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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 CoRE Working Group A. Castellani 3 Internet-Draft University of Padova 4 Intended status: Informational S. Loreto 5 Expires: December 24, 2016 Ericsson 6 A. Rahman 7 InterDigital Communications, LLC 8 T. Fossati 9 Nokia 10 E. Dijk 11 Philips Lighting 12 June 22, 2016 14 Guidelines for HTTP-to-CoAP Mapping Implementations 15 draft-ietf-core-http-mapping-11 17 Abstract 19 This document provides reference information for implementing a 20 cross-protocol network proxy that performs translation from the HTTP 21 protocol to the CoAP protocol. This will enable a HTTP client to 22 access resources on a CoAP server through the proxy. This document 23 describes how a HTTP request is mapped to a CoAP request, and then 24 how a CoAP response is mapped back to a HTTP response. This includes 25 guidelines for URI mapping, media type mapping and additional proxy 26 implementation issues. This document covers the Reverse, Forward and 27 Interception cross-protocol proxy cases. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on December 24, 2016. 46 Copyright Notice 48 Copyright (c) 2016 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 3. HTTP-to-CoAP Proxy . . . . . . . . . . . . . . . . . . . . . 5 66 4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6 67 5. URI Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 6 68 5.1. URI Terminology . . . . . . . . . . . . . . . . . . . . . 7 69 5.2. Null Mapping . . . . . . . . . . . . . . . . . . . . . . 8 70 5.3. Default Mapping . . . . . . . . . . . . . . . . . . . . . 8 71 5.3.1. Optional Scheme Omission . . . . . . . . . . . . . . 8 72 5.3.2. Encoding Caveats . . . . . . . . . . . . . . . . . . 9 73 5.4. URI Mapping Template . . . . . . . . . . . . . . . . . . 9 74 5.4.1. Simple Form . . . . . . . . . . . . . . . . . . . . . 9 75 5.4.2. Enhanced Form . . . . . . . . . . . . . . . . . . . . 11 76 5.5. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 12 77 5.5.1. Examples . . . . . . . . . . . . . . . . . . . . . . 12 78 6. Media Type Mapping . . . . . . . . . . . . . . . . . . . . . 14 79 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14 80 6.2. 'application/coap-payload' Media Type . . . . . . . . . . 15 81 6.3. Loose Media Type Mapping . . . . . . . . . . . . . . . . 16 82 6.4. Media Type to Content Format Mapping Algorithm . . . . . 17 83 6.5. Content Transcoding . . . . . . . . . . . . . . . . . . . 18 84 6.5.1. General . . . . . . . . . . . . . . . . . . . . . . . 18 85 6.5.2. CoRE Link Format . . . . . . . . . . . . . . . . . . 19 86 6.5.3. Diagnostic Messages . . . . . . . . . . . . . . . . . 19 87 7. Response Code Mapping . . . . . . . . . . . . . . . . . . . . 19 88 8. Additional Mapping Guidelines . . . . . . . . . . . . . . . . 22 89 8.1. Caching and Congestion Control . . . . . . . . . . . . . 22 90 8.2. Cache Refresh via Observe . . . . . . . . . . . . . . . . 22 91 8.3. Use of CoAP Blockwise Transfer . . . . . . . . . . . . . 23 92 8.4. CoAP Multicast . . . . . . . . . . . . . . . . . . . . . 24 93 8.5. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 24 95 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 96 9.1. New 'core.hc' Resource Type . . . . . . . . . . . . . . . 24 97 9.2. New 'coap-payload' Internet Media Type . . . . . . . . . 25 98 10. Security Considerations . . . . . . . . . . . . . . . . . . . 26 99 10.1. Multicast . . . . . . . . . . . . . . . . . . . . . . . 26 100 10.2. Traffic Overflow . . . . . . . . . . . . . . . . . . . . 27 101 10.3. Handling Secured Exchanges . . . . . . . . . . . . . . . 27 102 10.4. URI Mapping . . . . . . . . . . . . . . . . . . . . . . 28 103 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 104 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 105 12.1. Normative References . . . . . . . . . . . . . . . . . . 29 106 12.2. Informative References . . . . . . . . . . . . . . . . . 30 107 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 31 108 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 110 1. Introduction 112 CoAP [RFC7252] has been designed with the twofold aim to be an 113 application protocol specialized for constrained environments and to 114 be easily used in Representational State Transfer (REST) based 115 architectures such as the Web. The latter goal has led to defining 116 CoAP to easily interoperate with HTTP [RFC7230] through an 117 intermediary proxy which performs cross-protocol conversion. 119 Section 10 of [RFC7252] describes the fundamentals of the CoAP-to- 120 HTTP and the HTTP-to-CoAP cross-protocol mapping process. However, 121 [RFC7252] focuses on the basic mapping of request methods and simple 122 response code mapping between HTTP and CoAP, and it leaves many 123 details of the cross-protocol proxy for future definition. 124 Therefore, a primary goal of this informational document is to define 125 a consistent set of guidelines that an HTTP-to-CoAP proxy 126 implementation should adhere to. The key benefit to adhering to such 127 guidelines is to reduce variation between proxy implementations, 128 thereby increasing interoperability between an HTTP client and a CoAP 129 server independent of the proxy that implements the cross-protocol 130 mapping. (For example, a proxy conforming to these guidelines made 131 by vendor A can be easily replaced by a proxy from vendor B that also 132 conforms to the guidelines.) 134 This document is organized as follows: 136 o Section 2 defines proxy terminology; 138 o Section 3 introduces the HTTP-to-CoAP proxy; 140 o Section 4 lists use cases in which HTTP clients need to contact 141 CoAP servers; 143 o Section 5 introduces a null, default and advanced HTTP-to-CoAP URI 144 mapping syntax; 146 o Section 6 describes how to map HTTP media types to CoAP content 147 formats and vice versa; 149 o Section 7 describes how to map CoAP responses to HTTP responses; 151 o Section 8 describes additional mapping guidelines related to 152 caching, congestion, timeouts, etc.; 154 o Section 10 discusses possible security impact of HTTP-to-CoAP 155 protocol mapping. 157 2. Terminology 159 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 160 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 161 "OPTIONAL" in this document are to be interpreted as described in 162 [RFC2119]. 164 HC Proxy: a proxy performing a cross-protocol mapping, in the context 165 of this document an HTTP-to-CoAP (HC) mapping. Specifically, the HC 166 proxy acts as an HTTP server and a CoAP client. The HC Proxy can 167 take on the role of a Forward, Reverse or Interception Proxy. 169 Forward Proxy (or Forward HC Proxy): a message forwarding agent that 170 is selected by the HTTP client, usually via local configuration 171 rules, to receive requests for some type(s) of absolute URI and to 172 attempt to satisfy those requests via translation to the protocol 173 indicated by the absolute URI. The user decides (is willing) to use 174 the proxy as the forwarding/de-referencing agent for a predefined 175 subset of the URI space. In [RFC7230] this is called a Proxy. 176 [RFC7252] defines Forward-Proxy similarly. 178 Reverse Proxy (or Reverse HC Proxy): as in [RFC7230], a receiving 179 agent that acts as a layer above some other server(s) and translates 180 the received requests to the underlying server's protocol. A Reverse 181 HC Proxy behaves as an origin (HTTP) server on its connection from 182 the HTTP client. The HTTP client uses the "origin-form" 183 (Section 5.3.1 of [RFC7230]) as a request-target URI. 185 Interception Proxy (or Interception HC Proxy) [RFC3040]: a proxy that 186 receives inbound HTTP traffic flows through the process of traffic 187 redirection; transparent to the HTTP client. 