Per Resource Events

1.1. Motivation

HTTP was originally designed to transfer a static documents within a single request and response. If the document changes, HTTP does not automatically update clients with the new versions. This design was adequate for web pages that were mostly static and written by hand.¶

But web-applications today are dynamic. They provide (near-)instantaneous updates across multiple clients and servers. The many workarounds developed over the years to provide real-time updates over HTTP have often proven to be inadequate. Web programmers instead resort to implementing custom messaging systems over alternate protocols such as WebSockets, which requires additional layers of code in the form of non-standard JavaScript frameworks to synchronize changes of state.¶

Per Resource Events is a minimal protocol built on top of HTTP that allows clients to receive notifications directly from a resource of interest. Unlike other HTTP based solutions, Per Resource Events Protocol supports the use of arbitrary media-types for notifications, which can be negotiated just like representations; thus giving implementers a lot of flexibility to customize notifications according to the needs of their application.¶

1.2. How it Works

GET /foo HTTP/1.1
Host: example.org

GET /foo HTTP/1.1
Host: example.org
Accept-Events: "prep"

HTTP/1.1 200 OK
Last-Modified: Sat, 1 April 2023 10:11:12 GMT
Transfer-Encoding: chunked
ETag: 1234abcd
Content-Type: text/plain

Hello World!

HTTP/1.1 200 OK
Last-Modified: Sat, 1 April 2023 10:11:12 GMT
Transfer-Encoding: chunked
ETag: 1234abcd
Events: protocol="prep", status=200
Vary: Accept-Events
Accept-Events: "prep"; accept="message/rfc822"
Content-Type: multipart/mixed; boundary="MAIN-SEPARATOR"

--MAIN SEPARATOR
Content-Type: text/plain

Hello World!

--MAIN-SEPARATOR
Content-Type: multipart/digest; boundary="MESSAGE-SEPARATOR"

--MESSAGE-SEPARATOR

Method: PUT
Date: Sat, 1 April 2023 10:11:12 GMT
Event-ID: 456
E-tag: 6789wxyz


--MESSAGE-SEPARATOR

Method: DELETE
Date: Sat, 1 April 2023 10:11:12 GMT
Event-ID: 789


--MESSAGE-SEPARATOR--
--MAIN-SEPARATOR--

HTTP/1.1 200 OK
Last-Modified: Sat, 1 April 2023 10:11:12 GMT
Transfer-Encoding: chunked
ETag: 1234abcd
Events: protocol="prep", status=200
Vary: Accept-Events
Accept-Events: "prep"; accept="message/rfc822"
Content-Type: multipart/mixed; boundary="MAIN-SEPARATOR"

--MAIN SEPARATOR
Content-Type: text/plain

Hello World!

--MAIN-SEPARATOR
Content-Type: multipart/digest; boundary="MESSAGE-SEPARATOR"

--MESSAGE-SEPARATOR

Method: PUT
Date: Sat, 1 April 2023 10:11:12 GMT
Event-ID: 456
E-tag: 6789wxyz


--MESSAGE-SEPARATOR

Method: DELETE
Date: Sat, 1 April 2023 10:11:12 GMT
Event-ID: 789


--MESSAGE-SEPARATOR--
--MAIN-SEPARATOR--

1.3. Scope

The Per Resource Events Protocol specifies:¶

• a mechanism for the discovery of notification capabilities per resource,¶

• a mechanism to request notifications from a resource,¶

• representation and semantics for the response that transmits notifications,¶

• a default representation for notifications and associated semantics when used to transmit notifications.¶

The Per Resource Events Protocol does not specify:¶

• a specific authentication and authorization mechanism to be used with the Per Resource Events Protocol. Implementations are encouraged to use existing approaches for authentication and authorization.¶

• representation and semantics for notifications (other than the default case).¶

  • Implementations are free to use any media-type for notifications, which can be negotiated just like representations.¶

  • Implementations are also free to define additional semantics for a given media-type, when used to transmit notifications using the Per Resource Events Protocol.¶

