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       diameter - Main API of the diameter application.


       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 6733.

       Basic usage consists of creating a representation of a locally implemented  Diameter  node
       and   its   capabilities   with   start_service/2,   adding   transport  capability  using
       add_transport/2 and sending Diameter requests and receiving Diameter answers with  call/4.
       Incoming  Diameter requests are communicated as callbacks to a diameter_app(3erl) callback
       modules as specified in the service configuration.

       Beware the difference between diameter (not capitalized) and Diameter  (capitalized).  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 6733.

       The diameter application must be started before calling most functions in this module.








           Types corresponding to RFC 6733 AVP Data Formats. Defined in diameter_dict(5).

         application_alias() = term():
           A name identifying a Diameter application in service configuration. Passed  to  call/4
           when sending requests defined by the application.

         application_module() = Mod | [Mod | ExtraArgs] | #diameter_callback{}:

         Mod = atom()
         ExtraArgs = list()

           A module implementing the callback interface defined in diameter_app(3erl), along with
           any extra arguments to be appended to those  documented.  Note  that  extra  arguments
           specific  to  an  outgoing request can be specified to call/4, in which case those are
           are appended to any module-specific extra arguments.

           Specifying a #diameter_callback{} record allows individual functions to be  configured
           in  place  of  the  usual  diameter_app(3erl) callbacks. See diameter_callback.erl for

           Options defining a Diameter application. Has one the following types.

           {alias, application_alias()}:
             A unique identifier for the application in the scope of the service. Defaults to the
             value of the dictionary option if unspecified.

           {dictionary, atom()}:
             The  name  of  an  encode/decode  module  for  the  Diameter messages defined by the
             application. These modules are generated from files whose format  is  documented  in

           {module, application_module()}:
             The callback module with which messages of the Diameter application are handled. See
             diameter_app(3erl) for the required interface and semantics.

           {state, term()}:
             The  initial  callback   state.   The   prevailing   state   is   passed   to   some
             diameter_app(3erl)  callbacks,  which  can  then return a new state. Defaults to the
             value of the alias option if unspecified.

           {call_mutates_state, true|false}:
             Specifies whether or  not  the  pick_peer/4  application  callback  can  modify  the
             application state. Defaults to false if unspecified.

           pick_peer/4  callbacks  are  serialized when this option is true, 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.

           {answer_errors, callback|report|discard}:
             Determines the manner in which incoming answer messages containing decode errors are

             If  callback then errors result in a handle_answer/4 callback in the same fashion as
             for  handle_request/3,  with  errors  communicated  in  the  errors  field  of   the
             #diameter_packet{}  passed  to  the  callback.  If  report then an answer containing
             errors is discarded without a callback and a warning report is written to  the  log.
             If  discard  then  an  answer  containing  errors  is  silently  discarded without a
             callback. In both the report and discard cases  the  return  value  for  the  call/4
             invocation  in  question  is  as  if a callback had taken place and returned {error,

             Defaults to report if unspecified.

           {request_errors, answer_3xxx|answer|callback}:
             Determines the manner in which incoming requests are handled  when  an  error  other
             than  3007  (DIAMETER_APPLICATION_UNSUPPORTED,  which  cannot  be associated with an
             application callback module), is detected.

             If answer_3xxx then requests are answered without a handle_request/3 callback taking
             place.  If  answer  then even 5xxx errors are answered without a callback unless the
             connection in question has configured the RFC 3588 common dictionary as noted below.
             If callback then a handle_request/3 callback always takes place and its return value
             determines the answer sent to the peer, if any.

             Defaults to answer_3xxx if unspecified.

           Answers sent by diameter set the E-bit in the Diameter Header. Since RFC  3588  allows
           only  3xxx  result  codes  in  an  answer-message,  answer  has  the same semantics as
           answer_3xxx   when   the   transport   in   question   has   been   configured    with
           diameter_gen_base_rfc3588  as  its  common dictionary. Since RFC 6733 allows both 3xxx
           and 5xxx result codes in an answer-message, a transport with diameter_gen_base_rfc6733
           as its common dictionary does distinguish between answer_3xxx and answer.

