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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: January 2, 2017 Ericsson 6 A. Rahman 7 InterDigital Communications, LLC 8 T. Fossati 9 Nokia 10 E. Dijk 11 Philips Lighting 12 July 1, 2016 14 Guidelines for HTTP-to-CoAP Mapping Implementations 15 draft-ietf-core-http-mapping-12 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 January 2, 2017. 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 . . . . . . . . . . . . . . . . . . . . . . . . . 7 68 5.1. URI Terminology . . . . . . . . . . . . . . . . . . . . . 8 69 5.2. Null Mapping . . . . . . . . . . . . . . . . . . . . . . 8 70 5.3. Default Mapping . . . . . . . . . . . . . . . . . . . . . 8 71 5.3.1. Optional Scheme Omission . . . . . . . . . . . . . . 9 72 5.3.2. Encoding Caveats . . . . . . . . . . . . . . . . . . 9 73 5.4. URI Mapping Template . . . . . . . . . . . . . . . . . . 10 74 5.4.1. Simple Form . . . . . . . . . . . . . . . . . . . . . 10 75 5.4.2. Enhanced Form . . . . . . . . . . . . . . . . . . . . 11 76 5.5. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 13 77 5.5.1. Examples . . . . . . . . . . . . . . . . . . . . . . 13 78 6. Media Type Mapping . . . . . . . . . . . . . . . . . . . . . 15 79 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15 80 6.2. 'application/coap-payload' Media Type . . . . . . . . . . 16 81 6.3. Loose Media Type Mapping . . . . . . . . . . . . . . . . 17 82 6.4. Media Type to Content Format Mapping Algorithm . . . . . 18 83 6.5. Content Transcoding . . . . . . . . . . . . . . . . . . . 19 84 6.5.1. General . . . . . . . . . . . . . . . . . . . . . . . 19 85 6.5.2. CoRE Link Format . . . . . . . . . . . . . . . . . . 20 86 6.5.3. Diagnostic Messages . . . . . . . . . . . . . . . . . 20 87 7. Response Code Mapping . . . . . . . . . . . . . . . . . . . . 20 88 8. Additional Mapping Guidelines . . . . . . . . . . . . . . . . 23 89 8.1. Caching and Congestion Control . . . . . . . . . . . . . 23 90 8.2. Cache Refresh via Observe . . . . . . . . . . . . . . . . 23 91 8.3. Use of CoAP Blockwise Transfer . . . . . . . . . . . . . 24 92 8.4. CoAP Multicast . . . . . . . . . . . . . . . . . . . . . 25 93 8.5. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 25 95 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 96 9.1. New 'core.hc' Resource Type . . . . . . . . . . . . . . . 25 97 9.2. New 'coap-payload' Internet Media Type . . . . . . . . . 26 98 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27 99 10.1. Multicast . . . . . . . . . . . . . . . . . . . . . . . 27 100 10.2. Traffic Overflow . . . . . . . . . . . . . . . . . . . . 28 101 10.3. Handling Secured Exchanges . . . . . . . . . . . . . . . 28 102 10.4. URI Mapping . . . . . . . . . . . . . . . . . . . . . . 29 103 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 104 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 105 12.1. Normative References . . . . . . . . . . . . . . . . . . 30 106 12.2. Informative References . . . . . . . . . . . . . . . . . 31 107 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 32 108 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 110 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 describes HTTP mappings that apply to protocol elements 135 defined in the base CoAP specification [RFC7252]. It is up to CoAP 136 protocol extensions (new methods, response codes, options, content- 137 formats) to describe their own HTTP mappings, if applicable. 139 This document is organized as follows: 141 o Section 2 defines proxy terminology; 142 o Section 3 introduces the HTTP-to-CoAP proxy; 144 o Section 4 lists use cases in which HTTP clients need to contact 145 CoAP servers; 147 o Section 5 introduces a null, default and advanced HTTP-to-CoAP URI 148 mapping syntax; 150 o Section 6 describes how to map HTTP media types to CoAP content 151 formats and vice versa; 153 o Section 7 describes how to map CoAP responses to HTTP responses; 155 o Section 8 describes additional mapping guidelines related to 156 caching, congestion, timeouts, etc.; 158 o Section 10 discusses possible security impact of HTTP-to-CoAP 159 protocol mapping. 161 2. Terminology 163 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 164 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 165 "OPTIONAL" in this document are to be interpreted as described in 166 [RFC2119]. 168 HC Proxy: a proxy performing a cross-protocol mapping, in the context 169 of this document an HTTP-to-CoAP (HC) mapping. Specifically, the HC 170 proxy acts as an HTTP server and a CoAP client. The HC Proxy can 171 take on the role of a Forward, Reverse or Interception Proxy. 173 Forward Proxy (or Forward HC Proxy): a message forwarding agent that 174 is selected by the HTTP client, usually via local configuration 175 rules, to receive requests for some type(s) of absolute URI and to 176 attempt to satisfy those requests via translation to the protocol 177 indicated by the absolute URI. The user decides (is willing) to use 178 the proxy as the forwarding/de-referencing agent for a predefined 179 subset of the URI space. In [RFC7230] this is called a Proxy. 180 [RFC7252] defines Forward-Proxy similarly. 182 Reverse Proxy (or Reverse HC Proxy): as in [RFC7230], a receiving 183 agent that acts as a layer above some other server(s) and translates 184 the received requests to the underlying server's protocol. A Reverse 185 HC Proxy behaves as an origin (HTTP) server on its connection from 186 the HTTP client. The HTTP client uses the "origin-form" 187 (Section 5.3.1 of [RFC7230]) as a request-target URI. 189 Interception Proxy (or Interception HC Proxy) [RFC3040]: a proxy that 190 receives inbound HTTP traffic flows through the process of traffic 191 redirection; transparent to the HTTP client. 193 Note that a Reverse Proxy appears to an HTTP client as an origin 194 server while a Forward Proxy does not. So, when communicating with a 195 Reverse Proxy a client may be unaware it is communicating with a 196 proxy at all. 198 3. HTTP-to-CoAP Proxy 200 A HC proxy is accessed by an HTTP client which wants to access a 201 resource on a CoAP server. The HC proxy handles the HTTP request by 202 mapping it to the equivalent CoAP request, which is then forwarded to 203 the appropriate CoAP server. The received CoAP response is then 204 mapped to an appropriate HTTP response and finally sent back to the 205 originating HTTP client. 207 See Figure 1 for an example deployment scenario. Here a HC proxy is 208 located at the boundary of the Constrained Network domain, to avoid 209 sending any HTTP traffic into the Constrained Network and to avoid 210 any (unsecured) CoAP multicast traffic outside the Constrained 211 Network. A DNS server (not shown) is used by the HTTP Client to 212 resolve the IP address of the HC proxy and optionally also used by 213 the HC proxy to resolve IP addresses of CoAP servers. 215 Constrained Network 216 .-------------------. 217 / .------. \ 218 / | CoAP | \ 219 / |server| \ 220 || '------' || 221 || || 222 .--------. HTTP Request .------------. CoAP Req .------. || 223 | HTTP |---------------->|HTTP-to-CoAP|----------->| CoAP | || 224 | Client |<----------------| Proxy |<-----------|Server| || 225 '--------' HTTP Response '------------' CoAP Resp '------' || 226 || || 227 || .------. || 228 || | CoAP | || 229 \ |server| .------. / 230 \ '------' | CoAP | / 231 \ |server| / 232 \ '------' / 233 '-----------------' 235 Figure 1: HTTP-To-CoAP Proxy Deployment Scenario 237 Normative requirements on the translation of HTTP requests to CoAP 238 requests and of the CoAP responses back to HTTP responses are defined 239 in Section 10.2 of [RFC7252]. However, [RFC7252] focuses on the 240 basic mapping of request methods and simple response code mapping 241 between HTTP and CoAP, and leaves many details of the cross-protocol 242 HC proxy for future definition. This document provides additional 243 guidelines and more details for the implementation of a HC Proxy, 244 which should be followed in addition to the normative requirements. 