tamps of the video stream ensure that the video is not played faster or slower than Chapter 3 the original, while the audio timestamps do precisely the same with the audio stream. However, a mechanism is needed to synchronize streams, and that's where the Real-time Transport Control Protocol (RTCP) [RFC 1889] comes in. Its function is to associate timestamps and a real-time clock. Every RTP session has a parallel RTCP session. Besides media synchro- nization, RTCP provides information about the members of the session and the quality of the communication. RTCP reports how many packets the net- work dropped during the session SO the sender knows what quality of recep- tion the receiver is experiencing. QoS Provisioning: Integrated Services and Differentiated Services The Internet's best-effort service model works fine for most applications when the network is under a reasonable load. However, when an IP net- work experiences heavier loads, best-effort service may not be adequate for end-to-end traffic. As delays increase, the network becomes lossier. IP pack- ets bombard the routers faster than they can cope with them, forcing a queue to develop. When queue size limits are exceeded, the router responds by dropping datagrams. For TCP traffic, dropped datagrams mean more retransmissions, leading to lower performance and poor Quality of Service (QoS). The user begins to observe end-to-end delay even with relatively forgiving applications such as file transfer. If the dropped datagrams belong to real-time traffic, the receiver will never be received and the datagrams will not be retransmitted. As a result, the quality suffers. In the case of voice transmissions, audio can quickly become unintelligible on the receiver's end. For applications needing a better-than-best-effort service, two different approaches fit the bill: integrated services [RFC 1633] and differentiated services (DiffServ) [RFC 2475]. Integrated Services The basic idea behind the integrated services architecture is to give differ- ent treatment in the 