           Options  available to call/4 when sending an outgoing Diameter request. Has one of the
           following types.

           {extra, list()}:
             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.

           {filter, peer_filter()}:
             A filter to apply  to  the  list  of  available  peers  before  passing  it  to  the
             pick_peer/4   callback  for  the  application  in  question.  Multiple  options  are
             equivalent a single all filter on the corresponding list  of  filters.  Defaults  to

           {timeout, Unsigned32()}:
             The number of milliseconds after which the request should timeout. Defaults to 5000.

             Causes  call/4  to  return  ok  as soon as the request in question has been encoded,
             instead of waiting for and returning the result from a subsequent handle_answer/4 or
             handle_error/4 callback.

           An invalid option will cause call/4 to fail.

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

           {'Origin-Host', DiameterIdentity()}:

           {'Origin-Realm', DiameterIdentity()}:

           {'Host-IP-Address', [Address()]}:
             An  address list is available to the start function of a transport module, which can
             return a new list for use in the subsequent CER or CEA. Host-IP-Address need not  be
             specified  if  the  transport  module  in  question  communicates an address list as
             described in diameter_transport(3erl)

           {'Vendor-Id', Unsigned32()}:

           {'Product-Name', UTF8String()}:

           {'Origin-State-Id', Unsigned32()}:
             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 0 (zero) is
             equivalent to not  setting  a  value,  as  documented  in  RFC  6733.  The  function
             origin_state_id/0  can  be  used  as  to  retrieve a value that is computed when the
             diameter application is started.

           {'Supported-Vendor-Id', [Unsigned32()]}:

           {'Auth-Application-Id', [Unsigned32()]}:

           {'Inband-Security-Id', [Unsigned32()]}:
             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.

           {'Acct-Application-Id', [Unsigned32()]}:

           {'Vendor-Specific-Application-Id', [Grouped()]}:

           {'Firmware-Revision', Unsigned32()}:

           Note that each tuple communicates one or more AVP values. It is an  error  to  specify
           duplicate tuples.

         evaluable() = {M,F,A} | fun() | [evaluable() | A]:
           An expression that can be evaluated as a function in the following sense.

         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) ->

           Applying an evaluable() E to an argument list A is meant in the sense of eval([E|A]).

         Beware  of  using  fun expressions of the form fun Name/Arity 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 start_service/2 or add_transport/2.

         peer_filter() = term():
           A filter passed to call/4 in  order  to  select  candidate  peers  for  a  pick_peer/4
           callback. Has one of the following types.

             Matches  any  peer.  This  is  a convenience that provides a filter equivalent to no

             Matches only those peers whose Origin-Host has the same value as Destination-Host in
             the  outgoing  request  in  question,  or any peer if the request does not contain a
             Destination-Host AVP.

             Matches only those peers whose Origin-Realm has the same value as  Destination-Realm
             in  the  outgoing request in question, or any peer if the request does not contain a
             Destination-Realm AVP.

           {host, any|DiameterIdentity()}:
             Matches only those peers whose Origin-Host has the specified value, or all peers  if
             the atom any.

           {realm, any|DiameterIdentity():
             Matches only those peers whose Origin-Realm has the specified value, or all peers if
             the atom any.

           {eval, evaluable()}:
             Matches only those peers for which  the  specified  evaluable()  returns  true  when
             applied  to  the  connection's  diameter_caps  record.  Any  other  return  value or
             exception is equivalent to false.

           {neg, peer_filter()}:
             Matches only those peers not matched by the specified filter.

           {all, [peer_filter()]}:
             Matches only those peers matched by each filter in the specified list.

           {any, [peer_filter()]}:
             Matches only those peers matched by at least one filter in the specified list.

           An invalid filter is equivalent to {any,[]}, a filter that matches no peer.

