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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: March 12, 2017 Ericsson 6 A. Rahman 7 InterDigital Communications, LLC 8 T. Fossati 9 Nokia 10 E. Dijk 11 Philips Lighting 12 September 8, 2016 14 Guidelines for HTTP-to-CoAP Mapping Implementations 15 draft-ietf-core-http-mapping-14 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 CoAP (Constrained Application Protocol). This will 22 enable a HTTP client to access resources on a CoAP server through the 23 proxy. This document describes how a HTTP request is mapped to a 24 CoAP request, and then how a CoAP response is mapped back to a HTTP 25 response. This includes guidelines for URI mapping, media type 26 mapping and additional proxy implementation issues. This document 27 covers the Reverse, Forward and Interception cross-protocol proxy 28 cases. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on March 12, 2017. 47 Copyright Notice 49 Copyright (c) 2016 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (http://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 65 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 3. HTTP-to-CoAP Proxy . . . . . . . . . . . . . . . . . . . . . 5 67 4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6 68 5. URI Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 7 69 5.1. URI Terminology . . . . . . . . . . . . . . . . . . . . . 8 70 5.2. Null Mapping . . . . . . . . . . . . . . . . . . . . . . 8 71 5.3. Default Mapping . . . . . . . . . . . . . . . . . . . . . 8 72 5.3.1. Optional Scheme Omission . . . . . . . . . . . . . . 9 73 5.3.2. Encoding Caveats . . . . . . . . . . . . . . . . . . 9 74 5.4. URI Mapping Template . . . . . . . . . . . . . . . . . . 10 75 5.4.1. Simple Form . . . . . . . . . . . . . . . . . . . . . 10 76 5.4.2. Enhanced Form . . . . . . . . . . . . . . . . . . . . 11 77 5.5. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 13 78 5.5.1. Examples . . . . . . . . . . . . . . . . . . . . . . 13 79 6. Media Type Mapping . . . . . . . . . . . . . . . . . . . . . 15 80 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15 81 6.2. 'application/coap-payload' Media Type . . . . . . . . . . 16 82 6.3. Loose Media Type Mapping . . . . . . . . . . . . . . . . 17 83 6.4. Media Type to Content Format Mapping Algorithm . . . . . 18 84 6.5. Content Transcoding . . . . . . . . . . . . . . . . . . . 19 85 6.5.1. General . . . . . . . . . . . . . . . . . . . . . . . 19 86 6.5.2. CoRE Link Format . . . . . . . . . . . . . . . . . . 20 87 6.5.3. Diagnostic Messages . . . . . . . . . . . . . . . . . 20 88 7. Response Code Mapping . . . . . . . . . . . . . . . . . . . . 20 89 8. Additional Mapping Guidelines . . . . . . . . . . . . . . . . 23 90 8.1. Caching and Congestion Control . . . . . . . . . . . . . 23 91 8.2. Cache Refresh via Observe . . . . . . . . . . . . . . . . 23 92 8.3. Use of CoAP Blockwise Transfer . . . . . . . . . . . . . 24 93 8.4. CoAP Multicast . . . . . . . . . . . . . . . . . . . . . 25 94 8.5. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 25 96 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 97 9.1. New 'core.hc' Resource Type . . . . . . . . . . . . . . . 25 98 9.2. New 'coap-payload' Internet Media Type . . . . . . . . . 26 99 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27 100 10.1. Multicast . . . . . . . . . . . . . . . . . . . . . . . 27 101 10.2. Traffic Overflow . . . . . . . . . . . . . . . . . . . . 28 102 10.3. Handling Secured Exchanges . . . . . . . . . . . . . . . 28 103 10.4. URI Mapping . . . . . . . . . . . . . . . . . . . . . . 29 104 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29 105 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 106 12.1. Normative References . . . . . . . . . . . . . . . . . . 30 107 12.2. Informative References . . . . . . . . . . . . . . . . . 31 108 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 32 109 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36 111 1. Introduction 113 CoAP (Constrained Application Protocol) [RFC7252] has been designed 114 with the twofold aim to be an application protocol specialized for 115 constrained environments and to be easily used in Representational 116 State Transfer (REST) based architectures such as the Web. The 117 latter goal has led to defining CoAP to easily interoperate with HTTP 118 [RFC7230] through an intermediary proxy which performs cross-protocol 119 conversion. 121 Section 10 of [RFC7252] describes the fundamentals of the CoAP-to- 122 HTTP and the HTTP-to-CoAP cross-protocol mapping process. However, 123 [RFC7252] focuses on the basic mapping of request methods and simple 124 response code mapping between HTTP and CoAP, and it leaves many 125 details of the cross-protocol proxy for future definition. 126 Therefore, a primary goal of this informational document is to define 127 a consistent set of guidelines that an HTTP-to-CoAP proxy 128 implementation should adhere to. The key benefit to adhering to such 129 guidelines is to reduce variation between proxy implementations, 130 thereby increasing interoperability between an HTTP client and a CoAP 131 server independent of the proxy that implements the cross-protocol 132 mapping. (For example, a proxy conforming to these guidelines made 133 by vendor A can be easily replaced by a proxy from vendor B that also 134 conforms to the guidelines.) 136 This document describes HTTP mappings that apply to protocol elements 137 defined in the base CoAP specification [RFC7252]. It is up to CoAP 138 protocol extensions (new methods, response codes, options, content- 139 formats) to describe their own HTTP mappings, if applicable. 141 This document is organized as follows: 143 o Section 2 defines proxy terminology; 144 o Section 3 introduces the HTTP-to-CoAP proxy; 146 o Section 4 lists use cases in which HTTP clients need to contact 147 CoAP servers; 149 o Section 5 introduces a null, default and advanced HTTP-to-CoAP URI 150 mapping syntax; 152 o Section 6 describes how to map HTTP media types to CoAP content 153 formats and vice versa; 155 o Section 7 describes how to map CoAP responses to HTTP responses; 157 o Section 8 describes additional mapping guidelines related to 158 caching, congestion, timeouts, etc.; 160 o Section 10 discusses possible security impact of HTTP-to-CoAP 161 protocol mapping. 163 2. Terminology 165 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 166 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 167 "OPTIONAL" in this document are to be interpreted as described in 168 [RFC2119]. 170 HC Proxy: a proxy performing a cross-protocol mapping, in the context 171 of this document an HTTP-to-CoAP (HC) mapping. Specifically, the HC 172 proxy acts as an HTTP server and a CoAP client. The HC Proxy can 173 take on the role of a Forward, Reverse or Interception Proxy. 175 Forward Proxy (or Forward HC Proxy): a message forwarding agent that 176 is selected by the HTTP client, usually via local configuration 177 rules, to receive requests for some type(s) of absolute URI and to 178 attempt to satisfy those requests via translation to the protocol 179 indicated by the absolute URI. The user decides (is willing) to use 180 the proxy as the forwarding/de-referencing agent for a predefined 181 subset of the URI space. In [RFC7230] this is called a Proxy. 182 [RFC7252] defines Forward-Proxy similarly. 184 Reverse Proxy (or Reverse HC Proxy): as in [RFC7230], a receiving 185 agent that acts as a layer above some other server(s) and translates 186 the received requests to the underlying server's protocol. A Reverse 187 HC Proxy behaves as an origin (HTTP) server on its connection from 188 the HTTP client. The HTTP client uses the "origin-form" 189 (Section 5.3.1 of [RFC7230]) as a request-target URI. 191 Interception Proxy (or Interception HC Proxy) [RFC3040]: a proxy that 192 receives inbound HTTP traffic flows through the process of traffic 193 redirection; transparent to the HTTP client. 