RFC 9176
Constrained RESTful Environments (CoRE) Resource Directory
Constrained RESTful Environments (CoRE) Resource Directory ๐
Okay, the title block first, bestie ๐
- RFC number: 9176
- Title: Constrained RESTful Environments (CoRE) Resource Directory
- Editors and authors: C. Amsรผss, Ed. (editor), Z. Shelby (Edge Impulse), M. Koster (PassiveLogic), C. Bormann (Universitรคt Bremen TZI), P. van der Stok (vanderstok consultancy)
- Category: Standards Track
- ISSN: 2070-1721
- Date: April 2022
- Obsoletes / Updates: the header doesn't list any, so this one doesn't obsolete or update anything else ๐ง
Abstract โจ
Okay so, in a lot of IoT setups, finding resources directly just isn't realistic ๐ญ Some nodes are asleep, and some networks are super inefficient with multicast. The fix is an entity called a Resource Directory (RD) ๐ฆ It stores info about resources that live on other servers, so you can run lookups for those resources. Links go in, and links come out, built from what the RD has stored. This doc spells out the web interfaces an RD supports, so web servers can find the RD and register, maintain, look up, and remove resource info. It also defines new target attributes that work together with an RD. It's giving organized and it's giving useful ๐
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9176.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.
Table of Contents ๐
1. Introduction
2. Terminology
3. Architecture and Use Cases
3.1. Principles
3.2. Architecture
3.3. RD Content Model
3.4. Link-Local Addresses and Zone Identifiers
3.5. Use Case: Cellular M2M
3.6. Use Case: Home and Building Automation
3.7. Use Case: Link Catalogues
4. RD Discovery and Other Interface-Independent Components
4.1. Finding a Resource Directory
4.1.1. Resource Directory Address Option (RDAO)
4.1.2. Using DNS-SD to Discover a Resource Directory
4.2. Payload Content Formats
4.3. URI Discovery
5. Registration
5.1. Simple Registration
5.2. Third-Party Registration
5.3. Operations on the Registration Resource
5.3.1. Registration Update
5.3.2. Registration Removal
5.3.3. Further Operations
5.3.4. Request Freshness
6. RD Lookup
6.1. Resource Lookup
6.2. Lookup Filtering
6.3. Resource Lookup Examples
6.4. Endpoint Lookup
7. Security Policies
7.1. Endpoint Name
7.1.1. Random Endpoint Names
7.2. Entered Links
7.3. Link Confidentiality
7.4. Segmentation
7.5. "First Come First Remembered": A Default Policy
8. Security Considerations
8.1. Discovery
8.2. Endpoint Identification and Authentication
8.3. Access Control
8.4. Denial-of-Service Attacks
8.5. Skipping Freshness Checks
9. IANA Considerations
9.1. Resource Types
9.2. IPv6 ND Resource Directory Address Option
9.3. RD Parameters Registry
9.3.1. Full Description of the "Endpoint Type" RD Parameter
9.4. Endpoint Type (et=) RD Parameter Values
9.5. Multicast Address Registration
9.6. Well-Known URIs
9.7. Service Name and Transport Protocol Port Number Registry
10. Examples
10.1. Lighting Installation
10.1.1. Installation Characteristics
10.1.2. RD Entries
10.2. OMA Lightweight M2M (LwM2M)
11. References
11.1. Normative References
11.2. Informative References
Appendix A. Groups Registration and Lookup
Appendix B. Web Links and the Resource Directory
B.1. A Simple Example
B.1.1. Resolving the URIs
B.1.2. Interpreting Attributes and Relations
B.2. A Slightly More Complex Example
B.3. Enter the Resource Directory
B.4. A Note on Differences between Link-Format and Link Header
Fields
Appendix C. Limited Link Format
Acknowledgments
Authors' Addresses1. Introduction ๐ธ
Over in the Constrained RESTful Environments (CoRE) work, there's a Representational State Transfer (REST) architecture built for constrained nodes (think limited RAM and ROM [RFC7228]) and constrained networks (like IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) [RFC4944]). It's already in use in Internet of Things (IoT) and machine-to-machine (M2M) apps, like smart energy and building automation โก
Discovering the resources a constrained server offers is super important in M2M land, because nobody's sitting there in the loop to fix things, and static interfaces are fragile ๐ When an HTTP web server does discovery, that's usually called web linking [RFC8288]. Describing and discovering resources on constrained web servers is handled by the CoRE Link Format [RFC6690]. BUT, [RFC6690] only explains how to discover resources from the server that hosts them, by querying /.well-known/core. In a lot of constrained setups, direct discovery just isn't practical, thanks to sleeping nodes or networks where multicast traffic is inefficient. The solution? A Resource Directory (RD), which holds info about resources on other servers, so you can run lookups for those resources ๐ง
This document specifies the web interfaces an RD supports, so web servers can discover the RD and register, maintain, look up, and remove information about resources. It also defines new target attributes that are useful with an RD ๐ The examples here show these interfaces used with the Constrained Application Protocol (CoAP) [RFC7252], but they can be applied in an equivalent manner to HTTP [RFC7230]. Iconic. ๐
2. Terminology ๐
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
Okay, quick bit of grammar. The term "byte" is used in its now customary sense as a synonym for "octet" ๐ค
This specification requires readers to be familiar with all the terms and concepts 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] ๐
This specification makes use of the following additional terminology:
- Resolve Against ๐: The expression "a URI reference is _resolved against_ a base URI" describes the process in [RFC3986], Section 5.2. Two corner cases are worth knowing: (1) if the URI reference is a (full) URI, resolving it against any base URI gives back the original full URI, and (2) resolving an empty URI reference gives the base URI without any fragment identifier.
- Resource Directory (RD) ๐: A web entity that stores information about web resources. It implements the REST interfaces defined in this specification for discovery, for creating, maintaining, and removing registrations, and for looking up the registered resources.
- Sector ๐๏ธ: In the context of an RD, a sector is a logical grouping of endpoints. The abbreviation
d=is used for the sector in query parameters, for compatibility with deployed implementations. - Endpoint (EP) ๐ฅ๏ธ: Endpoint (EP) is a term used to describe a web server or client in [RFC7252]. In this specification, an endpoint describes a web server that registers resources to the RD. An endpoint is identified by its endpoint name, which is included during registration, and it has a unique name within the associated sector of the registration.
- Registration Base URI ๐: The base URI of a registration is a URI that typically gives scheme and authority information about an endpoint. The registration base URI is provided at registration time, and the RD uses it to resolve relative references of the registration into URIs.
- Target ๐ฏ: The target of a link is the destination address (URI) of the link. It is sometimes identified with
href=or displayed as<target>. Relative targets need resolving with respect to the base URI (Section 5.2 of [RFC3986]).
This use of the term "target" is consistent with the use in [RFC8288].
- Context ๐งญ: The context of a link is the source address (URI) of the link, and it describes which resource is linked to the target. A link's context is made explicit in serialized links as the
anchor=attribute.
This use of the term "context" is consistent with the use in [RFC8288].
- Directory Resource ๐๏ธ: A directory resource is a resource in the RD that contains registration resources.
- Registration Resource ๐: A registration resource is a resource in the RD that contains information about an endpoint and its links.
- Commissioning Tool (CT) ๐ ๏ธ: A Commissioning Tool (CT) is a device that helps during installation events by assigning values to parameters, naming endpoints and groups, or adapting the installation to the needs of the applications.
- Registrant-EP ๐: A registrant-EP is the endpoint that is registered into the RD. The registrant-EP can register itself, or a CT can register it.
- Resource Directory Address Option (RDAO) ๐ก: A Resource Directory Address Option (RDAO) is a new IPv6 Neighbor Discovery option, defined for announcing an RD's address.
3. Architecture and Use Cases โจ
3.1. Principles โจ
Okay bestie, the RD is basically the group chat summary ๐ซถ It's primarily a tool to make discovery operations way more efficient than querying /.well-known/core on every connected device, or across boundaries that would otherwise limit those operations ๐
It gives you info about resources hosted by other devices, info you could otherwise only get by directly querying the /.well-known/core resource on those devices, whether with a unicast request or a multicast request ๐ฆ
Information SHOULD only be stored in the RD if it can be obtained by querying the described device's /.well-known/core resource directly. Main character energy means nothing gets stored that the device wouldn't say itself ๐
Data in the RD can only be provided by the device that hosts the data, or by a dedicated Commissioning Tool (CT). CTs act on behalf of endpoints that are too constrained, or generally unable, to present that information themselves ๐ง No other client can modify data in the RD ๐ And changes to the information in the RD do not propagate automatically back to the web servers where the information came from. Not me thinking the RD would just update the originals, lol ๐
3.2. Architecture ๐ธ
The RD architecture is illustrated in Figure 1. An RD is used as a repository of registrations describing resources hosted on other web servers, also called endpoints (EPs). An endpoint is a web server associated with a scheme, IP address, and port. One physical node can host one or more endpoints ๐ The RD implements a set of REST interfaces so endpoints can register and maintain their RD registrations, and so endpoints can look up resources from the RD. An RD can also be logically segmented by using sectors ๐
A mechanism to discover an RD using CoRE Link Format [RFC6690] is defined โจ
Registrations in the RD are soft state, so they need to be periodically refreshed. Keep those vibes fresh ๐
An endpoint uses specific interfaces to register, update, and remove a registration. It's also possible for an RD to fetch web links from endpoints and add their contents to its registrations ๐ฆ
At the first registration of an endpoint, a "registration resource" is created, and its location is returned to the registering endpoint. The registering endpoint uses this registration resource to manage the contents of its registrations ๐
A lookup interface for discovering any of the web links stored in the RD is provided using the CoRE Link Format ๐
Registration Lookup
Interface Interface
+----+ | |
| EP |---- | |
+----+ ---- | |
--|- +------+ |
+----+ | ----| | | +--------+
| EP | ---------|-----| RD |----|-----| Client |
+----+ | ----| | | +--------+
--|- +------+ |
+----+ ---- | |
| CT |---- | |
+----+Figure 1: The RD Architecture
A registrant-EP MAY keep concurrent registrations to more than one RD at the same time, if explicitly configured to do so. Honestly, typical EP implementations aren't expected to support that, so it's giving niche ๐ญ Any such registrations are independent of each other. The usual expectation, when multiple discovery mechanisms or addresses are configured, is that they form a fall-back path for a single registration ๐ซถ
3.3. RD Content Model ๐
The Entity-Relationship (ER) models shown in Figures 2 and 3 model the contents of /.well-known/core and the RD respectively, using 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 ๐
Some of the attribute values are URIs. Those values are always full URIs and never relative references in the information model. However, they can be expressed as relative references in serializations, and they often are ๐
These models give you an abstract view of the information expressed in link-format documents and an RD. They cover the concepts but not necessarily all details of an RD's operation. They're meant to give an overview, not to be a template for implementations ๐
+----------------------+
| /.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
oooooooooFigure 2: ER Model of the Content of /.well-known/core
Figure 2 models the contents of /.well-known/core, which contains a set of links belonging to the hosting web server ๐ท
The web server is free to choose which links it deems appropriate to be exposed in its /.well-known/core. Typically, the links describe resources that are served by the host, but the set can also contain links to resources on other servers (see examples in Section 5 of [RFC6690]). The set does not necessarily contain links to all resources served by the host ๐
A link has the following attributes (see Section 5 of [RFC8288]):
- Zero or more link relations: They describe relations between the link context and the link target.
In link-format serialization, they are expressed as space-separated values in the "rel" attribute, and they default to "hosts".
- A link context URI: It defines the source of the relation, e.g., _who_ "hosts" something.
In link-format serialization, it is expressed in the "anchor" attribute, and it defaults to the Origin of the target (practically, the target with its path and later components removed).
- A link target URI: It defines the destination of the relation (e.g., _what_ is hosted), and it is the topic of all target attributes.
In link-format serialization, it is expressed between angular brackets and is sometimes called the "href".