1.4. Audience

This specification is for:¶

• Server developers who wish to enable clients to listen for updates to particular resources.¶

• Application developers who wish to implement a client to listen for updates to particular resources.¶

2.1. Principles

The Per Resource Events Protocol treats notifications as a temporally extended representation of any resource. That is, a representation can describe not (just) the state of the resource but events on the resource. With HTTP allowing representations to provide a potentially unbounded stream of data, the Per Resource Events Protocol is able to communicate events on the resource as notifications.¶

2.2. Goals

The goals of the Per Resource Events Protocol are:¶

• to provide notifications only using the HTTP protocol STD97 [HTTP] so that the clients are able to request for update notifications using the Fetch API [FETCH].¶

• to provide updates directly from the resource of interest, obliviating the need to connect to another endpoint for notifications, minimizing round-trips between clients and servers and the need to co-ordinate responses between a resource and the endpoint.¶

• to allow arbitrary representations for notifications. Implementers shall be able to provide notifications that are potentially more expressive when compared to existing HTTP based messaging protocols such as Server Sent Events [SSE].¶

2.3. Constraints

To the extent feasible, the Per Resource Events Protocol:¶

• adheres to established practices and conventions. In particular, every attempt has been made to reuse existing protocols and specifications. Implementers shall be able to repurpose existing tools and libraries for implementing this specification.¶

• conforms to the REST Architectural Style [REST] and best practices for Building Protocols with HTTP [RFC9205]. This specification can, thus, be used to extend the capabilities of any existing HTTP resource to provide notifications. Implementers shall be able to scale notifications along with their data/applications.¶

2.4. Known Limitations

The Per Resource Events Protocol only allows notifications to be sent for events on a given resource. It is not possible for resources to send notifications for arbitrary events such as state changes on multiple resources or combinations thereof. Implementations using Per Resource Events Protocol have to create separate resources to realize such notification endpoints. But this is no different from APIs built on top of existing messaging protocols (See, for example, [WS] and [WEBSUB]).¶

Due to the limits on connections per domain, the Per Resource Events Protocol is not suitable for use with HTTP/1.1, especially when providing notifications for multiple resource from the same domain that might be accessed simultaneously. This limitation is identical to that of other HTTP based streaming based protocols such as Server-Sent Events [SSE]. Implementations are strongly encouraged to adopt HTTP/2 (or later) and implement mitigation strategies, such as to maximize the number of concurrent connections and to provide alternate locations for resources on different domains.¶

3.1. Document Conventions

All assertions, diagrams, examples and notes in this specification are non-normative.¶

3.2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.3. Classes of Products

The Per Resource Events Protocol identifies the following Classes of Products for conforming implementations. These products are referenced throughout this specification.¶

Resource Server

that builds on HTTP origin server ([HTTP], Section 3.6) by defining header fields ([HTTP], Section 6.3) and media types ([HTTP], Section 8.3.1).¶

Application Client

that builds on HTTP user agent ([HTTP], Section 3.5) by defining behaviour in terms of fetching [FETCH] across the platform.¶

3.4. Interoperability

Interoperability occurs between Application Client—Resource Server (Section 3.4) as defined by the Per Resource Events Protocol.¶

Application Client—Resource Server Interoperability

Interoperability of implementations for application clients and resource servers is tested by evaluating an implementation's ability to request and respond to HTTP messages that conform to the Per Resource Events Protocol.¶

3.5. Terminology

The following terms are defined:¶

Notification

A message sent to application clients (that had previously requested it) when an HTTP request made on a resource fulfils some specified criteria.¶

Base Response

The response that would have been returned, were notifications not requested for, in an otherwise identical HTTP request.¶

Event Field

A parameter for a protocol item in the Accept-Events header field or a member of the Events header field.¶