         The host and realm filters cause the Destination-Host and Destination-Realm AVPs  to  be
         extracted  from  the  outgoing  request,  assuming  it  to  be  a record- or list-valued
         diameter_codec:message(), and assuming at most one of each AVP. If this is not the  case
         then  the  {host|realm,  DiameterIdentity()} filters must be used to achieve the desired
         result.  An  empty  DiameterIdentity()  (which  should  not  be  typical)  matches   all
         hosts/realms for the purposes of filtering.

         A  host  filter  is  not typically desirable when setting Destination-Host since it will
         remove peer agents from the candidates list.

         service_event()    =    #diameter_event{service     =     service_name(),     info     =
           An event message sent to processes that have subscribed to these using subscribe/1.

         service_event_info() = term():
           The info field of a service_event() record. Can have one of the following types.


             The  service  is being started or stopped. No event precedes a start event. No event
             follows a stop event, and this  event  implies  the  termination  of  all  transport

           {up, Ref, Peer, Config, Pkt}:

           {up, Ref, Peer, Config}:

           {down, Ref, Peer, Config}:

           Ref    = transport_ref()
           Peer   = diameter_app:peer()
           Config = {connect|listen, [transport_opt()]}
           Pkt    = #diameter_packet{}

             The  RFC 3539 watchdog state machine has transitioned into (up) or out of (down) the
             OKAY state. If a #diameter_packet{} is present in an up event then there has been  a
             capabilities  exchange  on  a  newly established transport connection and the record
             contains the received CER or CEA.

             Note that a single up or down  event  for  a  given  peer  corresponds  to  multiple
             peer_up/3  or  peer_up/3  callbacks,  one  for  each  of  the  Diameter applications
             negotiated  during  capabilities  exchange.  That   is,   the   event   communicates
             connectivity  with  the peer as a whole while the callbacks communicate connectivity
             with respect to individual Diameter applications.

           {reconnect, Ref, Opts}:

           Ref  = transport_ref()
           Opts = [transport_opt()]

             A connecting transport is attempting to establish/reestablish a transport connection
             with a peer following connect_timer or watchdog_timer expiry.

           {closed, Ref, Reason, Config}:

           Ref = transport_ref()
           Config = {connect|listen, [transport_opt()]}

             Capabilities exchange has failed. Reason can have one of the following types.

             {'CER', Result, Caps, Pkt}:

             Result = ResultCode | {capabilities_cb, CB, ResultCode|discard}
             Caps = #diameter_caps{}
             Pkt  = #diameter_packet{}
             ResultCode = integer()
             CB = evaluable()

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

             {'CER', Caps, {ResultCode, Pkt}}:

             ResultCode = integer()
             Caps = #diameter_caps{}
             Pkt  = #diameter_packet{}

               An incoming CER contained errors and has been answered with the  indicated  result
               code.  Caps  contains  values  for  the  local  node only. Pkt contains the CER in

             {'CER', timeout}:
               An expected CER was not received within capx_timeout of connection establishment.

             {'CEA', Result, Caps, Pkt}:

             Result = integer() | atom() | {capabilities_cb, CB, ResultCode|discard}
             Caps = #diameter_caps{}
             Pkt  = #diameter_packet{}
             ResultCode = integer()

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

             {'CEA', Caps, Pkt}:

             Caps = #diameter_caps{}
             Pkt  = #diameter_packet{}

               An incoming CEA contained errors and has been rejected. Caps contains only  values
               for the local node. Pkt contains the CEA in question.

             {'CEA', timeout}:
               An expected CEA was not received within capx_timeout of connection establishment.

           {watchdog, Ref, PeerRef, {From, To}, Config}:

           Ref = transport_ref()
           PeerRef = diameter_app:peer_ref()
           From, To = initial | okay | suspect | down | reopen
           Config = {connect|listen, [transport_opt()]}

             An RFC 3539 watchdog state machine has changed state.