245 Note that the guidelines apply to all forms of an HC proxy (i.e. 246 Reverse, Forward, Intercepting) unless explicitly otherwise noted. 248 4. Use Cases 250 To illustrate the situations HTTP to CoAP protocol translation may be 251 used, three use cases are described below. 253 1. Legacy building control application without CoAP: A building 254 control application that uses HTTP but not CoAP can check the status 255 of CoAP sensors and/or control actuators via a HC proxy. 257 2. Making sensor data available to 3rd parties on the Web: For 258 demonstration or public interest purposes, a HC proxy may be 259 configured to expose the contents of a CoAP sensor to the world via 260 the web (HTTP and/or HTTPS). Some sensors may only accept secure 261 'coaps' requests, therefore the proxy is configured to translate 262 request to those devices accordingly. The HC proxy is furthermore 263 configured to only pass through GET requests in order to protect the 264 constrained network. 266 3. Smartphone and home sensor: A smartphone can access directly a 267 CoAP home sensor using a mutually authenticated 'https' request, 268 provided its home router runs a HC proxy and is configured with the 269 appropriate certificate. An HTML5 application on the smartphone can 270 provide a friendly UI using the standard (HTTP) networking functions 271 of HTML5. 273 A key point in the above use cases is the expected nature of the URI 274 to be used by the HTTP client initiating the HTTP request to the HC 275 proxy. Specifically, in use case #1, there will be no "coap" or 276 "coaps" related information embedded in the HTTP URI as it is a 277 legacy HTTP client sending the request. Use case #2 is also expected 278 to be similar. In contrast, in use case #3, it is expected that the 279 HTTP client will specifically embed "coap" or "coaps" related 280 information in the HTTP URI of the HTTP request to the HC proxy. 282 5. URI Mapping 284 Though, in principle, a CoAP URI could be directly used by a HTTP 285 client to de-reference a CoAP resource through a HC proxy, the 286 reality is that all major web browsers, networking libraries and 287 command line tools do not allow making HTTP requests using URIs with 288 a scheme "coap" or "coaps". 290 Thus, there is a need for web applications to embed or "pack" a CoAP 291 URI into a HTTP URI so that it can be (non-destructively) transported 292 from the HTTP client to the HC proxy. The HC proxy can then "unpack" 293 the CoAP URI and finally de-reference it via a CoAP request to the 294 target Server. 296 URI Mapping is the term used in the document to describe the process 297 through which the URI of a CoAP resource is transformed into an HTTP 298 URI so that: 300 o the requesting HTTP client can handle it; 302 o the receiving HC proxy can extract the intended CoAP URI 303 unambiguously. 305 To this end, the remainder of this section will identify: 307 o the default mechanism to map a CoAP URI into a HTTP URI; 308 o the URI template format to express a class of CoAP-HTTP URI 309 mapping functions; 311 o the discovery mechanism based on CoRE Link Format [RFC6690] 312 through which clients of a HC proxy can dynamically discover 313 information about the supported URI Mapping Template(s), as well 314 as the URI where the HC proxy function is anchored. 316 5.1. URI Terminology 318 In the remainder of this section, the following terms will be used 319 with a distinctive meaning: 321 HC Proxy URI: 322 URI which refers to the HC proxy function. It conforms to 323 syntax defined in Section 2.7 of [RFC7230]. 325 Target CoAP URI: 326 URI which refers to the (final) CoAP resource that has to be 327 de-referenced. It conforms to syntax defined in Section 6 of 328 [RFC7252]. Specifically, its scheme is either "coap" or 329 "coaps". 331 Hosting HTTP URI: 332 URI that conforms to syntax in Section 2.7 of [RFC7230]. Its 333 authority component refers to a HC proxy, whereas path (and 334 query) component(s) embed the information used by a HC proxy 335 to extract the Target CoAP URI. 337 5.2. Null Mapping 339 The null mapping is the case where there is no Target CoAP URI 340 appended to the HC Proxy URI. In other words, it is a "pure" HTTP 341 URI that is sent to the HC Proxy. This would typically occur in 342 situations like Use Case #1 described in Section 4, and the Proxy 343 would typically be a Reverse Proxy. In this scenario, the HC Proxy 344 will determine through its own proprietary algorithms what the Target 345 CoAP URI should be. 347 5.3. Default Mapping 349 The default mapping is for the Target CoAP URI to be appended as-is 350 to the HC Proxy URI, to form the Hosting HTTP URI. This is the URI 351 that will then be sent by the HTTP client in the HTTP request to the 352 HC proxy. 354 For example: given a HC Proxy URI http://p.example.com/hc and a 355 Target CoAP URI coap://s.example.com/light, the resulting Hosting 356 HTTP URI would be http://p.example.com/hc/coap://s.example.com/light. 358 Provided a correct Target CoAP URI, the Hosting HTTP URI resulting 359 from the default mapping is always syntactically correct. 360 Furthermore, the Target CoAP URI can always be extracted 361 unambiguously from the Hosting HTTP URI. Also, it is worth noting 362 that, using the default mapping, a query component in the target CoAP 363 resource URI is naturally encoded into the query component of the 364 Hosting URI, e.g.: coap://s.example.com/light?dim=5 becomes 365 http://p.example.com/hc/coap://s.example.com/light?dim=5. 367 There is no default for the HC Proxy URI. Therefore, it is either 368 known in advance, e.g. as a configuration preset, or dynamically 369 discovered using the mechanism described in Section 5.5. 371 The default URI mapping function SHOULD be implemented and activated 372 by default in a HC proxy, unless there are valid reasons, e.g. 373 application specific, to use a different mapping function. 375 5.3.1. Optional Scheme Omission 377 When found in a Hosting HTTP URI, the scheme (i.e., "coap" or 378 "coaps"), the scheme component delimiter (":"), and the double slash 379 ("//") preceding the authority MAY be omitted. In such case, a local 380 default - not defined by this document - applies. 382 So, http://p.example.com/hc/s.coap.example.com/foo could either 383 represent the target coap://s.coap.example.com/foo or 384 coaps://s.coap.example.com/foo depending on application specific 385 presets. 387 5.3.2. Encoding Caveats 389 When the authority of the Target CoAP URI is given as an IPv6address, 390 then the surrounding square brackets must be percent-encoded in the 391 Hosting HTTP URI, in order to comply with the syntax defined in 392 Section 3.3. of [RFC3986] for a URI path segment. E.g.: 393 coap://[2001:db8::1]/light?on becomes 394 http://p.example.com/hc/coap://%5B2001:db8::1%5D/light?on. 396 Everything else can be safely copied verbatim from the Target CoAP 397 URI to the Hosting HTTP URI. 399 5.4. URI Mapping Template 401 This section defines a format for the URI template [RFC6570] used by 402 a HC proxy to inform its clients about the expected syntax for the 403 Hosting HTTP URI. This will then be used by the HTTP client to 404 construct the URI to be sent in the HTTP request to the HC proxy. 406 When instantiated, an URI Mapping Template is always concatenated to 407 a HC Proxy URI provided by the HC proxy via discovery (see 408 Section 5.5), or by other means. 410 A simple form (Section 5.4.1) and an enhanced form (Section 5.4.2) 411 are provided to fit different users' requirements. 