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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-02) exists of draft-bormann-t2trg-rel-impl-00 == Outdated reference: A later version (-09) exists of draft-hartke-t2trg-coral-07 == Outdated reference: A later version (-46) exists of draft-ietf-ace-oauth-authz-22 -- Obsolete informational reference (is this intentional?): RFC 7230 (Obsoleted by RFC 9110, RFC 9112) Summary: 0 errors (**), 0 flaws (~~), 12 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 CoRE Z. Shelby 3 Internet-Draft ARM 4 Intended status: Standards Track M. Koster 5 Expires: September 12, 2019 SmartThings 6 C. Bormann 7 Universitaet Bremen TZI 8 P. van der Stok 9 consultant 10 C. Amsuess, Ed. 11 March 11, 2019 13 CoRE Resource Directory 14 draft-ietf-core-resource-directory-20 16 Abstract 18 In many IoT applications, direct discovery of resources is not 19 practical due to sleeping nodes, disperse networks, or networks where 20 multicast traffic is inefficient. These problems can be solved by 21 employing an entity called a Resource Directory (RD), which contains 22 information about resources held on other servers, allowing lookups 23 to be performed for those resources. The input to an RD is composed 24 of links and the output is composed of links constructed from the 25 information stored in the RD. This document specifies the web 26 interfaces that a Resource Directory supports for web servers to 27 discover the RD and to register, maintain, lookup and remove 28 information on resources. Furthermore, new target attributes useful 29 in conjunction with an RD are defined. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at https://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on September 12, 2019. 48 Copyright Notice 50 Copyright (c) 2019 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents 55 (https://trustee.ietf.org/license-info) in effect on the date of 56 publication of this document. Please review these documents 57 carefully, as they describe your rights and restrictions with respect 58 to this document. Code Components extracted from this document must 59 include Simplified BSD License text as described in Section 4.e of 60 the Trust Legal Provisions and are provided without warranty as 61 described in the Simplified BSD License. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 66 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 3. Architecture and Use Cases . . . . . . . . . . . . . . . . . 6 68 3.1. Principles . . . . . . . . . . . . . . . . . . . . . . . 6 69 3.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 7 70 3.3. RD Content Model . . . . . . . . . . . . . . . . . . . . 8 71 3.4. Link-local addresses and zone identifiers . . . . . . . . 12 72 3.5. Use Case: Cellular M2M . . . . . . . . . . . . . . . . . 12 73 3.6. Use Case: Home and Building Automation . . . . . . . . . 13 74 3.7. Use Case: Link Catalogues . . . . . . . . . . . . . . . . 13 75 4. RD discovery and other interface-independent components . . . 14 76 4.1. Finding a Resource Directory . . . . . . . . . . . . . . 15 77 4.1.1. Resource Directory Address Option (RDAO) . . . . . . 17 78 4.2. Payload Content Formats . . . . . . . . . . . . . . . . . 18 79 4.3. URI Discovery . . . . . . . . . . . . . . . . . . . . . . 19 80 5. Registration . . . . . . . . . . . . . . . . . . . . . . . . 21 81 5.1. Simple Registration . . . . . . . . . . . . . . . . . . . 26 82 5.2. Third-party registration . . . . . . . . . . . . . . . . 28 83 5.3. Operations on the Registration Resource . . . . . . . . . 28 84 5.3.1. Registration Update . . . . . . . . . . . . . . . . . 29 85 5.3.2. Registration Removal . . . . . . . . . . . . . . . . 32 86 5.3.3. Further operations . . . . . . . . . . . . . . . . . 32 87 6. RD Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 33 88 6.1. Resource lookup . . . . . . . . . . . . . . . . . . . . . 33 89 6.2. Lookup filtering . . . . . . . . . . . . . . . . . . . . 34 90 6.3. Resource lookup examples . . . . . . . . . . . . . . . . 36 91 6.4. Endpoint lookup . . . . . . . . . . . . . . . . . . . . . 38 92 7. Security policies . . . . . . . . . . . . . . . . . . . . . . 39 93 7.1. Secure RD discovery . . . . . . . . . . . . . . . . . . . 40 94 7.2. Secure RD filtering . . . . . . . . . . . . . . . . . . . 40 95 7.3. Secure endpoint Name assignment . . . . . . . . . . . . . 41 97 8. Security Considerations . . . . . . . . . . . . . . . . . . . 41 98 8.1. Endpoint Identification and Authentication . . . . . . . 41 99 8.2. Access Control . . . . . . . . . . . . . . . . . . . . . 42 100 8.3. Denial of Service Attacks . . . . . . . . . . . . . . . . 42 101 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 102 9.1. Resource Types . . . . . . . . . . . . . . . . . . . . . 43 103 9.2. IPv6 ND Resource Directory Address Option . . . . . . . . 43 104 9.3. RD Parameter Registry . . . . . . . . . . . . . . . . . . 43 105 9.3.1. Full description of the "Endpoint Type" Registration 106 Parameter . . . . . . . . . . . . . . . . . . . . . . 45 107 9.4. "Endpoint Type" (et=) RD Parameter values . . . . . . . . 45 108 9.5. Multicast Address Registration . . . . . . . . . . . . . 46 109 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 46 110 10.1. Lighting Installation . . . . . . . . . . . . . . . . . 46 111 10.1.1. Installation Characteristics . . . . . . . . . . . . 47 112 10.1.2. RD entries . . . . . . . . . . . . . . . . . . . . . 48 113 10.2. OMA Lightweight M2M (LWM2M) Example . . . . . . . . . . 50 114 10.2.1. The LWM2M Object Model . . . . . . . . . . . . . . . 51 115 10.2.2. LWM2M Register Endpoint . . . . . . . . . . . . . . 52 116 10.2.3. LWM2M Update Endpoint Registration . . . . . . . . . 54 117 10.2.4. LWM2M De-Register Endpoint . . . . . . . . . . . . . 54 118 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 54 119 12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 54 120 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 63 121 13.1. Normative References . . . . . . . . . . . . . . . . . . 63 122 13.2. Informative References . . . . . . . . . . . . . . . . . 64 123 Appendix A. Groups Registration and Lookup . . . . . . . . . . . 66 124 Appendix B. Web links and the Resource Directory . . . . . . . . 67 125 B.1. A simple example . . . . . . . . . . . . . . . . . . . . 67 126 B.1.1. Resolving the URIs . . . . . . . . . . . . . . . . . 68 127 B.1.2. Interpreting attributes and relations . . . . . . . . 68 128 B.2. A slightly more complex example . . . . . . . . . . . . . 69 129 B.3. Enter the Resource Directory . . . . . . . . . . . . . . 69 130 B.4. A note on differences between link-format and Link 131 headers . . . . . . . . . . . . . . . . . . . . . . . . . 71 132 Appendix C. Limited Link Format . . . . . . . . . . . . . . . . 72 133 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 72 135 1. Introduction 137 In the work on Constrained RESTful Environments (CoRE), a REST 138 architecture suitable for constrained nodes (e.g. with limited RAM 139 and ROM [RFC7228]) and networks (e.g. 6LoWPAN [RFC4944]) has been 140 established and is used in Internet-of-Things (IoT) or machine-to- 141 machine (M2M) applications such as smart energy and building 142 automation. 144 The discovery of resources offered by a constrained server is very 145 important in machine-to-machine applications where there are no 146 humans in the loop and static interfaces result in fragility. The 147 discovery of resources provided by an HTTP Web Server is typically 148 called Web Linking [RFC8288]. The use of Web Linking for the 149 description and discovery of resources hosted by constrained web 150 servers is specified by the CoRE Link Format [RFC6690]. However, 151 [RFC6690] only describes how to discover resources from the web 152 server that hosts them by querying "/.well-known/core". In many 153 constrained scenarios, direct discovery of resources is not practical 154 due to sleeping nodes, disperse networks, or networks where multicast 155 traffic is inefficient. These problems can be solved by employing an 156 entity called a Resource Directory (RD), which contains information 157 about resources held on other servers, allowing lookups to be 158 performed for those resources. 160 This document specifies the web interfaces that a Resource Directory 161 supports for web servers to discover the RD and to register, 162 maintain, lookup and remove information on resources. Furthermore, 163 new target attributes useful in conjunction with a Resource Directory 164 are defined. Although the examples in this document show the use of 165 these interfaces with CoAP [RFC7252], they can be applied in an 166 equivalent manner to HTTP [RFC7230]. 168 2. Terminology 170 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 171 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 172 "OPTIONAL" in this document are to be interpreted as described in 173 [RFC2119]. The term "byte" is used in its now customary sense as a 174 synonym for "octet". 176 This specification requires readers to be familiar with all the terms 177 and concepts that are discussed in [RFC3986], [RFC8288] and 178 [RFC6690]. Readers should also be familiar with the terms and 179 concepts discussed in [RFC7252]. To describe the REST interfaces 180 defined in this specification, the URI Template format is used 181 [RFC6570]. 183 This specification makes use of the following additional terminology: 185 resolve against 186 The expression "a URI-reference is _resolved against_ a base URI" 187 is used to describe the process of [RFC3986] Section 5.2. 188 Noteworthy corner cases are that if the URI-reference is a (full) 189 URI and resolved against any base URI, that gives the original 190 full URI, and that resolving an empty URI reference gives the base 191 URI without any fragment identifier. 193 Resource Directory 194 A web entity that stores information about web resources and 195 implements the REST interfaces defined in this specification for 196 registration and lookup of those resources. 198 Sector 199 In the context of a Resource Directory, a sector is a logical 200 grouping of endpoints. 202 The abbreviation "d=" is used for the sector in query parameters 203 for compatibility with deployed implementations. 205 Endpoint 206 Endpoint (EP) is a term used to describe a web server or client in 207 [RFC7252]. In the context of this specification an endpoint is 208 used to describe a web server that registers resources to the 209 Resource Directory. An endpoint is identified by its endpoint 210 name, which is included during registration, and has a unique name 211 within the associated sector of the registration. 213 Registration Base URI 214 The Base URI of a Registration is a URI that typically gives 215 scheme and authority information about an Endpoint. The 216 Registration Base URI is provided at registration time, and is 217 used by the Resource Directory to resolve relative references of 218 the registration into URIs. 220 Target 221 The target of a link is the destination address (URI) of the link. 222 It is sometimes identified with "href=", or displayed as 223 "". Relative targets need resolving with respect to the 224 Base URI (section 5.2 of [RFC3986]). 226 This use of the term Target is consistent with [RFC8288]'s use of 227 the term. 229 Context 230 The context of a link is the source address (URI) of the link, and 231 describes which resource is linked to the target. A link's 232 context is made explicit in serialized links as the "anchor=" 233 attribute. 235 This use of the term Context is consistent with [RFC8288]'s use of 236 the term. 238 Directory Resource 239 A resource in the Resource Directory (RD) containing registration 240 resources. 242 Registration Resource 243 A resource in the RD that contains information about an Endpoint 244 and its links. 246 Commissioning Tool 247 Commissioning Tool (CT) is a device that assists during the 248 installation of the network by assigning values to parameters, 249 naming endpoints and groups, or adapting the installation to the 250 needs of the applications. 252 Registrant-ep 253 Registrant-ep is the endpoint that is registered into the RD. The 254 registrant-ep can register itself, or a CT registers the 255 registrant-ep. 257 RDAO 258 Resource Directory Address Option. A new IPv6 Neigbhbor Discovery 259 option defined for announcing a Resource Directory's address. 261 3. Architecture and Use Cases 263 3.1. Principles 265 The Resource Directory is primarily a tool to make discovery 266 operations more efficient than querying /.well-known/core on all 267 connected devices, or across boundaries that would be limiting those 268 operations. 270 It provides information about resources hosted by other devices that 271 could otherwise only be obtained by directly querying the /.well- 272 known/core resource on these other devices, either by a unicast 273 request or a multicast request. 275 Information SHOULD only be stored in the resource directory if it can 276 be obtained by querying the described device's /.well-known/core 277 resource directly. 279 Data in the resource directory can only be provided by the device 280 which hosts those data or a dedicated Commissioning Tool (CT). These 281 CTs are thought to act on behalf of endpoints too constrained, or 282 generally unable, to present that information themselves. No other 283 client can modify data in the resource directory. Changes to the 284 information in the Resource Directory do not propagate automatically 285 back to the web servers from where the information originated. 287 3.2. Architecture 289 The resource directory architecture is illustrated in Figure 1. A 290 Resource Directory (RD) is used as a repository of registrations 291 describing resources hosted on other web servers, also called 292 endpoints (EP). An endpoint is a web server associated with a 293 scheme, IP address and port. A physical node may host one or more 294 endpoints. The RD implements a set of REST interfaces for endpoints 295 to register and maintain resource directory registrations, and for 296 endpoints to lookup resources from the RD. An RD can be logically 297 segmented by the use of Sectors. 299 A mechanism to discover an RD using CoRE Link Format [RFC6690] is 300 defined. 302 Registrations in the RD are soft state and need to be periodically 303 refreshed. 305 An endpoint uses specific interfaces to register, update and remove a 306 registration. It is also possible for an RD to fetch Web Links from 307 endpoints and add their contents to resource directory registrations. 309 At the first registration of an endpoint, a "registration resource" 310 is created, the location of which is returned to the registering 311 endpoint. The registering endpoint uses this registration resource 312 to manage the contents of registrations. 314 A lookup interface for discovering any of the Web Links stored in the 315 RD is provided using the CoRE Link Format. 317 Registration Lookup 318 Interface Interface 319 +----+ | | 320 | EP |---- | | 321 +----+ ---- | | 322 --|- +------+ | 323 +----+ | ----| | | +--------+ 324 | EP | ---------|-----| RD |----|-----| Client | 325 +----+ | ----| | | +--------+ 326 --|- +------+ | 327 +----+ ---- | | 328 | CT |---- | | 329 +----+ 331 Figure 1: The resource directory architecture. 333 A Registrant-EP MAY keep concurrent registrations to more than one RD 334 at the same time if explicitly configured to do so, but that is not 335 expected to be supported by typical EP implementations. Any such 336 registrations are independent of each other. The usual expectation 337 when multiple discovery mechanisms or addresses are configured is 338 that they constitute a fall-back path for a single registration. 340 3.3. RD Content Model 342 The Entity-Relationship (ER) models shown in Figure 2 and Figure 3 343 model the contents of /.well-known/core and the resource directory 344 respectively, with entity-relationship diagrams [ER]. Entities 345 (rectangles) are used for concepts that exist independently. 346 Attributes (ovals) are used for concepts that exist only in 347 connection with a related entity. Relations (diamonds) give a 348 semantic meaning to the relation between entities. Numbers specify 349 the cardinality of the relations. 351 Some of the attribute values are URIs. Those values are always full 352 URIs and never relative references in the information model. They 353 can, however, be expressed as relative references in serializations, 354 and often are. 356 These models provide an abstract view of the information expressed in 357 link-format documents and a Resource Directory. They cover the 358 concepts, but not necessarily all details of an RD's operation; they 359 are meant to give an overview, and not be a template for 360 implementations. 