189 Note that a Reverse Proxy appears to an HTTP client as an origin 190 server while a Forward Proxy does not. So, when communicating with a 191 Reverse Proxy a client may be unaware it is communicating with a 192 proxy at all. 194 3. HTTP-to-CoAP Proxy 196 A HC proxy is accessed by an HTTP client which wants to access a 197 resource on a CoAP server. The HC proxy handles the HTTP request by 198 mapping it to the equivalent CoAP request, which is then forwarded to 199 the appropriate CoAP server. The received CoAP response is then 200 mapped to an appropriate HTTP response and finally sent back to the 201 originating HTTP client. 203 See Figure 1 for an example deployment scenario. Here a HC proxy is 204 located at the boundary of the Constrained Network domain, to avoid 205 sending any HTTP traffic into the Constrained Network and to avoid 206 any (unsecured) CoAP multicast traffic outside the Constrained 207 Network. A DNS server (not shown) is used by the HTTP Client to 208 resolve the IP address of the HC proxy and optionally also used by 209 the HC proxy to resolve IP addresses of CoAP servers. 211 Constrained Network 212 .-------------------. 213 / .------. \ 214 / | CoAP | \ 215 / |server| \ 216 || '------' || 217 || || 218 .--------. HTTP Request .------------. CoAP Req .------. || 219 | HTTP |---------------->|HTTP-to-CoAP|----------->| CoAP | || 220 | Client |<----------------| Proxy |<-----------|Server| || 221 '--------' HTTP Response '------------' CoAP Resp '------' || 222 || || 223 || .------. || 224 || | CoAP | || 225 \ |server| .------. / 226 \ '------' | CoAP | / 227 \ |server| / 228 \ '------' / 229 '-----------------' 231 Figure 1: HTTP-To-CoAP Proxy Deployment Scenario 233 Normative requirements on the translation of HTTP requests to CoAP 234 requests and of the CoAP responses back to HTTP responses are defined 235 in Section 10.2 of [RFC7252]. However, [RFC7252] focuses on the 236 basic mapping of request methods and simple response code mapping 237 between HTTP and CoAP, and leaves many details of the cross-protocol 238 HC proxy for future definition. This document provides additional 239 guidelines and more details for the implementation of a HC Proxy, 240 which should be followed in addition to the normative requirements. 241 Note that the guidelines apply to all forms of an HC proxy (i.e. 242 Reverse, Forward, Intercepting) unless explicitly otherwise noted. 244 4. Use Cases 246 To illustrate the situations HTTP to CoAP protocol translation may be 247 used, three use cases are described below. 249 1. Legacy building control application without CoAP: A building 250 control application that uses HTTP but not CoAP can check the status 251 of CoAP sensors and/or control actuators via a HC proxy. 253 2. Making sensor data available to 3rd parties on the Web: For 254 demonstration or public interest purposes, a HC proxy may be 255 configured to expose the contents of a CoAP sensor to the world via 256 the web (HTTP and/or HTTPS). Some sensors may only accept secure 257 'coaps' requests, therefore the proxy is configured to translate 258 request to those devices accordingly. The HC proxy is furthermore 259 configured to only pass through GET requests in order to protect the 260 constrained network. 262 3. Smartphone and home sensor: A smartphone can access directly a 263 CoAP home sensor using a mutually authenticated 'https' request, 264 provided its home router runs a HC proxy and is configured with the 265 appropriate certificate. An HTML5 application on the smartphone can 266 provide a friendly UI using the standard (HTTP) networking functions 267 of HTML5. 269 A key point in the above use cases is the expected nature of the URI 270 to be used by the HTTP client initiating the HTTP request to the HC 271 proxy. Specifically, in use case #1, there will be no "coap" or 272 "coaps" related information embedded in the HTTP URI as it is a 273 legacy HTTP client sending the request. Use case #2 is also expected 274 to be similar. In contrast, in use case #3, it is expected that the 275 HTTP client will specifically embed "coap" or "coaps" related 276 information in the HTTP URI of the HTTP request to the HC proxy. 278 5. URI Mapping 280 Though, in principle, a CoAP URI could be directly used by a HTTP 281 client to de-reference a CoAP resource through a HC proxy, the 282 reality is that all major web browsers, networking libraries and 283 command line tools do not allow making HTTP requests using URIs with 284 a scheme "coap" or "coaps". 286 Thus, there is a need for web applications to embed or "pack" a CoAP 287 URI into a HTTP URI so that it can be (non-destructively) transported 288 from the HTTP client to the HC proxy. The HC proxy can then "unpack" 289 the CoAP URI and finally de-reference it via a CoAP request to the 290 target Server. 292 URI Mapping is the term used in the document to describe the process 293 through which the URI of a CoAP resource is transformed into an HTTP 294 URI so that: 296 o the requesting HTTP client can handle it; 298 o the receiving HC proxy can extract the intended CoAP URI 299 unambiguously. 301 To this end, the remainder of this section will identify: 303 o the default mechanism to map a CoAP URI into a HTTP URI; 305 o the URI template format to express a class of CoAP-HTTP URI 306 mapping functions; 308 o the discovery mechanism based on CoRE Link Format [RFC6690] 309 through which clients of a HC proxy can dynamically discover 310 information about the supported URI Mapping Template(s), as well 311 as the URI where the HC proxy function is anchored. 313 5.1. URI Terminology 315 In the remainder of this section, the following terms will be used 316 with a distinctive meaning: 318 HC Proxy URI: 319 URI which refers to the HC proxy function. It conforms to 320 syntax defined in Section 2.7 of [RFC7230]. 322 Target CoAP URI: 323 URI which refers to the (final) CoAP resource that has to be 324 de-referenced. It conforms to syntax defined in Section 6 of 325 [RFC7252]. Specifically, its scheme is either "coap" or 326 "coaps". 328 Hosting HTTP URI: 329 URI that conforms to syntax in Section 2.7 of [RFC7230]. Its 330 authority component refers to a HC proxy, whereas path (and 331 query) component(s) embed the information used by a HC proxy 332 to extract the Target CoAP URI. 334 5.2. Null Mapping 336 The null mapping is the case where there is no Target CoAP URI 337 appended to the HC Proxy URI. In other words, it is a "pure" HTTP 338 URI that is sent to the HC Proxy. This would typically occur in 339 situations like Use Case #1 described in Section 4, and the Proxy 340 would typically be a Reverse Proxy. In this scenario, the HC Proxy 341 will determine through its own proprietary algorithms what the Target 342 CoAP URI should be. 344 5.3. Default Mapping 346 The default mapping is for the Target CoAP URI to be appended as-is 347 to the HC Proxy URI, to form the Hosting HTTP URI. This is the URI 348 that will then be sent by the HTTP client in the HTTP request to the 349 HC proxy. 351 For example: given a HC Proxy URI http://p.example.com/hc and a 352 Target CoAP URI coap://s.example.com/light, the resulting Hosting 353 HTTP URI would be http://p.example.com/hc/coap://s.example.com/light. 355 Provided a correct Target CoAP URI, the Hosting HTTP URI resulting 356 from the default mapping is always syntactically correct. 357 Furthermore, the Target CoAP URI can always be extracted 358 unambiguously from the Hosting HTTP URI. Also, it is worth noting 359 that, using the default mapping, a query component in the target CoAP 360 resource URI is naturally encoded into the query component of the 361 Hosting URI, e.g.: coap://s.example.com/light?dim=5 becomes 362 http://p.example.com/hc/coap://s.example.com/light?dim=5. 364 There is no default for the HC Proxy URI. Therefore, it is either 365 known in advance, e.g. as a configuration preset, or dynamically 366 discovered using the mechanism described in Section 5.5. 368 The default URI mapping function SHOULD be implemented and activated 369 by default in a HC proxy, unless there are valid reasons, e.g. 370 application specific, to use a different mapping function. 