Proxy and Registrar Server
SIP is a peer-to-peer protocol. SipProxy is the program used for communications between user-agent and user-agent, forwarding the requests and replies from one to the other: a user-agent sends the requests to the proxy instead of directly to another user-agent. The proxy, in turn, connects to the destination user-agent, forwards the original request, receives the reply and forwards it to the initial user-agent.
If the SIP messages are exchanged in order to initiate an audio and/or video session, and once the session has been established, the exchange of audio/video occurs without the intermediation of the proxy.
Registration is another basic concept of SIP protocol. Registration is the method with which a SIP server identifies the current location of the user. The server which handles the acquisition of the registrations and makes them available to the proxy (which in this way can correctly forward the SIP messages to the correct recipients) is called a registrar server.
The BCS SipProxy application carries out the role of both stateful proxy and registrar server for a single domain (a number of instances can be run to handle several domains).
As well as carrying out all the tasks required of SIP protocol, SipProxy adds (or improves) a variety of interesting features.