The Mercure Protocol
draft-dunglas-mercure-01

Network Working Group                                         K. Dunglas
Internet-Draft                                         Les-Tilleuls.coop
Intended status: Informational                           11 October 2018
Expires: 14 April 2019

                          The Mercure Protocol
                        draft-dunglas-mercure-01

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.

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   This Internet-Draft will expire on 14 April 2019.

Copyright Notice

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Table of Contents

   1.  Terminology
   2.  Discovery
   3.  Subscriptions
   4.  Hub
   5.  Authorization
   6.  Re-Connection and State Reconciliation
   7.  Encryption
   8.  References
       8.1.  Normative References
       8.2.  Informative References
   Author's Address

1.  Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   *  Topic: An HTTP [RFC7230] or HTTPS [RFC2818] topic URL.  The unit
      to which one can subscribe to changes.

   *  Publisher: An owner of a topic.  Notifies the hub when the topic
      feed has been updated.  As in almost all pubsub systems, the
      publisher is unaware of the subscribers, if any.  Other pubsub
      systems might call the publisher the "source".  Typically a
      website or a web API.

   *  Subscriber: A client application that subscribes to real-time
      updates of topics (typically a Progressive Web App or a Mobile
      App).

   *  Hub: A server that handles subscription requests and distributes
      the content to subscribers when the corresponding topics have been
      updated (a Hub implementation is provided in this repository).
      Any hub MAY implement its own policies on who can use it.

2.  Discovery

   The publisher SHOULD 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 MAY subscribe to
   one or more of them.

   The publisher SHOULD include at least one Link Header [RFC5988] with
   "rel=mercure" (a hub link header).  The target URL of these links
   MUST be a hub implementing the Mercure protocol.

   Note: this relation type has not been registered yet [RFC5988].
   During the meantime, the relation type "https://git.io/mercure" can
   be used instead.

   The publisher MAY provide the following target attributes in the Link
   headers:

   *  "last-event-id": the globally unique identifier of the last event
      dispatched by the publisher at the time of the generation of this
      resource.  If provided, it MUST be passed to the hub through a
      query parameter called "Last-Event-ID" and will be used to ensure
      that possible updates having been made during between the resource
      generation time and the connection to the hub are not lost.  See
      section #Re-Connection-and-State-Reconciliation).  If this
      attribute is provided, the publisher MUST always set the "id"
      parameter when sending updates to the hub.

   *  "content-type": the content type of the updates that will pushed
      by the hub.  If omited, the subscriber MUST assume that the
      content type will be the same than the one of the original
      resource.  Setting the "content-type" attribute is especially
      useful to hint that partial updates will be pushed, using formats
      such as JSON Patch [RFC6902] or JSON Merge Patch [RFC7386].

   *  "key-set=<JWKS>": the key(s) to decrypt updates encoded in the
      JWKS (JSON Web Key Set) format (see the Encryption section).

   All these attributes are optional.

   The publisher MAY also include one Link Header [RFC5988] with
   "rel=self" (the self link header).  It SHOULD contain the canonical
   URL for the topic to which subscribers are expected to use for
   subscriptions.  If the Link with "rel=self" is ommitted, the current
   URL of the resource MUST be used as fallback.

   Minimal example:

   GET /books/foo.jsonld HTTP/1.1
   Host: example.com

   HTTP/1.1 200 Ok
   Content-type: application/ld+json
   Link: <https://hub.example.com/subscribe>; rel="mercure"

   {"@id": "/books/foo.jsonld", "foo": "bar"}

   Links embedded in HTML or XML documents (as defined in the WebSub
   recommendation) MAY also be supported by subscribers.

   Note: the discovery mechanism described in this section is strongly
   inspired from the one specified in the WebSub recommendation
   (https://www.w3.org/TR/websub/#discovery).

3.  Subscriptions

   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 (https://html.spec.whatwg.org/multipage/server-sent-
   events.html) to the hub's subscription URL advertised by the
   Publisher.  The connection SHOULD 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 "topic".  The value of
   these query parameters MUST be URI templates [RFC6570].

   Note: an URL is also a valid URI template.