362 +----------------------+ 363 | /.well-known/core | 364 +----------------------+ 365 | 366 | 1 367 ////////\\\\\\\ 368 < contains > 369 \\\\\\\\/////// 370 | 371 | 0+ 372 +--------------------+ 373 | link | 374 +--------------------+ 375 | 376 | 1 oooooooo 377 +-----o target o 378 | oooooooo 379 oooooooooooo 0+ | 380 o target o--------+ 381 o attribute o | 0+ oooooo 382 oooooooooooo +-----o rel o 383 | oooooo 384 | 385 | 1 ooooooooo 386 +-----o context o 387 ooooooooo 389 Figure 2: E-R Model of the content of /.well-known/core 391 The model shown in Figure 2 models the contents of /.well-known/core 392 which contains: 394 o a set of links belonging to the hosting web server 396 The web server is free to choose links it deems appropriate to be 397 exposed in its ".well-known/core". Typically, the links describe 398 resources that are served by the host, but the set can also contain 399 links to resources on other servers (see examples in [RFC6690] page 400 14). The set does not necessarily contain links to all resources 401 served by the host. 403 A link has the following attributes (see [RFC8288]): 405 o Zero or more link relations: They describe relations between the 406 link context and the link target. 408 In link-format serialization, they are expressed as space- 409 separated values in the "rel" attribute, and default to "hosts". 411 o A link context URI: It defines the source of the relation, e.g. 412 _who_ "hosts" something. 414 In link-format serialization, it is expressed in the "anchor" 415 attribute. It defaults to that document's URI. 417 o A link target URI: It defines the destination of the relation 418 (e.g. _what_ is hosted), and is the topic of all target 419 attributes. 421 In link-format serialization, it is expressed between angular 422 brackets, and sometimes called the "href". 424 o Other target attributes (e.g. resource type (rt), interface (if), 425 or content format (ct)). These provide additional information 426 about the target URI. 428 +----------------------+ 429 | resource-directory | 430 +----------------------+ 431 | 1 432 | 433 | 434 | 435 | 436 //////\\\\ 437 < contains > 438 \\\\\///// 439 | 440 0+ | 441 ooooooo 1 +---------------+ 442 o base o-------| registration | 443 ooooooo +---------------+ 444 | | 1 445 | +--------------+ 446 oooooooo 1 | | 447 o href o----+ /////\\\\ 448 oooooooo | < contains > 449 | \\\\\///// 450 oooooooo 1 | | 451 o ep o----+ | 0+ 452 oooooooo | +------------------+ 453 | | link | 454 oooooooo 0-1 | +------------------+ 455 o d o----+ | 456 oooooooo | | 1 oooooooo 457 | +-----o target o 458 oooooooo 1 | | oooooooo 459 o lt o----+ ooooooooooo 0+ | 460 oooooooo | o target o-----+ 461 | o attribute o | 0+ oooooo 462 ooooooooooo 0+ | ooooooooooo +-----o rel o 463 o endpoint o----+ | oooooo 464 o attribute o | 465 ooooooooooo | 1 ooooooooo 466 +----o context o 467 ooooooooo 469 Figure 3: E-R Model of the content of the Resource Directory 471 The model shown in Figure 3 models the contents of the resource 472 directory which contains in addition to /.well-known/core: 474 o 0 to n Registrations of endpoints, 475 A registration is associated with one endpoint. A registration 476 defines a set of links as defined for /.well-known/core. A 477 Registration has six types of attributes: 479 o an endpoint name ("ep", a Unicode string) unique within a sector 481 o a Registration Base URI ("base", a URI typically describing the 482 scheme://authority part) 484 o a lifetime ("lt"), 486 o a registration resource location inside the RD ("href"), 488 o optionally a sector ("d", a Unicode string) 490 o optional additional endpoint attributes (from Section 9.3) 492 The cardinality of "base" is currently 1; future documents are 493 invited to extend the RD specification to support multiple values 494 (e.g. [I-D.silverajan-core-coap-protocol-negotiation]). Its value 495 is used as a Base URI when resolving URIs in the links contained in 496 the endpoint. 498 Links are modelled as they are in Figure 2. 500 3.4. Link-local addresses and zone identifiers 502 Registration Base URIs can contain link-local IP addresses. To be 503 usable across hosts, those can not be serialized to contain zone 504 identifiers (see [RFC6874] Section 1). 506 Link-local addresses can only be used on a single link (therefore RD 507 servers can not announce them when queried on a different link), and 508 lookup clients using them need to keep track of which interface they 509 got them from. 511 Therefore, it is advisable in many scenarios to use addresses with 512 larger scope if available. 514 3.5. Use Case: Cellular M2M 516 Over the last few years, mobile operators around the world have 517 focused on development of M2M solutions in order to expand the 518 business to the new type of users: machines. The machines are 519 connected directly to a mobile network using an appropriate embedded 520 wireless interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing 521 short and wide range wireless interfaces. From the system design 522 point of view, the ambition is to design horizontal solutions that 523 can enable utilization of machines in different applications 524 depending on their current availability and capabilities as well as 525 application requirements, thus avoiding silo like solutions. One of 526 the crucial enablers of such design is the ability to discover 527 resources (machines -- endpoints) capable of providing required 528 information at a given time or acting on instructions from the end 529 users. 531 Imagine a scenario where endpoints installed on vehicles enable 532 tracking of the position of these vehicles for fleet management 533 purposes and allow monitoring of environment parameters. During the 534 boot-up process endpoints register with a Resource Directory, which 535 is hosted by the mobile operator or somewhere in the cloud. 536 Periodically, these endpoints update their registration and may 537 modify resources they offer. 539 When endpoints are not always connected, for example because they 540 enter a sleep mode, a remote server is usually used to provide proxy 541 access to the endpoints. Mobile apps or web applications for 542 environment monitoring contact the RD, look up the endpoints capable 543 of providing information about the environment using an appropriate 544 set of link parameters, obtain information on how to contact them 545 (URLs of the proxy server), and then initiate interaction to obtain 546 information that is finally processed, displayed on the screen and 547 usually stored in a database. Similarly, fleet management systems 548 provide the appropriate link parameters to the RD to look up for EPs 549 deployed on the vehicles the application is responsible for. 551 3.6. Use Case: Home and Building Automation 553 Home and commercial building automation systems can benefit from the 554 use of M2M web services. The discovery requirements of these 555 applications are demanding. Home automation usually relies on run- 556 time discovery to commission the system, whereas in building 557 automation a combination of professional commissioning and run-time 558 discovery is used. Both home and building automation involve peer- 559 to-peer interactions between endpoints, and involve battery-powered 560 sleeping devices. 562 3.7. Use Case: Link Catalogues 564 Resources may be shared through data brokers that have no knowledge 565 beforehand of who is going to consume the data. Resource Directory 566 can be used to hold links about resources and services hosted 567 anywhere to make them discoverable by a general class of 568 applications. 570 For example, environmental and weather sensors that generate data for 571 public consumption may provide data to an intermediary server, or 572 broker. Sensor data are published to the intermediary upon changes 573 or at regular intervals. Descriptions of the sensors that resolve to 574 links to sensor data may be published to a Resource Directory. 575 Applications wishing to consume the data can use RD Lookup to 576 discover and resolve links to the desired resources and endpoints. 577 The Resource Directory service need not be coupled with the data 578 intermediary service. Mapping of Resource Directories to data 579 intermediaries may be many-to-many. 581 Metadata in web link formats like [RFC6690] which may be internally 582 stored as triples, or relation/attribute pairs providing metadata 583 about resource links, need to be supported by Resource Directories . 584 External catalogues that are represented in other formats may be 585 converted to common web linking formats for storage and access by 586 Resource Directories. Since it is common practice for these to be 587 URN encoded, simple and lossless structural transforms should 588 generally be sufficient to store external metadata in Resource 589 Directories. 591 The additional features of Resource Directory allow sectors to be 592 defined to enable access to a particular set of resources from 593 particular applications. This provides isolation and protection of 594 sensitive data when needed. Application groups with multicast 595 addresses may be defined to support efficient data transport. 597 4. RD discovery and other interface-independent components 599 This and the following sections define the required set of REST 600 interfaces between a Resource Directory (RD), endpoints and lookup 601 clients. Although the examples throughout these sections assume the 602 use of CoAP [RFC7252], these REST interfaces can also be realized 603 using HTTP [RFC7230]. Only multicast discovery operations are not 604 possible on HTTP, and Simple Registration can not be executed as base 605 attribute (which is mandatory for HTTP) can not be used there. In 606 all definitions in these sections, both CoAP response codes (with dot 607 notation) and HTTP response codes (without dot notation) are shown. 608 An RD implementing this specification MUST support the discovery, 609 registration, update, lookup, and removal interfaces. 611 All operations on the contents of the Resource Directory MUST be 612 atomic and idempotent. 614 For several operations, interface templates are given in list form; 615 those describe the operation participants, request codes, URIs, 616 content formats and outcomes. Sections of those templates contain 617 normative content about Interaction, Method, URI Template and URI 618 Template Variables as well as the details of the Success condition. 619 The additional sections on options like Content-Format and on Failure 620 codes give typical cases that an implementation of the RD should deal 621 with. Those serve to illustrate the typical responses to readers who 622 are not yet familiar with all the details of CoAP based interfaces; 623 they do not limit what a server may respond under atypical 624 circumstances. 626 REST clients (registrant-EPs / CTs, lookup clients, RD servers during 627 simple registrations) MUST be prepared to receive any unsuccessful 628 code and act upon it according to its definition, options and/or 629 payload to the best of their capabilities, falling back to failing 630 the operation if recovery is not possible. In particular, they 631 should retry the request upon 5.03 (Service Unavailable; 503 in HTTP) 632 according to the Max-Age (Retry-After in HTTP) option, and fall back 633 to link-format when receiving 4.15 (Unsupported Content Format; 415 634 in HTTP). 636 A resource directory MAY make the information submitted to it 637 available to further directories, if it can ensure that a loop does 638 not form. The protocol used between directories to ensure loop-free 639 operation is outside the scope of this document. 641 4.1. Finding a Resource Directory 643 A (re-)starting device may want to find one or more resource 644 directories for discovery purposes. 646 The device may be pre-configured to exercise specific mechanisms for 647 finding the resource directory: 649 1. It may be configured with a specific IP address for the RD. That 650 IP address may also be an anycast address, allowing the network 651 to forward RD requests to an RD that is topologically close; each 652 target network environment in which some of these preconfigured 653 nodes are to be brought up is then configured with a route for 654 this anycast address that leads to an appropriate RD. (Instead 655 of using an anycast address, a multicast address can also be 656 preconfigured. The RD servers then need to configure one of 657 their interfaces with this multicast address.) 659 2. It may be configured with a DNS name for the RD and use DNS to 660 return the IP address of the RD; it can find a DNS server to 661 perform the lookup using the usual mechanisms for finding DNS 662 servers. 664 3. It may be configured to use a service discovery mechanism such as 665 DNS-SD [RFC6763]. The present specification suggests configuring 666 the service with name rd._sub._coap._udp, preferably within the 667 domain of the querying nodes. 669 For cases where the device is not specifically configured with a way 670 to find a resource directory, the network may want to provide a 671 suitable default. 673 1. If the address configuration of the network is performed via 674 SLAAC, this is provided by the RDAO option Section 4.1.1. 676 2. If the address configuration of the network is performed via 677 DHCP, this could be provided via a DHCP option (no such option is 678 defined at the time of writing). 680 Finally, if neither the device nor the network offers any specific 681 configuration, the device may want to employ heuristics to find a 682 suitable resource directory. 684 The present specification does not fully define these heuristics, but 685 suggests a number of candidates: 687 1. In a 6LoWPAN, just assume the Border Router (6LBR) can act as a 688 resource directory (using the ABRO option to find that 689 [RFC6775]). Confirmation can be obtained by sending a Unicast to 690 "coap://[6LBR]/.well-known/core?rt=core.rd*". 692 2. In a network that supports multicast well, discovering the RD 693 using a multicast query for /.well-known/core as specified in 694 CoRE Link Format [RFC6690]: Sending a Multicast GET to 695 "coap://[MCD1]/.well-known/core?rt=core.rd*". RDs within the 696 multicast scope will answer the query. 698 When answering a multicast request directed at a link-local address, 699 the RD may want to respond from a routable address; this makes it 700 easier for registrants to use one of their own routable addresses for 701 registration. 703 As some of the RD addresses obtained by the methods listed here are 704 just (more or less educated) guesses, endpoints MUST make use of any 705 error messages to very strictly rate-limit requests to candidate IP 706 addresses that don't work out. For example, an ICMP Destination 707 Unreachable message (and, in particular, the port unreachable code 708 for this message) may indicate the lack of a CoAP server on the 709 candidate host, or a CoAP error response code such as 4.05 "Method 710 Not Allowed" may indicate unwillingness of a CoAP server to act as a 711 directory server. 713 If multiple candidate addresses are discovered, the device may pick 714 any of them initially, unless the discovery method indicates a more 715 precise selection scheme. 717 4.1.1. Resource Directory Address Option (RDAO) 719 The Resource Directory Address Option (RDAO) using IPv6 Neighbor 720 Discovery (ND) carries information about the address of the Resource 721 Directory (RD). This information is needed when endpoints cannot 722 discover the Resource Directory with a link-local or realm-local 723 scope multicast address, for instance because the endpoint and the RD 724 are separated by a Border Router (6LBR). In many circumstances the 725 availability of DHCP cannot be guaranteed either during commissioning 726 of the network. The presence and the use of the RD is essential 727 during commissioning. 729 It is possible to send multiple RDAO options in one message, 730 indicating as many resource directory addresses. 732 The RDAO format is: 734 0 1 2 3 735 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 1 736 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 737 | Type | Length = 3 | Valid Lifetime | 738 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 739 | Reserved | 740 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 741 | | 742 + + 743 | | 744 + RD Address + 745 | | 746 + + 747 | | 748 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 750 Fields: 752 Type: 38 754 Length: 8-bit unsigned integer. The length of 755 the option in units of 8 bytes. 756 Always 3. 758 Valid Lifetime: 16-bit unsigned integer. The length of 759 time in units of 60 seconds (relative to 760 the time the packet is received) that 761 this Resource Directory address is valid. 762 A value of all zero bits (0x0) indicates 763 that this Resource Directory address 764 is not valid anymore. 766 Reserved: This field is unused. It MUST be 767 initialized to zero by the sender and 768 MUST be ignored by the receiver. 770 RD Address: IPv6 address of the RD. 772 Figure 4: Resource Directory Address Option 774 4.2. Payload Content Formats 776 Resource Directory implementations using this specification MUST 777 support the application/link-format content format (ct=40). 779 Resource Directories implementing this specification MAY support 780 additional content formats. 