413 Both forms are expressed as level 2 URI templates [RFC6570] to take 414 care of the expansion of values that are allowed to include reserved 415 URI characters. The syntax of all URI formats is specified in this 416 section in Augmented Backus-Naur Form (ABNF) [RFC5234]. 418 5.4.1. Simple Form 420 The simple form MUST be used for mappings where the Target CoAP URI 421 is going to be copied (using rules of Section 5.3.2) at some fixed 422 position into the Hosting HTTP URI. 424 The "tu" template variable is intended to be used in a template 425 definition to represent a Target CoAP URI: 427 tu = coap-URI / coaps-URI ; from [RFC7252], Section 6.1 and 6.2 429 The same considerations as in Section 5.3.1 apply, in that the CoAP 430 scheme may be omitted from the Hosting HTTP URI. 432 5.4.1.1. Examples 434 All the following examples (given as a specific URI mapping template, 435 a Target CoAP URI, and the produced Hosting HTTP URI) use 436 http://p.example.com/hc as the HC Proxy URI. Note that these 437 examples all define mapping templates that deviate from the default 438 template of Section 5.3 to be able to illustrate the use of the above 439 template variables. 441 1. Target CoAP URI is a query argument of the Hosting HTTP URI: 443 ?target_uri={+tu} 445 coap://s.example.com/light 447 http://p.example.com/hc?target_uri=coap://s.example.com/light 449 or 451 coaps://s.example.com/light 453 http://p.example.com/hc?target_uri=coaps://s.example.com/light 455 2. Target CoAP URI in the path component of the Hosting HTTP URI 456 (i.e., the default URI Mapping template): 458 /{+tu} 460 coap://s.example.com/light 462 http://p.example.com/hc/coap://s.example.com/light 464 or 466 coaps://s.example.com/light 468 http://p.example.com/hc/coaps://s.example.com/light 470 3. "coap" URI is a query argument of the Hosting HTTP URI; client 471 decides to omit scheme because a default scheme is agreed 472 beforehand between client and proxy: 474 ?coap_uri={+tu} 476 coap://s.example.com/light 478 http://p.example.com/hc?coap_uri=s.example.com/light 480 5.4.2. Enhanced Form 482 The enhanced form can be used to express more sophisticated mappings 483 of the Target CoAP URI into the Hosting HTTP URI, i.e., mappings that 484 do not fit into the simple form. 486 There MUST be at most one instance of each of the following template 487 variables in a template definition: 489 s = "coap" / "coaps" ; from [RFC7252], Sections 6.1 and 6.2 490 hp = host [":" port] ; from [RFC3986], Sections 3.2.2 and 3.2.3 491 p = path-abempty ; from [RFC3986], Section 3.3 492 q = query ; from [RFC3986], Section 3.4 493 qq = [ "?" query ] ; qq is empty if and only if 'query' is empty 495 The qq form is used when the path and the (optional) query components 496 are to be copied verbatim from the Target CoAP URI into the Hosting 497 HTTP URI, i.e. as "{+p}{+qq}". Instead, the q form is used when the 498 query and path are mapped as separate entities, e.g. as in 499 "coap_path={+p}&coap_query={+q}". 501 5.4.2.1. Examples 503 All the following examples (given as a specific URI mapping template, 504 a Target CoAP URI, and the produced Hosting HTTP URI) use 505 http://p.example.com/hc as the HC Proxy URI. 507 1. Target CoAP URI components in path segments, and optional query 508 in query component: 510 /{+s}/{+hp}{+p}{+qq} 512 coap://s.example.com/light 514 http://p.example.com/hc/coap/s.example.com/light 516 or 518 coap://s.example.com/light?on 520 http://p.example.com/hc/coap/s.example.com/light?on 522 2. Target CoAP URI components split in individual query arguments: 524 ?s={+s}&hp={+hp}&p={+p}&q={+q} 526 coap://s.example.com/light 528 http://p.example.com/hc?s=coap&hp=s.example.com&p=/light&q= 530 or 532 coaps://s.example.com/light?on 534 http://p.example.com/hc?s=coaps&hp=s.example.com&p=/light&q=on 536 5.5. Discovery 538 In order to accommodate site specific needs while allowing third 539 parties to discover the proxy function, the HC proxy SHOULD publish 540 information related to the location and syntax of the HC proxy 541 function using the CoRE Link Format [RFC6690] interface. 543 To this aim a new Resource Type, "core.hc", is defined in this 544 document. It can be used as the value for the "rt" attribute in a 545 query to the /.well-known/core in order to locate the URI where the 546 HC proxy function is anchored, i.e. the HC Proxy URI. 548 Along with it, the new target attribute "hct" is defined in this 549 document. This attribute MAY be returned in a "core.hc" link to 550 provide the URI Mapping Template associated to the mapping resource. 551 The default template given in Section 5.3, i.e., {+tu}, MUST be 552 assumed if no "hct" attribute is found in the returned link. If a 553 "hct" attribute is present in the returned link, then a client MUST 554 use it to create the Hosting HTTP URI. 556 The URI mapping SHOULD be discoverable (as specified in [RFC6690]) on 557 both the HTTP and the CoAP side of the HC proxy, with one important 558 difference: on the CoAP side the link associated to the "core.hc" 559 resource needs an explicit anchor referring to the HTTP origin, while 560 on the HTTP interface the link context is already the HTTP origin 561 carried in the request's Host header, and doesn't have to be made 562 explicit. 564 5.5.1. Examples 566 o The first example exercises the CoAP interface, and assumes that 567 the default template, {+tu}, is used. For example, in use case #3 568 in section Section 4, the smartphone may discover the public HC 569 proxy before leaving the home network. Then when outside the home 570 network, the smartphone will be able to query the appropriate home 571 sensor. 573 Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc 575 Res: 2.05 Content 576 ;anchor="http://p.example.com";rt="core.hc" 578 o The second example - also on the CoAP side of the HC proxy - uses 579 a custom template, i.e., one where the CoAP URI is carried inside 580 the query component, thus the returned link carries the URI 581 template to be used in an explicit "hct" attribute: 583 Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc 585 Res: 2.05 Content 586 ;anchor="http://p.example.com"; 587 rt="core.hc";hct="?uri={+tu}" 589 On the HTTP side, link information can be serialized in more than one 590 way: 592 o using the 'application/link-format' content type: 594 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 595 Host: p.example.com 597 Res: HTTP/1.1 200 OK 598 Content-Type: application/link-format 599 Content-Length: 18 601 ;rt="core.hc" 603 o using the 'application/link-format+json' content type as defined 604 in [I-D.ietf-core-links-json]: 606 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 607 Host: p.example.com 609 Res: HTTP/1.1 200 OK 610 Content-Type: application/link-format+json 611 Content-Length: 31 613 [{"href":"/hc","rt":"core.hc"}] 615 o using the Link header: 617 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 618 Host: p.example.com 620 Res: HTTP/1.1 200 OK 621 Link: ;rt="core.hc" 623 6. Media Type Mapping 625 6.1. Overview 627 A HC proxy needs to translate HTTP media types (Section 3.1.1.1 of 628 [RFC7231]) and content encodings (Section 3.1.2.2 of [RFC7231]) into 629 CoAP content formats (Section 12.3 of [RFC7252]) and vice versa. 631 Media type translation can happen in GET, PUT or POST requests going 632 from HTTP to CoAP, and in 2.xx (i.e., successful) responses going 633 from CoAP to HTTP. Specifically, PUT and POST need to map both the 634 Content-Type and Content-Encoding HTTP headers into a single CoAP 635 Content-Format option, whereas GET needs to map Accept and Accept- 636 Encoding HTTP headers into a single CoAP Accept option. To generate 637 the HTTP response, the CoAP Content-Format option is mapped back to a 638 suitable HTTP Content-Type and Content-Encoding combination. 