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       diameter - Main API of the diameter application.


       This  module  provides  the  interface  with which a user creates a service that sends and
       receives messages using the Diameter protocol as defined in RFC 3588.

       Basic usage consists of creating a representation of a locally implemented  Diameter  peer
       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 application and Diameter  application.  The  former
       refers to the Erlang application named diameter whose main api is defined here, the latter
       to an application of the Diameter protocol in the  sense  of  RFC  3588.  More  generally,
       capitalized Diameter refers to the RFC and diameter to this implementation.

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






           Types corresponding to RFC 3588 AVP Data Formats. Defined in diameter_dict(5).

         application_alias() = term():
           A  name  identifying  a  Diameter  application  in  service  configuration  passed  to
           start_service/2  and  passed  to  call/4  when  sending  requests  belonging  to   the

         application_module() = Mod | [Mod | ExtraArgs]:

         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 for the  interface.  Any  extra
           arguments  are  appended  to  the documented list of arguments for each function. Note
           that additional arguments specific to an outgoing request be specified to  call/4,  in
           which case the call-specific arguments are appended to any specified with the callback

           Options defining a Diameter application as configured in an application option  passed
           to start_service/2.

           {alias, application_alias()}:
             An  unique  identifier  for  the  application in the scope of the service. Optional,
             defaults to the value of the dictionary option.

           {dictionary, atom()}:
             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 diameter_dict(5).

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

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

           {call_mutates_state, true|false}:
             Specifies whether or not the pick_peer/4 application callback (following from a call
             to call/4) can modifiy state, Defaults to false if unspecified.

             Note that pick_peer callbacks are serialized when these are allowed to modify state,
             which  is a potential performance bottleneck. A simple Diameter client may suffer no
             ill effects from using mutable state but a server or agent that responds to incoming
             request but sending its own requests should probably avoid it.

           {answer_errors, callback|report|discard}:
             Determines the manner in which incoming answer messages containing decode errors are
             handled. If callback then errors result in a handle_answer/4 callback  in  the  same
             fashion  as  for handle_request/3, in the errors field of the diameter_packet record
             passed into 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, failure}.

             Defaults to report if unspecified.

           Options available to call/4 when sending an outgoing Diameter request.

           {extra, list()}:
             Extra arguments to append to callbacks to the callback module in question. These are
             appended  to  any  extra  arguments  configured  with  the callback itself. Multiple
             options append to the argument list.

           {filter, peer_filter()}:
             A filter to apply to the  list  of  available  peers  before  passing  them  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  used  in  outgoing  CER/CEA messages during capabilities exchange. Can be
           configured both on a service and a transport, the latter taking  precedence  over  the

           {'Origin-Host', DiameterIdentity()}:
             Value of the Origin-Host AVP in outgoing messages.

           {'Origin-Realm', DiameterIdentity()}:
             Value of the Origin-Realm AVP in outgoing messages.

           {'Host-IP-Address', [Address()]}:
             Values of Host-IP-Address AVPs. Optional.

             The  list  of  addresses  is  available to the start function of a transport module,
             which either uses them as is or returns a  new  list  (typically  as  configured  as
             transport_config()  on  the  transport  module in question) in order for the correct
             list of addresses to be sent in capabilities exchange messages.

           {'Vendor-Id', Unsigned32()}:
             Value of the Vendor-Id AVP sent in an outgoing capabilities exchange message.

           {'Product-Name', UTF8String()}:
             Value of the Product-Name AVP sent in an outgoing capabilities exchange message.

           {'Origin-State-Id', Unsigned32()}:
             Value of Origin-State-Id to be  included  in  outgoing  messages  sent  by  diameter
             itself.  In  particular,  the  AVP will be included in CER/CEA and DWR/DWA messages.

             Setting a value of 0 (zero) is equivalent to not setting a value  as  documented  in
             RFC  3588. The function origin_state_id/0 can be used as to retrieve a value that is
             set when the diameter application is started.

           {'Supported-Vendor-Id', [Unsigned32()]}:
             Values of  Supported-Vendor-Id  AVPs  sent  in  an  outgoing  capabilities  exchange
             message. Optional, defaults to the empty list.

           {'Auth-Application-Id', [Unsigned32()]}:
             Values  of  Auth-Application-Id  AVPs  sent  in  an  outgoing  capabilities exchange
             message. Optional, defaults to the empty list.

           {'Acct-Application-Id', [Unsigned32()]}:
             Values of  Acct-Application-Id  AVPs  sent  in  an  outgoing  capabilities  exchange
             message. Optional, defaults to the empty list.

           {'Vendor-Specific-Application-Id', [Grouped()]}:
             Values  of  Vendor-Specific-Application-Id  AVPs  sent  in  an outgoing capabilities
             exchange message. Optional, defaults to the empty list.

           {'Firmware-Revision', Unsigned32()}:
             Value of the  Firmware-Revision  AVP  sent  in  an  outgoing  capabilities  exchange
             message. Optional.

           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) ->

           Evaluating an evaluable()  E  on  an  argument  list  A  is  meant  in  the  sense  of

           Beware   of  using  local  funs  (that  is,  fun  expressions  not  of  the  form  fun
           Module: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.

         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
             filter at all.

             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 value, or all peers if the atom

           {eval, evaluable()}:
             Matches only those peers for which the specified evaluable()  returns  true  on  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 of the specified list.