- Other target attributes (e.g., resource type (
rt), interface (if), or content format (ct)): These provide additional information about the target URI โจ
+--------------+
+ RD +
+--------------+
| 1
|
|
|
|
//////\\\\
< contains >
\\\\\/////
|
0+ |
ooooooo 1 +---------------+
o base o-------| registration |
ooooooo +---------------+
| | 1
| +--------------+
oooooooo 1 | |
o href o----+ /////\\\\
oooooooo | < contains >
| \\\\\/////
oooooooo 1 | |
o ep o----+ | 0+
oooooooo | +------------------+
| | link |
oooooooo 0-1 | +------------------+
o d o----+ |
oooooooo | | 1 oooooooo
| +-----o target o
oooooooo 1 | | oooooooo
o lt o----+ ooooooooooo 0+ |
oooooooo | o target o-----+
| o attribute o | 0+ oooooo
ooooooooooo 0+ | ooooooooooo +-----o rel o
o endpoint o----+ | oooooo
o attribute o |
ooooooooooo | 1 ooooooooo
+----o context o
oooooooooFigure 3: ER Model of the Content of the RD
Figure 3 models the contents of the RD, which contains, in addition to /.well-known/core, 0 to n registrations of endpoints ๐ง
A registration is associated with one endpoint. A registration defines a set of links, as defined for /.well-known/core. A registration has six types of attributes:
- an endpoint name (
"ep", a Unicode string) unique within a sector - a registration base URI (
"base", a URI typically describing the scheme://authority part) - a lifetime (
"lt") - a registration resource location inside the RD (
"href") - optionally, a sector (
"d", a Unicode string) - optional additional endpoint attributes (from Section 9.3)
The cardinality of "base" is currently 1. Future documents are invited to extend the RD specification to support multiple values (e.g., [COAP-PROT-NEG]). Its value is used as a base URI when resolving URIs in the links contained in the endpoint ๐
Links are modeled as they are in Figure 2 ๐
3.4. Link-Local Addresses and Zone Identifiers ๐ฆ
Registration base URIs can contain link-local IP addresses. To be usable across hosts, those cannot be serialized to contain zone identifiers (see [RFC6874], Section 1). Zone identifiers, nope, not in this house ๐
Link-local addresses can only be used on a single link. That means RD servers cannot announce them when queried on a different link, and lookup clients using them need to keep track of which interface they got them from ๐งญ
Therefore, it is advisable in many scenarios to use addresses with larger scopes, if available ๐
3.5. Use Case: Cellular M2M ๐ฑ
Okay so, over the last few years, mobile operators around the world have gone all in on M2M solutions ๐ to expand their business to a brand new type of user: machines ๐ค. The machines connect directly to a mobile network through an appropriate embedded wireless interface (GSM/General Packet Radio Service (GPRS), Wideband Code Division Multiple Access (W-CDMA), LTE, etc.), or they connect via a gateway that offers short- and wide-range wireless interfaces. The dream is to build these systems out of reusable components ๐งฉ, which speeds up development and deployment and lets machines be shared across different applications. One crucial piece is discovering resources (and therefore the endpoints they're hosted on) that can provide the required information at a given time or act on instructions from end users. It's giving organized ๐
Imagine this: endpoints installed on vehicles track where the vehicles are for fleet management ๐ and also monitor environment parameters ๐ฟ. During the boot-up process, the endpoints register with an RD, which is hosted by the mobile operator or somewhere in the cloud โ๏ธ. Every so often, these endpoints update their registration and may modify the resources they offer โจ.
When endpoints aren't always connected (for example, because they enter a sleep mode ๐ด), a remote server is usually used to provide proxy access to them. Mobile apps or web applications for environment monitoring contact the RD and look up the endpoints that can provide environment information, using an appropriate set of link parameters ๐. They then get info on how to contact those endpoints (URLs of the proxy server) and start an interaction to obtain the information. That information gets processed, shown on the screen ๐ฒ, and usually stored in a database ๐๏ธ. Similarly, fleet management systems hand the appropriate link parameters to the RD so they can look up the EPs deployed on the vehicles their application is responsible for ๐๐
3.6. Use Case: Home and Building Automation ๐
Home and commercial building automation systems can totally benefit from IoT web services ๐ธ. These apps have demanding discovery requirements ๐ค. Home automation usually relies on run-time discovery to commission the system, whereas building automation uses a mix of professional commissioning and run-time discovery ๐ ๏ธ. Both involve peer-to-peer interactions between endpoints and involve battery-powered sleeping devices ๐. Both can use the common RD infrastructure to set up device interactions efficiently, but they can pick security policies that suit their own needs ๐.
There are two phases you can spot for a network servicing the system: (1) installation and (2) operation ๐ฆ. During the operational phase, the network is connected to the Internet through a border router (e.g., a 6LoWPAN Border Router (6LBR) [RFC6775]), and the nodes on the network can use the Internet services provided by the IP or network administrator ๐. During the installation phase, the network is completely stand-alone. No border router is connected, so the network only supports IP communication between the connected nodes ๐๏ธ. The installation phase is usually followed by the operational phase. Because an RD's operations work without hard dependencies on names or addresses, it can be used for discovery across both phases ๐
3.7. Use Case: Link Catalogues ๐
Resources may be shared through data brokers that have no idea beforehand who's going to consume the data ๐ต๏ธโโ๏ธ. An RD can hold links about resources and services hosted anywhere, which makes them discoverable by a general class of applications ๐.
For example, environmental and weather sensors that generate data for public consumption may provide that data to an intermediary server or broker ๐ฆ๏ธ. Sensor data gets published to the intermediary upon changes or at regular intervals โฐ. Descriptions of the sensors that resolve to links to sensor data may be published to an RD. Applications that want to consume the data can use RD lookup to discover and resolve links to the desired resources and endpoints ๐ฏ. The RD service doesn't need to be coupled with the data intermediary service. Mapping of RDs to data intermediaries may be many-to-many ๐ธ๏ธ.
Metadata in web link formats, such as the one defined in [RFC6690], may be stored internally as triples or relation/attribute pairs that provide metadata about resource links, and RDs need to support it ๐ . External catalogues represented in other formats may be converted to common web linking formats for storage and access by RDs ๐. Since it's common practice for these to be encoded in URNs [RFC8141], simple and lossless structural transforms should generally be enough to store external metadata in RDs โจ.
The additional features of an RD let sectors be defined, so a particular set of resources can be accessed from particular applications. This gives isolation and protection for sensitive data when needed ๐. Application groups with multicast addresses may be defined to support efficient data transport ๐.
4. RD Discovery and Other Interface-Independent Components โจ
This section and the following ones define the required set of REST interfaces between an RD, endpoints, and lookup clients ๐งฉ. Although the examples throughout these sections assume the use of CoAP [RFC7252], these REST interfaces can also be realized using HTTP [RFC7230]. The multicast discovery and simple registration operations are the exceptions, since they rely on mechanisms unavailable in HTTP ๐. In all definitions in these sections, both CoAP response codes (with dot notation) and HTTP response codes (without dot notation) are shown ๐. An RD implementing this specification MUST support the discovery, registration, update, lookup, and removal interfaces ๐.
All operations on the contents of the RD MUST be atomic and idempotent ๐ฏ.
For several operations, interface templates are given in list form. Those describe the operation participants, request codes, URIs, content formats, and outcomes ๐. The sections of those templates contain normative content about Interaction, Method, URI Template, and URI Template Variables, as well as the details of the Success condition. The additional sections for options (such as Content-Format) and for Failure codes give typical cases that an implementation of the RD should deal with ๐. Those are there to illustrate typical responses for readers who aren't yet familiar with all the details of CoAP-based interfaces. They do not limit how a server may respond under atypical circumstances ๐ซถ.
REST clients (registrant-EPs and CTs during registration and maintenance, lookup clients, and RD servers during simple registrations) must be prepared to receive any unsuccessful code and act upon it according to its definition, options, and/or payload, to the best of their capabilities. If recovery isn't possible, they fall back to failing the operation ๐ญ. In particular, they SHOULD retry the request upon 5.03 (Service Unavailable; 503 in HTTP) according to the Max-Age (Retry-After in HTTP) option, and they SHOULD fall back to link format when they receive 4.15 (Unsupported Content-Format; 415 in HTTP) ๐.
An RD MAY make the information submitted to it available to further directories (subject to security policies on link confidentiality) if it can ensure that a loop does not form ๐. The protocol used between directories to ensure loop-free operation is outside the scope of this document ๐ง
4.1. Finding a Resource Directory โจ
okay so a (re)starting device might wanna find one or more RDs before it can even discover their URIs ๐งญ. Depending on the network vibes, one or more of these techniques apply.
The device may be preconfigured to use specific mechanisms for finding the RD:
- It may be configured with a specific IP address for the RD. That IP address may also be an anycast address, letting the network forward RD requests to an RD that's topologically close ๐. Each target network where these preconfigured nodes get brought up is then configured with a route for this anycast address that leads to an appropriate RD. (Instead of an anycast address, a multicast address can also be preconfigured. The RD servers then need to configure one of their interfaces with this multicast address ๐.)
- It may be configured with a DNS name for the RD and use DNS to return the IP address of the RD. It can find a DNS server to do the lookup using the usual mechanisms for finding DNS servers ๐.
- It may be configured to use a service discovery mechanism, such as DNS-based Service Discovery (DNS-SD), as outlined in Section 4.1.2 ๐ง.
For cases where the device isn't specifically configured with a way to find an RD, the network may want to provide a suitable default ๐ .
- The IPv6 Neighbor Discovery option RDAO (Section 4.1.1) can do that ๐ฆ.
- When DHCP is in use, this could be provided via a DHCP option (no such option is defined at the time of writing) ๐คทโโ๏ธ.
Finally, if neither the device nor the network offers any specific configuration, the device may want to use heuristics to find a suitable RD ๐.
The present specification doesn't fully define these heuristics, but it suggests a number of candidates โจ:
- In a 6LoWPAN, just assume the 6LBR can act as an RD (using the Authoritative Border Router Option (ABRO) to find that [RFC6775]). Confirmation can be obtained by sending a unicast GET to
coap://[6LBR]/.well-known/core?rt=core.rd*๐ซถ. - In a network that supports multicast well, discover the RD using a multicast query for
/.well-known/core, as specified in CoRE Link Format [RFC6690], and send a Multicast GET tocoap://[ff0x::fe]/.well-known/core?rt=core.rd*. RDs within the multicast scope will answer the query ๐ฃ.
When answering a multicast request directed at a link-local group, the RD may want to respond from a routable address. This makes it easier for registrants to use one of their own routable addresses for registration ๐. When source addresses are selected using the mechanism described in [RFC6724], this can be achieved by applying the changes of its Section 10.4, picking public addresses in Rule 7 of its Section 5, and superseding Rule 8 with preferring the source address's precedence ๐.
Heads up bestie: some of the RD addresses you get from these methods are just (more or less educated) guesses ๐. So 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. A CoAP error response code, such as 4.05 (Method Not Allowed), may indicate that a CoAP server is unwilling to act as a directory server ๐.
The following RD discovery mechanisms are recommended ๐:
- In managed networks with border routers that need stand-alone operation, the RDAO is recommended (e.g., the operational phase described in Section 3.6) ๐ .
- In managed networks without border routers (no Internet services available), the use of a preconfigured anycast address is recommended (e.g., the installation phase described in Section 3.6) ๐ ๏ธ.
- In networks managed using DNS-SD, the use of DNS-SD for discovery, as described in Section 4.1.2, is recommended ๐ธ.
The use of multicast discovery in mesh networks is NOT RECOMMENDED ๐ โโ๏ธ.
4.1.1. Resource Directory Address Option (RDAO) ๐
The Resource Directory Address Option (RDAO) carries information about the address of the RD in RAs (Router Advertisements) of IPv6 Neighbor Discovery (ND). It's similar to how Recursive DNS Server (RDNSS) options [RFC8106] are sent ๐จ. This info is needed when endpoints can't discover the RD with a link-local or realm-local scope multicast address, for instance because the endpoint and the RD are separated by a 6LBR ๐ง. In lots of situations, DHCP availability can't be guaranteed during commissioning of the network either ๐ญ. The presence and use of the RD is essential during commissioning ๐.
It's possible to send multiple RDAOs in one message, indicating as many RD addresses ๐ง.
The RDAO format is ๐ฆ:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ RD Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Resource Directory Address OptionHere's the breakdown of the fields ๐ฐ:
- Type:
41 - Length: an 8-bit unsigned integer. It's the length of the option in units of 8 bytes. Always
3. - Reserved: this field is unused ๐ซง. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
- Valid Lifetime: a 32-bit unsigned integer. It's the length of time in seconds (relative to the time the packet is received) that this RD address is valid โณ. A value of all zero bits (
0x0) means this RD address is no longer valid ๐. - RD Address: the IPv6 address of the RD ๐.
4.1.2. Using DNS-SD to Discover a Resource Directory ๐
An RD can advertise its presence in DNS-SD [RFC6763] using the service names defined in this document: _core-rd._udp (for CoAP), _core-rd-dtls._udp (for CoAP over DTLS), _core-rd._tcp (for CoAP over TCP), or _core-rd-tls._tcp (for CoAP over TLS) ๐ก. (For the WebSocket transports of CoAP, no service is defined, since DNS-SD is typically unavailable in environments where CoAP over WebSockets is used ๐.)
The selection of the service indicates the protocol used, and the SRV record points the client to a host name and port to use as a starting point for the "URI discovery" steps of Section 4.3 ๐ฏ.
This section is a simplified, concrete application of the more generic mechanism specified in [CORE-RD-DNS-SD] ๐.
4.2. Payload Content Formats ๐
RDs implementing this specification MUST support the application/link-format content format (ct=40) ๐ซถ.
RDs implementing this specification MAY support additional content formats โจ.
Any additional content format supported by an RD implementing this specification SHOULD be able to express all the information expressible in link format. It MAY be able to express information that's inexpressible in link format, but those expressions SHOULD be avoided where possible ๐.
4.3. URI Discovery ๐ฆ
okay bestie, before an endpoint can even use an RD ๐ง it must first know the RD's address and port AND the URI paths for its REST APIs. this section explains how to find all that using the well-known interface of the CoRE Link Format [RFC6690], once you've found a host the way Section 4.1 describes ๐
so the move is: send a GET to /.well-known/core (multicast OR unicast ๐) with a resource type (rt) parameter [RFC6690] in the query string. rt=core.rd finds the RD registration URI. rt=core.rd-lookup* finds the lookup URIs, and rt=core.rd* finds literally all the RD operation URIs ๐ if it works, you get back a payload with a link format entry for each RD function, telling you the URI and its resource type.
when you're doing multicast discovery, which multicast IP address you use depends on the scope you need AND what your network can actually do multicast-wise. see Section 9.5 for the deets ๐ธ
an RD MAY give hints about which content formats it supports, using the ct target attribute in the links it exposes or registers. the example for this is further down ๐ and clients MAY use those hints to pick alternate content formats when talking to the RD โจ
HTTP doesn't support multicast ๐ so over HTTP, only unicast discovery works, through the HTTP /.well-known/core resource.
