RTP has always been a protocol that supports multiple participants
each sending their own media streams in an RTP session.
Unfortunately, many implementations are designed only for point to
point voice over IP with a single source in each end-point. Even
client implementations aimed at video conferences have often been
built with the assumption around central mixers that only deliver a
single media stream per media type. Thus any application that wants
to allow for more advanced usage, where multiple media streams are
sent and received by an end-point, has an issue with legacy
implementations. This document describes the problem and proposes a
signalling solution for how to use multiple SSRCs within one RTP
session and at the same time handle the legacy issues.