Open Screen Protocol Explainer

The Open Screen Protocol allows one networked device to discover another on the local network, create a secure connection, and request that the other device render an application or streaming media using Web APIs such as the Presentation API and Remote Playback API.

We call devices that implement this protocol Open Screen Agents.

Why should users care?

Currently there are several proprietary solutions that allow media accessed on one device to be played on a different device. Examples include Google Cast, Apple AirPlay, and Spotify Connect.

However, the application, browser and/or OS largely determine what devices work together for media playback. The choice of one device, OS or browser largely determines what other media devices are compatible for launching Web applications and remote media playback. This in turn may force users into using specific devices, OSes, or browsers in order to take advantage of second screen functionality. It also prevents some browsers from implementing second screen Web APIs at all.

Why should Web developers care?

Web developers who want to integrate second screen functionality into their applications currently need to write different code for each browser to interface with the devices supported by that browser. This discourages wide adoption of remote playback functionality. For applications that do add second screen support, they tend to pick one device type versus integrating multiple device-specific SDKs into their application.

One goal of Second Screen APIs is to allow developers to write one application for remote media playback and expect it to function as expected across as many devices and browsers as possible.

Assumptions and Goals

The primary goal of the Open Screen Protocol is to provide a complete network protocol to allow devices from different vendors to implement second screen APIs and work together.

The protocol specifically targets the requirements of second screen Web APIs: the Remote Playback API and the Presentation API.

A design goal is to re-use existing network and data standards like mDNS, QUIC and CBOR as building blocks, versus inventing our own.

The protocol also provides security and privacy guarantees, especially with regards to other devices on the network.

We assume that the protocol should work on devices with low-end CPUs and constrained memory, which are typical in special purpose devices like smart TVs and connected speakers. We also assume the protocol will be used on battery-powered devices.

Non-Goals

A non-goal is to enable every Web application to be able to successfully render content on every Open Screen Protocol device. Devices have a wide range of rendering capabilities and policies about what content they wish to support. The protocol allows devices to indicate whether they are compatible with a specific URL or media type.

Alternatives Considered

We considered adopting an existing protocol like Google Cast, DIAL, or HbbTV 2.0 instead of inventing a new protocol from scratch. However these have significant limitations that motivated the development of a new protocol:

• They were not free to implement by other vendors.
• They did not fully support the Web APIs as written.
• They did not have a security model that met the requirements of the Web APIs.
• They would be harder to extend to anticipated future use cases, like cross-LAN support, or future cross-device Web APIs.

Protocol Overview

An interaction between Open Screen agents consists of the following phases.

Phase 1. Discovery and connection.

Initially, an agent S advertises itself using mDNS as available for other agents to connect to. Another agent, C, initiates a connection using QUIC to the port and IP address advertised by S.

Phase 2. Authentication.

TLS 1.3 is used to secure the connection between C and S. If C and S have not authenticated before, they negotiate one party to present a one-time code and the other party to accept the code. (Typically the end user is asked to input into C a code presented on the screen of S.) SPAKE2 is used to generate a shared secret and verify the identities of the TLS 1.3 endpoints.

Phase 3. Capability exchange.

Once the connection is authenticated, the two agents exchange capabilities, which map to sets of messages that each agent understands. For example, one agent may be an initiator of media remote playback, and the other agent may be the receiver of remote playback requests.

Some capabilities are defined by the spec corresponding to the needs of the Remote Playback API and Presentation API. However, the set of capabilities is open ended and may be extended to accomodate future requirements, additional Web APIs, or device-specific features.

Phase 4. API-specific usage.

Once the two agents have learned each other’s capabilities, they can exchange messages at the request of Web applications through supported Web APIs.

For example, if C wishes to start a presentation on S, it would:

1. Send a presentation-url-availability-request to learn if S can use the URL to start a presentation.
2. Send a presentation-start-request to request that S start a presentation with the URL.
3. Send and receive presentation-connection-messages to exchange messages with the active presentation.
4. Disconnect from the presentation with presentation-connection-close-event or request its termination with presentation-termination-request.

All messages that are specific to the Open Screen Protocol are encoded with CBOR which provides a good tradeoff between performance, conciseness and Web compatibility.

Privacy and security

The protocol is responsible for enabling a secure network connection between two Open Screen agents. The spec includes guidelines for the presentation and consumption of the pairing code, how agents can track other agents that have been previously authenticated, and when an agent should be reauthenticated.

Other aspects of security and privacy for second screen applications using the protocol are covered in other specifications, handled by specific agent implementations, or may be in scope for future revisions of the protocol.

Relationship to other Web specifications

WebRTC-QUIC is a proposal to enable Web applications to create peer-to-peer QUIC connections. This could enable applications to implement parts of the Open Screen Protocol in script.

WebTransport allows Web applications to construct HTTP/3 connections to servers. It may be possible to implement parts of the Open Screen protocol in application script on top of an HTTP/3 WebTransport.

With either of these capabiliites, applications still cannot discover and authenticate to Open Screen agents on their own. Those aspects would need to be handled by the user agent, or new APIs developed to enable Web applications to take them on.