Merge pull request #24 from w3c/aboba-patch-11

Clarification on scalabiltyMode support

index.html

@@ -82,9 +82,9 @@ <h2>Terminology</h2>
  (<dfn>MRST</dfn>), also defined in [[RFC7656]] Section 3.7, refers to
  implementations that transmit all layers within a single transport, using
  multiple RTP streams with a distinct SSRC for each layer. This specification
- only supports <a>SRST</a> transport. Codecs with RTP payload specifications
- supporting <a>SRST</a> transport include VP8 [[RFC7741]], VP9 [[VP9-PAYLOAD]]
- and H.264/SVC [[RFC6190]].</p>
+ only supports <a>SRST</a> transport, not <a>MRST</a>. Codecs with RTP payload
+ specifications supporting <a>SRST</a> transport include VP8 [[RFC7741]], VP9
+ [[VP9-PAYLOAD]] and H.264/SVC [[RFC6190]].</p>
  <p>This specification references objects, methods, internal slots and dictionaries defined in
  [[!WEBRTC]], including the <dfn>RTCPeerConnection</dfn> object (defined in Section 4.4),
  the <dfn>RTCError</dfn> object (defined in Section 11.1),
@@ -261,6 +261,12 @@ <h3>Scalability modes</h3>
  identifiers and characteristics, are provided in the table below. Additional information
  on the scalability modes, including dependency diagrams, is available in [[AV1]]
  Section 6.7.5.</p>
+ <p>While [[AV1]] and VP9 [[VP9-PAYLOAD]] implementations can support
+ all the modes defined in the table, other codecs cannot. For example, VP8
+ [[RFC7741]] only supports temporal scalability (e.g. "L1T2", "L1T3"). H.264/SVC
+ [[RFC6190]], which supports both temporal and spatial scalability, only permits
+ transport of simulcast on distinct SSRCs, so that it does not support the
+ "S" modes (where multiple encodings are transported on a single SSRC).</p>
  <table style="border-width:0; width:60%" border="1">
  <tbody>
  <tr>