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.

SIP 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.
SIP Registration Figure 1
In this example UA1 sends a REGISTER request to the registrar with an AOR of [email protected] and a contact address of [email protected] where represents the physical IP address of the device on which UA1 resides. When the SIP proxy server that is responsible for the domain receives a request for [email protected], it performs a lookup to the location service. The location service informs the proxy that the AOR [email protected] maps to [email protected] and now the proxy knows where UA1 can be reached and it can forward the request to UA1.

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