782 Any additional content format supported by a Resource Directory 783 implementing this specification SHOULD be able to express all the 784 information expressible in link-format. It MAY be able to express 785 information that is inexpressible in link-format, but those 786 expressions SHOULD be avoided where possible. 788 4.3. URI Discovery 790 Before an endpoint can make use of an RD, it must first know the RD's 791 address and port, and the URI path information for its REST APIs. 792 This section defines discovery of the RD and its URIs using the well- 793 known interface of the CoRE Link Format [RFC6690]. A complete set of 794 RD discovery methods is described in Section 4.1. 796 Discovery of the RD registration URI path is performed by sending 797 either a multicast or unicast GET request to "/.well-known/core" and 798 including a Resource Type (rt) parameter [RFC6690] with the value 799 "core.rd" in the query string. Likewise, a Resource Type parameter 800 value of "core.rd-lookup*" is used to discover the URIs for RD Lookup 801 operations, core.rd* is used to discover all URI paths for RD 802 operations. Upon success, the response will contain a payload with a 803 link format entry for each RD function discovered, indicating the URI 804 of the RD function returned and the corresponding Resource Type. 805 When performing multicast discovery, the multicast IP address used 806 will depend on the scope required and the multicast capabilities of 807 the network (see Section 9.5). 809 A Resource Directory MAY provide hints about the content-formats it 810 supports in the links it exposes or registers, using the "ct" target 811 attribute, as shown in the example below. Clients MAY use these 812 hints to select alternate content-formats for interaction with the 813 Resource Directory. 815 HTTP does not support multicast and consequently only unicast 816 discovery can be supported using HTTP. The well-known entry points 817 SHOULD be provided to enable unicast discovery. 819 An implementation of this resource directory specification MUST 820 support query filtering for the rt parameter as defined in [RFC6690]. 822 While the link targets in this discovery step are often expressed in 823 path-absolute form, this is not a requirement. Clients of the RD 824 SHOULD therefore accept URIs of all schemes they support, both as 825 URIs and relative references, and not limit the set of discovered 826 URIs to those hosted at the address used for URI discovery. 828 The URI Discovery operation can yield multiple URIs of a given 829 resource type. The client of the RD can use any of the discovered 830 addresses initially. 832 The discovery request interface is specified as follows (this is 833 exactly the Well-Known Interface of [RFC6690] Section 4, with the 834 additional requirement that the server MUST support query filtering): 836 Interaction: EP and Client -> RD 838 Method: GET 840 URI Template: /.well-known/core{?rt} 842 URI Template Variables: 844 rt := Resource Type. SHOULD contain one of the values "core.rd", 845 "core.rd-lookup*", "core.rd-lookup-res", "core.rd-lookup-ep", 846 or "core.rd*" 848 Accept: absent, application/link-format or any other media type 849 representing web links 851 The following response is expected on this interface: 853 Success: 2.05 "Content" or 200 "OK" with an application/link-format 854 or other web link payload containing one or more matching entries 855 for the RD resource. 857 The following example shows an endpoint discovering an RD using this 858 interface, thus learning that the directory resource location, in 859 this example, is /rd, and that the content-format delivered by the 860 server hosting the resource is application/link-format (ct=40). Note 861 that it is up to the RD to choose its RD locations. 863 Req: GET coap://[MCD1]/.well-known/core?rt=core.rd* 865 Res: 2.05 Content 866 ;rt="core.rd";ct=40, 867 ;rt="core.rd-lookup-ep";ct=40, 868 ;rt="core.rd-lookup-res";ct=40, 870 Figure 5: Example discovery exchange 872 The following example shows the way of indicating that a client may 873 request alternate content-formats. The Content-Format code attribute 874 "ct" MAY include a space-separated sequence of Content-Format codes 875 as specified in Section 7.2.1 of [RFC7252], indicating that multiple 876 content-formats are available. The example below shows the required 877 Content-Format 40 (application/link-format) indicated as well as a 878 CBOR and JSON representation from [I-D.ietf-core-links-json] (which 879 have no numeric values assigned yet, so they are shown as TBD64 and 880 TBD504 as in that draft). The RD resource locations /rd, and /rd- 881 lookup are example values. The server in this example also indicates 882 that it is capable of providing observation on resource lookups. 884 [ The RFC editor is asked to replace this and later occurrences of 885 MCD1 with the assigned IPv6 site-local address for "all CoRE Resource 886 Directories". ] 888 Req: GET coap://[MCD1]/.well-known/core?rt=core.rd* 890 Res: 2.05 Content 891 ;rt="core.rd";ct="40 65225", 892 ;rt="core.rd-lookup-res";ct="40 TBD64 TBD504";obs, 893 ;rt="core.rd-lookup-ep";ct="40 TBD64 TBD504", 895 From a management and maintenance perspective, it is necessary to 896 identify the components that constitute the RD server. The 897 identification refers to information about for example client-server 898 incompatibilities, supported features, required updates and other 899 aspects. The URI discovery address, a described in section 4 of 900 [RFC6690] can be used to find the identification. 902 It would typically be stored in an implementation information link 903 (as described in [I-D.bormann-t2trg-rel-impl]): 905 Req: GET /.well-known/core?rel=impl-info 907 Res: 2.05 Content 908 ; 909 rel="impl-info" 911 Note that depending on the particular server's architecture, such a 912 link could be anchored at the RD server's root, at the discovery site 913 (as in this example) or at individual RD components. The latter is 914 to be expected when different applications are run on the same 915 server. 917 5. Registration 919 After discovering the location of an RD, a registrant-ep or CT MAY 920 register the resources of the registrant-ep using the registration 921 interface. This interface accepts a POST from an endpoint containing 922 the list of resources to be added to the directory as the message 923 payload in the CoRE Link Format [RFC6690] or other representations of 924 web links, along with query parameters indicating the name of the 925 endpoint, and optionally the sector, lifetime and base URI of the 926 registration. It is expected that other specifications will define 927 further parameters (see Section 9.3). The RD then creates a new 928 registration resource in the RD and returns its location. The 929 receiving endpoint MUST use that location when refreshing 930 registrations using this interface. Registration resources in the RD 931 are kept active for the period indicated by the lifetime parameter. 932 The creating endpoint is responsible for refreshing the registration 933 resource within this period using either the registration or update 934 interface. The registration interface MUST be implemented to be 935 idempotent, so that registering twice with the same endpoint 936 parameters ep and d (sector) does not create multiple registration 937 resources. 939 The following rules apply for a registration request targeting a 940 given (ep, d) value pair: 942 o When the (ep, d) value pair of the registration-request is 943 different from any existing registration, a new registration is 944 generated. 946 o When the (ep, d) value pair of the registration-request is equal 947 to an existing registration, the content and parameters of the 948 existing registration are replaced with the content of the 949 registration request. 951 The posted link-format document can (and typically does) contain 952 relative references both in its link targets and in its anchors, or 953 contain empty anchors. The RD server needs to resolve these 954 references in order to faithfully represent them in lookups. They 955 are resolved against the base URI of the registration, which is 956 provided either explicitly in the "base" parameter or constructed 957 implicitly from the requester's URI as constructed from its network 958 address and scheme. 960 For media types to which Appendix C applies (i.e. documents in 961 application/link-format), the RD only needs to accept representations 962 in Limited Link Format as described there. Its behavior with 963 representations outside that subset is implementation defined. 965 The registration request interface is specified as follows: 967 Interaction: EP -> RD 969 Method: POST 971 URI Template: {+rd}{?ep,d,lt,base,extra-attrs*} 972 URI Template Variables: 974 rd := RD registration URI (mandatory). This is the location of 975 the RD, as obtained from discovery. 977 ep := Endpoint name (mostly mandatory). The endpoint name is an 978 identifier that MUST be unique within a sector. As the 979 endpoint name is a Unicode string, it is encoded in UTF-8 (and 980 possibly pct-encoding) during variable expansion (see [RFC6570] 981 Section 3.2.1). The endpoint name MUST NOT contain any 982 character in the inclusive ranges 0-31 or 127-159. The maximum 983 length of this parameter is 63 UTF-8 encoded bytes. If the RD 984 is configured to recognize the endpoint (e.g. based on its 985 security context), the RD assigns an endpoint name based on a 986 set of configuration parameter values. 988 d := Sector (optional). The sector to which this endpoint 989 belongs. When this parameter is not present, the RD MAY 990 associate the endpoint with a configured default sector or 991 leave it empty. The sector is encoded like the ep parameter, 992 and is limited to 63 UTF-8 encoded bytes as well. The endpoint 993 name and sector name are not set when one or both are set in an 994 accompanying authorization token. 996 lt := Lifetime (optional). Lifetime of the registration in 997 seconds. Range of 60-4294967295. If no lifetime is included 998 in the initial registration, a default value of 90000 (25 999 hours) SHOULD be assumed. 1001 base := Base URI (optional). This parameter sets the base URI of 1002 the registration, under which the relative links in the payload 1003 are to be interpreted. The specified URI typically does not 1004 have a path component of its own, and MUST be suitable as a 1005 base URI to resolve any relative references given in the 1006 registration. The parameter is therefore usually of the shape 1007 "scheme://authority" for HTTP and CoAP URIs. The URI SHOULD 1008 NOT have a query or fragment component as any non-empty 1009 relative part in a reference would remove those parts from the 1010 resulting URI. 1012 In the absence of this parameter the scheme of the protocol, 1013 source address and source port of the registration request are 1014 assumed. The Base URI is consecutively constructed by 1015 concatenating the used protocol's scheme with the characters 1016 "://", the requester's source address as an address literal and 1017 ":" followed by its port (if it was not the protocol's default 1018 one) in analogy to [RFC7252] Section 6.5. 1020 This parameter is mandatory when the directory is filled by a 1021 third party such as an commissioning tool. 1023 If the registrant-ep uses an ephemeral port to register with, 1024 it MUST include the base parameter in the registration to 1025 provide a valid network path. 1027 A registrant that can not be reached by potential lookup 1028 clients at the address it registers from (e.g. because it is 1029 behind some form of Network Address Translation (NAT)) MUST 1030 provide a reachable base address with its registration. 1032 If the Base URI contains a link-local IP literal, it MUST NOT 1033 contain a Zone Identifier, and MUST be local to the link on 1034 which the registration request is received. 1036 Endpoints that register with a base that contains a path 1037 component can not meaningfully use [RFC6690] Link Format due to 1038 its prevalence of the Origin concept in relative reference 1039 resolution. Those applications should use different 1040 representations of links to which Appendix C is not applicable 1041 (e.g. [I-D.hartke-t2trg-coral]). 1043 extra-attrs := Additional registration attributes (optional). 1044 The endpoint can pass any parameter registered at Section 9.3 1045 to the directory. If the RD is aware of the parameter's 1046 specified semantics, it processes it accordingly. Otherwise, 1047 it MUST store the unknown key and its value(s) as an endpoint 1048 attribute for further lookup. 1050 Content-Format: application/link-format or any other indicated media 1051 type representing web links 1053 The following response is expected on this interface: 1055 Success: 2.01 "Created" or 201 "Created". The Location-Path option 1056 or Location header MUST be included in the response. This 1057 location MUST be a stable identifier generated by the RD as it is 1058 used for all subsequent operations on this registration resource. 1059 The registration resource location thus returned is for the 1060 purpose of updating the lifetime of the registration and for 1061 maintaining the content of the registered links, including 1062 updating and deleting links. 1064 A registration with an already registered ep and d value pair 1065 responds with the same success code and location as the original 1066 registration; the set of links registered with the endpoint is 1067 replaced with the links from the payload. 1069 The location MUST NOT have a query or fragment component, as that 1070 could conflict with query parameters during the Registration 1071 Update operation. Therefore, the Location-Query option MUST NOT 1072 be present in a successful response. 1074 If the registration fails, including request timeouts, or if delays 1075 from Service Unavailable responses with Max-Age or Retry-After 1076 accumulate to exceed the registrant's configured timeouts, it SHOULD 1077 pick another registration URI from the "URI Discovery" step and if 1078 there is only one or the list is exhausted, pick other choices from 1079 the "Finding a Resource Directory" step. Care has to be taken to 1080 consider the freshness of results obtained earlier, e.g. of the 1081 result of a "/.well-known/core" response, the lifetime of an RDAO 1082 option and of DNS responses. Any rate limits and persistent errors 1083 from the "Finding a Resource Directory" step must be considered for 1084 the whole registration time, not only for a single operation. 1086 The following example shows a registrant-ep with the name "node1" 1087 registering two resources to an RD using this interface. The 1088 location "/rd" is an example RD location discovered in a request 1089 similar to Figure 5. 1091 Req: POST coap://rd.example.com/rd?ep=node1 1092 Content-Format: 40 1093 Payload: 1094 ;ct=41;rt="temperature-c";if="sensor"; 1095 anchor="coap://spurious.example.com:5683", 1096 ;ct=41;rt="light-lux";if="sensor" 1098 Res: 2.01 Created 1099 Location-Path: /rd/4521 1101 Figure 6: Example registration payload 1103 A Resource Directory may optionally support HTTP. Here is an example 1104 of almost the same registration operation above, when done using 1105 HTTP. 1107 Req: POST /rd?ep=node1&base=http://[2001:db8:1::1] HTTP/1.1 1108 Host: example.com 1109 Content-Type: application/link-format 1110 Payload: 1111 ;ct=41;rt="temperature-c";if="sensor"; 1112 anchor="coap://spurious.example.com:5683", 1113 ;ct=41;rt="light-lux";if="sensor" 1115 Res: 201 Created 1116 Location: /rd/4521 1118 5.1. Simple Registration 1120 Not all endpoints hosting resources are expected to know how to 1121 upload links to an RD as described in Section 5. Instead, simple 1122 endpoints can implement the Simple Registration approach described in 1123 this section. An RD implementing this specification MUST implement 1124 Simple Registration. However, there may be security reasons why this 1125 form of directory discovery would be disabled. 1127 This approach requires that the registrant-ep makes available the 1128 hosted resources that it wants to be discovered, as links on its 1129 "/.well-known/core" interface as specified in [RFC6690]. The links 1130 in that document are subject to the same limitations as the payload 1131 of a registration (with respect to Appendix C). 1133 o The registrant-ep finds one or more addresses of the directory 1134 server as described in Section 4.1. 1136 o The registrant-ep sends (and regularly refreshes with) a POST 1137 request to the "/.well-known/core" URI of the directory server of 1138 choice. The body of the POST request is empty, and triggers the 1139 resource directory server to perform GET requests at the 1140 requesting registrant-ep's /.well-known/core to obtain the link- 1141 format payload to register. 1143 The registrant-ep includes the same registration parameters in the 1144 POST request as it would per Section 5. The registration base URI 1145 of the registration is taken from the registrant-ep's network 1146 address (as is default with regular registrations). 