195 Note that a Reverse Proxy appears to an HTTP client as an origin 196 server while a Forward Proxy does not. So, when communicating with a 197 Reverse Proxy a client may be unaware it is communicating with a 198 proxy at all. 200 3. HTTP-to-CoAP Proxy 202 A HC proxy is accessed by an HTTP client which wants to access a 203 resource on a CoAP server. The HC proxy handles the HTTP request by 204 mapping it to the equivalent CoAP request, which is then forwarded to 205 the appropriate CoAP server. The received CoAP response is then 206 mapped to an appropriate HTTP response and finally sent back to the 207 originating HTTP client. 209 See Figure 1 for an example deployment scenario. Here a HC proxy is 210 located at the boundary of the Constrained Network domain, to avoid 211 sending any HTTP traffic into the Constrained Network and to avoid 212 any (unsecured) CoAP multicast traffic outside the Constrained 213 Network. A DNS server (not shown) is used by the HTTP Client to 214 resolve the IP address of the HC proxy and optionally also used by 215 the HC proxy to resolve IP addresses of CoAP servers. 217 Constrained Network 218 .-------------------. 219 / .------. \ 220 / | CoAP | \ 221 / |server| \ 222 || '------' || 223 || || 224 .--------. HTTP Request .------------. CoAP Req .------. || 225 | HTTP |---------------->|HTTP-to-CoAP|----------->| CoAP | || 226 | Client |<----------------| Proxy |<-----------|Server| || 227 '--------' HTTP Response '------------' CoAP Resp '------' || 228 || || 229 || .------. || 230 || | CoAP | || 231 \ |server| .------. / 232 \ '------' | CoAP | / 233 \ |server| / 234 \ '------' / 235 '-----------------' 237 Figure 1: HTTP-To-CoAP Proxy Deployment Scenario 239 Normative requirements on the translation of HTTP requests to CoAP 240 requests and of the CoAP responses back to HTTP responses are defined 241 in Section 10.2 of [RFC7252]. However, [RFC7252] focuses on the 242 basic mapping of request methods and simple response code mapping 243 between HTTP and CoAP, and leaves many details of the cross-protocol 244 HC proxy for future definition. This document provides additional 245 guidelines and more details for the implementation of a HC Proxy, 246 which should be followed in addition to the normative requirements. 247 Note that the guidelines apply to all forms of an HC proxy (i.e., 248 Reverse, Forward, Intercepting) unless explicitly otherwise noted. 250 4. Use Cases 252 To illustrate the situations HTTP to CoAP protocol translation may be 253 used, three use cases are described below. 255 1. Legacy building control application without CoAP: A building 256 control application that uses HTTP but not CoAP can check the status 257 of CoAP sensors and/or control actuators via a HC proxy. 259 2. Making sensor data available to 3rd parties on the Web: For 260 demonstration or public interest purposes, a HC proxy may be 261 configured to expose the contents of a CoAP sensor to the world via 262 the web (HTTP and/or HTTPS). Some sensors may only accept secure 263 'coaps' requests, therefore the proxy is configured to translate 264 request to those devices accordingly. The HC proxy is furthermore 265 configured to only pass through GET requests in order to protect the 266 constrained network. 268 3. Smartphone and home sensor: A smartphone can access directly a 269 CoAP home sensor using a mutually authenticated 'https' request, 270 provided its home router runs a HC proxy and is configured with the 271 appropriate certificate. An HTML5 [W3C.REC-html5-20141028] 272 application on the smartphone can provide a friendly UI using the 273 standard (HTTP) networking functions of HTML5. 275 A key point in the above use cases is the expected nature of the URI 276 to be used by the HTTP client initiating the HTTP request to the HC 277 proxy. Specifically, in use case #1, there will be no "coap" or 278 "coaps" related information embedded in the HTTP URI as it is a 279 legacy HTTP client sending the request. Use case #2 is also expected 280 to be similar. In contrast, in use case #3, it is expected that the 281 HTTP client will specifically embed "coap" or "coaps" related 282 information in the HTTP URI of the HTTP request to the HC proxy. 284 5. URI Mapping 286 Though, in principle, a CoAP URI could be directly used by a HTTP 287 client to de-reference a CoAP resource through a HC proxy, the 288 reality is that all major web browsers, networking libraries and 289 command line tools do not allow making HTTP requests using URIs with 290 a scheme "coap" or "coaps". 292 Thus, there is a need for web applications to embed or "pack" a CoAP 293 URI into a HTTP URI so that it can be (non-destructively) transported 294 from the HTTP client to the HC proxy. The HC proxy can then "unpack" 295 the CoAP URI and finally de-reference it via a CoAP request to the 296 target Server. 298 URI Mapping is the term used in the document to describe the process 299 through which the URI of a CoAP resource is transformed into an HTTP 300 URI so that: 302 o the requesting HTTP client can handle it; 304 o the receiving HC proxy can extract the intended CoAP URI 305 unambiguously. 307 To this end, the remainder of this section will identify: 309 o the default mechanism to map a CoAP URI into a HTTP URI; 310 o the URI template format to express a class of CoAP-HTTP URI 311 mapping functions; 313 o the discovery mechanism based on CoRE Link Format [RFC6690] 314 through which clients of a HC proxy can dynamically discover 315 information about the supported URI Mapping Template(s), as well 316 as the URI where the HC proxy function is anchored. 318 5.1. URI Terminology 320 In the remainder of this section, the following terms will be used 321 with a distinctive meaning: 323 HC Proxy URI: 324 URI which refers to the HC proxy function. It conforms to 325 syntax defined in Section 2.7 of [RFC7230]. 327 Target CoAP URI: 328 URI which refers to the (final) CoAP resource that has to be 329 de-referenced. It conforms to syntax defined in Section 6 of 330 [RFC7252]. Specifically, its scheme is either "coap" or 331 "coaps". 333 Hosting HTTP URI: 334 URI that conforms to syntax in Section 2.7 of [RFC7230]. Its 335 authority component refers to a HC proxy, whereas path (and 336 query) component(s) embed the information used by a HC proxy 337 to extract the Target CoAP URI. 339 5.2. Null Mapping 341 The null mapping is the case where there is no Target CoAP URI 342 appended to the HC Proxy URI. In other words, it is a "pure" HTTP 343 URI that is sent to the HC Proxy. This would typically occur in 344 situations like Use Case #1 described in Section 4, and the Proxy 345 would typically be a Reverse Proxy. In this scenario, the HC Proxy 346 will determine through its own proprietary algorithms what the Target 347 CoAP URI should be. 349 5.3. Default Mapping 351 The default mapping is for the Target CoAP URI to be appended as-is 352 to the HC Proxy URI, to form the Hosting HTTP URI. This is the URI 353 that will then be sent by the HTTP client in the HTTP request to the 354 HC proxy. 356 For example: given a HC Proxy URI http://p.example.com/hc/ and a 357 Target CoAP URI coap://s.example.com/light, the resulting Hosting 358 HTTP URI would be http://p.example.com/hc/coap://s.example.com/light. 360 Provided a correct Target CoAP URI, the Hosting HTTP URI resulting 361 from the default mapping is always syntactically correct. 362 Furthermore, the Target CoAP URI can always be extracted 363 unambiguously from the Hosting HTTP URI. Also, it is worth noting 364 that, using the default mapping, a query component in the target CoAP 365 resource URI is naturally encoded into the query component of the 366 Hosting URI, e.g., coap://s.example.com/light?dim=5 becomes 367 http://p.example.com/hc/coap://s.example.com/light?dim=5. 369 There is no default for the HC Proxy URI. Therefore, it is either 370 known in advance, e.g., as a configuration preset, or dynamically 371 discovered using the mechanism described in Section 5.5. 373 The default URI mapping function SHOULD be implemented and activated 374 by default in a HC proxy, unless there are valid reasons, e.g., 375 application specific, to use a different mapping function. 377 5.3.1. Optional Scheme Omission 379 When found in a Hosting HTTP URI, the scheme (i.e., "coap" or 380 "coaps"), the scheme component delimiter (":"), and the double slash 381 ("//") preceding the authority MAY be omitted. In such case, a local 382 default - not defined by this document - applies. 