RDs implementing this spec MUST support query filtering for the rt parameter, as defined in [RFC6690]. non-negotiable, periodt ๐
the link targets in this step are often written in path-absolute form, but that's not a rule. so clients SHOULD accept URIs of every scheme they support, whether they show up as full URIs or relative references, and they should NOT limit the discovered URIs to only the ones hosted at the address they used for discovery. main character energy ๐
with security policies where the client needs the RD to be authorized to act as an RD, that authorization can be limited to the resources where the authorized RD advertises the right resource types ๐ if a client has links it can't trust yet, it can repeat the "URI discovery" step at the /.well-known/core resource of the host it got them from, and grab the resource type info from an authorized source instead ๐ซถ
one discovery can turn up multiple URIs of the same resource type, and the client is free to try out any of them ๐
here's the discovery request interface, spelled out. it's exactly the well-known interface of [RFC6690], Section 4, with one extra rule: the server MUST support query filtering ๐ฅ
Interaction: EP, CT, or Client -> RD
Method: GET
URI Template: /.well-known/core{?rt}
URI Template Variables:
rt := Resource Type. SHOULD contain one of the values "core.rd",
"core.rd-lookup*", "core.rd-lookup-res", "core.rd-lookup-ep",
or "core.rd*"
Accept: absent, application/link-format, or any other media type
representing web linksthe expected success response is 2.05 (Content) or 200 (OK) with an application/link-format or other web link payload containing one or more matching entries for the RD resource โจ
Success: 2.05 (Content) or 200 (OK) with an application/link-format
or other web link payload containing one or more matching entries
for the RD resource.now the example ๐ง the endpoint discovers an RD with this interface and learns that the directory resource lives at /rd, and that the server delivers content as application/link-format (ct=40). fyi the RD gets to pick its own RD locations, so /rd is just what it chose here ๐
Req: GET coap://[ff02::fe]/.well-known/core?rt=core.rd*
Res: 2.05 Content
Payload:
</rd>;rt=core.rd;ct=40,
</rd-lookup/ep>;rt=core.rd-lookup-ep;ct=40,
</rd-lookup/res>;rt=core.rd-lookup-res;ct=40
Figure 5: Example Discovery Exchangenext, the way a server says a client can ask for alternate content formats ๐ the Content-Format code attribute ct MAY hold a space-separated list of Content-Format codes, as specified in Section 7.2.1 of [RFC7252], so you can signal that multiple formats are available. in this example, the required Content-Format 40 (application/link-format) is listed, plus Concise Binary Object Representation (CBOR) and JSON representations in the style of [CORE-LINKS-JSON] (those use the experimental values 65060 and 65050, just for this example). /rd and /rd-lookup are example values too ๐ธ and this server also says it can do observation on resource lookups, which is the obs bit ๐
Req: GET coap://[ff02::fe]/.well-known/core?rt=core.rd*
Res: 2.05 Content
Payload:
</rd>;rt=core.rd;ct=40,
</rd-lookup/res>;rt=core.rd-lookup-res;ct="40 65060 65050";obs,
</rd-lookup/ep>;rt=core.rd-lookup-ep;ct="40 65060 65050"
Figure 6: Example Discovery Exchange Indicating Additional
Content-Formatsnow, for maintenance, management, and debugging ๐ ๏ธ it can be super useful to know which components make up the RD server. that info helps you track down client-server incompatibilities, supported features, required updates, and other stuff ๐ the well-known interface in Section 4 of [RFC6690] can get you that data.
this info would typically be stored in an implementation information link (as described in [T2TRG-REL-IMPL]) ๐
Req: GET /.well-known/core?rel=impl-info
Res: 2.05 Content
Payload:
<http://software.example.com/shiny-resource-directory/1.0beta1>;
rel=impl-info
Figure 7: Example Exchange of Obtaining Implementation
Information Using the Relation Type Currently Proposed in
[T2TRG-REL-IMPL]last bit: depending on the server's architecture, that link could be anchored at the RD server's root (like in this example) or at individual RD components ๐งฉ you'd expect the second one when different applications run on the same server โจ
5. Registration โจ
Okay bestie, so once a registrant-EP or CT has found where the RD lives ๐บ๏ธ, it MAY register the resources of that registrant-EP through the registration interface ๐. This interface takes a POST from the endpoint. The message payload is the list of resources to add to the directory, written in the CoRE Link Format [RFC6690] or in any other representation of web links ๐. The POST also carries query parameters for the endpoint's name 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) ๐งฉ.
The RD then creates a brand new registration resource and hands back its location ๐. The receiving endpoint MUST use that location when refreshing registrations using this interface ๐. Registrations stay alive for the period set by the lifetime parameter โฐ. The creating endpoint is responsible for refreshing the registration resource within that period, using either the registration or update interface. The registration interface MUST be implemented to be idempotent, so registering twice with the same endpoint parameters ep and d (sector) doesn't make multiple registration resources ๐
โโ๏ธ. Iconic behaviour, honestly ๐
.
The following rules apply to a registration request that targets a given (ep, d) value pair:
- When the (
ep,d) value pair of the registration request is different from any existing registration, a new registration is generated ๐ฑ. - When the (
ep,d) value pair of the registration request is equal to an existing registration, the content and parameters of the existing registration are replaced with the content of the registration request. As with changes to registration resources, security policies (Section 7) usually require such requests to come from the same device ๐.
The posted link-format document can (and typically does) contain relative references both in its link targets and in its anchors ๐งต. It can also contain empty anchors ๐ซฅ. The RD server needs to resolve these references so it can represent them faithfully in lookups ๐. 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 built from its network address and scheme ๐๏ธ.
For media types to which Appendix C applies (i.e., documents in application/link-format), request bodies MUST be expressed in Limited Link Format ๐. Here's the registration request interface, spelled out:
Interaction: EP or CT -> RD
Method: POST
URI Template: {+rd}{?ep,d,lt,base,extra-attrs*}Now let's look at the URI template variables, one by one ๐:
rd:= RD registration URI (mandatory). This is the location of the RD, as obtained from discovery ๐งญ.ep:= Endpoint name (mostly mandatory). The endpoint name is an identifier that MUST be unique within a sector ๐ชช. Because the endpoint name is a Unicode string, it is encoded in UTF-8 (and possibly percent encoded) during variable expansion (see [RFC6570], Section 3.2.1). The endpoint name MUST NOT contain any character in the inclusive ranges0-31or127-159๐ซ. The maximum length of this parameter is 63 bytes encoded in UTF-8 ๐. If the RD is configured to recognize the endpoint that is to be authorized to use exactly one endpoint name, the RD assigns that name. In that case, giving the endpoint name becomes optional for the client. If the client gives any other endpoint name, it is not authorized to perform the registration ๐ โโ๏ธ.d:= Sector (optional). This is the sector this endpoint belongs to ๐ท๏ธ. When this parameter isn't present, the RD MAY associate the endpoint with a configured default sector (possibly based on the endpoint's authorization), or leave it empty ๐คทโโ๏ธ. The sector is encoded like theepparameter and is limited to 63 bytes encoded in UTF-8 as well ๐.lt:= Lifetime (optional). This is the lifetime of the registration in seconds, with a range of1-4294967295โณ. If no lifetime is included in the initial registration, a default value of90000(25 hours) SHOULD be assumed ๐.base:= Base URI (optional). This parameter sets the base URI of the registration, which is where the relative links in the payload are interpreted ๐งญ. The specified URI typically doesn't have a path component of its own, and it MUST be suitable as a base URI to resolve any relative references given in the registration ๐. So it's usually shaped likescheme://authorityfor HTTP and CoAP URIs ๐. The URI SHOULD NOT have a query or fragment component, since any non-empty relative part would remove those parts from the resulting URI โ ๏ธ.
If this parameter is missing, the scheme of the protocol, the source address, and the source port of the registration request are assumed ๐ต๏ธโโ๏ธ. The base URI is built by concatenating the protocol's scheme with the characters ://, the requester's source address as an address literal, and : followed by its port (if it wasn't the protocol's default one). This is analogous to the process described in [RFC7252], Section 6.5 ๐งฉ.
This parameter is mandatory when the directory is filled by a third party, such as a commissioning tool ๐ ๏ธ. If the registrant-EP uses an ephemeral port to register with, it MUST include the base parameter in the registration to provide a valid network path ๐ค๏ธ. A registrant that can't be reached by potential lookup clients at the address it registers from (e.g., because it's behind some form of Network Address Translation (NAT)) MUST provide a reachable base address with its registration ๐ .
If the base URI contains a link-local IP literal, it MUST NOT contain a Zone Identifier, and it MUST be local to the link on which the registration request is received ๐ก.
Endpoints that register with a base that contains a path component can't efficiently express their registrations in Limited Link Format (Appendix C). Those applications should use different representations of links to which Appendix C isn't applicable (e.g., [CORE-CORAL]) ๐ฆ.
extra-attrs:= Additional registration attributes (optional). The endpoint can pass any parameter registered in Section 9.3 to the directory ๐. If the RD knows the parameter's specified semantics, it processes the parameter accordingly. Otherwise, it MUST store the unknown key and its value(s) as an endpoint attribute for further lookup ๐.
The content type for this interface is:
Content-Format: application/link-format or any other indicated media
type representing web linksAnd here's what a successful response looks like ๐ฅณ:
Success: 2.01 (Created) or 201 (Created).The Location-Path option or Location header field MUST be included in the response ๐ฎ. This location MUST be a stable identifier generated by the RD, because it's used for all subsequent operations on this registration resource ๐. The registration resource location returned this way is for updating the lifetime of the registration and for maintaining the content of the registered links, including updating and deleting links โ๏ธ.
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 gets replaced with the links from the payload ๐.
The location MUST NOT have a query or fragment component, since that could conflict with query parameters during the registration update operation ๐. Therefore, the Location-Query option MUST NOT be present in a successful response ๐ซ.
If the registration fails, including request timeouts, or if delays from Service Unavailable responses with Max-Age or Retry-After accumulate to exceed the registrant's configured timeouts, it SHOULD pick another registration URI from the "URI discovery" step of Section 4.3. If there's only one URI or the list is exhausted, it can pick other choices from the "finding a resource directory" step of Section 4.1 ๐ญ.
Care has to be taken to consider the freshness of results obtained earlier, e.g., the result of a /.well-known/core response, the lifetime of an RDAO, and DNS responses ๐ฐ๏ธ. 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 ๐ง .
Here's an example of a registrant-EP named "node1" registering two resources to an RD using this interface ๐. The location "/rd" is an example RD location discovered in a request similar to Figure 5 ๐.
Req: POST coap://rd.example.com/rd?ep=node1
Content-Format: 40
Payload:
</sensors/temp>;rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="/sensors/temp";rel=describedby
Res: 2.01 Created
Location-Path: /rd/4521
Figure 8: Example Registration PayloadAn RD may optionally support HTTP too ๐ธ. Here's almost the same registration operation as above, but done using HTTP ๐.
Req:
POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1
Host: rd.example.com
Content-Type: application/link-format
</sensors/temp>;rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="/sensors/temp";rel=describedby
Res:
HTTP/1.1 201 Created
Location: /rd/4521
Figure 9: Example Registration Payload as Expressed Using HTTP5.1. Simple Registration ๐ฆ
okay so not every endpoint hosting resources is gonna know how to upload its links to an RD, like, the way Section 5 describes ๐ฅบ. That's where simple endpoints come in โจ. They can use the simple registration approach from this section. An RD implementing this specification MUST implement simple registration. ngl there may be security reasons why this form of directory discovery would be disabled, so it's not always switched on ๐ .
This approach needs the registrant-EP to make the hosted resources it wants discovered available 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) ๐.
- The registrant-EP finds one or more addresses of the directory server, as described in Section 4.1 ๐.
- The registrant-EP sends (and regularly refreshes with) a POST request to the /.well-known/rd URI of the directory server of choice ๐ฎ. The body of the POST request is empty, and it triggers the resource directory server to perform GET requests (redone before lifetime expiry) at the requesting registrant-EP's /.well-known/core to obtain the link-format payload to register. Main character energy, the RD does the work for you ๐.
The registrant-EP includes the same registration parameters in the POST request as it would with a regular registration, per Section 5. The registration base URI of the registration is taken from the registrant-EP's network address (as is default with regular registrations) ๐ธ.
The following is an example request from the registrant-EP to the RD (unanswered until the next step) ๐ง:
Req: POST /.well-known/rd?lt=6000&ep=node1
(No payload)Figure 10: First-Half Example Exchange of a Simple Registration
- The RD queries the registrant-EP's discovery resource to determine the success of the operation ๐ต๏ธโโ๏ธ. It SHOULD keep a cache of the discovery resource and not query it again as long as it is fresh ๐ง.
The following is an example request from the RD to the registrant-EP:
Req: GET /.well-known/core
Accept: 40
Res: 2.05 Content
Content-Format: 40
Payload:
</sen/temp>Figure 11: Example Exchange of the RD Querying the Simple Endpoint
With this response, the RD would answer the previous step's request:
Res: 2.04 ChangedFigure 12: Second-Half Example Exchange of a Simple Registration
The sequence of fetching the registration content before sending a successful response was chosen to make responses reliable ๐ฏ, and the point about caching was chosen to still allow very constrained registrants ๐ฅน. 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 then 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 ๐ซถ.