640 An HTTP request carrying a Content-Type and Content-Encoding 641 combination which the HC proxy is unable to map to an equivalent CoAP 642 Content-Format, SHALL elicit a 415 (Unsupported Media Type) response 643 by the HC proxy. 645 On the content negotiation side, failure to map Accept and Accept-* 646 headers SHOULD be silently ignored: the HC proxy SHOULD therefore 647 forward as a CoAP request with no Accept option. The HC proxy thus 648 disregards the Accept/Accept-* header fields by treating the response 649 as if it is not subject to content negotiation, as mentioned in 650 Sections 5.3.* of [RFC7231]. However, a HC proxy implementation is 651 free to attempt mapping a single Accept header in a GET request to 652 multiple CoAP GET requests, each with a single Accept option, which 653 are then tried in sequence until one succeeds. Note that an HTTP 654 Accept */* MUST be mapped to a CoAP request without Accept option. 656 While the CoAP to HTTP direction has always a well defined mapping 657 (with the exception examined in Section 6.2), the HTTP to CoAP 658 direction is more problematic because the source set, i.e., 659 potentially 1000+ IANA registered media types, is much bigger than 660 the destination set, i.e., the mere 6 values initially defined in 661 Section 12.3 of [RFC7252]. 663 Depending on the tight/loose coupling with the application(s) for 664 which it proxies, the HC proxy could implement different media type 665 mappings. 667 When tightly coupled, the HC proxy knows exactly which content 668 formats are supported by the applications, and can be strict when 669 enforcing its forwarding policies in general, and the media type 670 mapping in particular. 672 On the other side, when the HC proxy is a general purpose application 673 layer gateway, being too strict could significantly reduce the amount 674 of traffic that it would be able to successfully forward. In this 675 case, the "loose" media type mapping detailed in Section 6.3 MAY be 676 implemented. 678 The latter grants more evolution of the surrounding ecosystem, at the 679 cost of allowing more attack surface. In fact, as a result of such 680 strategy, payloads would be forwarded more liberally across the 681 unconstrained/constrained network boundary of the communication path. 682 Therefore, when applied, other forms of access control must be set in 683 place to avoid unauthorized users to deplete or abuse systems and 684 network resources. 686 6.2. 'application/coap-payload' Media Type 688 If the HC proxy receives a CoAP response with a Content-Format that 689 it does not recognize (e.g. because the value has been registered 690 after the proxy has been deployed, or the CoAP server uses an 691 experimental value which is not registered), then the HC proxy SHALL 692 return a generic "application/coap-payload" media type with numeric 693 parameter "cf" as defined in Section 9.2. 695 For example, the CoAP content format '60' ("application/cbor") would 696 be represented by "application/coap-payload;cf=60", if the HC Proxy 697 doesn't recognize the content format '60'. 699 A HTTP client may use the media type "application/coap-payload" as a 700 means to send a specific content format to a CoAP server via a HC 701 Proxy if the client has determined that the HC Proxy does not 702 directly support the type mapping it needs. This case may happen 703 when dealing for example with newly registered, yet to be registered, 704 or experimental CoAP content formats. 706 6.3. Loose Media Type Mapping 708 By structuring the type information in a super-class (e.g. "text") 709 followed by a finer grained sub-class (e.g. "html"), and optional 710 parameters (e.g. "charset=utf-8"), Internet media types provide a 711 rich and scalable framework for encoding the type of any given 712 entity. 714 This approach is not applicable to CoAP, where Content Formats 715 conflate an Internet media type (potentially with specific 716 parameters) and a content encoding into one small integer value. 718 To remedy this loss of flexibility, we introduce the concept of a 719 "loose" media type mapping, where media types that are 720 specializations of a more generic media type can be aliased to their 721 super-class and then mapped (if possible) to one of the CoAP content 722 formats. For example, "application/soap+xml" can be aliased to 723 "application/xml", which has a known conversion to CoAP. In the 724 context of this "loose" media type mapping, "application/octet- 725 stream" can be used as a fallback when no better alias is found for a 726 specific media type. 728 Table 1 defines the default lookup table for the "loose" media type 729 mapping. Given an input media type, the table returns its best 730 generalized media type using the most specific match i.e. the table 731 entries are compared to the input in top to bottom order until an 732 entry matches. 734 +---------------------+--------------------------+ 735 | Internet media type | Generalized media type | 736 +---------------------+--------------------------+ 737 | application/*+xml | application/xml | 738 | application/*+json | application/json | 739 | text/xml | application/xml | 740 | text/* | text/plain | 741 | */* | application/octet-stream | 742 +---------------------+--------------------------+ 744 Table 1: Media type generalization lookup table 746 The "loose" media type mapping is an OPTIONAL feature. 747 Implementations supporting this kind of mapping should provide a 748 flexible way to define the set of media type generalizations allowed. 750 6.4. Media Type to Content Format Mapping Algorithm 752 This section defines the algorithm used to map an HTTP Internet media 753 type to its correspondent CoAP content format. 755 The algorithm uses the mapping table defined in Section 12.3 of 756 [RFC7252] plus, possibly, any locally defined extension of it. 757 Optionally, the table and lookup mechanism described in Section 6.3 758 can be used if the implementation chooses so. 760 Note that the algorithm may have side effects on the associated 761 representation (see also Section 6.5). 763 In the following: 765 o C-T, C-E, and C-F stand for the values of the Content-Type (or 766 Accept) HTTP header, Content-Encoding (or Accept-Encoding) HTTP 767 header, and Content-Format CoAP option respectively. 769 o If C-E is not given it is assumed to be "identity". 771 o MAP is the mandatory lookup table, GMAP is the optional 772 generalized table. 774 INPUT: C-T and C-E 775 OUTPUT: C-F or Fail 777 1. if no C-T: return Fail 778 2. C-F = MAP[C-T, C-E] 779 3. if C-F is not None: return C-F 780 4. if C-E is not "identity": 781 5. if C-E is supported (e.g. gzip): 782 6. decode the representation accordingly 783 7. set C-E to "identity" 784 8. else: 785 9. return Fail 786 10. repeat steps 2. and 3. 787 11. if C-T allows a non-lossy transformation into \ 788 12. one of the supported C-F: 789 13. transcode the representation accordingly 790 14. return C-F 791 15. if GMAP is defined: 792 16. C-F = GMAP[C-T] 793 17. if C-F is not None: return C-F 794 18. return Fail 796 Figure 2 798 6.5. Content Transcoding 800 6.5.1. General 802 Payload content transcoding (e.g. see steps 11-14 of Figure 2) is an 803 OPTIONAL feature. Implementations supporting this feature should 804 provide a flexible way to define the set of transcodings allowed. 806 As noted in Section 6.4, the process of mapping the media type can 807 have side effects on the forwarded entity body. This may be caused 808 by the removal or addition of a specific content encoding, or because 809 the HC proxy decides to transcode the representation to a different 810 (compatible) format. The latter proves useful when an optimized 811 version of a specific format exists. For example an XML-encoded 812 resource could be transcoded to Efficient XML Interchange (EXI) 813 format, or a JSON-encoded resource into CBOR [RFC7049], effectively 814 achieving compression without losing any information. 