| CoRE | Z. Shelby |
| Internet-Draft | ARM |
| Intended status: Standards Track | M. Koster |
| Expires: June 23, 2019 | SmartThings |
| C. Bormann | |
| Universitaet Bremen TZI | |
| P. van der Stok | |
| consultant | |
| C. Amsüss, Ed. | |
| December 20, 2018 |
CoRE Resource Directory
draft-ietf-core-resource-directory-18
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 contains information about resources held on other servers, allowing lookups to be performed for those resources. The input to an RD is composed of links and the output is composed of links constructed from the information stored in the RD. This document specifies the web interfaces that a Resource Directory supports for web servers to discover the RD and to register, maintain, lookup and remove information on resources. Furthermore, new target attributes useful in conjunction with an RD are defined.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
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 June 23, 2019.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 [RFC8288]. The use of Web Linking for the description and discovery of resources hosted by constrained web servers is specified by the CoRE Link Format [RFC6690]. However, [RFC6690] only describes how to discover resources from the web server that hosts them by querying /.well-known/core. In many M2M scenarios, direct discovery of resources is not practical due to sleeping nodes, disperse networks, or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which contains information about resources held on other servers, allowing lookups to be performed for those resources.
This document specifies the web interfaces that a Resource Directory supports for web servers to discover the RD and to register, maintain, lookup and remove information on resources. Furthermore, new target 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].
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119]. The term “byte” is used in its now customary sense as a synonym for “octet”.
This specification requires readers to be familiar with all the terms and concepts that are discussed in [RFC3986], [RFC8288] 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:
For several operations, interface templates are given in list form; those describe the operation participants, request codes, URIs, content formats and outcomes. Sections of those templates contain normative content about Interaction, Method, URI Template and URI Template Variables as well as the details of the Success condition. The additional sections on options like Content-Format and on Failure codes give typical cases that an implementation of the RD should deal with. Those serve to illustrate the typical responses to readers who are not yet familiar with all the details of CoAP based interfaces; they do not limit what a server may respond under atypical circumstances.
The Resource Directory is primarily a tool to make discovery operations more efficient than querying /.well-known/core on all connected devices, or across boundaries that would be limiting those operations.
It provides information about resources hosted by other devices that could otherwise only be obtained by directly querying the /.well-known/core resource on these other devices, either by a unicast request or a multicast request.
Only information SHOULD be stored in the resource directory that can be obtained by querying the described device’s /.well-known/core resource directly.
Data in the resource directory can only be provided by the device which hosts those data or a dedicated Commissioning Tool (CT). These CTs are thought to act on behalf of endpoints too constrained, or generally unable, to present that information themselves. No other client can modify data in the resource directory. Changes to the information in the Resource Directory do not propagate automatically back to the web servers from where the information originated.
The resource directory architecture is illustrated in Figure 1. A Resource Directory (RD) is used as a repository of registrations describing resources hosted on other web servers, also called endpoints (EP). An endpoint is a web server associated with a scheme, IP address and port. A physical node may host one or more endpoints. The RD implements a set of REST interfaces for endpoints to register and maintain resource directory registrations, and for endpoints to lookup resources from the RD. An RD can be logically segmented by the use of Sectors.
A mechanism to discover an RD using CoRE Link Format [RFC6690] is defined.
Registrations in the RD are soft state and need to be periodically refreshed.
An endpoint uses specific interfaces to register, update and remove a registration. It is also possible for an RD to fetch Web Links from endpoints and add their contents to resource directory registrations.
At the first registration of an endpoint, a “registration resource” is created, the location of which is returned to the registering endpoint. The registering endpoint uses this registration resource to manage the contents of registrations.
A lookup interface for discovering any of the Web Links stored in the RD is provided using the CoRE Link Format.
Registration Lookup
Interface Interface
+----+ | |
| EP |---- | |
+----+ ---- | |
--|- +------+ |
+----+ | ----| | | +--------+
| EP | ---------|-----| RD |----|-----| Client |
+----+ | ----| | | +--------+
--|- +------+ |
+----+ ---- | |
| EP |---- | |
+----+
Figure 1: The resource directory architecture.
A Registrant-EP MAY keep concurrent registrations to more than one RD at the same time if explicitly configured to do so, but that is not expected to be supported by typical EP implementations. Any such registrations are independent of each other. The usual expectation when multiple discovery mechanisms or addresses are configured is that they constitute a fall-back path for a single registration.
The Entity-Relationship (ER) models shown in Figure 2 and Figure 3 model the contents of /.well-known/core and the resource directory respectively, with entity-relationship diagrams [ER]. Entities (rectangles) are used for concepts that exist independently. Attributes (ovals) are used for concepts that exist only in connection with a related entity. Relations (diamonds) give a semantic meaning to the relation between entities. Numbers specify the cardinality of the relations.
Some of the attribute values are URIs. Those values are always full URIs and never relative references in the information model. They can, however, be expressed as relative references in serializations, and often are.
These models provide an abstract view of the information expressed in link-format documents and a Resource Directory. They cover the concepts, but not necessarily all details of an RD’s operation; they are meant to give an overview, and not be a template for implementations.
+----------------------+
| /.well-known/core |
+----------------------+
|
| 1
////////\\\\\\\
< contains >
\\\\\\\\///////
|
| 0+
+--------------------+
| link |
+--------------------+
|
| 1 oooooooo
+-----o target o
| oooooooo
oooooooooooo 0+ |
o target o--------+
o attribute o | 0+ oooooo
oooooooooooo +-----o rel o
| oooooo
|
| 1 ooooooooo
+-----o context o
ooooooooo
Figure 2: E-R Model of the content of /.well-known/core
The model shown in Figure 2 models the contents of /.well-known/core which contains:
The web server is free to choose links it deems appropriate to be exposed in its .well-known/core. Typically, the links describe resources that are served by the host, but the set can also contain links to resources on other servers (see examples in [RFC6690] page 14). The set does not necessarily contain links to all resources served by the host.
A link has the following attributes (see [RFC8288]):
+----------------------+
| resource-directory |
+----------------------+
| 1
|
|
|
|
//////\\\\
< contains >
\\\\\/////
|
0+ |
ooooooo 1 +---------------+
o base o-------| registration |
ooooooo +---------------+
| | 1
| +--------------+
oooooooo 1 | |
o href o----+ /////\\\\
oooooooo | < contains >
| \\\\\/////
oooooooo 1 | |
o ep o----+ | 0+
oooooooo | +------------------+
| | link |
oooooooo 0-1 | +------------------+
o d o----+ |
oooooooo | | 1 oooooooo
| +-----o target o
oooooooo 1 | | oooooooo
o lt o----+ ooooooooooo 0+ |
oooooooo | o target o-----+
| o attribute o | 0+ oooooo
ooooooooooo 0+ | ooooooooooo +-----o rel o
o endpoint o----+ | oooooo
o attribute o |
ooooooooooo | 1 ooooooooo
+----o context o
ooooooooo
Figure 3: E-R Model of the content of the Resource Directory
The model shown in Figure 3 models the contents of the resource directory which contains in addition to /.well-known/core:
A registration is associated with one endpoint. A registration defines a set of links as defined for /.well-known/core. A Registration has six types of attributes:
The cardinality of “base” is currently 1; future documents are invited to extend the RD specification to support multiple values (e.g. [I-D.silverajan-core-coap-protocol-negotiation]). Its value is used as a Base URI when resolving URIs in the links contained in the endpoint.
Links are modelled as they are in Figure 2.
