if its endpoint would match the
criterion, and vice versa, an endpoint link matches a search
criterion if any of its resource links matches it.
Note that href is a valid search criterion and matches target
references. Like all search criteria, on a resource lookup, it can
match the target reference of the resource link itself but also the
registration resource of the endpoint that registered it. Queries
for resource link targets MUST be in URI form (i.e., not relative
references) and are matched against a resolved link target. Queries
for endpoints SHOULD be expressed in path-absolute form if possible
and MUST be expressed in URI form otherwise; the RD SHOULD recognize
either. The anchor attribute is usable for resource lookups and, if
queried, MUST be in URI form as well.
Additional query parameters "page" and "count" 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 a count of
10 and page of 0 will return the first 10 links in the result set
(links 0-9). Specifying a count of 10 and page of 1 will return the
next 'page' containing links 10-19, and so on. Unlike block-wise
transfer of a complete result set, these parameters ensure that each
chunk of results can be interpreted on its own. This simplifies the
processing but can result in duplicate or missed items when
coinciding with changes from the registration interface.
Endpoints that are interested in a lookup result repeatedly or
continuously can use mechanisms such as ETag caching, resource
observation [RFC 7641], or any future mechanism that might allow more
efficient observations of collections. These are advertised,
detected, and used according to their own specifications and can be
used with the lookup interface as with any other resource.
When resource observation is used, every time the set of matching
links changes or the content of a matching link changes, the RD sends
a notification with the matching link set. The notification contains
the successful current response to the given request, especially with
respect to representing zero matching links (see "Success" item
below).
The lookup interface is specified as follows:
Interaction: Client -> RD
Method: GET
URI Template: {+type-lookup-location}{?page,count,search*}
URI Template Variables:
type-lookup-location := RD lookup URI for a given lookup type
(mandatory). The address is discovered as described in
Section 4.3.
search := Search criteria for limiting the number of results
(optional). The search criteria are an associative array,
expressed in a form-style query, as per the URI Template (see
[RFC 6570], Sections 2.4.2 and 3.2.8).
page := Page (optional). This 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). The number of results is limited to
this parameter value. If the page parameter is also present,
the response MUST only include 'count' links starting with the
(page * count) link in the result set from the query. If the
count parameter is not present, then the response MUST return
all matching links in the result set. Link numbering starts
with zero.
Accept: absent, application/link-format, or any other indicated
media type representing web links
The following responses codes are defined for this interface:
Success: 2.05 (Content) or 200 (OK) with an application/link-format
or other web link payload containing matching entries for the
lookup.
The payload can contain zero links (which is an empty payload in
the link format described in [RFC 6690] but could also be [] in
JSON-based formats), indicating that no entities matched the
request.
6.3. Resource Lookup Examples
The examples in this section assume the existence of CoAP hosts with
a default CoAP port 61616. HTTP hosts are possible and do not change
the nature of the examples.
The following example shows a client performing a resource lookup
with the example resource lookup locations discovered in Figure 5:
Req: GET /rd-lookup/res?rt=tag:example.org,2020:temperature
Res: 2.05 Content
Payload:
<coap://[2001:db8:3::123]:61616/temp>;
rt="tag:example.org,2020:temperature"
Figure 19: Example of a Resource Lookup
A client that wants to be notified of new resources as they show up
can use this observation:
Req: GET /rd-lookup/res?rt=tag:example.org,2020:light
Observe: 0
Res: 2.05 Content
Observe: 23
Payload: empty
(at a later point in time)
Res: 2.05 Content
Observe: 24
Payload:
<coap://[2001:db8:3::124]/west>;rt="tag:example.org,2020:light",
<coap://[2001:db8:3::124]/south>;rt="tag:example.org,2020:light",
<coap://[2001:db8:3::124]/east>;rt="tag:example.org,2020:light"
Figure 20: Example of an Observing Resource Lookup
The following example shows a client performing a paginated resource
lookup:
Req: GET /rd-lookup/res?page=0&count=5
Res: 2.05 Content
Payload:
<coap://[2001:db8:3::123]:61616/res/0>;ct=60,
<coap://[2001:db8:3::123]:61616/res/1>;ct=60,
<coap://[2001:db8:3::123]:61616/res/2>;ct=60,
<coap://[2001:db8:3::123]:61616/res/3>;ct=60,
<coap://[2001:db8:3::123]:61616/res/4>;ct=60
Req: GET /rd-lookup/res?page=1&count=5
Res: 2.05 Content
Payload:
<coap://[2001:db8:3::123]:61616/res/5>;ct=60,
<coap://[2001:db8:3::123]:61616/res/6>;ct=60,
<coap://[2001:db8:3::123]:61616/res/7>;ct=60,
<coap://[2001:db8:3::123]:61616/res/8>;ct=60,
<coap://[2001:db8:3::123]:61616/res/9>;ct=60
Figure 21: Example of Paginated Resource Lookup
The following example shows a client performing a lookup of all
resources of all endpoints of a given endpoint type. It assumes that
two endpoints (with endpoint names sensor1 and sensor2) have
previously registered with their respective addresses
(coap://sensor1.example.com and coap://sensor2.example.com) and
posted the very payload of the 6th response of Section 5 of
[RFC 6690].
