TerminalCapabilitySetAck TerminalCapabilitySet TerminalCapabilitySetAck MasterSlaveDetermination MasterSlaveDeterminationAck OpenLogicalChannel OpenLogicalChannelAck OpenLogicalChannel OpenLogicalChannelAck RTP media session Figure 8.2 H.323 call flow example. Comparison to H.323 gatekeeper allows the call to continue by sending an Admission Confirmation (ACF) message. The ACF indicates to the calling terminal that end-point mes- sage routing, or the direct exchange of H.225 call signaling messages with the called terminal, is to be used. Alternatively, the gatekeeper can require gatekeeper routed signaling, where the gatekeeper acts like a proxy and forwards all signaling messages between the terminals. The gatekeeper has also translated the destina- tion in the ARQ into a transport address that was returned in the ACF. The calling terminal is now able to open a TCP connection to the called terminal using the transport address returned in the ACF and send a Q.931 Setup message to the called terminal. The called terminal responds with a Call Proceeding response to the calling terminal. The called terminal must also get permission from the gatekeeper before it accepts the call, SO an ARQ is sent to the gatekeeper. When it receives the ACF from the gatekeeper, the called terminal begins alerting the user, and sends an Alerting message to the calling terminal. When the user at the calling terminal answers, a Con- nect message is sent. There is no acknowledgment of messages because all these messages are sent using TCP, which provides reliable transport. These call signaling messages used in H.323 are a subset of the Q.931 recommendation that covers ISDN D-channel signaling. The next stage is the connection negotiation, which is handled by H.245 control signaling messages. A second TCP connection between the two terminals is opened by the calling terminal using the port number selected by the called ter- minal and returned in the Connect message. The Terminal Capabili- tySet message sent contains the media capabiliti