The SIP registration process

The SIP registration process is shown as follows:

The SIP protocol employs a component called Registrar. It is a server that accepts REGISTER requests and saves the information received in these packets on the location server for their managed domains. The SIP protocol has a discovery capacity; in other words, if a user starts a session with another user, the SIP protocol has to discover an existent host where the user can be reached. The discovery process is done (among others) by a Registrar server that receives the request and finds the location to send it. This is based in a location database maintained by the Registrar server per domain. The Registrar server can accept other types of information, not only the client's IP addresses. It can receive other information such as Call Processing Language (CPL) scripts on the server.

Before a telephone can receive calls, it needs to be registered with the location database. In this database, we will have all the phones associated with their respective IP addresses. In our example, you will see the sip user, 8500@opensips.org, registered with the IP address, 200.180.1.1.

RFC 3665 defines best practices to implement a minimum set of functionalities for a SIP IP communications network. In the following table, the flows are defined according to RFC 3665 for registration transactions. According to RFC 3665, there are five basic flows associated with the process of registering a user agent.