1148 Example request from registrant-EP to RD (unanswered until the 1149 next step): 1151 Req: POST /.well-known/core?lt=6000&ep=node1 1152 (No payload) 1154 o The Resource Directory queries the registrant-ep's discovery 1155 resource to determine the success of the operation. It SHOULD 1156 keep a cache of the discovery resource and not query it again as 1157 long as it is fresh. 1159 Example request from the RD to the registrant-EP: 1161 Req: GET /.well-known/core 1162 Accept: 40 1164 Res: 2.05 Content 1165 Content-Format: 40 1166 Payload: 1167 1169 With this response, the RD would answer the previous step's request: 1171 Res: 2.04 Changed 1173 The sequence of fetching the registration content before sending a 1174 successful response was chosen to make responses reliable, and the 1175 caching item was chosen to still allow very constrained registrants. 1176 Registrants MUST be able to serve a GET request to "/.well-known/ 1177 core" after having requested registration. Constrained devices MAY 1178 regard the initial request as temporarily failed when they need RAM 1179 occupied by their own request to serve the RD's GET, and retry later 1180 when the RD already has a cached representation of their discovery 1181 resources. Then, the RD can reply immediately and the registrant can 1182 receive the response. 1184 The simple registration request interface is specified as follows: 1186 Interaction: EP -> RD 1188 Method: POST 1190 URI Template: /.well-known/core{?ep,d,lt,extra-attrs*} 1192 URI Template Variables are as they are for registration in Section 5. 1193 The base attribute is not accepted to keep the registration interface 1194 simple; that rules out registration over CoAP-over-TCP or HTTP that 1195 would need to specify one. 1197 The following response is expected on this interface: 1199 Success: 2.04 "Changed". 1201 For the second interaction triggered by the above, the registrant-ep 1202 takes the role of server and the RD the role of client. (Note that 1203 this is exactly the Well-Known Interface of [RFC6690] Section 4): 1205 Interaction: RD -> EP 1207 Method: GET 1208 URI Template: /.well-known/core 1210 The following response is expected on this interface: 1212 Success: 2.05 "Content". 1214 The RD MUST delete registrations created by simple registration after 1215 the expiration of their lifetime. Additional operations on the 1216 registration resource cannot be executed because no registration 1217 location is returned. 1219 5.2. Third-party registration 1221 For some applications, even Simple Registration may be too taxing for 1222 some very constrained devices, in particular if the security 1223 requirements become too onerous. 1225 In a controlled environment (e.g. building control), the Resource 1226 Directory can be filled by a third party device, called a 1227 Commissioning Tool (CT). The commissioning tool can fill the 1228 Resource Directory from a database or other means. For that purpose 1229 scheme, IP address and port of the URI of the registered device is 1230 the value of the "base" parameter of the registration described in 1231 Section 5. 1233 It should be noted that the value of the "base" parameter applies to 1234 all the links of the registration and has consequences for the anchor 1235 value of the individual links as exemplified in Appendix B. An 1236 eventual (currently non-existing) "base" attribute of the link is not 1237 affected by the value of "base" parameter in the registration. 1239 5.3. Operations on the Registration Resource 1241 This section describes how the registering endpoint can maintain the 1242 registrations that it created. The registering endpoint can be the 1243 registrant-ep or the CT. An endpoint SHOULD NOT use this interface 1244 for registrations that it did not create. The registrations are 1245 resources of the RD. 1247 After the initial registration, the registering endpoint retains the 1248 returned location of the Registration Resource for further 1249 operations, including refreshing the registration in order to extend 1250 the lifetime and "keep-alive" the registration. When the lifetime of 1251 the registration has expired, the RD SHOULD NOT respond to discovery 1252 queries concerning this endpoint. The RD SHOULD continue to provide 1253 access to the Registration Resource after a registration time-out 1254 occurs in order to enable the registering endpoint to eventually 1255 refresh the registration. The RD MAY eventually remove the 1256 registration resource for the purpose of garbage collection. If the 1257 Registration Resource is removed, the corresponding endpoint will 1258 need to be re-registered. 1260 The Registration Resource may also be used cancel the registration 1261 using DELETE, and to perform further operations beyond the scope of 1262 this specification. 1264 These operations are described below. 1266 5.3.1. Registration Update 1268 The update interface is used by the registering endpoint to refresh 1269 or update its registration with an RD. To use the interface, the 1270 registering endpoint sends a POST request to the registration 1271 resource returned by the initial registration operation. 1273 An update MAY update the lifetime or the base URI registration 1274 parameters "lt", "base" as in Section 5. Parameters that are not 1275 being changed SHOULD NOT be included in an update. Adding parameters 1276 that have not changed increases the size of the message but does not 1277 have any other implications. Parameters MUST be included as query 1278 parameters in an update operation as in Section 5. 1280 A registration update resets the timeout of the registration to the 1281 (possibly updated) lifetime of the registration, independent of 1282 whether a "lt" parameter was given. 1284 If the base URI of the registration is changed in an update, relative 1285 references submitted in the original registration or later updates 1286 are resolved anew against the new base. 1288 The registration update operation only describes the use of POST with 1289 an empty payload. Future standards might describe the semantics of 1290 using content formats and payloads with the POST method to update the 1291 links of a registration (see Section 5.3.3). 1293 The update registration request interface is specified as follows: 1295 Interaction: EP -> RD 1297 Method: POST 1299 URI Template: {+location}{?lt,base,extra-attrs*} 1301 URI Template Variables: 1303 location := This is the Location returned by the RD as a result 1304 of a successful earlier registration. 1306 lt := Lifetime (optional). Lifetime of the registration in 1307 seconds. Range of 60-4294967295. If no lifetime is included, 1308 the previous last lifetime set on a previous update or the 1309 original registration (falling back to 90000) SHOULD be used. 1311 base := Base URI (optional). This parameter updates the Base URI 1312 established in the original registration to a new value. If 1313 the parameter is set in an update, it is stored by the RD as 1314 the new Base URI under which to interpret the relative links 1315 present in the payload of the original registration, following 1316 the same restrictions as in the registration. If the parameter 1317 is not set in the request but was set before, the previous Base 1318 URI value is kept unmodified. If the parameter is not set in 1319 the request and was not set before either, the source address 1320 and source port of the update request are stored as the Base 1321 URI. 1323 extra-attrs := Additional registration attributes (optional). As 1324 with the registration, the RD processes them if it knows their 1325 semantics. Otherwise, unknown attributes are stored as 1326 endpoint attributes, overriding any previously stored endpoint 1327 attributes of the same key. 1329 Content-Format: none (no payload) 1331 The following responses are expected on this interface: 1333 Success: 2.04 "Changed" or 204 "No Content" if the update was 1334 successfully processed. 1336 Failure: 4.04 "Not Found" or 404 "Not Found". Registration does not 1337 exist (e.g. may have been removed). 1339 If the registration fails in any way, including "Not Found" and 1340 request timeouts, or if the time indicated in a Service Unabailable 1341 Max-Age/Retry-After exceeds the remaining lifetime, the registering 1342 endpoint SHOULD attempt registration again. 1344 The following example shows how the registering endpoint updates its 1345 registration resource at an RD using this interface with the example 1346 location value: /rd/4521. 1348 Req: POST /rd/4521 1350 Res: 2.04 Changed 1351 The following example shows the registering endpoint updating its 1352 registration resource at an RD using this interface with the example 1353 location value: /rd/4521. The initial registration by the 1354 registering endpoint set the following values: 1356 o endpoint name (ep)=endpoint1 1358 o lifetime (lt)=500 1360 o Base URI (base)=coap://local-proxy-old.example.com:5683 1362 o payload of Figure 6 1364 The initial state of the Resource Directory is reflected in the 1365 following request: 1367 Req: GET /rd-lookup/res?ep=endpoint1 1369 Res: 2.01 Content 1370 Payload: 1371 ;ct=41; 1372 rt="temperature"; anchor="coap://spurious.example.com:5683", 1373 ;ct=41; 1374 rt="light-lux"; if="sensor"; 1375 anchor="coap://local-proxy-old.example.com:5683" 1377 The following example shows the registering endpoint changing the 1378 Base URI to "coaps://new.example.com:5684": 1380 Req: POST /rd/4521?base=coaps://new.example.com:5684 1382 Res: 2.04 Changed 1384 The consecutive query returns: 1386 Req: GET /rd-lookup/res?ep=endpoint1 1388 Res: 2.01 Content 1389 Payload: 1390 ;ct=41;rt="temperature"; 1391 anchor="coap://spurious.example.com:5683", 1392 ;ct=41;rt="light-lux"; 1393 if="sensor"; anchor="coaps://new.example.com:5684", 1395 5.3.2. Registration Removal 1397 Although RD registrations have soft state and will eventually timeout 1398 after their lifetime, the registering endpoint SHOULD explicitly 1399 remove an entry from the RD if it knows it will no longer be 1400 available (for example on shut-down). This is accomplished using a 1401 removal interface on the RD by performing a DELETE on the endpoint 1402 resource. 1404 The removal request interface is specified as follows: 1406 Interaction: EP -> RD 1408 Method: DELETE 1410 URI Template: {+location} 1412 URI Template Variables: 1414 location := This is the Location returned by the RD as a result 1415 of a successful earlier registration. 1417 The following responses are expected on this interface: 1419 Success: 2.02 "Deleted" or 204 "No Content" upon successful deletion 1421 Failure: 4.04 "Not Found" or 404 "Not Found". Registration does not 1422 exist (e.g. may already have been removed). 1424 The following examples shows successful removal of the endpoint from 1425 the RD with example location value /rd/4521. 1427 Req: DELETE /rd/4521 1429 Res: 2.02 Deleted 1431 5.3.3. Further operations 1433 Additional operations on the registration can be specified in future 1434 documents, for example: 1436 o Send iPATCH (or PATCH) updates ([RFC8132]) to add, remove or 1437 change the links of a registration. 1439 o Use GET to read the currently stored set of links in a 1440 registration resource. 1442 Those operations are out of scope of this document, and will require 1443 media types suitable for modifying sets of links. 1445 6. RD Lookup 1447 To discover the resources registered with the RD, a lookup interface 1448 must be provided. This lookup interface is defined as a default, and 1449 it is assumed that RDs may also support lookups to return resource 1450 descriptions in alternative formats (e.g. JSON or CBOR link format 1451 [I-D.ietf-core-links-json]) or using more advanced interfaces (e.g. 1452 supporting context or semantic based lookup) on different resources 1453 that are discovered independently. 1455 RD Lookup allows lookups for endpoints and resources using attributes 1456 defined in this document and for use with the CoRE Link Format. The 1457 result of a lookup request is the list of links (if any) 1458 corresponding to the type of lookup. Thus, an endpoint lookup MUST 1459 return a list of endpoints and a resource lookup MUST return a list 1460 of links to resources. 1462 The lookup type is selected by a URI endpoint, which is indicated by 1463 a Resource Type as per Table 1 below: 1465 +-------------+--------------------+-----------+ 1466 | Lookup Type | Resource Type | Mandatory | 1467 +-------------+--------------------+-----------+ 1468 | Resource | core.rd-lookup-res | Mandatory | 1469 | Endpoint | core.rd-lookup-ep | Mandatory | 1470 +-------------+--------------------+-----------+ 1472 Table 1: Lookup Types 1474 6.1. Resource lookup 1476 Resource lookup results in links that are semantically equivalent to 1477 the links submitted to the RD. The links and link parameters 1478 returned by the lookup are equal to the submitted ones, except that 1479 the target and anchor references are fully resolved. 1481 Links that did not have an anchor attribute are therefore returned 1482 with the base URI of the registration as the anchor. Links of which 1483 href or anchor was submitted as a (full) URI are returned with these 1484 attributes unmodified. 1486 Above rules allow the client to interpret the response as links 1487 without any further knowledge of the storage conventions of the RD. 1488 The Resource Directory MAY replace the registration base URIs with a 1489 configured intermediate proxy, e.g. in the case of an HTTP lookup 1490 interface for CoAP endpoints. 1492 If the base URI of a registration contains a link-local address, the 1493 RD MUST NOT show its links unless the lookup was made from the same 1494 link. The RD MUST NOT include zone identifiers in the resolved URIs. 1496 6.2. Lookup filtering 1498 Using the Accept Option, the requester can control whether the 1499 returned list is returned in CoRE Link Format ("application/link- 1500 format", default) or in alternate content-formats (e.g. from 1501 [I-D.ietf-core-links-json]). 1503 The page and count parameters are used to obtain lookup results in 1504 specified increments using pagination, where count specifies how many 1505 links to return and page specifies which subset of links organized in 1506 sequential pages, each containing 'count' links, starting with link 1507 zero and page zero. Thus, specifying count of 10 and page of 0 will 1508 return the first 10 links in the result set (links 0-9). Count = 10 1509 and page = 1 will return the next 'page' containing links 10-19, and 1510 so on. 1512 Multiple search criteria MAY be included in a lookup. All included 1513 criteria MUST match for a link to be returned. The Resource 1514 Directory MUST support matching with multiple search criteria. 1516 A link matches a search criterion if it has an attribute of the same 1517 name and the same value, allowing for a trailing "*" wildcard 1518 operator as in Section 4.1 of [RFC6690]. Attributes that are defined 1519 as "link-type" match if the search value matches any of their values 1520 (see Section 4.1 of [RFC6690]; e.g. "?if=core.s" matches ";if="abc 1521 core.s";"). A resource link also matches a search criterion if its 1522 endpoint would match the criterion, and vice versa, an endpoint link 1523 matches a search criterion if any of its resource links matches it. 1525 Note that "href" is a valid search criterion and matches target 1526 references. Like all search criteria, on a resource lookup it can 1527 match the target reference of the resource link itself, but also the 1528 registration resource of the endpoint that registered it. Queries 1529 for resource link targets MUST be in URI form (i.e. not relative 1530 references) and are matched against a resolved link target. Queries 1531 for endpoints SHOULD be expressed in path-absolute form if possible 1532 and MUST be expressed in URI form otherwise; the RD SHOULD recognize 1533 either. 1535 Endpoints that are interested in a lookup result repeatedly or 1536 continuously can use mechanisms like ETag caching, resource 1537 observation ([RFC7641]), or any future mechanism that might allow 1538 more efficient observations of collections. These are advertised, 1539 detected and used according to their own specifications and can be 1540 used with the lookup interface as with any other resource. 1542 When resource observation is used, every time the set of matching 1543 links changes, or the content of a matching link changes, the RD 1544 sends a notification with the matching link set. The notification 1545 contains the successful current response to the given request, 1546 especially with respect to representing zero matching links (see 1547 "Success" item below). 1549 The lookup interface is specified as follows: 1551 Interaction: Client -> RD 1553 Method: GET 1555 URI Template: {+type-lookup-location}{?page,count,search*} 1557 URI Template Variables: 1559 type-lookup-location := RD Lookup URI for a given lookup type 1560 (mandatory). The address is discovered as described in 1561 Section 4.3. 1563 search := Search criteria for limiting the number of results 1564 (optional). 1566 page := Page (optional). Parameter cannot be used without the 1567 count parameter. Results are returned from result set in pages 1568 that contain 'count' links starting from index (page * count). 