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In this case, it *MAY NOT* implement the endpoint to publish updates. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: Also, when the client is a web browser, the JWT SHOULD not be made accessible to JavaScript scripts for resilience against Cross-site Scription (XSS) attacks (https://www.owasp.org/index.php/Cross-site_Scripting_(XSS)). It's the main reason why, when the client is a web browser, using "HttpOnly" cookies as the authorization mechanism SHOULD always be preferred. -- The document date (9 January 2019) is 1927 days in the past. Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 2818 (Obsoleted by RFC 9110) ** Obsolete normative reference: RFC 5988 (Obsoleted by RFC 8288) ** Obsolete normative reference: RFC 7230 (Obsoleted by RFC 9110, RFC 9112) -- Obsolete informational reference (is this intentional?): RFC 7386 (Obsoleted by RFC 7396) Summary: 6 errors (**), 0 flaws (~~), 5 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group K. Dunglas 3 Internet-Draft Les-Tilleuls.coop 4 Intended status: Informational 9 January 2019 5 Expires: 13 July 2019 7 The Mercure Protocol 8 draft-dunglas-mercure-03 10 Abstract 12 Mercure is a protocol enabling the pushing of data updates to web 13 browsers and other HTTP clients in a fast, reliable and battery- 14 efficient way. It is especially useful for publishing real-time 15 updates of resources served through web APIs to reactive web and 16 mobile apps. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at https://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on 13 July 2019. 35 Copyright Notice 37 Copyright (c) 2019 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents (http://trustee.ietf.org/ 42 license-info) in effect on the date of publication of this document. 43 Please review these documents carefully, as they describe your rights 44 and restrictions with respect to this document. Code Components 45 extracted from this document must include Simplified BSD License text 46 as described in Section 4.e of the Trust Legal Provisions and are 47 provided without warranty as described in the Simplified BSD License. 49 Table of Contents 51 1. Terminology 52 2. Discovery 53 3. Subscriptions 54 4. Publication 55 5. Authorization 56 5.1. Publishers 57 5.2. Subscribers 58 6. Reconnection and State Reconciliation 59 7. Encryption 60 8. Security Considerations 61 9.1. Normative References 62 9.2. Informative References 63 Author's Address 65 1. Terminology 67 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, 68 *SHOULD NOT*, RECOMMENDED, MAY, and OPTIONAL, when they appear in 69 this document, are to be interpreted as described in [RFC2119]. 71 * Topic: An HTTP [RFC7230] or HTTPS [RFC2818] topic URL. The unit 72 to which one can subscribe to changes. 74 * Publisher: An owner of a topic. Notifies the hub when the topic 75 feed has been updated. As in almost all pubsub systems, the 76 publisher is unaware of the subscribers, if any. Other pubsub 77 systems might call the publisher the "source". Typically a 78 website or a web API, but can also be a web browser. 80 * Subscriber: A client application that subscribes to real-time 81 updates of topics. Typically a Progressive Web App or a Mobile 82 App, but can also be a server. 84 * Target: A subscriber, or a group of subscribers. A publisher is 85 able to securely dispatch updates to specific targets. Using an 86 HTTP [RFC7230] or HTTPS [RFC2818] URL to identify targets is 87 RECOMMENDED. 89 * Hub: A server that handles subscription requests and distributes 90 the content to subscribers when the corresponding topics have been 91 updated (a hub implementation is provided in this repository). 92 Any hub MAY implement its own policies on who can use it. 94 2. Discovery 96 If the publisher is a server, it SHOULD advertise the URL of one or 97 more hubs to the subscriber, allowing it to receive live updates when 98 topics are updated. If more than one hub URL is specified, it is 99 RECOMMENDED that the publisher notifies each hub, so the subscriber 100 MAY subscribe to one or more of them. 102 The publisher SHOULD include at least one Link Header [RFC5988] with 103 "rel=mercure" (a hub link header). The target URL of these links 104 MUST be a hub implementing the Mercure protocol. 106 Note: this relation type has not been registered yet [RFC5988]. In 107 the meantime, the relation type "https://git.io/mercure" MAY be used 108 instead. 