 present in the SDP message body is stored by the gateway, which sends Alerting and Connect messages. Messages are sent to the gatekeeper, which proxies them to the calling H.323 terminal. The gateway holds off sending the ACK response to the INVITE until the H.245 media exchange is completed between the H.323 ter- minal and the gateway. Once that is complete, the negotiated media capabilities are returned in the ACK and the media session begins. address (h323alias=Stibitz) gatekeeper routed signaling Setup Cd address (h323alias=Stibitz) Cg address (h323alias=Burroughs) Setup Call Flow Examples H.323 Gatekeeper Sip/H323 Proxy SIP PC Terminal gateway server 1.28.18.57 2.3.4.5 3.4.5.6 4.30.19.04 ARQ M1 AFC M2 Setup M3 Setup M4 ARQ M5 AFC M6 INVITE M7 INVITE M9 100 M8 180 M10 Alerting M13 Alerting M12 180 M11 200 M15 200 M14 Connect M17 Connect M16 TCS M18 TCS M19 TCSAck M20 TSCAck M21 TCS M23 TCS M22 TCSAck M24 TSCAck M25 MSD M26 MSD M27 MSDAck M29 MSDAck M28 OLC M30 OLC M31 OLCAck M33 OLCAck M32 OLC M35 OLC M34 OLCAck M36 OLCAck M37 ACK M38 ACK M39 RTP media packets Figure 10.6 H.323 to SIP call. Cd address (h323alias=Stibitz) Cg address (h323alias=Burroughs) INVITE sip:stibitz@proxy.com SIP/2.0 Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544 <TCP used for transport Max-Forwards: 70 From:<sip:burroughs@h323-gateway.com>;tag=3q526g45 To: sip:stibitz@proxy.com> Call-ID: 526272332146783569054 CSeq: 43252 INVITE <CSeq initialized to 43252 Contact:<sip:burroughs@h323-gateway.com;transport=tcp> SIP: Understanding the Session Initiation Protocol Content-Length: 0 100 Trying Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544 From: <sip:burroughs@h323-gateway.com> To: k<sip:stibitz@proxy.com> Call-ID: 526272332146783569054 CSeq: 43252 INVITE Content-Length: 0 INVITE sip:gstibitz123@snailmail.com SIP/2.0 Via: SIP/2.0/TCP 3.4.5.6:5060;branch=z9hG4bK452.1 Via: SIP/2.0/TCP ..3.4.5:4344;branch=z9hG4bK544 Max-Forwards: 69 From: sip:burroughs@h323-gateway.com>;tag=3q526g45 To: <sip:stibitz@proxy.com> Call-ID: 526272332146783569054 C