1569 Page numbering starts with zero. 1571 count := Count (optional). Number of results is limited to this 1572 parameter value. If the page parameter is also present, the 1573 response MUST only include 'count' links starting with the 1574 (page * count) link in the result set from the query. If the 1575 count parameter is not present, then the response MUST return 1576 all matching links in the result set. Link numbering starts 1577 with zero. 1579 Accept: absent, application/link-format or any other indicated 1580 media type representing web links 1582 The following responses codes are defined for this interface: 1584 Success: 2.05 "Content" or 200 "OK" with an "application/link- 1585 format" or other web link payload containing matching entries for 1586 the lookup. The payload can contain zero links (which is an empty 1587 payload in [RFC6690] link format, but could also be "[]" in JSON 1588 based formats), indicating that no entities matched the request. 1590 6.3. Resource lookup examples 1592 The examples in this section assume the existence of CoAP hosts with 1593 a default CoAP port 61616. HTTP hosts are possible and do not change 1594 the nature of the examples. 1596 The following example shows a client performing a resource lookup 1597 with the example resource look-up locations discovered in Figure 5: 1599 Req: GET /rd-lookup/res?rt=temperature 1601 Res: 2.05 Content 1602 ;rt="temperature"; 1603 anchor="coap://[2001:db8:3::123]:61616" 1605 A client that wants to be notified of new resources as they show up 1606 can use observation: 1608 Req: GET /rd-lookup/res?rt=light 1609 Observe: 0 1611 Res: 2.05 Content 1612 Observe: 23 1613 Payload: empty 1615 (at a later point in time) 1617 Res: 2.05 Content 1618 Observe: 24 1619 Payload: 1620 ;rt="light"; 1621 anchor="coap://[2001:db8:3::124]", 1622 ;rt="light"; 1623 anchor="coap://[2001:db8:3::124]", 1624 ;rt="light"; 1625 anchor="coap://[2001:db8:3::124]" 1627 The following example shows a client performing a paginated resource 1628 lookup 1629 Req: GET /rd-lookup/res?page=0&count=5 1631 Res: 2.05 Content 1632 ;rt=sensor;ct=60; 1633 anchor="coap://[2001:db8:3::123]:61616", 1634 ;rt=sensor;ct=60; 1635 anchor="coap://[2001:db8:3::123]:61616", 1636 ;rt=sensor;ct=60; 1637 anchor="coap://[2001:db8:3::123]:61616", 1638 ;rt=sensor;ct=60; 1639 anchor="coap://[2001:db8:3::123]:61616", 1640 ;rt=sensor;ct=60; 1641 anchor="coap://[2001:db8:3::123]:61616" 1643 Req: GET /rd-lookup/res?page=1&count=5 1645 Res: 2.05 Content 1646 ;rt=sensor;ct=60; 1647 anchor="coap://[2001:db8:3::123]:61616", 1648 ;rt=sensor;ct=60; 1649 anchor="coap://[2001:db8:3::123]:61616", 1650 ;rt=sensor;ct=60; 1651 anchor="coap://[2001:db8:3::123]:61616", 1652 ;rt=sensor;ct=60; 1653 anchor="coap://[2001:db8:3::123]:61616", 1654 ;rt=sensor;ct=60; 1655 anchor="coap://[2001:db8:3::123]:61616" 1657 The following example shows a client performing a lookup of all 1658 resources from endpoints of all endpoints of a given endpoint type. 1659 It assumes that two endpoints (with endpoint names "sensor1" and 1660 "sensor2") have previously registered with their respective addresses 1661 "coap://sensor1.example.com" and "coap://sensor2.example.com", and 1662 posted the very payload of the 6th request of section 5 of [RFC6690]. 1664 It demonstrates how absolute link targets stay unmodified, while 1665 relative ones are resolved: 1667 Req: GET /rd-lookup/res?et=oic.d.sensor 1669 ;ct=40;title="Sensor Index"; 1670 anchor="coap://sensor1.example.com", 1671 ;rt="temperature-c"; 1672 if="sensor"; anchor="coap://sensor1.example.com", 1673 ;rt="light-lux"; 1674 if="sensor"; anchor="coap://sensor1.example.com", 1675 ;rel="describedby"; 1676 anchor="coap://sensor1.example.com/sensors/temp", 1677 ;rel="alternate"; 1678 anchor="coap://sensor1.example.com/sensors/temp", 1679 ;ct=40;title="Sensor Index"; 1680 anchor="coap://sensor2.example.com", 1681 ;rt="temperature-c"; 1682 if="sensor"; anchor="coap://sensor2.example.com", 1683 ;rt="light-lux"; 1684 if="sensor"; anchor="coap://sensor2.example.com", 1685 ;rel="describedby"; 1686 anchor="coap://sensor2.example.com/sensors/temp", 1687 ;rel="alternate"; 1688 anchor="coap://sensor2.example.com/sensors/temp" 1690 6.4. Endpoint lookup 1692 The endpoint lookup returns registration resources which can only be 1693 manipulated by the registering endpoint. 1695 Endpoint registration resources are annotated with their endpoint 1696 names (ep), sectors (d, if present) and registration base URI (base; 1697 reports the registrant-ep's address if no explicit base was given) as 1698 well as a constant resource type (rt="core.rd-ep"); the lifetime (lt) 1699 is not reported. Additional endpoint attributes are added as target 1700 attributes to their endpoint link unless their specification says 1701 otherwise. 1703 Links to endpoints SHOULD be presented in path-absolute form or, if 1704 required, as absolute references. (This avoids the RFC6690 1705 ambiguities.) 1707 Base addresses that contain link-local addresses MUST NOT include 1708 zone identifiers, and such registrations MUST NOT be shown unless the 1709 lookup was made from the same link from which the registration was 1710 made. 1712 While Endpoint Lookup does expose the registration resources, the RD 1713 does not need to make them accessible to clients. Clients SHOULD NOT 1714 attempt to dereference or manipulate them. 1716 A Resource Directory can report endpoints in lookup that are not 1717 hosted at the same address. Lookup clients MUST be prepared to see 1718 arbitrary URIs as registration resources in the results and treat 1719 them as opaque identifiers; the precise semantics of such links are 1720 left to future specifications. 1722 The following example shows a client performing an endpoint type (et) 1723 lookup with the value oic.d.sensor (which is currently a registered 1724 rt value): 1726 Req: GET /rd-lookup/ep?et=oic.d.sensor 1728 Res: 2.05 Content 1729 ;base="coap://[2001:db8:3::127]:61616";ep="node5"; 1730 et="oic.d.sensor";ct="40";rt="core.rd-ep", 1731 ;base="coap://[2001:db8:3::129]:61616";ep="node7"; 1732 et="oic.d.sensor";ct="40";d="floor-3";rt="core.rd-ep" 1734 7. Security policies 1736 The Resource Directory (RD) provides assistance to applications 1737 situated on a selection of nodes to discover endpoints on connected 1738 nodes. This section discusses different security aspects of 1739 accessing the RD. 1741 The contents of the RD are inserted in two ways: 1743 1. The node hosting the discoverable endpoint fills the RD with the 1744 contents of /.well-known/core by: 1746 * Storing the contents directly into RD (see Section 5) 1748 * Requesting the RD to load the contents from /.well-known/core 1749 (see Section 5.1) 1751 2. A Commissioning Tool (CT) fills the RD with endpoint information 1752 for a set of discoverable nodes. (see Section 5 with 1753 base=authority parameter value) 1755 In both cases, the nodes filling the RD should be authenticated and 1756 authorized to change the contents of the RD. An Authorization Server 1757 (AS) is responsible to assign a token to the registering node to 1758 authorize the node to discover or register endpoints in a given RD 1759 [I-D.ietf-ace-oauth-authz]. 1761 It can be imagined that an installation is divided in a set of 1762 security regions, each one with its own RD(s) to discover the 1763 endpoints that are part of a given security region. An endpoint that 1764 wants to discover an RD, responsible for a given region, needs to be 1765 authorized to learn the contents of a given RD. Within a region, for 1766 a given RD, a more fine-grained security division is possible based 1767 on the values of the endpoint registration parameters. Authorization 1768 to discover endpoints with a given set of filter values is 1769 recommended for those cases. 1771 When a node registers its endpoints, criteria are needed to authorize 1772 the node to enter them. An important aspect is the uniqueness of the 1773 (endpoint name, and optional sector) pair within the RD. Consider 1774 the two cases separately: (1) CT registers endpoints, and (2) the 1775 registering node registers its own endpoint(s). 1777 o A CT needs authorization to register a set of endpoints. This 1778 authorization can be based on the region, i.e. a given CT is 1779 authorized to register any endpoint (endpoint name, sector) into a 1780 given RD, or to register an endpoint with (endpoint name, sector) 1781 value pairs assigned by the AS, or can be more fine-grained, 1782 including a subset of registration parameter values. 1784 o A given endpoint that registers itself, needs to proof its 1785 possession of its unique (endpoint name, sector) value pair. 1786 Alternatively, the AS can authorize the endpoint to register with 1787 an (endpoint name, sector) value pair assigned by the AS. 1789 A separate document needs to specify these aspects to ensure 1790 interoperability between registering nodes and RD. The subsections 1791 below give some hints how to handle a subset of the different 1792 aspects. 1794 7.1. Secure RD discovery 1796 The Resource Server (RS) discussed in [I-D.ietf-ace-oauth-authz] is 1797 equated to the RD. The client (C) needs to discover the RD as 1798 discussed in Section 4.1. C can discover the related AS by sending a 1799 request to the RD. The RD denies the request by sending the address 1800 of the related AS, as discussed in section 5.1 of 1801 [I-D.ietf-ace-oauth-authz]. The client MUST send an authorization 1802 request to the AS. When appropriate, the AS returns a token that 1803 specifies the authorization permission which needs to be specified in 1804 a separate document. 1806 7.2. Secure RD filtering 1808 The authorized parameter values for the queries by a given endpoint 1809 must be registered by the AS. The AS communicates the parameter 1810 values in the token. A separate document needs to specify the 1811 parameter value combinations and their storage in the token. The RD 1812 decodes the token and checks the validity of the queries of the 1813 client. 1815 7.3. Secure endpoint Name assignment 1817 This section only considers the assignment of a name to the endpoint 1818 based on an automatic mechanism without use of AS. More elaborate 1819 protocols are out of scope. The registering endpoint is authorized 1820 by the AS to discover the RD and add registrations. A token is 1821 provided by the AS and communicated from registering endpoint to RD. 1822 It is assumed that DTLS is used to secure the channel between 1823 registering endpoint and RD, where the registering endpoint is the 1824 DTLS client. Assuming that the client is provided by a certificate 1825 at manufacturing time, the certificate is uniquely identified by the 1826 CN field and the serial number. The RD can assign a unique endpoint 1827 name by using the certificate identifier as endpoint name. Proof of 1828 possession of the endpoint name by the registering endpoint is 1829 checked by encrypting the certificate identifier with the private key 1830 of the registering endpoint, which the RD can decrypt with the public 1831 key stored in the certificate. Even simpler, the authorized 1832 registering endpoint can generate a random number (or string) that 1833 identifies the endpoint. The RD can check for the improbable 1834 replication of the random value. The RD MUST check that registering 1835 endpoint uses only one random value for each authorized endpoint. 1837 8. Security Considerations 1839 The security considerations as described in Section 5 of [RFC8288] 1840 and Section 6 of [RFC6690] apply. The "/.well-known/core" resource 1841 may be protected e.g. using DTLS when hosted on a CoAP server as 1842 described in [RFC7252]. DTLS or TLS based security SHOULD be used on 1843 all resource directory interfaces defined in this document. 1845 8.1. Endpoint Identification and Authentication 1847 An Endpoint (name, sector) pair is unique within the et of endpoints 1848 registered by the RD. An Endpoint MUST NOT be identified by its 1849 protocol, port or IP address as these may change over the lifetime of 1850 an Endpoint. 1852 Every operation performed by an Endpoint on a resource directory 1853 SHOULD be mutually authenticated using Pre-Shared Key, Raw Public Key 1854 or Certificate based security. 1856 Consider the following threat: two devices A and B are registered at 1857 a single server. Both devices have unique, per-device credentials 1858 for use with DTLS to make sure that only parties with authorization 1859 to access A or B can do so. 1861 Now, imagine that a malicious device A wants to sabotage the device 1862 B. It uses its credentials during the DTLS exchange. Then, it 1863 specifies the endpoint name of device B as the name of its own 1864 endpoint in device A. If the server does not check whether the 1865 identifier provided in the DTLS handshake matches the identifier used 1866 at the CoAP layer then it may be inclined to use the endpoint name 1867 for looking up what information to provision to the malicious device. 1869 Section 7.3 specifies an example that removes this threat for 1870 endpoints that have a certificate installed. 1872 8.2. Access Control 1874 Access control SHOULD be performed separately for the RD registration 1875 and Lookup API paths, as different endpoints may be authorized to 1876 register with an RD from those authorized to lookup endpoints from 1877 the RD. Such access control SHOULD be performed in as fine-grained a 1878 level as possible. For example access control for lookups could be 1879 performed either at the sector, endpoint or resource level. 1881 8.3. Denial of Service Attacks 1883 Services that run over UDP unprotected are vulnerable to unknowingly 1884 become part of a DDoS attack as UDP does not require return 1885 routability check. Therefore, an attacker can easily spoof the 1886 source IP of the target entity and send requests to such a service 1887 which would then respond to the target entity. This can be used for 1888 large-scale DDoS attacks on the target. Especially, if the service 1889 returns a response that is order of magnitudes larger than the 1890 request, the situation becomes even worse as now the attack can be 1891 amplified. DNS servers have been widely used for DDoS amplification 1892 attacks. There is also a danger that NTP Servers could become 1893 implicated in denial-of-service (DoS) attacks since they run on 1894 unprotected UDP, there is no return routability check, and they can 1895 have a large amplification factor. The responses from the NTP server 1896 were found to be 19 times larger than the request. A Resource 1897 Directory (RD) which responds to wild-card lookups is potentially 1898 vulnerable if run with CoAP over UDP. Since there is no return 1899 routability check and the responses can be significantly larger than 1900 requests, RDs can unknowingly become part of a DDoS amplification 1901 attack. 1903 9. IANA Considerations 1904 9.1. Resource Types 1906 IANA is asked to enter the following values into the Resource Type 1907 (rt=) Link Target Attribute Values sub-registry of the Constrained 1908 Restful Environments (CoRE) Parameters registry defined in [RFC6690]: 1910 +--------------------+--------------------------+-------------------+ 1911 | Value | Description | Reference | 1912 +--------------------+--------------------------+-------------------+ 1913 | core.rd | Directory resource of an | RFCTHIS Section | 1914 | | RD | 4.3 | 1915 | core.rd-lookup-res | Resource lookup of an RD | RFCTHIS Section | 1916 | | | 4.3 | 1917 | core.rd-lookup-ep | Endpoint lookup of an RD | RFCTHIS Section | 1918 | | | 4.3 | 1919 | core.rd-ep | Endpoint resource of an | RFCTHIS Section 6 | 1920 | | RD | | 1921 +--------------------+--------------------------+-------------------+ 1923 9.2. IPv6 ND Resource Directory Address Option 1925 This document registers one new ND option type under the sub-registry 1926 "IPv6 Neighbor Discovery Option Formats": 1928 o Resource Directory Address Option (38) 1930 9.3. RD Parameter Registry 1932 This specification defines a new sub-registry for registration and 1933 lookup parameters called "RD Parameters" under "CoRE Parameters". 1934 Although this specification defines a basic set of parameters, it is 1935 expected that other standards that make use of this interface will 1936 define new ones. 1938 Each entry in the registry must include 1940 o the human readable name of the parameter, 1942 o the short name as used in query parameters or target attributes, 1944 o indication of whether it can be passed as a query parameter at 1945 registration of endpoints, as a query parameter in lookups, or be 1946 expressed as a target attribute, 1948 o validity requirements if any, and 1950 o a description. 1952 The query parameter MUST be both a valid URI query key [RFC3986] and 1953 a token as used in [RFC8288]. 1955 The description must give details on whether the parameter can be 1956 updated, and how it is to be processed in lookups. 