816 However, it should be noted that in certain cases, transcoding can 817 lose information in a non-obvious manner. For example, encoding an 818 XML document using schema-informed EXI encoding leads to a loss of 819 information when the destination does not know the exact schema 820 version used by the encoder, which means that whenever the HC proxy 821 transcodes an application/XML to application/EXI in-band metadata 822 could be lost. Therefore, the implementer should always carefully 823 verify such lossy payload transformations before triggering the 824 transcoding. 826 6.5.2. CoRE Link Format 828 The CoRE Link Format [RFC6690] is a set of links (i.e., URIs and 829 their formal relationships) which is carried as content payload in a 830 CoAP response. These links usually include CoAP URIs that might be 831 translated by the HC proxy to the correspondent HTTP URIs using the 832 implemented URI mapping function (see Section 5). Such a process 833 would inspect the forwarded traffic and attempt to re-write the body 834 of resources with an application/link-format media type, mapping the 835 embedded CoAP URIs to their HTTP counterparts. Some potential issues 836 with this approach are: 838 1. The client may be interested to retrieve original (unaltered) 839 CoAP payloads through the HC proxy, not modified versions. 841 2. Tampering with payloads is incompatible with resources that are 842 integrity protected (although this is a problem with transcoding 843 in general). 845 3. The HC proxy needs to fully understand [RFC6690] syntax and 846 semantics, otherwise there is an inherent risk to corrupt the 847 payloads. 849 Therefore, CoRE Link Format payload should only be transcoded at the 850 risk and discretion of the proxy implementer. 852 6.5.3. Diagnostic Messages 854 CoAP responses may, in certain error cases, contain a diagnostic 855 message in the payload explaining the error situation, as described 856 in Section 5.5.2 of [RFC7252]. If present, the CoAP response 857 diagnostic payload SHOULD be copied in the HTTP response body. The 858 CoAP diagnostic message MUST NOT be copied into the HTTP reason- 859 phrase, since it potentially contains CR-LF characters which are 860 incompatible with HTTP reason-phrase syntax. 862 7. Response Code Mapping 864 Table 2 defines the HTTP response status codes to which each CoAP 865 response code SHOULD be mapped. Multiple appearances of a HTTP 866 status code in the second column indicates multiple equivalent HTTP 867 responses are possible based on the same CoAP response code, 868 depending on the conditions cited in the Notes (third column and text 869 below table). 871 +-----------------------------+-----------------------------+-------+ 872 | CoAP Response Code | HTTP Status Code | Notes | 873 +-----------------------------+-----------------------------+-------+ 874 | 2.01 Created | 201 Created | 1 | 875 | 2.02 Deleted | 200 OK | 2 | 876 | | 204 No Content | 2 | 877 | 2.03 Valid | 304 Not Modified | 3 | 878 | | 200 OK | 4 | 879 | 2.04 Changed | 200 OK | 2 | 880 | | 204 No Content | 2 | 881 | 2.05 Content | 200 OK | | 882 | 4.00 Bad Request | 400 Bad Request | | 883 | 4.01 Unauthorized | 403 Forbidden | 5 | 884 | 4.02 Bad Option | 400 Bad Request | 6 | 885 | 4.02 Bad Option | 500 Internal Server Error | 6 | 886 | 4.03 Forbidden | 403 Forbidden | | 887 | 4.04 Not Found | 404 Not Found | | 888 | 4.05 Method Not Allowed | 400 Bad Request | 7 | 889 | 4.06 Not Acceptable | 406 Not Acceptable | | 890 | 4.12 Precondition Failed | 412 Precondition Failed | | 891 | 4.13 Request Ent. Too Large | 413 Request Repr. Too Large | | 892 | 4.15 Unsupported Media Type | 415 Unsupported Media Type | | 893 | 5.00 Internal Server Error | 500 Internal Server Error | | 894 | 5.01 Not Implemented | 501 Not Implemented | | 895 | 5.02 Bad Gateway | 502 Bad Gateway | | 896 | 5.03 Service Unavailable | 503 Service Unavailable | 8 | 897 | 5.04 Gateway Timeout | 504 Gateway Timeout | | 898 | 5.05 Proxying Not Supported | 502 Bad Gateway | 9 | 899 +-----------------------------+-----------------------------+-------+ 901 Table 2: CoAP-HTTP Response Code Mappings 903 Notes: 905 1. A CoAP server may return an arbitrary format payload along with 906 this response. If present, this payload MUST be returned as 907 entity in the HTTP 201 response. Section 7.3.2 of [RFC7231] does 908 not put any requirement on the format of the entity. (In the 909 past, [RFC2616] did.) 911 2. The HTTP code is 200 or 204 respectively for the case that a CoAP 912 server returns a payload or not. [RFC7231] Section 5.3 requires 913 code 200 in case a representation of the action result is 914 returned for DELETE/POST/PUT, and code 204 if not. Hence, a 915 proxy MUST transfer any CoAP payload contained in a CoAP 2.02 916 response to the HTTP client using a 200 OK response. 918 3. HTTP code 304 (Not Modified) is sent if the HTTP client performed 919 a conditional HTTP request and the CoAP server responded with 920 2.03 (Valid) to the corresponding CoAP validation request. Note 921 that Section 4.1 of [RFC7232] puts some requirements on header 922 fields that must be present in the HTTP 304 response. 924 4. A 200 response to a CoAP 2.03 occurs only when the HC proxy, for 925 efficiency reasons, is running a local cache. An unconditional 926 HTTP GET which produces a cache-hit, could trigger a re- 927 validation (i.e. a conditional GET) on the CoAP side. The proxy 928 receiving 2.03 updates the freshness of its cached representation 929 and returns it to the HTTP client. 931 5. A HTTP 401 Unauthorized (Section 3.1 of [RFC7235]) response is 932 not applicable because there is no equivalent in CoAP of WWW- 933 Authenticate which is mandatory in a HTTP 401 response. 935 6. If the proxy has a way to determine that the Bad Option is due to 936 the straightforward mapping of a client request header into a 937 CoAP option, then returning HTTP 400 (Bad Request) is 938 appropriate. In all other cases, the proxy MUST return HTTP 500 939 (Internal Server Error) stating its inability to provide a 940 suitable translation to the client's request. 942 7. A CoAP 4.05 (Method Not Allowed) response SHOULD normally be 943 mapped to a HTTP 400 (Bad Request) code, because the HTTP 405 944 response would require specifying the supported methods - which 945 are generally unknown. In this case the HC Proxy SHOULD also 946 return a HTTP reason-phrase in the HTTP status line that starts 947 with the string "CoAP server returned 4.05" in order to 948 facilitate troubleshooting. However, if the HC proxy has more 949 granular information about the supported methods for the 950 requested resource (e.g. via a Resource Directory 951 ([I-D.ietf-core-resource-directory])) then it MAY send back a 952 HTTP 405 (Method Not Allowed) with a properly filled in "Allow" 953 response-header field (Section 7.4.1 of [RFC7231]). 955 8. The value of the HTTP "Retry-After" response-header field is 956 taken from the value of the CoAP Max-Age Option, if present. 958 9. This CoAP response can only happen if the proxy itself is 959 configured to use a CoAP forward-proxy (Section 5.7 of [RFC7252]) 960 to execute some, or all, of its CoAP requests. 962 8. Additional Mapping Guidelines 964 8.1. Caching and Congestion Control 966 A HC proxy should cache CoAP responses, and reply whenever applicable 967 with a cached representation of the requested resource. 969 If the HTTP client drops the connection after the HTTP request was 970 made, a HC proxy should wait for the associated CoAP response and 971 cache it if possible. Subsequent requests to the HC proxy for the 972 same resource can use the result present in cache, or, if a response 973 has still to come, the HTTP requests will wait on the open CoAP 974 request. 