4. Use of RTP by Endpoints That Send Multiple Media Streams

RTP is inherently a group communication protocol. Each endpoint in an RTP session will use one or more SSRCs, as will some types of RTP-level middlebox. Accordingly, unless restrictions on the number of SSRCs have been signaled, RTP endpoints can expect to receive RTP data packets sent using a number of different SSRCs, within a single RTP session. This can occur irrespective of whether the RTP session is running over a point-to-point connection or a multicast group, since middleboxes can be used to connect multiple transport connections together into a single RTP session (the RTP session is defined by the shared SSRC space, not by the transport connections). Furthermore, if RTP mixers are used, some SSRCs might only be visible in the contributing source (CSRC) list of an RTP packet and in RTCP, and might not appear directly as the SSRC of an RTP data packet.

Every RTP endpoint will have an allocated share of the available session bandwidth, as determined by signaling and congestion control. The endpoint needs to keep its total media sending rate within this share. However, endpoints that send multiple RTP streams do not necessarily need to subdivide their share of the available bandwidth independently or uniformly to each RTP stream and its SSRCs. In particular, an endpoint can vary the bandwidth allocation to different streams depending on their needs, and it can dynamically change the bandwidth allocated to different SSRCs (for example, by using a variable-rate codec), provided the total sending rate does not exceed its allocated share. This includes enabling or disabling RTP streams, or their redundancy streams, as more or less bandwidth becomes available.