A SIP registrar enables client applications to associate a user name with a specific network address. Client registration informs the SIP proxy server that the client exists.
A SIP proxy server is an entry point into a larger network of proxy servers. SIP messages that arrive at one proxy are routed to an appropriate destination, which is usually another proxy server or an end point, such as a desktop computer or a mobile device. Although SIP messages can be sent directly between devices, they are usually routed through a proxy server.
For example, suppose Diggory wants to start a video conference with Polly. Polly is on the road and her mobile phone sends a message to a registrar that associates her name with the mobile phone’s network address. When Diggory tries to set up the video conference with Polly, his application uses SIP to ask the registrar for Polly’s current network location.