CoRE Z. Shelby
Internet-Draft ARM
Intended status: Standards Track M. Koster
Expires: May 3, 2018 SmartThings
C. Bormann
Universitaet Bremen TZI
P. van der Stok
consultant
C. Amsuess, Ed.
Energy Harvesting Solutions
October 30, 2017
CoRE Resource Directory
draft-ietf-core-resource-directory-12
Abstract
In many M2M applications, direct discovery of resources is not
practical due to sleeping nodes, disperse networks, or networks where
multicast traffic is inefficient. These problems can be solved by
employing an entity called a Resource Directory (RD), which hosts
descriptions of resources held on other servers, allowing lookups to
be performed for those resources. This document specifies the web
interfaces that a Resource Directory supports in order for web
servers to discover the RD and to register, maintain, lookup and
remove resource descriptions. Furthermore, new link attributes
useful in conjunction with an RD are defined.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 3, 2018.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architecture and Use Cases . . . . . . . . . . . . . . . . . 5
3.1. Principles . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Content model . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Use Case: Cellular M2M . . . . . . . . . . . . . . . . . 11
3.5. Use Case: Home and Building Automation . . . . . . . . . 12
3.6. Use Case: Link Catalogues . . . . . . . . . . . . . . . . 12
4. Finding a Resource Directory . . . . . . . . . . . . . . . . 13
4.1. Resource Directory Address Option (RDAO) . . . . . . . . 14
5. Resource Directory . . . . . . . . . . . . . . . . . . . . . 15
5.1. Content Formats . . . . . . . . . . . . . . . . . . . . . 16
5.2. URI Discovery . . . . . . . . . . . . . . . . . . . . . . 16
5.3. Registration . . . . . . . . . . . . . . . . . . . . . . 18
5.3.1. Simple Registration . . . . . . . . . . . . . . . . . 22
5.3.2. Third-party registration . . . . . . . . . . . . . . 23
5.4. Operations on the Registration Resource . . . . . . . . . 23
5.4.1. Registration Update . . . . . . . . . . . . . . . . . 24
5.4.2. Registration Removal . . . . . . . . . . . . . . . . 26
5.4.3. Read Endpoint Links . . . . . . . . . . . . . . . . . 27
5.4.4. Update Endpoint Links . . . . . . . . . . . . . . . . 28
6. RD Groups . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.1. Register a Group . . . . . . . . . . . . . . . . . . . . 29
6.2. Group Removal . . . . . . . . . . . . . . . . . . . . . . 31
7. RD Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.1. Resource lookup . . . . . . . . . . . . . . . . . . . . . 32
7.2. Endpoint and group lookup . . . . . . . . . . . . . . . . 33
7.3. Lookup filtering . . . . . . . . . . . . . . . . . . . . 33
7.4. Lookup examples . . . . . . . . . . . . . . . . . . . . . 35
8. Security Considerations . . . . . . . . . . . . . . . . . . . 38
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8.1. Endpoint Identification and Authentication . . . . . . . 38
8.2. Access Control . . . . . . . . . . . . . . . . . . . . . 39
8.3. Denial of Service Attacks . . . . . . . . . . . . . . . . 39
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
9.1. Resource Types . . . . . . . . . . . . . . . . . . . . . 40
9.2. IPv6 ND Resource Directory Address Option . . . . . . . . 40
9.3. RD Parameter Registry . . . . . . . . . . . . . . . . . . 40
9.3.1. Full description of the "Endpoint Type" Registration
Parameter . . . . . . . . . . . . . . . . . . . . . . 42
9.4. "Endpoint Type" (et=) RD Parameter values . . . . . . . . 42
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 42
10.1. Lighting Installation . . . . . . . . . . . . . . . . . 43
10.1.1. Installation Characteristics . . . . . . . . . . . . 43
10.1.2. RD entries . . . . . . . . . . . . . . . . . . . . . 44
10.2. OMA Lightweight M2M (LWM2M) Example . . . . . . . . . . 47
10.2.1. The LWM2M Object Model . . . . . . . . . . . . . . . 47
10.2.2. LWM2M Register Endpoint . . . . . . . . . . . . . . 49
10.2.3. LWM2M Update Endpoint Registration . . . . . . . . . 50
10.2.4. LWM2M De-Register Endpoint . . . . . . . . . . . . . 51
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 51
12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 51
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
13.1. Normative References . . . . . . . . . . . . . . . . . . 56
13.2. Informative References . . . . . . . . . . . . . . . . . 57
Appendix A. Web links and the Resource Directory . . . . . . . . 58
A.1. A simple example . . . . . . . . . . . . . . . . . . . . 58
A.1.1. Resolving the URIs . . . . . . . . . . . . . . . . . 59
A.1.2. Interpreting attributes and relations . . . . . . . . 59
A.2. A slightly more complex example . . . . . . . . . . . . . 59
A.3. Enter the Resource Directory . . . . . . . . . . . . . . 60
A.4. A note on differences between link-format and Link
headers . . . . . . . . . . . . . . . . . . . . . . . . . 62
Appendix B. Syntax examples for Protocol Negotiation . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63
1. Introduction
The work on Constrained RESTful Environments (CoRE) aims at realizing
the REST architecture in a suitable form for the most constrained
nodes (e.g., 8-bit microcontrollers with limited RAM and ROM) and
networks (e.g. 6LoWPAN). CoRE is aimed at machine-to-machine (M2M)
applications such as smart energy and building automation.
The discovery of resources offered by a constrained server is very
important in machine-to-machine applications where there are no
humans in the loop and static interfaces result in fragility. The
discovery of resources provided by an HTTP Web Server is typically
called Web Linking [RFC5988]. The use of Web Linking for the
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description and discovery of resources hosted by constrained web
servers is specified by the CoRE Link Format [RFC6690]. However,
[RFC6690] only describes how to discover resources from the web
server that hosts them by querying "/.well-known/core". In many M2M
scenarios, direct discovery of resources is not practical due to
sleeping nodes, disperse networks, or networks where multicast
traffic is inefficient. These problems can be solved by employing an
entity called a Resource Directory (RD), which hosts descriptions of
resources held on other servers, allowing lookups to be performed for
those resources.
This document specifies the web interfaces that a Resource Directory
supports in order for web servers to discover the RD and to register,
maintain, lookup and remove resource descriptions. Furthermore, new
link attributes useful in conjunction with a Resource Directory are
defined. Although the examples in this document show the use of
these interfaces with CoAP [RFC7252], they can be applied in an
equivalent manner to HTTP [RFC7230].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. The term "byte" is used in its now customary sense as a
synonym for "octet".
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC5988] and [RFC6690]. Readers
should also be familiar with the terms and concepts discussed in
[RFC7252]. To describe the REST interfaces defined in this
specification, the URI Template format is used [RFC6570].
This specification makes use of the following additional terminology:
Resource Directory
A web entity that stores information about web resources and
implements the REST interfaces defined in this specification for
registration and lookup of those resources.
Domain
In the context of a Resource Directory, a domain is a logical
grouping of endpoints.
Group
In the context of a Resource Directory, a group is a logical
grouping of endpoints for the purpose of group communications.
All groups within a domain have unique names.
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Endpoint
Endpoint (EP) is a term used to describe a web server or client in
[RFC7252]. In the context of this specification an endpoint is
used to describe a web server that registers resources to the
Resource Directory. An endpoint is identified by its endpoint
name, which is included during registration, and has a unique name
within the associated domain of the registration.
Context
A Context is a base URL that gives scheme and (typically)
authority information about an Endpoint. The Context of an
Endpoint is provided at registration time, and is used by the
Resource Directory to resolve relative references inside the
registration into absolute URIs.
Directory Resource
A resource in the Resource Directory (RD) containing registration
resources.
Group Resource
A resource in the RD containing registration resources of the
Endpoints that form a group.
Registration Resource
A resource in the RD that contains information about an Endpoint
and its links.
Commissioning Tool
Commissioning Tool (CT) is a device that assists during the
installation of the network by assigning values to parameters,
naming endpoints and groups, or adapting the installation to the
needs of the applications.
RDAO
Resource Directory Address Option.
3. Architecture and Use Cases
3.1. Principles
The Resource Directory is primarily a tool to make discovery
operations more efficient than querying /.well-known/core on all
connected device, or across boundaries that would be limiting those
operations.
It provides a cache (in the high-level sense, not as defined in
[RFC7252]/[RFC2616]) of data that could otherwise only be obtained by
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directly querying the /.well-known/core resource on the target
device, or by accessing those resources with a multicast request.
From that, it follows that no information should be stored in the
resource directory that cannot be discovered from querying the
described device's /.well-known/core resource directly.
It also follows that data in the resource directory can only be
provided by the device whose descriptions are cached or a dedicated
Commissioning Tool (CT). These CTs are thought to act on behalf
agents too constrained, or generally unable, to present that
information themselves. No other client can modify data in the
resource directory or even expect those changes to propagate back to
its source.
3.2. Architecture
The resource directory architecture is illustrated in Figure 1. A
Resource Directory (RD) is used as a repository for Web Links
[RFC5988] about resources hosted on other web servers, which are
called endpoints (EP). An endpoint is a web server associated with a
scheme, IP address and port, thus a physical node may host one or
more endpoints. The RD implements a set of REST interfaces for
endpoints to register and maintain sets of Web Links (called resource
directory registration entries), and for clients to lookup resources
from the RD or maintain groups. Endpoints themselves can also act as
clients. An RD can be logically segmented by the use of Domains.
The domain an endpoint is associated with can be defined by the RD or
configured by an outside entity. This information hierarchy is shown
in Figure 2.
A mechanism to discover an RD using CoRE Link Format [RFC6690] is
defined.
Endpoints proactively register and maintain resource directory
registration entries on the RD, which are soft state and need to be
periodically refreshed.
An endpoint is provided with interfaces to register, update and
remove a resource directory registration entry. It is also possible
for an RD to fetch Web Links from endpoints and add them as resource
directory registration entries.
At the first registration of a set of entries, a "registration
resource" is created, the location of which is returned to the
registering endpoint. The registering endpoint uses this
registration resource to manage the contents of the registration
entry.
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A lookup interface for discovering any of the Web Links held in the
RD is provided using the CoRE Link Format.
Registration Lookup, Group
Interface Interfaces
+----+ | |
| EP |---- | |
+----+ ---- | |
--|- +------+ |
+----+ | ----| | | +--------+
| EP | ---------|-----| RD |----|-----| Client |
+----+ | ----| | | +--------+
--|- +------+ |
+----+ ---- | |
| EP |---- | |
+----+
Figure 1: The resource directory architecture.
+------------+
| Domain | <-- Name
+------------+
| |
| +------------+
| | Group | <-- Name, Scheme, IP, Port
| +------------+
| |
+------------+
| Endpoint | <-- Name, Scheme, IP, Port
+------------+
|
|
+------------+
| Resource | <-- Target, Parameters
+------------+
Figure 2: The resource directory information hierarchy.
3.3. Content model
The Entity-Relationship (ER) models shown in Figure 3 and Figure 4
model the contents of /.well-known/core and the resource directory
respectively, with entity-relationship diagrams [ER]. Entities
(rectangles) are used for concepts that exist independently.
Attributes (ovals) are used for concepts that exist only in
connection with a related entity. Relations (diamonds) give a
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semantic meaning to the relation between entities. Numbers specify
the cardinality of the relations.
