The IMS: IP Multimedia Concepts And Services, Second Edition

12.8. Registration

SIP supports the concept of user mobility and discovery. A user can make herself available for communication by explicitly binding her AOR to a certain host address. This allows user mobility since the user can register from any device that supports SIP, including personal computers, wireless devices and cellular phones.

Discovery of the intended recipient of a SIP request is typically the function of SIP intermediary servers: for example, the user creates a binding to the registrar, which acts as a front end to a location server where all the bindings are stored, and then a proxy server, receiving a request that is destined to a domain it is responsible for, contacts that location server to retrieve the exact location of that intended recipient.

A user creates a binding by placing her AOR in the To header and the host address in the Contact header.

A user can be registered at many devices simultaneously by sending a REGISTER request from each device. Similarly, a user can create multiple bindings from the same device; this can be achieved by sending one REGISTER request with multiple bindings to the AOR. To do this, a user adds multiple contact headers in the REGISTER request.

A user can discover all the current bindings to her AOR using a process called "registration fetching". This is accomplished by sending a REGISTER request without a contact header. The registrar returns all the current bindings in the register response. Each binding has its own contact ...