372 5.3.1. Optional Scheme Omission 374 When found in a Hosting HTTP URI, the scheme (i.e., "coap" or 375 "coaps"), the scheme component delimiter (":"), and the double slash 376 ("//") preceding the authority MAY be omitted. In such case, a local 377 default - not defined by this document - applies. 379 So, http://p.example.com/hc/s.coap.example.com/foo could either 380 represent the target coap://s.coap.example.com/foo or 381 coaps://s.coap.example.com/foo depending on application specific 382 presets. 384 5.3.2. Encoding Caveats 386 When the authority of the Target CoAP URI is given as an IPv6address, 387 then the surrounding square brackets must be percent-encoded in the 388 Hosting HTTP URI, in order to comply with the syntax defined in 389 Section 3.3. of [RFC3986] for a URI path segment. E.g.: 390 coap://[2001:db8::1]/light?on becomes 391 http://p.example.com/hc/coap://%5B2001:db8::1%5D/light?on. 393 Everything else can be safely copied verbatim from the Target CoAP 394 URI to the Hosting HTTP URI. 396 5.4. URI Mapping Template 398 This section defines a format for the URI template [RFC6570] used by 399 a HC proxy to inform its clients about the expected syntax for the 400 Hosting HTTP URI. This will then be used by the HTTP client to 401 construct the URI to be sent in the HTTP request to the HC proxy. 403 When instantiated, an URI Mapping Template is always concatenated to 404 a HC Proxy URI provided by the HC proxy via discovery (see 405 Section 5.5), or by other means. 407 A simple form (Section 5.4.1) and an enhanced form (Section 5.4.2) 408 are provided to fit different users' requirements. 410 Both forms are expressed as level 2 URI templates [RFC6570] to take 411 care of the expansion of values that are allowed to include reserved 412 URI characters. The syntax of all URI formats is specified in this 413 section in Augmented Backus-Naur Form (ABNF) [RFC5234]. 415 5.4.1. Simple Form 417 The simple form MUST be used for mappings where the Target CoAP URI 418 is going to be copied (using rules of Section 5.3.2) at some fixed 419 position into the Hosting HTTP URI. 421 The "tu" template variable is intended to be used in a template 422 definition to represent a Target CoAP URI: 424 tu = coap-URI / coaps-URI ; from [RFC7252], Section 6.1 and 6.2 426 The same considerations as in Section 5.3.1 apply, in that the CoAP 427 scheme may be omitted from the Hosting HTTP URI. 429 5.4.1.1. Examples 431 All the following examples (given as a specific URI mapping template, 432 a Target CoAP URI, and the produced Hosting HTTP URI) use 433 http://p.example.com/hc as the HC Proxy URI. Note that these 434 examples all define mapping templates that deviate from the default 435 template of Section 5.3 to be able to illustrate the use of the above 436 template variables. 438 1. Target CoAP URI is a query argument of the Hosting HTTP URI: 440 ?target_uri={+tu} 442 coap://s.example.com/light 444 http://p.example.com/hc?target_uri=coap://s.example.com/light 446 or 448 coaps://s.example.com/light 450 http://p.example.com/hc?target_uri=coaps://s.example.com/light 452 2. Target CoAP URI in the path component of the Hosting HTTP URI 453 (i.e., the default URI Mapping template): 455 /{+tu} 457 coap://s.example.com/light 459 http://p.example.com/hc/coap://s.example.com/light 461 or 463 coaps://s.example.com/light 465 http://p.example.com/hc/coaps://s.example.com/light 467 3. "coap" URI is a query argument of the Hosting HTTP URI; client 468 decides to omit scheme because a default scheme is agreed 469 beforehand between client and proxy: 471 ?coap_uri={+tu} 473 coap://s.example.com/light 475 http://p.example.com/hc?coap_uri=s.example.com/light 477 5.4.2. Enhanced Form 479 The enhanced form can be used to express more sophisticated mappings, 480 i.e., those that do not fit into the simple form. 482 There MUST be at most one instance of each of the following template 483 variables in a template definition: 485 s = "coap" / "coaps" ; from [RFC7252], Sections 6.1 and 6.2 486 hp = host [":" port] ; from [RFC3986] Sections 3.2.2 and 3.2.3 487 p = path-abempty ; from [RFC3986] Section 3.3 488 q = query ; from [RFC3986] Section 3.4 489 qq = [ "?" query ] ; qq is empty iff 'query' is empty 491 5.4.2.1. Examples 493 All the following examples (given as a specific URI mapping template, 494 a Target CoAP URI, and the produced Hosting HTTP URI) use 495 http://p.example.com/hc as the HC Proxy URI. 497 1. Target CoAP URI components in path segments, and optional query 498 in query component: 500 {+s}{+hp}{+p}{+qq} 502 coap://s.example.com/light 504 http://p.example.com/hc/coap/s.example.com/light 506 or 508 coap://s.example.com/light?on 510 http://p.example.com/hc/coap/s.example.com/light?on 512 2. Target CoAP URI components split in individual query arguments: 514 ?s={+s}&hp={+hp}&p={+p}&q={+q} 516 coap://s.example.com/light 518 http://p.example.com/hc?s=coap&hp=s.example.com&p=/light&q= 520 or 522 coaps://s.example.com/light?on 524 http://p.example.com/hc?s=coaps&hp=s.example.com&p=/light&q=on 526 5.5. Discovery 528 In order to accommodate site specific needs while allowing third 529 parties to discover the proxy function, the HC proxy SHOULD publish 530 information related to the location and syntax of the HC proxy 531 function using the CoRE Link Format [RFC6690] interface. 533 To this aim a new Resource Type, "core.hc", is defined in this 534 document. It can be used as the value for the "rt" attribute in a 535 query to the /.well-known/core in order to locate the URI where the 536 HC proxy function is anchored, i.e. the HC Proxy URI. 538 Along with it, the new target attribute "hct" is defined in this 539 document. This attribute MAY be returned in a "core.hc" link to 540 provide the URI Mapping Template associated to the mapping resource. 541 The default template given in Section 5.3, i.e., {+tu}, MUST be 542 assumed if no "hct" attribute is found in the returned link. If a 543 "hct" attribute is present in the returned link, then a client MUST 544 use it to create the Hosting HTTP URI. 546 The URI mapping SHOULD be discoverable (as specified in [RFC6690]) on 547 both the HTTP and the CoAP side of the HC proxy, with one important 548 difference: on the CoAP side the link associated to the "core.hc" 549 resource needs an explicit anchor referring to the HTTP origin, while 550 on the HTTP interface the link context is already the HTTP origin 551 carried in the request's Host header, and doesn't have to be made 552 explicit. 554 5.5.1. Examples 556 o The first example exercises the CoAP interface, and assumes that 557 the default template, {+tu}, is used. For example, in use case #3 558 in section Section 4, the smartphone may discover the public HC 559 proxy before leaving the home network. Then when outside the home 560 network, the smartphone will be able to query the appropriate home 561 sensor. 563 Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc 565 Res: 2.05 Content 566 ;anchor="http://p.example.com";rt="core.hc" 568 o The second example - also on the CoAP side of the HC proxy - uses 569 a custom template, i.e., one where the CoAP URI is carried inside 570 the query component, thus the returned link carries the URI 571 template to be used in an explicit "hct" attribute: 573 Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc 575 Res: 2.05 Content 576 ;anchor="http://p.example.com"; 577 rt="core.hc";hct="?uri={+tu}" 579 On the HTTP side, link information can be serialized in more than one 580 way: 582 o using the 'application/link-format' content type: 584 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 585 Host: p.example.com 587 Res: HTTP/1.1 200 OK 588 Content-Type: application/link-format 589 Content-Length: 18 591 ;rt="core.hc" 593 o using the 'application/link-format+json' content type as defined 594 in [I-D.ietf-core-links-json]: 596 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 597 Host: p.example.com 599 Res: HTTP/1.1 200 OK 600 Content-Type: application/link-format+json 601 Content-Length: 31 603 [{"href":"/hc","rt":"core.hc"}] 605 o using the Link header: 607 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 608 Host: p.example.com 610 Res: HTTP/1.1 200 OK 611 Link: ;rt="core.hc" 613 6. Media Type Mapping 615 6.1. Overview 617 A HC proxy needs to translate HTTP media types (Section 3.1.1.1 of 618 [RFC7231]) and content encodings (Section 3.1.2.2 of [RFC7231]) into 619 CoAP content formats (Section 12.3 of [RFC7252]) and vice versa. 