Some of the attribute values are URIs. Those values are always full
URIs and never relative references in the information model. They
can, however, be expressed as relative references in serializations,
and often are.
These models provide an abstract view of the information expressed in
link-format documents and a Resource Directory. They cover the
concepts, but not necessarily all details of an RD's operation; they
are meant to give an overview, and not be a template for
implementations.
+----------------------+
| /.well-known/core |
+----------------------+
|
| 1
////////\\\\\\\
< contains >
\\\\\\\\///////
|
| 0+
+--------------------+
| link |
+--------------------+
|
| 1 oooooooo
+-----o target o
0+ | oooooooo
oooooooooooo |
o target o--------+
o attribute o | 0+ oooooo
oooooooooooo +-----o rel o
| oooooo
|
| 1 ooooooooo
+-----o context o
ooooooooo
Figure 3: E-R Model of the content of /.well-known/core
The model shown in Figure 3 models the contents of /.well-known/core
which contains:
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o a set of links belonging to the host
The host is free to choose links it deems appropriate to be exposed
in its ".well-known/core". Typically, the links describe resources
that are served by the host, but the set can also contain links to
resources on other servers (see examples in [RFC6690] page 14). The
set does not necessarily contain links to all resources served by the
host.
A link has the following attributes:
o Zero or more link relations: They describe a 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".
o A link context URI: It defines the source of the relation, eg.
_who_ "hosts" something.
In link-format serialization, it is expressed in the "anchor"
attribute. There, it can be a relative reference, in which case
it gets resolved against the URI of the ".well-known/core"
document it was obtained from . It defaults to that document's
URI.
In the serialization, the context also serves as the Base URI for
resolving the target reference.
o A link target URI: It defines the destination of the relation (eg.
_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". If it is a relative URI,
it gets resolved against the link context URI.
o Other target attributes (eg. resource type (rt), interface (if),
cor content-type (ct)). These provide additional information
about the target URI.
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+----------------------+
| resource-directory |
+----------------------+
|
| oooooooooooo 0-1
| o MC address o---+
| oooooooooooo |
| |
//////\\\\ 0+ +--------+
< contains >----------------| group |
\\\\\///// +--------+
| |
0-n | | 1+
ooooooo 1 +---------------+ ///////\\\\\\
o con o-------| registration |---------< composed of >
ooooooo +---------------+ \\\\\\\//////
| |
| +--------------+
oooooooo 1 | |
o loc o----+ /////\\\\
oooooooo | < contains >
| \\\\\/////
oooooooo 1 | |
o ep o----+ | 0+
oooooooo | +------------------+
| | link |
oooooooo 0-1 | +------------------+
o d o----+ |
oooooooo | | 1 oooooooo
| +-----o target o
oooooooo 0-1 | | oooooooo
o lt o----+ ooooooooooo 0+ |
oooooooo | o target o-----+
| o attribute o | 0+ oooooo
ooooooooooo 0+ | ooooooooooo +-----o rel o
o endpoint o----+ | oooooo
o attribute o |
ooooooooooo | 1 ooooooooo
+----o context o
ooooooooo
Figure 4: E-R Model of the content of the Resource Directory
The model shown in Figure 4 models the contents of the resource
directory which contains in addition to /.well-known/core:
o 0 to n Registration (entries),
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o 0 or more Groups
A Group has no or one Multicast address attribute and is composed of
0 or more endpoints. A registration is associated with one endpoint
(ep). An endpoint can be part of 0 or more Groups . A registration
defines a set of links as defined for /.well-known/core. A
Registration has six attributes:
o one ep (endpoint with a unique name)
o one con (a string describing the scheme://authority part)
o one lt (lifetime),
o one loc (location in the RD)
o optional one d (domain for query filtering),
o optional additional endpoint attributes (from Section 9.3)
The cardinality of con is currently 1. Its value is used as a Base
URI when resolving URIs in the links contained in the endpoint.
Links are modelled as they are in Figure 3.
3.4. Use Case: Cellular M2M
Over the last few years, mobile operators around the world have
focused on development of M2M solutions in order to expand the
business to the new type of users: machines. The machines are
connected directly to a mobile network using an appropriate embedded
wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing
short and wide range wireless interfaces. From the system design
point of view, the ambition is to design horizontal solutions that
can enable utilization of machines in different applications
depending on their current availability and capabilities as well as
application requirements, thus avoiding silo like solutions. One of
the crucial enablers of such design is the ability to discover
resources (machines -- endpoints) capable of providing required
information at a given time or acting on instructions from the end
users.
Imagine a scenario where endpoints installed on vehicles enable
tracking of the position of these vehicles for fleet management
purposes and allow monitoring of environment parameters. During the
boot-up process endpoints register with a Resource Directory, which
is hosted by the mobile operator or somewhere in the cloud.
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Periodically, these endpoints update their registration and may
modify resources they offer.
When endpoints are not always connected, for example because they
enter a sleep mode, a remote server is usually used to provide proxy
access to the endpoints. Mobile apps or web applications for
environment monitoring contact the RD, look up the endpoints capable
of providing information about the environment using appropriate set
of link parameters, obtain information on how to contact them (URLs
of the proxy server) and then initiate interaction to obtain
information that is finally processed, displayed on the screen and
usually stored in a database. Similarly, fleet management systems
provide the appropriate link parameters to the RD to look up for EPs
deployed on the vehicles the application is responsible for.
3.5. Use Case: Home and Building Automation
Home and commercial building automation systems can benefit from the
use of M2M web services. The discovery requirements of these
applications are demanding. Home automation usually relies on run-
time discovery to commission the system, whereas in building
automation a combination of professional commissioning and run-time
discovery is used. Both home and building automation involve peer-
to-peer interactions between endpoints, and involve battery-powered
sleeping devices.
3.6. Use Case: Link Catalogues
Resources may be shared through data brokers that have no knowledge
beforehand of who is going to consume the data. Resource Directory
can be used to hold links about resources and services hosted
anywhere to make them discoverable by a general class of
applications.
For example, environmental and weather sensors that generate data for
public consumption may provide the data to an intermediary server, or
broker. Sensor data are published to the intermediary upon changes
or at regular intervals. Descriptions of the sensors that resolve to
links to sensor data may be published to a Resource Directory.
Applications wishing to consume the data can use RD Lookup to
discover and resolve links to the desired resources and endpoints.
The Resource Directory service need not be coupled with the data
intermediary service. Mapping of Resource Directories to data
intermediaries may be many-to-many.
Metadata in web link formats like [RFC6690] are supplied by Resource
Directories, which may be internally stored as triples, or relation/
attribute pairs providing metadata about resource links. External
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catalogs that are represented in other formats may be converted to
common web linking formats for storage and access by Resource
Directories. Since it is common practice for these to be URN
encoded, simple and lossless structural transforms should generally
be sufficient to store external metadata in Resource Directories.
The additional features of Resource Directory allow domains to be
defined to enable access to a particular set of resources from
particular applications. This provides isolation and protection of
sensitive data when needed. Resource groups may defined to allow
batched reads from multiple resources.
4. Finding a Resource Directory
A device coming up may want to find one or more resource directories
to make itself known with.
The device may be pre-configured to exercise specific mechanisms for
finding the resource directory:
o 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 directory servers then need to configure
one of their interfaces with this multicast address.)
o It may be configured with a DNS name for the RD and a resource-
record type to look up under this name; it can find a DNS server
to perform the lookup using the usual mechanisms for finding DNS
servers.
o It may be configured to use a service discovery mechanism such as
DNS-SD [RFC6763]. The present specification suggests configuring
the service with name rd._sub._coap._udp, preferably within the
domain of the querying nodes.
For cases where the device is not specifically configured with a way
to find a resource directory, the network may want to provide a
suitable default.
o If the address configuration of the network is performed via
SLAAC, this is provided by the RDAO option Section 4.1.
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o If the address configuration of the network is performed via DHCP,
this could be provided via a DHCP option (no such option is
defined at the time of writing).
Finally, if neither the device nor the network offer any specific
configuration, the device may want to employ heuristics to find a
suitable resource directory.
The present specification does not fully define these heuristics, but
suggests a number of candidates:
o In a 6LoWPAN, just assume the Edge Router (6LBR) can act as a
resource directory (using the ABRO option to find that [RFC6775]).
Confirmation can be obtained by sending a Unicast to
"coap://[6LBR]/.well-known/core?rt=core.rd*".
o In a network that supports multicast well, discovering the RD
using a multicast query for /.well-known/core as specified in CoRE
Link Format [RFC6690]: Sending a Multicast GET to
"coap://[ff02::1]/.well-known/core?rt=core.rd*". RDs within the
multicast scope will answer the query.
As some of the RD addresses obtained by the methods listed here are
just (more or less educated) guesses, endpoints MUST make use of any
error messages to very strictly rate-limit requests to candidate IP
addresses that don't work out. For example, an ICMP Destination
Unreachable message (and, in particular, the port unreachable code
for this message) may indicate the lack of a CoAP server on the
candidate host, or a CoAP error response code such as 4.05 "Method
Not Allowed" may indicate unwillingness of a CoAP server to act as a
directory server.
4.1. Resource Directory Address Option (RDAO)
The Resource Directory Option (RDAO) using IPv6 neighbor Discovery
(ND) carries information about the address of the Resource Directory
(RD). This information is needed when endpoints cannot discover the
Resource Directory with link-local multicast address because the
endpoint and the RD are separated by a border Router (6LBR). In many
circumstances the availability of DHCP cannot be guaranteed either
during commissioning of the network. The presence and the use of the
RD is essential during commissioning.
It is possible to send multiple RDAO options in one message,
indicating as many resource directory addresses.
The lifetime 0x0 means that the RD address is invalid and to be
removed.
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The RDAO format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 3 | Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ RD Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 38
Length: 8-bit unsigned integer. The length of
the option in units of 8 bytes.
Always 3.
Valid Lifetime: 16-bit unsigned integer. The length of
time in units of 60 seconds (relative to
the time the packet is received) that
this Resource Directory address is valid.
A value of all zero bits (0x0) indicates
that this Resource Directory address
is not valid anymore.
Reserved: This field is unused. It MUST be
initialized to zero by the sender and
MUST be ignored by the receiver.
RD Address: IPv6 address of the RD.
Figure 5: Resource Directory Address Option
5. Resource Directory
This section defines the required set of REST interfaces between a
Resource Directory (RD) and endpoints. Although the examples
throughout this section assume the use of CoAP [RFC7252], these REST
interfaces can also be realized using HTTP [RFC7230]. In all
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definitions in this section, both CoAP response codes (with dot
notation) and HTTP response codes (without dot notation) are shown.
An RD implementing this specification MUST support the discovery,
registration, update, lookup, and removal interfaces defined in this
section.
All operations on the contents of the Resource Directory MUST be
atomic and idempotent.
A resource directory MAY make the information submitted to it
available to further directories, if it can ensure that a loop does
not form. The protocol used between directories to ensure loop-free
operation is outside the scope of this document.
5.1. Content Formats
Resource Directory implementations using this specification MUST
support the application/link-format content format (ct=40).
Resource Directories implementing this specification MAY support
additional content formats.
Any additional content format supported by a Resource Directory
implementing this specification MUST have an equivalent serialization
in the application/link-format content format.