The simple registration request interface is specified as follows:
Interaction: EP -> RD
Method: POST
URI Template: /.well-known/rd{?ep,d,lt,extra-attrs*}URI Template Variables are the same as for registration in Section 5. The base attribute is not accepted to keep the registration interface simple, which rules out registration over CoAP-over-TCP or HTTP that would need to specify one ๐ซ. For some time during this document's development, the URI Template /.well-known/core{?ep,...} was in use instead ๐ฐ๏ธ.
The following response is expected on this interface:
Success: 2.04 (Changed)For the second interaction triggered by the above, the registrant-EP takes the role of server and the RD takes the role of client. (Note that this is exactly the well-known interface of [RFC6690], Section 4 ๐.)
Interaction: RD -> EP
Method: GET
URI Template: /.well-known/coreThe following response is expected on this interface:
Success: 2.05 (Content)When the RD uses any authorization credentials to access the endpoint's discovery resource, or when it is deployed in a location where third parties might reach it but not the endpoint, it SHOULD verify that the apparent registrant-EP intends to register with the given registration parameters before revealing the obtained discovery information to lookup clients ๐ก๏ธ. An easy way to do that is to verify the simple registration request's sender address using the Echo option, as described in [RFC9175], Section 2.4 ๐.
The RD MUST delete registrations created by simple registration after the expiration of their lifetime. No additional operations on the registration resource can be executed, because no registration location is returned ๐.
5.2. Third-Party Registration ๐
For some applications, even simple registration may be too taxing for some very constrained devices, in particular if the security requirements get too onerous ๐ฎโ๐จ.
In a controlled environment (e.g., building control), the RD can be filled by a third-party device, called a Commissioning Tool (CT) ๐ ๏ธ. The CT can fill the RD from a database or other means. For that purpose, the IP address and port of the URI of the registered device is the value of the "base" parameter of the registration described in Section 5 ๐๏ธ.
It should be noted that the value of the "base" parameter applies to all the links of the registration, and it has consequences for the anchor value of the individual links, as exemplified in Appendix B ๐. A potential (currently nonexistent) "base" attribute of the link is not affected by the value of the "base" parameter in the registration ๐.
5.3. Operations on the Registration Resource ๐ท
This section describes how the registering endpoint can maintain the registrations that it created ๐งโ๐ป. The registering endpoint can be the registrant-EP or the CT. The registrations are resources of the RD ๐.
An endpoint should not use this interface for registrations that it did not create. This is usually enforced by security policies, which, in general, require equivalent credentials for creation of and operations on a registration ๐ท.
After the initial registration, the registering endpoint keeps the returned location of the registration resource for further operations, including refreshing the registration to extend the lifetime and "keep-alive" the registration ๐. When the lifetime of the registration has expired, the RD SHOULD NOT respond to discovery queries concerning this endpoint ๐ โโ๏ธ. The RD SHOULD continue to provide access to the registration resource after a registration timeout occurs, so the registering endpoint can eventually refresh the registration ๐. The RD MAY eventually remove the registration resource for the purpose of garbage collection ๐๏ธ. If the registration resource is removed, the corresponding endpoint will need to be reregistered ๐ญ.
The registration resource may also be used to cancel the registration using DELETE, and to perform further operations beyond the scope of this specification ๐ฎโจ.
Operations on the registration resource are sensitive to reordering, and Section 5.3.4 describes how order is restored ๐.
The operations on the registration resource are described below ๐.
5.3.1. Registration Update
okay so the update interface is how the registering endpoint refreshes or updates its registration with an RD ๐ฆ. to use it, the endpoint sends a POST to the registration resource it got back from the initial registration โจ
an update MAY change registration parameters like lifetime, base URI, or others ๐ parameters that aren't changing should not be included in an update. adding unchanged parameters just bloats the message, with no other effect ๐ parameters go in as query parameters in an update, same as in Section 5.
a registration update resets the timeout to the (possibly updated) lifetime of the registration, whether or not an lt parameter was given ๐
if the base URI changes in an update, relative references from the original registration or later updates get resolved anew against the new base ๐ธ
the registration update only describes POST with an empty payload. future standards might describe content formats and payloads for POST to update the links of a registration (see Section 5.3.3) ๐ง
here's the interface spec, verbatim:
The update registration request interface is specified as follows:
Interaction: EP or CT -> RD
Method: POST
URI Template: {+location}{?lt,base,extra-attrs*}
URI Template Variables:
location := This is the location returned by the RD as a result
of a successful earlier registration.
lt := Lifetime (optional). This is the lifetime of the
registration in seconds, with a range of 1-4294967295. If no
lifetime is included, the previous last lifetime set on a
previous update or the original registration (falling back to
90000) SHOULD be used.
base := Base URI (optional). This parameter updates the base URI
established in the original registration to a new value and is
subject to the same restrictions as in the registration.
If the parameter is set in an update, it is stored by the RD as
the new base URI under which to interpret the relative links
present in the payload of the original registration.
If the parameter is not set in the request but was set before,
the previous base URI value is kept unmodified.
If the parameter is not set in the request and was not set
before either, the source address and source port of the update
request are stored as the base URI.
extra-attrs := Additional registration attributes (optional). As
with the registration, the RD processes them if it knows their
semantics. Otherwise, unknown attributes are stored as
endpoint attributes, overriding any previously stored endpoint
attributes of the same key.
Note that this default behavior does not allow removing an
endpoint attribute in an update. For attributes whose
functionality depends on the endpoints' ability to remove them
in an update, it can make sense to define a value whose
presence is equivalent to the absence of a value. As an
alternative, an extension can define different updating rules
for their attributes. That necessitates either discovering
whether the RD is aware of that extension or tolerating the
default behavior.
Content-Format: none (no payload)the RD answers like this:
The following responses are expected on this interface:
Success: 2.04 (Changed) or 204 (No Content) if the update was
successfully processed.
Failure: 4.04 (Not Found) or 404 (Not Found). Registration does not
exist (e.g., may have been removed).if the registration update fails in any way, including "Not Found" and request timeouts, or if the time indicated in a Service Unavailable Max-Age/Retry-After exceeds the remaining lifetime, the registering endpoint SHOULD attempt registration again ๐
here's the endpoint resetting the timeout on its registration resource at an RD, using the example location /rd/4521:
Req: POST /rd/4521
Res: 2.04 ChangedFigure 13: Example Update of a Registration
now for a fuller example. the endpoint's initial registration set these values:
- endpoint name (ep)=endpoint1
- lifetime (lt)=500
- base URI (base)=coap://local-proxy-old.example.com
- payload of Figure 8
the RD's current state looks like this:
Req: GET /rd-lookup/res?ep=endpoint1
Res: 2.05 Content
Payload:
<coap://local-proxy-old.example.com/sensors/temp>;
rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="coap://local-proxy-old.example.com/sensors/temp";
rel=describedbyFigure 14: Example Lookup Before a Change to the Base Address
next, the endpoint changes the base URI to coaps://new.example.com:5684:
Req: POST /rd/4521?base=coaps://new.example.com
Res: 2.04 ChangedFigure 15: Example Registration Update that Changes the Base Address
and the follow-up lookup comes back like this:
Req: GET /rd-lookup/res?ep=endpoint1
Res: 2.05 Content
Payload:
<coaps://new.example.com/sensors/temp>;
rt=temperature-c;if=sensor,
<http://www.example.com/sensors/temp>;
anchor="coaps://new.example.com/sensors/temp";
rel=describedbyFigure 16: Example Lookup After a Change to the Base Address
5.3.2. Registration Removal
okay bestie, RD registrations are soft state and will eventually time out after their lifetime ๐ฐ๏ธ but the registering endpoint SHOULD explicitly remove its entry if it knows it won't be available anymore (like on shutdown) ๐ this happens with a removal interface on the RD, which is a DELETE on the endpoint resource.
The removal request interface is specified as follows:
Interaction: EP or CT -> RD
Method: DELETE
URI Template: {+location}
URI Template Variables:
location := This is the location returned by the RD as a result
of a successful earlier registration.
The following responses are expected on this interface:
Success: 2.02 (Deleted) or 204 (No Content) upon successful
deletion.
Failure: 4.04 (Not Found) or 404 (Not Found). Registration does not
exist (e.g., may already have been removed).successful removal of the endpoint from the RD with the example location /rd/4521 looks like this:
Req: DELETE /rd/4521
Res: 2.02 DeletedFigure 17: Example of a Registration Removal
5.3.3. Further Operations
future documents can specify more operations on the registration, for example:
- Send iPATCH (or PATCH) updates [RFC8132] to add, remove, or change the links of a registration.
- Use GET to read the currently stored set of links in a registration resource.
those operations are out of scope for this document, and they'll need media types that can modify sets of links ๐
5.3.4. Request Freshness
sis, some security mechanisms usable with an RD allow out-of-order request processing, or don't even require replay protection at all ๐ญ the RD needs to make sure operations on the registration resource run in an order that doesn't distort the client's intentions ๐ซถ
this ordering is expressed in terms of freshness, as defined in [RFC9175]. requests that change a resource's state need to be fresh relative to the latest request that changed that state in a conflicting way.
an RD SHOULD determine a request's freshness, and it MUST use the Echo option if it needs request freshness and can't determine it any other way. an endpoint MUST support the use of the Echo option. (one reason an RD wouldn't require freshness is when no relevant registration properties are covered by its security policies ๐)
5.3.4.1. Efficient Use of Echo by an RD
to keep the latency and traffic that freshness adds to a minimum, RDs should avoid naive (sufficient but inefficient) freshness criteria โก
here are some simple mechanisms the RD can use:
- State counter. the RD can keep a monotonous counter that increments whenever a registration changes. for every registration resource, it stores the post-increment value of that resource's last change. requests altering them need to have at least that value encoded in their Echo option, and are otherwise rejected with a 4.01 (Unauthorized) and the current counter value as the Echo value. if other applications on the same server use Echo as well, that encoding may include a prefix indicating it pertains to the RD's counter.
the value associated with a resource needs to be kept across the removal of registrations if the same registration resource is to be reused.
the counter can be reset (and the values of removed resources forgotten) when all previous security associations are reset.
this is the "Persistent Counter" method of [RFC9175], Appendix A.
- Preemptive Echo values. the current state counter can be sent in an Echo option not only when requests are rejected with 4.01 (Unauthorized), but also with successful responses. that way, clients can be given Echo values sufficient for their next request on a regular basis ๐ this is also described in Section 2.3 of [RFC9175]
while endpoints may discard received Echo values at leisure between requests, they are encouraged to keep these values for the next request to avoid extra round trips ๐
- if the RD can ensure that only one security association has modifying access to any registration at any given time, and that security association provides order on the requests, that order is sufficient to show request freshness ๐
5.3.4.2. Examples of Echo Usage
this shows an endpoint that has forgotten the server's latest Echo value and temporarily reduces its registration lifetime ๐ฅบ
Req: POST /rd/4521?lt=7200
Res: 4.01 Unauthorized
Echo: 0x0123
(EP tries again immediately.)
Req: POST /rd/4521?lt=7200
Echo: 0x0123
Res: 2.04 Changed
Echo: 0x0124
(Later, the EP regains its confidence in its long-term reachability.)
Req: POST /rd/4521?lt=90000
Echo: 0x0124
Res: 2.04 Changed
Echo: 0x0247Figure 18: Example Update of a Registration
the other examples don't show Echo options, for two reasons: (1) simplicity, and (2) they lack the context that would give any example values meaning โจ
6. RD Lookup ๐โจ
Okay bestie, so the RD needs a lookup interface so people can actually find the stuff registered with it ๐ง. This doc defines a default one, but RDs are also allowed to get extra fancy ๐ . We're talking resource descriptions in other formats, like JSON or CBOR link format ([CORE-LINKS-JSON]), or smarter lookups like context- or semantic-based ones on resources that got discovered on their own.
RD lookup lets you search for endpoints and resources using the attributes defined in this doc, and it's built for the CoRE Link Format ๐. The result of a lookup request is the list of links (if any) that match the type of lookup. So an endpoint lookup MUST return a list of endpoints, and a resource lookup MUST return a list of links to resources. No exceptions, periodt ๐ค.
The lookup type a lookup resource implements is shown by its resource type, as per Table 1:
+=============+====================+===========+
| Lookup Type | Resource Type | Mandatory |
+=============+====================+===========+
| Resource | core.rd-lookup-res | Mandatory |
+-------------+--------------------+-----------+
| Endpoint | core.rd-lookup-ep | Mandatory |
+-------------+--------------------+-----------+
Table 1: Lookup Types6.1. Resource Lookup
Resource lookup gives you links that are semantically equivalent to the links the registrant submitted to the RD ๐. The links and link parameters you get back are equal to the originally submitted ones, with two tweaks: the target reference is fully resolved, and the anchor reference is fully resolved too, but only if it shows up in the lookup result at all.
Links that didn't have an anchor attribute in the registration come back without one. Links where href or anchor was submitted as a (full) URI come back with that attribute unchanged ๐ฆ.
These rules mean the client can read the response as plain links without knowing anything about how the RD stores things ๐ง .
The RD MAY swap the registration base URIs for a configured intermediate proxy, like when you're using an HTTP lookup interface for CoAP endpoints ๐ธ.
