Abstract

This specification defines an API to enable web content to access external presentation-type displays and use them for presenting web content.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is a work in progress and is subject to change. Some sections are still incomplete or underspecified. Security and privacy considerations need to be adjusted based on feedback and experience. Some open issues are noted inline; please check the group's issue tracker on GitHub for all open issues. Feedback from early experimentations is encouraged to allow the Second Screen Presentation Working Group to evolve the specification based on implementation issues.

This document was published by the Second Screen Presentation Working Group as a Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-secondscreen@w3.org (subscribe, archives). All comments are welcome.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 August 2014 W3C Process Document.

Table of Contents

1. Introduction

This section is non-normative.

This specification aims to make presentation displays such as projectors or connected TVs, available to the Web and takes into account displays that are attached using wired (HDMI, DVI, or similar) and wireless technologies (Miracast, Chromecast, DLNA, AirPlay, or similar).

Devices with limited screen size lack the ability to show content to a larger audience, for example, a group of colleagues in a conference room, or friends and family at home. Showing content on an external large presentation display helps to improve the perceived quality and impact of the presented content.

At its core, this specification enables an exchange of messages between a page that acts as the controller and another page that represents the presentation shown in the presentation display. How the messages are transmitted is left to the UA in order to allow the use of presentation display devices that can be attached in a wide variety of ways. For example, when a presentation display device is attached using HDMI or Miracast, the same UA that acts as the controller renders the presentation. Instead of displaying the presentation in another window on the same device, however, it can use whatever means the operating system provides for using the external presentation displays. In such a case, both the controller and presentation run on the same UA and the operating system is used to route the presentation display output to the presentation display. This is commonly referred to as the 1-UA case. This specification imposes no requirements on the presentation display devices connected in such a manner.

If the presentation display is able to render HTML documents and communicate with the controller, the controller does not need to render the presentation. In this case, the UA acts as a proxy that requests the presentation display to show and render the presentation itself. This is commonly referred to as the 2-UA case. This way of attaching to displays could be enhanced in the future by defining a standard protocol for delivering these types of messages that display devices could choose to implement.

The API defined here is intended to be used with UAs that attach to presentation display devices through any of the above means.

2. Use cases and requirements

Use cases and requirements are captured in a separate Presentation API Use Cases and Requirements document.

3. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, SHOULD, and SHOULD NOT are to be interpreted as described in [RFC2119].

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and terminate these steps") are to be interpreted with the meaning of the key word ("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

4. Terminology

The terms browsing context, event handler, event handler event type, firing an event, navigate, queue a task, trusted event, allowed to show a popup are defined in [HTML5].

This document provides interface definitions using the Web IDL standard ([WEBIDL]). The terms Promise, ArrayBuffer, ArrayBufferView and DOMException are defined in [WEBIDL].

The terms resolving a Promise, and rejecting a Promise are used as explained in [PROMGUIDE].

The term URL is defined in the WHATWG URL standard [URL].

The term Blob is defined in the File API specification [FILEAPI].

The term RTCDataChannel is defined in the WebRTC API specification ([WEBRTC]).

5. Examples

This section shows example codes that highlight the usage of main features of the Presentation API. In these examples, controller.html implements the controller and presentation.html implements the presentation. Both pages are served from the domain http://example.org (http://example.org/controller.html and http://example.org/presentation.html). Please refer to the comments in the code examples for further details.