5.2. URI Discovery
Before an endpoint can make use of an RD, it must first know the RD's
address and port, and the URI path information for its REST APIs.
This section defines discovery of the RD and its URIs using the well-
known interface of the CoRE Link Format [RFC6690]. A complete set of
RD discovery methods is described in Section 4.
Discovery of the RD registration URI path is performed by sending
either a multicast or unicast GET request to "/.well-known/core" and
including a Resource Type (rt) parameter [RFC6690] with the value
"core.rd" in the query string. Likewise, a Resource Type parameter
value of "core.rd-lookup*" is used to discover the URIs for RD Lookup
operations, and "core.rd-group" is used to discover the URI path for
RD Group operations. Upon success, the response will contain a
payload with a link format entry for each RD function discovered,
indicating the URI path of the RD function returned and the
corresponding Resource Type. When performing multicast discovery,
the multicast IP address used will depend on the scope required and
the multicast capabilities of the network.
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A Resource Directory MAY provide hints about the content-formats it
supports in the links it exposes or registers, using the "ct" link
attribute, as shown in the example below. Clients MAY use these
hints to select alternate content-formats for interaction with the
Resource Directory.
HTTP does not support multicast and consequently only unicast
discovery can be supported using HTTP. Links to Resource Directories
MAY be registered in other Resource Directories, and well-known entry
points SHOULD be provided to enable the bootstrapping of unicast
discovery.
An RD implementation of this specification MUST support query
filtering for the rt parameter as defined in [RFC6690].
The discovery request interface is specified as follows:
Interaction: EP -> RD
Method: GET
URI Template: /.well-known/core{?rt}
URI Template Variables:
rt := Resource Type (optional). MAY contain one of the values
"core.rd", "core.rd-lookup*", "core.rd-lookup-res", "core.rd-
lookup-ep", "core.rd-lookup-gp", "core.rd-group" or "core.rd*"
Content-Format: application/link-format (if any)
Content-Format: application/link-format+json (if any)
Content-Format: application/link-format+cbor (if any)
The following response codes are defined for this interface:
Success: 2.05 "Content" or 200 "OK" with an application/link-format,
application/link-format+json, or application/link-format+cbor
payload containing one or more matching entries for the RD
resource.
Failure: 4.00 "Bad Request" or 400 "Bad Request" is returned in case
of a malformed request for a unicast request.
Failure: No error response to a multicast request.
HTTP support : YES (Unicast only)
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The following example shows an endpoint discovering an RD using this
interface, thus learning that the directory resource is, in this
example, at /rd, and that the content-format delivered by the server
hosting the resource is application/link-format (ct=40). Note that
it is up to the RD to choose its RD resource paths.
Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*
Res: 2.05 Content
</rd>;rt="core.rd";ct=40,
</rd-lookup/ep>;rt="core.rd-lookup-ep";ct=40,
</rd-lookup/res>;rt="core.rd-lookup-res";ct=40,
</rd-lookup/gp>;rt="core.rd-lookup-gp";ct=40,
</rd-group>;rt="core.rd-group";ct=40
Figure 6: Example discovery exchange
The following example shows the way of indicating that a client may
request alternate content-formats. The Content-Format code attribute
"ct" MAY include a space-separated sequence of Content-Format codes
as specified in Section 7.2.1 of [RFC7252], indicating that multiple
content-formats are available. The example below shows the required
Content-Format 40 (application/link-format) indicated as well as the
the CBOR and JSON representation of link format. The RD resource
paths /rd, /rd-lookup, and /rd-group are example values.
[ The RFC editor is asked to replace these and later occurrences of
TBD64 and TBD504 with the numeric ID values assigned by IANA to
application/link-format+cbor and application/link-format+json,
respectively, as they are defined in I-D.ietf-core-links-json. ]
Req: GET coap://[ff02::1]/.well-known/core?rt=core.rd*
Res: 2.05 Content
</rd>;rt="core.rd";ct="40 65225",
</rd-lookup/res>;rt="core.rd-lookup-res";ct="40 TBD64 TBD504",
</rd-lookup/ep>;rt="core.rd-lookup-ep";ct="40 TBD64 TBD504",
</rd-lookup/gp>;rt="core.rd-lookup-gp";ct=40 TBD64 TBD504",
</rd-group>;rt="core.rd-group";ct="40 TBD64 TBD504"
5.3. Registration
After discovering the location of an RD, an endpoint MAY register its
resources using the registration interface. This interface accepts a
POST from an endpoint containing the list of resources to be added to
the directory as the message payload in the CoRE Link Format
[RFC6690], JSON CoRE Link Format (application/link-format+json), or
CBOR CoRE Link Format (application/link-format+cbor)
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[I-D.ietf-core-links-json], along with query parameters indicating
the name of the endpoint, and optionally its domain and the lifetime
of the registration. It is expected that other specifications will
define further parameters (see Section 9.3). The RD then creates a
new registration resource in the RD and returns its location. An
endpoint MUST use that location when refreshing registrations using
this interface. Registration resources in the RD are kept active for
the period indicated by the lifetime parameter. The endpoint is
responsible for refreshing the registration resource within this
period using either the registration or update interface. The
registration interface MUST be implemented to be idempotent, so that
registering twice with the same endpoint parameters ep and d does not
create multiple registration resources. A new registration resource
may be created at any time to supersede an existing registration,
replacing the registration parameters and links.
An empty payload is considered a malformed request.
The posted link-format document can (and typically does) contain
relative references both in its link targets and in its anchors, or
contain empty anchors. The RD server needs to resolve these
references in order to faithfully represent them in lookups. The
Base URI against which they are resolved is the context of the
registration, which is provided either explicitly in the "con"
parameter or constructed implicitly from the requester's network
address. When resolving relative target references, the server first
resolves the context of that link, and then interprets the target as
a reference relative to that context (see Appendix A.4).
The registration request interface is specified as follows:
Interaction: EP -> RD
Method: POST
URI Template: {+rd}{?ep,d,lt,con,extra-attrs*}
URI Template Variables:
rd := RD registration URI (mandatory). This is the location of
the RD, as obtained from discovery.
ep := Endpoint name (mostly mandatory). The endpoint name is an
identifier that MUST be unique within a domain. The maximum
length of this parameter is 63 bytes. If the RD is configured
to recognize the endpoint (eg. based on its security context),
the endpoint can elide the endpoint name, and assign one based
on the configuration.
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d := Domain (optional). The domain to which this endpoint
belongs. The maximum length of this parameter is 63 bytes.
When this parameter is elided, the RD MAY associate the
endpoint with a configured default domain.
lt := Lifetime (optional). Lifetime of the registration in
seconds. Range of 60-4294967295. If no lifetime is included
in the initial registration, a default value of 86400 (24
hours) SHOULD be assumed.
con := Context (optional). This parameter sets the Default Base
URI under which the request's links are to be interpreted. The
URI MUST NOT have a path component of its own, but MUST be
suitable as a base URI to resolve any relative references given
in the registration. The parameter is therefore of the shape
"scheme://authority" for HTTP and CoAP URIs. In the absence of
this parameter the scheme of the protocol, source address and
source port of the registration request are assumed. This
parameter is mandatory when the directory is filled by a third
party such as an commissioning tool. If the endpoint uses an
ephemeral port to register with, it MUST include the con
parameter in the registration to provide a valid network path.
If the endpoint which is located behind a NAT gateway is
registering with a Resource Directory which is on the network
service side of the NAT gateway, the endpoint MUST use a
persistent port for the outgoing registration in order to
provide the NAT gateway with a valid network address for
replies and incoming requests.
extra-attrs := Additional registration attributes (optional).
The endpoint can pass any parameter registered at Section 9.3
to the directory. If the RD is aware of the parameter's
specified semantics, it processes it accordingly. Otherwise,
it MUST store the unknown key and its value(s) as an endpoint
attribute for further lookup.
Content-Format: application/link-format
Content-Format: application/link-format+json
Content-Format: application/link-format+cbor
The following response codes are defined for this interface:
Success: 2.01 "Created" or 201 "Created". The Location header
option MUST be included in the response when a new registration
resource is created. This Location MUST be a stable identifier
generated by the RD as it is used for all subsequent operations on
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this registration resource. The registration resource location
thus returned is for the purpose of updating the lifetime of the
registration and for maintaining the content of the registered
links, including updating and deleting links. A registration with
an already registered ep and d value pair responds with the same
success code and Location as the original registration; the set of
links registered with the endpoint is replaced with the links from
the payload.
Failure: 4.00 "Bad Request" or 400 "Bad Request". Malformed
request.
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
The following example shows an endpoint with the name "node1"
registering two resources to an RD using this interface. The
location "/rd" is an example RD location discovered in a request
similar to Figure 6.
Req: POST coap://rd.example.com/rd?ep=node1
Content-Format: 40
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor",
</sensors/light>;ct=41;rt="light-lux";if="sensor"
Res: 2.01 Created
Location: /rd/4521
A Resource Directory may optionally support HTTP. Here is an example
of almost the same registration operation above, when done using HTTP
and the JSON Link Format.
Req: POST /rd?ep=node1&con=http://[2001:db8:1::1] HTTP/1.1
Host : example.com
Content-Type: application/link-format+json
Payload:
[
{"href": "/sensors/temp", "ct": "41", "rt": "temperature-c", "if": "sensor"},
{"href": "/sensors/light", "ct": "41", "rt": "light-lux", "if": "sensor"}
]
Res: 201 Created
Location: /rd/4521
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5.3.1. Simple Registration
Not all endpoints hosting resources are expected to know how to
upload links to a RD as described in Section 5.3. Instead, simple
endpoints can implement the Simple Registration approach described in
this section. An RD implementing this specification MUST implement
Simple Registration. However, there may be security reasons why this
form of directory discovery would be disabled.
This approach requires that the endpoint makes available the hosted
resources that it wants to be discovered, as links on its "/.well-
known/core" interface as specified in [RFC6690].
The endpoint then finds one or more addresses of the directory server
as described in Section 4.
An endpoint finally asks the directory server to probe it for
resources and publish them as follows:
It sends (and regularly refreshes with) a POST request to the
"/.well-known/core" URI of the directory server of choice. The body
of the POST request is empty, which triggers the resource directory
server to perform GET requests at the requesting server's default
discovery URI to obtain the link-format payload to register.
The endpoint includes the same registration parameters in the POST
request as it would per Section 5.3. The context of the registration
is taken from the requesting server's URI.
The endpoints MUST be deleted after the expiration of their lifetime.
Additional operations cannot be executed because no registration
location is returned.
The following example shows an endpoint using Simple Registration, by
simply sending an empty POST to a resource directory.
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Req:(to RD server from [2001:db8:2::1])
POST /.well-known/core?lt=6000&ep=node1
Content-Format: 40
No payload
Res: 2.04 Changed
(later)
Req: (from RD server to [2001:db8:2::1])
GET /.well-known/core
Accept: 40
Res: 2.05 Content
Payload:
</sen/temp>
5.3.2. Third-party registration
For some applications, even Simple Registration may be too taxing for
certain very constrained devices, in particular if the security
requirements become too onerous.
In a controlled environment (e.g. building control), the Resource
Directory can be filled by a third device, called a commissioning
tool. The commissioning tool can fill the Resource Directory from a
database or other means. For that purpose the scheme, IP address and
port of the registered device is indicated in the Context parameter
of the registration described in Section 5.3.