Registration requests to the RD may arrive from link-local IP addresses. When building a Registration Base URI from that source IP address (which would become part of the resolved URIs in resource lookup), its link-local IP literal typically contains a zone identifier of the RD, and is not usable across hosts (see [RFC6874] Section 1).
Therefore, RD servers SHOULD reject registrations which use of URIs containing link-local IP addresses.
Over the last few years, mobile operators around the world have focused on development of M2M solutions in order to expand the business to the new type of users: machines. The machines are connected directly to a mobile network using an appropriate embedded wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing short and wide range wireless interfaces. From the system design point of view, the ambition is to design horizontal solutions that can enable utilization of machines in different applications depending on their current availability and capabilities as well as application requirements, thus avoiding silo like solutions. One of the crucial enablers of such design is the ability to discover resources (machines — endpoints) capable of providing required information at a given time or acting on instructions from the end users.
Imagine a scenario where endpoints installed on vehicles enable tracking of the position of these vehicles for fleet management purposes and allow monitoring of environment parameters. During the boot-up process endpoints register with a Resource Directory, which is hosted by the mobile operator or somewhere in the cloud. Periodically, these endpoints update their registration and may modify resources they offer.
When endpoints are not always connected, for example because they enter a sleep mode, a remote server is usually used to provide proxy access to the endpoints. Mobile apps or web applications for environment monitoring contact the RD, look up the endpoints capable of providing information about the environment using an appropriate set of link parameters, obtain information on how to contact them (URLs of the proxy server), and then initiate interaction to obtain information that is finally processed, displayed on the screen and usually stored in a database. Similarly, fleet management systems provide the appropriate link parameters to the RD to look up for EPs deployed on the vehicles the application is responsible for.
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.
Resources may be shared through data brokers that have no knowledge beforehand of who is going to consume the data. Resource Directory can be used to hold links about resources and services hosted anywhere to make them discoverable by a general class of applications.
For example, environmental and weather sensors that generate data for public consumption may provide data to an intermediary server, or broker. Sensor data are published to the intermediary upon changes or at regular intervals. Descriptions of the sensors that resolve to links to sensor data may be published to a Resource Directory. Applications wishing to consume the data can use RD Lookup to discover and resolve links to the desired resources and endpoints. The Resource Directory service need not be coupled with the data intermediary service. Mapping of Resource Directories to data intermediaries may be many-to-many.
Metadata in web link formats like [RFC6690] which may be internally stored as triples, or relation/attribute pairs providing metadata about resource links, need to be supported by Resource Directories . External catalogues that are represented in other formats may be converted to common web linking formats for storage and access by Resource Directories. Since it is common practice for these to be URN encoded, simple and lossless structural transforms should generally be sufficient to store external metadata in Resource Directories.
The additional features of Resource Directory allow sectors to be defined to enable access to a particular set of resources from particular applications. This provides isolation and protection of sensitive data when needed. Application groups with multicast addresses may be defined to support efficient data transport.
A (re-)starting device may want to find one or more resource directories for discovery purposes.
The device may be pre-configured to exercise specific mechanisms for finding the resource directory:
For cases where the device is not specifically configured with a way to find a resource directory, the network may want to provide a suitable default.
Finally, if neither the device nor the network offers any specific configuration, the device may want to employ heuristics to find a suitable resource directory.
The present specification does not fully define these heuristics, but suggests a number of candidates:
When answering a link-local multicast request, the RD SHOULD NOT respond with their link-local addresses but use a routable one; otherwise the registrant-ep would later need to pick an explicit base address to avoid the issue of Section 3.4.
As some of the RD addresses obtained by the methods listed here are just (more or less educated) guesses, endpoints MUST make use of any error messages to very strictly rate-limit requests to candidate IP addresses that don’t work out. For example, an ICMP Destination Unreachable message (and, in particular, the port unreachable code for this message) may indicate the lack of a CoAP server on the candidate host, or a CoAP error response code such as 4.05 “Method Not Allowed” may indicate unwillingness of a CoAP server to act as a directory server.
If multiple candidate addresses are discovered, the device may pick any of them initially, unless the discovery method indicates a more precise selection scheme.
The Resource Directory Address Option (RDAO) using IPv6 Neighbor Discovery (ND) carries information about the address of the Resource Directory (RD). This information is needed when endpoints cannot discover the Resource Directory with a link-local or realm-local scope multicast address because the endpoint and the RD are separated by a Border Router (6LBR). In many circumstances the availability of DHCP cannot be guaranteed either during commissioning of the network. The presence and the use of the RD is essential during commissioning.
It is possible to send multiple RDAO options in one message, indicating as many resource directory addresses.
The RDAO format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 3 | Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ RD Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 4: Resource Directory Address Option
This section defines the required set of REST interfaces between a Resource Directory (RD) and endpoints. Although the examples throughout this section assume the use of CoAP [RFC7252], these REST interfaces can also be realized using HTTP [RFC7230]. In all definitions in this section, both CoAP response codes (with dot notation) and HTTP response codes (without dot notation) are shown. An RD implementing this specification MUST support the discovery, registration, update, lookup, and removal interfaces defined in this section.
All operations on the contents of the Resource Directory MUST be atomic and idempotent.
A resource directory MAY make the information submitted to it available to further directories, if it can ensure that a loop does not form. The protocol used between directories to ensure loop-free operation is outside the scope of this document.
Resource Directory implementations using this specification MUST support the application/link-format content format (ct=40).
Resource Directories implementing this specification MAY support additional content formats.
Any additional content format supported by a Resource Directory implementing this specification SHOULD be able to express all the information expressible in link-format. It MAY be able to express information that is inexpressible in link-format, but those expressions SHOULD be avoided where possible.
Before an endpoint can make use of an RD, it must first know the RD’s address and port, and the URI path information for its REST APIs. This section defines discovery of the RD and its URIs using the well-known interface of the CoRE Link Format [RFC6690]. A complete set of RD discovery methods is described in Section 4.
Discovery of the RD registration URI path is performed by sending either a multicast or unicast GET request to /.well-known/core and including a Resource Type (rt) parameter [RFC6690] with the value “core.rd” in the query string. Likewise, a Resource Type parameter value of “core.rd-lookup*” is used to discover the URIs for RD Lookup operations, core.rd* is used to discover all URI paths for RD operations. Upon success, the response will contain a payload with a link format entry for each RD function discovered, indicating the URI of the RD function returned and the corresponding Resource Type. When performing multicast discovery, the multicast IP address used will depend on the scope required and the multicast capabilities of the network (see Section 9.5).
A Resource Directory MAY provide hints about the content-formats it supports in the links it exposes or registers, using the “ct” target attribute, as shown in the example below. Clients MAY use these hints to select alternate content-formats for interaction with the Resource Directory.
HTTP does not support multicast and consequently only unicast discovery can be supported using HTTP. The well-known entry points SHOULD be provided to enable unicast discovery.
An implementation of this resource directory specification MUST support query filtering for the rt parameter as defined in [RFC6690].
While the link targets in this discovery step are often expressed in path-absolute form, this is not a requirement. Clients of the RD SHOULD therefore accept URIs of all schemes they support, both as URIs and relative references, and not limit the set of discovered URIs to those hosted at the address used for URI discovery.
The URI Discovery operation can yield multiple URIs of a given resource type. The client of the RD can use any of the discovered addresses initially.
The discovery request interface is specified as follows (this is exactly the Well-Known Interface of [RFC6690] Section 4, with the additional requirement that the server MUST support query filtering):
The following response codes are defined for this interface:
The following example shows an endpoint discovering an RD using this interface, thus learning that the directory resource location, in this example, is /rd, and that the content-format delivered by the server hosting the resource is application/link-format (ct=40). Note that it is up to the RD to choose its RD locations.