It demonstrates how absolute link targets stay unmodified, while
relative ones are resolved:
Req: GET /rd-lookup/res?et=tag:example.com,2020:platform
Res: 2.05 Content
Payload:
<coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index",
<coap://sensor1.example.com/sensors/temp>;rt=temperature-c;if=sensor,
<coap://sensor1.example.com/sensors/light>;rt=light-lux;if=sensor,
<http://www.example.com/sensors/t123>;rel=describedby;
anchor="coap://sensor1.example.com/sensors/temp",
<coap://sensor1.example.com/t>;rel=alternate;
anchor="coap://sensor1.example.com/sensors/temp",
<coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index",
<coap://sensor2.example.com/sensors/temp>;rt=temperature-c;if=sensor,
<coap://sensor2.example.com/sensors/light>;rt=light-lux;if=sensor,
<http://www.example.com/sensors/t123>;rel=describedby;
anchor="coap://sensor2.example.com/sensors/temp",
<coap://sensor2.example.com/t>;rel=alternate;
anchor="coap://sensor2.example.com/sensors/temp"
Figure 22: Example of a Resource Lookup from Multiple Endpoints
6.4. Endpoint Lookup
The endpoint lookup returns links to and information about
registration resources, which themselves can only be manipulated by
the registering endpoint.
Endpoint registration resources are annotated with their endpoint
names (ep), sectors (d, if present), and registration base URI (base;
reports the registrant-EP's address if no explicit base was given),
as well as a constant resource type (rt="core.rd-ep"); the lifetime
(lt) is not reported. Additional endpoint attributes are added as
target attributes to their endpoint link unless their specification
says otherwise.
Links to endpoints SHOULD be presented in path-absolute form or, if
required, as (full) URIs. (This ensures that the output conforms to
Limited Link Format, as described in Appendix C.)
Base addresses that contain link-local addresses MUST NOT include
zone identifiers, and such registrations MUST NOT be shown unless the
lookup was made from the same link from which the registration was
made.
While the 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.
An RD can report registrations in lookup whose URI scheme and
authority differ from that of the lookup resource. 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 lookup
that is limited to endpoints of endpoint type
tag:example.com,2020:platform:
Req: GET /rd-lookup/ep?et=tag:example.com,2020:platform
Res: 2.05 Content
Payload:
</rd/1234>;base="coap://[2001:db8:3::127]:61616";ep=node5;
et="tag:example.com,2020:platform";ct=40;rt=core.rd-ep,
</rd/4521>;base="coap://[2001:db8:3::129]:61616";ep=node7;
et="tag:example.com,2020:platform";ct=40;d=floor-3;
rt=core.rd-ep
Figure 23: Example of Endpoint Lookup
7. Security Policies
The security policies that are applicable to an RD strongly depend on
the application and are not set out normatively here.
This section provides a list of aspects that applications should
consider when describing their use of the RD, without claiming to
cover all cases. It uses terminology of [ACE-OAUTH-AUTHZ], in which
the RD acts as the Resource Server (RS), and both registrant-EPs and
lookup clients act as Clients (C) with support from an Authorization
Server (AS), without the intention of ruling out other schemes (e.g.,
those based on certificates/Public Key Infrastructures (PKIs)).
Any, all, or none of the below can apply to an application. Which
are relevant depends on its protection objectives.
Security policies are set by configuration of the RD or by choice of
the implementation. Lookup clients (and, where relevant, endpoints)
can only trust an RD to uphold them if it is authenticated and
authorized to serve as an RD according to the application's
requirements.
7.1. Endpoint Name
Whenever an RD needs to provide trustworthy results to clients doing
endpoint lookup or resource lookup with filtering on the endpoint
name, the RD must ensure that the registrant is authorized to use the
given endpoint name. This applies both to registration and later to
operations on the registration resource. It is immaterial whether
the client is the registrant-EP itself or a CT is doing the
registration. The RD cannot tell the difference, and CTs may use
authorization credentials authorizing only operations on that
particular endpoint name or a wider range of endpoint names.
It is up to the concrete security policy to describe how the endpoint
name and sector are transported when certificates are used. For
example, it may describe how SubjectAltName dNSName entries are
mapped to endpoint and domain names.
7.1.1. Random Endpoint Names
Conversely, in applications where the RD does not check the endpoint
name, the authorized registering endpoint can generate a random
number (or string) that identifies the endpoint. The RD should then
remember unique properties of the registrant, associate them with the
registration for as long as its registration resource is active
(which may be longer than the registration's lifetime), and require
the same properties for operations on the registration resource.
Registrants that are prepared to pick a different identifier when
their initial attempt (or attempts, in the unlikely case of two
subsequent collisions) at registration is unauthorized should pick an
identifier at least twice as long as would be needed to enumerate the
expected number of registrants; registrants without any such recovery
options should pick significantly longer endpoint names (e.g., using
Universally Unique Identifier (UUID) URNs [RFC 4122]).
7.2. Entered Links
When lookup clients expect that certain types of links can only
originate from certain endpoints, then the RD needs to apply
filtering to the links an endpoint may register.
For example, if clients use an RD to find a server that provides
firmware updates, then any registrant that wants to register (or
update) links to firmware sources will need to provide suitable
credentials to do so, independently of its endpoint name.
Note that the impact of having undesirable links in the RD depends on
the application. If the client requires the firmware server to
present credentials as a firmware server, a fraudulent link's impact
is limited to the client revealing its intention to obtain updates
and slowing down the client until it finds a legitimate firmware
server; if the client accepts any credentials from the server as long
as they fit the provided URI, the impact is larger.