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

           Note that the host and realm filters examine the outgoing request as passed to call/4,
           assuming   that   this  is  a  record-  or  list-valued  message()  as  documented  in
           diameter_app(3erl), and that the message contains 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. Note also that an empty host/realm (which should not  be  typical)
           is equivalent to an unspecified one for the purposes of filtering.

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

         service_event() = #diameter_event{}:
           Event message sent to processes that have subscribed using subscribe/1.

           The info field of the event record can be one of the following.

           {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{}

             Reports  that  the RFC 3539 watchdog state machine has transitioned into (up) or out
             of (down) the open state. If a diameter_packet record is present in an up tuple then
             there has been an exchange of capabilities exchange messages and the record contains
             the received CER or CEA, otherwise the connection has reestablished without the loss
             or transport connectivity.

             Note  that  a  single  up/down  event  for  a  given  peer  corresponds  to  as many
             peer_up/down callbacks as there are Diameter applications shared  by  the  peer,  as
             determined   during   capablilities   exchange.  That  is,  the  event  communicates
             connectivity with the peer as a whole while the callbacks  communicate  connectivity
             with respect to individual Diameter applications.

           {reconnect, Ref, Opts}:

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

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

           For  forward   compatibility,   a   subscriber   should   be   prepared   to   receive
  of forms other than those documented 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.

           Options  accepted  by  start_service/2.  Can  be any capability() tuple as well as the

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

             A service must define one application for each Diameter application  it  intends  to
             support.  For  an outgoing Diameter request, the application is specified by passing
             the desired application's application_alias()  to  call/4,  while  for  an  incoming
             request  the application identifier in the message header determines the application
             (and callback module), the application identifier being specified in the  dictionary
             file defining the application.

           Options accepted by add_transport/2.

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

             The   interface   required   of   a    transport    module    is    documented    in

           {transport_config, term()}:
             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.

           {applications, [application_alias()]}:
             The  list  of  Diameter  applications  to  which  usage  of  the transport should be
             restricted. Defaults to all applications configured on the service in question.

           {capabilities, [capability()]}:
             AVP's  used  to  construct  outgoing  CER/CEA  messages.  Any  AVP  specified  takes
             precedence over a corresponding value specified for the service in question.

           {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

           {reconnect_timer, Tc}:

           Tc = Unsigned32()

             For  a  connecting transport, the RFC 3588 Tc timer, in milliseconds. Note that this
             timer determines the frequency with which the transport will attempt to establish  a
             connection  with  its  peer  only  before 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.

             For  a  listening  transport,  the timer specifies the time after which a previously
             connected peer will be forgotten: a connection after this time  is  regarded  as  an
             initial connection rather than a reestablishment, causing the RFC 3539 state machine
             to pass to state OPEN  rather  than  REOPEN.  Note  that  these  semantics  are  not
             goverened  by  the  RFC  and  that a listening transport's reconnect_timer should be
             greater than its peers's Tc plus jitter.

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

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


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


                 SvcName = service_name()
                 Options = [transport_opt()]
                 Ref = ref()
                 Reason = term()

              Add transport capability to a service.

              The service will start a transport process(es) in order to establish  a  connection
              with  the peer, either by connecting to the peer (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
              established. It is not an error to add a transport to a service that  has  not  yet
              been configured: a service can be started after configuring transports.

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


                 SvcName = service_name()
                 App = application_alias()
                 Request = diameter_app:message() | term()
                 Answer = term()
                 Options = [call_opt()]

              Send a Diameter request message and possibly return the answer or error.

              App  identifies  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 has been specified to cause an
              earlier return, the call returns either when an answer message is received from the
              peer  or an error occurs. In the case of an answer, the return value is as returned
              by a handle_answer/4 callback. In the case of an error, 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 from the peer,
              the cases being documented below.

              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 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
                  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.

       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


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

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

              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 the corresponding call. The
              remaining forms are equivalent to an arity-3 fun as follows.

              Pred = fun(reference(), list()):  fun(Ref, _, Opts) -> Pred(Ref, Opts) end
              Pred = fun(list()):               fun(_, _, Opts) -> Pred(Opts) end
              Pred = reference():               fun(Ref, _, _)  -> Pred == Ref end
              Pred = list():                    fun(_, _, Opts) -> [] == Pred -- Opts end
              Pred = true:                      fun(_, _, _) -> true end
              Pred = false:                     fun(_, _, _) -> false end
              Pred = {M,F,A}:  fun(Ref, Type, Opts) -> apply(M, F, [Ref, Type, Opts | A]) end

              Removing a transport causes all associated transport connections to  be  broken.  A
              base  application DPR message with Disconnect-Cause DO_NOT_WANT_TO_TALK_TO_YOU 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.

       service_info(SvcName, Item) -> Value


                 SvcName = service_name()
                 Value = term()

              Return information about a started service.

       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 3588. 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 will typically be 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 peer, specifying the capabilities
              of the peer to be used during capabilities exchange and the  Diameter  applications
              that it supports. Transports are added to a service using add_transport/2.

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

              Stop the diameter application.

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


                 SvcName = service_name()
                 Reason = term()

              Stop a diameter service.

       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.

       unsubscribe(SvcName) -> true


                 SvcName = service_name()

              Unsubscribe to event messages from a service.


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