4.1. Accept-Events

The Accept-Events header field can be used by application clients to specify their preferred protocol for receiving notifications. For example, the Accept-Events header field can be used to indicate that the request is specifically limited to a small set of desired protocols, as in the case for notifications with the Per Resource Events Protocol.¶

When sent by a resource server in a response, the Accept-Events header field provides information about which notification protocols are preferred in a subsequent request to the same resource.¶

An application client MAY generate a request for notifications regardless of having received an Accept-Events header field. The information only provides advice for the sake of improving performance and reducing unnecessary network transfers.¶

Conversely, an application client MUST NOT assume that receiving an Accept-Events header field means that future requests will return notifications. The content might change, the server might only support notifications requests at certain times or under certain conditions, or a different intermediary might process the next request.¶

4.2. Events

The Events header field is sent by a resource server to provide event fields in response to a request for notifications.¶

Where the Accept-Events header field sent in the request is ignored, a resource server MUST NOT send the Events header field in a response.¶

Conversely, a resource server MUST send the Events header field in a response, if the Accept-Events header field sent in the request is not ignored.¶

5.1. Event Fields

The Per Resource Events Protocol reuses existing HTTP fields ([HTTP], Section 5) as event fields. Any HTTP field MAY be used as an event field. For the limited context of notifications using the Per Resource Events Protocol, an event field with the same name as an HTTP field MUST have identical semantics to that HTTP field, unless otherwise specified.¶

This specification restricts Accept-Events and Events as Structured header fields* [HTTP-SF]. From this it follows:¶

• An event field whose value is anything except a bare Item, that is, an Item without Parameters, MUST be specified inside an Inner List.¶

• Unless otherwise specified, an event field that is not already defined as a Structured Field, therefore, MUST be handled as a Retrofit Structured Field [HTTP-Retrofit] when such handling is defined.¶

• An event field that is not already defined as a Structured Field but cannot be handled as a Retrofit Structured Field either, MUST be explicitly specified by the implementation.¶

5.2. Methods

For the Per Resource Events Protocol, HEAD ([HTTP], Section 9.3.2) and GET ([HTTP], Section 9.3.1) are the only methods in response to which notifications are advertised.¶

A resource server MUST NOT send the Accept-Events header field with prep as a protocol in response to a request with any method other than HEAD or GET.¶

For the Per Resource Events Protocol, GET ([HTTP], Section 9.3.1) is the only method by which notifications are requested and for which notifications response is defined.¶

An application client MUST NOT send the Accept-Events header field with prep as a protocol in a request with any method other than GET.¶

A resource server MUST NOT send the Events header field with the parameter protocol with a value of prep in response to a request with any method other than GET.¶

A resource server MUST NOT send the Events header field except in response to a GET request.¶

5.3. Status Codes

The Per Resource Events Protocol reuses existing HTTP status codes ([HTTP], Section 15) to describe the result of the request for notifications and the semantics of notifications in the response.¶

In response to a request where Accept-Events header field indicates prep as the preferred protocol, a resource server that supports notifications using the Per Resource Events Protocol MUST communicate the status code for the notifications response using the status parameter in the Events header field.¶

For the limited context of notifications using the Per Resource Events Protocol, the status code communicated using the status parameter in the Events header field MUST have identical semantics to the corresponding HTTP status code, unless otherwise specified.¶

6.1. Request

An application client can discover the ability of a resource server to deliver PREP notifications for a target resource by sending a HEAD ([HTTP], Section 9.3.2) request.¶

6.2. Not Available

In the response to a HEAD request, a resource server that does not provide notifications for the target resource using the Per Resource Events Protocol MUST NOT list prep as as one of the available protocols in the Accept-Events header field.¶

HEAD /foo HTTP/1.1
Host: example.org

6.3. Available

In response to a HEAD request, a resource server that serves notifications for the target resource using the Per Resource Events Protocol SHOULD include the Accept-Events header field, which MUST list prep as one of the available protocols.¶

Associated with prep list item, the resource server MUST include an accept event field with at least one acceptable media-type for notifications.¶

