Gatekeeper is a call and management function for network terminals (such as telephone) gateways, and it is an important component of the Voip network system. In IP telephony, gatekeepers are at the top level and are used to manage IP telephony gateways. They are also an important component of the Voip network system. A hierarchical top-level gatekeeper cloud can contain multiple top-level gatekeepers. The connection between top-level gatekeepers, the connection between primary gatekeepers and top-level gatekeepers, and the connection between different operators are determined by each operator based on their own situation. When the operator's network size is not very large, the functions of the top-level gatekeeper can be completed by one of the first level gatekeepers. (1) Top level gatekeeper. The top-level gatekeeper is responsible for managing the primary gatekeeper belonging to the operator, mainly responsible for address resolution between primary gatekeepers, interoperability and address exchange between video conferencing networks of different operators; Also responsible for the management of international business, namely the establishment and dismantling of international calls. (2) First level gatekeeper. The primary gatekeeper is mainly responsible for the address resolution work between all secondary gatekeepers and IP telephone terminal agents under the jurisdiction of the primary gatekeeper. In the case of a two-level network, all functions of the secondary gatekeeper are classified as the primary gatekeeper. (3) Secondary gatekeeper. The secondary gatekeeper is mainly responsible for resolving and authenticating the addresses of users in their respective areas, preventing illegal users from accessing and registering illegal gateways; Responsible for providing routing information to the affiliated gateway, including port information of the called gateway, etc. (4) Interworking between gatekeepers. Communication between gatekeepers completes call establishment between different regions. The interconnection between top-level gatekeepers and primary gatekeepers, or between primary and secondary gatekeepers, adopts the RAS protocol, mainly transmitting user address resolution information. (5) Interworking between gatekeepers and gateways. The RAS protocol is used for communication between the gatekeeper and the gateway, and the RAS signaling uses H.225.0 messages to complete registration, access control, bandwidth conversion, status information transmission, and disconnection operations between the gatekeeper and the endpoint. The RAS signaling channel is independent of the call signaling channel and the H.245 call control channel. The RAS signaling channel is a non reliable channel and uses UDP to transmit signaling