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 ， 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 information 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 terminal and the gateway. Once that is complete, the negotiated media capabilities are returned in the ACK and the media session begins.
M1ARQaddress(h323alias=Stibitz)M2ACFgatekeeper routed signalingM3SetupCd address(h323alias=Stibitz)Cg address(h323alias=Burroughs)M4SetupCd address(h323alias=Stibitz)Cg address(h323alias=Burroughs)M5ARQM6ACFM7INVITE sip:stibitz@proxy.com SIP/2.0Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544 ?TCP used for transportMax-Forwards: 70From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>Call-ID: 526272332146783569054CSeq: 43252 INVITE?CSeq initialized to 43252Contact: <sip:burroughs@h323-gateway.com;transport=tcp>Content-Length: 0M8100 TryingVia: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544From: <sip:burroughs@h323-gateway.com>To: <sip:stibitz@proxy.com>Call-ID: 526272332146783569054CSeq: 43252 INVITEContent-Length: 0M9INVITE sip:gstibitz123@snailmail.com SIP/2.0Via: SIP/2.0/TCP 3.4.5.6:5060;branch=z9hG4bK452.1Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544Max-Forwards: 69From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>Call-ID: 526272332146783569054CSeq: 43252 INVITEContact: <sip:burroughs@h323-gateway.com;transport=tcp>M10SIP/2.0 180 RingingVia: SIP/2.0/TCP 3.4.5.6:5060; branch=z9hG4bK452.1Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>;tag=1926Call-ID: 526272332146783569054CSeq: 43252 INVITEContact: <sip:gstibitz123@snailmail.com;transport=tcp>Content-Length: 0M11SIP/2.0 180 RingingVia: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544From: <sip:burroughs@h323-gateway.com>To: <sip:stibitz@proxy.com>;tag=1926Call-ID: 526272332146783569054CSeq: 43252 INVITEContact: <sip:gstibitz123@snailmail.com;transport=tcp>Content-Length: 0M12AlertingM13AlertingM14SIP/2.0 200 OKVia: SIP/2.0/TCP 3.4.5.6:5060; branch=z9hG4bK452.1Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>;tag=1926Call-ID: 526272332146783569054CSeq: 43252 INVITEContact: <sip:gstibitz123@snailmail.com;transport=tcp>Content-Type: application/sdpContent-Length: 134v=0o=George 2890844576 2890844576 IN IP4 4.30.19.04s=-t=0 0c=IN IP4 4.30.19.04m=audio 5004 RTP/AVP 0a=rtpmap:0 PCMU/8000M15SIP/2.0 200 OKVia: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK544From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>;tag=1926Call-ID: 526272332146783569054CSeq: 43252 INVITEContact: <sip:gstibitz123@snailmail.com>Content-Type: application/sdpContent-Length: 134v=0o=George 2890844576 2890844576 IN IP4 4.30.19.04s=-t=0 0c=IN IP4 4.30.19.04m=audio 5004 RTP/AVP 0a=rtpmap:0 PCMU/8000M16ConnectM17ConnectM18TerminalCapabilitySetM19TerminalCapabilitySetM20TerminalCapabilitySetAckM21TerminalCapabilitySetAckM22TerminalCapabilitySetM23TerminalCapabilitySetM24TerminalCapabilitySetAckM25TerminalCapabilitySetAckM26MasterSlaveDeterminationM27MasterSlaveDeterminationM28MasterSlaveDeterminationAckM29MasterSlaveDeterminationAckM30OpenLogicalChannelg711uLaw 1.28.18.57 60002M31OpenLogicalChannelg711uLaw 1.28.18.57 60002M32OpenLogicalChannelAckM33OpenLogicalChannelAckCall Flow Examples 181M34OpenLogicalChannelg711uLaw 4.30.19.04 5004M35OpenLogicalChannelg711uLaw 4.30.19.04 5004M36OpenLogicalChannelAckM37OpenLogicalChannelAckM38ACK sip:gstibitz123@snailmail.com SIP/2.0Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK32Max-Forwards: 70From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>;tag=1926Call-ID: 526272332146783569054CSeq: 43252 ACKContent-Type: application/sdpContent-Length: 130v=0o=- 2890844577 2890844577 IN IP4 1.28.18.57s=-t=0 0c=IN IP4 1.28.18.57m=audio 60002 RTP/AVP 0a=rtpmap:0 PCMU/8000M39ACK sip:gstibitz123@snailmail.com SIP/2.0Via: SIP/2.0/TCP 3.4.5.6:5060;branch=z9hG4bK452.1Via: SIP/2.0/TCP 2.3.4.5:4344;branch=z9hG4bK532Max-Forwards: 69From: <sip:burroughs@h323-gateway.com>;tag=3q526g45To: <sip:stibitz@proxy.com>;tag=1926Call-ID: 526272332146783569054CSeq: 43252 ACKContent-Type: application/sdpContent-Length: 130v=0o=- 2890844577 2890844577 IN IP4 1.28.18.57s=-t=0 0c=IN IP4 1.28.18.57m=audio 60002 RTP/AVP 0a=rtpmap:0 PCMU/8000
