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+# Assess interoperability of Presentation API implementations
+
+This document discusses the interoperability of potential Presentation API implementations in particular interoperability
+between user agents in case of [2-UA implementations][2ua] and between user agents and presentation displays in case of [1-UA implementations][1ua].
+For this, state of the art technologies and standards that may be useful to implement the Presentation API will be considered.
+
+## Interoperability between user agents (2-UA)
+
+The following diagram shows an overview of the main components for the 2-UA implementation. Each of the user agents implements
+part of the Presentation API (for controlling browsing context or presenting browsing context). To ensure interoperability
+between user agents running on controller devices and user agents running on presentation devices the following aspects should be considered:
+
+* **Discovery**: to discover presentation devices, the controlling user agent should use the same technology as the presenting
+user agent uses to advertise itself. **Potential technologies**: SSDP, mDNS or BLE (BLE Beacon)
+* **Launch**: after discovery, the controlling user agent should know how to launch a new presentation or join an existing one.
+Common message formats should be also used. **Potential technologies**: DIAL (uses SSDP for discovery), Google Cast (part of it).
+* **Communication**: also for communication both user agents should use same technologies/protocols to ensure interoperability.
+**Potential technologies**: WebSockets, WebRTC, Raw Socket, Google Cast (part of it)
+* **Signaling**: common language and message formats (e.g. using JSON or XML) should be used to exchange signaling information between the two user agents.
+
+![2-UA implementation][2ua-img]
+
+## Interoperability between user agents and presentation displays (1-UA)
+
+The following diagram shows an overview of the main components for the 1-UA Implementation. There are less requirements
+as in the 2-UA case to ensure interoperability between controlling user agents and presentation displays.
+The controlling user agent implements the Presentation API for both controlling and presenting browsing contexts and the presentation
+page is rendered in silent mode (not visible for the user). The user agent captures the UI output of the presentation page rendered
+in silent mode and sends the frames to a presentation device like a projector or wireless display. This means the user agent and the
+presentation device should use a common streaming technology to ensure interoperability. More than likely, the user agent will use System APIs
+offered by the OS to show content on the presentation device. But other technologies not offered directly by the OS are also possible, for example
+ Google Cast extension for chrome supports Tab cast to Chromecast using WebRTC. **Potentials Technologies**: HDMI, Intel WiDi, Miracast, Airplay, MHL.
+
+![1-UA implementation][1ua-img]
+
+[1ua]: http://w3c.github.io/presentation-api/#1-ua
+[2ua]: http://w3c.github.io/presentation-api/#2-ua
+[1ua-img]: ./images/interoperability-1ua.png
+[2ua-img]: ./images/interoperability-2ua.png
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