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+<?xml version="1.0" encoding="latin1" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+<header>
+<copyright>
+<year>2011</year>
+<holder>Ericsson AB. All Rights Reserved.</holder>
+</copyright>
+
+<legalnotice>
+The contents of this file are subject to the Erlang Public License,
+Version 1.1, (the "License"); you may not use this file except in
+compliance with the License. You should have received a copy of the
+Erlang Public License along with this software. If not, it can be
+retrieved online at http://www.erlang.org/.
+    
+Software distributed under the License is distributed on an "AS IS"
+basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+the License for the specific language governing rights and limitations
+under the License.
+    
+</legalnotice>
+
+<title>Introduction</title>
+<prepared></prepared>
+<responsible></responsible>
+<docno></docno>
+<approved></approved>
+<checked></checked>
+<date></date>
+<rev></rev>
+<file>diameter_intro.xml</file>
+</header>
+
+<p>
+The diameter application is an implementation of the Diameter protocol
+as defined by RFC 3588.
+It supports arbitrary Diameter applications by way of a
+<em>dictionary</em> interface that allows messages and AVP's 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.</p>
+
+<p>
+A Diameter peer is implemented by configuring a <em>service</em> and
+one or more <em>transports</em> using the interface module
+<seealso marker="diameter">diameter</seealso>.
+The service configuration defines the Diameter applications to be
+supported by the peer 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: <seealso
+marker="diameter_tcp">diameter_tcp</seealso> and <seealso
+marker="diameter_sctp">diameter_sctp</seealso> for transport over TCP
+(using <c>gen_tcp</c>) and SCTP (using <c>gen_sctp</c>) respectively.
+Other transports can be provided by any module that implements
+diameter's <seealso marker="diameter_transport">transport
+interface</seealso>.</p>
+
+<p>
+While a service typically implements a single Diameter peer (as
+identified by an Origin-Host AVP), transports can themselves be
+associated with capabilities AVP's so that a single service be used to
+implement more than one Diameter peer.</p>
+
+<p>
+Each Diameter application defined on a service is configured with a
+callback module that implements the <seealso
+marker="diameter_app">application interface</seealso> 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 peer(s) it
+implements.</p>
+
+<p>
+Each Diameter application is also configured with one or more
+dictionary modules
+that provide encode/decode functionality for outgoing/incoming
+Diameter messages.
+A module is generated from a <seealso
+marker="diameter_dict">specification file</seealso> using the <seealso
+marker="diameterc">diameterc</seealso> utility.
+Dictionaries for the RFC 3588 Diameter Common Messages, Base
+Accounting and Relay applications are provided by the diameter
+application.</p>
+
+</chapter>
+