ral SIP Application Servers have been developed such as WeSIP AS [14], implemented by Voztelecom company and Broadband Research Group from Universitat Politecnica de Catalunya (UPC), IBM Websphere, BEA WebLogic SIP Server or Glassfish, among others. 2.2.2.2 Developing Advanced Media Services Using Open Source Tools Open SIP Express Router (OpenSer) [15], Asterisk [16] and RTP Proxy are the three key components to build an open source VoIP platform. OpenSer is one of the most widely used SIP Proxies, and is usually the entry point for most SIP networks, providing authentication, registration, and other basic functionalities. It's a VoIP server based on SIP used to build a large-scale IP telephony infrastructure. It can operate as Registering, Proxy, Redirect, etc. When oper- ating with SIP standard, it makes interoperability with other SIP systems (from other vendors) easy. OpenSer is composed of a core system and several modules, with support of presence, authentication by AAA (e.g., RADIUS), C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Sipserviet SIP_factory : string Supported : string Timer_service string log() : void log() void service() : void SipServlet() : void doAck() : void doBye() void doCancel() : void doErrorResponse() : void dolnfo() : void dolnvite() : void doMessage() : void doNotify() : void doOptions() : void doPrack() : void doProvisionalResponse() : void doRedirectResponse() : void doRegister() : void doRequest() : void doResponse() : void doSubscribe() : void doSuccessResponse() : void FIGURE 2.6 SIPServlets API. remote calls (XML-RPC) and Call Processing Language (CPL). It also offers a Web-based interface application/server where it can monitor the state of the server and negotiate all features. The OpenSer configuration file is more than just a typical configuration file. It combines both static settings and a dynamic programming environment. In fact, configuration file is a program that is executed for each message received by the OpenSer. Also, O