621 Media type translation can happen in GET, PUT or POST requests going 622 from HTTP to CoAP, and in 2.xx (i.e., successful) responses going 623 from CoAP to HTTP. Specifically, PUT and POST need to map both the 624 Content-Type and Content-Encoding HTTP headers into a single CoAP 625 Content-Format option, whereas GET needs to map Accept and Accept- 626 Encoding HTTP headers into a single CoAP Accept option. To generate 627 the HTTP response, the CoAP Content-Format option is mapped back to a 628 suitable HTTP Content-Type and Content-Encoding combination. 630 An HTTP request carrying a Content-Type and Content-Encoding 631 combination which the HC proxy is unable to map to an equivalent CoAP 632 Content-Format, SHALL elicit a 415 (Unsupported Media Type) response 633 by the HC proxy. 635 On the content negotiation side, failure to map Accept and Accept-* 636 headers SHOULD be silently ignored: the HC proxy SHOULD therefore 637 forward as a CoAP request with no Accept option. The HC proxy thus 638 disregards the Accept/Accept-* header fields by treating the response 639 as if it is not subject to content negotiation, as mentioned in 640 Sections 5.3.* of [RFC7231]. However, a HC proxy implementation is 641 free to attempt mapping a single Accept header in a GET request to 642 multiple CoAP GET requests, each with a single Accept option, which 643 are then tried in sequence until one succeeds. Note that an HTTP 644 Accept */* MUST be mapped to a CoAP request without Accept option. 646 While the CoAP to HTTP direction has always a well defined mapping 647 (with the exception examined in Section 6.2), the HTTP to CoAP 648 direction is more problematic because the source set, i.e., 649 potentially 1000+ IANA registered media types, is much bigger than 650 the destination set, i.e., the mere 6 values initially defined in 651 Section 12.3 of [RFC7252]. 653 Depending on the tight/loose coupling with the application(s) for 654 which it proxies, the HC proxy could implement different media type 655 mappings. 657 When tightly coupled, the HC proxy knows exactly which content 658 formats are supported by the applications, and can be strict when 659 enforcing its forwarding policies in general, and the media type 660 mapping in particular. 662 On the other side, when the HC proxy is a general purpose application 663 layer gateway, being too strict could significantly reduce the amount 664 of traffic that it would be able to successfully forward. In this 665 case, the "loose" media type mapping detailed in Section 6.3 MAY be 666 implemented. 668 The latter grants more evolution of the surrounding ecosystem, at the 669 cost of allowing more attack surface. In fact, as a result of such 670 strategy, payloads would be forwarded more liberally across the 671 unconstrained/constrained network boundary of the communication path. 672 Therefore, when applied, other forms of access control must be set in 673 place to avoid unauthorized users to deplete or abuse systems and 674 network resources. 676 6.2. 'application/coap-payload' Media Type 678 If the HC proxy receives a CoAP response with a Content-Format that 679 it does not recognize (e.g. because the value has been registered 680 after the proxy has been deployed, or the CoAP server uses an 681 experimental value which is not registered), then the HC proxy SHALL 682 return a generic "application/coap-payload" media type with numeric 683 parameter "cf" as defined in Section 9.2. 685 For example, the CoAP content format '60' ("application/cbor") would 686 be represented by "application/coap-payload;cf=60", if the HC Proxy 687 doesn't recognize the content format '60'. 689 A HTTP client may use the media type "application/coap-payload" as a 690 means to send a specific content format to a CoAP server via a HC 691 Proxy if the client has determined that the HC Proxy does not 692 directly support the type mapping it needs. This case may happen 693 when dealing for example with newly registered, yet to be registered, 694 or experimental CoAP content formats. 696 6.3. Loose Media Type Mapping 698 By structuring the type information in a super-class (e.g. "text") 699 followed by a finer grained sub-class (e.g. "html"), and optional 700 parameters (e.g. "charset=utf-8"), Internet media types provide a 701 rich and scalable framework for encoding the type of any given 702 entity. 704 This approach is not applicable to CoAP, where Content Formats 705 conflate an Internet media type (potentially with specific 706 parameters) and a content encoding into one small integer value. 708 To remedy this loss of flexibility, we introduce the concept of a 709 "loose" media type mapping, where media types that are 710 specializations of a more generic media type can be aliased to their 711 super-class and then mapped (if possible) to one of the CoAP content 712 formats. For example, "application/soap+xml" can be aliased to 713 "application/xml", which has a known conversion to CoAP. In the 714 context of this "loose" media type mapping, "application/octet- 715 stream" can be used as a fallback when no better alias is found for a 716 specific media type. 718 Table 1 defines the default lookup table for the "loose" media type 719 mapping. Given an input media type, the table returns its best 720 generalized media type using the most specific match i.e. the table 721 entries are compared to the input in top to bottom order until an 722 entry matches. 724 +---------------------+--------------------------+ 725 | Internet media type | Generalized media type | 726 +---------------------+--------------------------+ 727 | application/*+xml | application/xml | 728 | application/*+json | application/json | 729 | text/xml | application/xml | 730 | text/* | text/plain | 731 | */* | application/octet-stream | 732 +---------------------+--------------------------+ 734 Table 1: Media type generalization lookup table 736 The "loose" media type mapping is an OPTIONAL feature. 737 Implementations supporting this kind of mapping should provide a 738 flexible way to define the set of media type generalizations allowed. 740 6.4. Media Type to Content Format Mapping Algorithm 742 This section defines the algorithm used to map an HTTP Internet media 743 type to its correspondent CoAP content format. 745 The algorithm uses the mapping table defined in Section 12.3 of 746 [RFC7252] plus, possibly, any locally defined extension of it. 747 Optionally, the table and lookup mechanism described in Section 6.3 748 can be used if the implementation chooses so. 750 Note that the algorithm may have side effects on the associated 751 representation (see also Section 6.5). 753 In the following: 755 o C-T, C-E, and C-F stand for the values of the Content-Type (or 756 Accept) HTTP header, Content-Encoding (or Accept-Encoding) HTTP 757 header, and Content-Format CoAP option respectively. 759 o If C-E is not given it is assumed to be "identity". 761 o MAP is the mandatory lookup table, GMAP is the optional 762 generalized table. 764 INPUT: C-T and C-E 765 OUTPUT: C-F or Fail 767 1. if no C-T: return Fail 768 2. C-F = MAP[C-T, C-E] 769 3. if C-F is not None: return C-F 770 4. if C-E is not "identity": 771 5. if C-E is supported (e.g. gzip): 772 6. decode the representation accordingly 773 7. set C-E to "identity" 774 8. else: 775 9. return Fail 776 10. repeat steps 2. and 3. 