If the base URI of a registration contains a link-local address, the RD MUST NOT show its links unless the lookup was made from the link where the registered endpoint can be reached. And the RD MUST NOT include zone identifiers in the resolved URIs. Not me letting zone IDs leak into the results ๐.
6.2. Lookup Filtering
Using the Accept option, the requester decides whether the returned list comes back in CoRE Link Format (application/link-format, which is the default) or in an alternate content format (like the ones from [CORE-LINKS-JSON]) โจ.
You MAY include multiple search criteria in a lookup. Every criterion you include MUST match for a link to be returned ๐. And the RD MUST support matching with multiple search criteria, no cap ๐ฅ.
A link matches a search criterion if it has an attribute with the same name and the same value. A trailing * wildcard is allowed, as in Section 4.1 of [RFC6690] ๐งฉ. Attributes defined as relation-types (in the link-format ABNF) match if the search value matches any of their values (see Section 4.1 of [RFC6690]). For example, ?if=tag:example.net,2020:sensor matches ;if="example.regname tag:example.net,2020:sensor";.
A resource link also matches a search criterion if its endpoint would match that criterion. And the other way around: an endpoint link matches a search criterion if any of its resource links matches it ๐.
Note that href is a valid search criterion and matches target references ๐. 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 (so, not relative references), and they're 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 one. The anchor attribute is usable for resource lookups and, if you query it, it MUST be in URI form as well ๐ .
Two extra query parameters, "page" and "count", let you get lookup results in chunks using pagination ๐. count says how many links to return, and page says which subset you want. The links are organized into sequential pages, each containing count links, starting with link zero and page zero. So a count of 10 and a page of 0 gives you the first 10 links in the result set (links 0-9). A count of 10 and a page of 1 gives you the next page, containing links 10-19, and so on ๐.
Unlike block-wise transfer of a complete result set, these parameters make sure each chunk of results can be interpreted on its own. That keeps processing simple, but it can cause duplicate or missed items if it happens at the same time as changes from the registration interface ๐ญ.
Endpoints that want a lookup result over and over, or continuously, can use mechanisms like ETag caching, resource observation [RFC7641], or any future mechanism that might allow more efficient observations of collections ๐. These are advertised, detected, and used according to their own specifications, and they can be used with the lookup interface like 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 ๐. The notification contains the successful current response to the given request. This holds especially when there are zero matching links (see the "Success" item below) ๐.
The lookup interface is specified as follows:
- Interaction:
Client -> RD - Method:
GET - URI Template:
{+type-lookup-location}{?page,count,search*} - URI Template Variables:
type-lookup-location :=RD lookup URI for a given lookup type (mandatory). The address is discovered as described in Section 4.3.search :=Search criteria for limiting the number of results (optional). The search criteria are an associative array, expressed in a form-style query, as per the URI Template (see [RFC6570], Sections 2.4.2 and 3.2.8).page :=Page (optional). This parameter cannot be used without the count parameter. Results are returned from the result set in pages that containcountlinks, starting from index (page * count). Page numbering starts with zero.count :=Count (optional). The number of results is limited to this parameter value. If the page parameter is also present, the response MUST only includecountlinks, starting with the (page * count) link in the result set from the query. If the count parameter is not present, then the response MUST return all matching links in the result set. Link numbering starts with zero.
- Accept:
absent,application/link-format, or any other indicated media type representing web links
The following response codes are defined for this interface:
- Success:
2.05 (Content)or200 (OK)with an application/link-format or other web link payload containing matching entries for the lookup. The payload can contain zero links (which is an empty payload in the link format described in [RFC6690], but could also be[]in JSON-based formats), indicating that no entities matched the request โจ.
6.3. Resource Lookup Examples
The examples in this section assume the existence of CoAP hosts with a default CoAP port of 61616 ๐. HTTP hosts are possible too, and they don't change the nature of the examples.
The following example shows a client performing a resource lookup with the example resource lookup locations discovered in Figure 5:
Req: GET /rd-lookup/res?rt=tag:example.org,2020:temperature
Res: 2.05 Content
Payload:
<coap://[2001:db8:3::123]:61616/temp>;
rt="tag:example.org,2020:temperature"
Figure 19: Example of a Resource LookupA client that wants to be notified of new resources as they show up can use this observation ๐:
Req: GET /rd-lookup/res?rt=tag:example.org,2020:light
Observe: 0
Res: 2.05 Content
Observe: 23
Payload: empty
(at a later point in time)
Res: 2.05 Content
Observe: 24
Payload:
<coap://[2001:db8:3::124]/west>;rt="tag:example.org,2020:light",
<coap://[2001:db8:3::124]/south>;rt="tag:example.org,2020:light",
<coap://[2001:db8:3::124]/east>;rt="tag:example.org,2020:light"
Figure 20: Example of an Observing Resource LookupThe following example shows a client performing a paginated resource lookup ๐:
Req: GET /rd-lookup/res?page=0&count=5
Res: 2.05 Content
Payload:
<coap://[2001:db8:3::123]:61616/res/0>;ct=60,
<coap://[2001:db8:3::123]:61616/res/1>;ct=60,
<coap://[2001:db8:3::123]:61616/res/2>;ct=60,
<coap://[2001:db8:3::123]:61616/res/3>;ct=60,
<coap://[2001:db8:3::123]:61616/res/4>;ct=60
Req: GET /rd-lookup/res?page=1&count=5
Res: 2.05 Content
Payload:
<coap://[2001:db8:3::123]:61616/res/5>;ct=60,
<coap://[2001:db8:3::123]:61616/res/6>;ct=60,
<coap://[2001:db8:3::123]:61616/res/7>;ct=60,
<coap://[2001:db8:3::123]:61616/res/8>;ct=60,
<coap://[2001:db8:3::123]:61616/res/9>;ct=60
Figure 21: Example of Paginated Resource LookupThe following example shows a client looking up all resources of all endpoints of a given endpoint type ๐. It assumes that two endpoints (with endpoint names sensor1 and sensor2) have already registered with their respective addresses (coap://sensor1.example.com and coap://sensor2.example.com), and that they posted the very payload of the 6th response of Section 5 of [RFC6690].
It shows how absolute link targets stay unmodified while relative ones get resolved ๐ฆ:
Req: GET /rd-lookup/res?et=tag:example.com,2020:platform
Res: 2.05 Content
Payload:
<coap://sensor1.example.com/sensors>;ct=40;title="Sensor Index",
<coap://sensor1.example.com/sensors/temp>;rt=temperature-c;if=sensor,
<coap://sensor1.example.com/sensors/light>;rt=light-lux;if=sensor,
<http://www.example.com/sensors/t123>;rel=describedby;
anchor="coap://sensor1.example.com/sensors/temp",
<coap://sensor1.example.com/t>;rel=alternate;
anchor="coap://sensor1.example.com/sensors/temp",
<coap://sensor2.example.com/sensors>;ct=40;title="Sensor Index",
<coap://sensor2.example.com/sensors/temp>;rt=temperature-c;if=sensor,
<coap://sensor2.example.com/sensors/light>;rt=light-lux;if=sensor,
<http://www.example.com/sensors/t123>;rel=describedby;
anchor="coap://sensor2.example.com/sensors/temp",
<coap://sensor2.example.com/t>;rel=alternate;
anchor="coap://sensor2.example.com/sensors/temp"
Figure 22: Example of a Resource Lookup from Multiple Endpoints6.4. Endpoint Lookup ๐ฆ
okay so the endpoint lookup gives you links to registration resources and info about them, but bestie, only the endpoint that registered something can actually change it ๐
Endpoint registration resources come with a bunch of labels: their endpoint names (ep), sectors (d, if there is one), and a registration base URI (base). If no explicit base was given, base shows the registrant-EP's address instead ๐ They also get a fixed resource type of rt="core.rd-ep". The lifetime (lt) is NOT reported, so don't go looking for it ๐
โโ๏ธ Any extra endpoint attributes get added as target attributes on the endpoint link, unless their spec says to do something different.
Links to endpoints SHOULD be written in path-absolute form, or as full URIs if that's needed. (This keeps the output valid for Limited Link Format, which is in Appendix C ๐)
Base addresses with link-local addresses MUST NOT include zone identifiers ๐ซ And those registrations MUST NOT be shown unless the lookup was made from the same link where the registration came from. Link-local stuff stays local, periodt ๐ฟ
The endpoint lookup does expose the registration resources, but the RD doesn't have to make them reachable for clients. Clients SHOULD NOT try to dereference or manipulate them. Hands off, sweetie ๐
An RD is allowed to report registrations whose URI scheme and authority are different from the lookup resource's. So lookup clients MUST be ready to see arbitrary URIs as registration resources and treat them as opaque identifiers ๐ฎ What those links actually mean is left for future specs to figure out. Not our problem rn โจ
Here's a client doing an endpoint lookup that's limited to endpoints of endpoint type tag:example.com,2020:platform:
Req: GET /rd-lookup/ep?et=tag:example.com,2020:platform
Res: 2.05 Content
Payload:
</rd/1234>;base="coap://[2001:db8:3::127]:61616";ep=node5;
et="tag:example.com,2020:platform";ct=40;rt=core.rd-ep,
</rd/4521>;base="coap://[2001:db8:3::129]:61616";ep=node7;
et="tag:example.com,2020:platform";ct=40;d=floor-3;
rt=core.rd-ep
Figure 23: Example of Endpoint Lookup7. Security Policies ๐
okay the security policies for an RD depend a LOT on the app, so they're not set in stone here ๐ This section is just a list of things apps should think about when they describe how they use the RD. It's not claiming to cover every case, fr fr.