             For forward compatibility, a subscriber should be prepared to receive info fields of
             forms other than the above.

         service_name() = term():
           The name of a service as passed to start_service/2  and  with  which  the  service  is
           identified.  There  can be at most one service with a given name on a given node. Note
           that erlang:make_ref/0 can be used to generate a service name that is somewhat unique.

           An option passed to start_service/2. Can be any capability() as well as the following.

           {application, [application_opt()]}:
             Defines a Diameter application supported by the service.

             A service must configure one tuple for  each  Diameter  application  it  intends  to
             support.  For  an  outgoing  request,  the relevant application_alias() is passed to
             call/4, while for an incoming request the  application  identifier  in  the  message
             header   determines   the   application,  the  identifier  being  specified  in  the
             application's dictionary file.

           The capabilities advertised by a node  must  match  its  configured  applications.  In
           particular,  application  configuration  must be matched by corresponding capability()
           configuration, of *-Application-Id AVPs in particular.

           {restrict_connections, false | node | nodes | [node()] | evaluable()}:
             Specifies the degree to which the service allows multiple transport  connections  to
             the same peer, as identified by its Origin-Host at capabilities exchange.

             If  [node()]  then  a connection is rejected if another already exists on any of the
             specified nodes. Types false, node, nodes and  evaluable()  are  equivalent  to  [],
             [node()],  [node()|nodes()] and the evaluated value respectively, evaluation of each
             expression taking place whenever a new connection is to be  established.  Note  that
             false  allows  an  unlimited  number  of connections to be established with the same

             Multiple connections are independent and governed by their  own  peer  and  watchdog
             state machines.

             Defaults to nodes.

           {sequence, {H,N} | evaluable()}:
             Specifies  a  constant value H for the topmost 32-N bits of of 32-bit End-to-End and
             Hop-by-Hop Identifiers generated by the service, either explicitly or  as  a  return
             value of a function to be evaluated at start_service/2. In particular, an identifier
             Id is mapped to a new identifier as follows.

           (H bsl N) bor (Id band ((1 bsl N) - 1))

             Note that RFC 6733 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
             appropriate values of N: at a rate of  R  requests  per  second,  an  N-bit  counter
             traverses  all of its values in (1 bsl N) div (R*60) minutes, so the bound is 4*R*60
             =< 1 bsl N.

             N must lie in the range 0..32 and H must be a non-negative integer less than  1  bsl

             Defaults to {0,32}.

           Multiple  Erlang  nodes  implementing the same Diameter node should be configured with
           different sequence masks to ensure that each node uses a unique  range  of  End-to-End
           and Hop-by-Hop Identifiers for outgoing requests.

           {share_peers, boolean() | [node()] | evaluable()}:
             Specifies  nodes  to which peer connections established on the local Erlang node are
             communicated. Shared peers become available in the remote candidates list passed  to
             pick_peer/4 callbacks on remote nodes whose services are configured to use them: see
             use_shared_peers below.

             If false then peers are not shared. If [node()]  then  peers  are  shared  with  the
             specified  list  of  nodes.  If  evaluable()  then  peers  are shared with the nodes
             returned by the specified function, evaluated whenever  a  peer  connection  becomes
             available  or  a  remote  service  requests information about local connections. The
             value true is equivalent to fun erlang:nodes/0.  The  value  node()  in  a  list  is
             ignored,  so  a  collection of services can all be configured to share with the same
             list of nodes.

             Defaults to false.

           Peers are only shared with services of the  same  name  for  the  purpose  of  sending
           outgoing  requests.  Since the value of the application_opt() alias, passed to call/4,
           is the handle for identifying a peer as a  suitable  candidate,  services  that  share
           peers  must use the same aliases to identify their supported applications. They should
           typically also configure identical capabilities(), since by sharing  peer  connections
           they  are  distributing  the  implementation of a single Diameter node across multiple
           Erlang nodes.

           {spawn_opt, [term()]}:
             An options list passed to erlang:spawn_opt/2 when spawning a process for an incoming
             Diameter  request, unless the transport in question specifies another value. Options
             monitor and link are ignored.

             Defaults to the empty list.