An RD may also require that links are only registered if the
registrant is authorized to publish information about the anchor (or
even target) of the link. One way to do this is to demand that the
registrant present the same credentials in its role as a registering
client that it would need to present in its role as a server when
contacted at the resources' URI. These credentials may include using
the address and port that are part of the URI. Such a restriction
places severe practical limitations on the links that can be
registered.
As above, the impact of undesirable links depends on the extent to
which the lookup client relies on the RD. To avoid the limitations,
RD applications should consider prescribing that lookup clients only
use the discovered information as hints and describe which pieces of
information need to be verified because they impact the application's
security. A straightforward way to verify such information is to
request it again from an authorized server, typically the one that
hosts the target resource. That is similar to what happens in
Section 4.3 when the "URI discovery" step is repeated.
7.3. Link Confidentiality
When registrants publish information in the RD that is not available
to any client that would query the registrant's /.well-known/core
interface, or when lookups to that interface are subject to stricter
firewalling than lookups to the RD, the RD may need to limit which
lookup clients may access the information.
In this case, the endpoint (and not the lookup clients) needs to be
careful to check the RD's authorization. The RD needs to check any
lookup client's authorization before revealing information directly
(in resource lookup) or indirectly (when using it to satisfy a
resource lookup search criterion).
7.4. Segmentation
Within a single RD, different security policies can apply.
One example of this are multi-tenant deployments separated by the
sector (d) parameter. Some sectors might apply limitations on the
endpoint names available, while others use a random identifier
approach to endpoint names and place limits on the entered links
based on their attributes instead.
Care must be taken in such setups to determine the applicable access
control measures to each operation. One easy way to do that is to
mandate the use of the sector parameter on all operations, as no
credentials are suitable for operations across sector borders anyway.
7.5. "First Come First Remembered": A Default Policy
The "First Come First Remembered" policy is provided both as a
reference example for a security policy definition and as a policy
that implementations may choose to use as default policy in the
absence of any other configuration. It is designed to enable
efficient discovery operations even in ad hoc settings.
Under this policy, the RD accepts registrations for any endpoint name
that is not assigned to an active registration resource and only
accepts registration updates from the same endpoint. The policy is
minimal in that it does not make any promises to lookup clients about
the claims of Sections 7.2 and 7.3, and promises about the claims in
Section 7.1 are limited to the lifetime of that endpoint's
registration. It does however promise the endpoint that, for the
duration of its registration, its links will be discoverable on the
RD.
When a registration or operation is attempted, the RD MUST determine
the client's subject name or public key:
* If the client's credentials indicate any subject name that is
certified by any authority that the RD recognizes (which may be
the system's trust anchor store), all such subject names are
stored. With credentials based on CWT or JWT (as common with
Authentication and Authorization for Constrained Environments
(ACE)), the Subject (sub) claim is stored as a single name, if it
exists. With X.509 certificates, the Common Name (CN) and the
complete list of SubjectAltName entries are stored. In both
cases, the authority that certified the claim is stored along with
the subject, as the latter may only be locally unique.
* Otherwise, if the client proves possession of a private key, the
matching public key is stored. This applies both to raw public
keys and to the public keys indicated in certificates that failed
the above authority check.
* If neither is present, a reference to the security session itself
is stored. With (D)TLS, that is the connection itself or the
session resumption information, if available. With OSCORE, that
is the security context.
As part of the registration operation, that information is stored
along with the registration resource.
The RD MUST accept all registrations whose registration resource is
not already active, as long as they are made using a security layer
supported by the RD.
Any operation on a registration resource, including registrations
that lead to an existing registration resource, MUST be rejected by
the RD unless all the stored information is found in the new
request's credentials.
Note that, even though subject names are compared in this policy,
they are never directly compared to endpoint names, and an endpoint
cannot expect to "own" any particular endpoint name outside of an
active registration -- even if a certificate says so. It is an
accepted shortcoming of this approach that the endpoint has no
indication of whether the RD remembers it by its subject name or
public key; recognition by subject happens on a best-effort basis
(given the RD may not recognize any authority). Clients MUST be
prepared to pick a different endpoint name when rejected by the RD
initially or after a change in their credentials; picking an endpoint
name, as per Section 7.1.1, is an easy option for that.
For this policy to be usable without configuration, clients should
not set a sector name in their registrations. An RD can set a
default sector name for registrations accepted under this policy,
which is especially useful in a segmented setup where different
policies apply to different sectors. The configuration of such a
behavior, as well as any other configuration applicable to such an RD
(i.e., the set of recognized authorities), is out of scope for this
document.
8. Security Considerations
The security considerations as described in Section 5 of [RFC 8288]
and Section 6 of [RFC 6690] apply. The /.well-known/core resource may
be protected, e.g., using DTLS when hosted on a CoAP server, as
described in [RFC 7252].
Access that is limited or affects sensitive data SHOULD be protected,
e.g., using (D)TLS or OSCORE [RFC 8613]; which aspects of the RD this
affects depends on the security policies of the application (see
Section 7).
8.1. Discovery
Most steps in discovery of the RD, and possibly its resources, are
not covered by CoAP's security mechanisms. This will not endanger
the security properties of the registrations and lookup itself (where
the client requires authorization of the RD if it expects any
security properties of the operation) but may leak the client's
intention to third parties and allow them to slow down the process.
To mitigate that, clients can retain the RD's address, use secure
discovery options (such as configured addresses), and send queries
for RDs in a very general form (e.g., ?rt=core.rd* rather than
?rt=core.rd-lookup-ep).
