4. A fourth, optional function is to convey minimal session
control information, for example participant identification
to be displayed in the user interface. This is most likely
to be useful in "loosely controlled" sessions where
participants enter and leave without membership control or
parameter negotiation. RTCP serves as a convenient channel
to reach all the participants, but it is not necessarily
expected to support all the control communication
requirements of an application. A higher-level session
control protocol, which is beyond the scope of this
document, may be needed.
Functions 1-3 are mandatory when RTP is used in the IP multicast
environment, and are recommended for all environments. RTP
application designers are advised to avoid mechanisms that can only
work in unicast mode and will not scale to larger numbers.
Schulzrinne, et al Standards Track [Page 16]
RFC 1889 RTP January 1996
6.1 RTCP Packet Format
This specification defines several RTCP packet types to carry a
variety of control information:
SR: Sender report, for transmission and reception statistics from
participants that are active senders
RR: Receiver report, for reception statistics from participants that
are not active senders
SDES: Source description items, including CNAME
BYE: Indicates end of participation
APP: Application specific functions
Each RTCP packet begins with a fixed part similar to that of RTP data
packets, followed by structured elements that may be of variable
length according to the packet type but always end on a 32-bit
boundary. The alignment requirement and a length field in the fixed
part are included to make RTCP packets "stackable". Multiple RTCP
packets may be concatenated without any intervening separators to
form a compound RTCP packet that is sent in a single packet of the
lower layer protocol, for example UDP. There is no explicit count of
individual RTCP packets in the compound packet since the lower layer
protocols are expected to provide an overall length to determine the
end of the compound packet.
Each individual RTCP packet in the compound packet may be processed
independently with no requirements upon the order or combination of
packets. However, in order to perform the functions of the protocol,
the following constraints are imposed:
o Reception statistics (in SR or RR) should be sent as often as
bandwidth constraints will allow to maximize the resolution of
the statistics, therefore each periodically transmitted
compound RTCP packet should include a report packet.