           {use_shared_peers, boolean() | [node()] | evaluable()}:
             Specifies nodes from which communicated peers  are  made  available  in  the  remote
             candidates list of pick_peer/4 callbacks.

             If  false  then  remote  peers  are  not  used. If [node()] then only peers from the
             specified list of nodes are used. If evaluable() then only  peers  returned  by  the
             specified  function  are  used,  evaluated  whenever  a  remote service communicates
             information about an available peer connection. The value true is equivalent to  fun
             erlang:nodes/0. The value node() in a list is ignored.

             Defaults to false.

           A service that does not use shared peers will always pass the empty list as the second
           argument of pick_peer/4 callbacks.

           Sending a request over a peer connection on a  remote  node  is  less  efficient  than
           sending  it  over  a  local  connection.  It  may  be  preferable  to  make use of the
           service_opt() restrict_connections and maintain a dedicated connection  on  each  node
           from which requests are sent.

           An option passed to add_transport/2. Has one of the following types.

           {applications, [application_alias()]}:
             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.

           The  capabilities  advertised  by  a  node  must match its configured applications. In
           particular, setting applications on  a  transport  typically  implies  having  to  set
           matching *-Application-Id AVPs in a capabilities() tuple.

           {capabilities, [capability()]}:
             AVPs  used  to  construct outgoing CER/CEA messages. Values take precedence over any
             specified on the service in question.

             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

           {capabilities_cb, evaluable()}:
             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
             transport_ref() and #diameter_caps{} record of the connection.

             The return value can have one of the following types.

               Accept the connection.

               Causes an incoming CER to be answered with the specified Result-Code.

               Causes an incoming CER to be discarded without CEA being sent.

               Equivalent to returning 3010, DIAMETER_UNKNOWN_PEER.

             Returning anything but ok  or  a  2xxx  series  result  code  causes  the  transport
             connection to be broken. Multiple capabilities_cb options can be specified, in which
             case the corresponding callbacks are applied until either all return ok or one  does

           {capx_timeout, Unsigned32()}:
             The  number  of  milliseconds  after which a transport process having an established
             transport connection will  be  terminated  if  the  expected  capabilities  exchange
             message  (CER or CEA) is not received from the peer. For a connecting transport, the
             timing of connection attempts is governed by connect_timer or watchdog_timer expiry.
             For a listening transport, the peer determines the timing.

             Defaults to 10000.

           {connect_timer, Tc}:

           Tc = Unsigned32()

             For  a  connecting  transport,  the  RFC  6733 Tc timer, in milliseconds. This timer
             determines the frequency with which a transport attempts  to  establish  an  initial
             connection  with  its  peer  following  transport  configuration.  Once  an  initial
             connection  has  been  established,  watchdog_timer  determines  the  frequency   of
             reconnection attempts, as required by RFC 3539.

             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 reestablishment, causing the RFC 3539 state machine
             to pass to state OKAY rather than REOPEN. Note that these semantics are not governed
             by the RFC and that a listening transport's connect_timer should be greater than its
             peer's Tw plus jitter.

             Defaults to 30000 for a connecting transport and 60000 for a listening transport.

           {disconnect_cb, evaluable()}:
             A callback invoked prior  to  terminating  the  transport  process  of  a  transport
             connection  having watchdog state OKAY. Applied to application|service|transport and
             the transport_ref() and diameter_app:peer() in question: application indicates  that
             the  diameter  application is being stopped, service that the service in question is
             being stopped by stop_service/1, and transport that the  transport  in  question  is
             being removed by remove_transport/2.

             The return value can have one of the following types.

             {dpr, [option()]}:
               Causes Disconnect-Peer-Request to be sent to the peer, the transport process being
               terminated following reception of Disconnect-Peer-Answer or timeout.  An  option()
               can be one of the following.

               {cause, 0|rebooting|1|busy|2|goaway}:
                 The  Disconnect-Cause  to  send,  REBOOTING, BUSY and DO_NOT_WANT_TO_TALK_TO_YOU
                 respectively. Defaults to rebooting for  Reason=service|application  and  goaway
                 for Reason=transport.

