Media/WebRTC/Architecture

Overall Architecture

At a high level, there are five major components we need to integrate to build a functional WebRTC stack into Firefox.

• The MediaStream components that provide generic media support.
• The WebRTC.org contributed code that handles RTP and codecs.
• The SIPCC signaling stack.
• The DataChannel management code and the libsctp code that it drives.
• The transport stack (mtransport) stack which drives DTLS, ICE, etc.

These are managed/integrated by the PeerConnection code which provides the PeerConnection API and maintains all the relevant state.

In addition, there is the GetUserMedia() [GUM] code which handles media acquisition. However, the GUM code has no direct contact with the rest of the WebRTC stack, since the stack itself solely manipulates MediaStreams and does not care how they were created.

Here is an example sequence of events from the caller's perspective:

1. JS calls create one or more MediaStream objects via the GetUserMedia() API. The GUM code works with the MediaStream code and returns a MediaStream object.
2. JS calls new PeerConnection() which creates a PeerConnection object. [QUESTION: does this create a CC_Call right here?]
3. JS calls pc.AddStream() to add a stream to the PeerConnection.
4. JS calls pc.CreateOffer() to create an offer.
5. Inside PeerConnection.createOffer(), the following steps happen:
   1. A Create offer request is sent to the CCAPP_Task
   2. An appropriate number of WebRTC streams are set up to match the number of streams.
   3. Some number of mtransports are set up (to match the appropriate number of streams) [OPEN QUESTION: is this done by PeerConnection or inside SIPCC?.
6. Asynchronously, SIPCC creates the SDP and it gets passed up to the PeerConnection.
7. The PeerConnection forwards the SDP response to the DOM which fires the JS createOffer callback.
8. The JS forwards the offer and then calls pc.SetLocalDescription(). This causes:
   1. Attachment of the mtransport to the WebRTC.org streams via ExternalRenderer/ExternalTransport
9. When the remote SDP is received, the JS calls pc.SetRemoteDescription() which forwards to the CCAPP_Task in the same manner as createOffer() and setLocalDescription(). This causes:
   1. Forwarding of the ICE candidates to mtransport. At the time when the first candidates are received, mtransport can start ICE negotiation.
10. Once ICE completes, DTLS negotiation starts
11. Once DTLS negotiation completes, media can flow. [QUESTION: Should we hold off on attaching the mtransport to WebRTC.org until this is ready?]

Thread Diagram

Signaling System: CreateOffer

http://www.websequencediagrams.com/?lz=dGl0bGUgU2lnbmFsaW5nIFRocmVhZHMgKENyZWF0ZU9mZmVyKQpwYXJ0aWNpcGFudCAiRE9NACAHIiBhcyBET00AEw1QQyBhcyBQQwAnDkNDQVBQX1Rhc2sAMAUACgUASg1TVFMgYXMgU1RTCgpET00gLT4gUEM6IERpc3BhdGNoAIECDlBDIC0-AD8GOiBGRUFUVVJFX0NSRUFURV9PRkZFUgAbB1BDOiAAgUMGIFRyYW5zcG9ydEZsb3dcbltEVExTLCBJQ0VdCgCBFwUAYBEAXg1ET00ACRIKbm90ZSBsZWZ0IG9mAB0GSlMgY2FsbGJhY2tcbndpdGggb2ZmZXIKClNUUwCBQhFJQ0UgY2FuZGlkYXRlcwBVFgAUEABPJUlDRQoKCg&s=default

Thread diagram for transport subsystem

http://www.websequencediagrams.com/?lz=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&s=default