384 So, http://p.example.com/hc/s.coap.example.com/foo could either 385 represent the target coap://s.coap.example.com/foo or 386 coaps://s.coap.example.com/foo depending on application specific 387 presets. 389 5.3.2. Encoding Caveats 391 When the authority of the Target CoAP URI is given as an IPv6address, 392 then the surrounding square brackets must be percent-encoded in the 393 Hosting HTTP URI, in order to comply with the syntax defined in 394 Section 3.3. of [RFC3986] for a URI path segment. E.g.: 395 coap://[2001:db8::1]/light?on becomes 396 http://p.example.com/hc/coap://%5B2001:db8::1%5D/light?on. 398 Everything else can be safely copied verbatim from the Target CoAP 399 URI to the Hosting HTTP URI. 401 5.4. URI Mapping Template 403 This section defines a format for the URI template [RFC6570] used by 404 a HC proxy to inform its clients about the expected syntax for the 405 Hosting HTTP URI. This will then be used by the HTTP client to 406 construct the URI to be sent in the HTTP request to the HC proxy. 408 When instantiated, an URI Mapping Template is always concatenated to 409 a HC Proxy URI provided by the HC proxy via discovery (see 410 Section 5.5), or by other means. 412 A simple form (Section 5.4.1) and an enhanced form (Section 5.4.2) 413 are provided to fit different users' requirements. 415 Both forms are expressed as level 2 URI templates [RFC6570] to take 416 care of the expansion of values that are allowed to include reserved 417 URI characters. The syntax of all URI formats is specified in this 418 section in Augmented Backus-Naur Form (ABNF) [RFC5234]. 420 5.4.1. Simple Form 422 The simple form MUST be used for mappings where the Target CoAP URI 423 is going to be copied (using rules of Section 5.3.2) at some fixed 424 position into the Hosting HTTP URI. 426 The "tu" template variable is intended to be used in a template 427 definition to represent a Target CoAP URI: 429 tu = [ ( "coap:" / "coaps:" ) "//" ] host [ ":" port ] path-abempty 430 [ "?" query ] 432 Note that the same considerations as in Section 5.3.1 apply, in that 433 the CoAP scheme may be omitted from the Hosting HTTP URI. 435 5.4.1.1. Examples 437 All the following examples (given as a specific URI mapping template, 438 a Target CoAP URI, and the produced Hosting HTTP URI) use 439 http://p.example.com/hc/ as the HC Proxy URI. Note that these 440 examples all define mapping templates that deviate from the default 441 template of Section 5.3 to be able to illustrate the use of the above 442 template variables. 444 1. Target CoAP URI is a query argument of the Hosting HTTP URI: 446 ?target_uri={+tu} 448 coap://s.example.com/light 450 http://p.example.com/hc/?target_uri=coap://s.example.com/light 452 or 454 coaps://s.example.com/light 456 http://p.example.com/hc/?target_uri=coaps://s.example.com/light 458 2. Target CoAP URI in the path component of the Hosting HTTP URI: 460 forward/{+tu} 462 coap://s.example.com/light 464 http://p.example.com/hc/forward/coap://s.example.com/light 466 or 468 coaps://s.example.com/light 470 http://p.example.com/hc/forward/coaps://s.example.com/light 472 3. "coap" URI is a query argument of the Hosting HTTP URI; client 473 decides to omit scheme because a default scheme is agreed 474 beforehand between client and proxy: 476 ?coap_uri={+tu} 478 coap://s.example.com/light 480 http://p.example.com/hc/?coap_uri=s.example.com/light 482 5.4.2. Enhanced Form 484 The enhanced form can be used to express more sophisticated mappings 485 of the Target CoAP URI into the Hosting HTTP URI, i.e., mappings that 486 do not fit into the simple form. 488 There MUST be at most one instance of each of the following template 489 variables in a template definition: 491 s = "coap" / "coaps" ; from [RFC7252], Sections 6.1 and 6.2 492 hp = host [":" port] ; from [RFC3986], Sections 3.2.2 and 3.2.3 493 p = path-abempty ; from [RFC3986], Section 3.3 494 q = query ; from [RFC3986], Section 3.4 495 qq = [ "?" query ] ; qq is empty if and only if 'query' is empty 497 The qq form is used when the path and the (optional) query components 498 are to be copied verbatim from the Target CoAP URI into the Hosting 499 HTTP URI, i.e., as "{+p}{+qq}". Instead, the q form is used when the 500 query and path are mapped as separate entities, e.g., as in 501 "coap_path={+p}&coap_query={+q}". 503 5.4.2.1. Examples 505 All the following examples (given as a specific URI mapping template, 506 a Target CoAP URI, and the produced Hosting HTTP URI) use 507 http://p.example.com/hc/ as the HC Proxy URI. 509 1. Target CoAP URI components in path segments, and optional query 510 in query component: 512 {+s}/{+hp}{+p}{+qq} 514 coap://s.example.com/light 516 http://p.example.com/hc/coap/s.example.com/light 518 or 520 coap://s.example.com/light?on 522 http://p.example.com/hc/coap/s.example.com/light?on 524 2. Target CoAP URI components split in individual query arguments: 526 ?s={+s}&hp={+hp}&p={+p}&q={+q} 528 coap://s.example.com/light 530 http://p.example.com/hc/?s=coap&hp=s.example.com&p=/light&q= 532 or 534 coaps://s.example.com/light?on 536 http://p.example.com/hc/?s=coaps&hp=s.example.com&p=/light&q=on 538 5.5. Discovery 540 In order to accommodate site specific needs while allowing third 541 parties to discover the proxy function, the HC proxy SHOULD publish 542 information related to the location and syntax of the HC proxy 543 function using the CoRE Link Format [RFC6690] interface. 545 To this aim a new Resource Type, "core.hc", is defined in this 546 document. It can be used as the value for the "rt" attribute in a 547 query to the /.well-known/core in order to locate the URI where the 548 HC proxy function is anchored, i.e., the HC Proxy URI. 550 Along with it, the new target attribute "hct" is defined in this 551 document. This attribute MAY be returned in a "core.hc" link to 552 provide the URI Mapping Template associated to the mapping resource. 553 The default template given in Section 5.3, i.e., {+tu}, MUST be 554 assumed if no "hct" attribute is found in the returned link. If a 555 "hct" attribute is present in the returned link, then a client MUST 556 use it to create the Hosting HTTP URI. 558 The URI mapping SHOULD be discoverable (as specified in [RFC6690]) on 559 both the HTTP and the CoAP side of the HC proxy, with one important 560 difference: on the CoAP side the link associated to the "core.hc" 561 resource needs an explicit anchor referring to the HTTP origin, while 562 on the HTTP interface the link context is already the HTTP origin 563 carried in the request's Host header, and doesn't have to be made 564 explicit. 566 5.5.1. Examples 568 o The first example exercises the CoAP interface, and assumes that 569 the default template, {+tu}, is used. For example, in use case #3 570 in section Section 4, the smartphone may discover the public HC 571 proxy before leaving the home network. Then when outside the home 572 network, the smartphone will be able to query the appropriate home 573 sensor. 575 Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc 577 Res: 2.05 Content 578 ;anchor="http://p.example.com";rt="core.hc" 580 o The second example - also on the CoAP side of the HC proxy - uses 581 a custom template, i.e., one where the CoAP URI is carried inside 582 the query component, thus the returned link carries the URI 583 template to be used in an explicit "hct" attribute: 585 Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc 587 Res: 2.05 Content 588 ;anchor="http://p.example.com"; 589 rt="core.hc";hct="?uri={+tu}" 591 On the HTTP side, link information can be serialized in more than one 592 way: 594 o using the 'application/link-format' content type: 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 601 Content-Length: 18 603 ;rt="core.hc" 605 o using the 'application/link-format+json' content type as defined 606 in [I-D.ietf-core-links-json]: 608 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 609 Host: p.example.com 611 Res: HTTP/1.1 200 OK 612 Content-Type: application/link-format+json 613 Content-Length: 31 615 [{"href":"/hc/","rt":"core.hc"}] 617 o using the Link header: 619 Req: GET /.well-known/core?rt=core.hc HTTP/1.1 620 Host: p.example.com 622 Res: HTTP/1.1 200 OK 623 Link: ;rt="core.hc" 625 6. Media Type Mapping 627 6.1. Overview 629 A HC proxy needs to translate HTTP media types (Section 3.1.1.1 of 630 [RFC7231]) and content encodings (Section 3.1.2.2 of [RFC7231]) into 631 CoAP content formats (Section 12.3 of [RFC7252]) and vice versa. 