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(3) 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(4).

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.

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

A module implementing the callback interface defined in diameter_app(3), 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 module.

Specifying a #diameter_callback{} record allows individual functions to be configured in place of the usual diameter_app(3) callbacks, with default implementations provided by module diameter_callback unless otherwise specified. See that module for details.


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(4).

{module, application_module()}

A callback module with which messages of the Diameter application are handled. See diameter_app(3) 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(3) 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 none.

{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 former.

{'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. Optional.

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.

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

Values of Inband-Security-Id AVPs sent in an outgoing capabilities exchange message. Optional, 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()]}

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) -> eval([F]).

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

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 any.

{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(3), 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{}

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/peer_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.

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

Capabilities exchange has failed. Reason can be one of the following.

{'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. The capabilities record contains pairs of values for the the local node and remote peer. The packet record contains the CER in question. In the case of rejection by a capabilities callback, the tuple indicates 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. The capabilities record contains only values for the the local node. The packet record contains the CER in question.

{'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. The capabilities record contains pairs of values for the the local node and remote peer. The packet record contains the CEA in question. In the case of rejection by a capabilities callback, the tuple indicates the rejecting callback.

{'CEA', Caps, Pkt}
Caps = #diameter_caps{} Pkt = #diameter_packet{}

An incoming CER contained errors and has been rejected. The capabilities record contains only values for the the local node. The packet record contains the CEA in question.

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.


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

{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(3). Defaults to diameter_tcp if unspecified.

The interface required of a transport module is documented in diameter_transport(3).

{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.

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 diameter_tcp(3) but not over SCTP as implemented by diameter_sctp(3).

{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 reference (as returned by add_transport/2) and diameter_caps record of the connection. Returning ok accepts the connection. Returning integer() causes an incoming CER to be answered with the specified Result-Code. Returning discard causes an incoming CER to be discarded. Returning unknown is 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 not.

{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 unspecified.

{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 remove_transport/2.


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(3). 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 started.

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 explicitly.

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.