777 11. if C-T allows a non-lossy transformation into \ 778 12. one of the supported C-F: 779 13. transcode the representation accordingly 780 14. return C-F 781 15. if GMAP is defined: 782 16. C-F = GMAP[C-T] 783 17. if C-F is not None: return C-F 784 18. return Fail 786 Figure 2 788 6.5. Content Transcoding 790 6.5.1. General 792 Payload content transcoding (e.g. see steps 11-14 of Figure 2) is an 793 OPTIONAL feature. Implementations supporting this feature should 794 provide a flexible way to define the set of transcodings allowed. 796 As noted in Section 6.4, the process of mapping the media type can 797 have side effects on the forwarded entity body. This may be caused 798 by the removal or addition of a specific content encoding, or because 799 the HC proxy decides to transcode the representation to a different 800 (compatible) format. The latter proves useful when an optimized 801 version of a specific format exists. For example an XML-encoded 802 resource could be transcoded to Efficient XML Interchange (EXI) 803 format, or a JSON-encoded resource into CBOR [RFC7049], effectively 804 achieving compression without losing any information. 806 However, it should be noted that in certain cases, transcoding can 807 lose information in a non-obvious manner. For example, encoding an 808 XML document using schema-informed EXI encoding leads to a loss of 809 information when the destination does not know the exact schema 810 version used by the encoder, which means that whenever the HC proxy 811 transcodes an application/XML to application/EXI in-band metadata 812 could be lost. Therefore, the implementer should always carefully 813 verify such lossy payload transformations before triggering the 814 transcoding. 816 6.5.2. CoRE Link Format 818 The CoRE Link Format [RFC6690] is a set of links (i.e., URIs and 819 their formal relationships) which is carried as content payload in a 820 CoAP response. These links usually include CoAP URIs that might be 821 translated by the HC proxy to the correspondent HTTP URIs using the 822 implemented URI mapping function (see Section 5). Such a process 823 would inspect the forwarded traffic and attempt to re-write the body 824 of resources with an application/link-format media type, mapping the 825 embedded CoAP URIs to their HTTP counterparts. Some potential issues 826 with this approach are: 828 1. The client may be interested to retrieve original (unaltered) 829 CoAP payloads through the HC proxy, not modified versions. 831 2. Tampering with payloads is incompatible with resources that are 832 integrity protected (although this is a problem with transcoding 833 in general). 835 3. The HC proxy needs to fully understand [RFC6690] syntax and 836 semantics, otherwise there is an inherent risk to corrupt the 837 payloads. 839 Therefore, CoRE Link Format payload should only be transcoded at the 840 risk and discretion of the proxy implementer. 842 6.5.3. Diagnostic Messages 844 CoAP responses may, in certain error cases, contain a diagnostic 845 message in the payload explaining the error situation, as described 846 in Section 5.5.2 of [RFC7252]. If present, the CoAP response 847 diagnostic payload SHOULD be copied in the HTTP response body. The 848 CoAP diagnostic message MUST NOT be copied into the HTTP reason- 849 phrase, since it potentially contains CR-LF characters which are 850 incompatible with HTTP reason-phrase syntax. 852 7. Response Code Mapping 854 Table 2 defines the HTTP response status codes to which each CoAP 855 response code SHOULD be mapped. Multiple appearances of a HTTP 856 status code in the second column indicates multiple equivalent HTTP 857 responses are possible based on the same CoAP response code, 858 depending on the conditions cited in the Notes (third column and text 859 below table). 861 +-----------------------------+-----------------------------+-------+ 862 | CoAP Response Code | HTTP Status Code | Notes | 863 +-----------------------------+-----------------------------+-------+ 864 | 2.01 Created | 201 Created | 1 | 865 | 2.02 Deleted | 200 OK | 2 | 866 | | 204 No Content | 2 | 867 | 2.03 Valid | 304 Not Modified | 3 | 868 | | 200 OK | 4 | 869 | 2.04 Changed | 200 OK | 2 | 870 | | 204 No Content | 2 | 871 | 2.05 Content | 200 OK | | 872 | 4.00 Bad Request | 400 Bad Request | | 873 | 4.01 Unauthorized | 403 Forbidden | 5 | 874 | 4.02 Bad Option | 400 Bad Request | 6 | 875 | 4.02 Bad Option | 500 Internal Server Error | 6 | 876 | 4.03 Forbidden | 403 Forbidden | | 877 | 4.04 Not Found | 404 Not Found | | 878 | 4.05 Method Not Allowed | 400 Bad Request | 7 | 879 | 4.06 Not Acceptable | 406 Not Acceptable | | 880 | 4.12 Precondition Failed | 412 Precondition Failed | | 881 | 4.13 Request Ent. Too Large | 413 Request Repr. Too Large | | 882 | 4.15 Unsupported Media Type | 415 Unsupported Media Type | | 883 | 5.00 Internal Server Error | 500 Internal Server Error | | 884 | 5.01 Not Implemented | 501 Not Implemented | | 885 | 5.02 Bad Gateway | 502 Bad Gateway | | 886 | 5.03 Service Unavailable | 503 Service Unavailable | 8 | 887 | 5.04 Gateway Timeout | 504 Gateway Timeout | | 888 | 5.05 Proxying Not Supported | 502 Bad Gateway | 9 | 889 +-----------------------------+-----------------------------+-------+ 891 Table 2: CoAP-HTTP Response Code Mappings 893 Notes: 895 1. A CoAP server may return an arbitrary format payload along with 896 this response. If present, this payload MUST be returned as 897 entity in the HTTP 201 response. Section 7.3.2 of [RFC7231] does 898 not put any requirement on the format of the entity. (In the 899 past, [RFC2616] did.) 901 2. The HTTP code is 200 or 204 respectively for the case that a CoAP 902 server returns a payload or not. [RFC7231] Section 5.3 requires 903 code 200 in case a representation of the action result is 904 returned for DELETE/POST/PUT, and code 204 if not. Hence, a 905 proxy MUST transfer any CoAP payload contained in a CoAP 2.02 906 response to the HTTP client using a 200 OK response. 908 3. HTTP code 304 (Not Modified) is sent if the HTTP client performed 909 a conditional HTTP request and the CoAP server responded with 910 2.03 (Valid) to the corresponding CoAP validation request. Note 911 that Section 4.1 of [RFC7232] puts some requirements on header 912 fields that must be present in the HTTP 304 response. 914 4. A 200 response to a CoAP 2.03 occurs only when the HC proxy, for 915 efficiency reasons, is running a local cache. An unconditional 916 HTTP GET which produces a cache-hit, could trigger a re- 917 validation (i.e. a conditional GET) on the CoAP side. The proxy 918 receiving 2.03 updates the freshness of its cached representation 919 and returns it to the HTTP client. 921 5. A HTTP 401 Unauthorized (Section 3.1 of [RFC7235]) response is 922 not applicable because there is no equivalent in CoAP of WWW- 923 Authenticate which is mandatory in a HTTP 401 response. 925 6. If the proxy has a way to determine that the Bad Option is due to 926 the straightforward mapping of a client request header into a 927 CoAP option, then returning HTTP 400 (Bad Request) is 928 appropriate. In all other cases, the proxy MUST return HTTP 500 929 (Internal Server Error) stating its inability to provide a 930 suitable translation to the client's request. 932 7. A CoAP 4.05 (Method Not Allowed) response SHOULD normally be 933 mapped to a HTTP 400 (Bad Request) code, because the HTTP 405 934 response would require specifying the supported methods - which 935 are generally unknown. In this case the HC Proxy SHOULD also 936 return a HTTP reason-phrase in the HTTP status line that starts 937 with the string "CoAP server returned 4.05" in order to 938 facilitate troubleshooting. However, if the HC proxy has more 939 granular information about the supported methods for the 940 requested resource (e.g. via a Resource Directory 941 ([I-D.ietf-core-resource-directory])) then it MAY send back a 942 HTTP 405 (Method Not Allowed) with a properly filled in "Allow" 943 response-header field (Section 7.4.1 of [RFC7231]). 