Key elements of the SIP system architecture are the User Agent, the Registrar Server and the Proxy Server. The User Agent is a terminal such as a telephone, computer or mobile attempting communicate via Voice-over-IP and which has an SIP address. If they recognise each other, they can communicate directly. The Registrar Server controls locating the subscriber. For this purpose the User Agents regularly report to the registrar using their SIP and IP addresses. Using this information, the server is able to address connection requests. The Proxy Server lastly can be used as a client or server and send requests on behalf of a client. Other important elements in the SIP architecture are Redirect Server, Session Border Controller and Gateways.