s packets at irregu- lar intervals, will have an irregularly increasing Timestamp, which can be used to play back the packets at the correct interval. The RTP Control Protocol (RTCP) is a related protocol also defined in RFC 3550 that allows participants in an RTP session to send each other quality reports and statistics, and exchange some basic identity information. The four types of RTCP packets are shown in Table 7.3. RTCP has been designed to scale to very large conferences. Because RTCP traffic is all overhead, the band- width allocated to these messages remains fixed regardless of the number of par- ticipants. That is, the more participants on a conference, the less frequently RTCP packets are sent. For example, in a basic two-participant audio RTP ses- sion, the RTP/AVP profile states that RTCP packets are to be sent about every 5 seconds; for four participants, RTCP packets can be sent every 10 seconds. Sender reports (SR) or receiver reports (RR) packets are sent the most fre- quently, with the other packet types being sent less frequently. The use of reports allows feedback on the quality of the connection including information such as: Number of packets sent and received; Number of packets lost; Packet jitter. Table 7.3 RTCP Packet Types Packet Description Sender report Sent by a participant that both sends and receives RTP packets Receiver report Sent by a participant that only receives RTP packets Source description Contain information about the participant in the session including e-mail address, phone number, and host Sent to terminate the RTP session Application specific Defined by a particular profile Extended report Extended report and summaries SIP: Understanding the Session Initiation Protocol In a multimedia session established with SIP, the information needed to select codecs and send the RTP packets to the right location is carried in the SDP message body. Under some scenarios, it can be desirable to change codecs during an RTP session. An example of this relates to the transport of dual to