SIP Registrar

The SIP Registrar is a key server component in the SIP infrastructure, primarily responsible for handling the registration of SIP users within a network. Its main function is to maintain a dynamic directory of user agents and their corresponding locations, enabling SIP servers to route calls and other SIP messages to the correct destination. By keeping an up-to-date record of user locations, the SIP Registrar ensures that communication within the SIP ecosystem is seamless and efficient. 

Functionality: 

User Registration: SIP clients (user agents) send REGISTER requests (more details further) to the SIP Registrar to inform it of their current location (typically, their IP address, port and transport). The SIP Registrar updates its location database with this information, enabling it to direct incoming calls and messages to the user's current endpoint.