1958 The mechanisms around new RD parameters should be designed in such a 1959 way that they tolerate RD implementations that are unaware of the 1960 parameter and expose any parameter passed at registration or updates 1961 on in endpoint lookups. (For example, if a parameter used at 1962 registration were to be confidential, the registering endpoint should 1963 be instructed to only set that parameter if the RD advertises support 1964 for keeping it confidential at the discovery step.) 1966 Initial entries in this sub-registry are as follows: 1968 +--------------+-------+---------------+-----+----------------------+ 1969 | Full name | Short | Validity | Use | Description | 1970 +--------------+-------+---------------+-----+----------------------+ 1971 | Endpoint | ep | | RLA | Name of the | 1972 | Name | | | | endpoint, max 63 | 1973 | | | | | bytes | 1974 | Lifetime | lt | 60-4294967295 | R | Lifetime of the | 1975 | | | | | registration in | 1976 | | | | | seconds | 1977 | Sector | d | | RLA | Sector to which this | 1978 | | | | | endpoint belongs | 1979 | Registration | base | URI | RLA | The scheme, address | 1980 | Base URI | | | | and port and path at | 1981 | | | | | which this server is | 1982 | | | | | available | 1983 | Page | page | Integer | L | Used for pagination | 1984 | Count | count | Integer | L | Used for pagination | 1985 | Endpoint | et | | RLA | Semantic name of the | 1986 | Type | | | | endpoint (see | 1987 | | | | | Section 9.4) | 1988 +--------------+-------+---------------+-----+----------------------+ 1990 Table 2: RD Parameters 1992 (Short: Short name used in query parameters or target attributes. 1993 Use: R = used at registration, L = used at lookup, A = expressed in 1994 target attribute 1996 The descriptions for the options defined in this document are only 1997 summarized here. To which registrations they apply and when they are 1998 to be shown is described in the respective sections of this document. 2000 The IANA policy for future additions to the sub-registry is "Expert 2001 Review" as described in [RFC8126]. The evaluation should consider 2002 formal criteria, duplication of functionality (Is the new entry 2003 redundant with an existing one?), topical suitability (E.g. is the 2004 described property actually a property of the endpoint and not a 2005 property of a particular resource, in which case it should go into 2006 the payload of the registration and need not be registered?), and the 2007 potential for conflict with commonly used target attributes (For 2008 example, "if" could be used as a parameter for conditional 2009 registration if it were not to be used in lookup or attributes, but 2010 would make a bad parameter for lookup, because a resource lookup with 2011 an "if" query parameter could ambiguously filter by the registered 2012 endpoint property or the [RFC6690] target attribute). It is expected 2013 that the registry will receive between 5 and 50 registrations in 2014 total over the next years. 2016 9.3.1. Full description of the "Endpoint Type" Registration Parameter 2018 An endpoint registering at an RD can describe itself with endpoint 2019 types, similar to how resources are described with Resource Types in 2020 [RFC6690]. An endpoint type is expressed as a string, which can be 2021 either a URI or one of the values defined in the Endpoint Type sub- 2022 registry. Endpoint types can be passed in the "et" query parameter 2023 as part of extra-attrs at the Registration step, are shown on 2024 endpoint lookups using the "et" target attribute, and can be filtered 2025 for using "et" as a search criterion in resource and endpoint lookup. 2026 Multiple endpoint types are given as separate query parameters or 2027 link attributes. 2029 Note that Endpoint Type differs from Resource Type in that it uses 2030 multiple attributes rather than space separated values. As a result, 2031 Resource Directory implementations automatically support correct 2032 filtering in the lookup interfaces from the rules for unknown 2033 endpoint attributes. 2035 9.4. "Endpoint Type" (et=) RD Parameter values 2037 This specification establishes a new sub-registry under "CoRE 2038 Parameters" called '"Endpoint Type" (et=) RD Parameter values'. The 2039 registry properties (required policy, requirements, template) are 2040 identical to those of the Resource Type parameters in [RFC6690], in 2041 short: 2043 The review policy is IETF Review for values starting with "core", and 2044 Specification Required for others. 2046 The requirements to be enforced are: 2048 o The values MUST be related to the purpose described in 2049 Section 9.3.1. 2051 o The registered values MUST conform to the ABNF reg-rel-type 2052 definition of [RFC6690] and MUST NOT be a URI. 2054 o It is recommended to use the period "." character for 2055 segmentation. 2057 The registry initially contains one value: 2059 o "core.rd-group": An application group as described in Appendix A. 2061 9.5. Multicast Address Registration 2063 IANA has assigned the following multicast addresses for use by CoAP 2064 nodes: 2066 IPv4 - "all CoRE resource directories" address, from the "IPv4 2067 Multicast Address Space Registry" equal to "All CoAP Nodes", 2068 224.0.1.187. As the address is used for discovery that may span 2069 beyond a single network, it has come from the Internetwork Control 2070 Block (224.0.1.x, RFC 5771). 2072 IPv6 - "all CoRE resource directories" address MCD1 (suggestions 2073 FF0X::FE), from the "IPv6 Multicast Address Space Registry", in the 2074 "Variable Scope Multicast Addresses" space (RFC 3307). Note that 2075 there is a distinct multicast address for each scope that interested 2076 CoAP nodes should listen to; CoAP needs the Link-Local and Site-Local 2077 scopes only. 2079 10. Examples 2081 Two examples are presented: a Lighting Installation example in 2082 Section 10.1 and a LWM2M example in Section 10.2. 2084 10.1. Lighting Installation 2086 This example shows a simplified lighting installation which makes use 2087 of the Resource Directory (RD) with a CoAP interface to facilitate 2088 the installation and start-up of the application code in the lights 2089 and sensors. In particular, the example leads to the definition of a 2090 group and the enabling of the corresponding multicast address as 2091 described in Appendix A. No conclusions must be drawn on the 2092 realization of actual installation or naming procedures, because the 2093 example only "emphasizes" some of the issues that may influence the 2094 use of the RD and does not pretend to be normative. 2096 10.1.1. Installation Characteristics 2098 The example assumes that the installation is managed. That means 2099 that a Commissioning Tool (CT) is used to authorize the addition of 2100 nodes, name them, and name their services. The CT can be connected 2101 to the installation in many ways: the CT can be part of the 2102 installation network, connected by WiFi to the installation network, 2103 or connected via GPRS link, or other method. 2105 It is assumed that there are two naming authorities for the 2106 installation: (1) the network manager that is responsible for the 2107 correct operation of the network and the connected interfaces, and 2108 (2) the lighting manager that is responsible for the correct 2109 functioning of networked lights and sensors. The result is the 2110 existence of two naming schemes coming from the two managing 2111 entities. 2113 The example installation consists of one presence sensor, and two 2114 luminaries, luminary1 and luminary2, each with their own wireless 2115 interface. Each luminary contains three lamps: left, right and 2116 middle. Each luminary is accessible through one endpoint. For each 2117 lamp a resource exists to modify the settings of a lamp in a 2118 luminary. The purpose of the installation is that the presence 2119 sensor notifies the presence of persons to a group of lamps. The 2120 group of lamps consists of: middle and left lamps of luminary1 and 2121 right lamp of luminary2. 2123 Before commissioning by the lighting manager, the network is 2124 installed and access to the interfaces is proven to work by the 2125 network manager. 2127 At the moment of installation, the network under installation is not 2128 necessarily connected to the DNS infra structure. Therefore, SLAAC 2129 IPv6 addresses are assigned to CT, RD, luminaries and sensor shown in 2130 Table 3 below: 2132 +--------------------+----------------+ 2133 | Name | IPv6 address | 2134 +--------------------+----------------+ 2135 | luminary1 | 2001:db8:4::1 | 2136 | luminary2 | 2001:db8:4::2 | 2137 | Presence sensor | 2001:db8:4::3 | 2138 | Resource directory | 2001:db8:4::ff | 2139 +--------------------+----------------+ 2141 Table 3: interface SLAAC addresses 2143 In Section 10.1.2 the use of resource directory during installation 2144 is presented. 2146 10.1.2. RD entries 2148 It is assumed that access to the DNS infrastructure is not always 2149 possible during installation. Therefore, the SLAAC addresses are 2150 used in this section. 2152 For discovery, the resource types (rt) of the devices are important. 2153 The lamps in the luminaries have rt: light, and the presence sensor 2154 has rt: p-sensor. The endpoints have names which are relevant to the 2155 light installation manager. In this case luminary1, luminary2, and 2156 the presence sensor are located in room 2-4-015, where luminary1 is 2157 located at the window and luminary2 and the presence sensor are 2158 located at the door. The endpoint names reflect this physical 2159 location. The middle, left and right lamps are accessed via path 2160 /light/middle, /light/left, and /light/right respectively. The 2161 identifiers relevant to the Resource Directory are shown in Table 4 2162 below: 2164 +----------------+------------------+---------------+---------------+ 2165 | Name | endpoint | resource path | resource type | 2166 +----------------+------------------+---------------+---------------+ 2167 | luminary1 | lm_R2-4-015_wndw | /light/left | light | 2168 | luminary1 | lm_R2-4-015_wndw | /light/middle | light | 2169 | luminary1 | lm_R2-4-015_wndw | /light/right | light | 2170 | luminary2 | lm_R2-4-015_door | /light/left | light | 2171 | luminary2 | lm_R2-4-015_door | /light/middle | light | 2172 | luminary2 | lm_R2-4-015_door | /light/right | light | 2173 | Presence | ps_R2-4-015_door | /ps | p-sensor | 2174 | sensor | | | | 2175 +----------------+------------------+---------------+---------------+ 2177 Table 4: Resource Directory identifiers 2179 It is assumed that the CT knows the RD's address, and has performed 2180 URI discovery on it that returned a response like the one in the 2181 Section 4.3 example. 2183 The CT inserts the endpoints of the luminaries and the sensor in the 2184 RD using the registration base URI parameter (base) to specify the 2185 interface address: 2187 Req: POST coap://[2001:db8:4::ff]/rd 2188 ?ep=lm_R2-4-015_wndw&base=coap://[2001:db8:4::1]&d=R2-4-015 2189 Payload: 2190 ;rt="light", 2191 ;rt="light", 2192 ;rt="light" 2194 Res: 2.01 Created 2195 Location-Path: /rd/4521 2197 Req: POST coap://[2001:db8:4::ff]/rd 2198 ?ep=lm_R2-4-015_door&base=coap://[2001:db8:4::2]&d=R2-4-015 2199 Payload: 2200 ;rt="light", 2201 ;rt="light", 2202 ;rt="light" 2204 Res: 2.01 Created 2205 Location-Path: /rd/4522 2207 Req: POST coap://[2001:db8:4::ff]/rd 2208 ?ep=ps_R2-4-015_door&base=coap://[2001:db8:4::3]d&d=R2-4-015 2209 Payload: 2210 ;rt="p-sensor" 2212 Res: 2.01 Created 2213 Location-Path: /rd/4523 2215 The sector name d=R2-4-015 has been added for an efficient lookup 2216 because filtering on "ep" name is more awkward. The same sector name 2217 is communicated to the two luminaries and the presence sensor by the 2218 CT. 2220 The group is specified in the RD. The base parameter is set to the 2221 site-local multicast address allocated to the group. In the POST in 2222 the example below, the resources supported by all group members are 2223 published. 2225 Req: POST coap://[2001:db8:4::ff]/rd 2226 ?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::1] 2227 Payload: 2228 ;rt="light", 2229 ;rt="light", 2230 ;rt="light" 2232 Res: 2.01 Created 2233 Location-Path: /rd/501 2234 After the filling of the RD by the CT, the application in the 2235 luminaries can learn to which groups they belong, and enable their 2236 interface for the multicast address. 2238 The luminary, knowing its sector and being configured to join any 2239 group containing lights, searches for candidate groups and joins 2240 them: 2242 Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep 2243 ?d=R2-4-015&et=core.rd-group&rt=light 2245 Res: 2.05 Content 2246 ;ep="grp_R2-4-015";et="core.rd-group"; 2247 base="coap://[ff05::1]";rt="core.rd-ep" 2249 From the returned base parameter value, the luminary learns the 2250 multicast address of the multicast group. 2252 Alternatively, the CT can communicate the multicast address directly 2253 to the luminaries by using the "coap-group" resource specified in 2254 [RFC7390]. 2256 Req: POST coap://[2001:db8:4::1]/coap-group 2257 Content-Format: application/coap-group+json 2258 Payload: 2259 { "a": "[ff05::1]", "n": "grp_R2-4-015"} 2261 Res: 2.01 Created 2262 Location-Path: /coap-group/1 2264 Dependent on the situation, only the address, "a", or the name, "n", 2265 is specified in the coap-group resource. 2267 The presence sensor can learn the presence of groups that support 2268 resources with rt=light in its own sector by sending the same 2269 request, as used by the luminary. The presence sensor learns the 2270 multicast address to use for sending messages to the luminaries. 2272 10.2. OMA Lightweight M2M (LWM2M) Example 2274 This example shows how the OMA LWM2M specification makes use of 2275 Resource Directory (RD). 2277 OMA LWM2M is a profile for device services based on CoAP(OMA Name 2278 Authority). LWM2M defines a simple object model and a number of 2279 abstract interfaces and operations for device management and device 2280 service enablement. 2282 An LWM2M server is an instance of an LWM2M middleware service layer, 2283 containing a Resource Directory along with other LWM2M interfaces 2284 defined by the LWM2M specification. 2286 CoRE Resource Directory (RD) is used to provide the LWM2M 2287 Registration interface. 2289 LWM2M does not provide for registration sectors and does not 2290 currently use the rd-lookup interface. 2292 The LWM2M specification describes a set of interfaces and a resource 2293 model used between a LWM2M device and an LWM2M server. Other 2294 interfaces, proxies, and applications are currently out of scope for 2295 LWM2M. 2297 The location of the LWM2M Server and RD URI path is provided by the 2298 LWM2M Bootstrap process, so no dynamic discovery of the RD is used. 2299 LWM2M Servers and endpoints are not required to implement the /.well- 2300 known/core resource. 2302 10.2.1. The LWM2M Object Model 2304 The OMA LWM2M object model is based on a simple 2 level class 2305 hierarchy consisting of Objects and Resources. 2307 An LWM2M Resource is a REST endpoint, allowed to be a single value or 2308 an array of values of the same data type. 2310 An LWM2M Object is a resource template and container type that 2311 encapsulates a set of related resources. An LWM2M Object represents 2312 a specific type of information source; for example, there is a LWM2M 2313 Device Management object that represents a network connection, 2314 containing resources that represent individual properties like radio 2315 signal strength. 2317 Since there may potentially be more than one of a given type object, 2318 for example more than one network connection, LWM2M defines instances 2319 of objects that contain the resources that represent a specific 2320 physical thing. 2322 The URI template for LWM2M consists of a base URI followed by Object, 2323 Instance, and Resource IDs: 2325 {/base-uri}{/object-id}{/object-instance}{/resource-id}{/resource- 2326 instance} 2328 The five variables given here are strings. base-uri can also have 2329 the special value "undefined" (sometimes called "null" in RFC 6570). 2331 Each of the variables object-instance, resource-id, and resource- 2332 instance can be the special value "undefined" only if the values 2333 behind it in this sequence also are "undefined". As a special case, 2334 object-instance can be "empty" (which is different from "undefined") 2335 if resource-id is not "undefined". 2337 base-uri := Base URI for LWM2M resources or "undefined" for default 2338 (empty) base URI 2340 object-id := OMNA (OMA Name Authority) registered object ID (0-65535) 2342 object-instance := Object instance identifier (0-65535) or 2343 "undefined"/"empty" (see above)) to refer to all instances of an 2344 object ID 2346 resource-id := OMNA (OMA Name Authority) registered resource ID 2347 (0-65535) or "undefined" to refer to all resources within an instance 2349 resource-instance := Resource instance identifier or "undefined" to 2350 refer to single instance of a resource 2352 LWM2M IDs are 16 bit unsigned integers represented in decimal (no 2353 leading zeroes except for the value 0) by URI format strings. For 2354 example, a LWM2M URI might be: 2356 /1/0/1 2358 The base uri is empty, the Object ID is 1, the instance ID is 0, the 2359 resource ID is 1, and the resource instance is "undefined". This 2360 example URI points to internal resource 1, which represents the 2361 registration lifetime configured, in instance 0 of a type 1 object 2362 (LWM2M Server Object). 