5.4. Operations on the Registration Resource
After the initial registration, an endpoint should retain the
returned location of the Registration Resource for further
operations, including refreshing the registration in order to extend
the lifetime and "keep-alive" the registration. When the lifetime of
the registration has expired, the RD SHOULD NOT respond to discovery
queries concerning this endpoint. The RD SHOULD continue to provide
access to the Registration Resource after a registration time-out
occurs in order to enable the registering endpoint to eventually
refresh the registration. The RD MAY eventually remove the
registration resource for the purpose of resource recovery and
garbage collection. If the Registration Resource is removed, the
endpoint will need to re-register.
The Registration Resource may also be used to inspect the
registration resource using GET, update the registration link
contents, or cancel the registration using DELETE.
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These operations are described in this section.
5.4.1. Registration Update
The update interface is used by an endpoint to refresh or update its
registration with an RD. To use the interface, the endpoint sends a
POST request to the registration resource returned in the Location
header option in the response returned from the initial registration
operation.
An update MAY update the lifetime- or the context- registration
parameters "lt", "con" as in Section 5.3. Parameters that are not
being changed SHOULD NOT be included in an update. Adding parameters
that have not changed increases the size of the message but does not
have any other implications. Parameters MUST be included as query
parameters in an update operation as in Section 5.3.
A registration update resets the timeout of the registration to the
(possibly updated) lifetime of the registration, independent of
whether a "lt" parameter was given.
If the context of the registration is changed in an update explicitly
or implicitly, relative references submitted in the original
registration or later updates are resolved anew against the new
context (like in the original registration).
This operation only describes the use of POST with an empty payload.
As with modification of individual using iPATCH or PATCH as proposed
in Section 5.4.4, future standards might describe the semantics of
using content formats and payloads with the POST method to update the
links of a registration.
The update registration request interface is specified as follows:
Interaction: EP -> RD
Method: POST
URI Template: {+location}{?lt,con,extra-attrs*}
URI Template Variables:
location := This is the Location returned by the RD as a result
of a successful earlier registration.
lt := Lifetime (optional). Lifetime of the registration in
seconds. Range of 60-4294967295. If no lifetime is included,
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the previous last lifetime set on a previous update or the
original registration (falling back to 86400) SHOULD be used.
con := Context (optional). This parameter updates the context
established in the original registration to a new value. If
the parameter is set in an update, it is stored by the RD as
the new Base URI under which to interpret the links of the
registration, following the same restrictions as in the
registration. If the parameter is not set and was set
explicitly before, the previous context value is kept
unmodified. If the parameter is not set and was not set
explicitly before either, the source address and source port of
the update request are stored as the context.
extra-attrs := Additional registration attributes (optional). As
with the registration, the RD processes them if it knows their
semantics. Otherwise, unknown attributes are stored as
endpoint attributes, overriding any previously stored endpoint
attributes of the same key.
Content-Format: none (no payload)
The following response codes are defined for this interface:
Success: 2.04 "Changed" or 204 "No Content" if the update was
successfully processed.
Failure: 4.00 "Bad Request" or 400 "Bad Request". Malformed
request.
Failure: 4.04 "Not Found" or 404 "Not Found". Registration does not
exist (e.g. may have expired).
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
The following example shows an endpoint updating its registration
resource at an RD using this interface with the example location
value: /rd/4521.
Req: POST /rd/4521
Res: 2.04 Changed
The following example shows an endpoint updating its registration
resource at an RD using this interface with the example location
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value: /rd/4521. The initial registration by the client set the
following values:
o endpoint name (ep)=endpoint1
o lifetime (lt)=500
o context (con)=coap://local-proxy-old.example.com:5683
The initial state of the Resource Directory is reflected in the
following request:
Req: GET /rd-lookup/res?ep=endpoint1
Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature";anchor="coap://local-proxy-old.example.com:5683",
</sensors/light>;ct=41;rt="light-lux";if="sensor";anchor="coap://local-proxy-old.example.com:5683"
The following example shows an EP changing the context to
"coaps://new.example.com:5684":
Req: POST /rd/4521?con=coaps://new.example.com:5684
Res: 2.04 Changed
The consecutive query returns:
Req: GET /rd-lookup/res?ep=endpoint1
Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature";anchor="coaps://new.example.com:5684",
</sensors/light>;ct=41;rt="light-lux";if="sensor";anchor="coaps://new.example.com:5684",
5.4.2. Registration Removal
Although RD entries have soft state and will eventually timeout after
their lifetime, an endpoint SHOULD explicitly remove its entry from
the RD if it knows it will no longer be available (for example on
shut-down). This is accomplished using a removal interface on the RD
by performing a DELETE on the endpoint resource.
Removed endpoints are implicitly removed from the groups to which
they belong.
The removal request interface is specified as follows:
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Interaction: EP -> RD
Method: DELETE
URI Template: {+location}
URI Template Variables:
location := This is the Location returned by the RD as a result
of a successful earlier registration.
The following responses codes are defined for this interface:
Success: 2.02 "Deleted" or 204 "No Content" upon successful deletion
Failure: 4.00 "Bad Request" or 400 "Bad request". Malformed
request.
Failure: 4.04 "Not Found" or 404 "Not Found". Registration does not
exist (e.g. may have expired).
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
The following examples shows successful removal of the endpoint from
the RD with example location value /rd/4521.
Req: DELETE /rd/4521
Res: 2.02 Deleted
5.4.3. Read Endpoint Links
Some endpoints may wish to manage their links as a collection, and
may need to read the current set of links stored in the registration
resource, in order to determine link maintenance operations.
One or more links MAY be selected by using query filtering as
specified in [RFC6690] Section 4.1
If no links are selected, the Resource Directory SHOULD return an
empty payload.
The read request interface is specified as follows:
Interaction: EP -> RD
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Method: GET
URI Template: {+location}{?href,rel,rt,if,ct}
URI Template Variables:
location := This is the Location returned by the RD as a result
of a successful earlier registration.
href,rel,rt,if,ct := link relations and attributes specified in
the query in order to select particular links based on their
relations and attributes. "href" denotes the URI target of the
link. See [RFC6690] Sec. 4.1
The following responses codes are defined for this interface:
Success: 2.05 "Content" or 200 "OK" upon success with an
"application/link-format", "application/link-format+cbor", or
"application/link-format+json" payload.
Failure: 4.00 "Bad Request" or 400 "Bad Request". Malformed
request.
Failure: 4.04 "Not Found" or 404 "Not Found". Registration does not
exist (e.g. may have expired).
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
The following examples show successful read of the endpoint links
from the RD, with example location value /rd/4521.
Req: GET /rd/4521
Res: 2.01 Content
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor",
</sensors/light>;ct=41;rt="light-lux";if="sensor"
5.4.4. Update Endpoint Links
An iPATCH (or PATCH) update [RFC8132] adds, removes or changes links
of a registration by including link update information in the payload
of the update with a media type that still needs to be defined.
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6. RD Groups
This section defines the REST API for the creation, management, and
lookup of endpoints for group operations. Similar to endpoint
registration entries in the RD, groups may be created or removed.
However unlike an endpoint entry, a group entry consists of a list of
endpoints and does not have a lifetime associated with it. In order
to make use of multicast requests with CoAP, a group MAY have a
multicast address associated with it.
6.1. Register a Group
In order to create a group, a commissioning tool (CT) used to
configure groups, makes a request to the RD indicating the name of
the group to create (or update), optionally the domain the group
belongs to, and optionally the multicast address of the group. The
registration message is a list of links to registration resources of
the endpoints that belong to that group.
The commissioning tool SHOULD not send any target attributes with the
links to the registration resources, and the resource directory
SHOULD ignore any attributes that are set.
Configuration of the endpoints themselves is out of scope of this
specification. Such an interface for managing the group membership
of an endpoint has been defined in [RFC7390].
The registration request interface is specified as follows:
Interaction: CT -> RD
Method: POST
URI Template: {+rd-group}{?gp,d,con}
URI Template Variables:
rd-group := RD Group URI (mandatory). This is the location of
the RD Group REST API.
gp := Group Name (mandatory). The name of the group to be
created or replaced, unique within that domain. The maximum
length of this parameter is 63 bytes.
d := Domain (optional). The domain to which this group belongs.
The maximum length of this parameter is 63 bytes. Optional.
When this parameter is elided, the RD MAY associate the
endpoint with a configured default domain.
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con := Context (optional). This parameter sets the scheme,
address and port of the multicast address associated with the
group. When con is used, scheme and host are mandatory and
port parameter is optional.
Content-Format: application/link-format
Content-Format: application/link-format+json
Content-Format: application/link-format+cbor
The following response codes are defined for this interface:
Success: 2.01 "Created" or 201 "Created". The Location header
option MUST be returned in response to a successful group CREATE
operation. This Location MUST be a stable identifier generated by
the RD as it is used for delete operations of the group resource.
Failure: 4.00 "Bad Request" or 400 "Bad Request". Malformed
request.
Failure: 4.04 "Not Found" or 404 "Not Found". An Endpoint is not
registered in the RD (e.g. may have expired).
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
The following example shows an EP registering a group with the name
"lights" which has two endpoints. The RD group path /rd-group is an
example RD location discovered in a request similar to Figure 6.
Req: POST coap://rd.example.com/rd-group?gp=lights
&con=coap://[ff35:30:2001:db8::1]
Content-Format: 40
Payload:
</rd/4521>,
</rd/4522>
Res: 2.01 Created
Location: /rd-group/12
The href value is the path to the registration resource of the
Endpoint.
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6.2. Group Removal
A group can be removed simply by sending a removal message to the
location of the group registration resource which was returned when
initially registering the group. Removing a group MUST NOT remove
the endpoints of the group from the RD.
The removal request interface is specified as follows:
Interaction: CT -> RD
Method: DELETE
URI Template: {+location}
URI Template Variables:
location := This is the path of the group resource returned by
the RD as a result of a successful group registration.
The following responses codes are defined for this interface:
Success: 2.02 "Deleted" or 204 "No Content" upon successful deletion
Failure: 4.00 "Bad Request" or 400 "Bad Request". Malformed
request.
Failure: 4.04 "Not Found" or 404 "Not Found". Group does not exist.
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
The following examples shows successful removal of the group from the
RD with the example location value /rd-group/12.
Req: DELETE /rd-group/12
Res: 2.02 Deleted
7. RD Lookup
To discover the resources registered with the RD, a lookup interface
must be provided. This lookup interface is defined as a default, and
it is assumed that RDs may also support lookups to return resource
descriptions in alternative formats (e.g. Atom or HTML Link) or
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using more advanced interfaces (e.g. supporting context or semantic
based lookup).
RD Lookup allows lookups for groups, endpoints and resources using
attributes defined in this document and for use with the CoRE Link
Format. The result of a lookup request is the list of links (if any)
corresponding to the type of lookup. Thus, a group lookup MUST
return a list of groups, an endpoint lookup MUST return a list of
endpoints and a resource lookup MUST return a list of links to
resources.
