Network Working Group K. Dunglas Internet-Draft Les-Tilleuls.coop Intended status: Informational October 11, 2018 Expires: April 14, 2019 The Mercure Protocol draft-dunglas-mercure-00 Abstract Mercure is a protocol allowing to push data updates to web browsers and other HTTP clients in a fast, reliable and battery-efficient way. It is especially useful to publish real-time updates of resources served through web APIs, to reactive web and mobile apps. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on April 14, 2019. Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Dunglas Expires April 14, 2019 [Page 1]
Internet-Draft The Mercure Protocol October 2018 1. The keywords , , , , , , , , , and , when they appear in this document, are to be interpreted as described in [RFC2119]. 2. The publisher advertises the URL of one or more hubs to the subscriber, allowing it to receive live updates when topics are updated. If more than one hub URL is specified, it is expected that the publisher notifies each hub, so the subscriber subscribe to one or more of them. The publisher include at least one Link Header [RFC5988] with (a hub link header). The target URL of these links be a hub implementing the Mercure protocol. Note: this relation type has not been registered yet [RFC5988]. During the meantime, the relation type can be used instead. Dunglas Expires April 14, 2019 [Page 2]
Internet-Draft The Mercure Protocol October 2018 The publisher provide the following target attributes in the Link headers: All these attributes are optional. The publisher also include one Link Header [RFC5988] with (the self link header). It contain the canonical URL for the topic to which subscribers are expected to use for subscriptions. If the Link with is ommitted, the current URL of the resource be used as fallback. Minimal example: Links embedded in HTML or XML documents (as defined in the WebSub recommendation) also be supported by subscribers. Note: the discovery mechanism described in this section is strongly inspired from the one specified in the WebSub recommendation [1]. 3. The subscriber subscribes to an URL exposed by a hub to receive updates of one or many topics. To subscribe to updates, the client opens an HTTPS connection following the Server-Sent Events specification [2] to the hub's subscription URL advertised by the Publisher. The connection use HTTP/2 to leverage mutliplexing and other advanced features of this protocol. The subscriber specifies the list of topics to get updates for by using one or several query parameters named . The value of these query parameters be URI templates [RFC6570]. Note: an URL is also a valid URI template. Dunglas Expires April 14, 2019 [Page 3]
Internet-Draft The Mercure Protocol October 2018 The protocol doesn't specify the maximum number of parameters that can be sent, but the hub apply an arbitrary limit. The EventSource JavaScript interface [3] be used to establish the connection. Any other appropriate mechanism including but not limited to readable streams [4] and XMLHttpRequest [5] (used by popular polyfills) also be used. The hub sends updates concerning all subscribed resources matching the provided URI templates. The hub send these updates as text/event-stream compliant events [6]. The property contain the new version of the topic. It can be the full resource, or a partial update by using formats such as JSON Patch or JSON Merge Patch . All other properties defined in the Server-Sent Events specification be used and be supported by hubs. The resource be represented in a format with hypermedia capabilities such as JSON- LD [W3C.REC-json-ld-20140116], Atom [RFC4287], XML [W3C.REC-xml-20081126] or HTML [W3C.REC-html52-20171214]. Web Linking [RFC5988] be used to indicate the IRI of the resource sent in the event. When using Atom, XML or HTML as serialization format for the resource, the document Dunglas Expires April 14, 2019 [Page 4]
Internet-Draft The Mercure Protocol October 2018 contain a element with a relation containing the IRI of the resource. When using JSON-LD, the document contain an property containing the IRI of the resource. Example: 4. The hub receives updates from the publisher on a dedicated HTTPS endpoint. The connection use an encryption layer, such as TLS. HTTPS certificate can be obtained for free using Let's Encrypt [7]. When it receives an update, the hub dispatches it to subsribers using the established server-sent events connections. An application CAN send events directly to the subscribers, without using an external hub server, if it is able to do so. In this case, it implement the endpoint to publish updates. The endpoint to publish updates is an HTTPS URL accessed using the method. The request be encoded using the format and contains the following data: The request also contain an HTTP header containing the string followed by a valid JWS [RFC7515] in compact serialization that the hub will check to ensure that the publisher is authorized to publish the update. Dunglas Expires April 14, 2019 [Page 5]
