rom: J.C. Maxwell sip:james.maxwell@kings.cambridge.edu.uk> ;tag=5 To: k<sip:faraday@effect.org;user=ip>;tag=6321 Call-ID: mNjdwWjkBfWrd@7.9.18.12 CSeq: 55 CANCEL M15 SIP/2.0 487 Request Terminated <Final response to INVITE Via: SIP/2.0/UDP 7.9.18.12:60000;branch=z9hG4bk2 From: J.C. Maxwell<sip:james.maxwell@kings.cambridge.edu.uk> ;tag=5 To: <sip:faraday@effect.org;user=ip>;tag=6321 Call-ID: mNjdwWjkBfWrd@7.9.18.12 CSeq: 55 INVITE M16 ACK SIP/2.0 Via: SIP/2.0/UDP 7.9.18.12:60000;branch=z9hG4bk2 From: J.C. Maxwell sip:james.maxwell@kings.cambridge.edu.uk: ;tag=5 SIP: Understanding the Session Initiation Protocol To: <sip : faraday@effect.org;user=ip>;tag=6321 Call-ID: hNjdwWjkBfWrd@7.9.18.12 CSeq: 55 ACK 10.6 H.323 to SIP Call In this example, a H.323 terminal calls a SIP-enabled PC through a H.323/SIP gateway. The gateway does signaling translation between the protocols but allows the two end points to exchange media packets directly with each other. The full details of SIP/H.323 interworking are being developed in the SIP working group [4]. In this example, shown in Figure 10.6, the initial message exchange is between the calling H.323 terminal and the H.323 gatekeeper. The gatekeeper resolves the H.323 alias into an address served by the H.323/SIP gateway. The ACF response indicates that gatekeeper-routed signaling is required, SO the Q.931 and H.245 TCP connections are opened to the gatekeeper, which opens TCP connections to the gateway. The calling H.323 terminal sends a Q.931 Setup message to the gatekeeper, which proxies it to the H.323/SIP gateway. The gateway then looks up the H.323 alias and resolves it to the SIP URI of the called party. It constructs an INVITE from the Setup message and forwards it to a SIP proxy, which forwards it to the called party. Note that because the Setup message does not contain any media information, the INVITE does not contain any media information either. The called party sends a 180 Ringing then a 200 OK to indicate that the call has been answered. The media informa- tion