               {timeout, Unsigned32()}:
                 The  number  of  milliseconds after which the transport process is terminated if
                 DPA has not been received. Defaults to 1000.

               Equivalent to {dpr, []}.

               Causes the transport process  to  be  terminated  without  Disconnect-Peer-Request
               being sent to the peer.

               Equivalent to not having configured the callback.

             Multiple  disconnect_cb  options  can  be specified, in which case the corresponding
             callbacks are applied until one of them returns  a  value  other  than  ignore.  All
             callbacks returning ignore is equivalent to not having configured them.

             Defaults to a single callback returning dpr.

           {length_errors, exit|handle|discard}:
             Specifies how to deal with errors in the Message Length field of the Diameter Header
             in an incoming message. An error in this context is that the length is not at  least
             20 bytes (the length of a Header), is not a multiple of 4 (a valid length) or is not
             the length of the message in question, as  received  over  the  transport  interface
             documented in diameter_transport(3erl).

             If  exit then a warning report is emitted and the parent of the transport process in
             question exits, which causes the transport process itself to exit  as  described  in
             diameter_transport(3erl).  If  handle  then  the  message  is  processed as usual, a
             resulting  handle_request/3  or  handle_answer/4  callback  (if  one  takes   place)
             indicating  the  5015  error  (DIAMETER_INVALID_MESSAGE_LENGTH). If discard then the
             message in question is silently discarded.

             Defaults to exit.

           The default value reflects the fact that a  transport  module  for  a  stream-oriented
           transport like TCP may not be able to recover from a message length error since such a
           transport must use the Message Length header to divide the incoming byte  stream  into
           individual  Diameter  messages.  An  invalid  length leaves it with no reliable way to
           rediscover message boundaries, which may result in the failure of subsequent messages.
           See diameter_tcp(3erl) for the behaviour of that module.

           {spawn_opt, [term()]}:
             An options list passed to erlang:spawn_opt/2 when spawning a process for an incoming
             Diameter request. Options monitor and link are ignored.

             Defaults to the list configured on the service if not specified.

           {transport_config, term()}:

           {transport_config, term(), Unsigned32() | infinity}:
             A term passed as the  third  argument  to  the  start/3  function  of  the  relevant
             transport  module  in order to start a transport process. Defaults to the empty list
             if unspecified.

             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.

           {transport_module, diameter_sctp}
           {transport_config, SctpOpts, 5000}
           {transport_module, diameter_tcp}
           {transport_config, TcpOpts}

             To listen on both SCTP and TCP, define one transport for each.

           {transport_module, atom()}:
             A  module  implementing  a transport process as defined in diameter_transport(3erl).
             Defaults to diameter_tcp if unspecified.

             Multiple transport_module and transport_config options are  allowed.  The  order  of
             these  is significant in this case (and only in this case), a transport_module being
             paired with the first transport_config 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.

           {watchdog_config, [{okay|suspect, non_neg_integer()}]}:
             Specifies  configuration that alters the behaviour of the watchdog state machine. On
             key okay, the non-negative number of answered DWR messages before transitioning from
             REOPEN to OKAY. On key suspect, the number of watchdog timeouts before transitioning
             from OKAY to SUSPECT when DWR is unanswered, or 0 to not make the transition.

             Defaults to [{okay, 3}, {suspect,  1}].  Not  specifying  a  key  is  equivalent  to
             specifying the default value for that key.

           The  default  value  is  as  required by RFC 3539: changing it results in non-standard
           behaviour that should only be used to simulate misbehaving nodes during test.

           {watchdog_timer, TwInit}:

           TwInit = Unsigned32()
                  | {M,F,A}

             The RFC 3539 watchdog timer. An integer value is interpreted as the RFC's TwInit  in
             milliseconds, a jitter of +/- 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.

