IBM Israel Software Lab (ILSL)

The SIP Registrar/Proxy provides two essential functions in the SIP infrastructure:

• Proxy Server - receives SIP registrations, invitations to sessions, and other requests, and forwards them on behalf of the requestor Proxy servers can make flexible "routing decisions" to decide where to send a request. For example, if Bob's SIP phone returned a busy response, the proxy server could proxy the INVITE to Bob's voicemail server. A proxy server can also send an INVITE to a number of locations simultaneously.
• SIP Registrar - acts as a device location service upon initialization, and at periodic intervals, each device sends REGISTER messages to the SIP registrar. The REGISTER messages associate Bob's SIP or SIPS URI with the machine into which he is currently logged. The registrar saves this association in memory so that it can be used by the proxy. Similarly, more than one user can be registered on a single device at the same time.