# 1 Introduction

The diameter application is an implementation of the Diameter protocol as defined by RFC 6733. It supports arbitrary Diameter applications by way of a dictionary interface that allows messages and AVPs to be defined and input into diameter as configuration. It has support for all roles defined in the RFC: client, server and agent. This chapter provides a short overview of the application.

A Diameter node is implemented by configuring a service and one or more transports using the interface module diameter. The service configuration defines the Diameter applications to be supported by the node and, typically, the capabilities that it should send to remote peers at capabilities exchange upon the establishment of transport connections. A transport is configured on a service and provides protocol-specific send/receive functionality by way of a transport interface defined by diameter and implemented by a transport module. The diameter application provides two transport modules: diameter_tcp and diameter_sctp for transport over TCP (using gen_tcp) and SCTP (using gen_sctp) respectively. Other transports can be provided by any module that implements diameter's transport interface.

While a service typically implements a single Diameter node (as identified by an Origin-Host AVP), transports can themselves be associated with capabilities AVPs so that a single service can be used to implement more than one Diameter node.

Each Diameter application defined on a service is configured with a callback module that implements the application interface through which diameter communicates the connectivity of remote peers, requests peer selection for outgoing requests, and communicates the reception of incoming Diameter request and answer messages. An application using diameter implements these application callback modules to provide the functionality of the Diameter node(s) it implements.

Each Diameter application is also configured with a dictionary module that provide encode/decode functionality for outgoing/incoming Diameter messages belonging to the application. A dictionary module is generated from a dictionary file using the diameterc utility. Dictionaries for the RFC 6733 Diameter Common Messages, Base Accounting and Relay applications are provided with the diameter application.