Req: GET coap://[MCD1]/.well-known/core?rt=core.rd* Res: 2.05 Content </rd>;rt="core.rd";ct=40, </rd-lookup/ep>;rt="core.rd-lookup-ep";ct=40, </rd-lookup/res>;rt="core.rd-lookup-res";ct=40,
Figure 5: Example discovery exchange
The following example shows the way of indicating that a client may request alternate content-formats. The Content-Format code attribute “ct” MAY include a space-separated sequence of Content-Format codes as specified in Section 7.2.1 of [RFC7252], indicating that multiple content-formats are available. The example below shows the required Content-Format 40 (application/link-format) indicated as well as a CBOR and JSON representation from [I-D.ietf-core-links-json] (which have no numeric values assigned yet, so they are shown as TBD64 and TBD504 as in that draft). The RD resource locations /rd, and /rd-lookup are example values. The server in this example also indicates that it is capable of providing observation on resource lookups.
[ The RFC editor is asked to replace this and later occurrences of MCD1 with the assigned IPv6 site-local address for “all CoRE Resource Directories”. ]
Req: GET coap://[MCD1]/.well-known/core?rt=core.rd* Res: 2.05 Content </rd>;rt="core.rd";ct="40 65225", </rd-lookup/res>;rt="core.rd-lookup-res";ct="40 TBD64 TBD504";obs, </rd-lookup/ep>;rt="core.rd-lookup-ep";ct="40 TBD64 TBD504",
From a management and maintenance perspective, it is necessary to identify the components that constitute the RD server. The identification refers to information about for example client-server incompatibilities, supported features, required updates and other aspects. The URI discovery address, a described in section 4 of [RFC6690] can be used to find the identification.
It would typically be stored in an implementation information link (as described in [I-D.bormann-t2trg-rel-impl]):
Req: GET /.well-known/core?rel=impl-info
Res: 2.05 Content
<http://software.example.com/shiny-resource-directory/1.0beta1>;
rel="impl-info"
Note that depending on the particular server’s architecture, such a link could be anchored at the RD server’s root, at the discovery site (as in this example) or at individual RD components. The latter is to be expected when different applications are run on the same server.
After discovering the location of an RD, a registrant-ep or CT MAY register the resources of the registrant-ep using the registration interface. This interface accepts a POST from an endpoint containing the list of resources to be added to the directory as the message payload in the CoRE Link Format [RFC6690] or other representations of web links, along with query parameters indicating the name of the endpoint, and optionally the sector, lifetime and base URI of the registration. It is expected that other specifications will define further parameters (see Section 9.3). The RD then creates a new registration resource in the RD and returns its location. The receiving endpoint MUST use that location when refreshing registrations using this interface. Registration resources in the RD are kept active for the period indicated by the lifetime parameter. The creating endpoint is responsible for refreshing the registration resource within this period using either the registration or update interface. The registration interface MUST be implemented to be idempotent, so that registering twice with the same endpoint parameters ep and d (sector) does not create multiple registration resources.
The following rules apply for a registration request targeting a given (ep, d) value pair:
The posted link-format document can (and typically does) contain relative references both in its link targets and in its anchors, or contain empty anchors. The RD server needs to resolve these references in order to faithfully represent them in lookups. They are resolved against the base URI of the registration, which is provided either explicitly in the base parameter or constructed implicitly from the requester’s URI as constructed from its network address and scheme.
For media types to which Appendix C applies (i.e. documents in application/link-format), the RD only needs to accept representations in Limited Link Format as described there. Its behavior with representations outside that subset is implementation defined.
The registration request interface is specified as follows:
The following response codes are defined for this interface:
If the registration fails with a Service Unavailable response and a Max-Age option or Retry-After header, the registering endpoint SHOULD retry the operation after the time indicated. If the registration fails in another way, including request timeouts, or if the Service Unavailable error persists after several retries, or indicates a longer time than the endpoint is willing to wait, it SHOULD pick another registration URI from the “URI Discovery” step and if there is only one or the list is exhausted, pick other choices from the “Finding a Resource Directory” step. Care has to be taken to consider the freshness of results obtained earlier, e.g. of the result of a /.well-known/core response, the lifetime of an RDAO option and of DNS responses. Any rate limits and persistent errors from the “Finding a Resource Directory” step must be considered for the whole registration time, not only for a single operation.
The following example shows a registrant-ep with the name “node1” registering two resources to an RD using this interface. The location “/rd” is an example RD location discovered in a request similar to Figure 5.
Req: POST coap://rd.example.com/rd?ep=node1
Content-Format: 40
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor";
anchor="coap://spurious.example.com:5683",
</sensors/light>;ct=41;rt="light-lux";if="sensor"
Res: 2.01 Created
Location-Path: /rd/4521
Figure 6: Example registration payload
A Resource Directory may optionally support HTTP. Here is an example of almost the same registration operation above, when done using HTTP.
Req: POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1
Host: example.com
Content-Type: application/link-format
Payload:
</sensors/temp>;ct=41;rt="temperature-c";if="sensor";
anchor="coap://spurious.example.com:5683",
</sensors/light>;ct=41;rt="light-lux";if="sensor"
Res: 201 Created
Location: /rd/4521
Not all endpoints hosting resources are expected to know how to upload links to an RD as described in Section 5.3. Instead, simple endpoints can implement the Simple Registration approach described in this section. An RD implementing this specification MUST implement Simple Registration. However, there may be security reasons why this form of directory discovery would be disabled.
This approach requires that the registrant-ep makes available the hosted resources that it wants to be discovered, as links on its /.well-known/core interface as specified in [RFC6690]. The links in that document are subject to the same limitations as the payload of a registration (with respect to Appendix C).
The registrant-ep finds one or more addresses of the directory server as described in Section 4.
The registrant-ep asks the selected directory server to probe its /.well-known/core and publish the links as follows:
The registrant-ep sends (and regularly refreshes with) a POST request to the /.well-known/core URI of the directory server of choice. The body of the POST request is empty, and triggers the resource directory server to perform GET requests at the requesting registrant-ep’s /.well-known/core to obtain the link-format payload to register.
The registrant-ep includes the same registration parameters in the POST request as it would per Section 5.3. The registration base URI of the registration is taken from the registrant-ep’s network address (as is default with regular registrations).
The Resource Directory needs to query the registrant-ep’s discovery resource to determine the success of the operation. It SHOULD keep a cache of the discovery resource and not query it again as long as it is fresh.
(This is to accomodate constrained registrant devices that can not process an incoming and outgoing request at the same time. Registrants MUST be able to serve a GET request to /.well-known/core after having requested registration. Constrained devices MAY regard the initial request as temporarily failed when they need RAM occupied by their own request to serve the RD’s GET, and retry later when the RD already has a cached representation of their discovery resources. Then, the RD can reply immediately and the registrant can receive the response.)
The simple registration request interface is specified as follows:
URI Template Variables are as they are for registration in Section 5.3. The base attribute is not accepted to keep the registration interface simple; that rules out registration over CoAP-over-TCP or HTTP that would need to specify one.
The following response codes are defined for this interface:
For the second interaction triggered by the above, the registrant-ep takes the role of server and the RD the role of client. (Note that this is exactly the Well-Known Interface of [RFC6690] Section 4):
The following response codes are defined for this interface:
The registration resources MUST be deleted after the expiration of their lifetime. Additional operations on the registration resource cannot be executed because no registration location is returned.
