3.1.5.28.2 User agent behavior for a=x-mediabw attribute

For the bandwidth limits to be effective, the label value of the x-mediabw attribute MUST match the media-level a=label attribute of one or more media descriptions within the SDP message, and the label value MUST be recognized by the user agent receiving the SDP message.

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". If the label value is not recognized by the receiving user agent, the attribute MUST be ignored.

If the label value is recognized by the receiving user agent, and one or more media streams with that label exist within the media session, the bandwidth values in the attribute SHOULD<54><55> be applied. If there is more than one active media channel with the same label value (for example, "main-video"), the bandwidth limits apply to the total bandwidth consumed by all the media channels of that modality. The receiving user agent applies the receive bandwidth limit from the x-mediabw attribute as an upper bandwidth consumption limit to its send streams. (The bandwidth consumed by the send stream for the user agent might also be constrained by other factors, unrelated to the x-mediabw attribute.)