The IMS: IP Multimedia Concepts And Services, Second Edition

19.5. Diameter agents

Diameter agents can be deployed in a network to perform load balancing, distribute system administration and maintenance, concentrate requests and perform additional message processing. Diameter relay agents are protocol-transparent (i.e., they support the Diameter Base Protocol), and are also agnostic about Diameter applications. They simply accept requests and route messages based on the supported realms and known peers. Relay agents are usually used to reduce the configuration load on Diameter clients and servers: for example, NASs of a specific geographical area can be grouped behind a common relay agent, thus avoiding the need to configure each NAS with the information needed to communicate with other realms. Similarly, a Diameter server may have a relay agent that handles the configuration load caused by the addition and removal of individual NASs in the network.

Diameter redirect agents perform realm-to-server resolution. They use the Diameter Realm Routing Table to determine the next-hop Diameter peer to a particular request message. Instead of routing a request by itself, the redirect agent returns a special answer message that includes the identity of the next-hop peer. The originator of the request then contacts the next-hop peer directly. Since redirect agents operate in a stateless fashion, they provide scalability and centralized message routing. However, centralized message routing is always a compromise between ease of configuration and fail-over ...