Synchronization source (SSRC): The source of a stream of RTP packets,
identified by a 32-bit numeric SSRC identifier carried in the
RTP header so as not to be dependent upon the network address.
All packets from a synchronization source form part of the same
timing and sequence number space, so a receiver groups packets
by synchronization source for playback. Examples of
synchronization sources include the sender of a stream of
packets derived from a signal source such as a microphone or a
camera, or an RTP mixer (see below). A synchronization source
may change its data format, e.g., audio encoding, over time. The
SSRC identifier is a randomly chosen value meant to be globally
unique within a particular RTP session (see Section 8). A
participant need not use the same SSRC identifier for all the
RTP sessions in a multimedia session; the binding of the SSRC
identifiers is provided through RTCP (see Section 6.4.1). If a
participant generates multiple streams in one RTP session, for
example from separate video cameras, each must be identified as
a different SSRC.
Contributing source (CSRC): A source of a stream of RTP packets that
has contributed to the combined stream produced by an RTP mixer
(see below). The mixer inserts a list of the SSRC identifiers of
the sources that contributed to the generation of a particular
packet into the RTP header of that packet. This list is called
the CSRC list. An example application is audio conferencing
where a mixer indicates all the talkers whose speech was
combined to produce the outgoing packet, allowing the receiver
to indicate the current talker, even though all the audio
packets contain the same SSRC identifier (that of the mixer).
Schulzrinne, et al Standards Track [Page 8]
RFC 1889 RTP January 1996
End system: An application that generates the content to be sent in
RTP packets and/or consumes the content of received RTP packets.
An end system can act as one or more synchronization sources in
a particular RTP session, but typically only one.
Mixer: An intermediate system that receives RTP packets from one or
more sources, possibly changes the data format, combines the
packets in some manner and then forwards a new RTP packet. 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.
Thus, all data packets originating from a mixer will be
identified as having the mixer as their synchronization source.