5.1 Monitor availability of presentation displays example

Example 1
<!-- controller.html -->
<button id="castBtn" style="display: none;">Cast</button>
<script>
  // it is also possible to use relative presentation URL e.g. "presentation.html"
  var presUrl = "http://example.com/presentation.html";
  // the cast button is visible if at least one presentation display is available
  var castBtn = document.getElementById("castBtn");
  // show or hide cast button depending on display availability
  var handleAvailabilityChange = function(available) {
    castBtn.style.display = available ? "inline" : "none";
  };
  // Promise is resolved as soon as the presentation display availability is known.
  var request = new PresentationRequest(presUrl);
  request.getAvailability().then(function(availability) {
    // availability.value may be kept up-to-date by the UA as long as the availability
    // object is alive. It is advised for the web developers to discard the object
    // as soon as it's not needed.
    handleAvailabilityChange(availability.value);
    availability.onchange = function() { handleAvailabilityChange(this.value); };
  }).catch(function() {
    // Availability monitoring is not supported by the platform, so discovery of presentation
    // displays will happen only after request.start() is called.  Pretend the
    // devices are available for simplicity; or, one could implement the third state for the
    // button.
    handleAvailabilityChange(true);
  });
</script>

5.2 Starting a new presentation session example

Example 2
<!-- controller.html -->
<script>
  // Start new session.
  request.start()
    // the new started session will be passed to setSession on success
    .then(setSession)
    // user cancels the selection dialog or an error is occurred
    .catch(endSession);
</script>

5.3 Joining a presentation session example

Example 3
<!-- controller.html -->
<script>
  // read presId from localStorage if exists
  var presId = localStorage && localStorage["presId"] || null;
  // presId is mandatory when joining a session.
  if (presId) {
    request.join(presId)
      // The joined session will be passed to setSession on success
      .then(setSession)
      // no session found for presUrl and presId or an error is occurred
      .catch(endSession);
  }
</script>

5.4 Handling an event for a UA initiated presentation session example

Example 4
<!-- controller.html -->
<head>
  <!-- Setting presentation.defaultRequest allows the page to specify the
       PresentationRequest to use when the UA initiates a presentation session. -->
</head>
<script>
  navigator.presentation.defaultRequest = new PresentationRequest(defaultUrl);
  navigator.presentation.defaultRequest.onsessionconnect = function(evt) {
    setSession(evt.session);
  };
</script>

5.5 Monitor session's state and exchange data example

Example 5
<!-- controller.html -->
<script>
  var session;
  var setSession = function (theSession) {
    // end existing session, if any
    endSession();
    // set the new session
    session = theSession;
    if (session) {
      // save presId in localStorage
      localStorage && (localStorage["presId"] = session.id);
      // monitor session's state
      session.onstatechange = function () {
        if (this == session && this.state == "disconnected")
          endSession();
      };
      // register message handler
      session.onmessage = function (evt) {
        console.log("receive message", evt.data);
      };
      // send message to presentation page
      session.send("say hello");
    }
  };
  var endSession = function () {
    // close old session if exists
    session && session.close();
    // remove old presId from localStorage if exists
    localStorage && delete localStorage["presId"];
  };
</script>
Example 6
<!-- presentation.html -->
<script>
  var addSession = function(session) {
    session.onstatechange = function () {
      // session.state is either 'connected' or 'disconnected'
      console.log("session's state is now", session.state);
    };
    session.onmessage = function (evt) {
      if (evt.data == "say hello")
        session.send("hello");
    }
  });
  navigator.presentation.getSession().then(addSession);
  navigator.presentation.onsessionavailable = function(evt) {
    navigator.presentation.getSessions().then(function(sessions) {
      addSession(sessions[sessions.length-1]);
    });
  };
</script>

6. API

6.1 Common idioms

A presentation display refers to an external screen available to the user agent via an implementation specific connection technology.

A presentation session is an object relating a controlling browsing context to its presenting browsing context and enables two-way-messaging between them. Each presentation session has a presentation session state, a presentation session identifier to distinguish it from other presentation sessions, and a presentation session URL that is a URL used to create or resume the presentation session. A valid presentation session identifier consists of alphanumeric ASCII characters only, is at least 16 characters long, and is unique within the set of presentations.

A controlling browsing context (or controller for short) is a browsing context that has connected to a presentation session by calling start() or join(), or received a presentation session via a sessionconnect event.