8.2. Endpoint Identification and Authentication
An endpoint (name, sector) pair is unique within the set of endpoints
registered by the RD. An endpoint MUST NOT be identified by its
protocol, port, or IP address, as these may change over the lifetime
of an endpoint.
Every operation performed by an endpoint on an RD SHOULD be mutually
authenticated using a pre-shared key, a 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.
Endpoint authorization needs to be checked on registration and
registration resource operations independently of whether there are
configured requirements on the credentials for a given endpoint name
and sector (Section 7.1) or whether arbitrary names are accepted
(Section 7.1.1).
Simple registration could be used to circumvent address-based access
control. An attacker would send a simple registration request with
the victim's address as the source address and later look up the
victim's /.well-known/core content in the RD. Mitigation for this is
recommended in Section 5.1.
The registration resource path is visible to any client that is
allowed endpoint lookup and can be extracted by resource lookup
clients as well. The same goes for registration attributes that are
shown as target attributes or lookup attributes. The RD needs to
consider this in the choice of registration resource paths, as do
administrators or endpoints in their choice of attributes.
8.3. 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 look up 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.
The precise access controls necessary (and the consequences of
failure to enforce them) depend on the protection objectives of the
application and the security policies (Section 7) derived from them.
8.4. Denial-of-Service Attacks
Services that run over UDP unprotected are vulnerable to unknowingly
amplify and distribute a DoS attack, as UDP does not require a return
routability check. Since RD lookup responses can be significantly
larger than requests, RDs are prone to this.
[RFC 7252] describes this at length in its Section 11.3, including
some mitigation by using small block sizes in responses. [RFC 9175]
updates that by describing a source address verification mechanism
using the Echo option.
8.5. Skipping Freshness Checks
When RD-based applications are built in which request freshness
checks are not performed, these concerns need to be balanced:
* When alterations to registration attributes are reordered, an
attacker may create any combination of attributes ever set, with
the attack difficulty determined by the security layer's replay
properties.
For example, if Figure 18 were conducted without freshness
assurances, an attacker could later reset the lifetime back to
7200. Thus, the device is made unreachable to lookup clients.
* When registration updates without query parameters (which just
serve to restart the lifetime) can be reordered, an attacker can
use intercepted messages to give the appearance of the device
being alive to the RD.
This is unacceptable when the RD's security policy promises
reachability of endpoints (e.g., when disappearing devices would
trigger further investigation) but may be acceptable with other
policies.
9. IANA Considerations
9.1. Resource Types
IANA has added the following values to the "Resource Type (rt=) Link
Target Attribute Values" subregistry of the "Constrained RESTful
Environments (CoRE) Parameters" registry defined in [RFC 6690]:
+====================+=============================+=============+
| Value | Description | Reference |
+====================+=============================+=============+
| core.rd | Directory resource of an RD | RFC 9176, |
| | | Section 4.3 |
+--------------------+-----------------------------+-------------+
| core.rd-lookup-res | Resource lookup of an RD | RFC 9176, |
| | | Section 4.3 |
+--------------------+-----------------------------+-------------+
| core.rd-lookup-ep | Endpoint lookup of an RD | RFC 9176, |
| | | Section 4.3 |
+--------------------+-----------------------------+-------------+
| core.rd-ep | Endpoint resource of an RD | RFC 9176, |
| | | Section 6 |
+--------------------+-----------------------------+-------------+
Table 2: Additions to Resource Type (rt=) Link Target
Attribute Values Subregistry
9.2. IPv6 ND Resource Directory Address Option
IANA has registered one new ND option type in the "IPv6 Neighbor
Discovery Option Formats" subregistry of the "Internet Control
Message Protocol version 6 (ICMPv6) Parameters" registry:
+======+===================================+===========+
| Type | Description | Reference |
+======+===================================+===========+
| 41 | Resource Directory Address Option | RFC 9176 |
+------+-----------------------------------+-----------+
Table 3: Addition to IPv6 Neighbor Discovery Option
Formats Subregistry
9.3. RD Parameters Registry
This specification defines a new subregistry for registration and
lookup parameters called "RD Parameters" within the "Constrained
RESTful Environments (CoRE) Parameters" registry. Although this
specification defines a basic set of parameters, it is expected that
other standards that make use of this interface will define new ones.
Each entry in the registry must include:
* the human-readable name of the parameter,
* the short name, as used in query parameters or target attributes,
* syntax and validity requirements (if any),
* indication of whether it can be passed as a query parameter at
registration of endpoints, passed as a query parameter in lookups,
or expressed as a target attribute,
* a description, and
* a link to reference documentation.
The query parameter MUST be both a valid URI query key [RFC 3986] and
a token as used in [RFC 8288].
The reference documentation 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
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 subregistry are as follows:
+==============+=======+==============+=====+=====================+
| Name | Short | Validity | Use | Description |
+==============+=======+==============+=====+=====================+
| Endpoint | ep | Unicode* | RLA | Name of the |
| Name | | | | endpoint |
+--------------+-------+--------------+-----+---------------------+
| Lifetime | lt | 1-4294967295 | R | Lifetime of the |
| | | | | registration in |
| | | | | seconds |
+--------------+-------+--------------+-----+---------------------+
| Sector | d | Unicode* | RLA | Sector to which |
| | | | | this endpoint |
| | | | | belongs |
+--------------+-------+--------------+-----+---------------------+
| Registration | base | URI | RLA | The scheme, |
| Base URI | | | | address, port, and |
| | | | | path at which this |
| | | | | server is available |
+--------------+-------+--------------+-----+---------------------+
| Page | page | Integer | L | Used for pagination |
+--------------+-------+--------------+-----+---------------------+
| Count | count | Integer | L | Used for pagination |
+--------------+-------+--------------+-----+---------------------+
| Endpoint | et | RFC 9176, | RLA | Semantic type of |
| Type | | Section | | the endpoint (see |
| | | 9.3.1 | | RFC 9176, |
| | | | | Section 9.4) |
+--------------+-------+--------------+-----+---------------------+
Table 4: New RD Parameters Registry
Where:
Short: Short name used in query parameters or target attributes
Validity:
Unicode* = up to 63 bytes of UTF-8-encoded Unicode, with no
control characters as per Section 5
Use:
R = used at registration
L = used at lookup
A = expressed in the target attribute
The descriptions for the options defined in this document are only
summarized here. To which registrations they apply and when they are
to be shown are described in the respective sections of this
document. All their reference documentation entries point to this
document.
