Using the Offline Charging API

Converged Application Server provides an offline charging API to enable any deployed application to act as a CTF and issue offline charging events. This API supports both event-based and session-based charging events.

The classes in package com.bea.wcp.diameter.accounting provide general support for Diameter accounting messages and sessions. Table 3-2 summarizes the classes.

Table 3-2 Diameter Accounting Classes

Class	Description
ACR	An Accounting-Request message.
ACA	An Accounting-Answer message.
ClientSession	A Client-based accounting session.
RecordType	Accounting record type constants.

In addition, classes in package com.bea.wcp.diameter.charging support the Rf application specifically. Table 3-3 summarizes the classes.

Table 3-3 Diameter Rf Application Support Classes

Charging	Common definitions for 3GPP charging functions
RfApplication	Offline charging application
RfSession	Offline charging session

The RfApplication class can be used to directly send ACR requests for event-based charging. The application also has the option of directly modifying the ACR request before it is sent out. This is necessary in order for an application to add any custom AVPs to the request.

In particular, an application must set the Service-Information AVP it carries the service-specific parameters for the CDF. The Service-Information AVP of the ACR request is used to send the application-specific charging service information from the CTF (WLSS) to the CDF (Charging Server). This is a grouped AVP whose value depends on the application and its charging function. The Offline Charging API allows the application to set this information on the request before it is sent out.

For session-based accounting, the RfApplication class can also be used to create new accounting sessions for generating session-based charging events. Each accounting session is represented by an instance of RfSession, which encapsulates the accounting state machine for the session.

Accessing the Rf Application

If the Rf application is deployed, then applications deployed on Converged Application Server can obtain an instance of the application from the Diameter node (com.bea.wcp.diameter.Node class). Example 3-4 shows the sample Servlet code used to obtain the Diameter Node and access the Rf application.

Example 3-4 Accessing the Rf Application

ServletContext sc = getServletConfig().getServletContext();
Node node = sc.getAttribute("com.bea.wcp.diameter.Node");
RfApplication rfApp = (RfApplication) node.getApplication(Charging.RF_APPLICATION_ID);

Applications can safely use a single instance of RfApplication to issue offline charging requests concurrently, in multiple threads. Each instance of RfSession actually holds the per-session state unique to each call.

Implementing Session-Based Charging

For session-based charging requests, an application first uses the RfApplication to create an instance of RfSession. The application can then use the session object to create one or more charging requests.

The first charging request must be an ACR START request, followed by zero or more ACR INTERIM requests. The session ends with an ACR STOP request. Upon receipt of the corresponding ACA STOP message, the RfApplication automatically terminates the RfSession.

Example 3-5 shows the sample code used to start a new session-based accounting session.

Example 3-5 Starting a Session-Based Account Session

  RfSession session = rfApp.createSession();
  ACR acr = session.createACR(RecordType.START);
  acr.addAvp(Charging.SERVICE_INFORMATION, ...);
  ACA aca = acr.sendAndWait(1000);
  sipRequest.getApplicationSession().setAttribute("RfSession", session);  if (!aca.getResultCode().isSuccess()) {
    ... error ...
  }

In Example 3-5, the RfSession is stored as a SIP application session attribute so that it can be used to send additional accounting requests as the call progresses. Example 3-6 shows how to send an INTERIM request.

Example 3-6 Sending an INTERIM request

RfSession session = (RfSession)     req.getApplicationSession().getAttribute("RfSession");
  ACR acr = session.createACR(RecordType.INTERIM);
  ACA aca = acr.sendAndWait(1000);
  if (!aca.getResultCode().isSuccess()) {
    // Handle the error...
  }

An application may want to send one or more ACR INTERIM requests while a call is in progress. The frequency of ACR INTERIM requests is usually based on the Acct-Interim-Interval AVP value in the ACA START message sent by the CDF. For this reason, an application timer must be used to send ACR INTERIM requests at the requested interval. See 3GPP TS 32.299 for more details about interim requests.

