The Mercure Protocol

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/.¶

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This Internet-Draft will expire on 11 December 2020.¶

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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Table of Contents

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: The unit to which one can subscribe to changes. The topic SHOULD be identified by an IRI [RFC3987]. Using an HTTPS [RFC7230] or HTTP [RFC7230] URI [RFC3986] is RECOMMENDED.¶

• Update: The message containing the updated version of the topic. An update can be marked as private, consequently, it must be dispatched only to subscribers allowed to receive it.¶

• Topic selector: An expression matching one or several topics.¶

• 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, but can also be a web browser.¶

• Subscriber: A client application that subscribes to real-time updates of topics using topic selectors. Typically a web or a mobile application, but can also be a server.¶

• Subscription: A topic selector used by a subscriber to receive updates. A single subscriber can have several subscriptions, when it provides several topic selectors.¶

• Hub: A server that handles subscription requests and distributes the content to subscribers when the corresponding topics have been updated. Any hub MAY implement its own policies on who can use it.¶

2. Discovery

The discovery mechanism aims at identifying at least 2 URLs.¶

1. The URL of one or more hubs designated by the publisher.¶

2. The canonical URL for the topic to which subscribers are expected to use for subscriptions.¶

The URL of the hub MUST be the "well-known" [RFC5785] fixed path /.well-known/mercure.¶

If the publisher is a server, it SHOULD advertise 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, the publisher MUST notifies each hub, so the subscriber MAY subscribe to one or more of them.¶

Note: Publishers may wish to advertise and publish to more than one hub for fault tolerance and redundancy. If one hub fails to propagate an update to the document, then using multiple independent hub is a way to increase the likelihood of delivery to subscribers. As such, subscribers may subscribe to one or more of the advertised hubs.¶

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.¶

The publisher MAY provide the following target attributes in the Link Headers:¶

• last-event-id: the 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 7.¶

• content-type: the content type of the updates that will pushed by the hub. If omitted, the subscriber MUST assume that the content type will be the same as that 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: the URL of the key set to use to decrypt updates, encoded in the JWK set format (JSON Web Key Set) [RFC7517]. See Section 10. As this key set will contain a secret key, the publisher must ensure that only the subscriber can access to this URL. To do so, the authorization mechanism (see Section 6) can be reused.¶

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 omitted, the current URL of the resource MUST be used as a fallback.¶

Minimal example:¶

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

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

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

Links embedded in HTML or XML documents as defined in the WebSub recommendation [W3C.REC-websub-20180123] MAY also be supported by subscribers. If both a header and an embedded link are provided, the header MUST be preferred.¶

GET /books/foo HTTP/1.1
Host: example.com
Accept: application/ld+json

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: </books/foo.jsonld>; rel="self"
Link: <https://example.com/.well-known/mercure>; rel="mercure"

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

GET /books/foo HTTP/1.1
Host: example.com
Accept: text/html

HTTP/1.1 200 OK
Content-type: text/html
Link: </books/foo.html>; rel="self"
Link: <https://example.com/.well-known/mercure>; rel="mercure"

<!doctype html>
<title>foo: bar</title>

GET /books/foo HTTP/1.1
Host: example.com
Accept-Language: fr-FR

HTTP/1.1 200 OK
Content-type: application/ld+json
Content-Language: fr-FR
Link: </books/foo-fr-FR.jsonld>; rel="self"
Link: <https://example.com/.well-known/mercure>; rel="mercure"

{"@id": "/books/foo", "foo": "bar", "@context": {"@language": "fr-FR"}}

3. Topic Selectors

A topic selector is an expression intended to be matched by one or several topics. A topic selector can also be used to match other topic selectors for authorization purposes. See Section 6.¶

A topic selector can be any string including URI Templates [RFC6570] and the reserved string * that matches all topics. It is RECOMMENDED to use URI Templates or the reserved string * as topic selectors.¶

Note: URLs and IRIs are valid URI templates.¶

To determine if a string matches a selector, the following steps must be followed:¶

1. If the topic selector is * then the string matches the selector.¶

2. If the topic selector and the string are exactly the same, the string matches the selector. This characteristic allows to compare a URI Template with another one.¶

3. If the topic selector is a valid URI Template, and that the string matches this URI Template, the string matches the selector.¶

