36
V. Liberatore
Pervasive Computing
RTCP
*Control protocol
nWorks in conjunction with RTP
*Functions
nQoS monitoring and congestion control
*Convey feedback on quality of data delivery and information of membership
*Modify transmission rates
*Diagnostic
nSource identification
nSession size estimation and scaling
nSession control
*E.g., user names  to be displayed
*Reports
nParticipants periodically send RTCP report packets
nStatistics, e.g.,
*Number of packets sent
*Number of packets lost
*Inter-arrival jitter
QoS monitoring and congestion control
Provides feedback to an application about the quality of data distribution
Sender can adjust its transmission based on the receiver report feedback
Receivers can know whether a congestion is local, regional or global
Source identification
RTCP SDES (source description) packets contain textual information called canonical names as globally unique identifiers of the session participants
It may include user's name, telephone number, email address and other information
Session size estimation and scaling
All participants send periodic RTCP packets
Transmission rate of RTCP packets must be reduced as the number of participants increases
RFC 1889 limits RTCP traffic to less than 5% of session bandwidth, divided between sender reports (25%) and receiver reports (75%)
Session control
Optional function to convey minimal session control information, for example, participant identification to be displayed in the user interface