The following example shows a registrant-ep using Simple Registration, by simply sending an empty POST to a resource directory.
Req: (to RD server from [2001:db8:2::1]) POST /.well-known/core?lt=6000&ep=node1 No payload Req: (from RD server to [2001:db8:2::1]) GET /.well-known/core Accept: 40 Res: (to the RD from [2001:db8:2::1] ) 2.05 Content Content-Format: 40 Payload: </sen/temp> Res: (from the RD to [2001:db8:2::1]) 2.04 Changed
For some applications, even Simple Registration may be too taxing for some very constrained devices, in particular if the security requirements become too onerous.
In a controlled environment (e.g. building control), the Resource Directory can be filled by a third party device, called a Commissioning Tool (CT). The commissioning tool can fill the Resource Directory from a database or other means. For that purpose scheme, IP address and port of the URI of the registered device is the value of the “base” parameter of the registration described in Section 5.3.
It should be noted that the value of the “base” parameter applies to all the links of the registration and has consequences for the anchor value of the individual links as exemplified in Appendix B. An eventual (currently non-existing) “base” attribute of the link is not affected by the value of “base” parameter in the registration.
This section describes how the registering endpoint can maintain the registrations that it created. The registering endpoint can be the registrant-ep or the CT. An endpoint SHOULD NOT use this interface for registrations that it did not create. The registrations are resources of the RD.
After the initial registration, the registering endpoint retains the returned location of the Registration Resource for further operations, including refreshing the registration in order to extend the lifetime and “keep-alive” the registration. When the lifetime of the registration has expired, the RD SHOULD NOT respond to discovery queries concerning this endpoint. The RD SHOULD continue to provide access to the Registration Resource after a registration time-out occurs in order to enable the registering endpoint to eventually refresh the registration. The RD MAY eventually remove the registration resource for the purpose of garbage collection. If the Registration Resource is removed, the corresponding endpoint will need to be re-registered.
The Registration Resource may also be used cancel the registration using DELETE, and to perform further operations beyond the scope of this specification.
These operations are described below.
The update interface is used by the registering endpoint to refresh or update its registration with an RD. To use the interface, the registering endpoint sends a POST request to the registration resource returned by the initial registration operation.
An update MAY update the lifetime- or the context- registration parameters “lt”, “base” as in Section 5.3. Parameters that are not being changed SHOULD NOT be included in an update. Adding parameters that have not changed increases the size of the message but does not have any other implications. Parameters MUST be included as query parameters in an update operation as in Section 5.3.
A registration update resets the timeout of the registration to the (possibly updated) lifetime of the registration, independent of whether a lt parameter was given.
If the context of the registration is changed in an update, relative references submitted in the original registration or later updates are resolved anew against the new context.
The registration update operation only describes the use of POST with an empty payload. Future standards might describe the semantics of using content formats and payloads with the POST method to update the links of a registration (see Section 5.4.3).
The update registration request interface is specified as follows:
The following response codes are defined for this interface:
If the registration update fails with a “Service Unavailable” response and a Max-Age option or Retry-After header, the registering endpoint SHOULD retry the operation after the time indicated. If the registration fails in another way, including request timeouts, or if the time indicated exceeds the remaining lifetime, the registering endpoint SHOULD attempt registration again.
The following example shows how the registering endpoint updates its registration resource at an RD using this interface with the example location value: /rd/4521.
Req: POST /rd/4521 Res: 2.04 Changed
The following example shows the registering endpoint updating its registration resource at an RD using this interface with the example location value: /rd/4521. The initial registration by the registering endpoint set the following values:
The initial state of the Resource Directory is reflected in the following request:
Req: GET /rd-lookup/res?ep=endpoint1 Res: 2.01 Content Payload: <coap://local-proxy-old.example.com:5683/sensors/temp>;ct=41; rt="temperature"; anchor="coap://spurious.example.com:5683", <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 the registering endpoint changing the Base URI to coaps://new.example.com:5684:
Req: POST /rd/4521?base=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:
<coaps://new.example.com:5684/sensors/temp>;ct=41;rt="temperature";
anchor="coap://spurious.example.com:5683",
<coaps://new.example.com:5684/sensors/light>;ct=41;rt="light-lux";
if="sensor"; anchor="coaps://new.example.com:5684",
Although RD registrations have soft state and will eventually timeout after their lifetime, the registering endpoint SHOULD explicitly remove an entry from the RD if it knows it will no longer be available (for example on shut-down). This is accomplished using a removal interface on the RD by performing a DELETE on the endpoint resource.
The removal request interface is specified as follows:
The following response codes are defined for this interface:
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
Additional operations on the registration can be specified in future documents, for example:
Those operations are out of scope of this document, and will require media types suitable for modifying sets of links.
To discover the resources registered with the RD, a lookup interface must be provided. This lookup interface is defined as a default, and it is assumed that RDs may also support lookups to return resource descriptions in alternative formats (e.g. JSON or CBOR link format [I-D.ietf-core-links-json]) or using more advanced interfaces (e.g. supporting context or semantic based lookup) on different resources that are discovered independently.
RD Lookup allows lookups for endpoints and resources using attributes defined in this document and for use with the CoRE Link Format. The result of a lookup request is the list of links (if any) corresponding to the type of lookup. Thus, an endpoint lookup MUST return a list of endpoints and a resource lookup MUST return a list of links to resources.
The lookup type 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 |
Resource lookup results in links that are semantically equivalent to the links submitted to the RD. The links and link parameters returned by the lookup are equal to the submitted ones, except that the target and anchor references are fully resolved.
Links that did not have an anchor attribute are therefore returned with the base URI of the registration as the anchor. Links of which href or anchor was submitted as a (full) URI are returned with these attributes unmodified.
Above rules allow the client to interpret the response as links without any further knowledge of the storage conventions of the RD. The Resource Directory MAY replace the registration base URIs with a configured intermediate proxy, e.g. in the case of an HTTP lookup interface for CoAP endpoints.
Using the Accept Option, the requester can control whether the returned list is returned in CoRE Link Format (application/link-format, default) or in alternate content-formats (e.g. from [I-D.ietf-core-links-json]).
The page and count parameters are used to obtain lookup results in specified increments using pagination, where count specifies how many links to return and page specifies which subset of links organized in sequential pages, each containing ‘count’ links, starting with link zero and page zero. Thus, specifying count of 10 and page of 0 will return the first 10 links in the result set (links 0-9). Count = 10 and page = 1 will return the next ‘page’ containing links 10-19, and so on.
Multiple search criteria MAY be included in a lookup. All included criteria MUST match for a link to be returned. The Resource Directory MUST support matching with multiple search criteria.
A link matches a search criterion if it has an attribute of the same name and the same value, allowing for a trailing “*” wildcard operator as in Section 4.1 of [RFC6690]. Attributes that are defined as “link-type” match if the search value matches any of their values (see Section 4.1 of [RFC6690]; e.g. ?if=core.s matches ;if="abc core.s";). A resource link also matches a search criterion if its endpoint would match the criterion, and vice versa, an endpoint link matches a search criterion if any of its resource links matches it.
Note that href is a valid search criterion and matches target references. Like all search criteria, on a resource lookup it can match the target reference of the resource link itself, but also the registration resource of the endpoint that registered it. Queries for resource link targets MUST be in URI form (i.e. not relative references) and are matched against a resolved link target. Queries for endpoints SHOULD be expressed in path-absolute form if possible and MUST be expressed in URI form otherwise; the RD SHOULD recognize either.