             An integer value must be at least 6000 as required by RFC 3539. Defaults to 30000 if

           Unrecognized  options  are  silently  ignored   but   are   returned   unmodified   by
           service_info/2   and   can   be   referred   to   in  predicate  functions  passed  to

         transport_ref() = reference():
           An reference returned by add_transport/2 that identifies the configuration.


       add_transport(SvcName, {connect|listen, [Opt]}) -> {ok, Ref} | {error, Reason}


                 SvcName = service_name()
                 Opt = transport_opt()
                 Ref = transport_ref()
                 Reason = term()

              Add transport capability to a service.

              The service will start transport processes as required  in  order  to  establish  a
              connection  with  the  peer,  either  by  connecting  to  the  peer (connect) or by
              accepting incoming connection requests (listen). A connecting transport establishes
              transport  connections  with  at  most one peer, an listening transport potentially
              with many.

              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  peer_up/3  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.

              The  returned  reference  uniquely identifies the transport within the scope of the
              service. Note that the function returns before  a  transport  connection  has  been

              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.

       call(SvcName, App, Request, [Opt]) -> Answer | ok | {error, Reason}


                 SvcName = service_name()
                 App = application_alias()
                 Request = diameter_codec:message()
                 Answer = term()
                 Opt = call_opt()

              Send a Diameter request message.

              App specifies the  Diameter  application  in  which  the  request  is  defined  and
              callbacks  to  the corresponding callback module will follow as described below and
              in diameter_app(3erl). Unless the detach 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 handle_answer/4 callback. In  the
              error  case,  whether  or  not the error is returned directly by diameter or from a
              handle_error/4  callback  depends  on  whether  or  not  the  outgoing  request  is
              successfully  encoded  for  transmission  to  the  peer, the cases being documented

              If there are no suitable peers, or if pick_peer/4 rejects them by returning  false,
              then  {error,no_connection}  is  returned.  Otherwise  pick_peer/4 is followed by a
              prepare_request/3 callback, the message is encoded and then sent.

              There are several error cases which may prevent an answer from being  received  and
              passed to a handle_answer/4 callback:

                * If  the  initial encode of the outgoing request fails, then the request process
                  fails and {error,encode} is returned.

                * If the request is successfully encoded and sent but the answer times out then a
                  handle_error/4 callback takes place with Reason = timeout.

                * If  the request is successfully encoded and sent but the service in question is
                  stopped before an answer is received then a handle_error/4 callback takes place
                  with Reason = cancel.

                * 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 pick_peer/4 callback rejects the candidates, then  a  handle_error/4
                  callback  takes  place  with  Reason  =  failover. If a peer is selected then a
                  prepare_retransmit/3 callback takes place, after which the  semantics  are  the
                  same as following an initial prepare_request/3 callback.

                * If  an  encode error takes place during retransmission then the request process
                  fails and {error,failure} is returned.

                * If an application callback made in processing  the  request  fails  (pick_peer,
                  prepare_request, prepare_retransmit, handle_answer or handle_error) then either
                  {error,encode} or {error,failure} is returned depending on whether or not there
                  has been an attempt to send the request over the transport.

              Note that {error,encode} is the only return value which guarantees that the request
              has not been sent over the transport connection.

       origin_state_id() -> Unsigned32()

              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 Diameter Time(), at the time the diameter application was

       remove_transport(SvcName, Pred) -> ok | {error, Reason}


                 SvcName = service_name()
                 Pred = Fun | MFA | transport_ref() | list() | true | false

                 Fun = fun((transport_ref(), connect|listen, list()) -> boolean())
                  | fun((transport_ref(), list()) -> boolean())
                  | fun((list()) -> boolean())
                 MFA = {atom(), atom(), list()}
                 Reason = term()

              Remove previously added transports.

              Pred determines which transports to remove.  An  arity-3-valued  Pred  removes  all
              transports for which Pred(Ref, Type, Opts) returns true, where Type and Opts are as
              passed to add_transport/2 and Ref is as returned by it.  The  remaining  forms  are
              equivalent to an arity-3 fun as follows.