The IANA policy for future additions to the subregistry is Expert
Review, as described in [RFC 8126]. The evaluation should consider
formal criteria, duplication of functionality (i.e., is the new entry
redundant with an existing one?), topical suitability (e.g., is the
described property actually a property of the endpoint and not a
property of a particular resource, in which case it should go into
the payload of the registration and need not be registered?), and the
potential for conflict with commonly used target attributes (e.g., 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 target
attribute [RFC 6690]).
9.3.1. Full Description of the "Endpoint Type" RD Parameter
An endpoint registering at an RD can describe itself with endpoint
types, similar to how resources are described with resource types in
[RFC 6690]. An endpoint type is expressed as a string, which can be
either a URI or one of the values defined in the "Endpoint Type (et=)
RD Parameter Values" subregistry. Endpoint types can be passed in
the et query parameter as part of extra-attrs at the "registration"
step of Section 5, 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 the endpoint type differs from the resource type in that it
uses multiple attributes rather than space-separated values. As a
result, RDs implementing this specification 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 subregistry called "Endpoint
Type (et=) RD Parameter Values" within the "Constrained RESTful
Environments (CoRE) Parameters" registry. The registry properties
(required policy, requirements, and template) are identical to those
of the "Resource Type (rt=) Link Target Attribute Values" subregistry
defined in [RFC 6690]; in short, the review policy is IETF Review for
values starting with "core" and Specification Required for others.
The requirements to be enforced are:
* The values MUST be related to the purpose described in
Section 9.3.1.
* The registered values MUST conform to the ABNF reg-rel-type
definition of [RFC 6690] and MUST NOT be a URI.
* It is recommended to use the period "." character for
segmentation.
The initial contents of the registry are as follows:
+===============+====================================+===========+
| Value | Description | Reference |
+===============+====================================+===========+
| core.rd-group | An application group, as described | RFC 9176 |
| | in RFC 9176, Appendix A. | |
+---------------+------------------------------------+-----------+
Table 5: New Endpoint Type (et=) RD Parameter Values Registry
9.5. Multicast Address Registration
IANA has assigned the following multicast addresses for use by CoAP
nodes:
IPv4 -- "All CoRE Resource Directories" address 224.0.1.190, in the
"Internetwork Control Block (224.0.1.0 - 224.0.1.255
(224.0.1/24))" subregistry within the "IPv4 Multicast Address
Space Registry". As the address is used for discovery that may
span beyond a single network, it has come from the Internetwork
Control Block (224.0.1.x) [RFC 5771].
IPv6 -- "All CoRE Resource Directories" address ff0x::fe, in the
"Variable Scope Multicast Addresses" subregistry within the "IPv6
Multicast Address Space Registry" [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.
9.6. Well-Known URIs
IANA has registered the URI suffix "rd" in the "Well-Known URIs"
registry as follows:
+============+===================+===========+===========+
| URI Suffix | Change Controller | Reference | Status |
+============+===================+===========+===========+
| rd | IETF | RFC 9176 | permanent |
+------------+-------------------+-----------+-----------+
Table 6: Addition to Well-Known URIs Registry
9.7. Service Name and Transport Protocol Port Number Registry
IANA has added four new items to the "Service Name and Transport
Protocol Port Number Registry" as follows:
+==============+===========+=====================+===========+
| Service Name | Transport | Description | Reference |
| | Protocol | | |
+==============+===========+=====================+===========+
| core-rd | udp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
+--------------+-----------+---------------------+-----------+
| core-rd-dtls | udp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
| | | over DTLS | |
+--------------+-----------+---------------------+-----------+
| core-rd | tcp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
| | | over TCP | |
+--------------+-----------+---------------------+-----------+
| core-rd-tls | tcp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
| | | over TLS | |
+--------------+-----------+---------------------+-----------+
Table 7: Additions to Service Name and Transport Protocol
Port Number Registry
10. Examples
Two examples are presented: a lighting installation example in
Section 10.1 and a Lightweight M2M (LwM2M) example in Section 10.2.
10.1. Lighting Installation
This example shows a simplified lighting installation that makes use
of the RD with a CoAP interface to facilitate the installation and
startup of the application code in the lights and sensors. In
particular, the example leads to the definition of a group and the
enabling of the corresponding multicast address, as described in
Appendix A. No conclusions must be drawn on the realization of
actual installation or naming procedures, because the example only
emphasizes some of the issues that may influence the use of the RD
and does not pretend to be normative.
