Establishing VoIP Connections with SIP
Many VoIP networks use the IETF’s signaling protocol, SIP, to handle the setup and tear down of multimedia sessions between endpoints. This lightweight, text-based signaling protocol is transported over either Transmission Control Protocol (TCP) or UDP. SIP uses invitations to create Session Description Protocol (SDP) messages to carry out capability exchange and to setup call control channel use. These invitations allow participants to agree on a set of compatible media types.
The powerful SIP client-server application supports user mobility with two operating modes: proxy and redirect. In proxy mode (shown in Figure 3), SIP clients send requests to the proxy server. The proxy server either handles the requests or forwards them to other SIP servers. Proxy servers can insulate and hide SIP users by proxying the signaling messages. To the other users on the VoIP network, the signaling invitations look as if they are coming from the proxy SIP server.
Source: Juniper Networks, Inc. White Paper