ined as the number of generated, as well as transmission statistics, such as how packets lost over the total number of received packets many packets and bytes have been transmitted since the since the last SR or RR packet was sent. beginning of the RTP session. SR control packets can also be The transmission interval of RTCP packets is often spec- used to synchronize the playout of different media data. ified in proportion to the session bandwidth. It is recom- SR and RR reports are also often used for flow and mended that the fraction of the session bandwidth added for congestion control. For example, by analyzing the inter- RTCP be fixed at 5%. Some applications may specify the arrival jitter field of the sender report, we can measure minimal transmission interval to be, for example, 5 S. the jitter over a certain interval and indicate congestion. As defined in RFC 3550, when packet i is received, the 3.6.2.3.3 Video payload format interarrival jitter J(i) is calculated as The purposes of using a video payload format are to specify an efficient way to encapsulate data to form (3.6.1) a standard-compliant bit stream and to enhance the RTP Sender RTP Receiver SR(i) sent at 10:20:30.250 DLSR(i) = 5 seconds RR(i) sent with the last SR RR(i) received at timestamps of SR(i) and 10:20:36.750 DLSR = 5 seconds) Round-trip time = 10:20:36.750 - 10:20:30.250 - 5 = 1.5 (seconds) Figure 3.6-6 An example of a round-trip time calculation. Real-time communication over networks CHAPTER 3.6 resilience against packet losses. The payload here means different codecs has to be supported and implemented to media data that are packed in an RTP packet. Forming provide a unified video payload encapsulation. the media payload can be done in a thin layer between the media encoder and the RTP transport layer. Cur- 3.6.2.3.4 An example of a call setup process rently, payload formats defined in RFC 3016 and RFC Before moving to the next section, we provide an ex- 2429 for encapsulating MPEG-4 and H.263 video data ample and illustrate the pro