Internet-Draft The Mercure Protocol October 2018 5. If a topic is not public, the update request sent by the publisher to the hub also contain a list of keys named . Theirs values are . They can be, for instance a user ID, or a list of group IDs. To receive updates for private topics, the subscriber send a cookie called when connecting to the hub. The cookie be set by the publisher during the discovery. The cookie have the , . It have the flag if appropriate. Setting the cookie's to the path of the subscribe endpoint is also . When skipping the discovery mechanism, the client set the cookie itself (for security reasons, this is not recommended in the context of a web browser). Consequently if the subscriber is a web browser, both the publisher and the hub have to share the same second level domain to use the autorization feature. The flag be used to allow the publisher and the host to use different subdomains. By the Dunglas Expires April 14, 2019 [Page 6]
Internet-Draft The Mercure Protocol October 2018 specification, connections can only be estabilished using the HTTP method, and it is not possible to set custom HTTP headers (such as the one). However, cookies are supported, and can be included even in crossdomain requests if the CORS credentials are set [8]: The value of this cookie be a JWS in compact serialization. It have a claim named that contains an array of strings: the list of targets the user is authorized to receive updates for. For instance, valid targets can be a username or a list of group identifiers. The JWS be short lived, especially if the subscriber is a web browser. If one or more targets are specified, the update be sent to the subscriber by the hub, unless the claim of the subscriber contains at least one target specified for the topic by the publisher. When using the authorization mechanism, the connection between the subscriber and the hub use an encryption layer (HTTPS is required). 6. To allow re-establisment in case of connection lost, events dispatched by the hub include an property. The value contained in this property be a globally unique identifier. To do so, UUID [RFC4122] be used. Dunglas Expires April 14, 2019 [Page 7]
Internet-Draft The Mercure Protocol October 2018 According to the server-sent events specification, in case of connection lost the subscriber will try to automatically reconnect. During the reconnection the subscriber send the last received event id in a Last-Event-ID [9] HTTP header. The server-sent events specification doesn't allow to set this HTTP header during the first connection (before a re-connection occurs). In order to fetch any update dispatched between the initial resource generation by the publisher and the connection to he hub, the subscriber send the event id provided during the discovery in the link's attribute in a query parameter named when connecting to the hub. If both the HTTP header and the query parameter are present, the HTTP header take precedence. If the header or query parameter exists, the hub send to the subscriber all events published since the one having this identifier. The hub discard some messages for operational reasons. The subscriber assume that no update will be lost, and re-fetch the original topic to ensure this (for instance, after a long deconnection time). The hub also specify the reconnection time using the key, as specified in the server-sent events format. Dunglas Expires April 14, 2019 [Page 8]
Internet-Draft The Mercure Protocol October 2018 7. Using HTTPS doesn't prevent the hub to access to the update's content. Depending of the intended privacy of informations contained in the updates, it be necessary to prevent eavesdropping by the hub. To make sure that the message content can not be read by the hub, the publisher encode the message before sending it to the hub. The publisher use JSON Web Encryption [RFC7516] to encrypt the update content. The publisher provide the relevant encryption key(s) in the attribute of the Link HTTP header during the discovery. The attribute contain a key encoded using the JSON Web Key Set [RFC7517] format. Any other out-of-band mechanism be used instead to share the key between the publisher and the subscriber. Updates encyption is considered a best practice to prevent mass surveillance. This is especially relevant if the hub is managed by an external provider. 8. References 8.1. References 8.2. References 8.3. URIs [1] https://www.w3.org/TR/websub/#discovery [2] https://html.spec.whatwg.org/multipage/server-sent-events.html [3] https://html.spec.whatwg.org/multipage/server-sent- events.html#the-eventsource-interface Dunglas Expires April 14, 2019 [Page 9]
Internet-Draft The Mercure Protocol October 2018 [4] https://developer.mozilla.org/en-US/docs/Web/API/Streams_API/ Using_readable_streams [5] https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/ Using_XMLHttpRequest [6] https://html.spec.whatwg.org/multipage/server-sent- events.html#sse-processing-model [7] https://letsencrypt.org/ [8] https://html.spec.whatwg.org/multipage/server-sent- events.html#dom-eventsourceinit-withcredentials [9] https://html.spec.whatwg.org/multipage/iana.html#last-event-id Author's Address Kevin Dunglas Les-Tilleuls.coop 5 rue Hegel Lille 59000 France Email: kevin@les-tilleuls.coop Dunglas Expires April 14, 2019 [Page 10]