Endpoints that are interested in a lookup result repeatedly or continuously can use mechanisms like ETag caching, resource observation ([RFC7641]), or any future mechanism that might allow more efficient observations of collections. These are advertised, detected and used according to their own specifications and can be used with the lookup interface as with any other resource.
When resource observation is used, every time the set of matching links changes, or the content of a matching link changes, the RD sends a notification with the matching link set. The notification contains the successful current response to the given request, especially with respect to representing zero matching links (see “Success” item below).
The lookup interface is specified as follows:
The following responses codes are defined for this interface:
The examples in this section assume the existence of CoAP hosts with a default CoAP port 61616. HTTP hosts are possible and do not change the nature of the examples.
The following example shows a client performing a resource lookup with the example resource look-up locations discovered in Figure 5:
Req: GET /rd-lookup/res?rt=temperature
Res: 2.05 Content
<coap://[2001:db8:3::123]:61616/temp>;rt="temperature";
anchor="coap://[2001:db8:3::123]:61616"
A client that wants to be notified of new resources as they show up can use observation:
Req: GET /rd-lookup/res?rt=light
Observe: 0
Res: 2.05 Content
Observe: 23
Payload: empty
(at a later point in time)
Res: 2.05 Content
Observe: 24
Payload:
<coap://[2001:db8:3::124]/west>;rt="light";
anchor="coap://[2001:db8:3::124]",
<coap://[2001:db8:3::124]/south>;rt="light";
anchor="coap://[2001:db8:3::124]",
<coap://[2001:db8:3::124]/east>;rt="light";
anchor="coap://[2001:db8:3::124]"
The following example shows a client performing a paginated resource lookup
Req: GET /rd-lookup/res?page=0&count=5
Res: 2.05 Content
<coap://[2001:db8:3::123]:61616/res/0>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/1>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/2>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/3>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/4>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616"
Req: GET /rd-lookup/res?page=1&count=5
Res: 2.05 Content
<coap://[2001:db8:3::123]:61616/res/5>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/6>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/7>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/8>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616",
<coap://[2001:db8:3::123]:61616/res/9>;rt=sensor;ct=60;
anchor="coap://[2001:db8:3::123]:61616"
The following example shows a client performing a lookup of all resources from endpoints of all endpoints of a given endpoint type. It assumes that two endpoints (with endpoint names sensor1 and sensor2) have previously registered with their respective addresses coap://sensor1.example.com and coap://sensor2.example.com, and posted the very payload of the 6th request of section 5 of [RFC6690].
It demonstrates how absolute link targets stay unmodified, while relative ones are resolved:
Req: GET /rd-lookup/res?et=oic.d.sensor
<coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index";
anchor="coap://sensor1.example.com",
<coap://sensor1.example.com/sensors/temp>;rt="temperature-c";
if="sensor"; anchor="coap://sensor1.example.com",
<coap://sensor1.example.com/sensors/light>;rt="light-lux";
if="sensor"; anchor="coap://sensor1.example.com",
<http://www.example.com/sensors/t123>;rel="describedby";
anchor="coap://sensor1.example.com/sensors/temp",
<coap://sensor1.example.com/t>;rel="alternate";
anchor="coap://sensor1.example.com/sensors/temp",
<coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index";
anchor="coap://sensor2.example.com",
<coap://sensor2.example.com/sensors/temp>;rt="temperature-c";
if="sensor"; anchor="coap://sensor2.example.com",
<coap://sensor2.example.com/sensors/light>;rt="light-lux";
if="sensor"; anchor="coap://sensor2.example.com",
<http://www.example.com/sensors/t123>;rel="describedby";
anchor="coap://sensor2.example.com/sensors/temp",
<coap://sensor2.example.com/t>;rel="alternate";
anchor="coap://sensor2.example.com/sensors/temp"
The endpoint lookup returns registration resources which can only be manipulated by the registering endpoint.
Endpoint registration resources are annotated with their endpoint names (ep), sectors (d, if present) and registration base URI (base; reports the registrant-ep’s address if no explicit base was given) as well as a constant resource type (rt=”core.rd-ep”); the lifetime (lt) is not reported. Additional endpoint attributes are added as target attributes to their endpoint link unless their specification says otherwise.
Links to endpoints SHOULD be presented in path-absolute form or, if required, as absolute references. (This avoids the RFC6690 ambiguities.)
While Endpoint Lookup does expose the registration resources, the RD does not need to make them accessible to clients. Clients SHOULD NOT attempt to dereference or manipulate them.
A Resource Directory can report endpoints in lookup that are not hosted at the same address. Lookup clients MUST be prepared to see arbitrary URIs as registration resources in the results and treat them as opaque identifiers; the precise semantics of such links are left to future specifications.
The following example shows a client performing an endpoint type (et) lookup with the value oic.d.sensor (which is currently a registered rt value):
Req: GET /rd-lookup/ep?et=oic.d.sensor Res: 2.05 Content </rd/1234>;base="coap://[2001:db8:3::127]:61616";ep="node5"; et="oic.d.sensor";ct="40";rt="core.rd-ep", </rd/4521>;base="coap://[2001:db8:3::129]:61616";ep="node7"; et="oic.d.sensor";ct="40";d="floor-3";rt="core.rd-ep"
The Resource Directory (RD) provides assistance to applications situated on a selection of nodes to discover endpoints on connected nodes. This section discusses different security aspects of accessing the RD.
The contents of the RD are inserted in two ways:
In both cases, the nodes filling the RD should be authenticated and authorized to change the contents of the RD. An Authorization Server (AS) is responsible to assign a token to the registering node to authorize the node to discover or register endpoints in a given RD [I-D.ietf-ace-oauth-authz].
It can be imagined that an installation is divided in a set of security regions, each one with its own RD(s) to discover the endpoints that are part of a given security region. An endpoint that wants to discover an RD, responsible for a given region, needs to be authorized to learn the contents of a given RD. Within a region, for a given RD, a more fine-grained security division is possible based on the values of the endpoint registration parameters. Authorization to discover endpoints with a given set of filter values is recommended for those cases.
When a node registers its endpoints, criteria are needed to authorize the node to enter them. An important aspect is the uniqueness of the (endpoint name, and optional sector) pair within the RD. Consider the two cases separately: (1) CT registers endpoints, and (2) the registering node registers its own endpoint(s).
A separate document needs to specify these aspects to ensure interoperability between registering nodes and RD. The subsections below give some hints how to handle a subset of the different aspects.
The Resource Server (RS) discussed in [I-D.ietf-ace-oauth-authz] is equated to the RD. The client (C) needs to discover the RD as discussed in Section 4. C can discover the related AS by sending a request to the RD. The RD denies the request by sending the address of the related AS, as discussed in section 5.1 of [I-D.ietf-ace-oauth-authz]. The client MUST send an authorization request to the AS. When appropriate, the AS returns a token that specifies the authorization permission which needs to be specified in a separate document.
The authorized parameter values for the queries by a given endpoint must be registered by the AS. The AS communicates the parameter values in the token. A separate document needs to specify the parameter value combinations and their storage in the token. The RD decodes the token and checks the validity of the queries of the client.
This section only considers the assignment of a name to the endpoint based on an automatic mechanism without use of AS. More elaborate protocols are out of scope. The registering endpoint is authorized by the AS to discover the RD and add registrations. A token is provided by the AS and communicated from registering endpoint to RD. It is assumed that DTLS is used to secure the channel between registering endpoint and RD, where the registering endpoint is the DTLS client. Assuming that the client is provided by a certificate at manufacturing time, the certificate is uniquely identified by the CN field and the serial number. The RD can assign a unique endpoint name by using the certificate identifier as endpoint name. Proof of possession of the endpoint name by the registering endpoint is checked by encrypting the certificate identifier with the private key of the registering endpoint, which the RD can decrypt with the public key stored in the certificate. Even simpler, the authorized registering endpoint can generate a random number (or string) that identifies the endpoint. The RD can check for the improbable replication of the random value. The RD MUST check that registering endpoint uses only one random value for each authorized endpoint.
