Requirements for Open Screen Protocol
This document outlines functional and non-functional requirements of the Open Screen protocol.
Functional requirements derive from the two Web APIs that the protocol will support:
- The Presentation API
- The Remote Playback API
Non-functional requirements address efficiency, user experience, and security aspects of the protocols.
Presentation API Requirements
In 2-UA mode, the Presentation API allows one browser (the controlling user agent) to discover and initiate presentation of Web content on a second user agent (the receiving user agent, or receiver). We use the term presentation display to refer to the entire device and software responsible for implementing the receiver, including browser, OS, networking and screen.
A presentation is initiated at the request of a controlling browsing context (or controller), which creates a receiving browsing context (or presentation) to load a presentation request URL and exchange messages with the resulting document.
For detailed information about the behavior of the Presentation API operations described below, please consult the Presentation API specification.
Presentation Display Availability
A controlling user agent must be able to discover the presence of a presentation display connected to the same IPv4 or IPv6 subnet and reachable by IP multicast.
A controlling user agent must be able to obtain the IPv4 or IPv6 address of the display, a friendly name for the display, and an IP port number for establishing a network transport to the display.
A controlling user agent must be able to determine if the receiver is reasonably capable of displaying a specific presentation request URL.
- The controller must be able to start a new presentation on a receiver given a presentation request URL and presentation ID.
- The controller must be able to reconnect to an active presentation on the receiver, given its presentation request URL and presentation ID.
The controlling user agent or receiver must be able to close the connection between a controller and its presentation, and signal the other party with the reason why the connection was closed (
Multiple controllers must be able to connect to a presentation simultaneously (from one or more controlling user agents).
Presentation Connection Messaging
A message refers to the data passed with an invocation of
PresentationConnection.send() by the controller or presentation.
Messages sent by the controller must be delivered to the presentation (or vice versa) in a reliable and in-order fashion.
If a message cannot be delivered, then the controlling user agent must be able to signal the receiver (or vice versa) that the connection should be closed with reason
The controller and presentation must be able to send and receive
DOMStringmessages (represented as
stringtype in ECMAScript).
The controller and presentation must be able to send and receive binary messages (represented as
Blobobjects in HTML5, or
ArrayBufferViewtypes in ECMAScript).
The controlling user agent must be able to signal to the receiver to terminate a presentation, given its presentation request URL and presentation ID.
The receiver must be able to signal all connected controlling user agents when a presentation is terminated.
Remote Playback API Requirements
Hardware and Efficiency
It should be possible to implement an Open Screen presentation display using modest hardware requirements, similar to what is found in a low end smartphone, smart TV or streaming device. See the Device Specifications document for expected presentation display hardware specifications.
It should be possible to implement an Open Screen controlling user agent on a low-end smartphone. See the Device Specifications document for expected controlling user agent hardware specifications.
The discovery and connection protocols should minimize power consumption, especially on the controlling user agent which is likely to be battery powered.
Privacy and Security
The protocol should minimize the amount of information provided to a passive network observer about the identity of the user, activity on the controlling user agent and activity on the receiver.
The protocol should prevent passive network eavesdroppers from learning presentation URLs, presentation IDs, or the content of presentation messages passed between controllers and presentations.
The protocol should prevent active network attackers from impersonating a display and observing or altering data intended for the controller or presentation.
The controlling user agent should be able to discover quickly when a presentation display becomes available or unavailable (i.e., when it connects or disconnects from the network).
The controlling user agent should present sensible information to the user when a protocol operation fails. For example, if a controlling user agent is unable to start a presentation, it should be possible to report in the controlling user agent interface if it was a network error, authentication error, or the presentation content failed to load.
The controlling user agent should be able to remember authenticated presentation displays. This means it is not required for the user to intervene and re-authenticate each time the controlling user agent connects to a pre-authenticated display.
Message latency between the controller and a presentation should be minimized to permit interactive use. For example, it should be comfortable to type in a form in the controller and have the text appear in the presentation in real time. Real-time latency for gaming or mouse use is ideal, but not a requirement.
The controlling user agent initiating a presentation should communicate its preferred locale to the receiver, so it can render the presentation content in that locale.
- It should be possible to extend the control protocol (above the discovery and transport levels) with optional features not defined explicitly by the specification, to facilitate experimentation and enhancement of the base APIs.