4. Otherwise the string does not match the selector.¶

4. Subscription

The subscriber subscribes to a URL exposed by a hub to receive updates from one or many topics. To subscribe to updates, the client opens an HTTPS connection following the Server-Sent Events specification [W3C.REC-eventsource-20150203] to the hub's subscription URL advertised by the publisher. The GET HTTP method must be used. 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 from by using one or several query parameters named topic. The topic query parameters MUST contain topic selectors. See Section 3.¶

The protocol doesn't specify the maximum number of topic parameters that can be sent, but the hub MAY apply an arbitrary limit. A subscription is created for every provided topic parameter. See Section 8.1.¶

The EventSource JavaScript interface MAY be used to establish the connection. Any other appropriate mechanism including, but not limited to, readable streams [W3C.NOTE-streams-api-20161129] and XMLHttpRequest (used by popular polyfills) MAY also be used.¶

The hub sends to the subscriber updates for topics matching the provided topic selectors.¶

If an update is marked as private, the hub MUST NOT dispatch it to subscribers not authorized to receive it. See Section 6.¶

The hub MUST send these updates as text/event-stream compliant events [!@W3C.REC-eventsource-20150203].¶

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 MUST 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 the 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, the bar topic,
// and to any topic matching https://example.com/books/{name}
const url = new URL('https://example.com/.well-known/mercure');
url.searchParams.append('topic', 'https://example.com/foo');
url.searchParams.append('topic', 'bar');
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);
};

The hub MAY require subscribers and publishers to be authenticated, and MAY apply extra authorization rules not defined in this specification.¶

5. Publication

The publisher sends updates by issuing POST HTTPS requests on the hub URL. When it receives an update, the hub dispatches it to subscribers using the established server-sent events connections.¶

An application CAN send events directly to 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 request MUST be encoded using the application/x-www-form-urlencoded format [W3C.REC-html52-20171214] and contains the following name-value tuples:¶

• topic: The identifiers of the updated topic. It is RECOMMENDED to use an IRI as identifier. If this name is present several times, the first occurrence is considered to be the canonical IRI of the topic, and other ones are considered to be alternate IRIs. The hub MUST dispatch this update to subscribers that are subscribed to both canonical or alternate IRIs.¶

• data (optional): the content of the new version of this topic.¶

• private (optional): if this name is set, the update MUST NOT be dispatched to subscribers not authorized to receive it. See Section 6. It is recommended to set the value to on but it CAN contain any value including an empty string.¶

• id (optional): the topic's revision identifier: it will be used as the SSE's id property. The provided id MUST NOT start with the # character. The provided id SHOULD be a valid IRI. If omitted, the hub MUST generate a valid IRI [RFC3987]. An UUID [RFC4122] or a DID (@W3C.WD-did-core-20200421) MAY be used. Alternatively the hub MAY generate a relative URI composed of a fragment (starting with #). This is convenient to return an offset or a sequence that is unique for this hub. Even if provided, the hub MAY ignore the id provided by the client and generate its own id.¶

• type (optional): the SSE's event property (a specific event type).¶

• retry (optional): the SSE's retry property (the reconnection time).¶

In the event of success, the HTTP response's body MUST be the id associated to this update generated by the hub and a success HTTP status code MUST be returned. The publisher MUST be authorized to publish updates. See Section 6.¶

6. Authorization

To ensure that they are authorized, both publishers and subscribers must present a valid JWS [RFC7515] in compact serialization to the hub. This JWS SHOULD be short-lived, especially if the subscriber is a web browser. A different key MAY be used to sign subscribers' and publishers' tokens.¶

Two mechanisms are defined to present the JWS to the hub:¶

• using an Authorization HTTP header¶

• using a cookie¶

If the publisher or the subscriber is not a web browser, it SHOULD use an Authorization HTTP header. This Authorization header MUST contain the string Bearer followed by the JWS. The hub will check that the JWS conforms to the rules (defined later) ensuring that the client is authorized to publish or subscribe to updates.¶

By the EventSource specification [W3C.REC-eventsource-20150203], web browsers can not set custom HTTP headers for such connections, and they can only be established using the GET HTTP method. However, cookies are supported and can be included even in cross-domain requests if the CORS credentials are set:¶

If the publisher or the subscriber is a web browser, it SHOULD send a cookie called mercureAuthorization containing the JWS when connecting to the hub.¶

