al Mobile Telecommunication System (UMTS), LTE/LTE-Advanced), and wireless networks (e.g. WiMAX, Worldwide Interoperability for Microwave Access). In telecom 3.3 Service Architecture in NGN NGN networks with deployed IMS, QoS-enabled services from telecom operators are provided by using signaling through the IMS, which further communicates with RACF for the resource reservations in the NGN architecture. However, the most important signaling protocols used in the IMS and overall in the NGN are SIP and Diameter. 3.3.2 Session Initiation Protocol (SIP) SIP is a signaling protocol for IP networks, which is standardized by the IETF [6]. SIP appeared in the second half of the 1990s (when IP dominance in the data world became evident) and was finalized at the beginning of the twenty-first century. Although it is already a well-known and mature protocol, its major impact on the telecommunication world becomes more evident with the transition of legacy telecommunication networks to all-IP networks, i.e. to NGN, which has already started to happen in fixed networks and will continue to happen in mobile networks with existing 4G and 5G developments. SIP can be used as a component with other IETF standardized protocols for multime- dia, including HTTP, Real-Time Streaming Protocol (RTSP) [7], and Session Description Protocol (SDP) for describing multimedia sessions [8]. For transmission of real-time multimedia (e.g. voice, video streaming) SIP typically uses RTP [9]. SIP is a client-server protocol, based on SIP requests from the client to the server and responses in the opposite direction. For example, in the case of operator-grade IP telephony, the caller acts as a client and the called party acts as a server. A single call may involve several clients and servers, and a single host can be addressed as a client and as a server for a given call. In general, a SIP caller also may directly contact the called party by using its IP address (previously resolved from the SIP address), which is a peer-to-peer SIP communication. So,