o New receivers need to receive the CNAME for a source as soon
as possible to identify the source and to begin associating
media for purposes such as lip-sync, so each compound RTCP
packet should also include the SDES CNAME.
o The number of packet types that may appear first in the
compound packet should be limited to increase the number of
constant bits in the first word and the probability of
successfully validating RTCP packets against misaddressed RTP
Schulzrinne, et al Standards Track [Page 17]
RFC 1889 RTP January 1996
data packets or other unrelated packets.
Thus, all RTCP packets must be sent in a compound packet of at least
two individual packets, with the following format recommended:
Encryption prefix: If and only if the compound packet is to be
encrypted, it is prefixed by a random 32-bit quantity redrawn
for every compound packet transmitted.
SR or RR: The first RTCP packet in the compound packet must always
be a report packet to facilitate header validation as described
in Appendix A.2. This is true even if no data has been sent nor
received, in which case an empty RR is sent, and even if the
only other RTCP packet in the compound packet is a BYE.
Additional RRs: If the number of sources for which reception
statistics are being reported exceeds 31, the number that will
fit into one SR or RR packet, then additional RR packets should
follow the initial report packet.
SDES: An SDES packet containing a CNAME item must be included in
each compound RTCP packet. Other source description items may
optionally be included if required by a particular application,
subject to bandwidth constraints (see Section 6.2.2).
BYE or APP: Other RTCP packet types, including those yet to be
defined, may follow in any order, except that BYE should be the
last packet sent with a given SSRC/CSRC. Packet types may appear
more than once.
It is advisable for translators and mixers to combine individual RTCP
packets from the multiple sources they are forwarding into one
compound packet whenever feasible in order to amortize the packet
overhead (see Section 7). An example RTCP compound packet as might be
produced by a mixer is shown in Fig. 1. If the overall length of a
compound packet would exceed the maximum transmission unit (MTU) of
the network path, it may be segmented into multiple shorter compound
packets to be transmitted in separate packets of the underlying
protocol. Note that each of the compound packets must begin with an
SR or RR packet.
An implementation may ignore incoming RTCP packets with types unknown
to it. Additional RTCP packet types may be registered with the
Internet Assigned Numbers Authority (IANA).
