Connected: An Internet Encyclopedia
7.2 RTCP Processing in Translators
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RFC 1889
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7. RTP Translators and Mixers
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7.2 RTCP Processing in Translators
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
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.
- 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.
- 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.
Next: 7.3 RTCP Processing in Mixers
Connected: An Internet Encyclopedia
7.2 RTCP Processing in Translators