u-hyogo.ac.jp;user=phone> From: userA<sip:userA@ai.u-hyogo.ac.jp> Call-ID: 12-34-56-78@ai.u-hyogo.ac.jp CSeq: 1 INVITE Subject: Lunch? © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Contact: <sip:userA@ai.u-hyogo.ac.jpx Content-Type: application/sdp Content-Length: 123 m=audio 5060 udp sdp SDP [10] describes general information about multimedia session established using SIP. The general form of SDP is: x=parameterl parameterN . In this example, the m field means that a caller would like to communicate with an audio media by User Datagram Protocol (UDP) at port 5060. In SDP, an option field a is defined. It can be used to extend SDP to provide more information about the media. Response message: When a SIP server and a UAS accept a SIP message such as REGISTER and INVITE, the server and the UAS reply to the SIP message. The responses 2xx, 4xx, and 5xx mean a success (OK), a client error, and a server error, respectively. A UAS registers its own information using REGISTER message (Figure 7.3(1)). The information includes multiple contact addresses where the UAC can reach the UAS. When a UAC would like to communicate with a UAS, the UAC issues an INVITE message to the SIP server (Figure 7.3(2)). The SIP server searches for the UAS locations and routes the INVITE message using the contact URI (Figure 7.3(3)). If the UAS accepts the INVITE message, it replies a 2xx OK message to the server (Figure 7.3(4)). The server relays the OK message to the UAC (Figure 7.3(5)). After the channel is established, the communication starts between the UAC and the UAS (Figure 7.3(6)). server Proxy Redirection Registrar server server server REGISTER(1) INVITE(2) INVITE(3) OK(5) OK(4) connection(6) User agent client User agent server (UAC) (UAS) FIGURE 7.3 SIP call establishment. (From Nakamoto, Y. and N. Kuri, Proc of 6th IEEE Int Conf Comp Info Tech, 2006, p. 99. With permission.) © 2009 by Taylor & Francis Group, LLC Home Networking System with SIP Redesign Issues A control point and a device 