945 8. The value of the HTTP "Retry-After" response-header field is 946 taken from the value of the CoAP Max-Age Option, if present. 948 9. This CoAP response can only happen if the proxy itself is 949 configured to use a CoAP forward-proxy (Section 5.7 of [RFC7252]) 950 to execute some, or all, of its CoAP requests. 952 8. Additional Mapping Guidelines 954 8.1. Caching and Congestion Control 956 A HC proxy should cache CoAP responses, and reply whenever applicable 957 with a cached representation of the requested resource. 959 If the HTTP client drops the connection after the HTTP request was 960 made, a HC proxy should wait for the associated CoAP response and 961 cache it if possible. Subsequent requests to the HC proxy for the 962 same resource can use the result present in cache, or, if a response 963 has still to come, the HTTP requests will wait on the open CoAP 964 request. 966 According to [RFC7252], a proxy must limit the number of outstanding 967 requests to a given CoAP server to NSTART. To limit the amount of 968 aggregate traffic to a constrained network, the HC proxy should also 969 put a limit on the number of concurrent CoAP requests pending on the 970 same constrained network; further incoming requests may either be 971 queued or dropped (returning 503 Service Unavailable). This limit 972 and the proxy queueing/dropping behavior should be configurable. 974 Highly volatile resources that are being frequently requested may be 975 observed [RFC7641] by the HC proxy to keep their cached 976 representation fresh while minimizing the amount of CoAP traffic in 977 the constrained network. See Section 8.2. 979 8.2. Cache Refresh via Observe 981 There are cases where using the CoAP observe protocol [RFC7641] to 982 handle proxy cache refresh is preferable to the validation mechanism 983 based on ETag as defined in [RFC7252]. Such scenarios include, but 984 are not limited to, sleepy CoAP nodes -- with possibly high variance 985 in requests' distribution -- which would greatly benefit from a 986 server driven cache update mechanism. Ideal candidates for CoAP 987 observe are also crowded or very low throughput networks, where 988 reduction of the total number of exchanged messages is an important 989 requirement. 991 This subsection aims at providing a practical evaluation method to 992 decide whether refreshing a cached resource R is more efficiently 993 handled via ETag validation or by establishing an observation on R. 995 Let T_R be the mean time between two client requests to resource R, 996 let T_C be the mean time between two representation changes of R, and 997 let M_R be the mean number of CoAP messages per second exchanged to 998 and from resource R. If we assume that the initial cost for 999 establishing the observation is negligible, an observation on R 1000 reduces M_R iff T_R < 2*T_C with respect to using ETag validation, 1001 that is iff the mean arrival rate of requests for resource R is 1002 greater than half the change rate of R. 1004 When observing the resource R, M_R is always upper bounded by 2/T_C. 1006 8.3. Use of CoAP Blockwise Transfer 1008 A HC proxy SHOULD support CoAP blockwise transfers 1009 [I-D.ietf-core-block] to allow transport of large CoAP payloads while 1010 avoiding excessive link-layer fragmentation in constrained networks, 1011 and to cope with small datagram buffers in CoAP end-points as 1012 described in [RFC7252] Section 4.6. 1014 A HC proxy SHOULD attempt to retry a payload-carrying CoAP PUT or 1015 POST request with blockwise transfer if the destination CoAP server 1016 responded with 4.13 (Request Entity Too Large) to the original 1017 request. A HC proxy SHOULD attempt to use blockwise transfer when 1018 sending a CoAP PUT or POST request message that is larger than 1019 BLOCKWISE_THRESHOLD bytes. The value of BLOCKWISE_THRESHOLD is 1020 implementation-specific, for example it can be: 1022 o calculated based on a known or typical UDP datagram buffer size 1023 for CoAP end-points, or 1025 o set to N times the known size of a link-layer frame in a 1026 constrained network where e.g. N=5, or 1028 o preset to a known IP MTU value, or 1030 o set to a known Path MTU value. 1032 The value BLOCKWISE_THRESHOLD, or the parameters from which it is 1033 calculated, should be configurable in a proxy implementation. The 1034 maximum block size the proxy will attempt to use in CoAP requests 1035 should also be configurable. 1037 The HC proxy SHOULD detect CoAP end-points not supporting blockwise 1038 transfers. This can be done by checking for a 4.02 (Bad Option) 1039 response returned by an end-point in response to a CoAP request with 1040 a Block* Option, and subsequent absence of the 4.02 in response to 1041 the same request without Block* Options. This allows the HC proxy to 1042 be more efficient, not attempting repeated blockwise transfers to 1043 CoAP servers that do not support it. 1045 8.4. CoAP Multicast 1047 A HC proxy MAY support CoAP multicast. If it does, the HC proxy 1048 sends out a multicast CoAP request if the Target CoAP URI's authority 1049 is a multicast IP literal or resolves to a multicast IP address. If 1050 the HC proxy does not support CoAP multicast, it SHOULD respond 403 1051 (Forbidden) to any valid HTTP request that maps to a CoAP multicast 1052 request. 1054 Details related to supporting CoAP multicast are currently out of 1055 scope of this document since in a proxy scenario a HTTP client 1056 typically expects to receive a single response, not multiple. 1057 However, a HC proxy that implements CoAP multicast may include 1058 application-specific functions to aggregate multiple CoAP responses 1059 into a single HTTP response. We suggest using the "application/http" 1060 internet media type (Section 8.3.2 of [RFC7230]) to enclose a set of 1061 one or more HTTP response messages, each representing the mapping of 1062 one CoAP response. 1064 For further considerations related to the handling of multicast 1065 requests, see Section 10.1. 1067 8.5. Timeouts 1069 If the CoAP server takes a long time in responding, the HTTP client 1070 or any other proxy in between may timeout. Further discussion of 1071 timeouts in HTTP is available in Section 6.2.4 of [RFC7230]. 1073 A HC proxy MUST define an internal timeout for each pending CoAP 1074 request, because the CoAP server may silently die before completing 1075 the request. Assuming the Proxy uses confirmable CoAP requests, such 1076 timeout value T SHOULD be at least 1078 T = MAX_RTT + MAX_SERVER_RESPONSE_DELAY 1080 where MAX_RTT is defined in [RFC7252] and MAX_SERVER_RESPONSE_DELAY 1081 is defined in [RFC7390]. 1083 9. IANA Considerations 1085 9.1. New 'core.hc' Resource Type 1087 This document registers a new Resource Type (rt=) Link Target 1088 Attribute, 'core.hc', in the "Resource Type (rt=) Link Target 1089 Attribute Values" subregistry under the "Constrained RESTful 1090 Environments (CoRE) Parameters" registry. 1092 Attribute Value: core.hc 1093 Description: HTTP to CoAP mapping base resource. 1095 Reference: See Section 5.5. 1097 9.2. New 'coap-payload' Internet Media Type 1099 This document defines the "application/coap-payload" media type with 1100 a single parameter "cf". This media type represents any payload that 1101 a CoAP message can carry, having a content format that can be 1102 identified by a CoAP Content-Format parameter (an integer in range 1103 0-65535). The parameter "cf" is the integer defining the CoAP 1104 content format. 1106 Type name: application 1108 Subtype name: coap-payload 1110 Required parameters: 1112 cf - CoAP Content-Format integer in range 0-65535 denoting the 1113 content format of the CoAP payload carried. 1115 Optional parameters: None 1117 Encoding considerations: 1119 The specific CoAP content format encoding considerations for the 1120 selected Content-Format (cf parameter) apply. 