7. RTP Translators and Mixers
7.1 General Description
In addition to end systems, RTP supports the notion of "translators" and "mixers", which could be considered as "intermediate systems" at the RTP level.
Although this support adds some complexity to the protocol, the need for these functions has been clearly established by experiments with multicast audio and video applications in the Internet. Example uses of translators and mixers given in Section 2.3 stem from the presence of firewalls and low bandwidth connections.
Mixers and translators may be designed for a number of purposes. Some examples are:
- To act as a bridge between two different transport-level protocols (e.g., IP multicast and IP unicast)
- To allow participants with different capabilities to interoperate
- To reduce bandwidth requirements
- To improve the quality of service
7.2 RTCP Processing in Translators
A translator forwards RTP packets with their SSRC identifier intact; this has no significant effect on RTCP. Translators typically forward RTCP packets unchanged as well.
7.3 RTCP Processing in Mixers
A mixer receives streams of RTP packets from one or more sources, possibly changes the data format, combines the streams into a single stream, and forwards the new stream. Since the timing among multiple input sources will not generally be synchronized, the mixer will make timing adjustments among the streams and generate its own timing for the combined stream.
7.4 Cascaded Mixers
An RTP session may involve a collection of mixers and translators as shown in the examples of Section 2.4. If two mixers are cascaded, packets received by the second mixer from the first mixer should not be resent to the first mixer, unless this is the desired behavior.