H.323 Gatekeeper overview
H.323 Gatekeeper works together with the H.323 protocol filter to provide full communications capabilities to H.323-registered clients using applications that are compliant with H.323 Gatekeeper, such as NetMeeting 3.0 or later. You can register the well-known aliases of someone who uses NetMeeting 3.0 or later into the H.323 Gatekeeper registration database to provide registered clients with call routing and directory services. Client registration is typically accomplished using the H.323 Registration, Admission, and Status (H.323 RAS) protocol. You can also use the H.323 protocol filter to create PBX-style dial plans to route calls intelligently, based on the called party's address.
Registered clients can also use H.323 Gatekeeper so that they can be reached by way of their well-known alias to engage in video, audio, and data conferences when using applications that are compliant with H.323 Gatekeeper. H.323 Gatekeeper clients can communicate in local and wide area networks, across multiple firewalls, and over the Internet.
H.323 Gatekeeper service is required for all inbound calls through the ISA Server computer to a well-known alias. Any client who wants to be reachable using a well-known alias—someone@microsoft.com, for example—must register with H.323 Gatekeeper. Clients must register with the H.323 Gatekeeper if translation services are to be used when placing outbound calls. Outbound calls that do not require translation services may be placed without H.323 Gatekeeper.
For example, suppose you are in your company office and want to contact a person over the Internet. He is named Someone, and works at Microsoft. The person is registered at Microsoft with H.323 Gatekeeper as someone@microsoft.com. Because that person is registered with H.323 Gatekeeper, you can contact him using his well-known alias–someone@microsoft.com–even though there is no DNS listing for him on the Internet. If you placed a call to someone@microsoft.com using NetMeeting 3.0, the following steps would take place.
From within your company:
| • | You place a call from your office to someone@microsoft.com using NetMeeting 3.0. |
| • | NetMeeting 3.0 connects with your in-house H.323 Gatekeeper. |
| • | The H.323 Gatekeeper does not recognize microsoft.com as an internal address and forwards the call to the ISA Server computer within your company. |
| • | ISA Server looks up the address for microsoft.com and makes the query over the Internet to microsoft.com. |
At the destination:
| • | When the ISA Server computer at microsoft.com receives the query for someone@microsoft.com, it contacts the internal H.323 Gatekeeper at microsoft.com to obtain the correct in-house address. |
| • | The H.323 Gatekeeper at microsoft.com translates the alias into a network address for ISA Server. |
| • | The ISA Server sends a confirmation back to ISA Server at your company and establishes the connection. |
| • | From this point through the end of the communication, the ISA Server holds open the link established by H.323 Gatekeeper. |
| • | Someone is not required to have a valid, externally routable IP address, and the address will remain hidden from other endpoints by the ISA Server. |
You can set restrictions within the ISA Server H.323 Gatekeeper filter to permit or deny video, audio, T120 data, and application sharing. You can also set time restrictions to limit the hours available for H.323-compliant communications.
For more information, see H.323 Gatekeeper.