2364 10.2.2. LWM2M Register Endpoint 2366 LWM2M defines a registration interface based on the REST API, 2367 described in Section 5. The RD registration URI path of the LWM2M 2368 Resource Directory is specified to be "/rd". 2370 LWM2M endpoints register object IDs, for example , to indicate 2371 that a particular object type is supported, and register object 2372 instances, for example , to indicate that a particular instance 2373 of that object type exists. 2375 Resources within the LWM2M object instance are not registered with 2376 the RD, but may be discovered by reading the resource links from the 2377 object instance using GET with a CoAP Content-Format of application/ 2378 link-format. Resources may also be read as a structured object by 2379 performing a GET to the object instance with a Content-Format of 2380 senml+json. 2382 When an LWM2M object or instance is registered, this indicates to the 2383 LWM2M server that the object and its resources are available for 2384 management and service enablement (REST API) operations. 2386 LWM2M endpoints may use the following RD registration parameters as 2387 defined in Table 2 : 2389 ep - Endpoint Name 2390 lt - registration lifetime 2392 Endpoint Name, Lifetime, and LWM2M Version are mandatory parameters 2393 for the register operation, all other registration parameters are 2394 optional. 2396 Additional optional LWM2M registration parameters are defined: 2398 +-----------+-------+-------------------------------+---------------+ 2399 | Name | Query | Validity | Description | 2400 +-----------+-------+-------------------------------+---------------+ 2401 | Binding | b | {"U",UQ","S","SQ","US","UQS"} | Available | 2402 | Mode | | | Protocols | 2403 | | | | | 2404 | LWM2M | ver | 1.0 | Spec Version | 2405 | Version | | | | 2406 | | | | | 2407 | SMS | sms | | MSISDN | 2408 | Number | | | | 2409 +-----------+-------+-------------------------------+---------------+ 2411 Table 5: LWM2M Additional Registration Parameters 2413 The following RD registration parameters are not currently specified 2414 for use in LWM2M: 2416 et - Endpoint Type 2417 base - Registration Base URI 2419 The endpoint registration must include a payload containing links to 2420 all supported objects and existing object instances, optionally 2421 including the appropriate link-format relations. 2423 Here is an example LWM2M registration payload: 2425 ,,, 2426 This link format payload indicates that object ID 1 (LWM2M Server 2427 Object) is supported, with a single instance 0 existing, object ID 3 2428 (LWM2M Device object) is supported, with a single instance 0 2429 existing, and object 5 (LWM2M Firmware Object) is supported, with no 2430 existing instances. 2432 10.2.3. LWM2M Update Endpoint Registration 2434 The LwM2M update is really very similar to the registration update as 2435 described in Section 5.3.1, with the only difference that there are 2436 more parameters defined and available. All the parameters listed in 2437 that section are also available with the initial registration but are 2438 all optional: 2440 lt - Registration Lifetime 2441 b - Protocol Binding 2442 sms - MSISDN 2443 link payload - new or modified links 2445 A Registration update is also specified to be used to update the 2446 LWM2M server whenever the endpoint's UDP port or IP address are 2447 changed. 2449 10.2.4. LWM2M De-Register Endpoint 2451 LWM2M allows for de-registration using the delete method on the 2452 returned location from the initial registration operation. LWM2M de- 2453 registration proceeds as described in Section 5.3.2. 2455 11. Acknowledgments 2457 Oscar Novo, Srdjan Krco, Szymon Sasin, Kerry Lynn, Esko Dijk, Anders 2458 Brandt, Matthieu Vial, Jim Schaad, Mohit Sethi, Hauke Petersen, 2459 Hannes Tschofenig, Sampo Ukkola, Linyi Tian, Jan Newmarch, Matthias 2460 Kovatsch and Jaime Jimenez have provided helpful comments, 2461 discussions and ideas to improve and shape this document. Zach would 2462 also like to thank his colleagues from the EU FP7 SENSEI project, 2463 where many of the resource directory concepts were originally 2464 developed. 2466 12. Changelog 2468 changes from -19 to -20 2470 (Processing comments from the WG chair review) 2472 o Define the permissible characters in endpoint and sector names 2473 o Express requirements on NAT situations in more abstract terms 2475 o Shifted heading levels to have the interfaces on the same level 2477 o Group instructions for error handling into general section 2479 o Simple Registration: process reflowed into items list 2481 o Updated introduction to reflect state of CoRE in general, 2482 reference RFC7228 (defining "constrained") and use "IoT" term in 2483 addition to "M2M" 2485 o Update acknowledgements 2487 o Assorted editorial changes 2489 * Unify examples style 2491 * Terminology: RDAO defined and not only expanded 2493 * Add CT to Figure 1 2495 * Consistency in the use of the term "Content Format" 2497 changes from -18 to -19 2499 o link-local addresses: allow but prescribe split-horizon fashion 2500 when used, disallow zone identifiers 2502 o Remove informative references to documents not mentioned any more 2504 changes from -17 to -18 2506 o Rather than re-specifying link format (Modernized Link Format), 2507 describe a Limited Link Format that's the uncontested subset of 2508 Link Format 2510 o Acknowledging the -17 version as part of the draft 2512 o Move "Read endpoint links" operation to future specification like 2513 PATCH 2515 o Demote links-json to an informative reference, and removed them 2516 from exchange examples 2518 o Add note on unusability of link-local IP addresses, and describe 2519 mitigation. 2521 o Reshuffling of sections: Move additional operations and endpoint 2522 lookup back from appendix, and groups into one 2524 o Lookup interface tightened to not imply applicability for non 2525 link-format lookups (as those can have vastly different views on 2526 link cardinality) 2528 o Simple registration: Change sequence of GET and POST-response, 2529 ensuring unsuccessful registrations are reported as such, and 2530 suggest how devices that would have required the inverse behavior 2531 can still cope with it. 2533 o Abstract and introduction reworded to avoid the impression that 2534 resources are stored in full in the RD 2536 o Simplify the rules governing when a registration resource can or 2537 must be changed. 2539 o Drop a figure that has become useless due to the changes of and 2540 -13 and -17 2542 o Wording consistency fixes: Use "Registrations" and "target 2543 attributes" 2545 o Fix incorrect use of content negotiation in discovery interface 2546 description (Content-Format -> Accept) 2548 o State that the base attribute value is part of endpoint lookup 2549 even when implicit in the registration 2551 o Update references from RFC5988 to its update RFC8288 2553 o Remove appendix on protocol-negotiation (which had a note to be 2554 removed before publication) 2556 changes from -16 to -17 2558 (Note that -17 is published as a direct follow-up to -16, containing 2559 a single change to be discussed at IETF103) 2561 o Removed groups that are enumerations of registrations and have 2562 dedicated mechanism 2564 o Add groups that are enumerations of shared resources and are a 2565 special case of endpoint registrations 2567 changes from -15 to -16 2568 o Recommend a common set of resources for members of a group 2570 o Clarified use of multicast group in lighting example 2572 o Add note on concurrent registrations from one EP being possible 2573 but not expected 2575 o Refresh web examples appendix to reflect current use of Modernized 2576 Link Format 2578 o Add examples of URIs where Modernized Link Format matters 2580 o Editorial changes 2582 changes from -14 to -15 2584 o Rewrite of section "Security policies" 2586 o Clarify that the "base" parameter text applies both to relative 2587 references both in anchor and href 2589 o Renamed "Registree-EP" to Registrant-EP" 2591 o Talk of "relative references" and "URIs" rather than "relative" 2592 and "absolute" URIs. (The concept of "absolute URIs" of [RFC3986] 2593 is not needed in RD). 2595 o Fixed examples 2597 o Editorial changes 2599 changes from -13 to -14 2601 o Rename "registration context" to "registration base URI" (and 2602 "con" to "base") and "domain" to "sector" (where the abbreviation 2603 "d" stays for compatibility reasons) 2605 o Introduced resource types core.rd-ep and core.rd-gp 2607 o Registration management moved to appendix A, including endpoint 2608 and group lookup 2610 o Minor editorial changes 2612 * PATCH/iPATCH is clearly deferred to another document 2614 * Recommend against query / fragment identifier in con= 2615 * Interface description lists are described as illustrative 2617 * Rewording of Simple Registration 2619 o Simple registration carries no error information and succeeds 2620 immediately (previously, sequence was unspecified) 2622 o Lookup: href are matched against resolved values (previously, this 2623 was unspecified) 2625 o Lookup: lt are not exposed any more 2627 o con/base: Paths are allowed 2629 o Registration resource locations can not have query or fragment 2630 parts 2632 o Default life time extended to 25 hours 2634 o clarified registration update rules 2636 o lt-value semantics for lookup clarified. 2638 o added template for simple registration 2640 changes from -12 to -13 2642 o Added "all resource directory" nodes MC address 2644 o Clarified observation behavior 2646 o version identification 2648 o example rt= and et= values 2650 o domain from figure 2 2652 o more explanatory text 2654 o endpoints of a groups hosted by different RD 2656 o resolve RFC6690-vs-8288 resolution ambiguities: 2658 * require registered links not to be relative when using anchor 2660 * return absolute URIs in resource lookup 2662 changes from -11 to -12 2663 o added Content Model section, including ER diagram 2665 o removed domain lookup interface; domains are now plain attributes 2666 of groups and endpoints 2668 o updated chapter "Finding a Resource Directory"; now distinguishes 2669 configuration-provided, network-provided and heuristic sources 2671 o improved text on: atomicity, idempotency, lookup with multiple 2672 parameters, endpoint removal, simple registration 2674 o updated LWM2M description 2676 o clarified where relative references are resolved, and how context 2677 and anchor interact 2679 o new appendix on the interaction with RFCs 6690, 5988 and 3986 2681 o lookup interface: group and endpoint lookup return group and 2682 registration resources as link targets 2684 o lookup interface: search parameters work the same across all 2685 entities 2687 o removed all methods that modify links in an existing registration 2688 (POST with payload, PATCH and iPATCH) 2690 o removed plurality definition (was only needed for link 2691 modification) 2693 o enhanced IANA registry text 2695 o state that lookup resources can be observable 2697 o More examples and improved text 2699 changes from -09 to -10 2701 o removed "ins" and "exp" link-format extensions. 2703 o removed all text concerning DNS-SD. 2705 o removed inconsistency in RDAO text. 2707 o suggestions taken over from various sources 2709 o replaced "Function Set" with "REST API", "base URI", "base path" 2710 o moved simple registration to registration section 2712 changes from -08 to -09 2714 o clarified the "example use" of the base RD resource values /rd, 2715 /rd-lookup, and /rd-group. 2717 o changed "ins" ABNF notation. 2719 o various editorial improvements, including in examples 2721 o clarifications for RDAO 2723 changes from -07 to -08 2725 o removed link target value returned from domain and group lookup 2726 types 2728 o Maximum length of domain parameter 63 bytes for consistency with 2729 group 2731 o removed option for simple POST of link data, don't require a 2732 .well-known/core resource to accept POST data and handle it in a 2733 special way; we already have /rd for that 2735 o add IPv6 ND Option for discovery of an RD 2737 o clarify group configuration section 6.1 that endpoints must be 2738 registered before including them in a group 2740 o removed all superfluous client-server diagrams 2742 o simplified lighting example 2744 o introduced Commissioning Tool 2746 o RD-Look-up text is extended. 2748 changes from -06 to -07 2750 o added text in the discovery section to allow content format hints 2751 to be exposed in the discovery link attributes 2753 o editorial updates to section 9 2755 o update author information 2757 o minor text corrections 2758 Changes from -05 to -06 2760 o added note that the PATCH section is contingent on the progress of 2761 the PATCH method 2763 changes from -04 to -05 2765 o added Update Endpoint Links using PATCH 2767 o http access made explicit in interface specification 2769 o Added http examples 2771 Changes from -03 to -04: 2773 o Added http response codes 2775 o Clarified endpoint name usage 2777 o Add application/link-format+cbor content-format 2779 Changes from -02 to -03: 2781 o Added an example for lighting and DNS integration 2783 o Added an example for RD use in OMA LWM2M 2785 o Added Read Links operation for link inspection by endpoints 2787 o Expanded DNS-SD section 2789 o Added draft authors Peter van der Stok and Michael Koster 2791 Changes from -01 to -02: 2793 o Added a catalogue use case. 2795 o Changed the registration update to a POST with optional link 2796 format payload. Removed the endpoint type update from the update. 2798 o Additional examples section added for more complex use cases. 2800 o New DNS-SD mapping section. 2802 o Added text on endpoint identification and authentication. 2804 o Error code 4.04 added to Registration Update and Delete requests. 2806 o Made 63 bytes a SHOULD rather than a MUST for endpoint name and 2807 resource type parameters. 2809 Changes from -00 to -01: 2811 o Removed the ETag validation feature. 2813 o Place holder for the DNS-SD mapping section. 2815 o Explicitly disabled GET or POST on returned Location. 2817 o New registry for RD parameters. 2819 o Added support for the JSON Link Format. 2821 o Added reference to the Groupcomm WG draft. 2823 Changes from -05 to WG Document -00: 2825 o Updated the version and date. 2827 Changes from -04 to -05: 2829 o Restricted Update to parameter updates. 2831 o Added pagination support for the Lookup interface. 2833 o Minor editing, bug fixes and reference updates. 2835 o Added group support. 2837 o Changed rt to et for the registration and update interface. 2839 Changes from -03 to -04: 2841 o Added the ins= parameter back for the DNS-SD mapping. 2843 o Integrated the Simple Directory Discovery from Carsten. 2845 o Editorial improvements. 2847 o Fixed the use of ETags. 2849 o Fixed tickets 383 and 372 2851 Changes from -02 to -03: 2853 o Changed the endpoint name back to a single registration parameter 2854 ep= and removed the h= and ins= parameters. 2856 o Updated REST interface descriptions to use RFC6570 URI Template 2857 format. 2859 o Introduced an improved RD Lookup design as its own function set. 2861 o Improved the security considerations section. 2863 o Made the POST registration interface idempotent by requiring the 2864 ep= parameter to be present. 2866 Changes from -01 to -02: 2868 o Added a terminology section. 2870 o Changed the inclusion of an ETag in registration or update to a 2871 MAY. 2873 o Added the concept of an RD Domain and a registration parameter for 2874 it. 2876 o Recommended the Location returned from a registration to be 2877 stable, allowing for endpoint and Domain information to be changed 2878 during updates. 2880 o Changed the lookup interface to accept endpoint and Domain as 2881 query string parameters to control the scope of a lookup. 2883 13. References 2885 13.1. Normative References 2887 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2888 Requirement Levels", BCP 14, RFC 2119, 2889 DOI 10.17487/RFC2119, March 1997, 2890 . 2892 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2893 Resource Identifier (URI): Generic Syntax", STD 66, 2894 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2895 . 2897 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 2898 and D. Orchard, "URI Template", RFC 6570, 2899 DOI 10.17487/RFC6570, March 2012, 2900 . 2902 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 2903 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 2904 . 2906 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 2907 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 2908 . 2910 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2911 Writing an IANA Considerations Section in RFCs", BCP 26, 2912 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2913 . 2915 13.2. Informative References 2917 [ER] Chen, P., "The entity-relationship model---toward a 2918 unified view of data", ACM Transactions on Database 2919 Systems Vol. 1, pp. 9-36, DOI 10.1145/320434.320440, March 2920 1976. 