764 CHAPTER 30 Security Issues in VoIP Telecommunication Networks SIP networks demonstrated in Figure 30-1 are as follows: DNS server Location server Proxy server User agent A ua@domain.com User agent B ub@domain.com FIGURE 30-1 Main SIP components. · User agents (UAs): The User agent represents the endpoints of a communication (i.e., SIP Phone). The User agents are logically divided into UA server and UA client, depending on the terminal initiating and responding to the request. · Proxy servers: A proxy server is present in every SIP Domain. It acts as an intermedi- ate server, receiving connection requests from UAs, or other proxy servers. The proxy server communicates with the location server and the redirect server. · Redirect servers: A redirect server processes all proxy server requests and performs call re- routing for User agents. · Registrar servers: A registrar server is respon- sible for registering User agents. It sends the