The presenting browsing context (or presentation for short) is the browsing context responsible for rendering to a presentation display. A presenting browsing context can reside in the same user agent as the controlling browsing context or a different one.

The set of presentations, initially empty, contains the presentation sessions created by the controlling browsing contexts for the user agent (or a specific user profile within the user agent). The set of presentations is represented by a list of tuples, where each tuple consists of a presentation session URL, a presentation session identifier, and the presentation session itself. The presentation session URL and presentation session identifier uniquely identify the presentation session.

6.2 Interface PresentationSession

Each presentation session is represented by a PresentationSession object. 

enum PresentationSessionState {
    "connected",
    "disconnected"
    //, "resumed" 
};

enum BinaryType {
    "blob",
    "arraybuffer"
};

interface PresentationSession : EventTarget {
    readonly    attribute DOMString?               id;
    readonly    attribute PresentationSessionState state;
    void close();
                attribute EventHandler             onstatechange;

    // Communication
                attribute BinaryType               binaryType;
                attribute EventHandler             onmessage;
    void send(DOMString message);
    void send(Blob data);
    void send(ArrayBuffer data);
    void send(ArrayBufferView data);
};

The id attribute specifies the presentation session's presentation session identifier.

The state attribute represents the presentation session's current state. It can take one of the values of PresentationSessionState depending on connection state.

When the send() method is called on a PresentationSession object with a message, the user agent MUST run the algorithm to send a message through a PresentationSession.

When the close() method is called on a PresentationSession, the user agent MUST run the algorithm to close a presentation session with PresentationSession.

6.2.1 Sending a message through PresentationSession

Note
No specific transport for the connection between the controlling browsing context and the presenting browsing context is mandated, except that for multiple calls to send() it has to be ensured that messages are delivered to the other end reliably and in sequence. The transport should function equivalently to an RTCDataChannel in reliable mode.

Let presentation message data be the payload data to be transmitted between two browsing contexts. Let presentation message type be the type of that data, one of text and binary.

When the user agent is to send a message through a PresentationSession S, it MUST run the following steps:

  1. If the state property of PresentationSession is "disconnected", throw an InvalidStateError exception.
  2. Let presentation message type messageType be binary if data is one of ArrayBuffer, ArrayBufferView, or Blob. Let messageType be text if data is of type DOMString)
  3. Assign the destination browsing context as follows:
    1. Let the destination browsing context be the controlling browsing context if send() is called in the presenting browsing context.
    2. Let destination browsing context be the presenting browsing context if send() is called from the controlling browsing context.
  4. Using an implementation specific mechanism, transmit the contents of the data argument as presentation message data and presentation message type messageType to the destination browsing context side.

6.2.2 Receiving a message through PresentationSession

When the user agent has received a transmission from the remote side consisting of presentation message data and presentation message type, it MUST run the following steps:

  1. If the state property of PresentationSession is "disconnected", abort these steps.
  2. Let event be a newly created trusted event that uses the MessageEvent interface, with the event type message, which does not bubble, is not cancelable, and has no default action.
  3. Initialize the event's data attribute as follows:
    1. If the presentation message type is text, then initialize event's data attribute to the contents of presentation message data of type DOMString.
    2. If the presentation message type is binary, and binaryType is set to blob, then initialise event's data attribute to a new Blob object that represents presentation message data as its raw data.
    3. If the presentation message type is binary, and binaryType is set to arraybuffer, then initialise event's data attribute to a new ArrayBuffer object whose contents are presentation message data.
  4. Queue a task to fire event at PresentationSession.

6.2.3 Closing a PresentationSession

When the user agent is to close a presentation session using session, it MUST run the following steps:

  1. If presentation session state of session is not connected, then abort these steps.
  2. Set presentation session state of session to disconnected.
  3. Queue a task to run the following steps in order:
    1. For each known session in the set of presentations:
      1. If the presentation session identifier of known session and session are equal, run the following steps:
        1. Queue a task to fire an event named statechange at session.
Issue 35: Refine how to do session teardown/disconnect/closing

There are rough edges when it comes to closing the session, resuming it. We need to gain experience from implementations what happens, when the session is disconnected, user-closed, remote-end closed etc.

