An introduction to Diameter Routing Agent (I)

What is Diameter? According to Wikipedia, “Diameter is an authentication, authorization, and accounting protocol for computer networks and an alternative to RADIUS.” The fact is that, in comparison with RADIUS, Diameter introduced many improvements in different aspects (while still being backward-compatible) and came as a result of developments to eliminate RADIUS limitations.

  • The Diameter protocol is much more reliable, has network and transport layer security, and includes the addition of attribute value pairs (AVPs) and error notifications.
  • It can be used in both stateful and stateless models, and offers dynamic peer discovery and capability negotiation. The Diameter base protocol specifies the delivery mechanisms, error handling and accounting.
  • It is also more easily extendable via Diameter Applications which extend the base protocol by adding new commands and attributes.
  • Applications using the Diameter protocol are able to support interfaces such as Cx, Dh, Dx, Rf, Ro, Sh. Diameter Applications specify service-specific functions and AVPs.

Diameter fills the gap between the old world and the new world by being designed as a peer-to-peer architecture, while at the same time keeping the client/server concept in place. This is achieved by referring to the AAA elements as Diameter nodes. A Diameter node can act as a client, server, or agent. The Diameter node that receives the user connection request will act as the Diameter client. In most cases, a Diameter client will be a Network Access Server. After collecting user credentials, such as username and password, it will send an access request message to a Diameter node serving the request. The Diameter server node is the peer that authenticates the user based on the provided information. If the authentication process succeeds, the user’s access privileges are included in the response message and sent back to the corresponding Diameter client. Otherwise, an access rejection message is sent. A node acting as the Diameter server might actually act as a Diameter client in some situations. The Diameter protocol is designed as a peer-to-peer-based architecture in a more generic sense. Besides, a special Diameter node called Diameter agent is clearly defined in Diameter.

Why do operators need to switch to Diameter? Nowadays, in the “smartphones era,” mobile data traffic is skyrocketing. More and more people use their phones not only for voice but also for data. Laptop dongles, flat-rate plans, free social networking and network videos are only a part of what mobile operators are offering their clients. And this only increases data traffic. Cisco, for example, predicts that global mobile traffic will double in 2013 and triple in 2014.

Operators have already realized that their 3G networks are not equipped to sustain this high level of traffic growth. They are thus looking at all-Internet Protocol (IP) networks such as Long Term Evolution (LTE) and IP Multimedia Subsystem (IMS) for a solution. They have to find a way to provide higher and higher bandwidth required to support devices and applications. They are also interested to address cost effectively the growing gap between traffic and revenue growth. That is why Diameter came into the game. Within the IMS control and service plans, Diameter plays a central role in policy, charging, authentication and mobility management. We can easily say that Diameter becomes the signaling protocol of the future.

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