110 The publisher MAY provide the following target attributes in the Link 111 Headers: 113 * "last-event-id": the globally unique identifier of the last event 114 dispatched by the publisher at the time of the generation of this 115 resource. If provided, it MUST be passed to the hub through a 116 query parameter called "Last-Event-ID" and will be used to ensure 117 that possible updates having been made during between the resource 118 generation time and the connection to the hub are not lost. See 119 section #Re-Connection-and-State-Reconciliation. If this 120 attribute is provided, the publisher MUST always set the "id" 121 parameter when sending updates to the hub. 123 * "content-type": the content type of the updates that will pushed 124 by the hub. If omitted, the subscriber MUST assume that the 125 content type will be the same as that of the original resource. 126 Setting the "content-type" attribute is especially useful to hint 127 that partial updates will be pushed, using formats such as JSON 128 Patch [RFC6902] or JSON Merge Patch [RFC7386]. 130 * "key-set=": the key(s) to decrypt updates encoded in the 131 JWKS (JSON Web Key Set) format (see the Encryption section). 133 All these attributes are optional. 135 The publisher MAY also include one Link Header [RFC5988] with 136 "rel=self" (the self link header). It SHOULD contain the canonical 137 URL for the topic to which subscribers are expected to use for 138 subscriptions. If the Link with "rel=self" is omitted, the current 139 URL of the resource MUST be used as a fallback. 141 Minimal example: 143 GET /books/foo.jsonld HTTP/1.1 144 Host: example.com 146 HTTP/1.1 200 Ok 147 Content-type: application/ld+json 148 Link: ; rel="mercure" 150 {"@id": "/books/foo.jsonld", "foo": "bar"} 152 Links embedded in HTML or XML documents (as defined in the WebSub 153 recommendation) MAY also be supported by subscribers. 155 Note: the discovery mechanism described in this section is strongly 156 inspired from the one specified in the WebSub recommendation 157 (https://www.w3.org/TR/websub/#discovery). 159 3. Subscriptions 161 The subscriber subscribes to a URL exposed by a hub to receive 162 updates from one or many topics. To subscribe to updates, the client 163 opens an HTTPS connection following the Server-Sent Events 164 specification (https://html.spec.whatwg.org/multipage/server-sent- 165 events.html) to the hub's subscription URL advertised by the 166 publisher. The "GET" HTTP method must be used. The connection 167 SHOULD use HTTP/2 to leverage mutliplexing and other advanced 168 features of this protocol. 170 The subscriber specifies the list of topics to get updates from by 171 using one or several query parameters named "topic". The value of 172 these query parameters MUST be URI templates [RFC6570]. 174 Note: a URL is also a valid URI template. 176 The protocol doesn't specify the maximum number of "topic" parameters 177 that can be sent, but the hub MAY apply an arbitrary limit. 179 The EventSource JavaScript interface 180 (https://html.spec.whatwg.org/multipage/server-sent-events.html#the- 181 eventsource-interface) MAY be used to establish the connection. Any 182 other appropriate mechanism including, but not limited to, readable 183 streams (https://developer.mozilla.org/en- 184 US/docs/Web/API/Streams_API/Using_readable_streams) and 185 XMLHttpRequest (https://developer.mozilla.org/en- 186 US/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest) (used by popular 187 polyfills) MAY also be used. 189 The hub sends updates concerning all subscribed resources matching 190 the provided URI templates and the provided targets (see section 191 #Authorization). If no targets are specified, the update is 192 dispatched to all subscribers. The hub MUST send these updates as 193 text/event-stream compliant events 194 (https://html.spec.whatwg.org/multipage/server-sent-events.html#sse- 195 processing-model). 197 The "data" property MUST contain the new version of the topic. It 198 can be the full resource, or a partial update by using formats such 199 as JSON Patch "@RFC6902" or JSON Merge Patch "@RFC7386". 201 All other properties defined in the Server-Sent Events specification 202 MAY be used and SHOULD be supported by hubs. 204 The resource SHOULD be represented in a format with hypermedia 205 capabilities such as JSON-LD [W3C.REC-json-ld-20140116], Atom 206 [RFC4287], XML [W3C.REC-xml-20081126] or HTML [W3C.REC- 207 html52-20171214]. 209 Web Linking [RFC5988] SHOULD be used to indicate the IRI of the 210 resource sent in the event. When using Atom, XML or HTML as the 211 serialization format for the resource, the document SHOULD contain a 212 "link" element with a "self" relation containing the IRI of the 213 resource. When using JSON-LD, the document SHOULD contain an "@id" 214 property containing the IRI of the resource. 