initial draft for evaluating Data Channel as the transport protocol.

datachannel.md

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+# Data Channel
+
+[Data Channel](https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-13)
+is a non-media transport protocol in WebRTC framework, designed for exchanging
+data from peer to peer. It advertises the following benefits:
+
+- Implemented in major browsers (except Safari)
+- Supports heterogeneous network environment e.g. NAT, firewall.
+- Easily supports media transport for remote playback
+- Data framing format is already defined
+
+# Design and Specification
+
+- [WebRTC overview](https://tools.ietf.org/html/draft-ietf-rtcweb-overview-18)
+- IETF draft: [WebRTC Data Channel Establishment Protocol](https://tools.ietf.org/html/draft-ietf-rtcweb-data-protocol-09)
+- IETF draft: [Data Channel](https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-13)
+
+# Presentation API functionality
+
+Data Channel is designed to be a non-media transport for WebRTC, with built-in
+sub protocol negotiation mechanism defined.
+
+A Data Channel is built on top of SCTP/DTLS/UDP protocol stack, which supports
+reliable data transmition. Each Data Channel has a unique integer ID and a label.
+Mutiple Data Channels can share the same DTLS connection.
+
+Assume that a single Data Channel is used as a control channel to transmit control
+messages between the controller and receiver and one Data Channel is created for each
+connected Presentation Connection for transmitting application data.
+
+An extra signalling channel and protocol for exchanging ICE candidates is required
+to bootstrap the control channel. The bootstrapping procedure needs to be executed
+for the first connecting to that receiver before the Presentation Initiation.
+
+## Presentation Initiation
+
+1. Send a control message on control channel from the controller to the display to
+   start the presentation with ID and URL for the initial presentation connection.
+2. Receiver spawn a Data Channel on the same RTC connection of control channel, with
+   presentation ID as label.
+3. Get a reply on control channel with the ID and URL to confirm success
+   (or report an error).
+
+## Presentation Resumption
+
+1. Send a control message on control channel from the controller to the display
+   reconnect to a presentation with ID and URL for the initial presentation connection.
+2. Receiver spawn a Data Channel on the same RTC connection of control channel, with
+   presentation ID as label, if there is a corresponding presentation to be resumed.
+2. Get a reply on control channel with the ID, URL, and stream ID to confirm success
+   (or report an error).
+
+## Presentation Connections
+
+Multiple connections from a controller can be handled by creating unique Data
+Channels for each peer. Unique stream ID will be selected for muxing/demuxing
+messages on the underlying SCTP transport, see
+[draft-ietf-rtcweb-data-protocol-09](https://tools.ietf.org/html/draft-ietf-rtcweb-data-protocol-09#section-6).
+
+## Presentation Connection Messaging
+
+Binary or text messages can be sent on the Data Channel corresponding to the
+presentation connection.  The framing of these messages is defined in
+[draft-ietf-rtcweb-data-channel](https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-13#section-6.6).
+
+Either side may close the Presentation Connection by following procedure:
+1. Send a control message with close reason on control channel to remote peer.
+2. While receiving the control message, initiate the
+   [Data Channel close procedure](https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-13#section-6.6)
+3. Consider Presentation Connection is closed while Data Channel is closed.
+
+## Presentation Termination
+
+For controller initiated termination:
+1. A controller send a control message with URL and ID on control channel to
+   instruct a display to terminate a presentation.
+2. The receiver sends a reply to all associated control channels to inform all
+   remote peers that a presentation is about to be terminated.
+
+For receiver initiated termination:
+1. The receiver directly sends out reply to all associated control channels to
+   inform all remote peers that a presentation is about to be terminated.
+
+# Remote Playback API functionality
+
+WebRTC framework supports media codec negociation and real-time media transport.
+
+**TODO:** Fill in when Remote Playback equirements are known.
+See [Issue #3](https://github.com/webscreens/openscreenprotocol/issues/3).
+
+# Reliability
+
+WebRTC is designed to be operated on heterogeneous network environment, using
+ICE for address discovery and built-in negotiation/re-negotiation mechanism.
+
+*TODO:* Get reliability data.
+
+# Latency to establish a new connection
+
+There are three sources of latency:
+1. Connecting a reliable channel the first time for for ICE negotiation.
+2. Establishing the first Data Channel.
+3. Adding an additional Data Channel on top of an existing peer-to-peer connection.
+
+*TODO:* Get connection latency data.
+
+# Latency to transmit a messages
+
+Data on latency of message transmission.
+
+Data Channel is based on SCTP, which allows for out of order packet delivery and
+may improve the latency to deliver presentation connection messages on a
+congested or lossy network.
+
+*TODO:* Get supporting data.
+
+# Ease of implementation / deployment
+
+Four major browsers (Chrome/Firefox/Edge/Opera) already implement and ship in
+the latest release version. An open source library [WebRTC.org](https://webrtc.org/)
+is ready with production level quality.
+
+A common signaling channel/protocol for exchanging ICE candidates needs to be defined.
+
+# Privacy
+
+By relying on UDP, the IP addresses of both endpoints are visible to network
+observers.
+
+DTLS is used for transport level security. Once DTLS is established, all the
+control message and data will be encrypted.
+
+# IPv4 and IPv6 support
+
+Data Channel depends on SCTP over UDP, which is supported for both IPv4 and IPv6.
+
+# Hardware requirements
+
+Major desktop, laptop, and high-end mobile phone should be able to create single
+RTC connection since major browser on desktop and mobile already ship WebRTC.
+
+*TODO*: Obtain data on hardware requirements
+
+# Network and power efficiency
+
+Maintain a long-lived Data Channel might not be power efficiency.
+
+*TODO*: Obtain data on network and power efficiency
+
+# Standardization status
+
+Data Channel has a standards track specification in the
+IETF [RTCWEB Working Group](https://datatracker.ietf.org/wg/rtcweb/charter/).
+
+Major browsers already implement and ship with interoperability tested.
+
+# Security
+
+Describe security architecture and what encryption and authentication mechanisms
+are supported.
+
+A general security study has be done by IETF RTCWEB working group, see
+[Security Overview](https://tools.ietf.org/html/draft-ietf-rtcweb-overview-18#section-5)
+
+Signaling channel for exchanging SDP must be encrypted to prevent man-in-the-middle
+attack.
+DTLS handshake for exchanging encryption key and setup encrypted channel for SCTP.
+STUN keepalive can be introduced for consent freshness.
+Identity Provider can be introduced for external authentication.
+
+
+# User Experience
+
+*TODO*: To be evaluated