What should the scalability mode be when no scalability mode is specified?

[alvestrand]

Consider the following code snippet:

  const pc = new RTCPeerConnection();
  t.add_cleanup(() => pc.close());
  const { sender } = pc.addTransceiver('video', {
    sendEncodings: [{}],
  });
  const param = sender.getParameters();
  const encoding = getFirstEncoding(param);

What should the value of "encoding.scalabilityMode" be?

Three alternatives:

• It should reflect the default scalability mode of the implementation
• It should be the empty string, to reflect that no scalaiblity mode is chosen
• It should be missing

At the moment the Chrome implementation chooses alternative 3.

[aboba]

I would say 'the default scalability mode'. This might be 'L1T1' for some codecs, but not for VP8.

[Orphis]

I don't think we should have an empty string there as it's not a valid scalabilityMode and we shouldn't special case it. Missing (undefined) is probably a better choice. As for what it does, I believe it should be up to the user agent to chose a sensible default.

We don't want to have to update all the existing applications so that they do not fallback to L1T1, nor do we want to have all the users understand all the pros and cons of having temporal layers and have to specify it from now on for all the browsers implementing the SVC spec. Experts probably won't care too much about the default value and supply their own default anyway.

[alvestrand]

I think we need to have the resulting scalability mode be implementation dependent, but there should be one that can be read out by getParameters.

[Orphis]

We probably want to specify that while it is implementation dependent, user agents may only chose to add temporal layers, no spatial layers. So pick any from available L1T* modes.

[aboba]

How about making the proposal for this Issue similar to that proposed for Issue 59?

• Before negotiation is completed, getParameters() should return the default mode of the most preferred codec.
• If negotiation has completed, getParameters() returns the currently used scalability mode, which can depend on the negotiated codec and its default scalability mode (both implementation dependent).

[jan-ivar]

Leaning toward choice 1.

[alvestrand]

Seems to me right to align with #59 - which is option 1 in the options list, so agreeing with Jan-Ivar.

[aboba]

Closed after merger of PR 64.