Specifying the Session Expiration

The Acct-Interim-Interval (AII) timer value is used to indicate the expiration time of an Rf accounting session. It is specified when ACR START is sent to the CDF to initiate the accounting session. The CDF responds with its own AII value, which must be used by the CTF to start a timer upon whose expiration an ACR INTERIM message must be sent. This INTERIM message informs the CDF that the session is still in use. Otherwise, the CDF terminates the session automatically.

It is the application's responsibility to send ACR INTERIM messages, because these are used to send updated Service-Information data to the CDF. Oracle recommends creating a ServletTimer that is set to expire according to the AII value. When the timer expires, the application must send an ACR INTERIM message with the updated service information data.

Sending Asynchronous Events

Applications generally use the synchronous sendAndWait() method. However, if latency is critical, an asynchronous API is provided wherein the application Servlet is asynchronously notified when an answer message is received from the CDF. To use the asynchronous API, an application first registers an instance of SessionListener in order to asynchronously receive messages delivered to the session, as shown in Example 3-7.

Example 3-7 Registering a SessionListener

  RfSession session = rfApp.createSession();
  session.setAttribute("SAS", sipReq.getApplicationSession());
  session.setListener(this);

Attributes can be stored in an RfSession instance similar to the way SIP application session attributes are stored. In the above example, the associated SIP application was stored as an RfSession so that it is available to the listener callback.

When a Diameter request or answer message is received from the CDF, the application Servlet is notified by calling the rcvMessage(Message msg) method. The associated SIP application session can then be retrieved from the RfSession if it was stored as a session attribute, as shown in Example 3-8.

Example 3-8 Retrieving the RfSession after a Notification

public void rcvMessage(Message msg) {
  if (msg.getCommand() != Command.ACA) {
    if (msg.isRequest()) {
      ((Request) msg).createAnswer(ResultCode.UNABLE_TO_COMPLY, "Unexpected
                    request").send();
    }
    return;
  }
  ACA aca = (ACA) msg;
  RfSession session = (RfSession) aca.getSession();
  SipApplicationSession appSession = (SipApplicationSession)
              session.getAttribute("SAS");
    ...
}

Implementing Event-Based Charging

For an event-based charging request, the charging request is a one-time event and the session is automatically terminated upon receipt of the corresponding EVENT ACA message. The sendAndWait(long timeout) method can be used to synchronously send the EVENT request and block the thread until a response has been received from the CDF. Example 3-9 shows an example that uses an RfSession for sending an event-based charging request.

Example 3-9 Event-Based Charging Using RfSession

RfSession session = rfApp.createSession();
ACR acr = session.createACR(RecordType.EVENT);
acr.addAvp(Charging.SERVICE_INFORMATION, ...);
ACA aca = acr.sendAndWait(1000);
if (!aca.getResultCode().isSuccess()) {
  // Send error response ...
}

For convenience, it is also possible send event-based charging requests using the RfApplication directly, as shown in Example 3-10.

Example 3-10 Event-Based Charging Using RfApplication

ACR acr = rfApp.createEventACR();
acr.addAvp(Charging.SERVICE_INFORMATION, ...);
ACA aca = acr.sendAndWait(1000);

Internally, the RfApplication creates an instance of RfSession associated with the ACR request, so this method is equivalent to creating the session explicitly.

For both session and event based accounting, the RfSession class automatically handles creating session IDs, as well as updating the Accounting-Record-Number AVP used to sequence messages within the same accounting session.

In the above cases the applications waits for up to 1000 ms to receive an answer from the CDF. If no answer is received within that time, the Diameter core delivers an UNABLE_TO_COMPLY error response to the application, and cancels the request. If no timeout is specified with sendAndWait(), then the default request timeout of 30 seconds is used. This default value can be configured using the Diameter console extension.

Using the Accounting Session State

The accounting session state for offline charging is serializable, so it can be stored as a SIP application session attribute. Because the client APIs are synchronous, it is not necessary to maintain any state for the accounting session once the Servlet has finished handling the call.

For event-based charging events it is not necessary for the application to maintain any accounting session state because it is only used internally, and is disposed once the ACA response has been received.