This document specifies a grouping mechanism for RTP media streams that can be used to specify relations betweeen media streams within different RTP sessions. This mechanism is used to signal the association between the RTP concept of SSRC and the WebRTC concept of "media stream" / "media stream track" using SDP signalling. This document is an input document for discussion. It should be discussed in the RTCWEB WG list, rtcweb@ietf.org.