The DIAMETER protocol  has been specified by the Internet Engineering Task Force (IETF) since 1998 as a renovated version of previous Authentication Authorization and Accounting (AAA) RADIUS protocols to offer more flexibility, reliability, scalability and security. In the design, DIAMETER base protocol offers only basic functions and services. Standardization bodies and vendors (e.g. IETF, 3GPP, ETSI) have extent the base protocol to create other specific applications (interfaces).
The structure of a DIAMETER message is depicted in Figure 1 including the message header and payload. In the header, the command code and flag fields are used to distinguish DIAMETER message from other ones. The Application Identifier (Application-ID) field is used to identify to which application the message belongs to and its values are defined and allocated by Internet Assigned Numbers Authority (IANA). The message payload encapsulates data in a sequence of Attribute Value Pairs (AVPs) field. Many DIAMETER commands and AVPs have already been defined and widely used. Nevertheless, standardization bodies and vendors can update them or define new ones, if necessary.
Figure 2: Emerging DIAMETER network (from Tekelec)
3GPP  has a lot of influence on the development of the DIAMETER protocol. Although IETF has specified DIAMETER base protocol and some applications, DIAMETER acceptance and deployment were still behind the ancestor RADIUS in a long time. Recognizing the value of DIAMETER versus RADIUS in mobile environments, 3GPP selected DIAMETER to be the main control protocol in IP Multimedia Subsystem (IMS) and NGN networks. The central role of DIAMETER protocol has continuously been confirmed when 3GPP introduced its new all-IP core network architecture Evolved Packet Core (EPC) and DIAMETER was the signaling protocol that every entity must support. The signaling planes of emerging mobile networks are now considered as DIAMETER networks . These networks communicate many service and network control components (e.g. policy server, billing system, gateway, mobility and session management, subscriber databases).