HTTP/1.1 200 OK
Accept: text/html
Accept-Events: "prep"; accept="message/rfc822"

8.1. Not Available

A resource server not implementing the Per Resource Events Protocol (or not supporting it for the target resource) can ignore a request for notifications and respond as if it received a normal request on that resource without impacting interoperability.¶

A resource server that does not provide notifications using the Per Resource Events Protocol MUST NOT:¶

• list prep as as one of the available protocols in the Accept-Events header field, if sent in the response.¶

• send the Events header field in the response with the protocol event field set to prep.¶

8.2. Error Response

A resource server MUST NOT include PREP notifications in a response, unless request results in one of the following status codes:¶

• 200 (OK) ([HTTP], Section 15.3.1),¶

• 204 (No Content) ([HTTP], Section 15.3.5),¶

• 206 (Partial Content) ([HTTP], Section 15.3.7),¶

• 226 (IM Used) ([Delta], Section 10.4.1).¶

A resource server that does not serve PREP notifications, on account of the response not having one of the above-mentioned status codes:¶

• SHOULD include the Events header fields where:¶

  • the protocol event field MUST be set to a value of prep,¶

  • the status event field MUST be set to 412 (Precondition Failed) ([HTTP], Section 15.5.13) status code.¶

8.3. Notification Errors

A resource server might still not be able to send notifications using the Per Resource Events Protocol requested by an application client despite a valid response.¶

A resource server unable to serve PREP notifications, even when the request results in a status code mentioned in Section 8.2:¶

• SHOULD include the Events header fields in the response where:¶

  • the protocol event field MUST be set to a value of prep,¶

  • the status event field MUST be set to appropriate status code indicating the reason for not serving notifications.¶

9.1. Headers

A resource server providing notifications using the Per Resource Events Protocol:¶

• MUST include the following header fields in the response:¶

  • Date ([HTTP], Section 6.6.1).¶

  • Content-Type ([HTTP], Section 8.3) with the media type set to multipart/mixed ([RFC2046], Section 5.1.3).¶

  • Events which MUST include the following event fields:¶

    • protocol set to the value prep.¶

    • status set to the 200 (OK) ([HTTP], Section 15.3.1) status code.¶

    • expires with an integer value in seconds indicating an interval after Date when notifications will no longer be sent and the response stream is closed.¶

    NOTE: Since caching is meaningless in the context of notifications, this specifications repurposes the deprecated Expires header field as an event field to specify when the notifications response ends.¶

• SHOULD include the following header fields in the response:¶

  • Vary ([HTTP], Section 12.5.5) which MUST include the following values:¶

    • Accept-Events.¶

    • Last-Event-ID, if the request for PREP notifications included the Last-Event-ID header field that was not ignored by the resource server.¶

  • Last-Modified ([HTTP], Section 8.8.2).¶

• MAY include the following header fields in the response:¶

  • Accept-Events which MUST list prep as one of the available notification protocols. Associated with the prep list item, the resource server:¶

    • MUST include an accept event field with at least one acceptable media-type for notifications.¶

9.2. Body

A resource server sending PREP Notifications MUST transmit a multipart message body ([RFC2046], Section 5.1) with a media type of multipart/mixed ([RFC2046], Section 5.1.3) with two body parts in the order specified below:¶

1. Base Response: the message body that would have been sent had notifications not been requested.¶

2. Notifications Response: the multipart response body with body parts containing notifications as defined in Section 9.2.2.¶

9.3. Termination

A resource server MUST end the notification response in any one of the following scenarios:¶

• Once the time specified in the expires parameter of the Events header field has elapsed.¶

• Immediately after sending a notification upon a DELETE request on the resource that results in a response with 200 (OK) ([HTTP], Section 15.3.1) or 204 (No Content) ([HTTP], Section 15.3.5) status codes.¶

A resource server MUST properly terminate the multipart responses as defined in [RFC2046], Section 5.1.2, before closing the response stream.¶