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 Wi-Fi to the installation network,
connected via GPRS link, or connected by another 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 the middle and left lamps of luminary1 and
the 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 infrastructure. Therefore,
Stateless Address Autoconfiguration (SLAAC) IPv6 addresses are
assigned to CT, RD, luminaries, and the sensor. The addresses shown
in Table 8 below stand in for these in the following examples.
+=================+================+
| Name | IPv6 address |
+=================+================+
| luminary1 | 2001:db8:4::1 |
+-----------------+----------------+
| luminary2 | 2001:db8:4::2 |
+-----------------+----------------+
| Presence sensor | 2001:db8:4::3 |
+-----------------+----------------+
| RD | 2001:db8:4::ff |
+-----------------+----------------+
Table 8: Addresses Used in the
Examples
In Section 10.1.2, the use of RD 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=tag:example.com,2020:light, and
the presence sensor has rt=tag:example.com,2020:p-sensor. The
endpoints have names that 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 RD are
shown in Table 9.
+=========+================+========+===============================+
|Name |Endpoint |Resource| Resource Type |
| | |Path | |
+=========+================+========+===============================+
|luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light |
| | |left | |
+---------+----------------+--------+-------------------------------+
|luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light |
| | |middle | |
+---------+----------------+--------+-------------------------------+
|luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light |
| | |right | |
+---------+----------------+--------+-------------------------------+
|luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light |
| | |left | |
+---------+----------------+--------+-------------------------------+
|luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light |
| | |middle | |
+---------+----------------+--------+-------------------------------+
|luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light |
| | |right | |
+---------+----------------+--------+-------------------------------+
|Presence |ps_R2-4-015_door|/ps | tag:example.com,2020:p-sensor |
|sensor | | | |
+---------+----------------+--------+-------------------------------+
Table 9: RD Identifiers
It is assumed that the CT has performed RD discovery and has received
a response like the one in the example in Section 4.3.
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
?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015
Payload:
</light/left>;rt="tag:example.com,2020:light",
</light/middle>;rt="tag:example.com,2020:light",
</light/right>;rt="tag:example.com,2020:light"
Res: 2.01 Created
Location-Path: /rd/4521
Req: POST coap://[2001:db8:4::ff]/rd
?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015
Payload:
</light/left>;rt="tag:example.com,2020:light",
</light/middle>;rt="tag:example.com,2020:light",
</light/right>;rt="tag:example.com,2020:light"
Res: 2.01 Created
Location-Path: /rd/4522
Req: POST coap://[2001:db8:4::ff]/rd
?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]&d=R2-4-015
Payload:
</ps>;rt="tag:example.com,2020:p-sensor"
Res: 2.01 Created
Location-Path: /rd/4523
Figure 24: Example of Registrations a CT Enters into an RD
The sector name d=R2-4-015 has been added for an efficient lookup
because filtering on the "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
?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]
Payload:
</light/left>;rt="tag:example.com,2020:light",
</light/middle>;rt="tag:example.com,2020:light",
</light/right>;rt="tag:example.com,2020:light"
Res: 2.01 Created
Location-Path: /rd/501
Figure 25: Example of a Multicast Group a CT Enters into an RD
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
?d=R2-4-015&et=core.rd-group&rt=light
Res: 2.05 Content
Payload:
</rd/501>;ep=grp_R2-4-015;et=core.rd-group;
base="coap://[ff05::1]";rt=core.rd-ep
Figure 26: Example of a Lookup Exchange to Find Suitable
Multicast Addresses
From the returned base parameter value, the luminary learns the
multicast address of the multicast group.
The presence sensor can learn the presence of groups that support
resources with rt=tag:example.com,2020: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)
OMA LwM2M is a profile for device services based on CoAP, providing
interfaces and operations for device management and device service
enablement.
An LwM2M server is an instance of an LwM2M middleware service layer,
containing an RD ([LwM2M], starting at page 36).
That RD only implements the registration interface, and no lookup is
implemented. Instead, the LwM2M server provides access to the
registered resources in a similar way to a reverse proxy.
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.
11. References
11.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC 3986, January 2005,
<https://www.rfc-editor.org/info/RFC 3986>.
[RFC 6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC 6570, March 2012,
<https://www.rfc-editor.org/info/RFC 6570>.
[RFC 6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC 6690, August 2012,
<https://www.rfc-editor.org/info/RFC 6690>.
[RFC 6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC 6763, February 2013,
<https://www.rfc-editor.org/info/RFC 6763>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<https://www.rfc-editor.org/info/RFC 7230>.
[RFC 7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC 7252, June 2014,
<https://www.rfc-editor.org/info/RFC 7252>.
[RFC 8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC 8126, June 2017,
<https://www.rfc-editor.org/info/RFC 8126>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC 8288, October 2017,
<https://www.rfc-editor.org/info/RFC 8288>.
[RFC 9175] Amsüss, C., Preuß Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC 9175, February 2022,
<https://www.rfc-editor.org/info/RFC 9175>.
11.2. Informative References
[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)", Work in Progress, Internet-Draft,
draft-ietf-ace-oauth-authz-46, 8 November 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
oauth-authz-46>.
[COAP-PROT-NEG]
Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
Work in Progress, Internet-Draft, draft-silverajan-core-
coap-protocol-negotiation-09, 2 July 2018,
<https://datatracker.ietf.org/doc/html/draft-silverajan-
core-coap-protocol-negotiation-09>.
[CORE-CORAL]
Amsüss, C. and T. Fossati, "The Constrained RESTful
Application Language (CoRAL)", Work in Progress, Internet-
Draft, draft-ietf-core-coral-05, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coral-05>.