This section is one aspect of this, but probably the example section needs to shed more light on how expected and unexpected disconnects are handled.

6.2.4 Event Handlers

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the PresentationSession interface:

Event handler Event handler event type
onmessage message
onstatechange statechange

6.3 Interface PresentationAvailability 

interface PresentationAvailability : EventTarget {
    readonly    attribute boolean      value;
                attribute EventHandler onchange;
};

A PresentationAvailability object is associated with a presentation display and represents its presentation display availability.

The value attribute MUST return the last value it was set to. The value is updated by the monitor the list of available presentation displays algorithm.

The onchange attribute is an event handler whose corresponding event handler event type is change.

6.3.1 The set of availability objects

The user agent MUST keep track of the set of availability objects requested through the getAvailability() method. The set of availability objects is represented as a set of tuples (A, availabilityUrl, P), initially empty, where:

  1. A is a live PresentationAvailability object;
  2. availabilityUrl is the availabilityUrl passed to getAvailability() to create A.
  3. P is the Promise that was returned by getAvailability() when A was created.

6.3.2 The list of available presentation displays

The user agent MUST keep a list of available presentation displays. This current list of presentation displays may be used for starting new presentations, and is populated based on an implementation specific discovery mechanism. It is set to the most recent result of the algorithm to monitor the list of available presentation displays.

While there are live PresentationAvailability objects, the user agent MAY monitor the list of available presentation displays continuously, so that pages can use the value property of an PresentationAvailability object to offer presentation only when there are available displays. However, the user agent may not support continuous availability monitoring; for example, because of platform or power consumption restrictions. In this case the Promise returned by getAvailability() MUST be rejected and the algorithm to monitor the list of available presentation displays will only run as part of the start a presentation session algorithm.

When there are no live PresentationAvailability objects (that is, the set of availability objects is empty), user agents SHOULD NOT monitor the list of available presentation displays to satisfy the power saving non-functional requirement. To further save power, the user agent MAY also keep track of whether the page holding a PresentationAvailability object is in the foreground. Using this information, implementation specific discovery of presentation displays can be resumed or suspended.

Some presentation displays may only be able to display a subset of Web content because of functional, security or hardware limitations. Examples are set-top boxes, smart TVs or networked speakers capable of rendering only audio. We say that such a display is a compatible presentation display for a display availability URL if the user agent can reasonably guarantee that the presentation of the URL on that display will succeed.

6.3.3 Monitor the list of available presentation displays

If set of availability objects is non-empty, or there is a pending request to start a presentation session, the user agent MUST monitor the list of available presentation displays by running the following steps.

  1. Queue a task to retrieve available presentation displays (using an implementation specific mechanism) and let newDisplays be this list.
  2. Wait for the completion of that task.
  3. For each member (A, availabilityUrl, P) of the set of availability objects:
    1. Set previousAvailability to the value of A's value property.
    2. Let newAvailability be true if newDisplays is not empty and at least one display in newDisplays is a compatible presentation display for availabilityUrl. Otherwise, set newAvailability to false.
    3. If previousAvailability is not equal to newAvailability, then queue a task to run the following steps:
      1. Set A's value property to newAvailability.
      2. If P is settled, then fire an event named change at A.
      3. If P is not settled, then resolve P with A.
  4. Set the list of available presentation displays to the value of newDisplays.

When a PresentationAvailability object is no longer alive (i.e., is eligible for garbage collection), the user agent SHOULD run the following steps:

  1. Find and remove any entry (A, availabilityUrl, P) in the set of availability objects for the newly deceased A.
  2. If the set of availability objects is now empty and there is no pending request to start a presentation session, cancel any pending task to monitor the list of available presentation displays for power saving purposes.
Note
The mechanism used to monitor presentation displays availability and determine the compatibility of a presentation display with a given URL is left to the user agent.

