Re: Proposed Resolution to Issue #84: Capabilities and Settings

This is a significant cleanup and covers a lot of things that were missing
before.  It feels good and complete.  Thanks for doing this work.


On Fri, Jun 13, 2014 at 1:51 PM, Bernard Aboba <Bernard.Aboba@microsoft.com>
wrote:

>  In ORTC  API, we have Capabilities which describe what an implementation
> can do, and Parameters, which describe what the developer wants to do.
>
>
>
> After having reworked the RTCRtpCapabilities object so as to be able to
> rewrite the Examples that formerly called the createParameters and
> filterParameters methods, we took a look at the implications for the
> RTCRtpParameters object.
>
>
>
> In the last Editor’s draft, the RTCRtpParameters object had made reference
> to the concepts of Capabilities and Settings from the getUserMedia()
> specification (specifically, the constrainable interface).
>
>
>
> This concerned some folks because of the overall complexity of the
> constrainable interface.   For example, in Issue #84, Shijun Sun noted that
> “ IMHO we should avoid the “Capabilities” stuff if it is not needed.”
>
>
>
> So we looked at whether we could simplify the RTCRtpParameters object.
>
>
>
> Also, we went over the RTP Usage Draft to make sure that ORTC API could be
> used to set any configuration parameter which that document “required to be
> signaled”.   A number of things were found to be missing, such as the
> ability to configure RTP/RTCP multiplexing and reduced-size RTCP.
>
>
>
> Furthermore, we looked at what configuration information was necessary for
> RTCP feedback messages.  Some of these relate to the transport layer (e.g.
> Generic NACK and TMMBR/TMMBN) so that they can apply to any codec, whereas
> other payload-specific messages (e.g. SLI, PLI, RPSI) are sent and
> processed within the codec, so they need to be configured at the codec
> level.   Rather than enabling RTCP feedback messages to be configured at
> both the RTP and per-codec levels, we decided that all RTCP configuration
> should be done at the codec level, so as to avoid complexity.
>
>
>
> With respect to Header Extensions, which could be configured for use with
> particular codecs, there was also a need for global consistency.  For
> example, the same header extension could be used with multiple codecs but
> with the same ID for all of them.  Therefore it made sense to have the ID
> reside at the RTP level, but there also needed to be a way of indicating
> whether the header extension would be used with a given codec or not.  For
> example, it would not make sense to use the Client to Mixer Audio extension
> with a video codec.
>
>
>
> After we went through the RTCRtpParameters again with a fine tooth comb,
> we found that we could eliminate the use of Capabilities and Settings from
> getUserMedia entirely, while at the same time providing the appropriate
> configuration knobs.    Here is what the proposed new dictionaries look
> like:
>
>
>
>
>
> dictionary RTCRtpParameters {
>
>     DOMString                                 receiverId = "";
>
> // Provide a sequence of codec configurations.
>
>     sequence<RTCRtpCodecParameters>           codecs;
>
> // Provide a sequence of all the header extensions to be supported on all
> the codecs.
>
>     sequence<RTCRtpHeaderExtensionParameters> headerExtensions;
>
> // Provide the encoding parameters for the codecs.
>
>     sequence<RTCRtpEncodingParameters>        encodings;
>
> // Provide information on RTCP settings applying to all codecs.
>
>     RTCRtcpParameters                         rtcp;
>
> };
>
>
>
> dictionary RTCRtpCodecParameters {
>
> // Specify the name of the codec.
>
>     DOMString                 name = "";
>
> // Specify the codec payload type.
>
>     unsigned short            payloadType;
>
>     unsigned long?            clockRate = null;
>
>     unsigned short?           numChannels = 1;
>
> // Indicate what RTCP feedback messages are to be used with this codec.
>
> // This includes both Transport Layer and Payload-specific messages.
>
>     sequence<RTCRtcpFeedback> rtcpFeedback;
>
> // Allow setting of codec-specific parameters (e.g. fmtp parameters).
>
>     Dictionary                parameters;
>
> // Specify the header extensions to be used with this codec.  The header
> extension IDs
>
> // are specified in RTCRtpParameters so they don't conflict.
>
>     sequence<DOMString>       headerExtensionURIs;
>
> };
>
>
>
> dictionary RTCRtpHeaderExtensionParameters {
>
>     DOMString      uri;
>
> // The extension ID.  Must be the same for all codecs enabling the header
> extension.
>
>     unsigned short id;
>
> // Whether the header extension is encrypted or not.
>
>     boolean        encrypt = false;
>
> };
>
>
>
> dictionary RTCRtpEncodingParameters {
>
> // The SSRC to which the encoding parameters apply, if available.
>
>     unsigned long?       ssrc = null;
>
> // Payload Type (and therefore, codec) to which the encoding parameters
> apply.
>
>     unsigned short?      codecPayloadType = null;
>
> // FEC configuration details, if used.
>
>     RTCRtpFecParameters? fec = null;
>
> // Retransmission configuration details, if used.
>
>     RTCRtpRtxParameters? rtx = null;
>
>     double               priority = 1.0;
>
> // Maximum bitrate.
>
>     double?              maxBitrate = null;
>
>     double               minQuality = 0;
>
>     double               framerateBias = 0.5;
>
> // Fraction of resolution to be used in this layer (for spatial
> scalability).
>
>     double               resolutionScale = null;
>
> // Fraction of framerate to be supported in this layer (for temporal
> scalability)
>
>     double               framerateScale = null;
>
> // Fraction of quality to be supported in this layer (for quality
> scalability)
>
>     double               qualityScale = null;
>
> // Whether this layer is active.
>
>     boolean              active = true;
>
> // The layer identifier (used for either simulcast or SVC layers)
>
>     DOMString?           encodingId;
>
> // The layers on which this layer depends (used only for SVC layers)
>
>     sequence<DOMString>  dependencyEncodingIds;
>
> };
>
>
>
> dictionary RTCRtcpParameters {
>
> // The SSRC used for RTCP.
>
>     unsigned long ssrc;
>
> // Whether compound RTCP is used, or reduced size RTCP as specified in RFC
> 5506 (false).
>
>     boolean       compound = true;
>
> // Whether RTP/RTCP multiplexing is used (as specified in RFC 5761).
>
>     boolean       mux = true;
>
> };
>
>
>
>