------------------->
Figure 3: Sample RTP network with end systems, mixers and translators
A collection of mixers and translators is shown in Figure 3 to
illustrate their effect on SSRC and CSRC identifiers. In the figure,
end systems are shown as rectangles (named E), translators as
triangles (named T) and mixers as ovals (named M). The notation "M1:
48(1,17)" designates a packet originating a mixer M1, identified with
M1's (random) SSRC value of 48 and two CSRC identifiers, 1 and 17,
copied from the SSRC identifiers of packets from E1 and E2.
7.2 RTCP Processing in Translators
In addition to forwarding data packets, perhaps modified, translators
and mixers must also process RTCP packets. In many cases, they will
take apart the compound RTCP packets received from end systems to
aggregate SDES information and to modify the SR or RR packets.
Retransmission of this information may be triggered by the packet
arrival or by the RTCP interval timer of the translator or mixer
itself.
A translator that does not modify the data packets, for example one
that just replicates between a multicast address and a unicast
address, may simply forward RTCP packets unmodified as well. A
Schulzrinne, et al Standards Track [Page 41]
RFC 1889 RTP January 1996
translator that transforms the payload in some way must make
corresponding transformations in the SR and RR information so that it
still reflects the characteristics of the data and the reception
quality. These translators must not simply forward RTCP packets. In
general, a translator should not aggregate SR and RR packets from
different sources into one packet since that would reduce the
accuracy of the propagation delay measurements based on the LSR and
DLSR fields.
SR sender information: A translator does not generate its own sender
information, but forwards the SR packets received from one cloud
to the others. The SSRC is left intact but the sender
information must be modified if required by the translation. If
a translator changes the data encoding, it must change the
"sender's byte count" field. If it also combines several data
packets into one output packet, it must change the "sender's
packet count" field. If it changes the timestamp frequency, it
must change the "RTP timestamp" field in the SR packet.