6.4 Interface PresentationRequest 

[Constructor(DOMString url)]
interface PresentationRequest : EventTarget {
    Promise<PresentationSession>      start();
    Promise<PresentationSession>      join(DOMString presentationId);
    Promise<PresentationAvailability> getAvailability();

                attribute EventHandler onsessionconnect;
};

When a PresentationRequest is constructed, the given url MUST be used as the presentation request URL which is the presentation session URL for the PresentationRequest instance.

6.4.1 Starting a presentation session

When the start method is called, the user agent MUST run the following steps to start a presentation session:

Input
presentationRequest, the PresentationRequest object
presentationUrl, the presentation request URL
Output
P, a Promise
  1. If the algorithm isn't allowed to show a popup, return a Promise rejected with a DOMException named "InvalidAccessError" and abort these steps.
  2. Let P be a new Promise.
  3. Return P.
  4. Run the following steps:
    1. Monitor the list of available presentation displays.
    2. Wait until the algorithm completes.
  5. If either of the following is true:
    1. The list of available presentation displays is empty;
    2. No member if the list of available presentation displays is a compatible presentation display for presentationUrl;
  6. Then run the following steps:
    1. Reject P with a DOMException named "NotFoundError".
    2. Abort all remaining steps.
  7. Queue a task T to request user permission for the use of a presentation display and selection of one presentation display.
    1. If T completes with the user granting permission to use a display, run the following steps:
      1. Let I be a new valid presentation session identifier unique among all presentation session identifiers for known sessions in the set of presentations.
      2. Create a new PresentationSession S.
      3. Set the presentation session identifier of S to I, and set the presentation session state of S to disconnected.
      4. Queue a task C to create a new browsing context on the user-selected presentation display and navigate to presentationUrl in it.
        1. If C completes successfully, run the following steps:
          1. Add the tuple {presentationUrl, S.id, S} to the set of presentations.
          2. Resolve P with S.
          3. Queue a task to fire an event named sessionconnect at presentationRequest with S as its session attribute.
          4. Establish a presentation connection with S.
        2. If C fails, run the following steps:
          1. Reject P with a DOMException named "OperationError".
    2. If T completes with the user denying permission, run the following steps:
      1. Reject P with a DOMException named "AbortError".
Note
The details of implementing the permission request and display selection are left to the user agent; for example it may show the user a dialog and allow the user to select an available display (granting permission), or cancel the selection (denying permission).
Note
The presentationUrl should name a resource accessible to the local or a remote user agent. This specification defines behavior for presentationUrl using the http or https schemes; behavior for other schemes is not defined by this specification.
Issue
Do we want to distinguish the permission-denied outcome from the no-screens-available outcome? Developers would be able to infer it anyway from getAvailability().

6.4.2 Joining a presentation session

When the join(presentationId) method is called, the user agent MUST run the following steps to join a presentation session:

Input
presentationRequest, the PresentationRequest object
presentationUrl, the presentation request URL
presentationId, the presentation session identifier
Output
P, a Promise
  1. Let P be a new Promise.
  2. Return P.
  3. Queue a task T to run the following steps in order:
    1. For each known session in the set of presentations,
      If the presentation session URL of known session is equal to presentationUrl, and the presentation session identifier of known session is equal to presentationId, run the following steps:
      1. Let S be the presentation session of known session.
      2. Resolve P with S.
      3. Queue a task to fire an event named sessionconnect at presentationRequest with S as its session attribute.
      4. Establish a presentation connection with S.
      5. Abort the remaining steps of T.
    2. Reject P with a DOMException named "NotFoundError".
Issue
If no matching presentation is found, we could leave the Promise pending in case a matching presentation is started in the future.