[CORE-LINKS-JSON]
Li, K., Rahman, A., and C. Bormann, Ed., "Representing
Constrained RESTful Environments (CoRE) Link Format in
JSON and CBOR", Work in Progress, Internet-Draft, draft-
ietf-core-links-json-10, 26 February 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
links-json-10>.
[CORE-RD-DNS-SD]
van der Stok, P., Koster, M., and C. Amsuess, "CoRE
Resource Directory: DNS-SD mapping", Work in Progress,
Internet-Draft, draft-ietf-core-rd-dns-sd-05, 7 July 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-rd-
dns-sd-05>.
[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,
<https://doi.org/10.1145/320434.320440>.
[LwM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification: Transport Bindings (Candidate
Version 1.1)", June 2018,
<https://openmobilealliance.org/RELEASE/LightweightM2M/
V1_1-20180612-C/OMA-TS-LightweightM2M_Transport-
V1_1-20180612-C.pdf>.
[RFC 3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, DOI 10.17487/RFC 3306,
August 2002, <https://www.rfc-editor.org/info/RFC 3306>.
[RFC 3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC 3307, DOI 10.17487/RFC 3307, August 2002,
<https://www.rfc-editor.org/info/RFC 3307>.
[RFC 3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849,
DOI 10.17487/RFC 3849, July 2004,
<https://www.rfc-editor.org/info/RFC 3849>.
[RFC 4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC 4122, July 2005,
<https://www.rfc-editor.org/info/RFC 4122>.
[RFC 4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC 4944, September 2007,
<https://www.rfc-editor.org/info/RFC 4944>.
[RFC 5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
DOI 10.17487/RFC 5771, March 2010,
<https://www.rfc-editor.org/info/RFC 5771>.
[RFC 6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC 6724, September 2012,
<https://www.rfc-editor.org/info/RFC 6724>.
[RFC 6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC 6775, November 2012,
<https://www.rfc-editor.org/info/RFC 6775>.
[RFC 6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC 6874,
February 2013, <https://www.rfc-editor.org/info/RFC 6874>.
[RFC 7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC 7228, May 2014,
<https://www.rfc-editor.org/info/RFC 7228>.
[RFC 7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC 7641, September 2015,
<https://www.rfc-editor.org/info/RFC 7641>.
[RFC 8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC 8106, March 2017,
<https://www.rfc-editor.org/info/RFC 8106>.
[RFC 8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC 8132, April 2017,
<https://www.rfc-editor.org/info/RFC 8132>.
[RFC 8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC 8141, April 2017,
<https://www.rfc-editor.org/info/RFC 8141>.
[RFC 8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC 8613, July 2019,
<https://www.rfc-editor.org/info/RFC 8613>.
[T2TRG-REL-IMPL]
Bormann, C., "impl-info: A link relation type for
disclosing implementation information", Work in Progress,
Internet-Draft, draft-bormann-t2trg-rel-impl-02, 27
September 2020, <https://datatracker.ietf.org/doc/html/
draft-bormann-t2trg-rel-impl-02>.
Appendix A. Groups Registration and Lookup
The RD-Group's usage pattern allows announcing application groups
inside an RD.
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
resources 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 5. The group address in the example is constructed from the
reserved 2001:db8:: prefix in [RFC 3849] as a unicast-prefix-based
site-local address (see [RFC 3306]).
Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
&base=coap://[ff35:30:2001:db8:f1::8000:1]
Content-Format: 40
Payload:
</light>;rt="tag:example.com,2020:light";
if="tag:example.net,2020:actuator",
</color-temperature>;if="tag:example.net,2020:parameter";u=K
Res: 2.01 Created
Location-Path: /rd/12
Figure 27: Example Registration of a Group
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.
The following example shows a client performing an endpoint lookup
for all groups:
Req: GET /rd-lookup/ep?et=core.rd-group
Res: 2.05 Content
Payload:
</rd/12>;ep=lights&et=core.rd-group;
base="coap://[ff35:30:2001:f1:db8::8000:1]";rt=core.rd-ep
Figure 28: Example Lookup of Groups
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
Res: 2.05 Content
Payload:
<coap://[ff35:30:2001:db8:f1::8000:1]/light>;
rt="tag:example.com,2020:light";
if="tag:example.net,2020:actuator",
<coap://[ff35:30:2001:db8:f1::8000:1]/color-temperature>;
if="tag:example.net,2020:parameter";u=K,
Figure 29: Example Lookup of Resources Inside Groups
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 ([RFC 3986]
defining URIs, [RFC 6690] defining link-format documents based on
[RFC 8288], which defines Link header fields, and [RFC 7252] 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.
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 ([RFC 7252], Section 7)
sends the following multicast request to learn about neighbors
supporting resources with resource-type "temperature".
The client sends a link-local multicast:
Req: GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature
Res: 2.05 Content
Payload:
</sensors/temp>;rt=temperature;ct=0
Figure 30: Example of Direct Resource Discovery
where the response is sent by the server, [2001:db8:f0::1]:5683.
While a practical client side implementation might just go ahead and
create a new request to [2001:db8:f0::1]:5683 with Uri-Path sensors
and temp, the full resolution steps for insertion into and retrieval
from the RD without any shortcuts are as follows.
B.1.1. Resolving the URIs
The client parses the single returned link. Its target (sometimes
called "href") is /sensors/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 against /sensors/temp.
The base URI of the requested resource can be composed from the
options of the CoAP GET request by following the steps of [RFC 7252],
Section 6.5 (with an addition at the end of Section 8.2) into
coap://[2001:db8:f0::1]/.well-known/core.