The security considerations as described in Section 5 of [RFC8288] 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.
An Endpoint (name, sector) pair is unique within the et of endpoints registered by the RD. An Endpoint MUST NOT be identified by its protocol, port or IP address as these may change over the lifetime of an Endpoint.
Every operation performed by an Endpoint on a resource directory SHOULD be mutually authenticated using Pre-Shared Key, Raw Public Key or Certificate based security.
Consider the following threat: two devices A and B are registered at a single server. Both devices have unique, per-device credentials for use with DTLS to make sure that only parties with authorization to access A or B can do so.
Now, imagine that a malicious device A wants to sabotage the device B. It uses its credentials during the DTLS exchange. Then, it specifies the endpoint name of device B as the name of its own endpoint in device A. If the server does not check whether the identifier provided in the DTLS handshake matches the identifier used at the CoAP layer then it may be inclined to use the endpoint name for looking up what information to provision to the malicious device.
Section 7.3 specifies an example that removes this threat for endpoints that have a certificate installed.
Access control SHOULD be performed separately for the RD registration and Lookup API paths, as different endpoints may be authorized to register with an RD from those authorized to lookup endpoints from the RD. Such access control SHOULD be performed in as fine-grained a level as possible. For example access control for lookups could be performed either at the sector, endpoint or resource level.
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.
IANA is asked to enter the following values into the Resource Type (rt=) Link Target Attribute Values sub-registry of the Constrained Restful Environments (CoRE) Parameters registry defined in [RFC6690]:
| Value | Description | Reference |
|---|---|---|
| core.rd | Directory resource of an RD | RFCTHIS Section 5.2 |
| core.rd-lookup-res | Resource lookup of an RD | RFCTHIS Section 5.2 |
| core.rd-lookup-ep | Endpoint lookup of an RD | RFCTHIS Section 5.2 |
| core.rd-ep | Endpoint resource of an RD | RFCTHIS Section 6 |
This document registers one new ND option type under the sub-registry “IPv6 Neighbor Discovery Option Formats”:
This specification defines a new sub-registry for registration and lookup parameters called “RD Parameters” under “CoRE Parameters”. Although this specification defines a basic set of parameters, it is expected that other standards that make use of this interface will define new ones.
Each entry in the registry must include
The query parameter MUST be both a valid URI query key [RFC3986] and a token as used in [RFC8288].
The description must give details on whether the parameter can be updated, and how it is to be processed in lookups.
The mechanisms around new RD parameters should be designed in such a way that they tolerate RD implementations that are unaware of the parameter and expose any parameter passed at registration or updates on in endpoint lookups. (For example, if a parameter used at registration were to be confidential, the registering endpoint should be instructed to only set that parameter if the RD advertises support for keeping it confidential at the discovery step.)
Initial entries in this sub-registry are as follows:
| Full name | Short | Validity | Use | Description |
|---|---|---|---|---|
| Endpoint Name | ep | RLA | Name of the endpoint, max 63 bytes | |
| Lifetime | lt | 60-4294967295 | R | Lifetime of the registration in seconds |
| Sector | d | RLA | Sector to which this endpoint belongs | |
| Registration Base URI | base | URI | RLA | The scheme, address and port and path at which this server is available |
| Page | page | Integer | L | Used for pagination |
| Count | count | Integer | L | Used for pagination |
| Endpoint Type | et | RLA | Semantic name of the endpoint (see Section 9.4) |
(Short: Short name used in query parameters or target attributes. Use: R = used at registration, L = used at lookup, A = expressed in target attribute
The descriptions for the options defined in this document are only summarized here. To which registrations they apply and when they are to be shown is described in the respective sections of this document.
The IANA policy for future additions to the sub-registry is “Expert Review” as described in [RFC8126]. The evaluation should consider formal criteria, duplication of functionality (Is the new entry redundant with an existing one?), topical suitability (E.g. is the described property actually a property of the endpoint and not a property of a particular resource, in which case it should go into the payload of the registration and need not be registered?), and the potential for conflict with commonly used target attributes (For example, if could be used as a parameter for conditional registration if it were not to be used in lookup or attributes, but would make a bad parameter for lookup, because a resource lookup with an if query parameter could ambiguously filter by the registered endpoint property or the [RFC6690] target attribute). It is expected that the registry will receive between 5 and 50 registrations in total over the next years.
An endpoint registering at an RD can describe itself with endpoint types, similar to how resources are described with Resource Types in [RFC6690]. An endpoint type is expressed as a string, which can be either a URI or one of the values defined in the Endpoint Type sub-registry. Endpoint types can be passed in the et query parameter as part of extra-attrs at the Registration step, are shown on endpoint lookups using the et target attribute, and can be filtered for using et as a search criterion in resource and endpoint lookup. Multiple endpoint types are given as separate query parameters or link attributes.
Note that Endpoint Type differs from Resource Type in that it uses multiple attributes rather than space separated values. As a result, Resource Directory implementations automatically support correct filtering in the lookup interfaces from the rules for unknown endpoint attributes.
This specification establishes a new sub-registry under “CoRE Parameters” called ‘“Endpoint Type” (et=) RD Parameter values’. The registry properties (required policy, requirements, template) are identical to those of the Resource Type parameters in [RFC6690], in short:
The review policy is IETF Review for values starting with “core”, and Specification Required for others.
The requirements to be enforced are:
The registry initially contains one value:
IANA has assigned the following multicast addresses for use by CoAP nodes:
IPv4 – “all CoRE resource directories” address, from the “IPv4 Multicast Address Space Registry” equal to “All CoAP Nodes”, 224.0.1.187. As the address is used for discovery that may span beyond a single network, it has come from the Internetwork Control Block (224.0.1.x, RFC 5771).
IPv6 – “all CoRE resource directories” address MCD1 (suggestions FF0X::FE), from the “IPv6 Multicast Address Space Registry”, in the “Variable Scope Multicast Addresses” space (RFC 3307). Note that there is a distinct multicast address for each scope that interested CoAP nodes should listen to; CoAP needs the Link-Local and Site-Local scopes only.
Two examples are presented: a Lighting Installation example in Section 10.1 and a LWM2M example in Section 10.2.
This example shows a simplified lighting installation which makes use of the Resource Directory (RD) with a CoAP interface to facilitate the installation and start-up of the application code in the lights and sensors. In particular, the example leads to the definition of a group and the enabling of the corresponding multicast address as described in Appendix A. No conclusions must be drawn on the realization of actual installation or naming procedures, because the example only “emphasizes” some of the issues that may influence the use of the RD and does not pretend to be normative.
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:
| Name | IPv6 address |
|---|---|
| luminary1 | 2001:db8:4::1 |
| luminary2 | 2001:db8:4::2 |
| Presence sensor | 2001:db8:4::3 |
| Resource directory | 2001:db8:4::ff |
In Section 10.1.2 the use of resource directory during installation is presented.
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 sensor | ps_R2-4-015_door | /ps | p-sensor |
It is assumed that the CT knows the RD’s address, and has performed URI discovery on it that returned a response like the one in the Section 5.2 example.
