r issue we have to deal with is error concealment. The techniques for this depend on the codec standards used. With waveform coders, the technique is to replay the previous packet, to interpolate, and to predict. With the CELP coders, coefficients are used to predict the gain, pitch, and SO on of the missing packet. There are also some advanced adaptation techniques, such as forward error correc- tion (FEC), in which extra packets with FEC are sent over some number of the packets. As another example, with layered coders, you send higher- or lower-fidelity or different encodings on different streams. Receivers then subscribe to appropriate layers. This is especially valuable for multicast. Media Transport Requirements Media transport requirements include identification of the payload, the type of codec used, the framing used, timing reconstruction, when to play out each packet within a stream, and how to synchronize multiple streams. Note that an external reference clock is needed to correlate time stamps of different RTP streams. Another requirement is sequencing-that is, how to play out in the correct order and how to detect losses. The media transport functions include moving bits, which is accomplished with RTP and SRTP. Feedback and statistics need to be provided as well, and this is accomplished using RTCP as well as the emerging standard RTCP XR, which is a reporting extension being developed by the IETF's AVT (Audio/Video Transport) working group. These are the main transport protocols: RTP-RTP, specified in RFC 3550, is a transport protocol for real-time applications. It is used by all VoIP signaling protocols, and it is based on User Datagram Protocol (UDP). RTP provides media transport functions as well as additional features. It is multicast friendly and easy to encrypt, it provides QoS feedback with RTCP, and it requires minimal session control. RTCP-RTCP is based on the periodic transmission of control packets to all participants in the session, using the same distribution mechanism as the data packe