1122 Security considerations: 1124 The specific CoAP content format security considerations for the 1125 selected Content-Format (cf parameter) apply. 1127 Interoperability considerations: 1129 Published specification: (this I-D - TBD) 1131 Applications that use this media type: 1133 HTTP-to-CoAP Proxies. 1135 Fragment identifier considerations: N/A 1137 Additional information: 1139 Deprecated alias names for this type: N/A 1140 Magic number(s): N/A 1142 File extension(s): N/A 1144 Macintosh file type code(s): N/A 1146 Person and email address to contact for further information: 1148 Esko Dijk ("esko@ieee.org") 1150 Intended usage: COMMON 1152 Restrictions on usage: 1154 An application (or user) can only use this media type if it has to 1155 represent a CoAP payload of which the specified CoAP Content-Format 1156 is an unrecognized number; such that a proper translation directly to 1157 the equivalent HTTP media type is not possible. 1159 Author: CoRE WG 1161 Change controller: IETF 1163 Provisional registration? (standards tree only): N/A 1165 10. Security Considerations 1167 The security concerns raised in Section 9.2 of [RFC7230] also apply 1168 to the HC proxy scenario. 1170 A HC proxy deployed at the boundary of a constrained network is an 1171 easy single point of failure for reducing availability. As such, 1172 special care should be taken in designing, developing and operating 1173 it, keeping in mind that, in most cases, it has fewer limitations 1174 than the constrained devices it is serving. 1176 The following sub paragraphs categorize and discuss a set of specific 1177 security issues related to the translation, caching and forwarding 1178 functionality exposed by a HC proxy. 1180 10.1. Multicast 1182 Multicast requests impose a non trivial cost on the constrained 1183 network and endpoints, and might be exploited as a DoS attack vector 1184 (see also Section 10.2). From a privacy perspective, they can be 1185 used to gather detailed information about the resources hosted in the 1186 constrained network. For these reasons, it is RECOMMENDED that 1187 requests to multicast resources are access controlled with a default- 1188 deny policy. It is RECOMMENDED that the requestor of a multicast 1189 resource is strongly authenticated. If privacy is a concern, for 1190 example whenever the HTTP request transits through the public 1191 Internet, the request SHOULD be transported over a mutually 1192 authenticated and encrypted TLS connection. 1194 10.2. Traffic Overflow 1196 Due to the typically constrained nature of CoAP nodes, particular 1197 attention should be given to the implementation of traffic reduction 1198 mechanisms (see Section 8.1), because inefficient proxy 1199 implementations can be targeted by unconstrained Internet attackers. 1200 Bandwidth or complexity involved in such attacks is very low. 1202 An amplification attack to the constrained network may be triggered 1203 by a multicast request generated by a single HTTP request which is 1204 mapped to a CoAP multicast resource, as discussed in Section 11.3 of 1205 [RFC7252]. 1207 The risk likelihood of this amplification technique is higher than an 1208 amplification attack carried out by a malicious constrained device 1209 (e.g. ICMPv6 flooding, like Packet Too Big, or Parameter Problem on 1210 a multicast destination [RFC4732]), since it does not require direct 1211 access to the constrained network. 1213 The feasibility of this attack which disrupts availability of the 1214 targeted CoAP server can be limited by access controlling the exposed 1215 multicast resources, so that only known/authorized users can access 1216 such URIs. 1218 10.3. Handling Secured Exchanges 1220 An HTTP request can be sent to the HC proxy over a secured 1221 connection. However, there may not always exist a secure connection 1222 mapping to CoAP. For example, a secure distribution method for 1223 multicast traffic is complex and may not be implemented (see 1224 [RFC7390]). 1226 A HC proxy should implement rules for security context translations. 1227 For example all "https" unicast requests are translated to "coaps" 1228 requests, or "https" requests are translated to unsecured "coap" 1229 requests. Another rule could specify the security policy and 1230 parameters used for DTLS connections. Such rules will largely depend 1231 on the application and network context in which the HC proxy 1232 operates. These rules should be configurable. 1234 It is RECOMMENDED that, by default, accessing a "coaps" URI is only 1235 allowed from a corresponding "https" URI. 1237 By default, a HC proxy SHOULD reject any secured client request if 1238 there is no configured security policy mapping. This recommendation 1239 may be relaxed in case the destination network is believed to be 1240 secured by other means. Assuming that CoAP nodes are isolated behind 1241 a firewall as in the HC proxy deployment shown in Figure 1, the HC 1242 proxy may be configured to translate the incoming HTTPS request using 1243 plain CoAP (NoSec mode). 1245 10.4. URI Mapping 1247 The following risks related to the URI mapping described in Section 5 1248 and its use by HC proxies have been identified: 1250 DoS attack on the constrained/CoAP network. 1251 Mitigation: by default deny any Target CoAP URI whose authority is 1252 (or maps to) a multicast address. Then explicitly white-list 1253 multicast resources/authorities that are allowed to be de- 1254 referenced. See also Section 8.4. 1256 Leaking information on the constrained/CoAP network resources and 1257 topology. 1258 Mitigation: by default deny any Target CoAP URI (especially 1259 /.well-known/core is a resource to be protected), and then 1260 explicitly white-list resources that are allowed to be seen from 1261 outside. 1263 The internal CoAP Target resource is totally transparent from 1264 outside. 1265 Mitigation: implement a HTTPS-only interface, which makes the 1266 Target CoAP URI totally opaque to a passive attacker. 1268 11. Acknowledgements 1270 An initial version of Table 2 in Section 7 has been provided in 1271 revision -05 of the CoRE CoAP I-D. Special thanks to Peter van der 1272 Stok for countless comments and discussions on this document, that 1273 contributed to its current structure and text. 1275 Thanks to Carsten Bormann, Zach Shelby, Michele Rossi, Nicola Bui, 1276 Michele Zorzi, Klaus Hartke, Cullen Jennings, Kepeng Li, Brian Frank, 1277 Peter Saint-Andre, Kerry Lynn, Linyi Tian, Dorothy Gellert, Francesco 1278 Corazza, Hannes Tschofenig and Jaime Jimenez for helpful comments and 1279 discussions that have shaped the document. 1281 The research leading to these results has received funding from the 1282 European Community's Seventh Framework Programme [FP7/2007-2013] 1283 under grant agreement n.251557. 1285 12. References 1287 12.1. Normative References 1289 [I-D.ietf-core-block] 1290 Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP", 1291 draft-ietf-core-block-20 (work in progress), April 2016. 1293 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1294 Requirement Levels", BCP 14, RFC 2119, 1295 DOI 10.17487/RFC2119, March 1997, 1296 . 1298 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1299 Resource Identifier (URI): Generic Syntax", STD 66, 1300 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1301 . 1303 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1304 Specifications: ABNF", STD 68, RFC 5234, 1305 DOI 10.17487/RFC5234, January 2008, 1306 . 1308 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 1309 and D. Orchard, "URI Template", RFC 6570, 1310 DOI 10.17487/RFC6570, March 2012, 1311 . 1313 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 1314 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 1315 . 1317 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1318 Protocol (HTTP/1.1): Message Syntax and Routing", 1319 RFC 7230, DOI 10.17487/RFC7230, June 2014, 1320 . 