Opus [0] is a versatile audio codec, with a variable sample rate and bitrate, suitable for both music and speech. It is defined in RFC 6716 [1] and required by WebRTC [2]. Opus can operate at various sample rates, from 8 KHz to 48 KHz, and at variable bitrates, from 6 kbit/sec to 510 kbit/sec. The RTP payload format defined for Opus in RFC 7587 [3] explains the use of media type parameters in SDP, and this article aims to analyze them and show in particular how "asymmetric streams" can be achieved. This is an example of SDP defining an Opus offer or answer: ...