976 According to [RFC7252], a proxy must limit the number of outstanding 977 requests to a given CoAP server to NSTART. To limit the amount of 978 aggregate traffic to a constrained network, the HC proxy should also 979 put a limit on the number of concurrent CoAP requests pending on the 980 same constrained network; further incoming requests may either be 981 queued or dropped (returning 503 Service Unavailable). This limit 982 and the proxy queueing/dropping behavior should be configurable. 984 Highly volatile resources that are being frequently requested may be 985 observed [RFC7641] by the HC proxy to keep their cached 986 representation fresh while minimizing the amount of CoAP traffic in 987 the constrained network. See Section 8.2. 989 8.2. Cache Refresh via Observe 991 There are cases where using the CoAP observe protocol [RFC7641] to 992 handle proxy cache refresh is preferable to the validation mechanism 993 based on ETag as defined in [RFC7252]. Such scenarios include, but 994 are not limited to, sleepy CoAP nodes -- with possibly high variance 995 in requests' distribution -- which would greatly benefit from a 996 server driven cache update mechanism. Ideal candidates for CoAP 997 observe are also crowded or very low throughput networks, where 998 reduction of the total number of exchanged messages is an important 999 requirement. 1001 This subsection aims at providing a practical evaluation method to 1002 decide whether refreshing a cached resource R is more efficiently 1003 handled via ETag validation or by establishing an observation on R. 1005 Let T_R be the mean time between two client requests to resource R, 1006 let T_C be the mean time between two representation changes of R, and 1007 let M_R be the mean number of CoAP messages per second exchanged to 1008 and from resource R. If we assume that the initial cost for 1009 establishing the observation is negligible, an observation on R 1010 reduces M_R if and only if T_R < 2*T_C with respect to using ETag 1011 validation, that is if and only if the mean arrival rate of requests 1012 for resource R is greater than half the change rate of R. 1014 When observing the resource R, M_R is always upper bounded by 2/T_C. 1016 8.3. Use of CoAP Blockwise Transfer 1018 A HC proxy SHOULD support CoAP blockwise transfers 1019 [I-D.ietf-core-block] to allow transport of large CoAP payloads while 1020 avoiding excessive link-layer fragmentation in constrained networks, 1021 and to cope with small datagram buffers in CoAP end-points as 1022 described in [RFC7252] Section 4.6. 1024 A HC proxy SHOULD attempt to retry a payload-carrying CoAP PUT or 1025 POST request with blockwise transfer if the destination CoAP server 1026 responded with 4.13 (Request Entity Too Large) to the original 1027 request. A HC proxy SHOULD attempt to use blockwise transfer when 1028 sending a CoAP PUT or POST request message that is larger than 1029 BLOCKWISE_THRESHOLD bytes. The value of BLOCKWISE_THRESHOLD is 1030 implementation-specific, for example it can be: 1032 o calculated based on a known or typical UDP datagram buffer size 1033 for CoAP end-points, or 1035 o set to N times the known size of a link-layer frame in a 1036 constrained network where e.g. N=5, or 1038 o preset to a known IP MTU value, or 1040 o set to a known Path MTU value. 1042 The value BLOCKWISE_THRESHOLD, or the parameters from which it is 1043 calculated, should be configurable in a proxy implementation. The 1044 maximum block size the proxy will attempt to use in CoAP requests 1045 should also be configurable. 1047 The HC proxy SHOULD detect CoAP end-points not supporting blockwise 1048 transfers. This can be done by checking for a 4.02 (Bad Option) 1049 response returned by an end-point in response to a CoAP request with 1050 a Block* Option, and subsequent absence of the 4.02 in response to 1051 the same request without Block* Options. This allows the HC proxy to 1052 be more efficient, not attempting repeated blockwise transfers to 1053 CoAP servers that do not support it. 1055 8.4. CoAP Multicast 1057 A HC proxy MAY support CoAP multicast. If it does, the HC proxy 1058 sends out a multicast CoAP request if the Target CoAP URI's authority 1059 is a multicast IP literal or resolves to a multicast IP address. If 1060 the HC proxy does not support CoAP multicast, it SHOULD respond 403 1061 (Forbidden) to any valid HTTP request that maps to a CoAP multicast 1062 request. 1064 Details related to supporting CoAP multicast are currently out of 1065 scope of this document since in a proxy scenario a HTTP client 1066 typically expects to receive a single response, not multiple. 1067 However, a HC proxy that implements CoAP multicast may include 1068 application-specific functions to aggregate multiple CoAP responses 1069 into a single HTTP response. We suggest using the "application/http" 1070 internet media type (Section 8.3.2 of [RFC7230]) to enclose a set of 1071 one or more HTTP response messages, each representing the mapping of 1072 one CoAP response. 1074 For further considerations related to the handling of multicast 1075 requests, see Section 10.1. 1077 8.5. Timeouts 1079 If the CoAP server takes a long time in responding, the HTTP client 1080 or any other proxy in between may timeout. Further discussion of 1081 timeouts in HTTP is available in Section 6.2.4 of [RFC7230]. 1083 A HC proxy MUST define an internal timeout for each pending CoAP 1084 request, because the CoAP server may silently die before completing 1085 the request. Assuming the Proxy uses confirmable CoAP requests, such 1086 timeout value T SHOULD be at least 1088 T = MAX_RTT + MAX_SERVER_RESPONSE_DELAY 1090 where MAX_RTT is defined in [RFC7252] and MAX_SERVER_RESPONSE_DELAY 1091 is defined in [RFC7390]. 1093 9. IANA Considerations 1095 9.1. New 'core.hc' Resource Type 1097 This document registers a new Resource Type (rt=) Link Target 1098 Attribute, 'core.hc', in the "Resource Type (rt=) Link Target 1099 Attribute Values" subregistry under the "Constrained RESTful 1100 Environments (CoRE) Parameters" registry. 1102 Attribute Value: core.hc 1103 Description: HTTP to CoAP mapping base resource. 1105 Reference: See Section 5.5. 1107 9.2. New 'coap-payload' Internet Media Type 1109 This document defines the "application/coap-payload" media type with 1110 a single parameter "cf". This media type represents any payload that 1111 a CoAP message can carry, having a content format that can be 1112 identified by a CoAP Content-Format parameter (an integer in range 1113 0-65535). The parameter "cf" is the integer defining the CoAP 1114 content format. 1116 Type name: application 1118 Subtype name: coap-payload 1120 Required parameters: 1122 cf - CoAP Content-Format integer in range 0-65535 denoting the 1123 content format of the CoAP payload carried. 1125 Optional parameters: None 1127 Encoding considerations: 1129 The specific CoAP content format encoding considerations for the 1130 selected Content-Format (cf parameter) apply. 1132 Security considerations: 1134 The specific CoAP content format security considerations for the 1135 selected Content-Format (cf parameter) apply. 1137 Interoperability considerations: 1139 Published specification: (this I-D - TBD) 1141 Applications that use this media type: 1143 HTTP-to-CoAP Proxies. 1145 Fragment identifier considerations: N/A 1147 Additional information: 1149 Deprecated alias names for this type: N/A 1150 Magic number(s): N/A 1152 File extension(s): N/A 1154 Macintosh file type code(s): N/A 1156 Person and email address to contact for further information: 1158 Esko Dijk ("esko@ieee.org") 1160 Intended usage: COMMON 1162 Restrictions on usage: 1164 An application (or user) can only use this media type if it has to 1165 represent a CoAP payload of which the specified CoAP Content-Format 1166 is an unrecognized number; such that a proper translation directly to 1167 the equivalent HTTP media type is not possible. 