th the user terminal and the SIP messages from the user is transferred over the IPSec, and the validity of these messages is verified at P-CSCF. Enabling Service Delivery in Next-Generation Networks 4.2.1.1.3 I-CSCF The Interrogating-CSCF (I-CSCF) is located in the home network. It accesses the SLF if required at user registration to identify the HSS in which the subscription information of that user is stored. Then, following the instruction from the HSS, registration processing for that user is taken over by the S-CSCF. I-CSCF facilitates the load balancing when there are multiple HSSs, and also plays the role to hide the network structure from outside. 4.2.1.2 NGN Transport Stratum In addition to the session and service control, there are other NGN features, which are Quality of Service (QoS) control and authentication functions. These functions are provided by Resource and Admission Control Functions (RACF) and Network Attachment Control Functions (NACF), respectively. RACF and NACF perform QoS control and authentication, in cooperation with the service control function in service stratum, and are located in the NGN Transport Stratum. 4.2.1.2.1 RACF While the quality was guaranteed for each service in conventional networks, in NGN all-IP is realized and various services and media are handled in an integrated manner. Even though all services are using IP network, there is a need to provide these services surely, according to Service Level Agreement (SLA). In particular, voice services without delay and loss must be provided, and emergency calls must be prioritized, and SO end-to-end QoS control is required. In NGN, when the ser- vice traffic passes through the network, packets are transferred according to their priority order. In this case, the priority can be controlled per call session with the authentication information of that session, and SO different service levels can be provided, such as emergency and ordinary calls within voice calls, or depending on the media type or the user type. RACF also performs