ng proxy server proxy server (triangle) SIP (peer-to-peer) SIP user agent SIP user agent (User A) (User B) DNS - Domain name system RTP - Real-time transport protocol SIP - Session initiation protocol Figure 3.5 SIP network architecture. requests to corresponding SIP servers), proxy server (used to enforce network policy), registrar server (provides binding IP address of the client with one or more SIP URIs), and location service (database that stores information about user location), as shown Figure 3.5. 3.3.3 Diameter Diameter is standardized by the IETF to overtake its predecessor, the RADIUS protocol. It is developed as a framework protocol for authentication, authorization, and account- ing (AAA), which works on the application layer. Diameter has been accepted by the 3GPP for the IMS, and with that also in the NGN. The Diameter protocol was initially specified by the IETF in RFC 3588, but a new version appeared in 2012 with the RFC 6733 [10]. Its name is influenced by the name of its predecessor, RADIUS - the diameter of a circle is twice larger than the radius, which points to the idea that Diameter is created to function "two times" better than RADIUS. Diameter is a message-based protocol (every such message is transported in a sep- arate IP packet). There are defined two types of Diameter messages: request messages and answer messages [10]. Each Diameter message contains a header followed by the packet body, which consists of one or more attribute-value pairs (AVPs). An AVP is used to encapsulate protocol-specific data, which may include (but is not limited to) QoS information, traffic information, routing information, AAA information, etc. The Diameter is defined as a framework for different applications to use for differ- ent purposes (so it is an extensible protocol over time). For differentiation between 3 QoS in NGN and Future Networks applications, IANA allocates an Application ID (32 bits) which is placed in the Diameter packet (i.e. message) header. The Diameter network architecture consists of t