1169 Author: CoRE WG 1171 Change controller: IETF 1173 Provisional registration? (standards tree only): N/A 1175 10. Security Considerations 1177 The security concerns raised in Section 9.2 of [RFC7230] also apply 1178 to the HC proxy scenario. 1180 A HC proxy deployed at the boundary of a constrained network is an 1181 easy single point of failure for reducing availability. As such, 1182 special care should be taken in designing, developing and operating 1183 it, keeping in mind that, in most cases, it has fewer limitations 1184 than the constrained devices it is serving. 1186 The following sub paragraphs categorize and discuss a set of specific 1187 security issues related to the translation, caching and forwarding 1188 functionality exposed by a HC proxy. 1190 10.1. Multicast 1192 Multicast requests impose a non trivial cost on the constrained 1193 network and endpoints, and might be exploited as a DoS attack vector 1194 (see also Section 10.2). From a privacy perspective, they can be 1195 used to gather detailed information about the resources hosted in the 1196 constrained network. For these reasons, it is RECOMMENDED that 1197 requests to multicast resources are access controlled with a default- 1198 deny policy. It is RECOMMENDED that the requestor of a multicast 1199 resource is strongly authenticated. If privacy is a concern, for 1200 example whenever the HTTP request transits through the public 1201 Internet, the request SHOULD be transported over a mutually 1202 authenticated and encrypted TLS connection. 1204 10.2. Traffic Overflow 1206 Due to the typically constrained nature of CoAP nodes, particular 1207 attention should be given to the implementation of traffic reduction 1208 mechanisms (see Section 8.1), because inefficient proxy 1209 implementations can be targeted by unconstrained Internet attackers. 1210 Bandwidth or complexity involved in such attacks is very low. 1212 An amplification attack to the constrained network may be triggered 1213 by a multicast request generated by a single HTTP request which is 1214 mapped to a CoAP multicast resource, as discussed in Section 11.3 of 1215 [RFC7252]. 1217 The risk likelihood of this amplification technique is higher than an 1218 amplification attack carried out by a malicious constrained device 1219 (e.g. ICMPv6 flooding, like Packet Too Big, or Parameter Problem on 1220 a multicast destination [RFC4732]), since it does not require direct 1221 access to the constrained network. 1223 The feasibility of this attack which disrupts availability of the 1224 targeted CoAP server can be limited by access controlling the exposed 1225 multicast resources, so that only known/authorized users can access 1226 such URIs. 1228 10.3. Handling Secured Exchanges 1230 An HTTP request can be sent to the HC proxy over a secured 1231 connection. However, there may not always exist a secure connection 1232 mapping to CoAP. For example, a secure distribution method for 1233 multicast traffic is complex and may not be implemented (see 1234 [RFC7390]). 1236 A HC proxy should implement rules for security context translations. 1237 For example all "https" unicast requests are translated to "coaps" 1238 requests, or "https" requests are translated to unsecured "coap" 1239 requests. Another rule could specify the security policy and 1240 parameters used for DTLS connections. Such rules will largely depend 1241 on the application and network context in which the HC proxy 1242 operates. These rules should be configurable. 1244 It is RECOMMENDED that, by default, accessing a "coaps" URI is only 1245 allowed from a corresponding "https" URI. 1247 By default, a HC proxy SHOULD reject any secured client request if 1248 there is no configured security policy mapping. This recommendation 1249 may be relaxed in case the destination network is believed to be 1250 secured by other means. Assuming that CoAP nodes are isolated behind 1251 a firewall as in the HC proxy deployment shown in Figure 1, the HC 1252 proxy may be configured to translate the incoming HTTPS request using 1253 plain CoAP (NoSec mode). 1255 10.4. URI Mapping 1257 The following risks related to the URI mapping described in Section 5 1258 and its use by HC proxies have been identified: 1260 DoS attack on the constrained/CoAP network. 1261 Mitigation: by default deny any Target CoAP URI whose authority is 1262 (or maps to) a multicast address. Then explicitly white-list 1263 multicast resources/authorities that are allowed to be de- 1264 referenced. See also Section 8.4. 1266 Leaking information on the constrained/CoAP network resources and 1267 topology. 1268 Mitigation: by default deny any Target CoAP URI (especially 1269 /.well-known/core is a resource to be protected), and then 1270 explicitly white-list resources that are allowed to be seen from 1271 outside. 1273 The internal CoAP Target resource is totally transparent from 1274 outside. 1275 Mitigation: implement a HTTPS-only interface, which makes the 1276 Target CoAP URI totally opaque to a passive attacker. 1278 11. Acknowledgements 1280 An initial version of Table 2 in Section 7 has been provided in 1281 revision -05 of the CoRE CoAP I-D. Special thanks to Peter van der 1282 Stok for countless comments and discussions on this document, that 1283 contributed to its current structure and text. 1285 Thanks to Carsten Bormann, Zach Shelby, Michele Rossi, Nicola Bui, 1286 Michele Zorzi, Klaus Hartke, Cullen Jennings, Kepeng Li, Brian Frank, 1287 Peter Saint-Andre, Kerry Lynn, Linyi Tian, Dorothy Gellert, Francesco 1288 Corazza, Hannes Tschofenig, Jaime Jimenez, Abhijan Bhattacharyya and 1289 Christian Groves for helpful comments and discussions that have 1290 shaped the document. 1292 The research leading to these results has received funding from the 1293 European Community's Seventh Framework Programme [FP7/2007-2013] 1294 under grant agreement n.251557. 1296 12. References 1298 12.1. Normative References 1300 [I-D.ietf-core-block] 1301 Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP", 1302 draft-ietf-core-block-20 (work in progress), April 2016. 1304 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1305 Requirement Levels", BCP 14, RFC 2119, 1306 DOI 10.17487/RFC2119, March 1997, 1307 . 1309 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1310 Resource Identifier (URI): Generic Syntax", STD 66, 1311 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1312 . 1314 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1315 Specifications: ABNF", STD 68, RFC 5234, 1316 DOI 10.17487/RFC5234, January 2008, 1317 . 1319 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 1320 and D. Orchard, "URI Template", RFC 6570, 1321 DOI 10.17487/RFC6570, March 2012, 1322 . 1324 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 1325 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 1326 . 1328 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1329 Protocol (HTTP/1.1): Message Syntax and Routing", 1330 RFC 7230, DOI 10.17487/RFC7230, June 2014, 1331 . 1333 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1334 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 1335 DOI 10.17487/RFC7231, June 2014, 1336 . 1338 [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1339 Protocol (HTTP/1.1): Conditional Requests", RFC 7232, 1340 DOI 10.17487/RFC7232, June 2014, 1341 . 1343 [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1344 Protocol (HTTP/1.1): Authentication", RFC 7235, 1345 DOI 10.17487/RFC7235, June 2014, 1346 . 