              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

              Removing a transport causes the corresponding transport processes to be terminated.
              Whether or not a DPR  message  is  sent  to  a  peer  is  controlled  by  value  of
              disconnect_cb configured on the transport.

       service_info(SvcName, Info) -> term()


                 SvcName = service_name()
                 Info = Item | [Info]
                 Item = atom()

              Return  information about a started service. Requesting info for an unknown service
              causes undefined to be returned. Requesting a list of items causes a tagged list to
              be returned.

              Item can be one of the following.












                  Return a capability value as configured with start_service/2.

                  Return the list of applications as configured with start_service/2.

                  Return   a   tagged   list  of  all  capabilities  values  as  configured  with

                  Return a list containing one entry for  each  of  the  service's  transport  as
                  configured  with  add_transport/2.  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 ""
                  configured with a single transport connected to "" might look
                  as follows.


                  Here  ref  is  a  transport_ref() and options the corresponding transport_opt()
                  list passed to add_transport/2.  The  watchdog  entry  shows  the  state  of  a
                  connection's  RFC  3539  watchdog  state machine. The peer entry identifies the
                  diameter_app:peer_ref() for which there will have been peer_up/3 callbacks  for
                  the  Diameter  applications  identified  by  the  apps  entry, common being the
                  application_alias(). The caps entry identifies the  capabilities  sent  by  the
                  local  node  and  received from the peer during capabilities exchange. The port
                  entry displays socket-level information about  the  transport  connection.  The
                  statistics    entry   presents   Diameter-level   counters,   an   entry   like
                  {{{0,280,1},recv},2} saying that  the  client  has  received  2  DWR  messages:
                  {0,280,1} = {Application_Id, Command_Code, R_Flag}.

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

                  A listening transport presents its information slightly differently since there
                  may  be  multiple  accepted  connections  for  the  same  transport_ref().  The
                  transport  info returned by a server with a single client connection might look
                  as follows.


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

                  Return  a  list containing one entry for every established transport connection
                  whose watchdog state machine is not in the down state. This is a flat  view  of
                  transport 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.


                  Note  that  there  may  be  multiple  entries with the same ref, in contrast to
                  transport info.

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

                  Return  transport  configuration  associated  with  a single peer, as passed to
                  add_transport/2. The returned list is empty if the peer is  unknown.  Otherwise
                  it  contains  the  ref, type and options tuples as in transport and connections
                  info above. For example:


       services() -> [SvcName]


                 SvcName = service_name()

              Return the list of started services.

       session_id(Ident) -> OctetString()


                 Ident = DiameterIdentity()

              Return a value for a Session-Id AVP.

              The value has the form required by section 8.8 of RFC 6733.  Ident  should  be  the
              Origin-Host  of  the peer from which the message containing the returned value will
              be sent.

       start() -> ok | {error, Reason}

              Start the diameter application.

              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 start/0

       start_service(SvcName, Options) -> ok | {error, Reason}


                 SvcName = service_name()
                 Options = [service_opt()]
                 Reason = term()

              Start a diameter service.

              A service defines a locally-implemented Diameter node, specifying the  capabilities
              to  be  advertised  during capabilities exchange. Transports are added to a service
              using add_transport/2.

              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.

       stop() -> ok | {error, Reason}

              Stop the diameter application.

       stop_service(SvcName) -> ok | {error, Reason}


                 SvcName = service_name()
                 Reason = term()

              Stop a diameter service.

              Stopping a service causes all associated transport connections to be broken. A  DPR
              message with be sent as in the case of remove_transport/2.

              Stopping  a  service  does not remove any associated transports: remove_transport/2
              must be called to remove transport configuration.

       subscribe(SvcName) -> true


                 SvcName = service_name()

              Subscribe to service_event() messages from a service.

              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
              transport-related events.

       unsubscribe(SvcName) -> true


                 SvcName = service_name()

              Unsubscribe to event messages from a service.


       diameter_app(3erl), diameter_transport(3erl), diameter_dict(5)