1322 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1323 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 1324 DOI 10.17487/RFC7231, June 2014, 1325 . 1327 [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1328 Protocol (HTTP/1.1): Conditional Requests", RFC 7232, 1329 DOI 10.17487/RFC7232, June 2014, 1330 . 1332 [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1333 Protocol (HTTP/1.1): Authentication", RFC 7235, 1334 DOI 10.17487/RFC7235, June 2014, 1335 . 1337 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1338 Application Protocol (CoAP)", RFC 7252, 1339 DOI 10.17487/RFC7252, June 2014, 1340 . 1342 [RFC7641] Hartke, K., "Observing Resources in the Constrained 1343 Application Protocol (CoAP)", RFC 7641, 1344 DOI 10.17487/RFC7641, September 2015, 1345 . 1347 12.2. Informative References 1349 [I-D.ietf-core-links-json] 1350 Li, K., Rahman, A., and C. Bormann, "Representing CoRE 1351 Formats in JSON and CBOR", draft-ietf-core-links-json-05 1352 (work in progress), April 2016. 1354 [I-D.ietf-core-resource-directory] 1355 Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE 1356 Resource Directory", draft-ietf-core-resource-directory-07 1357 (work in progress), March 2016. 1359 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1360 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1361 Transfer Protocol -- HTTP/1.1", RFC 2616, 1362 DOI 10.17487/RFC2616, June 1999, 1363 . 1365 [RFC3040] Cooper, I., Melve, I., and G. Tomlinson, "Internet Web 1366 Replication and Caching Taxonomy", RFC 3040, 1367 DOI 10.17487/RFC3040, January 2001, 1368 . 1370 [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet 1371 Denial-of-Service Considerations", RFC 4732, 1372 DOI 10.17487/RFC4732, December 2006, 1373 . 1375 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object 1376 Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, 1377 October 2013, . 1379 [RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for 1380 the Constrained Application Protocol (CoAP)", RFC 7390, 1381 DOI 10.17487/RFC7390, October 2014, 1382 . 1384 Appendix A. Change Log 1386 [Note to RFC Editor: Please remove this section before publication.] 1388 Changes from ietf-10 to ietf-11 (Chair review): 1390 o Removed cu/su distinction from the URI mapping template. 1392 o Addressed a few editorial issues. 1394 Changes from ietf-09 to ietf-10: 1396 o Addressed Ticket #401 - Clarified that draft covers not only 1397 Reverse HC Proxy but that many parts also apply to Forward and 1398 Interception Proxies. 1400 o Clarified that draft concentrates on the HTTP-to-CoAP mapping 1401 direction (i.e. the HC proxy is a HTTP server and a CoAP client). 1403 o Clarified the "null mapping" case where no CoAP URI information is 1404 embedded in the HTTP request URI. 1406 o Moved multicast related security text to the "Security 1407 Considerations" to consolidate all security information in one 1408 location. 1410 o Removed references to "placement" of proxy (e.g. server-side vs 1411 client-side) as is confusing and provides little added value. 1413 o Fixed version numbers on references that were corrupted in last 1414 revision due to outdated xml2rfc conversion tool local cache. 1416 o Various editorial improvements. 1418 Changes from ietf-08 to ietf-09: 1420 o Clean up requirements language as per Klaus' comment. 1422 Changes from ietf-07 to ietf-08: 1424 o Addressed WGLC review comments from Klaus Hartke as per the 1425 correspondence of March 9, 2016 on the CORE WG mailing list. 1427 Changes from ietf-06 to ietf-07: 1429 o Addressed Ticket #384 - Section 5.4.1 describes briefly 1430 (informative) how to discover CoAP resources from an HTTP client. 1432 o Addressed Ticket #378 - For HTTP media type to CoAP content format 1433 mapping and vice versa: a new draft (TBD) may be proposed in CoRE 1434 which describes an approach for automatic updating of the media 1435 type mapping. This was noted in Section 6.1 but is otherwise 1436 outside the scope of this draft. 1438 o Addressed Ticket #377 - Added IANA section that defines a new HTTP 1439 media type "application/coap-payload" and created new Section 6.2 1440 on how to use it. 1442 o Addressed Ticket #376 - Updated Table 2 (and corresponding note 7) 1443 to indicate that a CoAP 4.05 (Method Not Allowed) Response Code 1444 should be mapped to a HTTP 400 (Bad Request). 1446 o Added note to comply to ABNF when translating CoAP diagnostic 1447 payload to reason-phrase in Section 6.5.3. 1449 Changes from ietf-05 to ietf-06: 1451 o Fully restructured the draft, bringing introductory text more to 1452 the front and allocating main sections to each of the key topics; 1453 addressing Ticket #379; 1455 o Addressed Ticket #382, fix of enhanced form URI template 1456 definition of q in Section 5.3.2; 1458 o Addressed Ticket #381, found a mapping 4.01 to 401 Unauthorized in 1459 Section 7; 1461 o Addressed Ticket #380 (Add IANA registration for "core.hc" 1462 Resource Type) in Section 9; 1464 o Addressed Ticket #376 (CoAP 4.05 response can't be translated to 1465 HTTP 405 by HC proxy) in Section 7 by use of empty 'Allow' header; 1467 o Removed details on the pros and cons of HC proxy placement 1468 options; 1470 o Addressed review comments of Carsten Bormann; 1472 o Clarified failure in mapping of HTTP Accept headers (Section 6.3); 1473 o Clarified detection of CoAP servers not supporting blockwise 1474 (Section 8.3); 1476 o Changed CoAP request timeout min value to MAX_RTT + 1477 MAX_SERVER_RESPONSE_DELAY (Section 8.6); 1479 o Added security section item (Section 10.3) related to use of CoAP 1480 blockwise transfers; 1482 o Many editorial improvements. 1484 Changes from ietf-04 to ietf-05: 1486 o Addressed Ticket #366 (Mapping of CoRE Link Format payloads to be 1487 valid in HTTP Domain?) in Section 6.3.3.2 (Content Transcoding - 1488 CORE Link Format); 1490 o Addressed Ticket #375 (Add requirement on mapping of CoAP 1491 diagnostic payload) in Section 6.3.3.3 (Content Transcoding - 1492 Diagnostic Messages); 1494 o Addressed comment from Yusuke (http://www.ietf.org/mail- 1495 archive/web/core/current/msg05491.html) in Section 6.3.3.1 1496 (Content Transcoding - General); 1498 o Various editorial improvements. 1500 Changes from ietf-03 to ietf-04: 1502 o Expanded use case descriptions in Section 4; 1504 o Fixed/enhanced discovery examples in Section 5.4.1; 1506 o Addressed Ticket #365 (Add text on media type conversion by HTTP- 1507 CoAP proxy) in new Section 6.3.1 (Generalized media type mapping) 1508 and new Section 6.3.2 (Content translation); 1510 o Updated HTTPBis WG draft references to recently published RFC 1511 numbers. 1513 o Various editorial improvements. 1515 Changes from ietf-02 to ietf-03: 1517 o Closed Ticket #351 "Add security implications of proposed default 1518 HTTP-CoAP URI mapping"; 1520 o Closed Ticket #363 "Remove CoAP scheme in default HTTP-CoAP URI 1521 mapping"; 1523 o Closed Ticket #364 "Add discovery of HTTP-CoAP mapping 1524 resource(s)". 1526 Changes from ietf-01 to ietf-02: 1528 o Selection of single default URI mapping proposal as proposed to WG 1529 mailing list 2013-10-09. 1531 Changes from ietf-00 to ietf-01: 1533 o Added URI mapping proposals to Section 4 as per the Email 1534 proposals to WG mailing list from Esko. 1536 Authors' Addresses 1538 Angelo P. Castellani 1539 University of Padova 1540 Via Gradenigo 6/B 1541 Padova 35131 1542 Italy 1544 Email: angelo@castellani.net 1546 Salvatore Loreto 1547 Ericsson 1548 Hirsalantie 11 1549 Jorvas 02420 1550 Finland 1552 Email: salvatore.loreto@ericsson.com 1554 Akbar Rahman 1555 InterDigital Communications, LLC 1556 1000 Sherbrooke Street West 1557 Montreal H3A 3G4 1558 Canada 1560 Phone: +1 514 585 0761 1561 Email: Akbar.Rahman@InterDigital.com 1562 Thomas Fossati 1563 Nokia 1564 3 Ely Road 1565 Milton, Cambridge CB24 6DD 1566 UK 1568 Email: thomas.fossati@nokia.com 1570 Esko Dijk 1571 Philips Lighting 1572 High Tech Campus 34 1573 Eindhoven 5656 AE 1574 The Netherlands 1576 Email: esko.dijk@philips.com