   The protocol doesn't specify the maximum number of "topic" parameters
   that can be sent, but the hub MAY apply an arbitrary limit.

   The EventSource JavaScript interface
   (https://html.spec.whatwg.org/multipage/server-sent-events.html#the-
   eventsource-interface) MAY be used to establish the connection.  Any
   other appropriate mechanism including but not limited to readable
   streams (https://developer.mozilla.org/en-
   US/docs/Web/API/Streams_API/Using_readable_streams) and
   XMLHttpRequest (https://developer.mozilla.org/en-
   US/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest) (used by popular
   polyfills) MAY also be used.

   The hub sends updates concerning all subscribed resources matching
   the provided URI templates.  The hub MUST send these updates as text/
   event-stream compliant events
   (https://html.spec.whatwg.org/multipage/server-sent-events.html#sse-
   processing-model).

   The "data" property MUST contain the new version of the topic.  It
   can be the full resource, or a partial update by using formats such
   as JSON Patch "@RFC6902" or JSON Merge Patch "@RFC7386".

   All other properties defined in the Server-Sent Events specification
   MAY be used and SHOULD be supported by hubs.

   The resource SHOULD 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] SHOULD 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 SHOULD contain a
   "link" element with a "self" relation containing the IRI of the
   resource.  When using JSON-LD, the document SHOULD contain an "@id"
   property containing the IRI of the resource.

   Example:

   // The subscriber subscribes to updates for the https://example.com/foo topic
   // and to any topic matching https://example.com/books/{name}
   const url = new URL('https://hub.example.com/subscribe');
   url.searchParams.append('topic', 'https://example.com/foo');
   url.searchParams.append('topic', 'https://example.com/bar/{id}');

   const eventSource = new EventSource(url);

   // The callback will be called every time an update is published
   eventSource.onmessage = function ({data}) {
       console.log(data);
   };

4.  Hub

   The hub receives updates from the publisher on a dedicated HTTPS
   endpoint.  The connection MUST use an encryption layer, such as TLS.
   HTTPS certificate can be obtained for free using Let's Encrypt
   (https://letsencrypt.org/).

   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 *MAY NOT* implement the endpoint to publish updates.

   The endpoint to publish updates is an HTTPS URL accessed using the
   "POST" method.  The request MUST be encoded using the "application/x-
   www-form-urlencoded" format and contains the following data:

   *  "topic": IRIs of the updated topic.  If this key is present
      several times, the first occurence is considered to be the
      canonical URL of the topic, and other ones are considered to be
      alternate URLs.  The hub MUST dispatch this update to subscribers
      subscribed to both canonical or alternate URLs.

   *  "data": the content of the new version of this topic

   *  "target" (optional): target audience of this event, see the
      Authorization section for further information.

   *  "id" (optional): the topic's revision identifier, it will be used
      as the SSE's "id" property, if omited the hub MUST generate a
      valid UUID.

   *  "type" (optional): the SSE's "event" property (a specific event
      type)

   *  "retry" (optional): the SSE's "retry" property (the reconnection
      time)

   The request MUST also contain an "Authorization" HTTP header
   containing the string "Bearer" 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.

5.  Authorization

   If a topic is not public, the update request sent by the publisher to
   the hub MUST also contain a list of keys named "target".  Theirs
   values are "string".  They can be, for instance a user ID, or a list
   of group IDs.

   To receive updates for private topics, the subscriber MUST send a
   cookie called "mercureAuthorization" when connecting to the hub.

   The cookie SHOULD be set by the publisher during the discovery.  The
   cookie SHOULD have the "Secure", "HttpOnly".  It MAY have the
   "SameSite" flag if appropriate.  Setting the cookie's "Path" to the
   path of the subscribe endpoint is also RECOMMENDED.  When skipping
   the discovery mechanism, the client MAY 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 "Domain" flag MAY be used to allow the
   publisher and the host to use different subdomains.

   By the "EventSource" specification, connections can only be
   estabilished using the "GET" HTTP method, and it is not possible to
   set custom HTTP headers (such as the "Authorization" one).