The lookup type is selected by a URI endpoint, which is indicated by
a Resource Type as per Table 1 below:
+-------------+--------------------+-----------+
| Lookup Type | Resource Type | Mandatory |
+-------------+--------------------+-----------+
| Resource | core.rd-lookup-res | Mandatory |
| Endpoint | core.rd-lookup-ep | Mandatory |
| Group | core.rd-lookup-gp | Optional |
+-------------+--------------------+-----------+
Table 1: Lookup Types
7.1. Resource lookup
Resource lookup results in links that are semantically equivalent to
the links submitted to the RD if they were accessed on the endpoint
itself. The links and link parameters returned are equal to the
submitted ones except for anchor, which was resolved by the server
against the endpoint's context.
Links that did not have an anchor attribute are therefore returned
with the (explicitly or implicitly set) context URI of the
registration as the anchor. Links whose anchor was submitted as an
absolute URI are returned as they were registered. The hrefs of
links can always be served as they were submitted; the server MAY
return relative references in absolute form in to resource lookups,
but that results in needlessly verbose responses.
Above rules allow the client to interpret the response as links
without any further knowledge of what the RD does. The Resource
Directory MAY replace the contexts with a configured intermediate
proxy, e.g. in the case of an HTTP lookup interface for CoAP
endpoints.
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7.2. Endpoint and group lookup
Endpoint and group lookups result in links to registration resources
and group resources, respectively. Endpoint registration resources
are annotated with their endpoint names (ep), domains (d, if
present), context (con) and lifetime (lt, if present). Additional
endpoint attributes are added as link attributes to their endpoint
link unless their specification says otherwise. Group resources are
annotated with their group names (gp), domain (d, if present) and
multicast address (con, if present).
While Endpoint Lookup does expose the registration resources, the RD
does not need to make them accessible to clients. Clients SHOULD NOT
attempt to dereference or manipulate them.
7.3. Lookup filtering
Using the Accept Option, the requester can control whether this list
is returned in CoRE Link Format ("application/link-format", default)
or its alternate content-formats ("application/link-format+json" or
"application/link-format+cbor").
The page and count parameters are used to obtain lookup results in
specified increments using pagination, where count specifies how many
links to return and page specifies which subset of links organized in
sequential pages, each containing 'count' links, starting with link
zero and page zero. Thus, specifying count of 10 and page of 0 will
return the first 10 links in the result set (links 0-9). Count = 10
and page = 1 will return the next 'page' containing links 10-19, and
so on.
Multiple search criteria MAY be included in a lookup. All included
criteria MUST match for a link to be returned.
A link matches a search criterion if it has an attribute of the same
name and the same value, allowing for a trailing "*" wildcard
operator as in Section 4.1 of [RFC6690]. Attributes that are defined
as "link-type" match if the search value matches any of their values
(see Section 4.1 of [RFC6690]; eg. "?if=core.s" matches ";if="abc
core.s";"). A link also matches a search criterion if the link that
would be produced for any of its containing entities would match the
criterion: A search criterion matches an endpoint if it matches the
endpoint itself or any of the groups it is contained in, and one on a
resource if it matches the resource, the resource's endpoint, or any
of the endpoint's groups.
Note that "href" is also a valid search criterion and matches target
references. Like all search criteria, on a resource lookup it can
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match the target reference of the resource link itself, but also the
registration resource of the endpoint that registered it, or any
group resource that endpoint is contained in.
Clients that are interested in a lookup result repeatedly or
continuously can use mechanisms like ETag caching, resource
observation ([RFC7641]), or any future mechanism that might allow
more efficient observations of collections. These are advertised,
detected and used according to their own specifications and can be
used with the lookup interface as with any other resource.
The lookup interface is specified as follows:
Interaction: Client -> RD
Method: GET
URI Template: {+type-lookup-location}{?page,count,search*}
URI Template Variables:
type-lookup-location := RD Lookup URI for a given lookup type
(mandatory). The address is discovered as described in
Section 5.2.
search := Search criteria for limiting the number of results
(optional).
page := Page (optional). Parameter can not be used without the
count parameter. Results are returned from result set in pages
that contain 'count' links starting from index (page * count).
Page numbering starts with zero.
count := Count (optional). Number of results is limited to this
parameter value. If the page parameter is also present, the
response MUST only include 'count' links starting with the
(page * count) link in the result set from the query. If the
count parameter is not present, then the response MUST return
all matching links in the result set. Link numbering starts
with zero.
Content-Format: application/link-format (optional)
Content-Format: application/link-format+json (optional)
Content-Format: application/link-format+cbor (optional)
The following responses codes are defined for this interface:
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Success: 2.05 "Content" or 200 "OK" with an "application/link-
format", "application/link-format+cbor", or "application/link-
format+json" payload containing matching entries for the lookup.
The payload can contain zero links (which is an empty payload,
"80" (hex) or "[]" in the respective content format), indicating
that no entities matched the request.
Failure: No error response to a multicast request.
Failure: 4.00 "Bad Request" or 400 "Bad Request". Malformed
request.
Failure: 5.03 "Service Unavailable" or 503 "Service Unavailable".
Service could not perform the operation.
HTTP support: YES
7.4. Lookup examples
The examples in this section assume CoAP hosts with a default CoAP
port 61616. HTTP hosts are possible and do not change the nature of
the examples.
The following example shows a client performing a resource lookup
with the example resource look-up locations discovered in Figure 6:
Req: GET /rd-lookup/res?rt=temperature
Res: 2.05 Content
</temp>;rt="temperature";anchor="coap://[2001:db8:3::123]:61616"
The same lookup using the CBOR Link Format media type:
Req: GET /rd-lookup/res?rt=temperature
Accept: TBD64
Res: 2.05 Content
Content-Format: TBD64
Payload in Hex notation:
81A301652F74656D70096B74656D706572617475726503781E636F61703A2F2F5B323030
313A6462383A333A3A3132335D3A3631363136
Decoded payload:
[{1: "/temp", 9: "temperature", 3: "coap://[2001:db8:3::123]:61616"}]
A client that wants to be notified of new resources as they show up
can use observation:
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Req: GET /rd-lookup/res?rt=light
Observe: 0
Res: 2.05 Content
Observe: 23
Payload: empty
(at a later point in time)
Res: 2.05 Content
Observe: 24
Payload:
</west>;rt="light";anchor="coap://[2001:db8:3::124]",
</south>;rt="light";anchor="coap://[2001:db8:3::124]",
</east>;rt="light";anchor="coap://[2001:db8:3::124]"
The following example shows a client performing an endpoint type
lookup:
Req: GET /rd-lookup/ep?et=power-node
Res: 2.05 Content
</rd/1234>;con="coap://[2001:db8:3::127]:61616";ep="node5";
et="power-node";ct="40";lt="600",
</rd/4521>;con="coap://[2001:db8:3::129]:61616";ep="node7";
et="power-node";ct="40";lt="600";d="floor-3"
The following example shows a client performing a group lookup for
all groups:
Req: GET /rd-lookup/gp
Res: 2.05 Content
</rd-group/1>;gp="lights1";d="example.com";con="coap://[ff35:30:2001:db8::1]",
</rd-group/2>;gp="lights2";d="example.com";con="coap://[ff35:30:2001:db8::2]"
The following example shows a client performing a lookup for all
endpoints in a particular group:
Req: GET /rd-lookup/ep?gp=lights1
Res: 2.05 Content
</rd/abcd>;con="coap://[2001:db8:3::123]:61616";ep="node1";et="power-node";ct="40";lt="600",
</rd/efgh>;con="coap://[2001:db8:3::124]:61616";ep="node2";et="power-node";ct="40";lt="600"
The following example shows a client performing a lookup for all
groups the endpoint "node1" belongs to:
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Req: GET /rd-lookup/gp?ep=node1
Res: 2.05 Content
</rd-group/1>;gp="lights1"
The following example shows a client performing a paginated resource
lookup
Req: GET /rd-lookup/res?page=0&count=5
Res: 2.05 Content
</res/0>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/1>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/2>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/3>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/4>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616"
Req: GET /rd-lookup/res?page=1&count=5
Res: 2.05 Content
</res/5>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/6>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/7>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/8>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616",
</res/9>;rt=sensor;ct=60;anchor="coap://[2001:db8:3::123]:61616"
The following example shows a client performing a lookup of all
resources from endpoints of a given endpoint type. It assumes that
two endpoints (with endpoint names "sensor1" and "sensor2") have
previously registered with their respective addresses
"coap://sensor1.example.com" and "coap://sensor2.example.com", and
posted the very payload of the 6th request of section 5 of [RFC6690].
It demonstrates how the link targets stay unmodified, but the anchors
get constructed by the resource directory:
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Req: GET /rd-lookup/res?et=sensor-node
</sensors>;ct=40;title="Sensor Index";
anchor="coap://sensor1.example.com",
</sensors/temp>;rt="temperature-c";if="sensor";
anchor="coap://sensor1.example.com",
</sensors/light>;rt="light-lux";if="sensor";
anchor="coap://sensor1.example.com",
<http://www.example.com/sensors/t123>;rel="describedby";
anchor="coap://sensor1.example.com/sensors/temp",
</t>;rel="alternate";anchor="coap://sensor1.example.com/sensors/temp",
</sensors>;ct=40;title="Sensor Index";
anchor="coap://sensor2.example.com",
</sensors/temp>;rt="temperature-c";if="sensor";
anchor="coap://sensor2.example.com",
</sensors/light>;rt="light-lux";if="sensor";
anchor="coap://sensor2.example.com",
<http://www.example.com/sensors/t123>;rel="describedby";
;anchor="coap://sensor2.example.com/sensors/temp",
</t>;rel="alternate";anchor="coap://sensor2.example.com/sensors/temp"
8. Security Considerations
The security considerations as described in Section 7 of [RFC5988]
and Section 6 of [RFC6690] apply. The "/.well-known/core" resource
may be protected e.g. using DTLS when hosted on a CoAP server as
described in [RFC7252]. DTLS or TLS based security SHOULD be used on
all resource directory interfaces defined in this document.
8.1. Endpoint Identification and Authentication
An Endpoint is determined to be unique within (the domain of) an RD
by the Endpoint identifier parameter included during Registration,
and any associated TLS or DTLS security bindings. An Endpoint MUST
NOT be identified by its protocol, port or IP address as these may
change over the lifetime of an Endpoint.
Every operation performed by an Endpoint or Client on a resource
directory SHOULD be mutually authenticated using Pre-Shared Key, Raw
Public Key or Certificate based security.
Consider te following threat: two devices A and B are managed by a
single server. Both devices have unique, per-device credentials for
use with DTLS to make sure that only parties with authorization to
access A or B can do so.
Now, imagine that a malicious device A wants to sabotage the device
B. It uses its credentials during the TLS exchange. Then, it puts
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the endpoint name of device B. If the server does not check whether
the identifier provided in the DTLS handshake matches the identifier
used at the CoAP layer then it may be inclined to use the endpoint
name for looking up what information to provision to the malicious
device.
Therfore, Endpoints MUST include the Endpoint identifier in the
message, and this identifier MUST be checked by a resource directory
to match the Endpoint identifier included in the Registration
message.
8.2. Access Control
Access control SHOULD be performed separately for the RD
registration, Lookup, and group API paths, as different endpoints may
be authorized to register with an RD from those authorized to lookup
endpoints from the RD. Such access control SHOULD be performed in as
fine-grained a level as possible. For example access control for
lookups could be performed either at the domain, endpoint or resource
level.