Whenever possible, the mercureAuthorization cookie SHOULD be set during discovery (see Section 2) to improve the overall security. Consequently, if the cookie is set during the discovery, both the publisher and the hub have to share the same second level domain. The Domain attribute MAY be used to allow the publisher and the hub to use different subdomains. See Section 2.¶

The cookie SHOULD have the Secure, HttpOnly and SameSite attributes set. The cookie's Path attribute SHOULD also be set to the hub's URL. See Section 12.¶

When using authorization mechanisms, the connection MUST use an encryption layer such as HTTPS.¶

If both an Authorization HTTP header and a cookie named mercureAuthorization are presented by the client, the cookie MUST be ignored. If the client tries to execute an operation it is not allowed to, a 403 HTTP status code SHOULD be returned.¶

7. Reconnection, State Reconciliation and Event Sourcing

The protocol allows to reconciliate states after a reconnection. It can also be used to implement an Event store.¶

To allow re-establishment in case of connection lost, events dispatched by the hub MUST include an id property. The value contained in this id property SHOULD be an IRI [RFC3987]. An UUID [RFC4122] or a DID (@W3C.WD-did-core-20200421) MAY be used.¶

According to the server-sent events specification, in case of connection lost the subscriber will try to automatically re-connect. During the re-connection, the subscriber MUST send the last received event id in a Last-Event-ID HTTP header.¶

In order to fetch any update dispatched between the initial resource generation by the publisher and the connection to the hub, the subscriber MUST send the event id provided during the discovery in the last-event-id as the last event id. See Section 2.¶

EventSource implementations may not allow to set HTTP headers during the first connection (before a reconnection) and implementations in web browsers don't allow to set it.¶

To work around this problem, the hub MUST also allow to pass the last event id in a query parameter named Last-Event-ID.¶

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 HTTP header or query parameter exists, the hub SHOULD send all events published following the one bearing this identifier to the subscriber.¶

The reserved value earliest can be used to hint the hub to send all updates it has for the subscribed topics. According to its own policy, the hub MAY or MAY NOT fulfil this request.¶

The hub MAY discard some events for operational reasons. When the request contains a Last-Event-ID HTTP header or query parameter the hub MUST set a Last-Event-ID header on the HTTP response. The value of the Last-Event-ID response header MUST be the id of the event preceding the first one sent to the subscriber, or the reserved value earliest if there is no preceding event (it happens when the hub history is empty, when the subscriber requests the earliest event or when the subscriber requests an event that doesn't exist).¶

The subscriber SHOULD NOT assume that no events will be lost (it may happen, for example if the hub stores only a limited number of events in its history). In some cases (for instance when sending partial updates in the JSON Patch [RFC6902] format, or when using the hub as an event store), updates lost can cause data lost.¶

To detect if a data lost ocurred, the subscriber CAN compare the value of the Last-Event-ID response HTTP header with Last-Event-ID it requested. In case of data lost, the subscriber SHOULD re-fetch the original topic.¶

Note: Native EventSource implementations don't give access to headers associated with the HTTP response, however polyfills and server-sent events clients in most programming languages allow it.¶

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

8. Active Subscriptions

Mercure provides a mechanism to track active subscriptions. If the hub support this optional set of features, updates will be published when a subscription is created, or terminated, and a web API exposes the list of active subscriptions.¶

Variables are templated and expanded in accordance with [RFC6570].¶

{
   "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "type": "Subscription",
   "topic": "https://example.com/{selector}",
   "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "active": true,
   "payload": {"foo": "bar"}
}

GET /.well-known/subscriptions HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"
ETag: urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb
Cache-control: must-revalidate

{
   "@context": "https://mercure.rocks/",
   "id": "/.well-known/subscriptions",
   "type": "Subscriptions",
   "lastEventID": "urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb",
   "subscriptions": [
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "active": true,
         "payload": {"foo": "bar"}
      },
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2Fa-topic/urn%3Auuid%3A1e0cba4c-4bcd-44f0-ae8a-7b76f7ef1280",
         "type": "Subscription",
         "topic": "https://example.com/a-topic",
         "subscriber": "urn:uuid:1e0cba4c-4bcd-44f0-ae8a-7b76f7ef1280",
         "active": true,
         "payload": {"baz": "bat"}
      },
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Aa6c49794-5f74-4723-999c-3a7e33e51d49",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:a6c49794-5f74-4723-999c-3a7e33e51d49",
         "active": true,
         "payload": {"foo": "bap"}
      }
   ]
}

