Diameter is defined in terms of a base protocol and a set
of applications. This design allows the protocol to be extended
to new access technologies. The base protocol provides basic mechanisms
for reliable transport, message delivery, and error handling.
The base protocol must be used in conjunction with a Diameter
application. Each application relies on the services of the base
protocol to support a specific type of network access. The two major
applications are Mobile IPv4 and NASREQ (network access server requirements).
The NASREQ application supports dial-in PPP/IP and is the intended
replacement for RADIUS. The following figure depicts the Diameter
architecture:
The base protocol defines the basic Diameter message format.
Data is carried within a Diameter message as a collection of AVPs.
An AVP is like a RADIUS attribute. An AVP consists of multiple fields:
an AVP Code, a Length, Flags, and Data. Some AVPs are used by the
Diameter base protocol; other AVPs are intended for the Diameter
application (e.g. NASREQ); while yet others may be used by the higher-level
end-system application that employs Diameter.
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