It uses the terms from [ACE-OAUTH-AUTHZ], where the RD is the Resource Server (RS), and both registrant-EPs and lookup clients are Clients (C) getting help from an Authorization Server (AS). That doesn't rule out other schemes, like ones based on certificates and Public Key Infrastructures (PKIs) ๐
Any, all, or none of these can apply to an app. Which ones matter depends on what the app is trying to protect ๐ก๏ธ
Security policies get set by configuring the RD or by what the implementation chooses. Lookup clients (and endpoints too, where it matters) can only trust an RD to uphold them if the RD is authenticated and authorized to act as an RD for the app's requirements. Trust is earned, babe ๐
7.1. Endpoint Name ๐
Whenever an RD needs to give trustworthy results to clients doing endpoint lookup, or resource lookup filtered on the endpoint name, the RD must make sure the registrant is authorized to use that endpoint name. This applies to the registration AND later operations on the registration resource ๐ It doesn't matter whether the registrant-EP itself is the client or a CT is doing the registering. The RD can't tell the difference, and CTs can use credentials that only authorize operations on that one endpoint name, or on a wider range of names โจ
How the endpoint name and sector get carried over when certificates are used is up to the concrete security policy. For example, the policy might say how SubjectAltName dNSName entries map to endpoint and domain names ๐
7.1.1. Random Endpoint Names ๐ฒ
Now, in apps where the RD does NOT check the endpoint name, the authorized registering endpoint can make up a random number (or string) to identify itself. The RD should then remember unique properties of the registrant, tie them to the registration for as long as its registration resource is active (which can be longer than the registration's lifetime), and require those same properties for operations on the registration resource ๐ง
Registrants who are okay picking a different identifier when their first attempt gets rejected as unauthorized (or attempts, if there are two collisions in a row, which is super unlikely ๐) should pick an identifier at least twice as long as what it'd take to enumerate the expected number of registrants. Registrants with no backup plan should pick way longer endpoint names, like Universally Unique Identifier (UUID) URNs [RFC4122] ๐
7.2. Entered Links ๐
When lookup clients expect certain types of links to only come from certain endpoints, the RD needs to filter the links an endpoint is allowed to register ๐ง
Like, if clients use an RD to find a server that offers firmware updates, then any registrant that wants to register (or update) links to firmware sources needs suitable credentials to do it, no matter what its endpoint name is ๐
How bad an undesirable link is depends on the app. If the client requires the firmware server to present credentials as a firmware server, a fake link can only make the client reveal that it wants updates and slow it down until it finds a legit firmware server. If the client accepts ANY credentials from the server as long as they fit the provided URI, the damage is way bigger ๐ฌ
An RD may also require that links only get registered if the registrant is authorized to publish info about the link's anchor (or even its target). One way to do this is to make the registrant present the same credentials as a registering client that it would need to present as a server when contacted at the resource's URI. Those credentials can include using the address and port that are part of the URI. Heads up that this kind of restriction puts SEVERE practical limits on which links can be registered ๐
Like I said, the impact of bad links depends on how much the lookup client relies on the RD. To avoid those limits, RD apps should consider telling lookup clients to only use the discovered info as hints, and describe which pieces of info need verifying because they affect the app's security. A simple way to verify is to request it again from an authorized server, usually the one hosting the target resource. That's basically what happens in Section 4.3 when the "URI discovery" step gets repeated ๐
7.3. Link Confidentiality ๐คซ
When registrants publish info to the RD that isn't available to every client that would query the registrant's /.well-known/core interface, or when lookups to that interface get stricter firewalling than lookups to the RD, the RD may need to limit which lookup clients can access the info ๐
In this case, it's the endpoint (not the lookup clients) that needs to be careful about checking the RD's authorization. The RD needs to check any lookup client's authorization before revealing info directly (in resource lookup) or indirectly (when it's used to satisfy a resource lookup search criterion) ๐
7.4. Segmentation ๐ฐ
Within a single RD, different security policies can apply at the same time ๐
One example is multi-tenant deployments separated by the sector (d) parameter. Some sectors might limit which endpoint names are available, while others use a random identifier approach for endpoint names and put limits on entered links based on their attributes instead ๐งฉ
You have to be careful in setups like this to figure out which access control measures apply to each operation. One easy way is to require the sector parameter on ALL operations, since no credentials are suitable for operations across sector borders anyway ๐ซถ
7.5. "First Come First Remembered": A Default Policy โจ
okay bestie, this one is the default vibe for when nobody has set anything up ๐ It's called "First Come First Remembered" and it's both a reference example for writing security policies AND a policy implementations can just use by default if there's no other config ๐ It's built for efficient discovery even in ad hoc settings, so yes, very practical ๐ซถ
Under this policy, the RD accepts registrations for any endpoint name that is not assigned to an active registration resource ๐ธ It also only accepts registration updates from the same endpoint ๐ฅ The policy is super minimal ๐ It doesn't make any promises to lookup clients about the claims in Sections 7.2 and 7.3, and the promises about Section 7.1 only last for the lifetime of that endpoint's registration โณ What it does promise the endpoint is that, for the whole duration of its registration, its links will be discoverable on the RD ๐ฆ
When a registration or operation is attempted, the RD MUST determine the client's subject name or public key ๐
- If the client's credentials indicate any subject name that is certified by any authority the RD recognizes (which may be the system's trust anchor store), all of those subject names get stored โจ With credentials based on CWT or JWT (the usual suspects with Authentication and Authorization for Constrained Environments (ACE) ๐ง), the Subject (sub) claim is stored as a single name, if it exists ๐ With X.509 certificates, the Common Name (CN) and the complete list of SubjectAltName entries are stored ๐ In both cases, the authority that certified the claim is stored too, because the subject may only be locally unique ๐ซถ
- Otherwise, if the client proves possession of a private key, the matching public key is stored ๐ This applies to both raw public keys and to public keys in certificates that failed the authority check above ๐
- If neither of those is present, a reference to the security session itself is stored ๐ฆ With (D)TLS, that's the connection itself or the session resumption information, if available โจ With OSCORE, it's the security context ๐
As part of the registration operation, that information is stored along with the registration resource ๐
The RD MUST accept all registrations whose registration resource is not already active, as long as they're made using a security layer the RD supports ๐
Any operation on a registration resource, including registrations that lead to an existing registration resource, MUST be rejected by the RD unless all the stored information is found in the new request's credentials ๐ โโ๏ธ
Note that, even though subject names are compared in this policy, they are never directly compared to endpoint names ๐ An endpoint also can't expect to "own" any particular endpoint name outside of an active registration, even if a certificate says so ๐ญ It's an accepted shortcoming of this approach that the endpoint has no indication of whether the RD remembers it by subject name or public key ๐ Recognition by subject happens on a best-effort basis, since the RD may not recognize any authority ๐
Clients MUST be prepared to pick a different endpoint name when rejected by the RD initially or after a change in their credentials ๐ธ Picking an endpoint name, as per Section 7.1.1, is an easy option for that ๐
For this policy to be usable without configuration, clients should not set a sector name in their registrations ๐ง An RD can set a default sector name for registrations accepted under this policy, which is especially useful in a segmented setup where different policies apply to different sectors ๐ How that gets configured, and any other configuration for such an RD (like the set of recognized authorities), is out of scope for this document ๐ซง
8. Security Considerations ๐
The security considerations in Section 5 of [RFC8288] and Section 6 of [RFC6690] apply ๐ The /.well-known/core resource may be protected, for example using DTLS when hosted on a CoAP server, as described in [RFC7252] ๐ก๏ธ
Access that is limited or affects sensitive data SHOULD be protected, for example using (D)TLS or OSCORE [RFC8613] ๐ Which aspects of the RD this affects depends on the security policies of the application (see Section 7) ๐
8.1. Discovery ๐
Most steps in discovery of the RD, and possibly its resources, are not covered by CoAP's security mechanisms ๐ฌ This won't endanger the security properties of the registrations and lookup itself (where the client requires authorization of the RD if it expects any security properties of the operation), but it may leak the client's intention to third parties and let them slow down the process ๐ข
To mitigate that, clients can retain the RD's address, use secure discovery options (such as configured addresses), and send queries for RDs in a very general form (for example, ?rt=core.rd* rather than ?rt=core.rd-lookup-ep) ๐ง โจ
8.2. Endpoint Identification and Authentication ๐ชช
An endpoint (name, sector) pair is unique within the set of endpoints registered by the RD ๐ An endpoint MUST NOT be identified by its protocol, port, or IP address, because these may change over the lifetime of an endpoint ๐ซ
Every operation performed by an endpoint on an RD SHOULD be mutually authenticated using a pre-shared key, a raw public key, or certificate-based security ๐ซถ
Consider the following threat: two devices, A and B, are registered at a single server ๐ฏ 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 ๐
Now, imagine a malicious device A wants to sabotage device B ๐ It uses its credentials during the DTLS exchange ๐ญ Then it specifies the endpoint name of device B as the name of its own endpoint in device A ๐ 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 when looking up what information to provision to the malicious device ๐ฑ
Endpoint authorization needs to be checked on registration and registration resource operations, independently of whether there are configured requirements on the credentials for a given endpoint name and sector (Section 7.1) or whether arbitrary names are accepted (Section 7.1.1) ๐ง
Simple registration could be used to circumvent address-based access control ๐ต๏ธโโ๏ธ An attacker would send a simple registration request with the victim's address as the source address, then later look up the victim's /.well-known/core content in the RD ๐ค Mitigation for this is recommended in Section 5.1 ๐ฉน
The registration resource path is visible to any client that is allowed endpoint lookup, and it can be extracted by resource lookup clients as well ๐ The same goes for registration attributes that are shown as target attributes or lookup attributes ๐ The RD needs to consider this in its choice of registration resource paths, and administrators or endpoints need to consider it in their choice of attributes ๐งฉ
8.3. Access Control ๐ช
Access control SHOULD be performed separately for the RD registration and lookup API paths ๐๏ธ Different endpoints may be authorized to register with an RD than those authorized to look up endpoints from the RD ๐ซง Such access control SHOULD be performed at as fine-grained a level as possible ๐ฌ For example, access control for lookups could be performed at the sector, endpoint, or resource level ๐ท
The precise access controls necessary (and the consequences of failing to enforce them) depend on the protection objectives of the application and the security policies (Section 7) derived from them ๐ฏ
8.4. Denial-of-Service Attacks ๐
Services that run over UDP unprotected are vulnerable to unknowingly amplifying and distributing a DoS attack, because UDP does not require a return routability check ๐ฃ Since RD lookup responses can be significantly larger than requests, RDs are prone to this ๐ณ
[RFC7252] describes this at length in its Section 11.3, including some mitigation by using small block sizes in responses ๐ [RFC9175] updates that by describing a source address verification mechanism using the Echo option ๐
8.5. Skipping Freshness Checks โฐ
When RD-based applications are built in which request freshness checks are not performed, these concerns need to be balanced ๏ฟฝterrificโ๏ธ
- When alterations to registration attributes are reordered, an attacker may create any combination of attributes ever set, with the attack difficulty determined by the security layer's replay properties ๐
For example, if Figure 18 were conducted without freshness assurances, an attacker could later reset the lifetime back to 7200 ๐ฑ Thus, the device is made unreachable to lookup clients ๐ต
- When registration updates without query parameters (which just serve to restart the lifetime) can be reordered, an attacker can use intercepted messages to give the appearance of the device being alive to the RD ๐ป
This is unacceptable when the RD's security policy promises reachability of endpoints (for example, when disappearing devices would trigger further investigation), but it may be acceptable with other policies ๐คทโโ๏ธ
9. IANA Considerations ๐งพ
9.1. Resource Types ๐ท๏ธ
IANA has added the following values to the "Resource Type (rt=) Link Target Attribute Values" subregistry of the "Constrained RESTful Environments (CoRE) Parameters" registry defined in [RFC6690]:
+====================+=============================+=============+
| Value | Description | Reference |
+====================+=============================+=============+
| core.rd | Directory resource of an RD | RFC 9176, |
| | | Section 4.3 |
+--------------------+-----------------------------+-------------+
| core.rd-lookup-res | Resource lookup of an RD | RFC 9176, |
| | | Section 4.3 |
+--------------------+-----------------------------+-------------+
| core.rd-lookup-ep | Endpoint lookup of an RD | RFC 9176, |
| | | Section 4.3 |
+--------------------+-----------------------------+-------------+
| core.rd-ep | Endpoint resource of an RD | RFC 9176, |
| | | Section 6 |
+--------------------+-----------------------------+-------------+
Table 2: Additions to Resource Type (rt=) Link Target
Attribute Values Subregistry9.2. IPv6 ND Resource Directory Address Option ๐
IANA has registered one new ND option type in the "IPv6 Neighbor Discovery Option Formats" subregistry of the "Internet Control Message Protocol version 6 (ICMPv6) Parameters" registry ๐
+======+===================================+===========+
| Type | Description | Reference |
+======+===================================+===========+
| 41 | Resource Directory Address Option | RFC 9176 |
+------+-----------------------------------+-----------+
Table 3: Addition to IPv6 Neighbor Discovery Option
Formats Subregistry9.3. RD Parameters Registry ๐
Okay bestie, so this section creates a brand new subregistry called "RD Parameters" inside the "Constrained RESTful Environments (CoRE) Parameters" registry ๐ง. The spec gives a starter set of parameters, but other standards that use this interface are expected to add their own โจ
Each entry in the registry must include:
- the human-readable name of the parameter,
- the short name, as used in query parameters or target attributes,
- syntax and validity requirements (if any),
- indication of whether it can be passed as a query parameter at registration of endpoints, passed as a query parameter in lookups, or expressed as a target attribute,
- a description, and
- a link to reference documentation.
The query parameter MUST be both a valid URI query key [RFC3986] and a token as used in [RFC8288]. Literally the key is a must-match ๐
The reference documentation must give details on whether the parameter can be updated and how it is to be processed in lookups ๐
The mechanisms around new RD parameters should be designed in such a way that they tolerate RD implementations that are unaware of the parameter, and expose any parameter passed at registration or updates in endpoint lookups. (For example, if a parameter used at registration were to be confidential, the registering endpoint should be instructed to only set that parameter if the RD advertises support for keeping it confidential at the discovery step.) ๐ Confidential stuff needs a bouncer, no cap.
Initial entries in this subregistry are as follows:
+==============+=======+==============+=====+=====================+
| Name | Short | Validity | Use | Description |
+==============+=======+==============+=====+=====================+
| Endpoint | ep | Unicode* | RLA | Name of the |
| Name | | | | endpoint |
+--------------+-------+--------------+-----+---------------------+
| Lifetime | lt | 1-4294967295 | R | Lifetime of the |
| | | | | registration in |
| | | | | seconds |
+--------------+-------+--------------+-----+---------------------+
| Sector | d | Unicode* | RLA | Sector to which |
| | | | | this endpoint |
| | | | | belongs |
+--------------+-------+--------------+-----+---------------------+
| Registration | base | URI | RLA | The scheme, |
| Base URI | | | | address, port, and |
| | | | | path at which this |
| | | | | server is available |
+--------------+-------+--------------+-----+---------------------+
| Page | page | Integer | L | Used for pagination |
+--------------+-------+--------------+-----+---------------------+
| Count | count | Integer | L | Used for pagination |
+--------------+-------+--------------+-----+---------------------+
| Endpoint | et | RFC 9176, | RLA | Semantic type of |
| Type | | Section | | the endpoint (see |
| | | 9.3.1 | | RFC 9176, |
| | | | | Section 9.4) |
+--------------+-------+--------------+-----+---------------------+
Table 4: New RD Parameters RegistryWhere:
Short: Short name used in query parameters or target attributesValidity:Unicode*= up to 63 bytes of UTF-8-encoded Unicode, with no control characters as per Section 5
Use:R= used at registrationL= used at lookupA= expressed in the target attribute
The descriptions for the options defined in this document are only summarized here. To which registrations they apply and when they are to be shown are described in the respective sections of this document. All their reference documentation entries point to this document. ๐ธ
The IANA policy for future additions to the subregistry is Expert Review, as described in [RFC8126]. The evaluation should consider formal criteria, duplication of functionality (i.e., is the new entry redundant with an existing one?), topical suitability (e.g., is the described property actually a property of the endpoint and not a property of a particular resource, in which case it should go into the payload of the registration and need not be registered?), and the potential for conflict with commonly used target attributes (e.g., if could be used as a parameter for conditional registration if it were not to be used in lookup or attributes but would make a bad parameter for lookup because a resource lookup with an if query parameter could ambiguously filter by the registered endpoint property or the target attribute [RFC6690]). So basically the reviewers are the bouncers, and they check for dupes, wrong vibes, and clashes ๐
9.3.1. Full Description of the "Endpoint Type" RD Parameter ๐ฆ
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 (et=) RD Parameter Values" subregistry. Endpoint types can be passed in the et query parameter as part of extra-attrs at the "registration" step of Section 5, are shown on endpoint lookups using the et target attribute, and can be filtered for using et as a search criterion in resource and endpoint lookup. Multiple endpoint types are given as separate query parameters or link attributes.