The CT inserts the endpoints of the luminaries and the sensor in the RD using the registration base URI parameter (base) to specify the interface address:
Req: POST coap://[2001:db8:4::ff]/rd ?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015 Payload: </light/left>;rt="light", </light/middle>;rt="light", </light/right>;rt="light" Res: 2.01 Created Location-Path: /rd/4521
Req: POST coap://[2001:db8:4::ff]/rd ?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015 Payload: </light/left>;rt="light", </light/middle>;rt="light", </light/right>;rt="light" Res: 2.01 Created Location-Path: /rd/4522
Req: POST coap://[2001:db8:4::ff]/rd ?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]d&d=R2-4-015 Payload: </ps>;rt="p-sensor" Res: 2.01 Created Location-Path: /rd/4523
The sector name d=R2-4-015 has been added for an efficient lookup because filtering on “ep” name is more awkward. The same sector name is communicated to the two luminaries and the presence sensor by the CT.
The group is specified in the RD. The base parameter is set to the site-local multicast address allocated to the group. In the POST in the example below, the resources supported by all group members are published.
Req: POST coap://[2001:db8:4::ff]/rd ?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1] Payload: </light/left>;rt="light", </light/middle>;rt="light", </light/right>;rt="light" Res: 2.01 Created Location-Path: /rd/501
After the filling of the RD by the CT, the application in the luminaries can learn to which groups they belong, and enable their interface for the multicast address.
The luminary, knowing its sector and being configured to join any group containing lights, searches for candidate groups and joins them:
Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
?d=R2-4-015&et=core.rd-group&rt=light
Res: 2.05 Content
</rd/501>;ep="grp_R2-4-015";et="core.rd-group";
base="coap://[ff05::1]";rt="core.rd-ep"
From the returned base parameter value, the luminary learns the multicast address of the multicast group.
Alternatively, the CT can communicate the multicast address directly to the luminaries by using the “coap-group” resource specified in [RFC7390].
Req: POST coap://[2001:db8:4::1]/coap-group
Content-Format: application/coap-group+json
Payload:
{ "a": "[ff05::1]", "n": "grp_R2-4-015"}
Res: 2.01 Created
Location-Path: /coap-group/1
Dependent on the situation, only the address, “a”, or the name, “n”, is specified in the coap-group resource.
The presence sensor can learn the presence of groups that support resources with rt=light in its own sector by sending the same request, as used by the luminary. The presence sensor learns the multicast address to use for sending messages to the luminaries.
This example shows how the OMA LWM2M specification makes use of Resource Directory (RD).
OMA LWM2M is a profile for device services based on CoAP(OMA Name Authority). LWM2M defines a simple object model and a number of abstract interfaces and operations for device management and device service enablement.
An LWM2M server is an instance of an LWM2M middleware service layer, containing a Resource Directory along with other LWM2M interfaces defined by the LWM2M specification.
CoRE Resource Directory (RD) is used to provide the LWM2M Registration interface.
LWM2M does not provide for registration sectors and does not currently use the rd-lookup interface.
The LWM2M specification describes a set of interfaces and a resource model used between a LWM2M device and an LWM2M server. Other interfaces, proxies, and applications are currently out of scope for LWM2M.
The location of the LWM2M Server and RD URI path is provided by the LWM2M Bootstrap process, so no dynamic discovery of the RD is used. LWM2M Servers and endpoints are not required to implement the /.well-known/core resource.
The OMA LWM2M object model is based on a simple 2 level class hierarchy consisting of Objects and Resources.
An LWM2M Resource is a REST endpoint, allowed to be a single value or an array of values of the same data type.
An LWM2M Object is a resource template and container type that encapsulates a set of related resources. An LWM2M Object represents a specific type of information source; for example, there is a LWM2M Device Management object that represents a network connection, containing resources that represent individual properties like radio signal strength.
Since there may potentially be more than one of a given type object, for example more than one network connection, LWM2M defines instances of objects that contain the resources that represent a specific physical thing.
The URI template for LWM2M consists of a base URI followed by Object, Instance, and Resource IDs:
{/base-uri}{/object-id}{/object-instance}{/resource-id}{/resource-instance}
The five variables given here are strings. base-uri can also have the special value “undefined” (sometimes called “null” in RFC 6570). Each of the variables object-instance, resource-id, and resource-instance can be the special value “undefined” only if the values behind it in this sequence also are “undefined”. As a special case, object-instance can be “empty” (which is different from “undefined”) if resource-id is not “undefined”.
base-uri := Base URI for LWM2M resources or “undefined” for default (empty) base URI
object-id := OMNA (OMA Name Authority) registered object ID (0-65535)
object-instance := Object instance identifier (0-65535) or “undefined”/”empty” (see above)) to refer to all instances of an object ID
resource-id := OMNA (OMA Name Authority) registered resource ID (0-65535) or “undefined” to refer to all resources within an instance
resource-instance := Resource instance identifier or “undefined” to refer to single instance of a resource
LWM2M IDs are 16 bit unsigned integers represented in decimal (no leading zeroes except for the value 0) by URI format strings. For example, a LWM2M URI might be:
/1/0/1
The base uri is empty, the Object ID is 1, the instance ID is 0, the resource ID is 1, and the resource instance is “undefined”. This example URI points to internal resource 1, which represents the registration lifetime configured, in instance 0 of a type 1 object (LWM2M Server Object).
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:
| Name | Query | Validity | Description |
|---|---|---|---|
| Binding Mode | b | {“U”,UQ”,”S”,”SQ”,”US”,”UQS”} | Available Protocols |
| LWM2M Version | ver | 1.0 | Spec Version |
| SMS Number | sms | MSISDN |
The following RD registration parameters are not currently specified for use in LWM2M:
et - Endpoint Type base - Registration Base URI
The endpoint registration must include a payload containing links to all supported objects and existing object instances, optionally including the appropriate link-format relations.
Here is an example LWM2M registration payload:
</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.
The LwM2M update is really very similar to the registration update as described in Section 5.4.1, with the only difference that there are more parameters defined and available. All the parameters listed in that section are also available with the initial registration but are all optional:
lt - Registration Lifetime b - Protocol Binding sms - MSISDN link payload - new or modified links
A Registration update is also specified to be used to update the LWM2M server whenever the endpoint’s UDP port or IP address are changed.
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.
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.
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| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3986] | Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005. |
| [RFC6570] | Gregorio, J., Fielding, R., Hadley, M., Nottingham, M. and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, March 2012. |
| [RFC6690] | Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012. |
| [RFC6763] | Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013. |
| [RFC8126] | Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017. |
The RD-Groups usage pattern allows announcing application groups inside a Resource Directory.
Groups are represented by endpoint registrations. Their base address is a multicast address, and they SHOULD be entered with the endpoint type core.rd-group. The endpoint name can also be referred to as a group name in this context.
The registration is inserted into the RD by a Commissioning Tool, which might also be known as a group manager here. It performs third party registration and registration updates.
The links it registers SHOULD be available on all members that join the group. Depending on the application, members that lack some resource MAY be permissible if requests to them fail gracefully.
The following example shows a CT registering a group with the name “lights” which provides two resources. The directory resource path /rd is an example RD location discovered in a request similar to Figure 5.
Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
&base=coap://[ff35:30:2001:db8::1]
Content-Format: 40
Payload:
</light>;rt="light";if="core.a",
</color-temperature>;if="core.p";u="K"
Res: 2.01 Created
Location-Path: /rd/12
In this example, the group manager can easily permit devices that have no writable color-temperature to join, as they would still respond to brightness changing commands. Had the group instead contained a single resource that sets brightness and color temperature atomically, endpoints would need to support both properties.
