3. Use Cases for Multi-Stream Endpoints

This section discusses several use cases that have motivated the development of endpoints that sends RTP data using multiple SSRCs in a single RTP session.

3.1. Endpoints with Multiple Capture Devices

The most straightforward motivation for an endpoint to send multiple simultaneous RTP streams in a single RTP session is when an endpoint has multiple capture devices and, hence, can generate multiple media sources, of the same media type and characteristics. For example, telepresence systems of the type described by the CLUE Telepresence Framework [CLUE-FRAME] often have multiple cameras or microphones covering various areas of a room and, hence, send several RTP streams of each type within a single RTP session.

3.2. Multiple Media Types in a Single RTP Session

Recent work has updated RTP [MULTI-RTP] and Session Description Protocol (SDP) [SDP-BUNDLE] to remove the historical assumption in RTP that media sources of different media types would always be sent on different RTP sessions. In this work, a single endpoint's audio and video RTP streams (for example) are instead sent in a single RTP session to reduce the number of transport-layer flows used.

3.3. Multiple Stream Mixers

There are several RTP topologies that can involve a central device that itself generates multiple RTP streams in a session. An example is a mixer providing centralized compositing for a multi-capture scenario like that described in Section 3.1. In this case, the centralized node is behaving much like a multi-capturer endpoint, generating several similar and related sources.

A more complex example is the selective forwarding middlebox, described in Section 3.7 of [RFC7667]. This is a middlebox that receives RTP streams from several endpoints and then selectively forwards modified versions of some RTP streams toward the other endpoints to which it is connected. For each connected endpoint, a separate media source appears in the session for every other source connected to the middlebox, "projected" from the original streams, but at any given time many of them can appear to be inactive (and thus are receivers, not senders, in RTP). This sort of device is closer to being an RTP mixer than an RTP translator: it terminates RTCP reporting about the mixed streams; it can rewrite SSRCs, timestamps, and sequence numbers, as well as the contents of the RTP payloads; and it can turn sources on and off at will without appearing to generate packet loss. Each projected stream will typically preserve its original RTCP source description (SDES) information.

3.4. Multiple SSRCs for a Single Media Source

There are also several cases where multiple SSRCs can be used to send data from a single media source within a single RTP session. These include, but are not limited to, transport robustness tools, such as the RTP retransmission payload format [RFC4588], that require one SSRC to be used for the media data and another SSRC for the repair data. Similarly, some layered media encoding schemes, for example, H.264 Scalable Video Coding (SVC) [RFC6190], can be used in a configuration where each layer is sent using a different SSRC within a single RTP session.