Tab Mirroring over CASTV2

The app used by Chrome is "Chrome Mirroring", and has application ID 0F5096E8.

The app is launched as usual and advertises protocols urn:x-cast:com.google.cast.webrtc and urn:x-cast:com.google.cast.media.

Once the app is launched, Chrome issues a CONNECT message to the transportId with two additional fields :

• origin which value is {}
• userAgent which contains the browser UA string

Then an OFFER is sent over the urn:x-cast:com.google.cast.webrtc channel :

{
  "type": "OFFER",
  "seqNum": 22790009,
  "offer": {
    "supportedStreams": [
      {
        "index": 0,
        "type": "audio_source",
        "codecName": "opus",
        "rtpProfile": "cast",
        "rtpPayloadType": 127,
        "ssrc": 306348,
        "storeTime": 400,
        "targetDelay": 400,
        "aesKey": "e91d41098c89ffb1352e8f65aa760bd4",
        "aesIvMask": "cabed613fdaf330a5576cc9ba0373e77",
        "bitRate": 128000,
        "sampleRate": 48000,
        "timeBase": "1\/48000",
        "channels": 2,
        "receiverRtcpEventLog": true
      },
      {
        "index": 1,
        "type": "video_source",
        "codecName": "vp8",
        "rtpProfile": "cast",
        "rtpPayloadType": 96,
        "ssrc": 866107,
        "storeTime": 400,
        "targetDelay": 400,
        "aesKey": "e91d41098c89ffb1352e8f65aa760bd4",
        "aesIvMask": "cabed613fdaf330a5576cc9ba0373e77",
        "maxFrameRate": "30000\/1000",
        "timeBase": "1\/90000",
        "maxBitRate": 5000,
        "resolutions": [
          {
            "width": 1280,
            "height": 720
          }
        ],
        "receiverRtcpEventLog": true
      }
    ]
  }
}

The Chromecast responds with an ANSWER :

{
  "answer": {
    "sendIndexes": [ 0, 1 ],
    "ssrcs": [ 306349, 866108 ],
    "udpPort": 36916
  },
  "result": "ok",
  "type": "ANSWER"
}

Chrome follows up with a PRESENTATION message on urn:x-cast:com.google.cast.webrtc(here I was browsing www.google.com) :

{
  "sessionId": "D9322EE6-C345-4B9D-E6E5-0B7370F7FBAA",
  "seqNum": 22790010,
  "type": "PRESENTATION",
  "title": "www.google.com (Tab)",
  "icons": [
    {
      "url": "https:\/\/www.google.com\/favicon.ico"
    }
  ]
}

And then sends a GET_STATUS on the media channel which returns the playing state (PLAYING) and a few other informations.

Then UDP traffic kicks in and the tab appears on the TV screen. I guess the OFFER/ANSWER cycle is somehow translated to its SDP counterpart but not knowing the WebRTC internals I cannot say much more.