Merge branch 'anders/diameter_import/OTP-9321' into dev

* anders/diameter_import/OTP-9321:
  Initial commit of the diameter application.

1 files changed, 1123 insertions, 0 deletions

diff --git a/lib/diameter/doc/src/diameter.xml b/lib/diameter/doc/src/diameter.xml
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@@ -0,0 +1,1123 @@
+<?xml version="1.0" encoding="latin1" ?>
+<!DOCTYPE erlref SYSTEM "erlref.dtd">
+
+<erlref>
+<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>diameter(3)</title>
+
+<prepared>Anders Svensson</prepared>
+<responsible></responsible>
+<docno></docno>
+<approved></approved>
+<checked></checked>
+<date></date>
+<rev>%VSN%</rev>
+<file>diameter.xml</file>
+
+</header>
+
+<!-- ===================================================================== -->
+<!-- ===================================================================== -->
+
+<module>diameter</module>
+<modulesummary>Main API of the diameter application.</modulesummary>
+
+<description>
+<p>
+This module provides the interface with which a user
+creates a service that sends and receives messages using the
+Diameter protocol as defined in RFC 3588.</p>
+
+<p>
+Basic usage consists of creating a representation of a
+locally implemented Diameter peer and its capabilities with <seealso
+marker="#start_service">start_service/2</seealso>, adding transport
+capability using <seealso
+marker="#add_transport">add_transport/2</seealso> and sending Diameter
+requests and receiving Diameter answers with <seealso
+marker="#call">call/4</seealso>.
+Incoming Diameter requests are communicated as callbacks to a
+<seealso
+marker="diameter_app">diameter_app(3)</seealso> callback modules as
+specified in the service configuration.</p>
+
+<p>
+Beware the difference between <em>diameter application</em> and
+<em>Diameter application</em>.
+The former refers to the Erlang application named diameter whose main
+api is defined here, the latter to an application of the Diameter
+protocol in the sense of RFC 3588.
+More generally, capitalized Diameter refers to the RFC
+and diameter to this implementation.</p>
+
+<p>
+The diameter application must be started before calling functions in
+this module.</p>
+
+</description>
+
+<!-- ===================================================================== -->
+<!-- ===================================================================== -->
+<section>
+<title>DATA TYPES</title>
+
+<taglist>
+
+<tag><c>Address()</c></tag>
+<tag><c>DiameterIdentity()</c></tag>
+<tag><c>Time()</c></tag>
+<tag><c>Unsigned32()</c></tag>
+<tag><c>UTF8String()</c></tag>
+<item>
+<p>
+Types corresponding to RFC 3588 AVP Data Formats.
+Defined in <seealso marker="diameter_dict">diameter_dict(4)</seealso>.</p>
+
+<marker id="application_alias"/>
+</item>
+
+<tag><c>application_alias() = term()</c></tag>
+<item>
+<p>
+A name identifying a Diameter application in
+service configuration passed to <seealso
+marker="#start_service">start_service/2</seealso> and passed to
+<seealso marker="#call">call/4</seealso> when sending requests
+belonging to the application.</p>
+
+<marker id="application_module"/>
+</item>
+
+<tag><c>application_module() = Mod | [Mod | ExtraArgs]</c></tag>
+<item>
+<code>
+Mod = atom()
+ExtraArgs = list()
+</code>
+
+<p>
+A module implementing the callback interface defined in <seealso
+marker="diameter_app">diameter_app(3)</seealso>, along with any
+extra arguments to be appended to those documented for the interface.
+Any extra arguments are appended to the documented list of arguments for
+each function.
+Note that additional arguments specific to an outgoing request be
+specified to <seealso marker="#call">call/4</seealso>, in which case
+the call-specific arguments are appended to any specified with the
+callback module.</p>
+
+<marker id="application_opt"/>
+</item>
+
+<tag><c>application_opt()</c></tag>
+<item>
+
+<p>
+Options defining a Diameter application as configured in an
+<c>application</c> option passed to
+<seealso marker="#start_service">start_service/2</seealso>.</p>
+
+<taglist>
+
+<tag><c>{alias, application_alias()}</c></tag>
+<item>
+<p>
+An unique identifier for the application in the scope of the
+service.
+Optional, defaults to the value of the <c>dictionary</c> option.</p>
+</item>
+
+<tag><c>{dictionary, atom()}</c></tag>
+<item>
+<p>
+The name of an encode/decode module for the Diameter
+messages defined by the application.
+These modules are generated from a specification file whose format is
+documented in <seealso
+marker="diameter_dict">diameter_dict(4)</seealso>.</p>
+</item>
+
+<tag><c>{module, application_module()}</c></tag>
+<item>
+<p>
+A callback module with which messages of the Diameter application are
+handled.
+See <seealso marker="diameter_app">diameter_app(3)</seealso> for
+information on the required interface and semantics.</p>
+</item>
+
+<tag><c>{state, term()}</c></tag>
+<item>
+<p>
+The initial callback state.
+Defaults to the value of the <c>alias</c> option if unspecified.
+The prevailing state is passed to certain
+<seealso marker="diameter_app">diameter_app(3)</seealso>
+callbacks, which can then return a new state.</p>
+</item>
+
+<tag><c>{call_mutates_state, true|false}</c></tag>
+<item>
+<p>
+Specifies whether or not the <seealso
+marker="diameter_app#pick_peer">pick_peer/4</seealso>
+application callback (following from a call to
+<seealso marker="#call">call/4</seealso>)
+can modifiy state,
+Defaults to <c>false</c> if unspecified.</p>
+
+<p>
+Note that <seealso
+marker="diameter_app#pick_peer">pick_peer</seealso> callbacks are
+serialized when these are allowed to modify state, which is a
+potential performance bottleneck.
+A simple Diameter client may suffer no ill effects from using mutable
+state but a server or agent that responds to incoming request but
+sending its own requests should probably avoid it.</p>
+</item>
+
+<tag><c>{answer_errors, callback|report|discard}</c></tag>
+<item>
+<p>
+Determines the manner in which incoming answer messages containing
+decode errors are handled.
+If <c>callback</c> then errors result in a <seealso
+marker="diameter_app#handle_answer">handle_answer/4</seealso>
+callback in the same fashion as for <seealso
+marker="diameter_app#handle_request">handle_request/3</seealso>, in the
+<c>errors</c> field of the <c>diameter_packet</c> record passed into
+the callback.
+If <c>report</c> then an answer containing errors is discarded
+without a callback and a warning report is written to the log.
+If <c>discard</c> then an answer containing errors is silently
+discarded without a callback.
+In both the <c>report</c> and <c>discard</c> cases the return value
+for the <seealso marker="#call">call/4</seealso> invocation in
+question is as if a callback had taken place and returned
+<c>{error, failure}</c>.</p>
+
+<p>
+Defaults to <c>report</c> if unspecified.</p>
+</item>
+
+</taglist>
+
+<marker id="call_opt"/>
+</item>
+
+<tag><c>call_opt()</c></tag>
+<item>
+
+<p>
+Options available to <seealso marker="#call">call/4</seealso> when
+sending an outgoing Diameter request.</p>
+
+<taglist>
+
+<tag><c>{extra, list()}</c></tag>
+<item>
+<p>
+Extra arguments to append to callbacks to the callback
+module in question.
+These are appended to any extra arguments configured with the callback
+itself.
+Multiple options append to the argument list.</p>
+</item>
+
+<tag><c>{filter, peer_filter()}</c></tag>
+<item>
+<p>
+A filter to apply to the list of available peers before passing them to
+the <seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
+callback for the application in question.
+Multiple options are equivalent a single <c>all</c> filter on the
+corresponding list of filters.
+Defaults to <c>none</c>.</p>
+</item>
+
+<tag><c>{timeout, Unsigned32()}</c></tag>
+<item>
+<p>
+The number of milliseconds after which the request should
+timeout.
+Defaults to 5000.</p>
+</item>
+
+<tag><c>detach</c></tag>
+<item>
+<p>
+Causes <seealso marker="#call">call/4</seealso> to return <c>ok</c> as
+soon as the request in
+question has been encoded instead of waiting for and returning
+the result from a subsequent
+<seealso marker="diameter_app#handle_answer">handle_answer/4</seealso>
+or <seealso
+marker="diameter_app#handle_error">handle_error/4</seealso>
+callback.</p>
+</item>
+
+</taglist>
+
+<marker id="capability"/>
+</item>
+
+<tag><c>capability()</c></tag>
+<item>
+
+<p>
+AVP values used in outgoing CER/CEA messages during capabilities exchange.
+Can be configured both on a service and a transport, the latter taking
+precedence over the former.</p>
+
+<taglist>
+
+<tag><c>{'Origin-Host', DiameterIdentity()}</c></tag>
+<item>
+<p>
+Value of the Origin-Host AVP in outgoing messages.</p>
+</item>
+
+<tag><c>{'Origin-Realm', DiameterIdentity()}</c></tag>
+<item>
+<p>
+Value of the Origin-Realm AVP in outgoing messages.</p>
+</item>
+
+<tag><c>{'Host-IP-Address', [Address()]}</c></tag>
+<item>
+<p>
+Values of Host-IP-Address AVPs.
+Optional.</p>
+
+<p>
+The list of addresses is available to the start function of a
+transport module, which either uses them as is or returns a new list
+(typically as configured as <c>transport_config()</c> on the
+transport module in question) in order for the correct list of
+addresses to be sent in capabilities exchange messages.</p>
+</item>
+
+<tag><c>{'Vendor-Id', Unsigned32()}</c></tag>
+<item>
+<p>
+Value of the Vendor-Id AVP sent in an outgoing capabilities
+exchange message.</p>
+</item>
+
+<tag><c>{'Product-Name', UTF8String()}</c></tag>
+<item>
+<p>
+Value of the Product-Name AVP sent in an outgoing capabilities
+exchange message.</p>
+</item>
+
+<tag><c>{'Origin-State-Id', Unsigned32()}</c></tag>
+<item>
+<p>
+Value of Origin-State-Id to be included in outgoing messages sent by
+diameter itself.
+In particular, the AVP will be included in CER/CEA and DWR/DWA messages.
+Optional.</p>
+
+<p>
+Setting a value of <c>0</c> (zero) is equivalent to not setting a
+value as documented in RFC 3588.
+The function <seealso
+marker="#origin_state_id">origin_state_id/0</seealso>
+can be used as to retrieve a value that is set when the diameter
+application is started.</p>
+</item>
+
+<tag><c>{'Supported-Vendor-Id', [Unsigned32()]}</c></tag>
+<item>
+<p>
+Values of Supported-Vendor-Id AVPs sent in an outgoing
+capabilities exchange message.
+Optional, defaults to the empty list.</p>
+</item>
+
+<tag><c>{'Auth-Application-Id', [Unsigned32()]}</c></tag>
+<item>
+<p>
+Values of Auth-Application-Id AVPs sent in an outgoing
+capabilities exchange message.
+Optional, defaults to the empty list.</p>
+</item>
+
+<tag><c>{'Acct-Application-Id', [Unsigned32()]}</c></tag>
+<item>
+<p>
+Values of Acct-Application-Id AVPs sent in an outgoing
+capabilities exchange message.
+Optional, defaults to the empty list.</p>
+</item>
+
+<tag><c>{'Vendor-Specific-Application-Id', [Grouped()]}</c></tag>
+<item>
+<p>
+Values of Vendor-Specific-Application-Id AVPs sent in
+an outgoing capabilities exchange message.
+Optional, defaults to the empty list.</p>
+</item>
+
+<tag><c>{'Firmware-Revision', Unsigned32()}</c></tag>
+<item>
+<p>
+Value of the Firmware-Revision AVP sent in an outgoing capabilities
+exchange message.
+Optional.</p>
+</item>
+
+</taglist>
+
+<p>
+Note that each tuple communicates one or more AVP values.
+It is an error to specify duplicate tuples.</p>
+
+<marker id="evaluable"/>
+</item>
+
+<tag><c>evaluable() = {M,F,A} | fun() | [evaluable() | A]</c></tag>
+<item>
+<p>
+An expression that can be evaluated as a function in the following
+sense.</p>
+
+<code>
+eval([{M,F,A} | T]) ->
+    apply(M, F, T ++ A);
+eval([F|A]) ->
+    apply(F, A);
+eval(F) ->
+    eval([F]).
+</code>
+
+<p>
+Evaluating an evaluable() <c>E</c> on an argument list <c>A</c>
+is meant in the sense of <c>eval([E|A])</c>.</p>
+
+<marker id="peer_filter"/>
+</item>
+
+<tag><c>peer_filter() = term()</c></tag>
+<item>
+<p>
+A filter passed to <seealso marker="#call">call/4</seealso>
+in order to select candidate peers for a 
+<seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
+callback.
+Has one of the following types.</p>
+
+<taglist>
+
+<tag><c>none</c></tag>
+<item>
+<p>
+Matches any peer.
+This is a convenience that provides a filter equivalent to no
+filter at all.</p>
+</item>
+
+<tag><c>host</c></tag>
+<item>
+<p>
+Matches only those peers whose <c>Origin-Host</c> has the same value
+as <c>Destination-Host</c> in the outgoing request in question,
+or any peer if the request does not contain
+a <c>Destination-Host</c> AVP.</p>
+</item>
+
+<tag><c>realm</c></tag>
+<item>
+<p>
+Matches only those peers whose <c>Origin-Realm</c> has the same value
+as <c>Destination-Realm</c> in the outgoing request in question,
+or any peer if the request does not contain
+a <c>Destination-Realm</c> AVP.</p>
+</item>
+
+<tag><c>{host, any|UTF8String()}</c></tag>
+<item>
+<p>
+Matches only those peers whose <c>Origin-Host</c> has the
+specified value, or all peers if the atom <c>any</c>.</p>
+</item>
+
+<tag><c>{realm, any|UTF8String()</c></tag>
+<item>
+<p>
+Matches only those peers whose <c>Origin-Realm</c> has the
+value, or all peers if the atom <c>any</c>.</p>
+</item>
+
+<tag><c>{eval, evaluable()}</c></tag>
+<item>
+<p>
+Matches only those peers for which the specified evaluable() evaluates
+to true on the peer's <c>diameter_caps</c> record.</p>
+</item>
+
+<tag><c>{neg, peer_filter()}</c></tag>
+<item>
+<p>
+Matches only those peers not matched by the specified filter.</p>
+</item>
+
+<tag><c>{all, [peer_filter()]}</c></tag>
+<item>
+<p>
+Matches only those peers matched by each filter of the specified list.</p>
+</item>
+
+<tag><c>{any, [peer_filter()]}</c></tag>
+<item>
+<p>
+Matches only those peers matched by at least one filter of the
+specified list.</p>
+</item>
+
+</taglist>
+
+<marker id="service_event"/>
+</item>
+
+<tag><c>service_event() = #diameter_event{}</c></tag>
+<item>
+<p>
+Event message sent to processes that have subscribed using <seealso
+marker="#subscribe">subscribe/1</seealso>.</p>
+
+<p>
+The <c>info</c> field of the event record can be one of the
+following.</p>
+
+<taglist>
+
+<tag><c>{up, Ref, Peer, Config, Pkt}</c></tag>
+<tag><c>{up, Ref, Peer, Config}</c></tag>
+<tag><c>{down, Ref, Peer, Config}</c></tag>
+<item>
+<code>
+Ref    = transport_ref()
+Peer   = diameter_app:peer()
+Config = {connect|listen, [transport_opt()]}
+Pkt    = #diameter_packet{}
+</code>
+
+<p>
+Reports that the RFC 3539 watchdog state machine has
+transitioned into (<c>up</c>) or out of (<c>down</c>) the open
+state.
+If a <c>diameter_packet</c> record is present in an <c>up</c> tuple
+then there has been an exchange of capabilities exchange messages and
+the record contains the received CER or CEA, otherwise the
+connection has reestablished without the loss or transport
+connectivity.</p>
+
+<p>
+Note that a single up/down event for a given peer corresponds to
+as many peer_up/down callbacks as there are Diameter
+applications shared by the peer, as determined during capablilities
+exchange.
+That is, the event communicates connectivity with the
+peer as a whole while the callbacks communicate connectivity with
+respect to individual Diameter applications.</p>
+</item>
+
+<tag><c>{reconnect, Ref, Opts}</c></tag>
+<item>
+<code>
+Ref  = transport_ref()
+Opts = [transport_opt()]
+</code>
+
+<p>
+A connecting transport is attempting to establish/reestablish a
+transport connection with a peer following <c>reconnect_timer</c> or
+<c>watchdog_timer</c> expiry.</p>
+</item>
+
+</taglist>
+
+<p>
+For forward compatibility, a subscriber should be prepared to receive
+<c>diameter_event.info</c> of forms other than those documented
+above.</p>
+
+<marker id="service_name"/>
+</item>
+
+<tag><c>service_name() = term()</c></tag>
+<item>
+<p>
+The name of a service as passed to <seealso
+marker="#start_service">start_service/2</seealso> and with which the
+service is identified.
+There can be at most one service with a given name on a given node.
+Note that <c>erlang:make_ref/0</c> can be used to generate a service name
+that is somewhat unique.</p>
+
+<marker id="service_opt"/>
+</item>
+
+<tag><c>service_opt()</c></tag>
+<item>
+<p>
+Options accepted by <seealso
+marker="#start_service">start_service/2</seealso>.
+Can be any <c>capability()</c> tuple as
+well as the following.</p>
+
+<taglist>
+
+<tag><c>{application, [application_opt()]}</c></tag>
+<item>
+<p>
+Defines a Diameter application supported by the service.</p>
+
+<p>
+A service must define one application for each Diameter application it
+intends to support.
+For an outgoing Diameter request, the application is specified by
+passing the desired application's <c>application_alias()</c> to
+<seealso marker="#call">call/4</seealso>, while for an
+incoming request the application identifier in the message
+header determines the application (and callback module), the
+application identifier being specified in the <seealso
+marker="diameter_dict">dictionary</seealso> file defining the
+application.</p>
+</item>
+
+</taglist>
+
+<marker id="transport_opt"/>
+</item>
+
+<tag><c>transport_opt()</c></tag>
+<item>
+<p>
+Options accepted by <seealso
+marker="#add_transport">add_transport/2</seealso>.</p>
+
+<taglist>
+<tag><c>{transport_module, atom()}</c></tag>
+<item>
+<p>
+A module implementing a transport process as defined in <seealso
+marker="diameter_transport">diameter_transport(3)</seealso>.
+Defaults to <c>diameter_tcp</c> if unspecified.</p>
+
+<p>
+The interface required of a transport module is documented in <seealso
+marker="diameter_transport">diameter_transport(3)</seealso>.</p>
+</item>
+
+<tag><c>{transport_config, term()}</c></tag>
+<item>
+<p>
+A term passed as the third argument to the <seealso
+marker="diameter_transport#start">start/3</seealso> function of
+the relevant <c>transport_module</c> in order to start a transport process.
+Defaults to the empty list if unspecified.</p>
+</item>
+
+<tag><c>{applications, [application_alias()]}</c></tag>
+<item>
+<p>
+The list of Diameter applications to which usage of the transport
+should be restricted.
+Defaults to all applications configured on the service
+in question.</p>
+</item>
+
+<tag><c>{capabilities, [capability()]}</c></tag>
+<item>
+<p>
+AVP's used to construct outgoing CER/CEA messages.
+Any AVP specified takes precedence over a corresponding value specified
+for the service in question.</p>
+</item>
+
+<tag><c>{watchdog_timer, TwInit}</c></tag>
+<item>
+<code>
+TwInit = Unsigned32()
+       | {M,F,A}
+</code>
+
+<p>
+The RFC 3539 watchdog timer.
+An integer value is interpreted as the RFC's TwInit in milliseconds,
+a jitter of &plusmn; 2 seconds being added at each rearming of the
+timer to compute the RFC's Tw.
+An MFA is expected to return the RFC's Tw directly, with jitter
+applied, allowing the jitter calculation to be performed by
+the callback.</p>
+
+<p>
+An integer value must be at least 6000 as required by RFC 3539.
+Defaults to 30000 if unspecified.</p>
+</item>
+
+<tag><c>{reconnect_timer, Tc}</c></tag>
+<item>
+<code>
+Tc = Unsigned32()
+</code>
+
+<p>
+For a connecting transport, the RFC 3588 Tc timer, in milliseconds.
+Note that this timer determines the frequency with which the transport
+will attempt to establish a connection with its peer only <em>before</em>
+an initial connection is established: once there is an initial
+connection it's watchdog_timer that determines the frequency of
+reconnection attempts, as required by RFC 3539.</p>
+
+<p>
+For a listening transport, the timer specifies the time after which a
+previously connected peer will be forgotten: a connection after this time is
+regarded as an initial connection rather than a reestablishment,
+causing the RFC 3539 state machine to pass to state OPEN rather than
+REOPEN.
+Note that these semantics are not goverened by the RFC and
+that a listening transport's reconnect_timer should be greater than its
+peers's Tc plus jitter.</p>
+
+<p>
+Defaults to 30000 for a connecting transport and 60000 for a listening
+transport.</p>
+</item>
+
+</taglist>
+
+<p>
+Unrecognized options are silently ignored but are returned unmodified
+by <seealso
+marker="#service_info_1">service_info/1,2</seealso> and can be referred to
+in predicate functions passed to <seealso
+marker="#remove_transport">remove_transport/2</seealso>.</p>
+
+</item>
+
+</taglist>
+
+</section>
+
+<marker id="add_transport"/>
+<funcs>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>add_transport(SvcName, {connect|listen, Options})
+        -> {ok, Ref} | {error, Reason}</name>
+<fsummary>Add transport capability to a service.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>Options = [transport_opt()]</v>
+<v>Ref = ref()</v>
+<v>Reason = term()</v>
+</type>
+<desc>
+<p>
+Add transport capability to a service.
+The service will start a transport process(es) in order to establish a
+connection with the peer, either by connecting to the peer
+(<c>connect</c>) or by  accepting incoming connection requests
+(<c>listen</c>).
+A connecting transport establishes transport connections with at most
+one peer, an listening transport potentially with many.</p>
+
+<p>
+The diameter application takes responsibility for exchanging
+CER/CEA with the peer.
+Upon successful completion of capabilities exchange the service
+calls each relevant application module's <seealso
+marker="diameter_app#peer_up">peer_up/3</seealso> callback
+after which the caller can exchange Diameter messages with the peer over
+the transport.
+In addition to CER/CEA, the service takes responsibility for the
+handling of DWR/DWA and required by RFC 3539 as well as for DPR/DPA.</p>
+
+<p>
+The returned reference uniquely identifies the transport within the
+scope of the service.
+Not that the function returns before a transport connection has been
+established.</p>
+
+<p>
+It is not an error to add a transport to a service that has not yet
+been configured: a service can be started after configuring
+transports.</p>
+
+<marker id="call"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>call(SvcName, App, Request, Options) -> Answer | {error, Reason}</name>
+<fsummary>Send a Diameter request message.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>App = application_alias()</v>
+<v>Request = diameter_app:message()</v>
+<v>Answer = term()</v>
+<v>Options = [call_opt()]</v>
+</type>
+<desc>
+<p>
+Send a Diameter request message and possibly return the answer or error.</p>
+
+<p>
+<c>App</c> identifies the Diameter application in which the request is
+defined and callbacks to the corresponding callback module
+will follow as described below and in <seealso
+marker="diameter_app">diameter_app(3)</seealso>.
+The call returns either when an answer message is received from the
+peer or an error occurs, unless the <c>detach</c> option has been
+specified.
+If <c>detach</c> is not specified then the form of an <c>Answer</c> is
+as returned from a <seealso
+marker="diameter_app#handle_answer">handle_answer/4</seealso> or
+<seealso
+marker="diameter_app#handle_error">handle_error/4</seealso>
+callback.</p>
+
+<p>
+If there are no suitable peers, or if
+<seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
+rejects them by returning 'false', then <c>{error, no_connection}</c>
+is returned.
+If <seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
+selects a candidate peer then a request process is spawned for the
+outgoing request, in which there is a
+<seealso
+marker="diameter_app#prepare_request">prepare_request/3</seealso>
+callback, the message is encoded and sent.</p>
+
+<p>
+There are several error cases which may prevent an
+answer from being received and passed to a
+<seealso marker="diameter_app#handle_answer">handle_answer/4</seealso>
+callback:</p>
+
+<list>
+
+<item>
+<p>
+If the initial encode of the outgoing request
+fails, then the request process fails and <c>{error, encode}</c>
+is returned.</p>
+</item>
+
+<item>
+<p>
+If the request is successfully encoded and sent but
+the answer times out then a
+<seealso marker="diameter_app#handle_error">handle_error/4</seealso>
+callback takes place with <c>Reason = timeout</c>.</p>
+</item>
+
+<item>
+<p>
+If the request is successfully encoded and sent but the service in
+question is stopped before an answer is received then a
+<seealso marker="diameter_app#handle_error">handle_error/4</seealso>
+callback takes place <c>Reason = cancel</c>.</p>
+</item>
+
+<item>
+<p>
+If the transport connection with the peer goes down after the request
+has been sent but before an answer has been received then an attempt
+is made to resend the request to an alternate peer.
+If no such peer is available, or if the subsequent
+<seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
+callback rejects the candidates, then a
+<seealso marker="diameter_app#handle_error">handle_error/4</seealso>
+callback takes place with <c>Reason = failover</c>.
+If a peer is selected then a
+<seealso
+marker="diameter_app#prepare_retransmit">prepare_retransmit/3</seealso>
+callback takes place, after which the semantics are the same as
+following an initial
+<seealso marker="diameter_app#prepare_request">
+prepare_request/3</seealso>
+callback.</p>
+</item>
+
+<item>
+<p>
+If an encode error takes place during
+retransmission then the request process fails and
+<c>{error, failure}</c> is returned.</p>
+</item>
+
+<item>
+<p>
+If an application callback made in processing the request fails
+(pick_peer, prepare_request, prepare_retransmit,
+handle_answer or handle_error) then either
+<c>{error, encode}</c> or <c>{error, failure}</c>
+is returned depending on whether or not there has been an
+attempt to send the request over the transport.</p>
+</item>
+
+</list>
+
+<p>
+Note that <c>{error, encode}</c> is the only return value which
+guarantees that the request has not been sent over the transport.</p>
+
+<marker id="origin_state_id"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>origin_state_id() -> Unsigned32()</name>
+<fsummary>Returns a reasonable Origin-State-Id.</fsummary>
+<desc>
+<p>
+Return a reasonable value for use as Origin-State-Id in
+outgoing messages.
+The value returned is the number of seconds since 19680120T031408Z
+(the first value that can be encoded as a Time())
+at the time the diameter application was started.</p>
+
+<marker id="remove_transport"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>remove_transport(SvcName, Pred) -> ok</name>
+<fsummary>Remove previously added transports.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>Pred = Fun | MFA | ref() | list() | true | false</v>
+<v></v>
+<v>Fun = fun((reference(), connect|listen, list()) -> boolean())</v>
+<v>&nbsp;&nbsp;&nbsp; | fun((reference(), list()) -> boolean())</v>
+<v>&nbsp;&nbsp;&nbsp; | fun((list()) -> boolean())</v>
+<v>MFA = {atom(), atom(), list()}</v>
+</type>
+<desc>
+<p>
+Remove previously added transports.</p>
+
+<p>
+<c>Pred</c> determines which transports to remove.
+An arity-3-valued <c>Pred</c> removes all transports for which
+<c>Pred(Ref, Type, Opts)</c> returns <c>true</c>, where <c>Type</c> and
+<c>Opts</c> are as passed to <seealso
+marker="#add_transport">add_transport/2</seealso> and <c>Ref</c> is
+as returned by the corresponding call.
+The remaining forms are equivalent to an arity-3 fun as follows.</p>
+
+<code>
+Pred = fun(reference(), list()):  fun(Ref, _, Opts) -> Pred(Ref, Opts) end
+Pred = fun(list()):               fun(_, _, Opts) -> Pred(Opts) end
+Pred = reference():               fun(Ref, _, _)  -> Pred == Ref end
+Pred = list():                    fun(_, _, Opts) -> [] == Pred -- Opts end
+Pred = true:                      fun(_, _, _) -> true end
+Pred = false:                     fun(_, _, _) -> false end
+Pred = {M,F,A}:  fun(Ref, Type, Opts) -> apply(M, F, [Ref, Type, Opts | A]) end
+</code>
+
+<p>
+Removing a transport causes all associated transport connections to
+be broken.
+A base application DPR message with
+Disconnect-Cause <c>DO_NOT_WANT_TO_TALK_TO_YOU</c> will be sent
+to each connected peer before disassociating the transport configuration
+from the service and terminating the transport upon reception of
+DPA or timeout.</p>
+
+<!-- TODO: document the timeout value, possibly make configurable. -->
+
+<marker id="service_info_1"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>service_info(SvcName) -> Info</name>
+<fsummary>Return information about a started service.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>Info = [{Item, Value}]</v>
+</type>
+<desc>
+<p>
+Return information about a started service.
+Equivalent to <c>service_info(SvcName, all)</c>.</p>
+
+<marker id="service_info_2"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>service_info(SvcName, Item) -> Value</name>
+<fsummary>Return specific information about a started service.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>Value = term()</v>
+</type>
+<desc>
+<p>
+Return specific information about a started service.</p>
+
+<marker id="services"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>services() -> [SvcName]</name>
+<fsummary>Return the list of started services.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+</type>
+<desc>
+<p>
+Return the list of started services.</p>
+
+<marker id="session_id"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>session_id(Ident) -> OctetString()</name>
+<fsummary>Return a value for a Session-Id AVP</fsummary>
+<type>
+<v>Ident = DiameterIdentity()</v>
+</type>
+<desc>
+<p>
+Return a value for a Session-Id AVP.
+The value has the form required by section 8.8 of RFC 3588.
+Ident should be the Origin-Host of the peer from which
+the message containing the returned value will be sent.</p>
+
+<marker id="start_service"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+<func>
+<name>start_service(SvcName, Options) -> ok | {error, Reason}</name>
+<fsummary>Start a Diameter service</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>Options = [service_opt()]</v>
+<v>Reason  = term()</v>
+</type>
+<desc>
+<p>
+Start a diameter service.
+A service defines a locally-implemented Diameter peer, specifying the
+capabilities of the peer to be used during capabilities exchange and
+the Diameter applications that it supports.
+Transports are added to a service using <seealso
+marker="#add_transport">add_transport/2</seealso>.</p>
+
+<marker id="stop_service"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+<func>
+<name>stop_service(SvcName) -> ok | {error, Reason}</name>
+<fsummary>Stops a Diameter service.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+<v>Reason = term()</v>
+</type>
+<desc>
+<p>
+Stop a diameter service.</p>
+
+<marker id="subscribe"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>subscribe(SvcName) -> true</name>
+<fsummary>Subscribe to event messages from a service.</fsummary>
+<type>
+<v>SvcName = service_name()</v>
+</type>
+<desc>
+<p>
+Subscribe to <c>service_event()</c> messages from a service.</p>
+
+<p>
+It is not an error to subscribe to events from a service
+that does not yet exist.</p>
+
+<marker id="unsubscribe"/>
+</desc>
+</func>
+
+<!-- ===================================================================== -->
+
+<func>
+<name>unsubscribe(SvcName) -> true</name>
+<fsummary></fsummary>
+<type>
+<v>SvcName = service_name()</v>
+</type>
+<desc>
+<p>
+Unsubscribe to event messages from a service.</p>
+
+</desc>
+</func>
+
+</funcs>
+
+<!-- ===================================================================== -->
+<!-- ===================================================================== -->
+<section>
+<title>SEE ALSO</title>
+
+<p>
+<seealso marker="diameter_app">diameter_app(3)</seealso>,
+<seealso marker="diameter_transport">diameter_transport(3)</seealso>,
+<seealso marker="diameter_dict">diameter_dict(4)</seealso></p>
+
+</section>
+
+</erlref>