SR/RR reception report blocks: A translator forwards reception
reports received from one cloud to the others. Note that these
flow in the direction opposite to the data. The SSRC is left
intact. If a translator combines several data packets into one
output packet, and therefore changes the sequence numbers, it
must make the inverse manipulation for the packet loss fields
and the "extended last sequence number" field. This may be
complex. In the extreme case, there may be no meaningful way to
translate the reception reports, so the translator may pass on
no reception report at all or a synthetic report based on its
own reception. The general rule is to do what makes sense for a
particular translation.
A translator does not require an SSRC identifier of its own, but may
choose to allocate one for the purpose of sending reports about what
it has received. These would be sent to all the connected clouds,
each corresponding to the translation of the data stream as sent to
that cloud, since reception reports are normally multicast to all
participants.
SDES: Translators typically forward without change the SDES
information they receive from one cloud to the others, but may,
for example, decide to filter non-CNAME SDES information if
bandwidth is limited. The CNAMEs must be forwarded to allow SSRC
identifier collision detection to work. A translator that
generates its own RR packets must send SDES CNAME information
about itself to the same clouds that it sends those RR packets.
Schulzrinne, et al Standards Track [Page 42]
RFC 1889 RTP January 1996
BYE: Translators forward BYE packets unchanged. Translators with
their own SSRC should generate BYE packets with that SSRC
identifier if they are about to cease forwarding packets.
APP: Translators forward APP packets unchanged.
7.3 RTCP Processing in Mixers
Since a mixer generates a new data stream of its own, it does not
pass through SR or RR packets at all and instead generates new
information for both sides.
SR sender information: A mixer does not pass through sender
information from the sources it mixes because the
characteristics of the source streams are lost in the mix. As a
synchronization source, the mixer generates its own SR packets
with sender information about the mixed data stream and sends
them in the same direction as the mixed stream.
SR/RR reception report blocks: A mixer generates its own reception
reports for sources in each cloud and sends them out only to the
same cloud. It does not send these reception reports to the
other clouds and does not forward reception reports from one
cloud to the others because the sources would not be SSRCs there
(only CSRCs).
