The SIP register method is used to create bindings between addresses-of-record (AORs) and contact addresses where a user can be reached. An AOR is a URI with a publicly known domain that can be mapped to another URI where a user may be available. A server that can process REGISTER requests is known as a registrar.
A SIP registrar’s role is to accept REGISTER requests and write the associated bindings to a location service. The location service is a logical entity and is simply a database that contains a list of these AOR to contact address bindings. Very often a registrar functions as a location service. It is also very common for a registrar/location service to be co-located with the proxy server for the same domain. The example below shows a typical registration transaction.
In this example UA1 sends a REGISTER request to the registrar with an AOR of email@example.com and a contact address of firstname.lastname@example.org where 192.168.0.10 represents the physical IP address of the device on which UA1 resides. When the SIP proxy server that is responsible for the vocal.com domain receives a request for email@example.com, it performs a lookup to the location service. The location service informs the proxy that the AOR firstname.lastname@example.org maps to email@example.com and now the proxy knows where UA1 can be reached and it can forward the request to UA1.