[w3ctag/design-reviews] Request for TAG Review of WebRTC-SVC (#396)

こんにちはTAG!

I'm requesting a TAG review of:

  - Name: [WebRTC-SVC]
  - Specification URL: [https://w3c.github.io/webrtc-svc/]
  - Explainer (containing user needs and example code)¹: [https://github.com/w3c/webrtc-svc/blob/master/explainer.md]
  - GitHub issues (if you prefer feedback filed there): [https://github.com/w3c/webrtc-svc/issues]
  - Tests: No WPT tests yet
  - Primary contacts (and their relationship to the specification): [pthatcherg, aboba]

Further details:

  - Relevant time constraints or deadlines: [Review by TPAC 2019 would be appreciated]
  - [X ] I have read and filled out the [Self-Review Questionnare on Security and Privacy](https://www.w3.org/TR/security-privacy-questionnaire/). The [assessment is here](https://w3c.github.io/webrtc-svc/#privacy-security).
  - [X ] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/)
  - The group where the work on this specification is: WEBRTC WG

We recommend the explainer to be in [Markdown](https://github.github.com/gfm/). On top of the [usual information expected in the explainer](https://w3ctag.github.io/explainers), it is strongly recommended to add:

  - Links to major pieces of multi-stakeholder review or discussion of this specification: 
  - Links to major unresolved issues or opposition with this specification:

You should also know that...

Relevant background:

Today, multiple browsers have implemented support for Scalable Video Coding (SVC) within their WebRTC implementations.  However, those implementations are proprietary and do not interoperate either at the protocol or API levels.

The goal of the WebRTC-SVC Extension to the WebRTC 1.0 API is to provide an interoperable way for browsers to support configuration of SVC within encoders.   The focus is on the VP8, VP9 and AV1 codecs which support SVC and require that compliant decoders be able to decode anything that an encoder can send.  This simplifies the API because there is no need to negotiate SVC support within Offer/Answer, allowing SVC support to be provided by adding an attribute to the RTCRtpEncodingParameters and RTCRtpCodecCapability dictionaries, without requiring changes to SDP.

The API expresses SVC capabilities and settings by using a shorthand designation for the desired SVC dependency structure known as the "scalability mode".  These scalability modes express commonly utilized configurations of spatial and temporal scalability.  It is also possible to configure spatial simulcast and temporal scalability, either using multiple RTP streams or a single stream (using "S" modes). 

We'd prefer the TAG provide feedback as (please select one):

  - [ ] open issues in our GitHub repo for each point of feedback
  - [X] open a single issue in our GitHub repo for the entire review
  - [ ] leave review feedback as a comment in this issue and @-notify [github usernames]

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_Please preview the issue and check that the links work before submitting._ In particular, if anything links to a URL which requires authentication (e.g. Google document), please make sure anyone with the link can access the document.

¹ For background, see our [explanation of how to write a good explainer](https://w3ctag.github.io/explainers).


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