8.3. Denial of Service Attacks
Services that run over UDP unprotected are vulnerable to unknowingly
become part of a DDoS attack as UDP does not require return
routability check. Therefore, an attacker can easily spoof the
source IP of the target entity and send requests to such a service
which would then respond to the target entity. This can be used for
large-scale DDoS attacks on the target. Especially, if the service
returns a response that is order of magnitudes larger than the
request, the situation becomes even worse as now the attack can be
amplified. DNS servers have been widely used for DDoS amplification
attacks. There is also a danger that NTP Servers could become
implicated in denial-of-service (DoS) attacks since they run on
unprotected UDP, there is no return routability check, and they can
have a large amplification factor. The responses from the NTP server
were found to be 19 times larger than the request. A Resource
Directory (RD) which responds to wild-card lookups is potentially
vulnerable if run with CoAP over UDP. Since there is no return
routability check and the responses can be significantly larger than
requests, RDs can unknowingly become part of a DDoS amplification
attack.
9. IANA Considerations
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9.1. Resource Types
"core.rd", "core.rd-group", "core.rd-lookup-ep", "core.rd-lookup-
res", and "core.rd-lookup-gp" resource types need to be registered
with the resource type registry defined by [RFC6690].
9.2. IPv6 ND Resource Directory Address Option
This document registers one new ND option type under the subregistry
"IPv6 Neighbor Discovery Option Formats":
o Resource Directory address Option (38)
9.3. RD Parameter Registry
This specification defines a new sub-registry for registration and
lookup parameters called "RD Parameters" under "CoRE Parameters".
Although this specification defines a basic set of parameters, it is
expected that other standards that make use of this interface will
define new ones.
Each entry in the registry must include * the human readable name of
the parameter, * the short name as used in query parameters or link
attributes, * indication of whether it can be passed as a query
parameter at registration of endpoints or groups, as a query
parameter in lookups, or be expressed as a link attribute, * validity
requirements if any, and * a description.
The query parameter MUST be both a valid URI query key [RFC3986] and
a parmname as used in [RFC5988].
The description must give details on which registrations they apply
to (Endpoint, group registrations or both? Can they be updated?),
and how they are to be processed in lookups.
The mechanisms around new RD parameters should be designed in such a
way that they tolerate RD implementations that are unaware of the
parameter and expose any parameter passed at registration or updates
on in endpoint lookups. (For example, if a parameter used at
registration were to be confidential, the registering endpoint should
be instructed to only set that parameter if the RD advertises support
for keeping it confidential at the discovery step.)
Initial entries in this sub-registry are as follows:
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+----------+-------+---------------+-----+--------------------------+
| Full | Short | Validity | Use | Description |
| name | | | | |
+----------+-------+---------------+-----+--------------------------+
| Endpoint | ep | | RLA | Name of the endpoint, |
| Name | | | | max 63 bytes |
| Lifetime | lt | 60-4294967295 | RLA | Lifetime of the |
| | | | | registration in seconds |
| Domain | d | | RLA | Domain to which this |
| | | | | endpoint belongs |
| Context | con | URI | RLA | The scheme, address and |
| | | | | port and path at which |
| | | | | this server is available |
| Group | gp | | RLA | Name of a group in the |
| Name | | | | RD |
| Page | page | Integer | L | Used for pagination |
| Count | count | Integer | L | Used for pagination |
| Endpoint | et | | RLA | Semantic name of the |
| Type | | | | endpoint (see Section |
| | | | | 9.4) |
+----------+-------+---------------+-----+--------------------------+
Table 2: RD Parameters
(Short: Short name used in query parameters or link attributes. Use:
R = used at registration, L = used at lookup, A = expressed in link
attribute
The descriptions for the options defined in this document are only
summarized here. To which registrations they apply and when they are
to be shown is described in the respective sections of this document.
The IANA policy for future additions to the sub-registry is "Expert
Review" as described in [RFC8126]. The evaluation should consider
formal criteria, duplication of functionality (Is the new entry
redundant with an existing one?), topical suitability (Eg. is the
described property actually a property of the endpoint and not a
property of a particular resource, in which case it should go into
the payload of the registration and need not be registered?), and the
potential for conflict with commonly used link attributes (For
example, "if" could be used as a parameter for conditional
registration if it were not to be used in lookup or attributes, but
would make a bad parameter for lookup, because a resource lookup with
an "if" query parameter could ambiguously filter by the registered
endpoint property or the [RFC6690] link attribute). It is expected
that the registry will receive between 5 and 50 registrations in
total over the next years.
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9.3.1. Full description of the "Endpoint Type" Registration Parameter
An endpoint registering at an RD can describe itself with endpoint
types, similar to how resources are described with Resource Types in
[RFC6690]. An endpoint type is expressed as a string, which can be
either a URI or one of the values defined in the Endpoint Type
subregistry. Endpoint types can be passed in the "et" query
parameter as part of extra-attrs at the Registration step, are shown
on endpoint lookups using the "et" target attribute, and can be
filtered for using "et" as a search criterion in resource and
endpoint lookup. Multiple endpoint types are given as separate query
parameters or link attributes.
Note that Endpoint Type differs from Resource Type in that it uses
multiple attributes rather than space separated values. As a result,
Resource Directory implementations automatically support correct
filtering in the lookup interfaces from the rules for unknown
endpoint attributes.
9.4. "Endpoint Type" (et=) RD Parameter values
This specification establishes a new sub-registry under "CoRE
Parameters" called '"Endpoint Type" (et=) RD Parameter values'. The
registry properties (required policy, requirements, template) are
identical to those of the Resource Type parameters in [RFC6690], in
short:
The review policy is IETF Review for values starting with "core", and
Specification Required for others.
The requirements to be enforced are:
o The values MUST be related to the purpose described in
Section 9.3.1.
o The registered values MUST conform to the ABNF reg-rel-type
definition of [RFC6690] and MUST NOT be a URI.
o It is recommended to use the period "." character for
segmentation.
The registry is initially empty.
10. Examples
Two examples are presented: a Lighting Installation example in
Section 10.1 and a LWM2M example in Section 10.2.
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10.1. Lighting Installation
This example shows a simplified lighting installation which makes use
of the Resource Directory (RD) with a CoAP interface to facilitate
the installation and start up of the application code in the lights
and sensors. In particular, the example leads to the definition of a
group and the enabling of the corresponding multicast address. No
conclusions must be drawn on the realization of actual installation
or naming procedures, because the example only "emphasizes" some of
the issues that may influence the use of the RD and does not pretend
to be normative.
10.1.1. Installation Characteristics
The example assumes that the installation is managed. That means
that a Commissioning Tool (CT) is used to authorize the addition of
nodes, name them, and name their services. The CT can be connected
to the installation in many ways: the CT can be part of the
installation network, connected by WiFi to the installation network,
or connected via GPRS link, or other method.
It is assumed that there are two naming authorities for the
installation: (1) the network manager that is responsible for the
correct operation of the network and the connected interfaces, and
(2) the lighting manager that is responsible for the correct
functioning of networked lights and sensors. The result is the
existence of two naming schemes coming from the two managing
entities.
The example installation consists of one presence sensor, and two
luminaries, luminary1 and luminary2, each with their own wireless
interface. Each luminary contains three lamps: left, right and
middle. Each luminary is accessible through one endpoint. For each
lamp a resource exists to modify the settings of a lamp in a
luminary. The purpose of the installation is that the presence
sensor notifies the presence of persons to a group of lamps. The
group of lamps consists of: middle and left lamps of luminary1 and
right lamp of luminary2.
Before commissioning by the lighting manager, the network is
installed and access to the interfaces is proven to work by the
network manager.
At the moment of installation, the network under installation is not
necessarily connected to the DNS infra structure. Therefore, SLAAC
IPv6 addresses are assigned to CT, RD, luminaries and sensor shown in
Table 3 below:
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+--------------------+----------------+
| Name | IPv6 address |
+--------------------+----------------+
| luminary1 | 2001:db8:4::1 |
| luminary2 | 2001:db8:4::2 |
| Presence sensor | 2001:db8:4::3 |
| Resource directory | 2001:db8:4::ff |
+--------------------+----------------+
Table 3: interface SLAAC addresses
In Section 10.1.2 the use of resource directory during installation
is presented.
10.1.2. RD entries
It is assumed that access to the DNS infrastructure is not always
possible during installation. Therefore, the SLAAC addresses are
used in this section.
For discovery, the resource types (rt) of the devices are important.
The lamps in the luminaries have rt: light, and the presence sensor
has rt: p-sensor. The endpoints have names which are relevant to the
light installation manager. In this case luminary1, luminary2, and
the presence sensor are located in room 2-4-015, where luminary1 is
located at the window and luminary2 and the presence sensor are
located at the door. The endpoint names reflect this physical
location. The middle, left and right lamps are accessed via path
/light/middle, /light/left, and /light/right respectively. The
identifiers relevant to the Resource Directory are shown in Table 4
below:
+----------------+------------------+---------------+---------------+
| Name | endpoint | resource path | resource type |
+----------------+------------------+---------------+---------------+
| luminary1 | lm_R2-4-015_wndw | /light/left | light |
| luminary1 | lm_R2-4-015_wndw | /light/middle | light |
| luminary1 | lm_R2-4-015_wndw | /light/right | light |
| luminary2 | lm_R2-4-015_door | /light/left | light |
| luminary2 | lm_R2-4-015_door | /light/middle | light |
| luminary2 | lm_R2-4-015_door | /light/right | light |
| Presence | ps_R2-4-015_door | /ps | p-sensor |
| sensor | | | |
+----------------+------------------+---------------+---------------+
Table 4: Resource Directory identifiers
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It is assumed that the CT knows of the RD's address, and has
performed URI discovery on it that gave a response like the one in
the Section 5.2 example.
The CT inserts the endpoints of the luminaries and the sensor in the
RD using the Context parameter (con) to specify the interface
address:
Req: POST coap://[2001:db8:4::ff]/rd
?ep=lm_R2-4-015_wndw&con=coap://[2001:db8:4::1]&d=R2-4-015
Payload:
</light/left>;rt="light",
</light/middle>;rt="light",
</light/right>;rt="light"
Res: 2.01 Created
Location: /rd/4521
Req: POST coap://[2001:db8:4::ff]/rd
?ep=lm_R2-4-015_door&con=coap://[2001:db8:4::2]&d=R2-4-015
Payload:
</light/left>;rt="light",
</light/middle>;rt="light",
</light/right>;rt="light"
Res: 2.01 Created
Location: /rd/4522
Req: POST coap://[2001:db8:4::ff]/rd
?ep=ps_R2-4-015_door&con=coap://[2001:db8:4::3]d&d=R2-4-015
Payload:
</ps>;rt="p-sensor"
Res: 2.01 Created
Location: /rd/4523
The domain name d=R2-4-015 has been added for an efficient lookup
because filtering on "ep" name is more awkward. The same domain name
is communicated to the two luminaries and the presence sensor by the
CT.
The group is specified in the RD. The Context parameter is set to
the site-local multicast address allocated to the group. In the POST
in the example below, these two endpoints and the endpoint of the
presence sensor are registered as members of the group.
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Req: POST coap://[2001:db8:4::ff]/rd-group
?gp=grp_R2-4-015&con=coap://[ff05::1]
Payload:
</rd/4521>,
</rd/4522>,
</rd/4523>
Res: 2.01 Created
Location: /rd-group/501
After the filling of the RD by the CT, the application in the
luminaries can learn to which groups they belong, and enable their
interface for the multicast address.
