WebRTC Codec Preferences

WebRTC specified mandatory to implement codecs for attaining the interoperability between browsers and to increase the success rate . Browser implementors may support other codecs apart from mandatory to implement codecs. This specification describes a mechanism for JavaScript application to check the available codecs, to give preferences among the available codecs and to remove some codecs if the application don't want to use them. This specification extends RTCPeerConnection by adding two new methods, getSupportedAudioCodecs and getSupportedVideoCodecs and the offer / answer options with sequences of audio and video codec list. This process will avoid the SDP mangling for removing and re-ordering of codecs.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

The getSupportedAudioCodes method of RTCPeerConnection returns the supported audio codecs. This function SHOULD be called before calling createOffer or createAnswer mehtod.

The getSupportedVideoCodes method of RTCPeerConnection returns the supported video codecs. This function SHOULD be called before calling createOffer or createAnswer mehtod.

createOffer and createAnswer methods supports optional "options" attribute. A sequence of audio and video codecs, namely preferedAudioCodecs and preferedVideoCodecs respectively, in applications prefered order SHOULD be included as optional constraints in RTCOfferAnswerOptions. Next sections explain about preferedAudioCodecs and preferdVideoCodecs.

preferedAudioCodecs optional constraint is a sequence of audio codecs in the applications prefered order. If the application does not specify any codec preference, then this attribute MUST not be added to RTCOfferAnswerOptions. If options attribute is not present in the createOffer/createAnswer, browser SHOULD arrange the codecs with previously specified preference, if specified earlier for that session, otherwise browser MAY arrange the codecs with its own preferences. The prefered codec sequence MAY contain the same number of audio codecs returned by getSupportedAudioCodecs. Audio codecs other than those supported by browser SHOULD be ignored, if present in preferedAudioCodecs. If the preferedAudioCodecs does not contain atleast one audio codec returned by getAudioCodecs, browser SHOULD fire an error event UNSUPPORTED_CODECS. The offer/answer SHOULD_NOT contain audo codecs other than those specified in preferedAudioCodecs and the order of preference SHOULD be with high priority for the codecs first in the preferedAudioCodecs sequence.

preferedVideoCodecs optional constraint is a sequence of video codecs in the applications prefered order. If the application does not specify any codec preference, then this attribute MUST not be added to RTCOfferAnswerOptions. If options attribute is not present in the createOffer/createAnswer, browser SHOULD arrange the codecs with previously specified preference, if specified earlier for that session, otherwise browser MAY arrange the codecs with its own preferences. The prefered codec sequence MAY contain the same number of video codecs returned by getSupportedVideoCodecs. Video codecs other than those supported by browser SHOULD be ignored, if present in preferedVideoCodecs. If the preferedVideoCodecs does not contain atleast one video codec returned by getVideoCodecs, browser SHOULD fire an error event UNSUPPORTED_CODECS. The offer/answer SHOULD_NOT contain audo codecs other than those specified by preferedVideoCodecs and the order of preference SHOULD be with high priority for the codecs first in the preferedVideoCodecs sequence.

TBD.

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TBD.