216 Example: 218 // The subscriber subscribes to updates for the https://example.com/foo topic 219 // and to any topic matching https://example.com/books/{name} 220 const url = new URL('https://example.com/hub'); 221 url.searchParams.append('topic', 'https://example.com/foo'); 222 url.searchParams.append('topic', 'https://example.com/bar/{id}'); 224 const eventSource = new EventSource(url); 226 // The callback will be called every time an update is published 227 eventSource.onmessage = function ({data}) { 228 console.log(data); 229 }; 231 The hub MAY require that subscribers are authorized to receive 232 updates. 234 4. Publication 236 The publisher send updates by issuing "POST" HTTPS requests on the 237 hub URL. When it receives an update, the hub dispatches it to 238 subscribers using the established server-sent events connections. 240 An application CAN send events directly to subscribers without using 241 an external hub server, if it is able to do so. In this case, it 242 *MAY NOT* implement the endpoint to publish updates. 244 The request MUST be encoded using the "application/x-www-form- 245 urlencoded" format and contain the following data: 247 * "topic": IRIs of the updated topic. If this key is present 248 several times, the first occurrence is considered to be the 249 canonical URL of the topic, and other ones are considered to be 250 alternate URLs. The hub MUST dispatch this update to subscribers 251 that are subscribed to both canonical or alternate URLs. 253 * "data": The content of the new version of this topic. 255 * "target" (optional): Target audience of this update. This key can 256 be present several times. See section #Authorization for further 257 information. 259 * "id" (optional): The topic's revision identifier: it will be used 260 as the SSE's "id" property. If omitted, the hub MUST generate a 261 valid globally unique id. It MAY be a UUID. Even if provided, 262 the hub MAY ignore the id provided by the client and generate its 263 own id. 265 * "type" (optional): The SSE's "event" property (a specific event 266 type). 268 * "retry" (optional): The SSE's "retry" property (the reconnection 269 time). 271 In the event of success, the HTTP response's body MUST be the "id" 272 associated to this update generated by the hub and a success HTTP 273 status code MUST be returned. The publisher MUST be authorized to 274 publish updates. See section #Authorization. 276 5. Authorization 278 To ensure that they are authorized, both publishers and subscribers 279 must present a valid JWS [RFC7515] in compact serialization to the 280 hub. This JWS SHOULD be short-lived, especially if the subscriber is 281 a web browser. A different key MAY be used to sign subscribers' and 282 publishers' tokens. 284 Two mechanisms are defined to present the JWS to the hub: 286 * using an "Authorization" HTTP header 288 * using a cookie 290 If the publisher or the subscriber is not a web browser, it SHOULD 291 use an "Authorization" HTTP header. This "Authorization" header MUST 292 contain the string "Bearer" followed by the JWS. The hub will check 293 that the JWS conforms to the rules (defined later) ensuring that the 294 client is authorized to publish or subscribe to updates. 296 By the "EventSource" specification, web browsers can not set custom 297 HTTP headers for such connections, and they can only be estabilished 298 using the "GET" HTTP method. However, cookies are supported and can 299 be included even in cross-domain requests if the CORS credentials are 300 set (https://html.spec.whatwg.org/multipage/server-sent- 301 events.html#dom-eventsourceinit-withcredentials): 303 If the publisher or the subscriber is a web browser, it SHOULD send a 304 cookie called "mercureAuthorization" containing the JWS when 305 connecting to the hub. 307 Whenever possible, the "mercureAuthorization" cookie SHOULD be set 308 during the discovery to improve the overall security. See section 309 #Discovery. Consequently, if the cookie is set during the discovery, 310 both the publisher and the hub have to share the same second level 311 domain. The "Domain" attribute MAY be used to allow the publisher 312 and the hub to use different subdomains. 314 The cookie SHOULD have the "Secure", "HttpOnly" and "SameSite" 315 attributes set. The cookie's "Path" attribute SHOULD also be set to 316 the hub's URL. See section #Security-Considerations. 318 When using authorization mechanisms, the connection MUST use an 319 encryption layer such as HTTPS. 321 If both an "Authorization" HTTP header and a cookie named 322 "mercureAuthorization" are presented by the client, the cookie MUST 323 be ignored. If the client tries to execute an operation it is not 324 allowed to, a 403 HTTP status code SHOULD be returned. 326 5.1. Publishers 328 Publishers MUST be authorized to dispatch updates to the hub, and 329 MUST prove that they are allowed to send updates. 