The luminary, knowing its domain, queries the RD for the endpoint
with rt=light and d=R2-4-015. The RD returns all endpoints in the
domain.
Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
?d=R2-4-015;rt=light
Res: 2.05 Content
</rd/4521>;con="coap://[2001:db8:4::1]",
ep="lm_R2-4-015_wndw",
</rd/4522>;con="coap://[2001:db8:4::2]",
ep="lm_R2-4-015_door"
Knowing its own IPv6 address, the luminary discovers its endpoint
name. With the endpoint name the luminary queries the RD for all
groups to which the endpoint belongs.
Req: GET coap://[2001:db8:4::ff]/rd-lookup/gp
?ep=lm_R2-4-015_wndw
Res: 2.05 Content
</rd-group/501>;gp="grp_R2-4-015";con="coap://[ff05::1]"
From the context parameter value, the luminary learns the multicast
address of the multicast group.
Alternatively, the CT can communicate the multicast address directly
to the luminaries by using the "coap-group" resource specified in
[RFC7390].
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Req: POST //[2001:db8:4::1]/coap-group
Content-Format: application/coap-group+json
{ "a": "[ff05::1]",
"n": "grp_R2-4-015"}
Res: 2.01 Created
Location-Path: /coap-group/1
Dependent on the situation, only the address, "a", or the name, "n",
is specified in the coap-group resource.
10.2. OMA Lightweight M2M (LWM2M) Example
This example shows how the OMA LWM2M specification makes use of
Resource Directory (RD).
OMA LWM2M is a profile for device services based on CoAP(OMA Name
Authority). LWM2M defines a simple object model and a number of
abstract interfaces and operations for device management and device
service enablement.
An LWM2M server is an instance of an LWM2M middleware service layer,
containing a Resource Directory along with other LWM2M interfaces
defined by the LWM2M specification.
CoRE Resource Directory (RD) is used to provide the LWM2M
Registration interface.
LWM2M does not provide for registration domains and does not
currently use the rd-group or rd-lookup interfaces.
The LWM2M specification describes a set of interfaces and a resource
model used between a LWM2M device and an LWM2M server. Other
interfaces, proxies, and applications are currently out of scope for
LWM2M.
The location of the LWM2M Server and RD URI path is provided by the
LWM2M Bootstrap process, so no dynamic discovery of the RD is used.
LWM2M Servers and endpoints are not required to implement the /.well-
known/core resource.
10.2.1. The LWM2M Object Model
The OMA LWM2M object model is based on a simple 2 level class
hierarchy consisting of Objects and Resources.
An LWM2M Resource is a REST endpoint, allowed to be a single value or
an array of values of the same data type.
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An LWM2M Object is a resource template and container type that
encapsulates a set of related resources. An LWM2M Object represents
a specific type of information source; for example, there is a LWM2M
Device Management object that represents a network connection,
containing resources that represent individual properties like radio
signal strength.
Since there may potentially be more than one of a given type object,
for example more than one network connection, LWM2M defines instances
of objects that contain the resources that represent a specific
physical thing.
The URI template for LWM2M consists of a base URI followed by Object,
Instance, and Resource IDs:
{/base-uri}{/object-id}{/object-instance}{/resource-id}{/resource-
instance}
The five variables given here are strings. base-uri can also have
the special value "undefined" (sometimes called "null" in RFC 6570).
Each of the variables object-instance, resource-id, and resource-
instance can be the special value "undefined" only if the values
behind it in this sequence also are "undefined". As a special case,
object-instance can be "empty" (which is different from "undefined")
if resource-id is not "undefined".
base-uri := Base URI for LWM2M resources or "undefined" for default
(empty) base URI
object-id := OMNA (OMA Name Authority) registered object ID (0-65535)
object-instance := Object instance identifier (0-65535) or
"undefined"/"empty" (see above)) to refer to all instances of an
object ID
resource-id := OMNA (OMA Name Authority) registered resource ID
(0-65535) or "undefined" to refer to all resources within an instance
resource-instance := Resource instance identifier or "undefined" to
refer to single instance of a resource
LWM2M IDs are 16 bit unsigned integers represented in decimal (no
leading zeroes except for the value 0) by URI format strings. For
example, a LWM2M URI might be:
/1/0/1
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The base uri is empty, the Object ID is 1, the instance ID is 0, the
resource ID is 1, and the resource instance is "undefined". This
example URI points to internal resource 1, which represents the
registration lifetime configured, in instance 0 of a type 1 object
(LWM2M Server Object).
10.2.2. LWM2M Register Endpoint
LWM2M defines a registration interface based on the REST API,
described in Section 5. The RD registration URI path of the LWM2M
Resource Directory is specified to be "/rd".
LWM2M endpoints register object IDs, for example </1>, to indicate
that a particular object type is supported, and register object
instances, for example </1/0>, to indicate that a particular instance
of that object type exists.
Resources within the LWM2M object instance are not registered with
the RD, but may be discovered by reading the resource links from the
object instance using GET with a CoAP Content-Format of application/
link-format. Resources may also be read as a structured object by
performing a GET to the object instance with a Content-Format of
senml+json.
When an LWM2M object or instance is registered, this indicates to the
LWM2M server that the object and its resources are available for
management and service enablement (REST API) operations.
LWM2M endpoints may use the following RD registration parameters as
defined in Table 2 :
ep - Endpoint Name
lt - registration lifetime
Endpoint Name, Lifetime, and LWM2M Version are mandatory parameters
for the register operation, all other registration parameters are
optional.
Additional optional LWM2M registration parameters are defined:
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+-----------+-------+-------------------------------+---------------+
| Name | Query | Validity | Description |
+-----------+-------+-------------------------------+---------------+
| Binding | b | {"U",UQ","S","SQ","US","UQS"} | Available |
| Mode | | | Protocols |
| | | | |
| LWM2M | ver | 1.0 | Spec Version |
| Version | | | |
| | | | |
| SMS | sms | | MSISDN |
| Number | | | |
+-----------+-------+-------------------------------+---------------+
Table 5: LWM2M Additional Registration Parameters
The following RD registration parameters are not currently specified
for use in LWM2M:
et - Endpoint Type
con - Context
The endpoint registration must include a payload containing links to
all supported objects and existing object instances, optionally
including the appropriate link-format relations.
Here is an example LWM2M registration payload:
</1>,</1/0>,</3/0>,</5>
This link format payload indicates that object ID 1 (LWM2M Server
Object) is supported, with a single instance 0 existing, object ID 3
(LWM2M Device object) is supported, with a single instance 0
existing, and object 5 (LWM2M Firmware Object) is supported, with no
existing instances.
10.2.3. LWM2M Update Endpoint Registration
The LwM2M update is really very similar to the registration update as
described in Section 5.4.1, with the only difference that there are
more parameters defined and available. All the parameters listed in
that section are also available with the initial registration but are
all optional:
lt - Registration Lifetime
b - Protocol Binding
sms - MSISDN
link payload - new or modified links
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A Registration update is also specified to be used to update the
LWM2M server whenever the endpoint's UDP port or IP address are
changed.
10.2.4. LWM2M De-Register Endpoint
LWM2M allows for de-registration using the delete method on the
returned location from the initial registration operation. LWM2M de-
registration proceeds as described in Section 5.4.2.
11. Acknowledgments
Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders
Brandt, Matthieu Vial, Jim Schaad, Mohit Sethi, Hauke Petersen,
Hannes Tschofenig, Sampo Ukkola, Linyi Tian, and Jan Newmarch have
provided helpful comments, discussions and ideas to improve and shape
this document. Zach would also like to thank his colleagues from the
EU FP7 SENSEI project, where many of the resource directory concepts
were originally developed.
12. Changelog
changes from -11 to -12
o added Content Model section, including ER diagram
o removed domain lookup interface; domains are now plain attributes
of groups and endpoints
o updated chapter "Finding a Resource Directory"; now distinguishes
configuration-provided, network-provided and heuristic sources
o improved text on: atomicity, idempotency, lookup with multiple
parameters, endpoint removal, simple registration
o updated LWM2M description
o clarified where relative references are resolved, and how context
and anchor interact
o new appendix on the interaction with RFCs 6690, 5988 and 3986
o lookup interface: group and endpoint lookup return group and
registration resources as link targets
o lookup interface: search parameters work the same across all
entities
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o removed all methods that modify links in an existing registration
(POST with payload, PATCH and iPATCH)
o removed plurality definition (was only needed for link
modification)
o enhanced IANA registry text
o More examples and improved text
changes from -09 to -10
o removed "ins" and "exp" link-format extensions.
o removed all text concerning DNS-SD.
o removed inconsistency in RDAO text.
o suggestions taken over from various sources
o replaced "Function Set" with "REST API", "base URI", "base path"
o moved simple registration to registration section
changes from -08 to -09
o clarified the "example use" of the base RD resource values /rd,
/rd-lookup, and /rd-group.
o changed "ins" ABNF notation.
o various editorial improvements, including in examples
o clarifications for RDAO
changes from -07 to -08
o removed link target value returned from domain and group lookup
types
o Maximum length of domain parameter 63 bytes for consistency with
group
o removed option for simple POST of link data, don't require a
.well-known/core resource to accept POST data and handle it in a
special way; we already have /rd for that
o add IPv6 ND Option for discovery of an RD
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o clarify group configuration section 6.1 that endpoints must be
registered before including them in a group
o removed all superfluous client-server diagrams
o simplified lighting example
o introduced Commissioning Tool
o RD-Look-up text is extended.
changes from -06 to -07
o added text in the discovery section to allow content format hints
to be exposed in the discovery link attributes
o editorial updates to section 9
o update author information
o minor text corrections
Changes from -05 to -06
o added note that the PATCH section is contingent on the progress of
the PATCH method
changes from -04 to -05
o added Update Endpoint Links using PATCH
o http access made explicit in interface specification
o Added http examples
Changes from -03 to -04:
o Added http response codes
o Clarified endpoint name usage
o Add application/link-format+cbor content-format
Changes from -02 to -03:
o Added an example for lighting and DNS integration
o Added an example for RD use in OMA LWM2M
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o Added Read Links operation for link inspection by endpoints
o Expanded DNS-SD section
o Added draft authors Peter van der Stok and Michael Koster
Changes from -01 to -02:
o Added a catalogue use case.
o Changed the registration update to a POST with optional link
format payload. Removed the endpoint type update from the update.
o Additional examples section added for more complex use cases.
o New DNS-SD mapping section.
o Added text on endpoint identification and authentication.
o Error code 4.04 added to Registration Update and Delete requests.
o Made 63 bytes a SHOULD rather than a MUST for endpoint name and
resource type parameters.
Changes from -00 to -01:
o Removed the ETag validation feature.
o Place holder for the DNS-SD mapping section.
o Explicitly disabled GET or POST on returned Location.
o New registry for RD parameters.
o Added support for the JSON Link Format.
o Added reference to the Groupcomm WG draft.
Changes from -05 to WG Document -00:
o Updated the version and date.
Changes from -04 to -05:
o Restricted Update to parameter updates.
o Added pagination support for the Lookup interface.
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o Minor editing, bug fixes and reference updates.
o Added group support.
o Changed rt to et for the registration and update interface.
