Now we come to the rtpmap lines for the formats included in the “m” lines above to map the payload types and the codecs. We also have a number of the fmtp lines to describe characteristics of the offered codecs. The order in which the formats are written in the media line is important as it defines the priority in which the browsers would like to use them.
Opus is one of the MTI audio codecs for WebRTC. It features a variable bit rate (6kbps-510kbps) and is not under any royalty so it can be freely implemented in any browser (unlike other codecs like as G.729). Opus support is starting to become common and it has become critical for most WebRTC applications.