HTTP/1.1 200 OK
Vary: Accept-Events
Accept-Events: "prep"; accept="message/rfc822"
Events: protocol="prep", status=200, vary="accept"
Last-Modified: Sat, 1 April 2023 10:11:12 GMT
Transfer-Encoding: chunked
Content-Type: multipart/mixed; boundary="main-boundary"

--main-boundary
Content-Type: text/html

<html>

--main-boundary
Content-Type: multipart/digest; boundary="next-message"

--next-message
Content-Type: message/rfc822

<message>

--next-message--
--main-boundary--

HTTP/1.1 200 OK
Vary: Accept-Events, Last-Event-ID
Accept-Events: "prep"; accept="message/rfc822"
Events: protocol="prep", status=200, vary="accept"
Last-Modified: Sat, 1 April 2023 10:11:12 GMT
Transfer-Encoding: chunked
Content-Type: multipart/mixed; boundary="main-boundary"

--main-boundary
Content-Type: text/html

--main-boundary
Content-Type: multipart/digest; boundary="next-message"

--next-message
Content-Type: message/rfc822

<message>
--next-message--
--main-boundary--

10.1. Request

An application clients MAY specify additional parameters to message/rfc822 notification media-type in the accept event field associated with the prep list item in the Accept-Events header field of the request.¶

10.2. Notification

A resource server implementing the Per Resource Events Protocol SHOULD send a notification to an application client when a request with one of the following methods generates a response with any of the following status codes:¶

The notification MUST NOT be sent before the success response has been sent to the user agent that initiated the request.¶

10.3. Headers

A resource server MUST include the following header fields in a message/rfc822 formatted PREP notification:¶

Method

identical to :method pseudo-header field in a HTTP/2 request ([HTTP/2], Section 8.3.1), that specifies the request method that triggered the notification.¶

Date

identical to Date header field in a HTTP response ([HTTP], Section 6.6.1), that specifies date and time at which the event that triggered the notification was completed.¶

Event-ID

an opaque identifier for the generated notification.¶

A resource server SHOULD include the following headers in a message/rfc822 formatted PREP notification when the state of the resource or its representation is modified as a result of the event:¶

ETag

identical to ETag header field ([HTTP], Section 8.8.3) in a HTTP response. The ETag sent MUST be for the same Content-Type as the representation that was sent in the notification response or that would have been sent in the absense of the Last-Events-ID header.¶

--random-boundary  <- From the Previous Message (do not include)
                   <- Empty part response headers & blank line
Method: PUT
Date: Sat, 1 April 2023 10:11:12 GMT
Event-ID: 1234
ETag: "abc123"


--random-boundary

A resource server MUST include the following headers in a message/rfc822 formatted PREP notification when a request modifies another resource:¶

Content-Location

identical to the Content-Location header field ([HTTP], Section 8.7).¶

--random-boundary  <- From the Previous Message (do not include)
                   <- Empty part response headers & blank line
Method: POST
Date: Sat, 1 April 2023 10:12:14 GMT
Event-ID: 1235
Content-Location: /foo


--random-boundary

10.4. Body

The body of a message/rfc822 formatted PREP notification might be used to provide a delta representation, the difference between the state of the representation before and after the most recent event on the resource.¶

If the application client does not specify delta parameter associated with a preferred notifications format of message/rfc822 in the accept event field of the ​request (Section 7) that results in message/rfc822 formatted PREP notifications, the notifications MUST NOT contain a body.¶

If state of the resource is not modified as a result of an HTTP operation, the resulting notification MUST NOT contain a body.¶

If the application client specifies a delta parameter associated with a preferred notifications format of message/rfc822 in the accept event field of the ​request (Section 7) that results in message/rfc822 formatted PREP notifications, a resource server:¶

• SHOULD send the delta representation as the body of the notification generated upon a PATCH ([RFC5789]) operation.¶

• MAY send the delta representation as the body of the notification generated upon a PUT ([HTTP], Section 9.3.4) or a POST ([HTTP], Section 9.3.3) operation.¶