331 To be allowed to publish an update, the JWT presented by the 332 publisher MUST contain a claim called "mercure", and this claim MUST 333 contain a "publish" key. "mercure.publish" MUST contain an array of 334 targets the publisher is allowed to dispatch updates to. 336 If "mercure.publish": 338 * is not defined, then the publisher MUST NOT be authorized to 339 dispatch any update 341 * contains an empty array, then the publisher is only allowed to 342 dispatch public updates 344 * contains the reserved string "*" as an array value, then the 345 publisher is authorized to dispatch updates to all targets 347 If a topic is not public, the "POST" request sent by the publisher to 348 the hub MUST contain a list of keys named "target". Their values 349 MUST be of type "string", and it is RECOMMENDED to use valid IRIs. 350 They can be, for instance, a user ID or a list of group IDs. If an 351 update contains at least one target the publisher is not authorized 352 for, the hub MUST NOT dispatch the update (even if some targets in 353 the list are allowed) and SHOULD return a 403 HTTP status code. 355 5.2. Subscribers 357 Subscribers MAY need to be authorized to connect to the hub. To 358 receive updates destined to specific targets, they MUST be 359 authorized, and MUST prove they belong to at least one of the 360 specified targets. If the subscriber is not authorized, it MUST NOT 361 receive any update having at least one target. 363 To receive updates destined for specific targets, the JWS presented 364 by the subscriber MUST have a claim named "mercure" with a key named 365 "subscribe" that contains an array of strings: a list of targets the 366 user is authorized to receive updates for. The targets SHOULD be 367 IRIs. 369 If at least one target is specified, the update MUST NOT be sent to 370 the subscriber by the hub, unless the "mercure.subscribe" array of 371 the JWS presented by the subscriber contains at least one of the 372 specified targets. 374 If the "mercure.subscribe" array contains the reserved string value 375 "*", then the subscriber is authorized to receive updates destined 376 for all targets. 378 6. Reconnection and State Reconciliation 380 To allow re-establishment in case of connection lost, events 381 dispatched by the hub SHOULD include an "id" property. The value 382 contained in this "id" property SHOULD be a globally unique 383 identifier. To do so, a UUID [RFC4122] MAY be used. 385 According to the server-sent events specification, in case of 386 connection lost the subscriber will try to automatically re-connect. 387 During the re-connection, the subscriber MUST send the last received 388 event id in a Last-Event-ID (https://html.spec.whatwg.org/multipage/ 389 iana.html#last-event-id) HTTP header. 391 The server-sent events specification doesn't allow this HTTP header 392 to be set during the first connection (before a reconnection). In 393 order to fetch any update dispatched between the initial resource 394 generation by the publisher and the connection to the hub, the 395 subscriber MUST send the event id provided during the discovery in 396 the "last-event-id" link's attribute in a query parameter named 397 "Last-Event-ID" when connecting to the hub. 399 If both the "Last-Event-ID" HTTP header and the query parameter are 400 present, the HTTP header MUST take precedence. 402 If the "Last-Event-ID" HTTP header or query parameter exists, the hub 403 SHOULD send all events published following the one bearing this 404 identifier to the subscriber. 406 The hub MAY discard some messages for operational reasons. The 407 subscriber MUST NOT assume that no update will be lost, and MUST re- 408 fetch the original topic to ensure this (for instance, after a long 409 disconnection time). 411 The hub MAY also specify the reconnection time using the "retry" key, 412 as specified in the server-sent events format. 414 7. Encryption 416 Using HTTPS does not prevent the hub from accessing the update's 417 content. Depending of the intended privacy of information contained 418 in the update, it MAY be necessary to prevent eavesdropping by the 419 hub. 421 To make sure that the message content can not be read by the hub, the 422 publisher MAY encode the message before sending it to the hub. The 423 publisher SHOULD use JSON Web Encryption [RFC7516] to encrypt the 424 update content. The publisher MAY provide the relevant encryption 425 key(s) in the "key-set" attribute of the Link HTTP header during the 426 discovery. The "key-set" attribute SHOULD contain a key encoded 427 using the JSON Web Key Set [RFC7517] format. Any other out-of-band 428 mechanism MAY be used instead to share the key between the publisher 429 and the subscriber. 