# ORTC shim for Edge

The `modules/RTC/ortc` folder contains a `RTCPeerConnection` shim for Edge based on ORTC among with other shims (such as `RTCSessionDescription`) and some utilities/helpers to deal with both, SDP and ORTC objects.


## Files in `ortc` folder


### RTCPeerConnection.js

Exports a `RTCPeerConnection` shim. The interface is based on the [W3C specification of 2015](https://www.w3.org/TR/2015/WD-webrtc-20150210/), which matches (mostly) the current implementation of Chrome.

It also implements Plan-B for multi-stream.


#### Limitations

* BUNDLE is assumed (single transport for all the local and remote media streams).
* `rtcp-mux` is assumed (not a real problem nowadays).
* Calling `createOffer()` is not implemented, so P2P mode is not supported (`RTCBrowserType.isP2PSupported()` returns `false`).
* Calling `setRemoteDescription()` with a SDP answer with mangled SSRC values is currently unsupported (those new SSRC values will not be used for sending media).
* If the app calls `createAnswer()`, mangles SSRC values, and applies by calling `setLocalDescription()`, those new SSRC values are currently ignored.
* Simulcast not supported (currently Edge supports `maxFramerate` per encoding but it does not support `maxBitrate`, `resolutionScale` or `framerateScale`, so it's not worth it).
* `RTCDataChannel` not supported (Edge does not implement it).
* `addIceCandidate()` not supported (it's never called anyway, at least in non P2P mode). Edge does not support Trickle-ICE which means that, after reading all the remote ICE candidates from the remote SDP and applying them, an "empty" candidate must be immediately applied (otherwise the `RTCIceTransport` never enters the "completed" state) and, after that, new remote candidates cannot be added.


### RTCSessionDescription.js

Exports a `RTCSessionDescription` shim.

The interface is the same as in the WebRTC specification (although internally it also handles a Object representation of the SDP generated by the [sdp-transform](https://www.npmjs.com/package/sdp-transform) library).


### errors.js

Exports some `Error` based classes needed in WebRTC (such as `InvalidStateError`).


### utils.js

Some utilities related to SDP and ORTC.


## Issues


### ICE error: Wrong MESSAGE-INTEGRITY

Randomly, ICE Binding Requests from Edge are replied with WRONG MESSAGE-INTEGRITY error by the bridge. It seems that Edge, sometimes, adds a extra byte in the `USERNAME` attribute producing such an issue.

* GitHub issue: https://github.com/jitsi/lib-jitsi-meet/issues/498
* Edge issue: https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/12332457/


### Wrong VP8 video from Edge to Chrome if ENABLE_VP8_PICID_REWRITING=True

When VP8 rewriting is enabled in the bridge, VP8 video produced by Edge is wrongly rendered in Chrome/Firefox.

* GitHub issue: https://github.com/jitsi/lib-jitsi-meet/issues/520


### InvalidStateError when calling send() / receive()

Randomly, sending a new track or receiving a new track fails with `InvalidStateError`. The issue reported in Edge tracker includes a way to reproduce it.

As per conversations with Edge developers, it seems that Edge has some kind of limitation in the number of VP8 streams it can encode/decode at the same time. However, in my tests I've been able to send a VP8 stream and receive 5 VP8 streams at the same time.

The problem happens much more often when calling `rtpSender.stop()` or `rtpReceiver.stop()` and then creating a new `RtpSender` or `RtpReceiver` and calling `send()` or `receive()` on it.

* GitHub issue: https://github.com/jitsi/lib-jitsi-meet/issues/519
* Edge issue: https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/12459320/


### pc.getStats(): no way to get usable and needed stats

WebRTC stats produced by Edge don't provide enough information to determine resolution, framerate or bitrate.

* GitHub issue: https://github.com/jitsi/lib-jitsi-meet/issues/523


### Device selection does not work

In Edge, the device selector UI shows grayed drop down lists for mic and webcam.

* GitHub issue: https://github.com/jitsi/lib-jitsi-meet/issues/530


### Other issues in Edge

Those issues/bugs in Edge do not affect lib-jitsi-meet because the code avoids them. However, for future changes, it's important to consider them:

* `RtpReceiver.track` returns a different `MediaStreamTrack` every time: https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/12399497/