1348 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1349 Application Protocol (CoAP)", RFC 7252, 1350 DOI 10.17487/RFC7252, June 2014, 1351 . 1353 [RFC7641] Hartke, K., "Observing Resources in the Constrained 1354 Application Protocol (CoAP)", RFC 7641, 1355 DOI 10.17487/RFC7641, September 2015, 1356 . 1358 12.2. Informative References 1360 [I-D.ietf-core-links-json] 1361 Li, K., Rahman, A., and C. Bormann, "Representing CoRE 1362 Formats in JSON and CBOR", draft-ietf-core-links-json-05 1363 (work in progress), April 2016. 1365 [I-D.ietf-core-resource-directory] 1366 Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE 1367 Resource Directory", draft-ietf-core-resource-directory-07 1368 (work in progress), March 2016. 1370 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1371 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1372 Transfer Protocol -- HTTP/1.1", RFC 2616, 1373 DOI 10.17487/RFC2616, June 1999, 1374 . 1376 [RFC3040] Cooper, I., Melve, I., and G. Tomlinson, "Internet Web 1377 Replication and Caching Taxonomy", RFC 3040, 1378 DOI 10.17487/RFC3040, January 2001, 1379 . 1381 [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet 1382 Denial-of-Service Considerations", RFC 4732, 1383 DOI 10.17487/RFC4732, December 2006, 1384 . 1386 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object 1387 Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, 1388 October 2013, . 1390 [RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for 1391 the Constrained Application Protocol (CoAP)", RFC 7390, 1392 DOI 10.17487/RFC7390, October 2014, 1393 . 1395 Appendix A. Change Log 1397 [Note to RFC Editor: Please remove this section before publication.] 1399 Changes from ietf-11 to ietf-12 (2nd WGLC): 1401 o Addressed a few editorial issues (including a clarification on 1402 when to use qq vs q in the URI mapping template). 1404 o Fixed missing slash in one template example. 1406 o Added para about the need for future CoAP protocol elements to 1407 define their own HTTP mappings. 1409 Changes from ietf-10 to ietf-11 (Chair review): 1411 o Removed cu/su distinction from the URI mapping template. 1413 o Addressed a few editorial issues. 1415 Changes from ietf-09 to ietf-10: 1417 o Addressed Ticket #401 - Clarified that draft covers not only 1418 Reverse HC Proxy but that many parts also apply to Forward and 1419 Interception Proxies. 1421 o Clarified that draft concentrates on the HTTP-to-CoAP mapping 1422 direction (i.e. the HC proxy is a HTTP server and a CoAP client). 1424 o Clarified the "null mapping" case where no CoAP URI information is 1425 embedded in the HTTP request URI. 1427 o Moved multicast related security text to the "Security 1428 Considerations" to consolidate all security information in one 1429 location. 1431 o Removed references to "placement" of proxy (e.g. server-side vs 1432 client-side) as is confusing and provides little added value. 1434 o Fixed version numbers on references that were corrupted in last 1435 revision due to outdated xml2rfc conversion tool local cache. 1437 o Various editorial improvements. 1439 Changes from ietf-08 to ietf-09: 1441 o Clean up requirements language as per Klaus' comment. 1443 Changes from ietf-07 to ietf-08: 1445 o Addressed WGLC review comments from Klaus Hartke as per the 1446 correspondence of March 9, 2016 on the CORE WG mailing list. 1448 Changes from ietf-06 to ietf-07: 1450 o Addressed Ticket #384 - Section 5.4.1 describes briefly 1451 (informative) how to discover CoAP resources from an HTTP client. 1453 o Addressed Ticket #378 - For HTTP media type to CoAP content format 1454 mapping and vice versa: a new draft (TBD) may be proposed in CoRE 1455 which describes an approach for automatic updating of the media 1456 type mapping. This was noted in Section 6.1 but is otherwise 1457 outside the scope of this draft. 1459 o Addressed Ticket #377 - Added IANA section that defines a new HTTP 1460 media type "application/coap-payload" and created new Section 6.2 1461 on how to use it. 1463 o Addressed Ticket #376 - Updated Table 2 (and corresponding note 7) 1464 to indicate that a CoAP 4.05 (Method Not Allowed) Response Code 1465 should be mapped to a HTTP 400 (Bad Request). 1467 o Added note to comply to ABNF when translating CoAP diagnostic 1468 payload to reason-phrase in Section 6.5.3. 1470 Changes from ietf-05 to ietf-06: 1472 o Fully restructured the draft, bringing introductory text more to 1473 the front and allocating main sections to each of the key topics; 1474 addressing Ticket #379; 1476 o Addressed Ticket #382, fix of enhanced form URI template 1477 definition of q in Section 5.3.2; 1479 o Addressed Ticket #381, found a mapping 4.01 to 401 Unauthorized in 1480 Section 7; 1482 o Addressed Ticket #380 (Add IANA registration for "core.hc" 1483 Resource Type) in Section 9; 1485 o Addressed Ticket #376 (CoAP 4.05 response can't be translated to 1486 HTTP 405 by HC proxy) in Section 7 by use of empty 'Allow' header; 1488 o Removed details on the pros and cons of HC proxy placement 1489 options; 1491 o Addressed review comments of Carsten Bormann; 1493 o Clarified failure in mapping of HTTP Accept headers (Section 6.3); 1495 o Clarified detection of CoAP servers not supporting blockwise 1496 (Section 8.3); 1498 o Changed CoAP request timeout min value to MAX_RTT + 1499 MAX_SERVER_RESPONSE_DELAY (Section 8.6); 1501 o Added security section item (Section 10.3) related to use of CoAP 1502 blockwise transfers; 1504 o Many editorial improvements. 1506 Changes from ietf-04 to ietf-05: 1508 o Addressed Ticket #366 (Mapping of CoRE Link Format payloads to be 1509 valid in HTTP Domain?) in Section 6.3.3.2 (Content Transcoding - 1510 CORE Link Format); 1512 o Addressed Ticket #375 (Add requirement on mapping of CoAP 1513 diagnostic payload) in Section 6.3.3.3 (Content Transcoding - 1514 Diagnostic Messages); 1516 o Addressed comment from Yusuke (http://www.ietf.org/mail- 1517 archive/web/core/current/msg05491.html) in Section 6.3.3.1 1518 (Content Transcoding - General); 1520 o Various editorial improvements. 1522 Changes from ietf-03 to ietf-04: 1524 o Expanded use case descriptions in Section 4; 1526 o Fixed/enhanced discovery examples in Section 5.4.1; 1528 o Addressed Ticket #365 (Add text on media type conversion by HTTP- 1529 CoAP proxy) in new Section 6.3.1 (Generalized media type mapping) 1530 and new Section 6.3.2 (Content translation); 1532 o Updated HTTPBis WG draft references to recently published RFC 1533 numbers. 1535 o Various editorial improvements. 1537 Changes from ietf-02 to ietf-03: 1539 o Closed Ticket #351 "Add security implications of proposed default 1540 HTTP-CoAP URI mapping"; 1542 o Closed Ticket #363 "Remove CoAP scheme in default HTTP-CoAP URI 1543 mapping"; 1545 o Closed Ticket #364 "Add discovery of HTTP-CoAP mapping 1546 resource(s)". 1548 Changes from ietf-01 to ietf-02: 1550 o Selection of single default URI mapping proposal as proposed to WG 1551 mailing list 2013-10-09. 1553 Changes from ietf-00 to ietf-01: 1555 o Added URI mapping proposals to Section 4 as per the Email 1556 proposals to WG mailing list from Esko. 1558 Authors' Addresses 1560 Angelo P. Castellani 1561 University of Padova 1562 Via Gradenigo 6/B 1563 Padova 35131 1564 Italy 1566 Email: angelo@castellani.net 1568 Salvatore Loreto 1569 Ericsson 1570 Hirsalantie 11 1571 Jorvas 02420 1572 Finland 1574 Email: salvatore.loreto@ericsson.com 1575 Akbar Rahman 1576 InterDigital Communications, LLC 1577 1000 Sherbrooke Street West 1578 Montreal H3A 3G4 1579 Canada 1581 Phone: +1 514 585 0761 1582 Email: Akbar.Rahman@InterDigital.com 1584 Thomas Fossati 1585 Nokia 1586 3 Ely Road 1587 Milton, Cambridge CB24 6DD 1588 UK 1590 Email: thomas.fossati@nokia.com 1592 Esko Dijk 1593 Philips Lighting 1594 High Tech Campus 34 1595 Eindhoven 5656 AE 1596 The Netherlands 1598 Email: esko.dijk@philips.com