Note that the endpoint type differs from the resource type in that it uses multiple attributes rather than space-separated values. As a result, RDs implementing this specification automatically support correct filtering in the lookup interfaces from the rules for unknown endpoint attributes. The multiple attributes thing is the whole plot twist ๐คฏ
9.4. Endpoint Type (et=) RD Parameter Values ๐
This specification establishes a new subregistry called "Endpoint Type (et=) RD Parameter Values" within the "Constrained RESTful Environments (CoRE) Parameters" registry. The registry properties (required policy, requirements, and template) are identical to those of the "Resource Type (rt=) Link Target Attribute Values" subregistry defined in [RFC6690]; in short, the review policy is IETF Review for values starting with "core" and Specification Required for others.
The requirements to be enforced are:
- The values MUST be related to the purpose described in Section 9.3.1.
- The registered values MUST conform to the ABNF reg-rel-type definition of [RFC6690] and MUST NOT be a URI.
- It is recommended to use the period "." character for segmentation.
The initial contents of the registry are as follows:
+===============+====================================+===========+
| Value | Description | Reference |
+===============+====================================+===========+
| core.rd-group | An application group, as described | RFC 9176 |
| | in RFC 9176, Appendix A. | |
+---------------+------------------------------------+-----------+
Table 5: New Endpoint Type (et=) RD Parameter Values Registry9.5. Multicast Address Registration ๐ก
IANA has assigned the following multicast addresses for use by CoAP nodes:
- IPv4 -- "All CoRE Resource Directories" address
224.0.1.190, in the "Internetwork Control Block (224.0.1.0 - 224.0.1.255 (224.0.1/24))" subregistry within the "IPv4 Multicast Address Space Registry". As the address is used for discovery that may span beyond a single network, it has come from the Internetwork Control Block (224.0.1.x) [RFC5771]. - IPv6 -- "All CoRE Resource Directories" address
ff0x::fe, in the "Variable Scope Multicast Addresses" subregistry within the "IPv6 Multicast Address Space Registry" [RFC3307]. 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. โจ Only two scopes, so it's a pretty chill list ๐
9.6. Well-Known URIs ๐
IANA has registered the URI suffix "rd" in the "Well-Known URIs" registry as follows:
+============+===================+===========+===========+
| URI Suffix | Change Controller | Reference | Status |
+============+===================+===========+===========+
| rd | IETF | RFC 9176 | permanent |
+------------+-------------------+-----------+-----------+
Table 6: Addition to Well-Known URIs Registry9.7. Service Name and Transport Protocol Port Number Registry ๐ฅ
IANA has added four new items to the "Service Name and Transport Protocol Port Number Registry" as follows:
+==============+===========+=====================+===========+
| Service Name | Transport | Description | Reference |
| | Protocol | | |
+==============+===========+=====================+===========+
| core-rd | udp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
+--------------+-----------+---------------------+-----------+
| core-rd-dtls | udp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
| | | over DTLS | |
+--------------+-----------+---------------------+-----------+
| core-rd | tcp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
| | | over TCP | |
+--------------+-----------+---------------------+-----------+
| core-rd-tls | tcp | Resource Directory | RFC 9176 |
| | | accessed using CoAP | |
| | | over TLS | |
+--------------+-----------+---------------------+-----------+
Table 7: Additions to Service Name and Transport Protocol
Port Number Registry10. Examples ๐ง
Two examples are presented: a lighting installation example in Section 10.1 and a Lightweight M2M (LwM2M) example in Section 10.2.
10.1. Lighting Installation ๐ก
This example shows a simplified lighting installation that makes use of the RD with a CoAP interface to facilitate the installation and startup of the application code in the lights and sensors. In particular, the example leads to the definition of a group and the enabling of the corresponding multicast address, as described in Appendix A. No conclusions must be drawn on the realization of actual installation or naming procedures, because the example only emphasizes some of the issues that may influence the use of the RD and does not pretend to be normative. Cute little lights story, but it's not the rulebook ๐ซถ
10.1.1. Installation Characteristics โจ
Okay so this example is for a managed installation, which means somebody is in charge of who gets to join ๐ฅ A Commissioning Tool (CT) is the thing that authorizes new nodes, names them, and names their services. The CT can hook up to the installation in a bunch of ways: it can be part of the installation network, connect over Wi-Fi to that network, connect over a GPRS link, or use some other method ๐
There are two naming authorities here, which is giving a lot of organization ๐ง. One is the network manager, who keeps the network and its connected interfaces working correctly. The other is the lighting manager, who keeps the networked lights and sensors working correctly. So you end up with two naming schemes, one from each manager ๐
The example setup has one presence sensor and two luminaries, luminary1 and luminary2. Each luminary has its own wireless interface ๐ธ Each luminary holds three lamps (left, right, and middle), and each luminary can be reached through one endpoint. Every lamp has a resource you can use to change its settings. The goal is for the presence sensor to tell a group of lamps when people are around. That group is the middle and left lamps of luminary1 plus the right lamp of luminary2 โจ
Before the lighting manager commissions anything, the network gets installed, and the network manager checks that the interfaces actually work ๐
Here's the catch: while the network is being installed, it might not be connected to DNS yet ๐ So CT, RD, the luminaries, and the sensor all get Stateless Address Autoconfiguration (SLAAC) IPv6 addresses. The addresses in Table 8 below stand in for those in the examples that follow.
+=================+================+
| Name | IPv6 address |
+=================+================+
| luminary1 | 2001:db8:4::1 |
+-----------------+----------------+
| luminary2 | 2001:db8:4::2 |
+-----------------+----------------+
| Presence sensor | 2001:db8:4::3 |
+-----------------+----------------+
| RD | 2001:db8:4::ff |
+-----------------+----------------+
Table 8: Addresses Used in the
ExamplesNext up, Section 10.1.2 shows how the RD gets used during installation ๐ฆ
10.1.2. RD Entries ๐
DNS isn't always reachable during installation, so this section sticks with the SLAAC addresses ๐
For discovery, the resource types (rt) of the devices are the key thing. The lamps in the luminaries have rt=tag:example.com,2020:light, and the presence sensor has rt=tag:example.com,2020:p-sensor. The endpoints get names that matter to the light installation manager. Here, luminary1, luminary2, and the presence sensor are all in room 2-4-015. luminary1 sits at the window, while luminary2 and the presence sensor sit at the door, and the endpoint names reflect that physical location. The middle, left, and right lamps are reached at /light/middle, /light/left, and /light/right, respectively. The identifiers the RD cares about are in Table 9 โจ
+=========+================+========+===============================+
|Name |Endpoint |Resource| Resource Type |
| | |Path | |
+=========+================+========+===============================+
|luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light |
| | |left | |
+---------+----------------+--------+-------------------------------+
|luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light |
| | |middle | |
+---------+----------------+--------+-------------------------------+
|luminary1|lm_R2-4-015_wndw|/light/ | tag:example.com,2020:light |
| | |right | |
+---------+----------------+--------+-------------------------------+
|luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light |
| | |left | |
+---------+----------------+--------+-------------------------------+
|luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light |
| | |middle | |
+---------+----------------+--------+-------------------------------+
|luminary2|lm_R2-4-015_door|/light/ | tag:example.com,2020:light |
| | |right | |
+---------+----------------+--------+-------------------------------+
|Presence |ps_R2-4-015_door|/ps | tag:example.com,2020:p-sensor |
|sensor | | | |
+---------+----------------+--------+-------------------------------+
Table 9: RD IdentifiersWe assume the CT has already done RD discovery and got back a response like the one in the example in Section 4.3 ๐
Now the CT adds the endpoints of the luminaries and the sensor to the RD. It uses the registration base URI parameter (base) to specify the interface address ๐
Req: POST coap://[2001:db8:4::ff]/rd
?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015
Payload:
</light/left>;rt="tag:example.com,2020:light",
</light/middle>;rt="tag:example.com,2020:light",
</light/right>;rt="tag:example.com,2020:light"
Res: 2.01 Created
Location-Path: /rd/4521
Req: POST coap://[2001:db8:4::ff]/rd
?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015
Payload:
</light/left>;rt="tag:example.com,2020:light",
</light/middle>;rt="tag:example.com,2020:light",
</light/right>;rt="tag:example.com,2020:light"
Res: 2.01 Created
Location-Path: /rd/4522
Req: POST coap://[2001:db8:4::ff]/rd
?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]&d=R2-4-015
Payload:
</ps>;rt="tag:example.com,2020:p-sensor"
Res: 2.01 Created
Location-Path: /rd/4523
Figure 24: Example of Registrations a CT Enters into an RDThe sector name d=R2-4-015 gets added so lookups stay efficient, since filtering on the ep name is more awkward ๐ง The CT gives the same sector name to the two luminaries and the presence sensor.
The group itself is specified in the RD. The base parameter is set to the site-local multicast address allocated to the group. In the POST below, the resources that every group member supports get published ๐
Req: POST coap://[2001:db8:4::ff]/rd
?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1]
Payload:
</light/left>;rt="tag:example.com,2020:light",
</light/middle>;rt="tag:example.com,2020:light",
</light/right>;rt="tag:example.com,2020:light"
Res: 2.01 Created
Location-Path: /rd/501
Figure 25: Example of a Multicast Group a CT Enters into an RDOnce the CT has filled up the RD, the luminaries' application can figure out which groups they belong to and turn on their interface for the multicast address ๐
The luminary already knows its sector and is set up to join any group that contains lights. So it searches for candidate groups and joins them ๐ก
Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
?d=R2-4-015&et=core.rd-group&rt=light
Res: 2.05 Content
Payload:
</rd/501>;ep=grp_R2-4-015;et=core.rd-group;
base="coap://[ff05::1]";rt=core.rd-ep
Figure 26: Example of a Lookup Exchange to Find Suitable
Multicast AddressesFrom the base parameter value in that response, the luminary learns the multicast address of the group ๐ซถ
The presence sensor can find out about groups that support resources with rt=tag:example.com,2020:light in its own sector by sending the same request the luminary sent. That's how the sensor learns which multicast address to use when it sends messages to the luminaries ๐ฆ
10.2. OMA Lightweight M2M (LwM2M)
OMA LwM2M is a profile for device services built on CoAP. It provides interfaces and operations for device management and device service enablement ๐
An LwM2M server is an instance of an LwM2M middleware service layer, and it contains an RD ([LwM2M], starting at page 36) ๐ธ
That RD only implements the registration interface, and no lookup is implemented. Instead, the LwM2M server gives access to the registered resources in a way that's similar to a reverse proxy ๐
The location of the LwM2M server and the RD URI path comes from the LwM2M bootstrap process, so no dynamic discovery of the RD is used ๐ง LwM2M servers and endpoints are not required to implement the /.well-known/core resource.
11. References โจ
11.1. Normative References
[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>.
[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>.
[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>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[RFC9175] Amsรผss, C., Preuร Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC9175, February 2022,
<https://www.rfc-editor.org/info/rfc9175>.11.2. Informative References โจ
okay bestie, here are the informative refs ๐ these ones aren't required reading to build the thing, but they're the good background stuff ๐ซถ
- [ACE-OAUTH-AUTHZ] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)", Work in Progress, Internet-Draft, draft-ietf-ace-oauth-authz-46, 8 November 2021,
https://datatracker.ietf.org/doc/html/draft-ietf-ace-oauth-authz-46. - [COAP-PROT-NEG] Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation", Work in Progress, Internet-Draft, draft-silverajan-core-coap-protocol-negotiation-09, 2 July 2018,
https://datatracker.ietf.org/doc/html/draft-silverajan-core-coap-protocol-negotiation-09. - [CORE-CORAL] Amsรผss, C. and T. Fossati, "The Constrained RESTful Application Language (CoRAL)", Work in Progress, Internet-Draft, draft-ietf-core-coral-05, 7 March 2022,
https://datatracker.ietf.org/doc/html/draft-ietf-core-coral-05. - [CORE-LINKS-JSON] Li, K., Rahman, A., and C. Bormann, Ed., "Representing Constrained RESTful Environments (CoRE) Link Format in JSON and CBOR", Work in Progress, Internet-Draft, draft-ietf-core-links-json-10, 26 February 2018,
https://datatracker.ietf.org/doc/html/draft-ietf-core-links-json-10. - [CORE-RD-DNS-SD] van der Stok, P., Koster, M., and C. Amsuess, "CoRE Resource Directory: DNS-SD mapping", Work in Progress, Internet-Draft, draft-ietf-core-rd-dns-sd-05, 7 July 2019,
https://datatracker.ietf.org/doc/html/draft-ietf-core-rd-dns-sd-05. - [ER] Chen, P., "The entity-relationship model--toward a unified view of data", ACM Transactions on Database Systems, Vol. 1, pp. 9-36, DOI 10.1145/320434.320440, March 1976,
https://doi.org/10.1145/320434.320440. - [LwM2M] Open Mobile Alliance, "Lightweight Machine to Machine Technical Specification: Transport Bindings (Candidate Version 1.1)", June 2018,
https://openmobilealliance.org/RELEASE/LightweightM2M/V1_1-20180612-C/OMA-TS-LightweightM2M_Transport-V1_1-20180612-C.pdf. - [RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306, August 2002,
https://www.rfc-editor.org/info/rfc3306. - [RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast Addresses", RFC 3307, DOI 10.17487/RFC3307, August 2002,
https://www.rfc-editor.org/info/rfc3307. - [RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix Reserved for Documentation", RFC 3849, DOI 10.17487/RFC3849, July 2004,
https://www.rfc-editor.org/info/rfc3849. - [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, July 2005,
https://www.rfc-editor.org/info/rfc4122. - [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
https://www.rfc-editor.org/info/rfc4944. - [RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI 10.17487/RFC5771, March 2010,
https://www.rfc-editor.org/info/rfc5771. - [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
https://www.rfc-editor.org/info/rfc6724. - [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. - [RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing IPv6 Zone Identifiers in Address Literals and Uniform Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874, February 2013,
https://www.rfc-editor.org/info/rfc6874. - [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014,
https://www.rfc-editor.org/info/rfc7228. - [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. - [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 Router Advertisement Options for DNS Configuration", RFC 8106, DOI 10.17487/RFC8106, March 2017,
https://www.rfc-editor.org/info/rfc8106. - [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. - [RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
https://www.rfc-editor.org/info/rfc8141. - [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
https://www.rfc-editor.org/info/rfc8613. - [T2TRG-REL-IMPL] Bormann, C., "impl-info: A link relation type for disclosing implementation information", Work in Progress, Internet-Draft, draft-bormann-t2trg-rel-impl-02, 27 September 2020,
https://datatracker.ietf.org/doc/html/draft-bormann-t2trg-rel-impl-02.