   However, cookies are supported, and can be included even in
   crossdomain requests if the CORS credentials are set
   (https://html.spec.whatwg.org/multipage/server-sent-events.html#dom-
   eventsourceinit-withcredentials):

   The value of this cookie MUST be a JWS in compact serialization.  It
   MUST have a claim named "mercureTargets" 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 SHOULD be short lived, especially if
   the subscriber is a web browser.

   If one or more targets are specified, the update MUST NOT be sent to
   the subscriber by the hub, unless the "mercureTargets" 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 MUST use an encryption layer (HTTPS is
   required).

6.  Re-Connection and State Reconciliation

   To allow re-establisment in case of connection lost, events
   dispatched by the hub SHOULD include an "id" property.  The value
   contained in this "id" property SHOULD be a globally unique
   identifier.  To do so, UUID [RFC4122] MAY be used.

   According to the server-sent events specification, in case of
   connection lost the subscriber will try to automatically reconnect.
   During the reconnection the subscriber MUST send the last received
   event id in a Last-Event-ID (https://html.spec.whatwg.org/multipage/
   iana.html#last-event-id) 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 MUST send the event id provided during the discovery in
   the "last-event-id" link's attribute in a query parameter named
   "Last-Event-ID" when connecting to the hub.

   If both the "Last-Event-ID" HTTP header and the query parameter are
   present, the HTTP header MUST take precedence.

   If the "Last-Event-ID" header or query parameter exists, the hub
   SHOULD send to the subscriber all events published since the one
   having this identifier.

   The hub MAY discard some messages for operational reasons.  The
   subscriber MUST NOT assume that no update will be lost, and MUST re-
   fetch the original topic to ensure this (for instance, after a long
   deconnection time).

   The hub MAY also specify the reconnection time using the "retry" key,
   as specified in the server-sent events format.

7.  Encryption

   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 MAY be necessary to prevent eavesdropping by the
   hub.

   To make sure that the message content can not be read by the hub, the
   publisher MAY encode the message before sending it to the hub.  The
   publisher SHOULD use JSON Web Encryption [RFC7516] to encrypt the
   update content.  The publisher MAY provide the relevant encryption
   key(s) in the "key-set" attribute of the Link HTTP header during the
   discovery.  The "key-set" attribute SHOULD contain a key encoded
   using the JSON Web Key Set [RFC7517] format.  Any other out-of-band
   mechanism MAY 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.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000, <https://www.rfc-
              editor.org/info/rfc2818>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005, <https://www.rfc-
              editor.org/info/rfc4122>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010, <https://www.rfc-
              editor.org/info/rfc5988>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012, <https://www.rfc-
              editor.org/info/rfc6570>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <https://www.rfc-editor.org/info/rfc7516>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015, <https://www.rfc-
              editor.org/info/rfc7517>.

8.2.  Informative References

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <https://www.rfc-editor.org/info/rfc4287>.

   [RFC6902]  Bryan, P., Ed. and M. Nottingham, Ed., "JavaScript Object
              Notation (JSON) Patch", RFC 6902, DOI 10.17487/RFC6902,
              April 2013, <https://www.rfc-editor.org/info/rfc6902>.

   [RFC7386]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7386,
              DOI 10.17487/RFC7386, October 2014, <https://www.rfc-
              editor.org/info/rfc7386>.

   [W3C.REC-html52-20171214]
              Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
              S. Moon, "HTML 5.2", World Wide Web Consortium
              Recommendation REC-html52-20171214, 14 December 2017,
              <https://www.w3.org/TR/2017/REC-html52-20171214>.

   [W3C.REC-json-ld-20140116]
              Sporny, M., Kellogg, G., and M. Lanthaler, "JSON-LD 1.0",
              World Wide Web Consortium Recommendation REC-json-ld-
              20140116, 16 January 2014, <http://www.w3.org/TR/2014/REC-
              json-ld-20140116>.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, 26 November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

Author's Address

   Kevin Dunglas
   Les-Tilleuls.coop
   5 rue Hegel
   Lille  59000
   France

   Email: kevin@les-tilleuls.coop