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3.1.5.15 Labeling a Media Description with an a=label Attribute

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All active media descriptions, except those with m=applicationsharing media type, in an SDP offer or SDP answer SHOULD contain an a=label attribute. The syntax of the a=label attribute is defined in [RFC4574].

The a=label attribute helps identify the modality of the media stream. In particular, the attribute can distinguish the modality of a media description with type m=video as either main video or panoramic video.

For the audio modality, the label value MUST be "main-audio". For the main video modality (which might comprise multiple media channels), the label value MUST be "main-video". For the panoramic video modality, the label value MUST be "panoramic-video". For the Video-based Screen Sharing modality, the label value MUST be “applicationsharing-video”.<27> Other label values are not recognized and MUST be ignored by the receiving user agent.