GET /.well-known/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"
ETag: urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb
Cache-control: must-revalidate

{
   "@context": "https://mercure.rocks/",
   "id": "/.well-known/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D",
   "type": "Subscriptions",
   "lastEventID": "urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb",
   "subscriptions": [
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
         "active": true,
         "payload": {"foo": "bar"}
      },
      {
         "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Aa6c49794-5f74-4723-999c-3a7e33e51d49",
         "type": "Subscription",
         "topic": "https://example.com/{selector}",
         "subscriber": "urn:uuid:a6c49794-5f74-4723-999c-3a7e33e51d49",
         "active": true,
         "payload": {"foo": "bap"}
      }
   ]
}

GET /.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6 HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Content-type: application/ld+json
Link: <https://example.com/.well-known/mercure>; rel="mercure"
ETag: urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb
Cache-control: must-revalidate

{
   "@context": "https://mercure.rocks/",
   "id": "/.well-known/mercure/subscriptions/https%3A%2F%2Fexample.com%2F%7Bselector%7D/urn%3Auuid%3Abb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "type": "Subscription",
   "topic": "https://example.com/{selector}",
   "subscriber": "urn:uuid:bb3de268-05b0-4c65-b44e-8f9acefc29d6",
   "active": true,
   "payload": {"foo": "bar"},
   "lastEventID": "urn:uuid:5e94c686-2c0b-4f9b-958c-92ccc3bbb4eb"
}

9. JSON-LD Context

The JSON-LD context available at https://mercure.rocks is the following:¶

{
"@context": {
   "@vocab": "_:",
   "mercure": "https://mercure.rocks/",
   "id": "@id",
   "type": "@type",
   "Subscription": "mercure:Subscription",
   "Subscriptions": "mercure:Subscriptions",
   "subscriptions": "mercure:subscriptions",
   "topic": "mercure:topic",
   "subscriber": "mercure:subscriber",
   "active": "mercure:active",
   "payload": "mercure:payload",
   "lastEventID": "mercure:lastEventID"
}

10. Encryption

Using HTTPS does not prevent the hub from accessing the update's content. Depending of the intended privacy of information contained in the update, 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 encrypt 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 URL pointing to the relevant encryption key(s) in the key-set attribute of the Link HTTP header during the discovery. See Section 2. The key-set attribute MUST 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.¶

Update encryption is considered a best practice to prevent mass surveillance. This is especially relevant if the hub is managed by an external provider.¶

11. IANA Considerations

12. Security Considerations

The confidentiality of the secret key(s) used to generate the JWSs is a primary concern. The secret key(s) MUST be stored securely. They MUST be revoked immediately in the event of compromission.¶

Possessing valid JWSs allows any client to subscribe, or to publish to the hub. Their confidentiality MUST therefore be ensured. To do so, JWSs MUST only be transmitted over secure connections.¶

Also, when the client is a web browser, the JWS SHOULD not be made accessible to JavaScript scripts for resilience against Cross-site Scripting (XSS) attacks. It's the main reason why, when the client is a web browser, using HttpOnly cookies as the authorization mechanism SHOULD always be preferred.¶

In the event of compromission, revoking JWSs before their expiration is often difficult. To that end, using short-lived tokens is strongly RECOMMENDED.¶

The publish endpoint of the hub may be targeted by Cross-Site Request Forgery (CSRF) attacks when the cookie-based authorization mechanism is used. Therefore, implementations supporting this mechanism MUST mitigate such attacks.¶

The first prevention method to implement is to set the mercureAuthorization cookie's SameSite attribute. However, some web browsers still not support this attribute and will remain vulnerable. Additionally, hub implementations SHOULD use the Origin and Referer HTTP headers set by web browsers to verify that the source origin matches the target origin. If none of these headers are available, the hub SHOULD discard the request.¶

CSRF prevention techniques, including those previously mentioned, are described in depth in OWASP's Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet.¶

13. Implementation Status

[RFC Editor Note: Please remove this entire seciton prior to publication as an RFC.]¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC6982]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to RFC 6982, "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit."¶

14. Acknowledgements

Parts of this specification, especially Section 2 have been adapted from the WebSub recommendation [W3C.REC-websub-20180123]. The editor wish to thanks all the authors of this specification.¶

Author's Address