633 Media type translation can happen in GET, PUT or POST requests going 634 from HTTP to CoAP, and in 2.xx (i.e., successful) responses going 635 from CoAP to HTTP. Specifically, PUT and POST need to map both the 636 Content-Type and Content-Encoding HTTP headers into a single CoAP 637 Content-Format option, whereas GET needs to map Accept and Accept- 638 Encoding HTTP headers into a single CoAP Accept option. To generate 639 the HTTP response, the CoAP Content-Format option is mapped back to a 640 suitable HTTP Content-Type and Content-Encoding combination. 642 An HTTP request carrying a Content-Type and Content-Encoding 643 combination which the HC proxy is unable to map to an equivalent CoAP 644 Content-Format, SHALL elicit a 415 (Unsupported Media Type) response 645 by the HC proxy. 647 On the content negotiation side, failure to map Accept and Accept-* 648 headers SHOULD be silently ignored: the HC proxy SHOULD therefore 649 forward as a CoAP request with no Accept option. The HC proxy thus 650 disregards the Accept/Accept-* header fields by treating the response 651 as if it is not subject to content negotiation, as mentioned in 652 Sections 5.3.* of [RFC7231]. However, a HC proxy implementation is 653 free to attempt mapping a single Accept header in a GET request to 654 multiple CoAP GET requests, each with a single Accept option, which 655 are then tried in sequence until one succeeds. Note that an HTTP 656 Accept */* MUST be mapped to a CoAP request without Accept option. 658 While the CoAP to HTTP direction has always a well defined mapping 659 (with the exception examined in Section 6.2), the HTTP to CoAP 660 direction is more problematic because the source set, i.e., 661 potentially 1000+ IANA registered media types, is much bigger than 662 the destination set, i.e., the mere 6 values initially defined in 663 Section 12.3 of [RFC7252]. 665 Depending on the tight/loose coupling with the application(s) for 666 which it proxies, the HC proxy could implement different media type 667 mappings. 669 When tightly coupled, the HC proxy knows exactly which content 670 formats are supported by the applications, and can be strict when 671 enforcing its forwarding policies in general, and the media type 672 mapping in particular. 674 On the other side, when the HC proxy is a general purpose application 675 layer gateway, being too strict could significantly reduce the amount 676 of traffic that it would be able to successfully forward. In this 677 case, the "loose" media type mapping detailed in Section 6.3 MAY be 678 implemented. 680 The latter grants more evolution of the surrounding ecosystem, at the 681 cost of allowing more attack surface. In fact, as a result of such 682 strategy, payloads would be forwarded more liberally across the 683 unconstrained/constrained network boundary of the communication path. 684 Therefore, when applied, other forms of access control must be set in 685 place to avoid unauthorized users to deplete or abuse systems and 686 network resources. 688 6.2. 'application/coap-payload' Media Type 690 If the HC proxy receives a CoAP response with a Content-Format that 691 it does not recognize (e.g., because the value has been registered 692 after the proxy has been deployed, or the CoAP server uses an 693 experimental value which is not registered), then the HC proxy SHALL 694 return a generic "application/coap-payload" media type with numeric 695 parameter "cf" as defined in Section 9.2. 697 For example, the CoAP content format '60' ("application/cbor") would 698 be represented by "application/coap-payload;cf=60", if the HC Proxy 699 doesn't recognize the content format '60'. 701 A HTTP client may use the media type "application/coap-payload" as a 702 means to send a specific content format to a CoAP server via a HC 703 Proxy if the client has determined that the HC Proxy does not 704 directly support the type mapping it needs. This case may happen 705 when dealing for example with newly registered, yet to be registered, 706 or experimental CoAP content formats. 708 6.3. Loose Media Type Mapping 710 By structuring the type information in a super-class (e.g., "text") 711 followed by a finer grained sub-class (e.g., "html"), and optional 712 parameters (e.g., "charset=utf-8"), Internet media types provide a 713 rich and scalable framework for encoding the type of any given 714 entity. 716 This approach is not applicable to CoAP, where Content Formats 717 conflate an Internet media type (potentially with specific 718 parameters) and a content encoding into one small integer value. 720 To remedy this loss of flexibility, we introduce the concept of a 721 "loose" media type mapping, where media types that are 722 specializations of a more generic media type can be aliased to their 723 super-class and then mapped (if possible) to one of the CoAP content 724 formats. For example, "application/soap+xml" can be aliased to 725 "application/xml", which has a known conversion to CoAP. In the 726 context of this "loose" media type mapping, "application/octet- 727 stream" can be used as a fallback when no better alias is found for a 728 specific media type. 730 Table 1 defines the default lookup table for the "loose" media type 731 mapping. It is expected that an implementation can refine it either 732 given application-specific knowledge, or because new Content-Formats 733 are defined. Given an input media type, the table returns its best 734 generalized media type using the most specific match i.e., the table 735 entries are compared to the input in top to bottom order until an 736 entry matches. 738 +---------------------+--------------------------+ 739 | Internet media type | Generalized media type | 740 +---------------------+--------------------------+ 741 | application/*+xml | application/xml | 742 | application/*+json | application/json | 743 | text/xml | application/xml | 744 | text/* | text/plain | 745 | */* | application/octet-stream | 746 +---------------------+--------------------------+ 748 Table 1: Media type generalization lookup table 750 The "loose" media type mapping is an OPTIONAL feature. 751 Implementations supporting this kind of mapping should provide a 752 flexible way to define the set of media type generalizations allowed. 754 6.4. Media Type to Content Format Mapping Algorithm 756 This section defines the algorithm used to map an HTTP Internet media 757 type to its correspondent CoAP content format. 759 The algorithm uses the mapping table defined in Section 12.3 of 760 [RFC7252] plus, possibly, any locally defined extension of it. 761 Optionally, the table and lookup mechanism described in Section 6.3 762 can be used if the implementation chooses so. 764 Note that the algorithm may have side effects on the associated 765 representation (see also Section 6.5). 767 In the following: 769 o C-T, C-E, and C-F stand for the values of the Content-Type (or 770 Accept) HTTP header, Content-Encoding (or Accept-Encoding) HTTP 771 header, and Content-Format CoAP option respectively. 773 o If C-E is not given it is assumed to be "identity". 775 o MAP is the mandatory lookup table, GMAP is the optional 776 generalized table. 778 INPUT: C-T and C-E 779 OUTPUT: C-F or Fail 781 1. if no C-T: return Fail 782 2. C-F = MAP[C-T, C-E] 783 3. if C-F is not None: return C-F 784 4. if C-E is not "identity": 785 5. if C-E is supported (e.g., gzip): 786 6. decode the representation accordingly 787 7. set C-E to "identity" 788 8. else: 789 9. return Fail 790 10. repeat steps 2. and 3. 791 11. if C-T allows a non-lossy transformation into \ 792 12. one of the supported C-F: 793 13. transcode the representation accordingly 794 14. return C-F 795 15. if GMAP is defined: 796 16. C-F = GMAP[C-T] 797 17. if C-F is not None: return C-F 798 18. return Fail 800 Figure 2 802 6.5. Content Transcoding 804 6.5.1. General 806 Payload content transcoding (e.g., see steps 11-14 of Figure 2) is an 807 OPTIONAL feature. Implementations supporting this feature should 808 provide a flexible way to define the set of transcodings allowed. 