431 Update encyption is considered a best practice to prevent mass 432 surveillance. This is especially relevant if the hub is managed by 433 an external provider. 435 8. Security Considerations 437 The confidentiality of the secret key(s) used to generate the JWTs is 438 a primary concern. The secret key(s) MUST be stored securely. They 439 MUST be revoked immediately in the event of compromission. 441 Possessing valid JWTs allows any client to subscribe, or to publish 442 to the hub. Their confidentiality MUST therefore be ensured. To do 443 so, JWTs MUST only be transmitted over secure connections. 445 Also, when the client is a web browser, the JWT SHOULD not be made 446 accessible to JavaScript scripts for resilience against Cross-site 447 Scription (XSS) attacks (https://www.owasp.org/index.php/Cross- 448 site_Scripting_(XSS)). It's the main reason why, when the client is 449 a web browser, using "HttpOnly" cookies as the authorization 450 mechanism SHOULD always be preferred. 452 In the event of compromission, revoking JWTs before their expiration 453 is often difficult. To that end, using short-lived tokens is 454 strongly RECOMMENDED. 456 The publish endpoint of the hub may be targeted by Cross-Site Request 457 Forgery (CSRF) attacks (https://www.owasp.org/index.php/Cross- 458 Site_Request_Forgery_(CSRF)) when the cookie-based authorization 459 mechanism is used. Therefore, implementations supporting this 460 mechanism MUST mitigate such attacks. 462 The first prevention method to implement is to set the 463 "mercureAuthorization" cookie's "SameSite" attribute. However, some 464 web browsers still not support this attribute 465 (https://caniuse.com/#feat=same-site-cookie-attribute) and will 466 remain vulnerable. Additionally, hub implementations SHOULD use the 467 "Origin" and "Referer" HTTP headers set by web browsers to verify 468 that the source origin matches the target origin. If none of these 469 headers are available, the hub SHOULD discard the request. 471 CSRF prevention techniques, including those previously mentioned, are 472 described in depth in OWASP's Cross-Site Request Forgery (CSRF) 473 Prevention Cheat Sheet (https://www.owasp.org/index.php/Cross- 474 Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet). 476 9.1. Normative References 478 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 479 Requirement Levels", BCP 14, RFC 2119, 480 DOI 10.17487/RFC2119, March 1997, 481 . 483 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, 484 DOI 10.17487/RFC2818, May 2000, 485 . 487 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 488 Unique IDentifier (UUID) URN Namespace", RFC 4122, 489 DOI 10.17487/RFC4122, July 2005, 490 . 492 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, 493 DOI 10.17487/RFC5988, October 2010, 494 . 496 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 497 and D. Orchard, "URI Template", RFC 6570, 498 DOI 10.17487/RFC6570, March 2012, 499 . 501 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 502 Protocol (HTTP/1.1): Message Syntax and Routing", 503 RFC 7230, DOI 10.17487/RFC7230, June 2014, 504 . 506 [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web 507 Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 508 2015, . 510 [RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", 511 RFC 7516, DOI 10.17487/RFC7516, May 2015, 512 . 514 [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, 515 DOI 10.17487/RFC7517, May 2015, 516 . 518 9.2. Informative References 520 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 521 Syndication Format", RFC 4287, DOI 10.17487/RFC4287, 522 December 2005, . 524 [RFC6902] Bryan, P., Ed. and M. Nottingham, Ed., "JavaScript Object 525 Notation (JSON) Patch", RFC 6902, DOI 10.17487/RFC6902, 526 April 2013, . 528 [RFC7386] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7386, 529 DOI 10.17487/RFC7386, October 2014, 530 . 532 [W3C.REC-html52-20171214] 533 Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and 534 S. Moon, "HTML 5.2", World Wide Web Consortium 535 Recommendation REC-html52-20171214, 14 December 2017, 536 . 538 [W3C.REC-json-ld-20140116] 539 Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0", 540 World Wide Web Consortium Recommendation REC-json-ld- 541 20140116, 16 January 2014, 542 . 544 [W3C.REC-xml-20081126] 545 Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and 546 F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth 547 Edition)", World Wide Web Consortium Recommendation REC- 548 xml-20081126, 26 November 2008, 549 . 551 Author's Address 553 Les-Tilleuls.coop 554 Kevin Dunglas 555 5 rue Hegel 556 59000 Lille 557 France 559 Email: kevin@les-tilleuls.coop