(R(i)-R(i - -1))-(TS(i) phony and VoIP applications, there is no CSRC present - TS(i - 1)), (3.6.2) in the RTP header. and R(i) and TS(i) are the arrival time and the timestamp 3.6.2.3.2 Real-time control protocols of packet i, respectively. Both are in RTP timestamp RTCP is used in conjunction with RTP to allow RTP units. It is up to the implementation to decide what session participants to monitor the quality of data de- action to take when congestion occurs. A typical solution livery. It is based on the periodic transmission of control is to reduce the transmission rate until congestion be- packets. There are five control packets defined in RFC comes alleviated. 3550: The round-trip time can also be estimated using last SR timestamp (LSR) and the delay since last SR (DLSR) in- SR: Send Report. This is sent by an RTP participant formation in both RR and SR control packets. Figure 3.6-6 that sends and receives the RTP packets; demonstrates one example of a round-trip time calculation. RR: Receiver Report. This is sent by an RTP partici- Assume that the RTP sender sends one SR packet at time pant that only receives the RTP packets; 10:20:30.250. The RTP receiver receives this SR and, after SDES: Source DEScription, including CNAME; 5s, sends an RR packet. In the RR control packet, LSR is the BYE: This is to indicate the end of the RTP timestamp in SR(i) and the DLSR is 5 S. When the RTP participation; sender receives this RR packet at time 10:20:36.750, it can APP: Application-specific functions. calculate the round-trip time by subtracting the sending time of SR(i) and the DLSR from the arrival time of RR(i), Both SR and RR control packets contain reception sta- which is 1.5 S as shown in Figure 3.6-6. tistics such as interarrival jitter and packet loss rate. Each The fraction of loss in SR and RR control packets can SR control packet further includes the sender's wallclock also be used for the video encoder to perform error time and the corresponding RTP timestamp when it is control. The packet loss rate is def