810 As noted in Section 6.4, the process of mapping the media type can 811 have side effects on the forwarded entity body. This may be caused 812 by the removal or addition of a specific content encoding, or because 813 the HC proxy decides to transcode the representation to a different 814 (compatible) format. The latter proves useful when an optimized 815 version of a specific format exists. For example an XML-encoded 816 resource could be transcoded to Efficient XML Interchange (EXI) 817 format, or a JSON-encoded resource into CBOR [RFC7049], effectively 818 achieving compression without losing any information. 820 However, it should be noted that in certain cases, transcoding can 821 lose information in a non-obvious manner. For example, encoding an 822 XML document using schema-informed EXI encoding leads to a loss of 823 information when the destination does not know the exact schema 824 version used by the encoder, which means that whenever the HC proxy 825 transcodes an application/XML to application/EXI in-band metadata 826 could be lost. Therefore, the implementer should always carefully 827 verify such lossy payload transformations before triggering the 828 transcoding. 830 6.5.2. CoRE Link Format 832 The CoRE Link Format [RFC6690] is a set of links (i.e., URIs and 833 their formal relationships) which is carried as content payload in a 834 CoAP response. These links usually include CoAP URIs that might be 835 translated by the HC proxy to the correspondent HTTP URIs using the 836 implemented URI mapping function (see Section 5). Such a process 837 would inspect the forwarded traffic and attempt to re-write the body 838 of resources with an application/link-format media type, mapping the 839 embedded CoAP URIs to their HTTP counterparts. Some potential issues 840 with this approach are: 842 1. The client may be interested to retrieve original (unaltered) 843 CoAP payloads through the HC proxy, not modified versions. 845 2. Tampering with payloads is incompatible with resources that are 846 integrity protected (although this is a problem with transcoding 847 in general). 849 3. The HC proxy needs to fully understand [RFC6690] syntax and 850 semantics, otherwise there is an inherent risk to corrupt the 851 payloads. 853 Therefore, CoRE Link Format payload should only be transcoded at the 854 risk and discretion of the proxy implementer. 856 6.5.3. Diagnostic Messages 858 CoAP responses may, in certain error cases, contain a diagnostic 859 message in the payload explaining the error situation, as described 860 in Section 5.5.2 of [RFC7252]. If present, the CoAP response 861 diagnostic payload SHOULD be copied in the HTTP response body. The 862 CoAP diagnostic message MUST NOT be copied into the HTTP reason- 863 phrase, since it potentially contains CR-LF characters which are 864 incompatible with HTTP reason-phrase syntax. 866 7. Response Code Mapping 868 Table 2 defines the HTTP response status codes to which each CoAP 869 response code SHOULD be mapped. Multiple appearances of a HTTP 870 status code in the second column indicates multiple equivalent HTTP 871 responses are possible based on the same CoAP response code, 872 depending on the conditions cited in the Notes (third column and text 873 below table). 875 +-----------------------------+-----------------------------+-------+ 876 | CoAP Response Code | HTTP Status Code | Notes | 877 +-----------------------------+-----------------------------+-------+ 878 | 2.01 Created | 201 Created | 1 | 879 | 2.02 Deleted | 200 OK | 2 | 880 | | 204 No Content | 2 | 881 | 2.03 Valid | 304 Not Modified | 3 | 882 | | 200 OK | 4 | 883 | 2.04 Changed | 200 OK | 2 | 884 | | 204 No Content | 2 | 885 | 2.05 Content | 200 OK | | 886 | 4.00 Bad Request | 400 Bad Request | | 887 | 4.01 Unauthorized | 403 Forbidden | 5 | 888 | 4.02 Bad Option | 400 Bad Request | 6 | 889 | 4.02 Bad Option | 500 Internal Server Error | 6 | 890 | 4.03 Forbidden | 403 Forbidden | | 891 | 4.04 Not Found | 404 Not Found | | 892 | 4.05 Method Not Allowed | 400 Bad Request | 7 | 893 | 4.06 Not Acceptable | 406 Not Acceptable | | 894 | 4.12 Precondition Failed | 412 Precondition Failed | | 895 | 4.13 Request Ent. Too Large | 413 Request Repr. Too Large | | 896 | 4.15 Unsupported Media Type | 415 Unsupported Media Type | | 897 | 5.00 Internal Server Error | 500 Internal Server Error | | 898 | 5.01 Not Implemented | 501 Not Implemented | | 899 | 5.02 Bad Gateway | 502 Bad Gateway | | 900 | 5.03 Service Unavailable | 503 Service Unavailable | 8 | 901 | 5.04 Gateway Timeout | 504 Gateway Timeout | | 902 | 5.05 Proxying Not Supported | 502 Bad Gateway | 9 | 903 +-----------------------------+-----------------------------+-------+ 905 Table 2: CoAP-HTTP Response Code Mappings 907 Notes: 909 1. A CoAP server may return an arbitrary format payload along with 910 this response. If present, this payload MUST be returned as 911 entity in the HTTP 201 response. Section 7.3.2 of [RFC7231] does 912 not put any requirement on the format of the entity. (In the 913 past, [RFC2616] did.) 915 2. The HTTP code is 200 or 204 respectively for the case that a CoAP 916 server returns a payload or not. [RFC7231] Section 5.3 requires 917 code 200 in case a representation of the action result is 918 returned for DELETE/POST/PUT, and code 204 if not. Hence, a 919 proxy MUST transfer any CoAP payload contained in a CoAP 2.02 920 response to the HTTP client using a 200 OK response. 922 3. HTTP code 304 (Not Modified) is sent if the HTTP client performed 923 a conditional HTTP request and the CoAP server responded with 924 2.03 (Valid) to the corresponding CoAP validation request. Note 925 that Section 4.1 of [RFC7232] puts some requirements on header 926 fields that must be present in the HTTP 304 response. 928 4. A 200 response to a CoAP 2.03 occurs only when the HC proxy, for 929 efficiency reasons, is running a local cache. An unconditional 930 HTTP GET which produces a cache-hit, could trigger a re- 931 validation (i.e., a conditional GET) on the CoAP side. The proxy 932 receiving 2.03 updates the freshness of its cached representation 933 and returns it to the HTTP client. 935 5. A HTTP 401 Unauthorized (Section 3.1 of [RFC7235]) response is 936 not applicable because there is no equivalent in CoAP of WWW- 937 Authenticate which is mandatory in a HTTP 401 response. 939 6. If the proxy has a way to determine that the Bad Option is due to 940 the straightforward mapping of a client request header into a 941 CoAP option, then returning HTTP 400 (Bad Request) is 942 appropriate. In all other cases, the proxy MUST return HTTP 500 943 (Internal Server Error) stating its inability to provide a 944 suitable translation to the client's request. 946 7. A CoAP 4.05 (Method Not Allowed) response SHOULD normally be 947 mapped to a HTTP 400 (Bad Request) code, because the HTTP 405 948 response would require specifying the supported methods - which 949 are generally unknown. In this case the HC Proxy SHOULD also 950 return a HTTP reason-phrase in the HTTP status line that starts 951 with the string "CoAP server returned 4.05" in order to 952 facilitate troubleshooting. However, if the HC proxy has more 953 granular information about the supported methods for the 954 requested resource (e.g., via a Resource Directory 955 ([I-D.ietf-core-resource-directory])) then it MAY send back a 956 HTTP 405 (Method Not Allowed) with a properly filled in "Allow" 957 response-header field (Section 7.4.1 of [RFC7231]). 959 8. The value of the HTTP "Retry-After" response-header field is 960 taken from the value of the CoAP Max-Age Option, if present. 962 9. This CoAP response can only happen if the proxy itself is 963 configured to use a CoAP forward-proxy (Section 5.7 of [RFC7252]) 964 to execute some, or all, of its CoAP requests. 966 8. Additional Mapping Guidelines 968 8.1. Caching and Congestion Control 970 A HC proxy should cache CoAP responses, and reply whenever applicable 971 with a cached representation of the requested resource. 973 If the HTTP client drops the connection after the HTTP request was 974 made, a HC proxy should wait for the associated CoAP response and 975 cache it if possible. Subsequent requests to the HC proxy for the 976 same resource can use the result present in cache, or, if a response 977 has still to come, the HTTP requests will wait on the open CoAP 978 request. 