Changes from -03 to -04:
o Added the ins= parameter back for the DNS-SD mapping.
o Integrated the Simple Directory Discovery from Carsten.
o Editorial improvements.
o Fixed the use of ETags.
o Fixed tickets 383 and 372
Changes from -02 to -03:
o Changed the endpoint name back to a single registration parameter
ep= and removed the h= and ins= parameters.
o Updated REST interface descriptions to use RFC6570 URI Template
format.
o Introduced an improved RD Lookup design as its own function set.
o Improved the security considerations section.
o Made the POST registration interface idempotent by requiring the
ep= parameter to be present.
Changes from -01 to -02:
o Added a terminology section.
o Changed the inclusion of an ETag in registration or update to a
MAY.
o Added the concept of an RD Domain and a registration parameter for
it.
o Recommended the Location returned from a registration to be
stable, allowing for endpoint and Domain information to be changed
during updates.
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o Changed the lookup interface to accept endpoint and Domain as
query string parameters to control the scope of a lookup.
13. References
13.1. Normative References
[I-D.ietf-core-links-json]
Li, K., Rahman, A., and C. Bormann, "Representing
Constrained RESTful Environments (CoRE) Link Format in
JSON and CBOR", draft-ietf-core-links-json-09 (work in
progress), July 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010, <https://www.rfc-
editor.org/info/rfc5988>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012, <https://www.rfc-
editor.org/info/rfc6570>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
13.2. Informative References
[ER] Chen, P., "The entity-relationship model---toward a
unified view of data", ACM Transactions on Database
Systems Vol. 1, pp. 9-36, DOI 10.1145/320434.320440, March
1976.
[I-D.arkko-core-dev-urn]
Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
Names for Device Identifiers", draft-arkko-core-dev-urn-04
(work in progress), July 2017.
[I-D.nottingham-rfc5988bis]
Nottingham, M., "Web Linking", draft-nottingham-
rfc5988bis-08 (work in progress), August 2017.
[I-D.silverajan-core-coap-protocol-negotiation]
Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
draft-silverajan-core-coap-protocol-negotiation-07 (work
in progress), October 2017.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999, <https://www.rfc-
editor.org/info/rfc2616>.
[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, <https://www.rfc-
editor.org/info/rfc7252>.
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[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014, <https://www.rfc-
editor.org/info/rfc7390>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015, <https://www.rfc-
editor.org/info/rfc7641>.
Appendix A. Web links and the Resource Directory
Understanding the semantics of a link-format document and its URI
references is a journey through different documents ([RFC3986]
defining URIs, [RFC6690] defining link-format documents based on
[RFC5988] which defines link headers, and [RFC7252] providing the
transport). This appendix summarizes the mechanisms and semantics at
play from an entry in ".well-known/core" to a resource lookup.
This text is primarily aimed at people entering the field of
Constrained Restful Environments from applications that previously
did not use web mechanisms.
A.1. A simple example
Let's start this example with a very simple host, "2001:db8:f0::1".
A client that follows classical CoAP Discovery ([RFC7252] Section 7),
sends the following multicast request to learn about neighbours
supporting resources with resource-type "temperature".
The client sends a link-local multicast:
GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature
RES 2.05 Content
</temp>;rt=temperature;ct=0
where the response is sent by the server, "[2001:db8:f0::1]:5683".
While the client - on the practical or implementation side - can just
go ahead and create a new request to "[2001:db8:f0::1]:5683" with
Uri-Path: "temp", the full resolution steps without any shortcuts
are:
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A.1.1. Resolving the URIs
The client parses the single returned record. The link's target
(sometimes called "href") is ""/temp"", which is a relative URI that
needs resolving. The Base URI to resolve that against is, in absence
of an "anchor" parameter, the URI of the requested resource as
described in [RFC6690] Section 2.1.
The URI of the requested resource can be composed by following the
steps of [RFC7252] section 6.5 (with an addition at the end of 8.2)
into ""coap://[2001:db8:f0::1]/.well-known/core"".
The record's target is resolved by replacing the path ""/.well-known/
core"" from the Base URI (section 5.2 [RFC3986]) with the relative
target URI ""/temp"" into ""coap://[2001:db8:f0::1]/temp"".
A.1.2. Interpreting attributes and relations
Some more information but the record's target can be obtained from
the payload: the resource type of the target is "temperature", and
its content type is text/plain (ct=0).
A relation in a web link is a three-part statement that the context
resource has a named relation to the target resource, like "_This
page_ has _its table of contents_ at _/toc.html_". In [RFC6690]
link-format documents, there is an implicit "host relation" specified
with default parameter: rel="hosts".
In our example, the context of the link is the URI of the requested
document itself. A full English expression of the "host relation"
is:
'"coap://[2001:db8:f0::1]/.well-known/core" is hosting the resource
"coap://[2001:db8:f0::1]/temp", which is of the resource type
"temperature" and can be accessed using the text/plain content
format.'
A.2. A slightly more complex example
Omitting the "rt=temperature" filter, the discovery query would have
given some more records in the payload:
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</temp>;rt=temperature;ct=0,
</light>;rt=light-lux;ct=0,
</t>;anchor="/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;anchor="/sensors/temp";
rel=describedby,
<t123.pdf>;rel=alternate;ct=65001;
anchor="http://www.example.com/sensors/t123"
Parsing the third record, the client encounters the "anchor"
parameter. It is a URI relative to the document's Base URI and is
thus resolved to ""coap://[2001:db8:f0::1]/sensors/temp"". That is
the context resource of the link, so the "rel" statement is not about
the target and the document Base URI any more, but about the target
and that address.
Thus, the third record could be read as
""coap://[2001:db8:f0::1]/sensors/temp" has an alternate
representation at "coap://[2001:db8:f0::1]/t"".
The fourth record can be read as ""coap://[2001:db8:f0::1]/sensors/
temp" is described by "http://www.example.com/sensors/t123""
In the last example the anchor is absolute, where a ""t123.pdf"" is
resolved relative to ""http://www.example.com/sensors/t123"", which
gives a statement that ""http://www.example.com/sensors/t123/
t123.pdf" is an alternate representation to
""http://www.example.com/sensors/t123" of which the content type is
PDF".
A.3. Enter the Resource Directory
The resource directory tries to carry the semantics obtainable by
classical CoAP discovery over to the resource lookup interface as
faithfully as possible.
For the following queries, we will assume that the simple host has
used Simple Registration to register at the resource directory that
was announced to it, sending this request from its UDP port
"[2001:db8:f0::1]:6553":
POST coap://[2001:db8:f01::ff]/.well-known/core?ep-simple-host1
The resource directory would have accepted the registration, and
queried the simple host's ".well-known/core" by itself. As a result,
the host is registered as an endpoint in the RD with the name
"simple-host1". The registration is active for 86400 seconds, and
the endpoint registration Base URI is ""coap://[2001:db8:f0::1]/""
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because that is the address the registration was sent from (and no
explicit "con=" was given).
If the client now queries the RD as it would previously have issued a
multicast request, it would go through the RD discovery steps by
fetching "coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-
lookup-res", obtain "coap://[2001:db8:f0::ff]/rd-lookup/res" as the
resource lookup endpoint, and issue a request to
"coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature" to receive
the following data:
</temp>;rt=temperature;ct=0;anchor="coap://[2001:db8:f0::1]"
This is not _literally_ the same response that it would have received
from a multicast request, but it would contain the (almost) same
statement:
'"coap://[2001:db8:f0::1]" is hosting the resource
"coap://[2001:db8:f0::1]/temp", which is of the resource type
"temperature" and can be accessed using the text/plain content
format.'
(The difference is whether "/" or "/.well-known/core" hosts the
resources, which is subject of ongoing discussion about RFC6690).
To complete the examples, the client could also query all resources
hosted at the endpoint with the known endpoint name "simple-host1".
A request to "coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1"
would return
</temp>;rt=temperature;ct=0;anchor="coap://[2001:db8:f0::1]",
</light>;rt=light-lux;ct=0;anchor="coap://[2001:db8:f0::1]",
</t>;anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=describedby,
<t123.pdf>;rel=alternate;ct=65001;
anchor="http://www.example.com/sensors/t123"
Note that the last link was not modified at all because its anchor
was already an absolute reference.
Had the simple host registered with an explicit context (eg.
"?ep=simple-host1&con=coap+tcp://simple-host1.example.com"), that
context would have been used to resolve the relative anchor values
instead, giving
</temp>;rt=temperature;ct=0;anchor="coap+tcp://simple-host1.example.com"
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and analogous records.
A.4. A note on differences between link-format and Link headers
While link-format and Link headers look very similar and are based on
the same model of typed links, there are some differences between
[RFC6690] and [RFC5988] that should be kept in mind when using or
implementing a Resource Directory:
o 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 headers
are practically ASCII strings that use percent encoding for non-
ASCII characters, stating the encoding explictly when required.
For example, while a Link header 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/Malmoe>;rel="live-environment-data""
o In a link-format document, if the anchor attribute is present, the
link target reference is resolved by using the the (resolved)
anchor value as Base URI for that link, while in Link headers, it
is resolved against the URI of the requested document.
This is explicit in [RFC6690] section 2.1 for link-format, and
spelled out in section B.2 of [I-D.nottingham-rfc5988bis] , which
obsoletes the older [RFC5988]. [RFC6690] is based on [RFC5988]
and has not been updated with clarifications from
[I-D.nottingham-rfc5988bis].
Appendix B. Syntax examples for Protocol Negotiation
[ This appendix should not show up in a published version of this
document. ]
The protocol negotiation that is being worked on in
[I-D.silverajan-core-coap-protocol-negotiation] makes use of the
Resource Directory.
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Until that document is update to use the latest resource-directory
specification, here are some examples of protocol negotiation with
the current Resource Directory:
An endpoint could register as follows:
Req: POST coap://rd.example.com/rd?ep=node1
&at=coap+tcp://[2001:db8:f1::2]
&at=coap://[2001:db8:f1::2]
Content-Format: 40
Payload:
</temperature>;ct=0;rt="temperature";if="core.s"
Res: 2.01 Created
Location: /rd/1234
A UDP client would then query:
Req: GET /rd-lookup/res?rt=temperature
Res: 2.05 Content
</temperature>;ct=0;rt="temperature";if="core.s";
anchor="coap://[2001:db8:f1::2]"
while a TCP capable client could say:
Req: GET /rd-lookup/res?rt=temperature&tt=tcp
Res: 2.05 Content
</temperature>;ct=0;rt="temperature";if="core.s";
anchor="coap+tcp://[2001:db8:f1::2]"
Authors' Addresses
Zach Shelby
ARM
150 Rose Orchard
San Jose 95134
USA
Phone: +1-408-203-9434
Email: zach.shelby@arm.com
Shelby, et al. Expires May 3, 2018 [Page 63]
Internet-Draft CoRE Resource Directory October 2017
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View 94043
USA
Phone: +1-707-502-5136
Email: Michael.Koster@smartthings.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Peter van der Stok
consultant
Phone: +31-492474673 (Netherlands), +33-966015248 (France)
Email: consultancy@vanderstok.org
URI: www.vanderstok.org
Christian Amsuess (editor)
Energy Harvesting Solutions
Hollandstr. 12/4
1020
Austria
Phone: +43-664-9790639
Email: c.amsuess@energyharvesting.at
Shelby, et al. Expires May 3, 2018 [Page 64]