ription Protocol (SDP) [8] is the most Table 3.6-2. A more detailed comparison can be found popular one. SDP specifies media information such as in [11]. media type, codec, author, title, encryption key, band- width, start time, and end time. SDP can be used for 3.6.2.3 Media transport and control capability exchange at the call set-up stage. An example of SDP message is shown here protocols m = audio 49170 RTP/AVP 102 The commonly used media transport and control pro- a = rtpmap 102 AMR/8000 tocols in IP voice and video telephony applications are a = fmtp : 102 maxptime=60; octect- RTP and RTCP, as defined in RFC 3550. RTP and RTCP align=1; mode-set=4 are designed to be independent of the underlying trans- m = video 49350 RTP/AVP 110 port and network layers. Applications usually run RTP and a = rtpmap: 110 MP4V-ES/90000 RTCP on top of UDP and IP, as shown in Figure 3.6-3. a = fmtp 110 profile-level-id=0; config=000001B 3.6.2.3.1 Real-time transport protocols The SDP message just given specifies the following RTP provides end-to-end delivery services for media data information. Audio is transported by the RTP/AVP pro- that have real-time characteristics. It defines useful in- tocol through port 49170, with payload number 102. The formation such as timestamp, sequence number, and RTP timestamp resolution is 1/8000 S. The audio is marker, to allow receivers to keep the order of the packets, coded by AMR with the maximum bit rate of 7.4 kbps. and to play out media at the proper pace. This is due to the Table 3.6-1 Mandatory header fields for six fundamental SIP methods, where M denotes mandatory. Header/Requests INVITE REGISTER CANCEL OPTIONS Call-ID Contact Max-Forwards Real-time communication over networks CHAPTER 3.6 specifies that if an encoded video frame is carried in Table 3.6-2 Comparisions between SIP and H.323. multiple RTP packets, the marker bit of the last packet Comparisons H.323 should be set to one to indicate the end of the frame. This is particularly useful for the RTP receiver to signal Encoding m