980 According to [RFC7252], a proxy must limit the number of outstanding 981 requests to a given CoAP server to NSTART. To limit the amount of 982 aggregate traffic to a constrained network, the HC proxy should also 983 put a limit on the number of concurrent CoAP requests pending on the 984 same constrained network; further incoming requests may either be 985 queued or dropped (returning 503 Service Unavailable). This limit 986 and the proxy queueing/dropping behavior should be configurable. 988 Highly volatile resources that are being frequently requested may be 989 observed [RFC7641] by the HC proxy to keep their cached 990 representation fresh while minimizing the amount of CoAP traffic in 991 the constrained network. See Section 8.2. 993 8.2. Cache Refresh via Observe 995 There are cases where using the CoAP observe protocol [RFC7641] to 996 handle proxy cache refresh is preferable to the validation mechanism 997 based on ETag as defined in [RFC7252]. Such scenarios include, but 998 are not limited to, sleepy CoAP nodes -- with possibly high variance 999 in requests' distribution -- which would greatly benefit from a 1000 server driven cache update mechanism. Ideal candidates for CoAP 1001 observe are also crowded or very low throughput networks, where 1002 reduction of the total number of exchanged messages is an important 1003 requirement. 1005 This subsection aims at providing a practical evaluation method to 1006 decide whether refreshing a cached resource R is more efficiently 1007 handled via ETag validation or by establishing an observation on R. 1009 Let T_R be the mean time between two client requests to resource R, 1010 let T_C be the mean time between two representation changes of R, and 1011 let M_R be the mean number of CoAP messages per second exchanged to 1012 and from resource R. If we assume that the initial cost for 1013 establishing the observation is negligible, an observation on R 1014 reduces M_R if and only if T_R < 2*T_C with respect to using ETag 1015 validation, that is if and only if the mean arrival rate of requests 1016 for resource R is greater than half the change rate of R. 1018 When observing the resource R, M_R is always upper bounded by 2/T_C. 1020 8.3. Use of CoAP Blockwise Transfer 1022 A HC proxy SHOULD support CoAP blockwise transfers [RFC7959] to allow 1023 transport of large CoAP payloads while avoiding excessive link-layer 1024 fragmentation in constrained networks, and to cope with small 1025 datagram buffers in CoAP end-points as described in [RFC7252] 1026 Section 4.6. 1028 A HC proxy SHOULD attempt to retry a payload-carrying CoAP PUT or 1029 POST request with blockwise transfer if the destination CoAP server 1030 responded with 4.13 (Request Entity Too Large) to the original 1031 request. A HC proxy SHOULD attempt to use blockwise transfer when 1032 sending a CoAP PUT or POST request message that is larger than 1033 BLOCKWISE_THRESHOLD bytes. The value of BLOCKWISE_THRESHOLD is 1034 implementation-specific, for example it can be: 1036 o calculated based on a known or typical UDP datagram buffer size 1037 for CoAP end-points, or 1039 o set to N times the known size of a link-layer frame in a 1040 constrained network where e.g., N=5, or 1042 o preset to a known IP MTU value, or 1044 o set to a known Path MTU value. 1046 The value BLOCKWISE_THRESHOLD, or the parameters from which it is 1047 calculated, should be configurable in a proxy implementation. The 1048 maximum block size the proxy will attempt to use in CoAP requests 1049 should also be configurable. 1051 The HC proxy SHOULD detect CoAP end-points not supporting blockwise 1052 transfers. This can be done by checking for a 4.02 (Bad Option) 1053 response returned by an end-point in response to a CoAP request with 1054 a Block* Option, and subsequent absence of the 4.02 in response to 1055 the same request without Block* Options. This allows the HC proxy to 1056 be more efficient, not attempting repeated blockwise transfers to 1057 CoAP servers that do not support it. 1059 8.4. CoAP Multicast 1061 A HC proxy MAY support CoAP multicast. If it does, the HC proxy 1062 sends out a multicast CoAP request if the Target CoAP URI's authority 1063 is a multicast IP literal or resolves to a multicast IP address. If 1064 the HC proxy does not support CoAP multicast, it SHOULD respond 403 1065 (Forbidden) to any valid HTTP request that maps to a CoAP multicast 1066 request. 1068 Details related to supporting CoAP multicast are currently out of 1069 scope of this document since in a proxy scenario a HTTP client 1070 typically expects to receive a single response, not multiple. 1071 However, a HC proxy that implements CoAP multicast may include 1072 application-specific functions to aggregate multiple CoAP responses 1073 into a single HTTP response. We suggest using the "application/http" 1074 internet media type (Section 8.3.2 of [RFC7230]) to enclose a set of 1075 one or more HTTP response messages, each representing the mapping of 1076 one CoAP response. 1078 For further considerations related to the handling of multicast 1079 requests, see Section 10.1. 1081 8.5. Timeouts 1083 If the CoAP server takes a long time in responding, the HTTP client 1084 or any other proxy in between may timeout. Further discussion of 1085 timeouts in HTTP is available in Section 6.2.4 of [RFC7230]. 1087 A HC proxy MUST define an internal timeout for each pending CoAP 1088 request, because the CoAP server may silently die before completing 1089 the request. Assuming the Proxy uses confirmable CoAP requests, such 1090 timeout value T SHOULD be at least 1092 T = MAX_RTT + MAX_SERVER_RESPONSE_DELAY 1094 where MAX_RTT is defined in [RFC7252] and MAX_SERVER_RESPONSE_DELAY 1095 is defined in [RFC7390]. 1097 9. IANA Considerations 1099 9.1. New 'core.hc' Resource Type 1101 This document registers a new Resource Type (rt=) Link Target 1102 Attribute, 'core.hc', in the "Resource Type (rt=) Link Target 1103 Attribute Values" subregistry under the "Constrained RESTful 1104 Environments (CoRE) Parameters" registry. 1106 Attribute Value: core.hc 1107 Description: HTTP to CoAP mapping base resource. 1109 Reference: See Section 5.5. 1111 9.2. New 'coap-payload' Internet Media Type 1113 This document defines the "application/coap-payload" media type with 1114 a single parameter "cf". This media type represents any payload that 1115 a CoAP message can carry, having a content format that can be 1116 identified by an integer in range 0-65535 corresponding to a CoAP 1117 Content-Format parameter ([RFC7252], Section 12.3). The parameter 1118 "cf" is the integer defining the CoAP content format. 1120 Type name: application 1122 Subtype name: coap-payload 1124 Required parameters: cf (CoAP Content-Format integer in range 0-65535 1125 denoting the content format of the CoAP payload carried, as defined 1126 by the "CoAP Content-Formats" subregistry that is part of the 1127 "Constrained RESTful Environments (CoRE) Parameters" registry.) 1129 Optional parameters: None 1131 Encoding considerations: Common use is BINARY. The specific CoAP 1132 content format encoding considerations for the selected Content- 1133 Format (cf parameter) apply. The encoding can vary based on the 1134 value of the cf parameter. 1136 Security considerations: The specific CoAP content format security 1137 considerations for the selected Content-Format (cf parameter) apply. 1139 Interoperability considerations: This media type can never be used 1140 directly in CoAP messages because there is no means available to 1141 encode the mandatory 'cf' parameter in CoAP. 1143 Published specification: (this I-D - TBD) 1145 Applications that use this media type: HTTP-to-CoAP Proxies. 1147 Fragment identifier considerations: CoAP does not support URI 1148 fragments; therefore a CoAP payload fragment cannot be identified. 