6.4.3 Establishing a presentation connection in a controlling browsing context

When the user agent is to establish a presentation connection using a presentation session S, it MUST run the following steps:

  1. If the presentation session state of S is connected, then:
    1. Abort all remaining steps.
  2. Queue a task T to connect the presentation session S to the presenting browsing context.
  3. If T completes successfully, run the following steps:
    1. Set the presentation session state of S to connected.
    2. Queue a task T to run the following steps in order:
      1. For each known session in the set of presentations:
        1. If the presentation session identifier of known session and S are equal, then run the following steps:
          1. Queue a task to fire an event named statechange at s.
Note
The mechanism that is used to present on the remote display and connect the controlling browsing context with the presented document is an implementation choice of the user agent. The connection must provide a two-way messaging abstraction capable of carrying DOMString payloads in a reliable and in-order fashion as described in the Send Message and Receive Message steps below.
Note
If T does not complete successfully, the user agent may choose to re-execute the Presentation Connection algorithm at a later time.
Issue
Do we want to notify the caller of a failure to connect, i.e. with an "error" onstatechange?
Issue
Do we want to pass the new state as a property of the statechange event?

6.4.4 Getting the presentation displays availability information

When the getAvailability() method is called, the user agent MUST run the following steps:

Input
presentationUrl, the presentation request URL
Output
P, a Promise
  1. Let P be a new Promise.
  2. Return P.
  3. If the user agent is unable to continuously monitor the list of available presentation displays, then:
    1. Reject P with a DOMException named "NotSupportedError".
    2. Abort all the remaining steps.
  4. Let A be a new PresentationAvailability object with its value property set to false if the list of available presentation displays is empty and true otherwise.
  5. Create a tuple (A, presentationUrl, P) and add it to the set of availability objects.
  6. Run the algorithm to monitor the list of available presentation displays.

6.4.5 Establishing a presentation connection in a presenting browsing context

When a new presenting browsing context has been created and navigated to the presentationUrl on a user-selected presentation display, the user agent MUST run the following steps:

  1. Queue a task to start monitoring incoming presentation sessions from controlling browsing contexts.

6.4.6 Event Handlers

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the PresentationRequest interface:

Event handler Event handler event type
onsessionconnect sessionconnect

6.4.7 Interface PresentationSessionConnectEvent 

[Constructor(DOMString type, optional PresentationSessionConnectEventInit eventInitDict)]
interface PresentationSessionConnectEvent : Event {
    [SameObject]
    readonly    attribute PresentationSession session;
};

dictionary PresentationSessionConnectEventInit : EventInit {
    PresentationSession session;
};

An event named sessionconnect is fired on a PresentationRequest when a session associated with the object is created. It is fired at the PresentationRequest instance, using the PresentationSessionConnectEvent interface, with the session attribute set to the PresentationSession object that was created. The event is fired for all sessions that are created for the controller, either by the controller calling start() or join(), or by the UA creating a session on the controller's behalf via defaultRequest.

The UA MUST fire the event as soon as it can create the PresentationSession associated with the event.

6.5 Interface Presentation 

partial interface Navigator {
    [SameObject]
    readonly    attribute Presentation presentation;
};

The presentation attribute is used to retrieve an instance of the Presentation interface. 

interface Presentation : EventTarget {
    // This API used by controlling browsing context.
                attribute PresentationRequest? defaultRequest;

    // This API used by presenting browsing context.
    Promise<PresentationSession>           getSession();
    Promise<sequence<PresentationSession>> getSessions();
                attribute EventHandler         onsessionavailable;
};

The defaultRequest MUST return the default presentation request if any, null otherwise.

6.5.1 Setting the default presentation request

If set by the controller, the defaultRequest SHOULD be used by the UA as the default presentation request for that controller. When the UA wishes to initiate a PresentationSession on the controller's behalf, it MUST start a presentation session using the default presentation request for the controller (as if the controller had called defaultRequest.start()).