2922 [I-D.bormann-t2trg-rel-impl] 2923 Bormann, C., "impl-info: A link relation type for 2924 disclosing implementation information", draft-bormann- 2925 t2trg-rel-impl-00 (work in progress), January 2018. 2927 [I-D.hartke-t2trg-coral] 2928 Hartke, K., "The Constrained RESTful Application Language 2929 (CoRAL)", draft-hartke-t2trg-coral-07 (work in progress), 2930 February 2019. 2932 [I-D.ietf-ace-oauth-authz] 2933 Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and 2934 H. Tschofenig, "Authentication and Authorization for 2935 Constrained Environments (ACE) using the OAuth 2.0 2936 Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-22 2937 (work in progress), March 2019. 2939 [I-D.ietf-core-links-json] 2940 Li, K., Rahman, A., and C. Bormann, "Representing 2941 Constrained RESTful Environments (CoRE) Link Format in 2942 JSON and CBOR", draft-ietf-core-links-json-10 (work in 2943 progress), February 2018. 2945 [I-D.silverajan-core-coap-protocol-negotiation] 2946 Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation", 2947 draft-silverajan-core-coap-protocol-negotiation-09 (work 2948 in progress), July 2018. 2950 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 2951 "Transmission of IPv6 Packets over IEEE 802.15.4 2952 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 2953 . 2955 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 2956 Bormann, "Neighbor Discovery Optimization for IPv6 over 2957 Low-Power Wireless Personal Area Networks (6LoWPANs)", 2958 RFC 6775, DOI 10.17487/RFC6775, November 2012, 2959 . 2961 [RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing 2962 IPv6 Zone Identifiers in Address Literals and Uniform 2963 Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874, 2964 February 2013, . 2966 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2967 Constrained-Node Networks", RFC 7228, 2968 DOI 10.17487/RFC7228, May 2014, 2969 . 2971 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2972 Protocol (HTTP/1.1): Message Syntax and Routing", 2973 RFC 7230, DOI 10.17487/RFC7230, June 2014, 2974 . 2976 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 2977 Application Protocol (CoAP)", RFC 7252, 2978 DOI 10.17487/RFC7252, June 2014, 2979 . 2981 [RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for 2982 the Constrained Application Protocol (CoAP)", RFC 7390, 2983 DOI 10.17487/RFC7390, October 2014, 2984 . 2986 [RFC7641] Hartke, K., "Observing Resources in the Constrained 2987 Application Protocol (CoAP)", RFC 7641, 2988 DOI 10.17487/RFC7641, September 2015, 2989 . 2991 [RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and 2992 FETCH Methods for the Constrained Application Protocol 2993 (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017, 2994 . 2996 [RFC8288] Nottingham, M., "Web Linking", RFC 8288, 2997 DOI 10.17487/RFC8288, October 2017, 2998 . 3000 Appendix A. Groups Registration and Lookup 3002 The RD-Groups usage pattern allows announcing application groups 3003 inside a Resource Directory. 3005 Groups are represented by endpoint registrations. Their base address 3006 is a multicast address, and they SHOULD be entered with the endpoint 3007 type "core.rd-group". The endpoint name can also be referred to as a 3008 group name in this context. 3010 The registration is inserted into the RD by a Commissioning Tool, 3011 which might also be known as a group manager here. It performs third 3012 party registration and registration updates. 3014 The links it registers SHOULD be available on all members that join 3015 the group. Depending on the application, members that lack some 3016 resource MAY be permissible if requests to them fail gracefully. 3018 The following example shows a CT registering a group with the name 3019 "lights" which provides two resources. The directory resource path 3020 /rd is an example RD location discovered in a request similar to 3021 Figure 5. 3023 Req: POST coap://rd.example.com/rd?ep=lights&et=core.rd-group 3024 &base=coap://[ff35:30:2001:db8::1] 3025 Content-Format: 40 3026 Payload: 3027 ;rt="light";if="core.a", 3028 ;if="core.p";u="K" 3030 Res: 2.01 Created 3031 Location-Path: /rd/12 3033 In this example, the group manager can easily permit devices that 3034 have no writable color-temperature to join, as they would still 3035 respond to brightness changing commands. Had the group instead 3036 contained a single resource that sets brightness and color 3037 temperature atomically, endpoints would need to support both 3038 properties. 3040 The resources of a group can be looked up like any other resource, 3041 and the group registrations (along with any additional registration 3042 parameters) can be looked up using the endpoint lookup interface. 3044 The following example shows a client performing and endpoint lookup 3045 for all groups. 3047 Req: GET /rd-lookup/ep?et=core.rd-group 3049 Res: 2.01 Content 3050 Payload: 3051 ;ep="GRP_R2-4-015";et="core.rd-group"; 3052 base="coap://[ff05::1]", 3053 ;ep=lights&et=core.rd-group; 3054 base="coap://[ff35:30:2001:db8::1]";rt="core.rd-ep" 3056 The following example shows a client performing a lookup of all 3057 resources of all endpoints (groups) with et=core.rd-group. 3059 Req: GET /rd-lookup/res?et=core.rd-group 3061 ;rt="light";if="core.a"; 3062 et="core.rd-group";anchor="coap://[ff35:30:2001:db8::1]", 3063 ;if="core.p";u="K"; 3064 et="core.rd-group"; 3065 anchor="coap://[ff35:30:2001:db8::1]" 3067 Appendix B. Web links and the Resource Directory 3069 Understanding the semantics of a link-format document and its URI 3070 references is a journey through different documents ([RFC3986] 3071 defining URIs, [RFC6690] defining link-format documents based on 3072 [RFC8288] which defines link headers, and [RFC7252] providing the 3073 transport). This appendix summarizes the mechanisms and semantics at 3074 play from an entry in ".well-known/core" to a resource lookup. 3076 This text is primarily aimed at people entering the field of 3077 Constrained Restful Environments from applications that previously 3078 did not use web mechanisms. 3080 The explanation of the steps makes some shortcuts in the more 3081 confusing details of [RFC6690], which are justified as all examples 3082 being in Limited Link Format. 3084 B.1. A simple example 3086 Let's start this example with a very simple host, "2001:db8:f0::1". 3087 A client that follows classical CoAP Discovery ([RFC7252] Section 7), 3088 sends the following multicast request to learn about neighbours 3089 supporting resources with resource-type "temperature". 3091 The client sends a link-local multicast: 3093 GET coap://[ff02::fd]:5683/.well-known/core?rt=temperature 3095 RES 2.05 Content 3096 ;rt=temperature;ct=0 3098 where the response is sent by the server, "[2001:db8:f0::1]:5683". 3100 While the client - on the practical or implementation side - can just 3101 go ahead and create a new request to "[2001:db8:f0::1]:5683" with 3102 Uri-Path: "temp", the full resolution steps for insertion into and 3103 retrieval from the RD without any shortcuts are: 3105 B.1.1. Resolving the URIs 3107 The client parses the single returned record. The link's target 3108 (sometimes called "href") is ""/temp"", which is a relative URI that 3109 needs resolving. The base URI is used to resolve the reference /temp against. 3112 The Base URI of the requested resource can be composed from the 3113 header options of the CoAP GET request by following the steps of 3114 [RFC7252] section 6.5 (with an addition at the end of 8.2) into 3115 ""coap://[2001:db8:f0::1]/.well-known/core"". 3117 Because ""/temp"" starts with a single slash, the record's target is 3118 resolved by replacing the path ""/.well-known/core"" from the Base 3119 URI (section 5.2 [RFC3986]) with the relative target URI ""/temp"" 3120 into ""coap://[2001:db8:f0::1]/temp"". 3122 B.1.2. Interpreting attributes and relations 3124 Some more information but the record's target can be obtained from 3125 the payload: the resource type of the target is "temperature", and 3126 its content format is text/plain (ct=0). 3128 A relation in a web link is a three-part statement that specifies a 3129 named relation between the so-called "context resource" and the 3130 target resource, like "_This page_ has _its table of contents_ at _/ 3131 toc.html_". In link format documents, there is an implicit "host 3132 relation" specified with default parameter: rel="hosts". 3134 In our example, the context resource of the link is the URI specified 3135 in the GET request "coap:://[2001:db8:f0::1]/.well-known/core". A 3136 full English expression of the "host relation" is: 3138 '"coap://[2001:db8:f0::1]/.well-known/core" is hosting the resource 3139 "coap://[2001:db8:f0::1]/temp", which is of the resource type 3140 "temperature" and can be accessed using the text/plain content 3141 format.' 3143 B.2. A slightly more complex example 3145 Omitting the "rt=temperature" filter, the discovery query would have 3146 given some more records in the payload: 3148 GET coap://[ff02::fd]:5683/.well-known/core 3150 RES 2.05 Content 3151 ;rt=temperature;ct=0, 3152 ;rt=light-lux;ct=0, 3153 ;anchor="/sensors/temp";rel=alternate, 3154 ;anchor="/sensors/temp"; 3155 rel="describedby" 3157 Parsing the third record, the client encounters the "anchor" 3158 parameter. It is a URI relative to the Base URI of the request and 3159 is thus resolved to ""coap://[2001:db8:f0::1]/sensors/temp"". That 3160 is the context resource of the link, so the "rel" statement is not 3161 about the target and the Base URI any more, but about the target and 3162 the resolved URI. Thus, the third record could be read as 3163 ""coap://[2001:db8:f0::1]/sensors/temp" has an alternate 3164 representation at "coap://[2001:db8:f0::1]/t"". 3166 Following the same resolution steps, the fourth record can be read as 3167 ""coap://[2001:db8:f0::1]/sensors/temp" is described by 3168 "http://www.example.com/sensors/t123"". 3170 B.3. Enter the Resource Directory 3172 The resource directory tries to carry the semantics obtainable by 3173 classical CoAP discovery over to the resource lookup interface as 3174 faithfully as possible. 3176 For the following queries, we will assume that the simple host has 3177 used Simple Registration to register at the resource directory that 3178 was announced to it, sending this request from its UDP port 3179 "[2001:db8:f0::1]:6553": 3181 POST coap://[2001:db8:f01::ff]/.well-known/core?ep=simple-host1 3183 The resource directory would have accepted the registration, and 3184 queried the simple host's ".well-known/core" by itself. As a result, 3185 the host is registered as an endpoint in the RD with the name 3186 "simple-host1". The registration is active for 90000 seconds, and 3187 the endpoint registration Base URI is ""coap://[2001:db8:f0::1]"" 3188 following the resolution steps described in Appendix B.1.1. It 3189 should be remarked that the Base URI constructed that way always 3190 yields a URI of the form: scheme://authority without path suffix. 3192 If the client now queries the RD as it would previously have issued a 3193 multicast request, it would go through the RD discovery steps by 3194 fetching "coap://[2001:db8:f0::ff]/.well-known/core?rt=core.rd- 3195 lookup-res", obtain "coap://[2001:db8:f0::ff]/rd-lookup/res" as the 3196 resource lookup endpoint, and issue a request to 3197 "coap://[2001:db8:f0::ff]/rd-lookup/res?rt=temperature" to receive 3198 the following data: 3200 ;rt=temperature;ct=0; 3201 anchor="coap://[2001:db8:f0::1]" 3203 This is not _literally_ the same response that it would have received 3204 from a multicast request, but it contains the equivalent statement: 3206 '"coap://[2001:db8:f0::1]" is hosting the resource 3207 "coap://[2001:db8:f0::1]/temp", which is of the resource type 3208 "temperature" and can be accessed using the text/plain content 3209 format.' 3211 (The difference is whether "/" or "/.well-known/core" hosts the 3212 resources, which does not matter in this application; if it did, the 3213 endpoint would have been more explicit. Actually, /.well-known/core 3214 does NOT host the resource but stores a URI reference to the 3215 resource.) 3217 To complete the examples, the client could also query all resources 3218 hosted at the endpoint with the known endpoint name "simple-host1". 3219 A request to "coap://[2001:db8:f0::ff]/rd-lookup/res?ep=simple-host1" 3220 would return 3222 ;rt=temperature;ct=0; 3223 anchor="coap://[2001:db8:f0::1]", 3224 ;rt=light-lux;ct=0; 3225 anchor="coap://[2001:db8:f0::1]", 3226 ; 3227 anchor="coap://[2001:db8:f0::1]/sensors/temp";rel=alternate, 3228 ; 3229 anchor="coap://[2001:db8:f0::1]/sensors/temp";rel="describedby" 3231 All the target and anchor references are already in absolute form 3232 there, which don't need to be resolved any further. 3234 Had the simple host done an equivalent full registration with a base= 3235 parameter (e.g. "?ep=simple-host1&base=coap+tcp://simple- 3236 host1.example.com"), that context would have been used to resolve the 3237 relative anchor values instead, giving 3239 ;rt=temperature;ct=0; 3240 anchor="coap+tcp://simple-host1.example.com" 3242 and analogous records. 3244 B.4. A note on differences between link-format and Link headers 3246 While link-format and Link headers look very similar and are based on 3247 the same model of typed links, there are some differences between 3248 [RFC6690] and [RFC8288], which are dealt with differently: 3250 o "Resolving the target against the anchor": [RFC6690] Section 2.1 3251 states that the anchor of a link is used as the Base URI against 3252 which the term inside the angle brackets (the target) is resolved, 3253 falling back to the resource's URI with paths stripped off (its 3254 "Origin"). In contrast to that, [RFC8288] Section B.2 describes 3255 that the anchor is immaterial to the resolution of the target 3256 reference. 3258 RFC6690, in the same section, also states that absent anchors set 3259 the context of the link to the target's URI with its path stripped 3260 off, while according to [RFC8288] Section 3.2, the context is the 3261 resource's base URI. 3263 The rules introduced in Appendix C ensure that an RD does not need 3264 to deal with those differences when processing input data. Lookup 3265 results are required to be absolute references for the same 3266 reason. 3268 o There is no percent encoding in link-format documents. 3270 A link-format document is a UTF-8 encoded string of Unicode 3271 characters and does not have percent encoding, while Link headers 3272 are practically ASCII strings that use percent encoding for non- 3273 ASCII characters, stating the encoding explicitly when required. 3275 For example, while a Link header in a page about a Swedish city 3276 might read 3278 "Link: ;rel="live-environment-data"" 3280 a link-format document from the same source might describe the 3281 link as 3283 ";rel="live-environment-data"" 3284 Parsers and producers of link-format and header data need to be 3285 aware of this difference. 3287 Appendix C. Limited Link Format 3289 The CoRE Link Format as described in [RFC6690] has been interpreted 3290 differently by implementers, and a strict implementation rules out 3291 some use cases of a Resource Directory (e.g. base values with path 3292 components). 3294 This appendix describes a subset of link format documents called 3295 Limited Link Format. The rules herein are not very limiting in 3296 practice - all examples in RFC6690, and all deployments the authors 3297 are aware of already stick to them - but ease the implementation of 3298 resource directory servers. 3300 It is applicable to representations in the application/link-format 3301 media type, and any other media types that inherit [RFC6690] 3302 Section 2.1. 3304 A link format representation is in Limited Link format if, for each 3305 link in it, the following applies: 3307 o All URI references either follow the URI or the path-absolute ABNF 3308 rule of RFC3986 (i.e. target and anchor each either start with a 3309 scheme or with a single slash), 3311 o if the anchor reference starts with a scheme, the target reference 3312 starts with a scheme as well (i.e. relative references in target 3313 cannot be used when the anchor is a full URI), and 3315 o the application does not care whether links without an explicitly 3316 given anchor have the origin's "/" or "/.well-known/core" resource 3317 as their link context. 3319 Authors' Addresses 3321 Zach Shelby 3322 ARM 3323 150 Rose Orchard 3324 San Jose 95134 3325 USA 3327 Phone: +1-408-203-9434 3328 Email: zach.shelby@arm.com 3329 Michael Koster 3330 SmartThings 3331 665 Clyde Avenue 3332 Mountain View 94043 3333 USA 3335 Phone: +1-707-502-5136 3336 Email: Michael.Koster@smartthings.com 3338 Carsten Bormann 3339 Universitaet Bremen TZI 3340 Postfach 330440 3341 Bremen D-28359 3342 Germany 3344 Phone: +49-421-218-63921 3345 Email: cabo@tzi.org 3347 Peter van der Stok 3348 consultant 3350 Phone: +31-492474673 (Netherlands), +33-966015248 (France) 3351 Email: consultancy@vanderstok.org 3352 URI: www.vanderstok.org 3354 Christian Amsuess (editor) 3355 Hollandstr. 12/4 3356 1020 3357 Austria 3359 Phone: +43-664-9790639 3360 Email: christian@amsuess.com