1149 Fragments are not applicable for this media type. 1151 Additional information: 1153 Deprecated alias names for this type: N/A 1154 Magic number(s): N/A 1156 File extension(s): N/A 1158 Macintosh file type code(s): N/A 1160 Person and email address to contact for further information: 1162 Esko Dijk ("esko@ieee.org") 1164 Intended usage: COMMON 1166 Restrictions on usage: 1168 An application (or user) can only use this media type if it has to 1169 represent a CoAP payload of which the specified CoAP Content-Format 1170 is an unrecognized number; such that a proper translation directly to 1171 the equivalent HTTP media type is not possible. 1173 Author: CoRE WG 1175 Change controller: IETF 1177 Provisional registration: No 1179 10. Security Considerations 1181 The security concerns raised in Section 9.2 of [RFC7230] also apply 1182 to the HC proxy scenario. 1184 A HC proxy deployed at the boundary of a constrained network is an 1185 easy single point of failure for reducing availability. As such, 1186 special care should be taken in designing, developing and operating 1187 it, keeping in mind that, in most cases, it has fewer limitations 1188 than the constrained devices it is serving. 1190 The following sub paragraphs categorize and discuss a set of specific 1191 security issues related to the translation, caching and forwarding 1192 functionality exposed by a HC proxy. 1194 10.1. Multicast 1196 Multicast requests impose a non trivial cost on the constrained 1197 network and endpoints, and might be exploited as a DoS attack vector 1198 (see also Section 10.2). From a privacy perspective, they can be 1199 used to gather detailed information about the resources hosted in the 1200 constrained network. For these reasons, it is RECOMMENDED that 1201 requests to multicast resources are access controlled with a default- 1202 deny policy. It is RECOMMENDED that the requestor of a multicast 1203 resource is strongly authenticated. If privacy is a concern, for 1204 example whenever the HTTP request transits through the public 1205 Internet, the request SHOULD be transported over a mutually 1206 authenticated and encrypted TLS connection. 1208 10.2. Traffic Overflow 1210 Due to the typically constrained nature of CoAP nodes, particular 1211 attention should be given to the implementation of traffic reduction 1212 mechanisms (see Section 8.1), because inefficient proxy 1213 implementations can be targeted by unconstrained Internet attackers. 1214 Bandwidth or complexity involved in such attacks is very low. 1216 An amplification attack to the constrained network may be triggered 1217 by a multicast request generated by a single HTTP request which is 1218 mapped to a CoAP multicast resource, as discussed in Section 11.3 of 1219 [RFC7252]. 1221 The risk likelihood of this amplification technique is higher than an 1222 amplification attack carried out by a malicious constrained device 1223 (e.g., ICMPv6 flooding, like Packet Too Big, or Parameter Problem on 1224 a multicast destination [RFC4732]), since it does not require direct 1225 access to the constrained network. 1227 The feasibility of this attack which disrupts availability of the 1228 targeted CoAP server can be limited by access controlling the exposed 1229 multicast resources, so that only known/authorized users can access 1230 such URIs. 1232 10.3. Handling Secured Exchanges 1234 An HTTP request can be sent to the HC proxy over a secured 1235 connection. However, there may not always exist a secure connection 1236 mapping to CoAP. For example, a secure distribution method for 1237 multicast traffic is complex and may not be implemented (see 1238 [RFC7390]). 1240 A HC proxy should implement rules for security context translations. 1241 For example all "https" unicast requests are translated to "coaps" 1242 requests, or "https" requests are translated to unsecured "coap" 1243 requests. Another rule could specify the security policy and 1244 parameters used for DTLS sessions [RFC7925]. Such rules will largely 1245 depend on the application and network context in which the HC proxy 1246 operates. These rules should be configurable. 1248 It is RECOMMENDED that, by default, accessing a "coaps" URI is only 1249 allowed from a corresponding "https" URI. 1251 By default, a HC proxy SHOULD reject any secured CoAP client request 1252 (i.e., one with a "coaps" scheme) if there is no configured security 1253 policy mapping. This recommendation may be relaxed in case the 1254 destination network is believed to be secured by other means. 1255 Assuming that CoAP nodes are isolated behind a firewall as in the HC 1256 proxy deployment shown in Figure 1, the HC proxy may be configured to 1257 translate the incoming HTTPS request using plain CoAP (NoSec mode). 1259 10.4. URI Mapping 1261 The following risks related to the URI mapping described in Section 5 1262 and its use by HC proxies have been identified: 1264 DoS attack on the constrained/CoAP network. 1265 Mitigation: by default deny any Target CoAP URI whose authority is 1266 (or maps to) a multicast address. Then explicitly white-list 1267 multicast resources/authorities that are allowed to be de- 1268 referenced. See also Section 8.4. 1270 Leaking information on the constrained/CoAP network resources and 1271 topology. 1272 Mitigation: by default deny any Target CoAP URI (especially 1273 /.well-known/core is a resource to be protected), and then 1274 explicitly white-list resources that are allowed to be seen from 1275 outside. 1277 The internal CoAP Target resource is totally transparent from 1278 outside. 1279 Mitigation: implement a HTTPS-only interface, which makes the 1280 Target CoAP URI totally opaque to a passive attacker. 1282 11. Acknowledgments 1284 An initial version of Table 2 in Section 7 has been provided in 1285 revision -05 of the CoRE CoAP I-D. Special thanks to Peter van der 1286 Stok for countless comments and discussions on this document, that 1287 contributed to its current structure and text. 1289 Thanks to Abhijan Bhattacharyya, Alexey Melnikov, Brian Frank, 1290 Carsten Bormann, Christian Amsuess, Christian Groves, Cullen 1291 Jennings, Dorothy Gellert, Francesco Corazza, Francis Dupont, Hannes 1292 Tschofenig, Jaime Jimenez, Kepeng Li, Kerry Lynn, Klaus Hartke, Linyi 1293 Tian, Michele Rossi, Michele Zorzi, Nicola Bui, Peter Saint-Andre, 1294 Sean Leonard, Zach Shelby for helpful comments and discussions that 1295 have shaped the document. 1297 The research leading to these results has received funding from the 1298 European Community's Seventh Framework Programme [FP7/2007-2013] 1299 under grant agreement n.251557. 1301 12. References 1303 12.1. Normative References 1305 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1306 Requirement Levels", BCP 14, RFC 2119, 1307 DOI 10.17487/RFC2119, March 1997, 1308 . 1310 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 1311 Resource Identifier (URI): Generic Syntax", STD 66, 1312 RFC 3986, DOI 10.17487/RFC3986, January 2005, 1313 . 1315 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1316 Specifications: ABNF", STD 68, RFC 5234, 1317 DOI 10.17487/RFC5234, January 2008, 1318 . 1320 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 1321 and D. Orchard, "URI Template", RFC 6570, 1322 DOI 10.17487/RFC6570, March 2012, 1323 . 1325 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 1326 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 1327 . 1329 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1330 Protocol (HTTP/1.1): Message Syntax and Routing", 1331 RFC 7230, DOI 10.17487/RFC7230, June 2014, 1332 . 1334 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1335 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 1336 DOI 10.17487/RFC7231, June 2014, 1337 . 1339 [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1340 Protocol (HTTP/1.1): Conditional Requests", RFC 7232, 1341 DOI 10.17487/RFC7232, June 2014, 1342 . 1344 [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1345 Protocol (HTTP/1.1): Authentication", RFC 7235, 1346 DOI 10.17487/RFC7235, June 2014, 1347 . 1349 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1350 Application Protocol (CoAP)", RFC 7252, 1351 DOI 10.17487/RFC7252, June 2014, 1352 . 1354 [RFC7641] Hartke, K., "Observing Resources in the Constrained 1355 Application Protocol (CoAP)", RFC 7641, 1356 DOI 10.17487/RFC7641, September 2015, 1357 . 1359 [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 1360 the Constrained Application Protocol (CoAP)", RFC 7959, 1361 DOI 10.17487/RFC7959, August 2016, 1362 . 