The user agent SHOULD initiate presentation using the default presentation request only when the user has expressed an intention to do so, for example by clicking a button in the browser.

Note
Not all user agents may support initiation of a presentation session outside of the content area. In this case setting defaultRequest has no effect.
Issue
It should be clear that user-intiated presentation via the user agent may have pre-selected the presentation display. In this case step 6 of start a presentation session is optional. It may be cleaner to define a separate set of steps for initiating a default presentation.

6.5.2 Monitoring incoming presentation sessions in a presenting browsing context

When the user agent is to start monitoring incoming presentation sessions in a presenting browsing context from controlling browsing contexts, it MUST run the following steps:

  1. Queue a task T to wait for and accept an incoming connection request from a controlling browsing context using an implementation specific mechanism.
  2. When a new connection request is received from a controlling browsing context, run the following steps:
    1. Create a new PresentationSession S.
    2. Let I be a new valid presentation session identifier unique among all presentation session identifiers for known sessions in the set of presentations.
    3. Set the presentation session identifier of S to I.
    4. Establish the connection between the controlling and presenting browsing contexts using an implementation specific mechanism.
    5. Set the presentation session state of S to connected.
    6. Add a tuple (undefined, presentation session identifier of S, S) to the set of presentations.
    7. Queue a task to fire an event named sessionavailable at presentation.

6.5.3 Getting the first connected presentation session on the startup of a presenting browsing context

When the getSession() method is called, the user agent MUST run the following steps:

  1. Let P be a new Promise.
  2. Return P.
  3. Queue a task T to run the following steps in order:
    1. If the set of presentations is empty, then wait until at least one element is added to the set of presentations.
    2. Let S be the first presentation session added to the set of presentations.
    3. Resolve P with S.
Note
If the set of presentations is empty, we leave the Promise pending until connecting request arrives from the controlling browsing context. If the first controlling browsing context disconnects after initial connection, then the Promise returned to subsequent calls to getSession() will resolve with a presentation session that has its presentation session state set to disconnected.

6.5.4 Getting all connected presentation sessions in a presenting browsing context

When the getSessions() method is called, the user agent MUST run the following steps:

  1. Let P be a new Promise.
  2. Return P.
  3. Queue a task to run the following steps in order:
    1. Let array be an empty array.
    2. For each known session in the set of presentations
      1. Add known session to array.
    3. Resolve P with array.

6.5.5 Event Handlers

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by objects implementing the Presentation interface:

Event handler Event handler event type
onsessionavailable sessionavailable

7. Security and privacy considerations

Issue 45: Security and privacy considerations

We should complete the Security and privacy considerations section to the Presentation API once we're better aware of the threats.

The spec has been evaluated against Self-Review Questionnaire: Security and Privacy, with results documented in this issue. Furthermore, the Privacy Interest Group (PING) has produced documentation that may be helpful, namely Fingerprinting Guidance for Web Specification Authors and Privacy Considerations for Web Protocols. The Web Security IG review and PING review of the specification have been initiated. This sections should be crafted based on feedback from the respective reviews.

Personally identifiable information

The change event fired on the PresentationAvailability object reveals one bit of information about the presence (or non-presence) of a presentation display typically discovered through the local area network. This could be used in conjunction with other information for fingerprinting the user. However, this information is also dependent on the user's local network context, so the risk is minimized.

Cross-origin access

A presentation session is allowed to be accessed across origins; the presentation URL and presentation ID used to create the presentation are the only information needed to join a session from any origin in that user agent. In other words, a presentation is not tied to a particular opening origin.

This design allows controlling contexts from different domains to connect to a shared presentation resource. The security of the presentation ID prevents arbitrary pages from connecting to an existing presentation.

This specification does not prohibit a user agent from publishing information about its set of presentations. The group envisions a user agent on another device (distinct from the controller or presentation) becoming authorized to join the presentation, either by user action or by discovering the presentation's URL and id.

