This document expands and clarifies the behavior of the Real-Time Transport Protocol (RTP) endpoints when they are using multiple synchronization sources (SSRCs), e.g. for sending multiple media streams, in a single RTP session. In particular, issues involving RTCP Control Protocol (RTCP) messages are described. This document updates RFC 3550 in regards to handling of multiple SSRCs per endpoint in RTP sessions. It also updates RFC 4585 to update and clarify the calculation of the timeout of SSRCs and the inclusion of feeback messages.