Because /sensors/temp starts with a single slash, the link's target
is resolved by replacing the path /.well-known/core from the base URI
([RFC 3986], Section 5.2) with the relative target URI /sensors/temp
into coap://[2001:db8:f0::1]/sensors/temp.
B.1.2. Interpreting Attributes and Relations
Some more information about the link's target can be obtained from
the payload: the resource type of the target is "temperature", and
its content format 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 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 implied to be
coap:://[2001:db8:f0::1] by the default value of the anchor (see
Appendix B.4). A full English expression of the "host relation" is:
coap://[2001:db8:f0::1] is hosting the resource
coap://[2001:db8:f0::1]/sensors/temp, which is of the resource
type "temperature" and can be read in 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 links in the payload:
Req: GET coap://[ff02::fd]:5683/.well-known/core
Res: 2.05 Content
Payload:
</sensors/temp>;rt=temperature;ct=0,
</sensors/light>;rt=light-lux;ct=0,
</t>;anchor="/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;anchor="/sensors/temp";
rel=describedby
Figure 31: Extended Example of Direct Resource Discovery
Parsing the third link, 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 link 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 link can be read as
coap://[2001:db8:f0::1]/sensors/temp is described by
http://www.example.com/sensors/t123.
B.3. Enter the Resource Directory
The RD 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 RD that was announced to
it, sending this request from its UDP port [2001:db8:f0::1]:6553:
Req: POST coap://[2001:db8:f0::ff]/.well-known/rd?ep=simple-host1
Res: 2.04 Changed
Figure 32: Example of a Simple Registration
The RD 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 ask it for all temperature resources:
Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature
Res: 2.05 Content
Payload:
<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0
Figure 33: Example Exchange Performing Resource Lookup
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]/sensors/temp, which is of the resource
type "temperature" and can be accessed using the text/plain
content format.
To complete the examples, the client could also query all resources
hosted at the endpoint with the known endpoint name "simple-host1":
Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1
Res: 2.05 Content
Payload:
<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0,
<coap://[2001:db8:f0::1]/sensors/light>;rt=light-lux;ct=0,
<coap://[2001:db8:f0::1]/t>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=describedby
Figure 34: Extended Example Exchange Performing Resource Lookup
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://sh1.example.com),
that context would have been used to resolve the relative anchor
values instead, giving the following and analogous links:
<coap+tcp://sh1.example.com/sensors/temp>;rt=temperature;ct=0
Figure 35: Example Payload of a Response to a Resource Lookup
with a Dedicated Base URI
B.4. A Note on Differences between Link-Format and Link Header Fields
While link-format and Link header fields look very similar and are
based on the same model of typed links, there are some differences
between [RFC 6690] and [RFC 8288]. When implementing an RD or
interacting with an RD, care must be taken to follow the behavior
described in [RFC 6690] whenever application/link-format
representations are used.
* "Default value of anchor": Under both [RFC 6690] and [RFC 8288],
relative references in the term inside the angle brackets (the
target) and the anchor attribute are resolved against the relevant
base URI (which usually is the URI used to retrieve the entity)
and independent of each other.
When, in a Link header [RFC 8288], the anchor attribute is absent,
the link's context is the URI of the selected representation (and
usually equal to the base URI).
In links per [RFC 6690], if the anchor attribute is absent, the
default value is the Origin of (for all relevant cases, the URI
reference / resolved against) the link's target.
* There is no percent encoding in link-format documents.
A link-format document is a UTF-8-encoded string of Unicode
characters and does not have percent encoding, while Link header
fields are practically ASCII strings that use percent encoding for
non-ASCII characters, stating the encoding explicitly when
required.
For example, while a Link header field in a page about a Swedish
city might read:
Link: </temperature/Malm%C3%B6>;rel=live-environment-data
a link-format document from the same source might describe the
link as:
</temperature/Malmö>;rel=live-environment-data
Appendix C. Limited Link Format
The CoRE Link Format, as described in [RFC 6690], has been interpreted
differently by implementers, and a strict implementation rules out
some use cases of an RD (e.g., base values with path components in
combination with absent anchors).
This appendix describes a subset of link format documents called the
Limited Link Format. The one rule herein is not very limiting in
practice -- all examples in [RFC 6690] and all deployments the authors
are aware of already stick to them -- but eases the implementation of
RD servers.
It is applicable to representations in the application/link-format
media type and any other media types that inherit [RFC 6690],
Section 2.1.
A link format representation is in the Limited Link Format if, for
each link in it, the following applies:
All URI references either follow the URI or the path-absolute ABNF
rule of [RFC 3986] (i.e., the target and anchor each either start with
a scheme or with a single slash).
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, Jan Newmarch, Matthias
Kovatsch, Jaime Jimenez, and Ted Lemon 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 RD concepts were originally developed.
Authors' Addresses
Christian Amsüss (editor)
Email: christian@amsuess.com
Zach Shelby
Edge Impulse
3031 Tisch Way
San Jose, 95128
United States of America
Email: zach@edgeimpulse.com
Michael Koster
PassiveLogic
524 H Street
Antioch, CA 94509
United States of America
Phone: +1-707-502-5136
Email: michaeljohnkoster@gmail.com
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Peter van der Stok
vanderstok consultancy
Email: stokcons@bbhmail.nl
RFC TOTAL SIZE: 149556 bytes
PUBLICATION DATE: Monday, April 25th, 2022
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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