1364 12.2. Informative References 1366 [I-D.ietf-core-links-json] 1367 Li, K., Rahman, A., and C. Bormann, "Representing CoRE 1368 Formats in JSON and CBOR", draft-ietf-core-links-json-06 1369 (work in progress), July 2016. 1371 [I-D.ietf-core-resource-directory] 1372 Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE 1373 Resource Directory", draft-ietf-core-resource-directory-08 1374 (work in progress), July 2016. 1376 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1377 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1378 Transfer Protocol -- HTTP/1.1", RFC 2616, 1379 DOI 10.17487/RFC2616, June 1999, 1380 . 1382 [RFC3040] Cooper, I., Melve, I., and G. Tomlinson, "Internet Web 1383 Replication and Caching Taxonomy", RFC 3040, 1384 DOI 10.17487/RFC3040, January 2001, 1385 . 1387 [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet 1388 Denial-of-Service Considerations", RFC 4732, 1389 DOI 10.17487/RFC4732, December 2006, 1390 . 1392 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object 1393 Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, 1394 October 2013, . 1396 [RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for 1397 the Constrained Application Protocol (CoAP)", RFC 7390, 1398 DOI 10.17487/RFC7390, October 2014, 1399 . 1401 [RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer 1402 Security (TLS) / Datagram Transport Layer Security (DTLS) 1403 Profiles for the Internet of Things", RFC 7925, 1404 DOI 10.17487/RFC7925, July 2016, 1405 . 1407 [W3C.REC-html5-20141028] 1408 Hickson, I., Berjon, R., Faulkner, S., Leithead, T., 1409 Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5", W3C 1410 Recommendation REC-html5-20141028, 2014, 1411 . 1413 Appendix A. Change Log 1415 [Note to RFC Editor: Please remove this section before publication.] 1417 Changes from ietf-13 to ietf-14: 1419 o Addressed Gen-ART and AD review comments. 1421 Changes from ietf-12 to ietf-13 (Christian Amsuess' comments): 1423 o More missing slashes in URI mapping template examples. 1425 Changes from ietf-11 to ietf-12 (2nd WGLC): 1427 o Addressed a few editorial issues (including a clarification on 1428 when to use qq vs q in the URI mapping template). 1430 o Fixed missing slash in one template example. 1432 o Added para about the need for future CoAP protocol elements to 1433 define their own HTTP mappings. 1435 Changes from ietf-10 to ietf-11 (Chair review): 1437 o Removed cu/su distinction from the URI mapping template. 1439 o Addressed a few editorial issues. 1441 Changes from ietf-09 to ietf-10: 1443 o Addressed Ticket #401 - Clarified that draft covers not only 1444 Reverse HC Proxy but that many parts also apply to Forward and 1445 Interception Proxies. 1447 o Clarified that draft concentrates on the HTTP-to-CoAP mapping 1448 direction (i.e., the HC proxy is a HTTP server and a CoAP client). 1450 o Clarified the "null mapping" case where no CoAP URI information is 1451 embedded in the HTTP request URI. 1453 o Moved multicast related security text to the "Security 1454 Considerations" to consolidate all security information in one 1455 location. 1457 o Removed references to "placement" of proxy (e.g., server-side vs 1458 client-side) as is confusing and provides little added value. 1460 o Fixed version numbers on references that were corrupted in last 1461 revision due to outdated xml2rfc conversion tool local cache. 1463 o Various editorial improvements. 1465 Changes from ietf-08 to ietf-09: 1467 o Clean up requirements language as per Klaus' comment. 1469 Changes from ietf-07 to ietf-08: 1471 o Addressed WGLC review comments from Klaus Hartke as per the 1472 correspondence of March 9, 2016 on the CORE WG mailing list. 1474 Changes from ietf-06 to ietf-07: 1476 o Addressed Ticket #384 - Section 5.4.1 describes briefly 1477 (informative) how to discover CoAP resources from an HTTP client. 1479 o Addressed Ticket #378 - For HTTP media type to CoAP content format 1480 mapping and vice versa: a new draft (TBD) may be proposed in CoRE 1481 which describes an approach for automatic updating of the media 1482 type mapping. This was noted in Section 6.1 but is otherwise 1483 outside the scope of this draft. 1485 o Addressed Ticket #377 - Added IANA section that defines a new HTTP 1486 media type "application/coap-payload" and created new Section 6.2 1487 on how to use it. 1489 o Addressed Ticket #376 - Updated Table 2 (and corresponding note 7) 1490 to indicate that a CoAP 4.05 (Method Not Allowed) Response Code 1491 should be mapped to a HTTP 400 (Bad Request). 1493 o Added note to comply to ABNF when translating CoAP diagnostic 1494 payload to reason-phrase in Section 6.5.3. 1496 Changes from ietf-05 to ietf-06: 1498 o Fully restructured the draft, bringing introductory text more to 1499 the front and allocating main sections to each of the key topics; 1500 addressing Ticket #379; 1502 o Addressed Ticket #382, fix of enhanced form URI template 1503 definition of q in Section 5.3.2; 1505 o Addressed Ticket #381, found a mapping 4.01 to 401 Unauthorized in 1506 Section 7; 1508 o Addressed Ticket #380 (Add IANA registration for "core.hc" 1509 Resource Type) in Section 9; 1511 o Addressed Ticket #376 (CoAP 4.05 response can't be translated to 1512 HTTP 405 by HC proxy) in Section 7 by use of empty 'Allow' header; 1514 o Removed details on the pros and cons of HC proxy placement 1515 options; 1517 o Addressed review comments of Carsten Bormann; 1519 o Clarified failure in mapping of HTTP Accept headers (Section 6.3); 1521 o Clarified detection of CoAP servers not supporting blockwise 1522 (Section 8.3); 1524 o Changed CoAP request timeout min value to MAX_RTT + 1525 MAX_SERVER_RESPONSE_DELAY (Section 8.6); 1527 o Added security section item (Section 10.3) related to use of CoAP 1528 blockwise transfers; 1530 o Many editorial improvements. 1532 Changes from ietf-04 to ietf-05: 1534 o Addressed Ticket #366 (Mapping of CoRE Link Format payloads to be 1535 valid in HTTP Domain?) in Section 6.3.3.2 (Content Transcoding - 1536 CORE Link Format); 1538 o Addressed Ticket #375 (Add requirement on mapping of CoAP 1539 diagnostic payload) in Section 6.3.3.3 (Content Transcoding - 1540 Diagnostic Messages); 1542 o Addressed comment from Yusuke (http://www.ietf.org/mail- 1543 archive/web/core/current/msg05491.html) in Section 6.3.3.1 1544 (Content Transcoding - General); 1546 o Various editorial improvements. 1548 Changes from ietf-03 to ietf-04: 1550 o Expanded use case descriptions in Section 4; 1552 o Fixed/enhanced discovery examples in Section 5.4.1; 1554 o Addressed Ticket #365 (Add text on media type conversion by HTTP- 1555 CoAP proxy) in new Section 6.3.1 (Generalized media type mapping) 1556 and new Section 6.3.2 (Content translation); 1558 o Updated HTTPBis WG draft references to recently published RFC 1559 numbers. 1561 o Various editorial improvements. 1563 Changes from ietf-02 to ietf-03: 1565 o Closed Ticket #351 "Add security implications of proposed default 1566 HTTP-CoAP URI mapping"; 1568 o Closed Ticket #363 "Remove CoAP scheme in default HTTP-CoAP URI 1569 mapping"; 1571 o Closed Ticket #364 "Add discovery of HTTP-CoAP mapping 1572 resource(s)". 1574 Changes from ietf-01 to ietf-02: 1576 o Selection of single default URI mapping proposal as proposed to WG 1577 mailing list 2013-10-09. 1579 Changes from ietf-00 to ietf-01: 1581 o Added URI mapping proposals to Section 4 as per the Email 1582 proposals to WG mailing list from Esko. 1584 Authors' Addresses 1586 Angelo P. Castellani 1587 University of Padova 1588 Via Gradenigo 6/B 1589 Padova 35131 1590 Italy 1592 Email: angelo@castellani.net 1594 Salvatore Loreto 1595 Ericsson 1596 Hirsalantie 11 1597 Jorvas 02420 1598 Finland 1600 Email: salvatore.loreto@ericsson.com 1602 Akbar Rahman 1603 InterDigital Communications, LLC 1604 1000 Sherbrooke Street West 1605 Montreal H3A 3G4 1606 Canada 1608 Phone: +1 514 585 0761 1609 Email: Akbar.Rahman@InterDigital.com 1611 Thomas Fossati 1612 Nokia 1613 3 Ely Road 1614 Milton, Cambridge CB24 6DD 1615 UK 1617 Email: thomas.fossati@nokia.com 1619 Esko Dijk 1620 Philips Lighting 1621 High Tech Campus 7 1622 Eindhoven 5656 AE 1623 The Netherlands 1625 Email: esko.dijk@philips.com