A resource server SHOULD send the delta representation in the content-type preferred by the client as specified by the delta parameter.¶

--random-boundary  <- From the Previous Message (do not include)
                   <- Empty part response headers & blank line
Method: PATCH
Date: Sat, 1 April 2023 10:14:16 GMT
Event-ID: 1236
ETag: "xyz987"
Content-type: application/example

<delta>

--random-boundary

12.1. Browser Fingerprinting

The Accept-Events header field provides an extra vector that can aid unique identification of user agent. Follow the advice in [HTTP], Section 17.13 to minimize the risk of fingerprinting.¶

12.2. Denial-of-Service Attacks

Resources that serve notifications, by virtue of keeping the response stream open for an extended period of time are more susceptible to Denial-of-Service attacks because the effort required to request notifications from the same resource is tiny compared to the time, memory, and bandwidth consumed by attempting to serve the notifications. Servers ought to ignore, coalesce, or reject egregious notification request, such as repeated notification requests to resource from the same origin.¶

13.1. Header Field Registration

IANA has registered the following entry in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" defined by [HTTP]:¶

13.2. Notifications Protocol Registry

The Accept-Events and Events header fields identify specific protocols for notifications. The "HTTP Notifications Protocol Registry", created and maintained by IANA at https://www.iana.org/assignments/http-parameters/, registers identifiers for notification protocols.¶

Notifications protocol registrations MUST include the following fields:¶

• Identifier¶

• Name¶

• Pointer to specification text¶

Initial registrations are given below:¶

Values to be added to this namespace require IETF Review (see [RFC8126], Section 4.8). Identified protocols MUST conform to the purpose of sending notifications as defined in Section 4 of this document.¶

14.1. Normative References

[Delta]

Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland, Y., van Hoff, A., and D. Hellerstein, "Delta encoding in HTTP", RFC 3229, DOI 10.17487/RFC3229, January 2002, <https://www.rfc-editor.org/rfc/rfc3229>.

[HTTP]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/rfc/rfc9110>.

[HTTP-Retrofit]

Nottingham, M., "Retrofit Structured Fields for HTTP", Work in Progress, Internet-Draft, draft-ietf-httpbis-retrofit-06, 31 March 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-retrofit-06>.

[HTTP-SF]

Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024, <https://www.rfc-editor.org/rfc/rfc9651>.

[HTTP/1.1]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112, June 2022, <https://www.rfc-editor.org/rfc/rfc9112>.

[HTTP/2]

Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, June 2022, <https://www.rfc-editor.org/rfc/rfc9113>.

[RFC2046]

Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996, <https://www.rfc-editor.org/rfc/rfc2046>.

[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/rfc/rfc2119>.

[RFC5789]

Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789, DOI 10.17487/RFC5789, March 2010, <https://www.rfc-editor.org/rfc/rfc5789>.

[RFC8126]

Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/rfc/rfc8126>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[SSE]

"Server-Sent Events", W3C REC eventsource, W3C eventsource, <https://www.w3.org/TR/eventsource/>.

14.2. Informative References

[FETCH]

"Fetch - Living Standard", <https://fetch.spec.whatwg.org>.

[REST]

Fielding, R. T., "Architectural Styles and the Design of Network-based Software Architectures", Doctoral Dissertation, University of California, Irvine, <https://roy.gbiv.com/pubs/dissertation/top.htm>.

[RFC7838]

Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative Services", RFC 7838, DOI 10.17487/RFC7838, April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.

[RFC9205]

Nottingham, M., "Building Protocols with HTTP", BCP 56, RFC 9205, DOI 10.17487/RFC9205, June 2022, <https://www.rfc-editor.org/rfc/rfc9205>.

[WEBSUB]

"WebSub", W3C REC websub, W3C websub, <https://www.w3.org/TR/websub/>.

[WS]

"The WebSocket API", W3C NOTE websockets, W3C websockets, <https://www.w3.org/TR/websockets/>.