Appendix A. Groups Registration and Lookup ๐
okay this appendix is about groups, and it's giving multicast main character energy ๐ the RD-Group usage pattern lets you announce application groups inside an RD ๐ธ
Groups are stored as endpoint registrations ๐ 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 called a group name in this context ๐
The registration gets inserted into the RD by a Commissioning Tool, which might also be known as a group manager here. It does third-party registration and registration updates ๐ง
The links it registers SHOULD be available on all members that join the group. Depending on the application, members that lack some resources MAY be permissible if requests to them fail gracefully ๐ฆ
Here's an example of a CT registering a group named "lights", which has two resources โจ The directory resource path /rd is an example RD location found in a request like Figure 5. The group address is built from the reserved 2001:db8:: prefix in [RFC3849] as a unicast-prefix-based site-local address (see [RFC3306]) ๐
Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group
&base=coap://[ff35:30:2001:db8:f1::8000:1]
Content-Format: 40
Payload:
</light>;rt="tag:example.com,2020:light";
if="tag:example.net,2020:actuator",
</color-temperature>;if="tag:example.net,2020:parameter";u=K
Res: 2.01 Created
Location-Path: /rd/12
Figure 27: Example Registration of a GroupIn this example, the group manager can easily let devices with no writable color-temperature join ๐ they'd still respond to brightness-changing commands, so they're totally fine. But if the group had a single resource that sets brightness and color-temperature atomically, endpoints would need to support both properties ๐
The resources of a group can be looked up like any other resource, and the group registrations (along with any extra registration parameters) can be looked up using the endpoint lookup interface ๐
Here's a client doing an endpoint lookup for all groups:
Req: GET /rd-lookup/ep?et=core.rd-group
Res: 2.05 Content
Payload:
</rd/12>;ep=lights&et=core.rd-group;
base="coap://[ff35:30:2001:f1:db8::8000:1]";rt=core.rd-ep
Figure 28: Example Lookup of GroupsAnd here's a client looking up all resources of all endpoints (groups) with et=core.rd-group:
Req: GET /rd-lookup/res?et=core.rd-group
Res: 2.05 Content
Payload:
<coap://[ff35:30:2001:db8:f1::8000:1]/light>;
rt="tag:example.com,2020:light";
if="tag:example.net,2020:actuator",
<coap://[ff35:30:2001:db8:f1::8000:1]/color-temperature>;
if="tag:example.net,2020:parameter";u=K,
Figure 29: Example Lookup of Resources Inside GroupsAppendix B. Web Links and the Resource Directory ๐ฆ
okay so understanding what a link-format document and its URI references actually mean is a whole journey through different docs ๐บ๏ธ there's [RFC3986] defining URIs, [RFC6690] defining link-format documents based on [RFC8288] (which defines Link header fields), and [RFC7252] providing the transport. This appendix sums up the mechanisms and semantics from an entry in /.well-known/core all the way to a resource lookup ๐
This text is mainly for people who are new to Constrained Restful Environments and are coming from applications that didn't use web mechanisms before ๐ท
B.1. A Simple Example ๐ง
Let's start with a very simple host, 2001:db8:f0::1 โจ A client using classical CoAP discovery ([RFC7252], Section 7) sends this multicast request to find neighbours that support resources with resource-type "temperature" ๐ซถ
The client sends a link-local multicast:
Req: GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature
Res: 2.05 Content
Payload:
</sensors/temp>;rt=temperature;ct=0
Figure 30: Example of Direct Resource DiscoveryThe response comes from the server, [2001:db8:f0::1]:5683 ๐
A practical client might just go ahead and make a new request to [2001:db8:f0::1]:5683 with Uri-Path sensors and temp, but the full steps for inserting into and retrieving from the RD, with no shortcuts, look like this ๐
B.1.1. Resolving the URIs ๐ฅ
The client parses the single returned link ๐ Its target (sometimes called "href") is /sensors/temp, which is a relative URI that needs resolving. The base URI coap://[ff02::fd]:5683/.well-known/core is used to resolve the reference against /sensors/temp ๐
The base URI of the requested resource can be built from the options of the CoAP GET request by following the steps of [RFC7252], Section 6.5 (with an addition at the end of Section 8.2), giving coap://[2001:db8:f0::1]/.well-known/core ๐
Because /sensors/temp starts with a single slash, the link's target gets resolved by replacing the path /.well-known/core from the base URI ([RFC3986], Section 5.2) with the relative target URI /sensors/temp. That gives coap://[2001:db8:f0::1]/sensors/temp ๐
B.1.2. Interpreting Attributes and Relations ๐ง โจ
okay so the payload is where the juicy details live, bestie ๐ the resource type of the target is "temperature", and its content format is text/plain (ct=0) ๐ธ
so a relation in a web link is a three-part statement, like a little sentence ๐ it names a relation between the "context resource" and the target resource. think "_This page_ has _its table of contents_ at _/toc.html_" ๐ and in link-format documents there's an implicit "host relation" with the default parameter rel="hosts" ๐ซถ
in our example, the context resource is implied to be coap:://[2001:db8:f0::1] by the default value of the anchor (see Appendix B.4) ๐ the full English version of the "host relation" is:
coap://[2001:db8:f0::1] is hosting the resource
coap://[2001:db8:f0::1]/sensors/temp, which is of the resource
type "temperature" and can be read in the text/plain content
format.B.2. A Slightly More Complex Example ๐ฆ
omitting the rt=temperature filter, the discovery query would have given some more links in the payload ๐
Req: GET coap://[ff02::fd]:5683/.well-known/core
Res: 2.05 Content
Payload:
</sensors/temp>;rt=temperature;ct=0,
</sensors/light>;rt=light-lux;ct=0,
</t>;anchor="/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;anchor="/sensors/temp";
rel=describedby
Figure 31: Extended Example of Direct Resource Discoveryokay so parsing the third link, the client hits the "anchor" parameter ๐ it's a URI relative to the base URI of the request, so it gets resolved to coap://[2001:db8:f0::1]/sensors/temp ๐ง that's the context resource of the link, so the "rel" statement isn't about the target and the base URI anymore, it's about the target and the resolved URI ๐ so the third link reads as:
coap://[2001:db8:f0::1]/sensors/temp has an alternate
representation at coap://[2001:db8:f0::1]/t.same resolution steps for the fourth link, which reads as coap://[2001:db8:f0::1]/sensors/temp is described by http://www.example.com/sensors/t123 ๐โจ
B.3. Enter the Resource Directory ๐
the RD tries to carry the semantics you get from classical CoAP discovery over to the resource lookup interface as faithfully as it can ๐
for the following queries, we'll assume the simple host used simple registration to register at the RD that was announced to it, sending this request from its UDP port [2001:db8:f0::1]:6553 ๐ฆ
Req: POST coap://[2001:db8:f0::ff]/.well-known/rd?ep=simple-host1
Res: 2.04 ChangedFigure 32: Example of a Simple Registration ๐
the RD accepted the registration and queried the simple host's /.well-known/core by itself ๐ซถ so the host is registered as an endpoint in the RD with the name "simple-host1" ๐ฅ the registration is active for 90000 seconds, and the endpoint registration base URI is coap://[2001:db8:f0::1], following the resolution steps in Appendix B.1.1 โจ it should be remarked that the base URI built that way always yields a URI of the form scheme://authority without a path suffix ๐
if the client now queries the RD the way it would have previously sent a multicast request, it goes through the RD discovery steps by fetching coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd-lookup-res, gets coap://[2001:db8:f0::ff]/rd-lookup/res as the resource lookup endpoint, and asks it for all temperature resources ๐ธ
Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature
Res: 2.05 Content
Payload:
<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0Figure 33: Example Exchange Performing Resource Lookup ๐ง
this is not _literally_ the same response it would've gotten from a multicast request, but it holds the equivalent statement ๐
coap://[2001:db8:f0::1] is hosting the resource
coap://[2001:db8:f0::1]/sensors/temp, which is of the resource
type "temperature" and can be accessed using the text/plain
content format.to wrap up the examples, the client could also query all resources hosted at the endpoint named "simple-host1" ๐
Req: GET coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1
Res: 2.05 Content
Payload:
<coap://[2001:db8:f0::1]/sensors/temp>;rt=temperature;ct=0,
<coap://[2001:db8:f0::1]/sensors/light>;rt=light-lux;ct=0,
<coap://[2001:db8:f0::1]/t>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate,
<http://www.example.com/sensors/t123>;
anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=describedbyFigure 34: Extended Example Exchange Performing Resource Lookup โจ
all the target and anchor references are already in absolute form there, so they don't need resolving any further ๐
if the simple host had done an equivalent full registration with a base= parameter (e.g., ?ep=simple-host1&base=coap+tcp://sh1.example.com), that context would've been used to resolve the relative anchor values instead, giving these analogous links ๐ฆ
<coap+tcp://sh1.example.com/sensors/temp>;rt=temperature;ct=0Figure 35: Example Payload of a Response to a Resource Lookup with a Dedicated Base URI ๐
B.4. A Note on Differences between Link-Format and Link Header Fields ๐ธ
okay bestie, link-format and Link header fields look super similar and both use the same model of typed links ๐ซถ but there are some differences between [RFC6690] and [RFC8288] ๐ when you're implementing an RD or talking to one, you have to be careful to follow the behavior described in [RFC6690] whenever application/link-format representations are used ๐
- "Default value of anchor": under both [RFC6690] and [RFC8288], relative references in the term inside the angle brackets (the target) and the anchor attribute get resolved against the relevant base URI (usually the URI used to retrieve the entity), and they're independent of each other ๐ง
When, in a Link header [RFC8288], the anchor attribute is absent, the link's context is the URI of the selected representation (and usually equal to the base URI) ๐
In links per [RFC6690], if the anchor attribute is absent, the default value is the Origin of (for all relevant cases, the URI reference / resolved against) the link's target ๐
- "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 ๐ Link header fields, on the other hand, are practically ASCII strings that use percent encoding for non-ASCII characters, stating the encoding explicitly when required โจ
for example, a Link header field in a page about a Swedish city might read:
Link: </temperature/Malm%C3%B6>;rel=live-environment-datawhile a link-format document from the same source might describe the link as:
</temperature/Malmรถ>;rel=live-environment-dataAppendix C. Limited Link Format ๐
okay so the CoRE Link Format, as described in [RFC6690], has been interpreted differently by implementers ๐ญ a strict implementation rules out some RD use cases (e.g., base values with path components in combination with absent anchors) ๐
this appendix describes a subset of link format documents called the Limited Link Format ๐ฆ the one rule here isn't very limiting in practice (all examples in [RFC6690] and all deployments the authors know of already stick to it) but it makes RD servers way easier to implement ๐ซถ
it applies to representations in the application/link-format media type and to any other media types that inherit [RFC6690], Section 2.1 ๐
a link format representation is in the Limited Link Format if, for each link in it, the following applies:
All URI references either follow the URI or the path-absolute ABNF rule of [RFC3986] (i.e., the target and anchor each either start with a scheme or with a single slash) ๐
Acknowledgments ๐ซถ
Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders Brandt, Matthieu Vial, Jim Schaad, Mohit Sethi, Hauke Petersen, Hannes Tschofenig, Sampo Ukkola, Linyi Tian, Jan Newmarch, Matthias Kovatsch, Jaime Jimenez, and Ted Lemon have provided helpful comments, discussions, and ideas to improve and shape this document ๐ธ Zach would also like to thank his colleagues from the EU FP7 SENSEI project, where many of the RD concepts were originally developed ๐
Authors' Addresses โจ
Christian Amsรผss (editor) Email: christian@amsuess.com
Zach Shelby Edge Impulse 3031 Tisch Way San Jose, 95128 United States of America Email: zach@edgeimpulse.com
Michael Koster PassiveLogic 524 H Street Antioch, CA 94509 United States of America Phone: +1-707-502-5136 Email: michaeljohnkoster@gmail.com
Carsten Bormann Universitรคt Bremen TZI Postfach 330440 D-28359 Bremen Germany Phone: +49-421-218-63921 Email: cabo@tzi.org
Peter van der Stok vanderstok consultancy Email: stokcons@bbhmail.nl
๐ฆ where this page comes from
- ietf: rfc9176 2022-04
- ๐ girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms