ructure requires additional periodic messages. 1.2.2 SIP Fundamentals SIP was developed by the Internet Engineering Task Force (IETF) as RFC 3261 [1]. SIP is a text-based protocol for session management in general, which is built upon standard Internet Protocols only. The protocol is predominantly used for multimedia session management, e.g., for establishing and terminat- ing voice over IP (VoIP) sessions. The SIP RFC specifies a set of entities on the basis of the client-server paradigm (see Figure 1.2). User agents (SIP-UA) send and receive SIP mes- sages to establish, modify, and terminate sessions. End-terminals register their current contact information (i.e., IP address and port of the endpoint) at a reg- istrar server using a REGISTER message. Then, the registrar stores this data in a location service (LOC), which represents some kind of database for con- tact information of participating user agents within a specific domain. Proxy servers control the SIP network traffic, as shown in Figure 1.2. For establishing a SIP session, the end-terminal sends an INVITE message, which contains the target end-terminal's address as SIP URI (format: username@domainname), to a locally predefined proxy server (Figure 1.2, message 1). The proxy server first checks the location of the target end-terminal. If the target's SIP-URI is within another domain, the message is forwarded to a proxy of the other C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Location service SIP-UA SIP-UA Proxy Proxy SIP-UA SIP-UA Terminal Terminal Domain A Domain B FIGURE 1.2 Message flow of standard SIP session establishment. domain, which is discovered using the Domain Name System (DNS) (Fig- ure 1.2, message 2). If the target is located within the same domain the proxy is able to locate the target's current contact address using the domain-local location service. With this information, the proxy is able to forward the mes- sage to the target user agent (Figure 1.2, Messages 3 and 4). Integration of P2P T