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AAA and Network Security for Mobile Access: Radius, Diameter, EAP, PKI and IP Mobility by Madjid Nakhjiri, Mahsa Nakhjiri

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7.2. Diameter Protocol

In this section, we will go through the basic building blocks of the Diameter base specification. We will describe the Diameter message format and the way different messages are distinguished and carry information around. We also go through the transport, reliability and security procedures of Diameter messaging.

7.2.1. Diameter Messages

As in RADIUS, Diameter messages are used for carrying various pieces of application or AAA information. The information, carried within a Diameter message, is typically called an attribute and is formed according to the attribute-value pair (AVP) format. For that reason Diameter attributes are frequently referred to as AVPs. The format for AVPs is described later on.

RADIUS is a server-client protocol, where requests are always issued by the client, while answers (challenge or accept/reject) are created by the server. In contrast, Diameter is a peer-to-peer protocol, which means both client and server can create either a request or an answer. Diameter messaging terminology does not include "message type", so one can say there are only two message types: requests and answers. Instead Diameter uses the concept of commands. The commands are distinguished from each through the use of a command code that specifies the type of function the Diameter message intends to perform. The action to be taken in conjunction with reception of each message is defined by the command code and the attributes included in the message. This will ...

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