The resources of a group can be looked up like any other resource, and the group registrations (along with any additional registration parameters) can be looked up using the endpoint lookup interface.
The following example shows a client performing and endpoint lookup for all groups.
Req: GET /rd-lookup/ep?et=core.rd-group
Res: 2.01 Content
Payload:
</rd/501>;ep="GRP_R2-4-015";et="core.rd-group";
base="coap://[ff05::1]",
</rd/12>;ep=lights&et=core.rd-group;
base="coap://[ff35:30:2001:db8::1]";rt="core.rd-ep"
The following example shows a client performing a lookup of all resources of all endpoints (groups) with et=core.rd-group.
Req: GET /rd-lookup/res?et=core.rd-group
<coap://[ff35:30:2001:db8::1]/light>;rt="light";if="core.a";
et="core.rd-group";anchor="coap://[ff35:30:2001:db8::1]",
<coap://[ff35:30:2001:db8::1]/color-temperature>;if="core.p";u="K";
et="core.rd-group";
anchor="coap://[ff35:30:2001:db8::1]"
Understanding the semantics of a link-format document and its URI references is a journey through different documents ([RFC3986] defining URIs, [RFC6690] defining link-format documents based on [RFC8288] which defines link headers, and [RFC7252] providing the transport). This appendix summarizes the mechanisms and semantics at play from an entry in .well-known/core to a resource lookup.
This text is primarily aimed at people entering the field of Constrained Restful Environments from applications that previously did not use web mechanisms.
The explanation of the steps makes some shortcuts in the more confusing details of [RFC6690], which are justified as all examples being in Limited Link Format.
Let’s start this example with a very simple host, 2001:db8:f0::1. A client that follows classical CoAP Discovery ([RFC7252] Section 7), sends the following multicast request to learn about neighbours supporting resources with resource-type “temperature”.
The client sends a link-local multicast:
GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature RES 2.05 Content </temp>;rt=temperature;ct=0
where the response is sent by the server, [2001:db8:f0::1]:5683.
While the client – on the practical or implementation side – can just go ahead and create a new request to [2001:db8:f0::1]:5683 with Uri-Path: temp, the full resolution steps for insertion into and retrieval from the RD without any shortcuts are:
The client parses the single returned record. The link’s target (sometimes called “href”) is “/temp”, which is a relative URI that needs resolving. The base URI <coap://[ff02::fd]:5683/.well-known/core> is used to resolve the reference /temp against.
The Base URI of the requested resource can be composed from the header options of the CoAP GET request by following the steps of [RFC7252] section 6.5 (with an addition at the end of 8.2) into “coap://[2001:db8:f0::1]/.well-known/core”.
Because “/temp” starts with a single slash, the record’s target is resolved by replacing the path “/.well-known/core” from the Base URI (section 5.2 [RFC3986]) with the relative target URI “/temp” into “coap://[2001:db8:f0::1]/temp”.
Some more information but the record’s target can be obtained from the payload: the resource type of the target is “temperature”, and its content type is text/plain (ct=0).
A relation in a web link is a three-part statement that specifies a named relation between the so-called “context resource” and the target resource, like “This page has its table of contents at /toc.html”. In link format documents, there is an implicit “host relation” specified with default parameter: rel=”hosts”.
In our example, the context resource of the link is the URI specified in the GET request “coap:://[2001:db8:f0::1]/.well-known/core”. A full English expression of the “host relation” is:
‘coap://[2001:db8:f0::1]/.well-known/core is hosting the resource coap://[2001:db8:f0::1]/temp, which is of the resource type “temperature” and can be accessed using the text/plain content format.’
Omitting the rt=temperature filter, the discovery query would have given some more records in the payload:
GET coap://[ff02::fd]:5683/.well-known/core
RES 2.05 Content
</temp>;rt=temperature;ct=0,
</light>;rt=light-lux;ct=0,
</t>;anchor="/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;anchor="/sensors/temp";
rel="describedby"
Parsing the third record, the client encounters the “anchor” parameter. It is a URI relative to the Base URI of the request and is thus resolved to “coap://[2001:db8:f0::1]/sensors/temp”. That is the context resource of the link, so the “rel” statement is not about the target and the Base URI any more, but about the target and the resolved URI. Thus, the third record could be read as “coap://[2001:db8:f0::1]/sensors/temp has an alternate representation at coap://[2001:db8:f0::1]/t”.
Following the same resolution steps, the fourth record can be read as “coap://[2001:db8:f0::1]/sensors/temp is described by http://www.example.com/sensors/t123”.
The resource directory tries to carry the semantics obtainable by classical CoAP discovery over to the resource lookup interface as faithfully as possible.
For the following queries, we will assume that the simple host has used Simple Registration to register at the resource directory that was announced to it, sending this request from its UDP port [2001:db8:f0::1]:6553:
POST coap://[2001:db8:f01::ff]/.well-known/core?ep=simple-host1
The resource directory would have accepted the registration, and queried the simple host’s .well-known/core by itself. As a result, the host is registered as an endpoint in the RD with the name “simple-host1”. The registration is active for 90000 seconds, and the endpoint registration Base URI is “coap://[2001:db8:f0::1]” following the resolution steps described in Appendix B.1.1. It should be remarked that the Base URI constructed that way always yields a URI of the form: scheme://authority without path suffix.
If the client now queries the RD as it would previously have issued a multicast request, it would go through the RD discovery steps by fetching coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-lookup-res, obtain coap://[2001:db8:f0::ff]/rd-lookup/res as the resource lookup endpoint, and issue a request to coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature to receive the following data:
<coap://[2001:db8:f0::1]/temp>;rt=temperature;ct=0;
anchor="coap://[2001:db8:f0::1]"
This is not literally the same response that it would have received from a multicast request, but it contains the equivalent statement:
‘coap://[2001:db8:f0::1] is hosting the resource coap://[2001:db8:f0::1]/temp, which is of the resource type “temperature” and can be accessed using the text/plain content format.’
(The difference is whether / or /.well-known/core hosts the resources, which does not matter in this application; if it did, the endpoint would have been more explicit. Actually, /.well-known/core does NOT host the resource but stores a URI reference to the resource.)
To complete the examples, the client could also query all resources hosted at the endpoint with the known endpoint name “simple-host1”. A request to coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1 would return
<coap://[2001:db8:f0::1]/temp>;rt=temperature;ct=0;
anchor="coap://[2001:db8:f0::1]",
<coap://[2001:db8:f0::1]/light>;rt=light-lux;ct=0;
anchor="coap://[2001:db8:f0::1]",
<coap://[2001:db8:f0::1]/t>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel="describedby"
All the target and anchor references are already in absolute form there, which don’t need to be resolved any further.
Had the simple host done an equivalent full registration with a base= parameter (e.g. ?ep=simple-host1&base=coap+tcp://simple-host1.example.com), that context would have been used to resolve the relative anchor values instead, giving
<coap+tcp://simple-host1.example.com/temp>;rt=temperature;ct=0;
anchor="coap+tcp://simple-host1.example.com"
and analogous records.
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 [RFC8288], which are dealt with differently:
The CoRE Link Format as described in [RFC6690] has been interpreted differently by implementers, and a strict implementation rules out some use cases of a Resource Directory (e.g. base values with path components).
This appendix describes a subset of link format documents called Limited Link Format. The rules herein are not very limiting in practice – all examples in RFC6690, and all deployments the authors are aware of already stick to them – but ease the implementation of resource directory servers.
It is applicable to representations in the application/link-format media type, and any other media types that inherit [RFC6690] Section 2.1.
A link format representation is in Limited Link format if, for each link in it, the following applies: