path: root/lib/diameter/doc/src/diameter.xml

<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
<header>

<copyright>
<year>2011</year><year>2012</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 can
implement a Diameter node 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 node 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</em> (not capitalised) and
<em>Diameter</em> (capitalised).
The former refers to the Erlang application named diameter whose main
api is defined here, the latter to Diameter protocol in the sense of
RFC 3588.</p>

<p>
The diameter application must be started before calling most functions
in this module.</p>

</description>

<!-- ===================================================================== -->
<!-- ===================================================================== -->
<section>
<title>DATA TYPES</title>

<taglist>

<tag><c>Address()</c></tag>
<tag><c>DiameterIdentity()</c></tag>
<tag><c>Grouped()</c></tag>
<tag><c>OctetString()</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#DATA_TYPES">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="#call">call/4</seealso> when sending requests
defined by the application.</p>

<marker id="application_module"/>
</item>

<tag><c>application_module() = Mod
                             | [Mod | ExtraArgs]
                             | #diameter_callback{}</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.
Note that extra arguments specific to an outgoing request can be
specified to <seealso marker="#call">call/4</seealso>, in which case
those are are appended to any module-specific extra arguments.</p>

<p>
Specifying a <c>#diameter_callback{}</c> record allows individual
functions to be configured in place of the usual <seealso
marker="diameter_app">diameter_app(3)</seealso> callbacks.
See that module for details.</p>

<marker id="application_opt"/>
</item>

<tag><c>application_opt()</c></tag>
<item>

<p>
Options defining a Diameter application.
Has one the following types.</p>

<taglist>

<tag><c>{alias, <seealso marker="#application_alias">application_alias()</seealso>}</c></tag>
<item>
<p>
An unique identifier for the application in the scope of the
service.
Defaults to the value of the <c>dictionary</c> option if
unspecified.</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, <seealso marker="#application_module">application_module()</seealso>}</c></tag>
<item>
<p>
The callback module with which messages of the Diameter application are
handled.
See <seealso marker="diameter_app">diameter_app(3)</seealso> for
the required interface and semantics.</p>
</item>

<tag><c>{state, term()}</c></tag>
<item>
<p>
The initial callback state.
The prevailing state is passed to some
<seealso marker="diameter_app">diameter_app(3)</seealso>
callbacks, which can then return a new state.
Defaults to the value of the <c>alias</c> option if unspecified.</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 can modify the application state,
Defaults to <c>false</c> if unspecified.</p>

<note>
<p>
<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 should
probably avoid it.</p>
</note>
</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>, with
errors communicated in the <c>errors</c> field of the
<c>#diameter_packet{}</c> record passed to 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.
Has one of the following types.</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 on the callback
itself.
Multiple options append to the argument list.</p>
</item>

<tag><c>{filter, <seealso marker="#peer_filter">peer_filter()</seealso>}</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, <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>}</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>

<p>
An invalid option will cause <seealso marker="#call">call/4</seealso>
to fail.</p>

<marker id="capability"/>
</item>

<tag><c>capability()</c></tag>
<item>

<p>
AVP values sent in outgoing CER or CEA messages during capabilities
exchange.
Can be configured both on a service and a transport, values specified
on the latter taking precedence over any specified on the former.
Has one of the following types.</p>

<taglist>

<tag><c>{'Origin-Host', <seealso marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso>}</c></tag>
<tag><c>{'Origin-Realm', <seealso marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso>}</c></tag>
<tag><c>{'Host-IP-Address', [<seealso marker="diameter_dict#DATA_TYPES">Address()</seealso>]}</c></tag>
<item>
<p>
An address list is available to the start function of a
<seealso marker="diameter_transport">transport module</seealso>, which
can return a new list for use in the subsequent CER or CEA.
Host-IP-Address need not be specified if the transport start function
returns an address list.</p>
</item>

<tag><c>{'Vendor-Id', <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>}</c></tag>
<tag><c>{'Product-Name', <seealso marker="diameter_dict#DATA_TYPES">UTF8String()</seealso>}</c></tag>
<tag><c>{'Origin-State-Id', <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>}</c></tag>
<item>
<p>
Origin-State-Id is optional but will be included in outgoing messages
sent by diameter itself: CER/CEA, DWR/DWA and DPR/DPA.
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 computed when the diameter
application is started.</p>
</item>

<tag><c>{'Supported-Vendor-Id', [<seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>]}</c></tag>
<tag><c>{'Auth-Application-Id', [<seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>]}</c></tag>
<tag><c>{'Inband-Security-Id', [<seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>]}</c></tag>
<item>
<p>
Inband-Security-Id defaults to the empty list, which is equivalent to a
list containing only 0 (= NO_INBAND_SECURITY).
If 1 (= TLS) is specified then TLS is selected if the CER/CEA received
from the peer offers it.</p>
</item>

<tag><c>{'Acct-Application-Id', [<seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>]}</c></tag>
<tag><c>{'Vendor-Specific-Application-Id', [<seealso marker="diameter_dict#DATA_TYPES">Grouped()</seealso>]}</c></tag>
<tag><c>{'Firmware-Revision', <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>}</c></tag>

</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] | T]) ->
    eval([F | T ++ A]);
eval([F|A]) ->
    apply(F, A);
eval(F) ->
    eval([F]).
</code>

<p>
Applying an <c><seealso marker="#evaluable">evaluable()</seealso></c>
<c>E</c> to an argument list <c>A</c>
is meant in the sense of <c>eval([E|A])</c>.</p>

<warning>
<p>
Beware of using fun expressions of the form <c>fun Name/Arity</c> in
situations in which the fun is not short-lived
and code is to be upgraded at runtime since any processes retaining
such a fun will have a reference to old code.
In particular, such a value is typically inappropriate in
configuration passed to <seealso
marker="#start_service">start_service/2</seealso> or
<seealso
marker="#add_transport">add_transport/2</seealso>.</p> 
</warning>

<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|<seealso marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso>}</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|<seealso marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso></c></tag>
<item>
<p>
Matches only those peers whose <c>Origin-Realm</c> has the
specified value, or all peers if the atom <c>any</c>.</p>
</item>

<tag><c>{eval, <seealso marker="#evaluable">evaluable()</seealso>}</c></tag>
<item>
<p>
Matches only those peers for which the specified <c><seealso
marker="#evaluable">evaluable()</seealso></c> returns
<c>true</c> on the connection's <c>diameter_caps</c> record.
Any other return value or exception is equivalent to <c>false</c>.</p>
</item>

<tag><c>{neg, <seealso marker="#peer_filter">peer_filter()</seealso>}</c></tag>
<item>
<p>
Matches only those peers not matched by the specified filter.</p>
</item>

<tag><c>{all, [<seealso marker="#peer_filter">peer_filter()</seealso>]}</c></tag>
<item>
<p>
Matches only those peers matched by each filter in the specified list.</p>
</item>

<tag><c>{any, [<seealso marker="#peer_filter">peer_filter()</seealso>]}</c></tag>
<item>
<p>
Matches only those peers matched by at least one filter in the
specified list.</p>
</item>

</taglist>

<p>
An invalid filter is equivalent to <c>{any,[]}</c>, a filter
that matches no peer.</p>

<note>
<p>
The <c>host</c> and <c>realm</c> filters examine the
outgoing request as passed to <seealso marker="#call">call/4</seealso>,
assuming that this is a record- or list-valued <c><seealso
marker="diameter_app#message">diameter_app:message()</seealso></c>,
and that the message contains at most one of each AVP.
If this is not the case then the <c>{host|realm, <seealso
marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso>}</c>
filters must be used to achieve the desired result.
An empty <c><seealso
marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso></c>
(which should not be typical)
matches all hosts/realms for the purposes of filtering.</p>
</note>

<warning>
<p>
A <c>host</c> filter is not typically desirable when setting
Destination-Host since it will remove peer agents from the
candidates list.</p>
</warning>

<marker id="service_event"/>
</item>

<tag><c>service_event() = #diameter_event{}</c></tag>
<item>
<p>
An event message sent to processes that have subscribed to these using
<seealso marker="#subscribe">subscribe/1</seealso>.</p>

<p>
The <c>info</c> field of the event record can have one of the
following types.</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    = <seealso marker="#transport_ref">transport_ref()</seealso>
Peer   = <seealso marker="diameter_app#peer">diameter_app:peer()</seealso>
Config = {connect|listen, [<seealso marker="#transport_opt">transport_opt()</seealso>]}
Pkt    = #diameter_packet{}
</code>

<p>
The RFC 3539 watchdog state machine has
transitioned into (<c>up</c>) or out of (<c>down</c>) the OKAY
state.
If a <c>#diameter_packet{}</c> record is present in an <c>up</c> event
then there has been a capabilties exchange on a newly established
transport connection and the record contains the received CER or CEA.
Otherwise a connection has reestablished without the loss or
connectivity.</p>

<p>
Note that a single <c>up</c>/<c>down</c> event for a given peer
corresponds to one <seealso marker="diameter_app#peer_up">peer_up/peer_down</seealso>
callback for each of the Diameter applications negotiated 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  = <seealso marker="#transport_ref">transport_ref()</seealso>
Opts = [<seealso marker="#transport_opt">transport_opt()</seealso>]
</code>

<p>
A connecting transport is attempting to establish/reestablish a
transport connection with a peer following <seealso
marker="#reconnect_timer">reconnect_timer</seealso> or
<seealso marker="#watchdog_timer">watchdog_timer</seealso>
expiry.</p>
</item>

<tag><c>{closed, Ref, Reason, Config}</c></tag>
<item>
<code>
Ref = <seealso marker="#transport_ref">transport_ref()</seealso>
Config = {connect|listen, [<seealso marker="#transport_opt">transport_opt()</seealso>]}
</code>

<p>
Capabilities exchange has failed.
<c>Reason</c> can have one of the following types.</p>

<taglist>

<tag><c>{'CER', Result, Caps, Pkt}</c></tag>
<item>
<code>
Result = ResultCode | {capabilities_cb, CB, ResultCode|discard}
Caps = #diameter_caps{}
Pkt  = #diameter_packet{}
ResultCode = integer()
CB = <seealso marker="#evaluable">evaluable()</seealso>
</code>

<p>
An incoming CER has been answered with the indicated result code or
discarded.
<c>Caps</c> contains pairs of values for the the local node and remote
peer.
<c>Pkt</c> contains the CER in question.
In the case of rejection by a capabilities callback, the tuple
contains the rejecting callback.</p>
</item>

<tag><c>{'CER', Caps, {ResultCode, Pkt}}</c></tag>
<item>
<code>
ResultCode = integer()
Caps = #diameter_caps{}
Pkt  = #diameter_packet{}
</code>

<p>
An incoming CER contained errors and has been answered with the
indicated result code.
<c>Caps</c> contains only values for the the local node.
<c>Pkt</c> contains the CER in question.</p>
</item>

<tag><c>{'CEA', Result, Caps, Pkt}</c></tag>
<item>
<code>
Result = integer() | atom() | {capabilities_cb, CB, ResultCode|discard}
Caps = #diameter_caps{}
Pkt  = #diameter_packet{}
ResultCode = integer()
</code>

<p>
An incoming CEA has been rejected for the indicated reason.
An integer-valued <c>Result</c> indicates the result code sent
by the peer.
<c>Caps</c> contains pairs of values for the the local node and remote
peer.
<c>Pkt</c> contains the CEA in question.
In the case of rejection by a capabilities callback, the tuple
contains the rejecting callback.</p>
</item>

<tag><c>{'CEA', Caps, Pkt}</c></tag>
<item>
<code>
Caps = #diameter_caps{}
Pkt  = #diameter_packet{}
</code>

<p>
An incoming CEA contained errors and has been rejected.
<c>Caps</c> contains only values for the the local node.
<c>Pkt</c> contains the CEA in question.</p>
</item>

</taglist>
</item>

<tag><c>{watchdog, Ref, PeerRef, {From, To}, Config}</c></tag>
<item>
<code>
Ref = <seealso marker="#transport_ref">transport_ref()</seealso>
PeerRef  = <seealso marker="diameter_app#peer_ref">diameter_app:peer_ref()</seealso>
From, To = initial | okay | suspect | down | reopen
Config = {connect|listen, [transport_opt()]}
</code>

<p>
An RFC 3539 watchdog state machine has changed state.</p>
</item>

</taglist>

<p>
For forward compatibility, a subscriber should be prepared to receive
info fields of forms other than the 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 <seealso marker="erts:erlang#make_ref-0">erlang:make_ref/0</seealso>
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>
An option passed to <seealso
marker="#start_service">start_service/2</seealso>.
Can be any <c><seealso marker="#capability">capability()</seealso></c> as
well as the following.</p>

<taglist>

<tag><c>{application, [<seealso marker="#application_opt">application_opt()</seealso>]}</c></tag>
<item>
<p>
Defines a Diameter application supported by the service.</p>

<p>
A service must configure one <c>application</c> for each Diameter
application it intends to support.
For an outgoing Diameter request, the relevant <c><seealso 
marker="#application_alias">application_alias()</seealso></c> is
passed to <seealso marker="#call">call/4</seealso>, while for an
incoming request the application identifier in the message
header determines the application, the identifier being specified in
the application's <seealso marker="diameter_dict">dictionary</seealso>
file.</p>
</item>

<tag><c>{restrict_connections, false
                             | node
                             | nodes
                             | [node()]
                             | diameter:evaluable()}</c></tag>
<item>
<p>
Specifies the degree to which multiple transport connections to the
same peer are accepted by the service.</p>

<p>
If type <c>[node()]</c> then a connection is rejected if another already
exists on any of the specified nodes.
Values of type <c>false</c>, <c>node</c>, <c>nodes</c> or
<c>diameter:evaluable()</c> are equivalent to values <c>[]</c>,
<c>[node()]</c>, <c>[node()|nodes()]</c> and the evaluated value,
respectively, evaluation of each expression taking place whenever a
new connection is to be established.
Note that <c>false</c> allows an unlimited number of connections to be
established with the same peer.</p>

<p>
Multiple connections are independent and governed
by their own peer and watchdog state machines.</p>

<p>
Defaults to <c>nodes</c>.</p>
</item>

<tag><c>{sequence, {H,N} | <seealso
   marker="diameter#evaluable">diameter:evaluable()</seealso>}</c></tag>
<item>
<p>
Specifies a constant value <c>H</c> for the topmost <c>32-N</c> bits of
of 32-bit End-to-End and Hop-by-Hop identifiers generated
by the service, either explicity or as a return value of a function
to be evaluated at <seealso
marker="diameter#start_service">diameter:start_service/2</seealso>.
In particular, an identifier <c>Id</c> is mapped to a new identifier
as follows.</p>
<code>
(H bsl N) bor (Id band ((1 bsl N) - 1))
</code>
<p>
Note that RFC 3588 requires that End-to-End identifiers remain unique
for a period of at least 4 minutes and that this and the call rate
places a lower bound on the appropriate values of <c>N</c>:
at a rate of <c>R</c> requests per second an <c>N</c>-bit counter
traverses all of its values in <c>(1 bsl N) div (R*60)</c> minutes so
the bound is <c>4*R*60 =&lt; 1 bsl N</c>.</p>

<p><c>N</c> must lie in the range <c>0..32</c> and <c>H</c> must be a
non-negative integer less than <c>1 bsl (32-N)</c>.</p>

<p>
Defaults to <c>{0,32}</c>.</p>
</item>

</taglist>

<marker id="transport_opt"/>
</item>

<tag><c>transport_opt()</c></tag>
<item>
<p>
An option passed to <seealso
marker="#add_transport">add_transport/2</seealso>.
Has one of the following types.</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>
Multiple <c>transport_module</c> and <c>transport_config</c>
options are allowed.
The order of these is significant in this case (and only in this case),
a <c>transport_module</c> being paired with the first
<c>transport_config</c> following it in the options list, or the
default value for trailing modules.
Transport starts will be attempted with each of the
modules in order until one establishes a connection within the
corresponding timeout (see below) or all fail.</p>
</item>

<tag><c>{transport_config, term()}</c></tag>
<tag><c>{transport_config, term(), <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>}</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>

<p>
The 3-tuple form additionally specifies an interval, in milliseconds,
after which a started transport process should be terminated if it has
not yet established a connection.
For example, the following options on a connecting transport
request a connection with one peer over SCTP or another
(typically the same) over TCP.</p>

<code>
{transport_module, diameter_sctp}
{transport_config, SctpOpts, 5000}
{transport_module, diameter_tcp}
{transport_config, TcpOpts}
</code>

<p>
To listen on both SCTP and TCP, define one transport for each.</p>
</item>

<tag><c>{applications, [<seealso marker="#application_alias">application_alias()</seealso>]}</c></tag>
<item>
<p>
The list of Diameter applications to which the transport should be
restricted.
Defaults to all applications configured on the service in question.
Applications not configured on the service in question are ignored.</p>
</item>

<tag><c>{capabilities, [<seealso marker="#capability">capability()</seealso>]}</c></tag>
<item>
<p>
AVP's used to construct outgoing CER/CEA messages.
Values take precedence over any specified on the service in
question.</p>

<p>
Specifying a capability as a transport option
may be particularly appropriate for Inband-Security-Id, in case
TLS is desired over TCP as implemented by
<seealso marker="diameter_tcp">diameter_tcp(3)</seealso>.</p>
</item>

<tag><c>{capabilities_cb, <seealso marker="#evaluable">evaluable()</seealso>}</c></tag>
<item>
<p>
A callback invoked upon reception of CER/CEA during capabilities
exchange in order to ask whether or not the connection should
be accepted.
Applied to the relevant <c><seealso
marker="#transport_ref">transport_ref()</seealso></c> and the
<c>#diameter_caps{}</c> record of the connection.
Returning <c>ok</c> accepts the connection.
Returning <c>integer()</c> causes an incoming
CER to be answered with the specified Result-Code.
Returning <c>discard</c> causes an incoming CER to
be discarded.
Returning <c>unknown</c> is equivalent to returning <c>3010</c>,
DIAMETER_UNKNOWN_PEER.
Returning anything but <c>ok</c> or a 2xxx series result
code causes the transport connection to be broken.</p>

<p>
Multiple <c>capabilities_cb</c> options can be specified, in which
case the corresponding callbacks are applied until either all return
<c>ok</c> or one does not.</p>

<marker id="watchdog_timer"/>
</item>

<tag><c>{watchdog_timer, TwInit}</c></tag>
<item>
<code>
TwInit = <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>
       | {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>

<marker id="reconnect_timer"/>
</item>

<tag><c>{reconnect_timer, Tc}</c></tag>
<item>
<code>
Tc = <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso>
</code>

<p>
For a connecting transport, the RFC 3588 Tc timer, in milliseconds.
Note that this timer determines the frequency with which a 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 OKAY rather than
REOPEN.
Note that these semantics are not goverened by the RFC and
that a listening transport's <c>reconnect_timer</c> should be greater
than its peer's Tw 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">service_info/2</seealso> and can be referred to
in predicate functions passed to <seealso
marker="#remove_transport">remove_transport/2</seealso>.</p>

<marker id="transport_ref"/>
</item>

<tag><c>transport_ref() = reference()</c></tag>
<item>
<p>
An reference returned by <seealso
marker="#add_transport">add_transport/2</seealso> that
identifies the configuration.</p>
</item>

</taglist>

</section>

<marker id="add_transport"/>
<funcs>

<!-- ===================================================================== -->

<func>
<name>add_transport(SvcName, {connect|listen, [Opt]})
        -> {ok, Ref} | {error, Reason}</name>
<fsummary>Add transport capability to a service.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></v>
<v>Opt = <seealso marker="#transport_opt">transport_opt()</seealso></v>
<v>Ref = <seealso marker="#transport_ref">transport_ref()</seealso></v>
<v>Reason = term()</v>
</type>
<desc>
<p>
Add transport capability to a service.</p>

<p>
The service will start transport processes as required 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.
Note that the function returns before a transport connection has been
established.</p>

<note>
<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
its transports.</p>
</note>

<marker id="call"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>call(SvcName, App, Request, [Opt]) -> Answer | ok | {error, Reason}</name>
<fsummary>Send a Diameter request message.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></v>
<v>App = <seealso marker="#application_alias">application_alias()</seealso></v>
<v>Request = <seealso marker="diameter_app#message">diameter_app:message()</seealso></v>
<v>Answer = term()</v>
<v>Opt = <seealso marker="#call_opt">call_opt()</seealso></v>
</type>
<desc>
<p>
Send a Diameter request message.</p>

<p>
<c>App</c> specifies 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>.
Unless the <c>detach</c> option is specified, the call returns either
when an answer message is received from the peer or an error occurs.
In the answer case, the return value is as returned by a
<seealso
marker="diameter_app#handle_answer">handle_answer/4</seealso>
callback.
In the error case, whether or not the error is returned directly
by diameter or from a <seealso
marker="diameter_app#handle_error">handle_error/4</seealso>
callback depends on whether or not the outgoing request is
successfully encoded for transmission to the peer, the cases being
documented below.</p>

<p>
If there are no suitable peers, or if
<seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
rejects them by returning <c>false</c>, then <c>{error,no_connection}</c>
is returned.
Otherwise <seealso marker="diameter_app#pick_peer">pick_peer/4</seealso>
is followed by a
<seealso
marker="diameter_app#prepare_request">prepare_request/3</seealso>
callback, the message is encoded and then 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 with <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 <em>not</em> been sent over the
transport connection.</p>

<marker id="origin_state_id"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>origin_state_id() -> <seealso marker="diameter_dict#DATA_TYPES">Unsigned32()</seealso></name>
<fsummary>Returns a reasonable Origin-State-Id.</fsummary>
<desc>
<p>
Return a reasonable value for use as Origin-State-Id in
outgoing messages.</p>

<p>
The value returned is the number of seconds since 19680120T031408Z,
the first value that can be encoded as a Diameter <c><seealso marker="diameter_dict#DATA_TYPES">Time()</seealso></c>,
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 = <seealso marker="#service_name">service_name()</seealso></v>
<v>Pred = Fun | MFA | <seealso marker="#transport_ref">transport_ref()</seealso> | list() | true | false</v>
<v></v>
<v>Fun = fun((<seealso marker="#transport_ref">transport_ref()</seealso>, connect|listen, list()) -> boolean())</v>
<v>&nbsp;&nbsp;&nbsp; | fun((<seealso marker="#transport_ref">transport_ref()</seealso>, 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 it.
The remaining forms are equivalent to an arity-3 fun as follows.</p>

<code>
Pred = fun(transport_ref(), list()):  fun(Ref, _, Opts) -> Pred(Ref, Opts) end
Pred = fun(list()):                   fun(_, _, Opts) -> Pred(Opts) end
Pred = transport_ref():               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 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"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>service_info(SvcName, Info) -> term()</name>
<fsummary>Return information about a started service.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></v>
<v>Info = Item | [Info]</v>
<v>Item = atom()</v>
</type>
<desc>
<p>
Return information about a started service.
Requesting info for an unknown service causes <c>undefined</c> to be
returned.
Requesting a list of items causes a tagged list to be
returned.</p>

<p>
<c>Item</c> can be one of the following.</p>

<taglist>

<tag><c>'Origin-Host'</c></tag>
<tag><c>'Origin-Realm'</c></tag>
<tag><c>'Vendor-Id'</c></tag>
<tag><c>'Product-Name'</c></tag>
<tag><c>'Origin-State-Id'</c></tag>
<tag><c>'Host-IP-Address'</c></tag>
<tag><c>'Supported-Vendor'</c></tag>
<tag><c>'Auth-Application-Id'</c></tag>
<tag><c>'Inband-Security-Id'</c></tag>
<tag><c>'Acct-Application-Id'</c></tag>
<tag><c>'Vendor-Specific-Application-Id'</c></tag>
<tag><c>'Firmware-Revision'</c></tag>
<item>
<p>
Return a capability value as configured with <seealso
marker="#start_service">start_service/2</seealso>.</p>
</item>

<tag><c>applications</c></tag>
<item>
<p>
Return the list of applications as configured with <seealso
marker="#start_service">start_service/2</seealso>.
</p>
</item>

<tag><c>capabilities</c></tag>
<item>
<p>
Return a tagged list of all capabilities values as configured with
<seealso
marker="#start_service">start_service/2</seealso>.</p>
</item>

<tag><c>transport</c></tag>
<item>
<p>
Return a list containing one entry for each of the service's transport
as configured with <seealso
marker="#add_transport">add_transport/2</seealso>.
Each entry is a tagged list containing both configuration and
information about established peer connections.
An example return value with for a client service with Origin-Host
"client.example.com" configured with a single transport connected to
"server.example.com" might look as follows.</p>

<code>
[[{ref,#Ref&lt;0.0.0.93>},
  {type,connect},
  {options,[{transport_module,diameter_tcp},
            {transport_config,[{ip,{127,0,0,1}},
                               {raddr,{127,0,0,1}},
                               {rport,3868},
                               {reuseaddr,true}]}]},
  {watchdog,{&lt;0.66.0>,{1346,171491,996448},okay}},
  {peer,{&lt;0.67.0>,{1346,171491,999906}}},
  {apps,[{0,common}]},
  {caps,[{origin_host,{"client.example.com","server.example.com"}},
         {origin_realm,{"example.com","example.com"}},
         {host_ip_address,{[{127,0,0,1}],[{127,0,0,1}]}},
         {vendor_id,{0,193}},
         {product_name,{"Client","Server"}},
         {origin_state_id,{[],[]}},
         {supported_vendor_id,{[],[]}},
         {auth_application_id,{[0],[0]}},
         {inband_security_id,{[],[0]}},
         {acct_application_id,{[],[]}},
         {vendor_specific_application_id,{[],[]}},
         {firmware_revision,{[],[]}},
         {avp,{[],[]}}]},
  {port,[{owner,&lt;0.69.0>},
         {module,diameter_tcp},
         {socket,{{127,0,0,1},48758}},
         {peer,{{127,0,0,1},3868}},
         {statistics,[{recv_oct,656},
                      {recv_cnt,6},
                      {recv_max,148},
                      {recv_avg,109},
                      {recv_dvi,19},
                      {send_oct,836},
                      {send_cnt,6},
                      {send_max,184},
                      {send_avg,139},
                      {send_pend,0}]}]},
  {statistics,[{{{0,258,0},recv},3},
               {{{0,258,1},send},3},
               {{{0,257,0},recv},1},
               {{{0,257,1},send},1},
               {{{0,258,0},recv,{'Result-Code',2001}},3},
               {{{0,280,1},recv},2},
               {{{0,280,0},send},2}]}]]
</code>

<p>
Here <c>ref</c> is a <c><seealso
marker="#transport_ref">transport_ref()</seealso></c> and <c>options</c>
the corresponding <c><seealso
marker="#transport_opt">transport_opt()</seealso></c> list passed to <seealso 
marker="#add_transport">add_transport/2</seealso>.
The <c>watchdog</c> entry shows the state of a connection's RFC 3539 watchdog
state machine.
The <c>peer</c> entry identifies the <c><seealso
marker="diameter_app#peer_ref">diameter_app:peer_ref()</seealso></c> for
which there will have been <seealso
marker="diameter_app#peer_up">peer_up</seealso> callbacks for the
Diameter applications identified by the <c>apps</c> entry,
<c>common</c> being the <c><seealso
marker="#application_alias">application_alias()</seealso></c>.
The <c>caps</c> entry identifies the capabilities sent by the local
node and received from the peer during capabilities exchange.
The <c>port</c> entry displays socket-level information about the
transport connection.
The <c>statistics</c> entry presents Diameter-level counters, 
an entry like <c>{{{0,280,1},recv},2}</c> saying that the client has
received 2 DWR messages: <c>{0,280,1} = {Application_Id, Command_Code,
R_Flag}</c>.</p>

<p>
Note that <c>watchdog</c>, <c>peer</c>, <c>apps</c>, <c>caps</c>
and <c>port</c> entries depend on connectivity
with the peer and may not be present.
Note also that the <c>statistics</c> entry presents values acuumulated
during the lifetime of the transport configuration.</p>

<p>
A listening transport presents its information slightly differently
since there may be multiple accepted connections for the same <c><seealso
marker="#transport_ref">transport_ref()</seealso></c>.
The <c>transport</c> info returned by a server with a single client
connection might look as follows.</p>

<code>
[[{ref,#Ref&lt;0.0.0.61>},
  {type,listen},
  {options,[{transport_module,diameter_tcp},
            {transport_config,[{reuseaddr,true},
                               {ip,{127,0,0,1}},
                               {port,3868}]}]},
  {accept,[[{watchdog,{&lt;0.56.0>,{1346,171481,226895},okay}},
            {peer,{&lt;0.58.0>,{1346,171491,999511}}},
            {apps,[{0,common}]},
            {caps,[{origin_host,{"server.example.com","client.example.com"}},
                   {origin_realm,{"example.com","example.com"}},
                   {host_ip_address,{[{127,0,0,1}],[{127,0,0,1}]}},
                   {vendor_id,{193,0}},
                   {product_name,{"Server","Client"}},
                   {origin_state_id,{[],[]}},
                   {supported_vendor_id,{[],[]}},
                   {auth_application_id,{[0],[0]}},
                   {inband_security_id,{[],[]}},
                   {acct_application_id,{[],[]}},
                   {vendor_specific_application_id,{[],[]}},
                   {firmware_revision,{[],[]}},
                   {avp,{[],[]}}]},
            {port,[{owner,&lt;0.62.0>},
                   {module,diameter_tcp},
                   {socket,{{127,0,0,1},3868}},
                   {peer,{{127,0,0,1},48758}},
                   {statistics,[{recv_oct,1576},
                                {recv_cnt,16},
                                {recv_max,184},
                                {recv_avg,98},
                                {recv_dvi,26},
                                {send_oct,1396},
                                {send_cnt,16},
                                {send_max,148},
                                {send_avg,87},
                                {send_pend,0}]}]}],
           [{watchdog,{&lt;0.72.0>,{1346,171491,998404},initial}}]]},
  {statistics,[{{{0,280,0},recv},7},
               {{{0,280,1},send},7},
               {{{0,258,0},send,{'Result-Code',2001}},3},
               {{{0,258,1},recv},3},
               {{{0,258,0},send},3},
               {{{0,280,1},recv},5},
               {{{0,280,0},send},5},
               {{{0,257,1},recv},1},
               {{{0,257,0},send},1}]}]]
</code>

<p>
The information presented here is as in the <c>connect</c> case except
that the client connections are grouped under an <c>accept</c> tuple.</p>

</item>

<tag><c>connections</c></tag>
<item>
<p>
Return a list containing one entry for every established transport
connection whose watchdog state machine is not in the <c>down</c>
state.
This is a flat view of <c>transport</c> info which lists only active
connections and for which Diameter-level statistics are accumulated
only for the lifetime of the transport connection.
A return value for the server above might look as follows.</p>

<code>
[[{ref,#Ref&lt;0.0.0.61>},
  {type,accept},
  {options,[{transport_module,diameter_tcp},
            {transport_config,[{reuseaddr,true},
                               {ip,{127,0,0,1}},
                               {port,3868}]}]},
  {watchdog,{&lt;0.56.0>,{1346,171481,226895},okay}},
  {peer,{&lt;0.58.0>,{1346,171491,999511}}},
  {apps,[{0,common}]},
  {caps,[{origin_host,{"server.example.com","client.example.com"}},
         {origin_realm,{"example.com","example.com"}},
         {host_ip_address,{[{127,0,0,1}],[{127,0,0,1}]}},
         {vendor_id,{193,0}},
         {product_name,{"Server","Client"}},
         {origin_state_id,{[],[]}},
         {supported_vendor_id,{[],[]}},
         {auth_application_id,{[0],[0]}},
         {inband_security_id,{[],[]}},
         {acct_application_id,{[],[]}},
         {vendor_specific_application_id,{[],[]}},
         {firmware_revision,{[],[]}},
         {avp,{[],[]}}]},
  {port,[{owner,&lt;0.62.0>},
         {module,diameter_tcp},
         {socket,{{127,0,0,1},3868}},
         {peer,{{127,0,0,1},48758}},
         {statistics,[{recv_oct,10124},
                      {recv_cnt,132},
                      {recv_max,184},
                      {recv_avg,76},
                      {recv_dvi,9},
                      {send_oct,10016},
                      {send_cnt,132},
                      {send_max,148},
                      {send_avg,75},
                      {send_pend,0}]}]},
  {statistics,[{{{0,280,0},recv},62},
               {{{0,280,1},send},62},
               {{{0,258,0},send,{'Result-Code',2001}},3},
               {{{0,258,1},recv},3},
               {{{0,258,0},send},3},
               {{{0,280,1},recv},66},
               {{{0,280,0},send},66},
               {{{0,257,1},recv},1},
               {{{0,257,0},send},1}]}]]
</code>

<p>
Note that there may be multiple entries with the same <c>ref</c>, in
contrast to <c>transport</c> info.</p>
</item>

<tag><c>statistics</c></tag>
<item>
<p>
Return a <c>{{Counter, Ref}, non_neg_integer()}</c> list of counter values.
<c>Ref</c> can be either a <c><seealso
marker="#transport_ref">transport_ref()</seealso></c>
or a <c><seealso
marker="diameter_app#peer_ref">diameter_app:peer_ref()</seealso></c>.
Entries for the latter are folded into corresponding entries for the
former as peer connections go down.
Entries for both are removed at <seealso
marker="#remove_transport">remove_transport/2</seealso>.
The Diameter-level statistics returned by <c>transport</c> and
<c>connections</c> info are based upon these entries.</p>
</item>

<tag><c><seealso marker="diameter_app#peer_ref">diameter_app:peer_ref()</seealso></c></tag>
<item>
<p>
Return transport configuration associated with a single peer, as
passed to <seealso marker="#add_transport">add_transport/2</seealso>.
The returned list is empty if the peer is unknown.
Otherwise it contains the <c>ref</c>, <c>type</c> and <c>options</c>
tuples as in <c>transport</c> and <c>connections</c> info above.
For example:</p>

<code>
[{ref,#Ref&lt;0.0.0.61>},
 {type,accept},
 {options,[{transport_module,diameter_tcp},
           {transport_config,[{reuseaddr,true},
                              {ip,{127,0,0,1}},
                              {port,3868}]}]}]
</code>
</item>

</taglist>

<marker id="services"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>services() -> [SvcName]</name>
<fsummary>Return the list of started services.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></v>
</type>
<desc>
<p>
Return the list of started services.</p>

<marker id="session_id"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>session_id(Ident) -> <seealso marker="diameter_dict#DATA_TYPES">OctetString()</seealso></name>
<fsummary>Return a value for a Session-Id AVP.</fsummary>
<type>
<v>Ident = <seealso marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso></v>
</type>
<desc>
<p>
Return a value for a Session-Id AVP.</p>

<p>
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"/>
</desc>
</func>

<!-- ===================================================================== -->
<func>
<name>start() -> ok | {error, Reason}</name>
<fsummary>Start the diameter application.</fsummary>
<desc>
<p>
Start the diameter application.</p>

<p>
The diameter application must be started before starting a service.
In a production system this is typically accomplished by a boot
file, not by calling <c>start/0</c> explicitly.</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 = <seealso marker="#service_name">service_name()</seealso></v>
<v>Options = [<seealso marker="#service_opt">service_opt()</seealso>]</v>
<v>Reason  = term()</v>
</type>
<desc>
<p>
Start a diameter service.</p>

<p>
A service defines a locally-implemented Diameter node, specifying the
capabilities to be advertised during capabilities exchange.
Transports are added to a service using <seealso
marker="#add_transport">add_transport/2</seealso>.
</p>

<note>
<p>
A transport can both override its service's
capabilities and restrict its supported Diameter applications so
"service = Diameter node as identified by Origin-Host" is not
necessarily the case.</p>
</note>

<marker id="stop_service"/>
</desc>
</func>

<!-- ===================================================================== -->
<func>
<name>stop() -> ok | {error, Reason}</name>
<fsummary>Stop the diameter application.</fsummary>
<desc>
<p>
Stop the diameter application.</p>

<p>
</p>

<marker id="stop_service"/>
</desc>
</func>

<!-- ===================================================================== -->
<func>
<name>stop_service(SvcName) -> ok | {error, Reason}</name>
<fsummary>Stop a Diameter service.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></v>
<v>Reason = term()</v>
</type>
<desc>
<p>
Stop a diameter service.</p>

<p>
Stopping a service causes all associated transport connections to be
broken.
A DPR message with be sent as in the case of <seealso
marker="#remove_transport">remove_transport/2</seealso>.</p>

<note>
<p>
Stopping a transport does not remove any associated transports:
<seealso marker="#remove_transport">remove_transport/2</seealso> must
be called to remove transport configuration.</p>
</note>

<marker id="subscribe"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>subscribe(SvcName) -> true</name>
<fsummary>Subscribe to event messages.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></v>
</type>
<desc>
<p>
Subscribe to <c><seealso
marker="#service_event">service_event()</seealso></c> messages from
a service.</p>

<p>
It is not an error to subscribe to events from a service
that does not yet exist.
Doing so before adding transports is required to guarantee the
reception of all related events.</p>

<marker id="unsubscribe"/>
</desc>
</func>

<!-- ===================================================================== -->

<func>
<name>unsubscribe(SvcName) -> true</name>
<fsummary>Unsubscribe to event messages.</fsummary>
<type>
<v>SvcName = <seealso marker="#service_name">service_name()</seealso></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>