Issue
This section should provide informative guidance as to what constitutes a reasonable context for a Web page to become authorized to control a presentation session.

Device Access

The presentation API abstracts away what "local" means for displays, meaning that it exposes network-accessible displays as though they were local displays. The Presentation API requires user permission for a page to access any display to mitigate issues that could arise, such as showing unwanted content on a display viewable by others.

Temporary identifiers and browser state

The presentation URL and presentation ID can be used to connect to a presentation session from another browsing context. They can be intercepted if an attacker can inject content into the controlling page.

Issue
Should we restrict the API to some extent in non secure contexts?

Incognito mode and clearing of browsing data

The content displayed on the presentation is different from the controller. In particular, if the user is logged in in both contexts, then logs out of the controlling browsing context, she will not be automatically logged out from the presenting browsing context. Applications that use authentication should pay extra care when communicating between devices.

The set of presentations known to the user agent should be cleared when the user requests to "clear browsing data."

Issue 14: Define user agent context for rendering the presentation

The spec should specify any restrictions on the presenting browsing context when the opening browsing context is in "incognito" mode.

From public-webscreens and related threads:

The specification should define the behavior of the following Web features for the presenting page across presentation sessions. The goals are the following:

  1. Ensure that the same presentation will behave correctly in the 1-UA and 2-UA cases
  2. Information does not leak from presentation session to session in cases where the controlling pages may represent different authorization domains (e.g., logged-in users).

The following Web features have been discussed (and there may be others to consider):

Issue
The spec should clarify what is to happen to the set of known presentations in "incognito" (private browsing context) mode.

Messaging between presentation sessions

This spec will not mandate communication protocols between the controlling browsing context and the presenting browsing context, but it should set some guarantees of message confidentiality and authenticity between corresponding presentation sessions.

Issue 80: Define security requirements for messaging channel between secure origins

public-secondscreen thread:

While discussing the Hbb 2.0 TV standard, the issue of how WebSockets would be used to communicate between the controlling Web application and the presenting TV Web application.

If these Web applications are provided on secure (https://) origins, some guarantees of message confidentiality and authenticity of either party should be made to meet the standards set out by the Mixed Content proposal.

This issue will be addressed by a spec change to spell out the security requirements for the messaging channel as part of the Security and Privacy sections of the spec.

8. Acknowledgments

Thanks to Wayne Carr, Louay Bassbouss, Anssi Kostiainen, 闵洪波 (Hongbo Min), Anton Vayvod, and Mark Foltz for help with editing, reviews and feedback to this draft.

A. References

A.1 Normative references

[FILEAPI]
Arun Ranganathan; Jonas Sicking. File API. 21 April 2015. W3C Working Draft. URL: http://www.w3.org/TR/FileAPI/
[HTML5]
Ian Hickson; Robin Berjon; Steve Faulkner; Travis Leithead; Erika Doyle Navara; Edward O'Connor; Silvia Pfeiffer. HTML5. 28 October 2014. W3C Recommendation. URL: http://www.w3.org/TR/html5/
[PROMGUIDE]
Domenic Denicola. Writing Promise-Using Specifications. finding. URL: http://www.w3.org/2001/tag/doc/promises-guide
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[URL]
Anne van Kesteren; Sam Ruby. URL. 9 December 2014. W3C Working Draft. URL: http://www.w3.org/TR/url-1/
[WEBIDL]
Cameron McCormack; Boris Zbarsky. WebIDL Level 1. 4 August 2015. W3C Working Draft. URL: http://www.w3.org/TR/WebIDL-1/

A.2 Informative references

[WEBRTC]
Adam Bergkvist; Daniel Burnett; Cullen Jennings; Anant Narayanan. WebRTC 1.0: Real-time Communication Between Browsers. 10 February 2015. W3C Working Draft. URL: http://www.w3.org/TR/webrtc/