e header fields and the CSeq: a decimal number used to uniquely identify message body may not appear in all messages. a request. All responses corresponding to a request use CHAPTER 3.6 Real-time communication over networks the same CSeq as the request. The CSeq number is Three-frame bundling is used, three audio frames are usually increased by one for a new request. bundled together to form an RTP packet. Audio packe- From: used to specify the originator. Example: From: tization should follow the rules defined in RFC 3267 [9]. John Smith'' <sip: johnsmith@test.com> Video is transported by the RTP/AVP protocol through Max-Forwards: an integer in the range of 0-255 used port 49350, with payload number 110. The RTP time- to specify the maximum number of hops that a message stamp resolution is 1/90,000 S. The video is coded by can take. The recommended initial value is 70. It is de- MPEG-4 video SVP L0 (simple visual profile level 0). creased by one as the message passes through a proxy or Video packetization should follow the rules defined in gateway. The proxy/gateway discards the message when RFC 3016 [10]. the value is dropped to zero. SIP is an IP telephony signaling protocol developed by To: used to specify the recipient of the request. the IETF, which competes with the H.323 protocol de- Via: used to record the path the request has been veloped by the ITU-T for the same application. The fun- traveled. The response walks through the same path in damental difference between the two protocols is that SIP the reverse order. is a text-based protocol and inherits the rich set of the The mandatory headers for the six fundamental re- IETF protocols, such as SDP, whereas H.323 is binary quests are shown in Table 3.6-1. encoded and utilizes many features from other ITU-T protocols, for example, H.245. Comparisons between the 3.6.2.2.3 Message body two protocols on features, packet loss recovery, security Although any format can be used as a message body, the mechanism, and capability exchange are listed in Session Desc