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Modes 1 and 2—Pre-Rel-11 Standalone TDF Operation (PCC / ePCC Rules Using Gx)

 

Modes 1 and 2 of TDF operations with services and subscriber sessions that are handled on Gx interfaces are described in the following sections. These modes are the only supported modes in Junos OS Services Control Gateway Release 14.1X55.

Gx Session Establishment Triggered by TDF

When a Gx-based interface is configured to be used by TDF towards the PCRF, the Gx session creation can be triggered by whether the Accounting Start message from the PDN GW or the user plane packet arriving for a new IP address.

The PCRF is configured with the TDF IP address to differentiate the request coming in from the PCRF with that from the request coming in from PCEF. This configuration is because number of AVPs do not make sense in TDF Gx session.

  • The UE requests for connection to the PDN. PDN GW receives Create Session Request with IMSI.

  • The PDN GW creates the Gx session with the PCRF.

  • The PDN GW triggers Accounting Start. The TDF is configured as one of the Accounting Servers for Accounting in PDN GW. The Accounting Start message must contain the IMSI and might contain the MSISDN. The TDF Acknowledges the Accounting Start. The TDF obtains the IMSI and the IPv4/v6 address information from the Accounting (Start) message. The CCR-I is constructed for the subscriber using the IMSI and the IP address information. The PCRF creates a new session for the user and responds with CCA-I. Optionally if the TDF receives an IPv4 or IPv6 packet for an IP address for which the Gx session does not exist, the TDF triggers Gx session establishment. The IP address in the Framed-IP-Address or IPv6 prefix in framed-IPv6-Address AVP identifies the user to the PCRF.

  • The TDF triggers the Gx CCR-I with PCRF.

  • The subscriber session creation is completed.

The TDF is configured whether the IP-CAN Type sent is 3GPP-EPS or 3GPP-GPRS. This AVP is optional, and is configurable to be sent or not.

For the Accounting Start on (S)Gi interface, the 3GPP vendor-specific sub-attribute 3GPP-IMSI contains the IMSI of the user. Because this is an optional parameter, it is possible to configure the PGW to include this parameter in Accounting-Start message. All other parameters of Accounting-Request (Start) can be ignored.

The Accounting Acknowledge message, which contains the Acct-Status-Type information element. indicates the start of accounting.

CCIR-Init Messages for Gx Session Creation

The CCR-I is sent to the network when TDF is triggered to create the Gx session. The trigger can be when RADIUS Accounting Start is received or when the user Plane packet is received. The following AVP elements are contained in the Credit Control request-Initial (CCR-I) messages sent when Gx session creation occurs:

The following AVPs are not required for TDF operation:

  • 3GPP-RAT-Type/RAT-Type

  • QoS-Negotiation/Upgrade

  • Default-EPS-Bearer-QoS

  • Supported-features

CCA-Init Messages for Gx Session Creation

Gx CCA-I messages are sent by PCRF to TDF for

  • Providing the TDF (when applicable, such as a case where RADIUS is not used to transmit to the TDF the identity of the subscriber) the Subscription-ID information.

  • Provisioning (or activating, in case they are preconfigured) PCC , ePCC rules in the TDF for application detection and related charging information.

  • Contextually, subscribing to event triggers such as notification of applications detection.

  • Contextually, requesting Revalidation timeout Or alternatively, to provide a Release-cause or permanent failure (for example, subscriber unknown) so that the Gx session from the TDF might be torn down

The following is the format of the CCA-I message with AVPs contained in it:

Gx Session Update Triggered by TDF

The TDF sends CCR-U to the PCRF in the following cases:

Note

Gx session update triggered by TDF is not supported in Release 14.1X55; only the PCRF-triggered session update mechanism is supported.

  • If the PCRF has subscribed to event triggers such as Application_Start, Application_Stop (detection of a specific application bound to a PCC or EPCC rule)

  • If there are event triggers related to charging (for example, OUT OF CREDIT, REALLOCATION OF CREDIT) on the Gy that needs to be relayed to the PCRF

  • If there are event triggers related to usage-monitoring such as threshold usage reaching for one PCC or EPCC rule

  • If PCC rules have been provisioned or activated with a Revalidation-Timeout indication by the PCRf. In such a case, TDF uses CCR-Update to provide PCRV an Event-Trigger that contains the value of REVALIDATION-Timeout

  • If the TDF receives certain triggers from OCS for which information is available in the PCRF. In such a case TDF uses CCR-U to provide PCRF with Event-Report-Indication with the list of event triggers TDF is interested in subscribing.

The TDF triggers an update procedure due to any of the following event triggers:

  • If configured, when a new application start is detected by the TDF

  • If configured, when a application stop is detected by the TDF (when application start was reported)

  • When usage monitoring is enabled and the usage quota is exhausted.

The TDF initiates the CCR-U message along with appropriate event trigger and reports. The PCRF might download new rules related to applications. Otherwise, the PCRF might download new rules towards the TDF. If the update has been triggered due to detection of new application, the PCRF might download new rules related to the detected application towards the PCRF.

Gx Session Detach Triggered by TDF

The TDF deletes the UE session based on the Accounting Stop procedure and aging of the subscriber based on inactivity.

For Accounting Stop, when the PDN GW receives a Delete Session Request or other detach triggers, the PDN GW triggers an Accounting Stop to all the Accounting Servers it had initiated the session. This message can be used by the TDF to trigger CCR-T towards the PCRF.

  • The UE initiates the Delete Session Request procedure for deleting the PDN connection.

  • The PDN GW deletes the context towards the PCRF.

  • The PDN GW initiates Accounting Stop towards all the servers. TDF receives Accounting Stop as it is configured as an Accounting Server.

  • The TDF sends CCR-T towards the PCRF.

  • The Delete Session Response packet completes the delete procedure.

The following is the format of the Gx CCR-T message with contained AVPs:

The following is the format of the Gx CCA-T message with contained AVPs:

Because the Accounting Stop message is not wholly reliable and to handle the scenarios of PDN GW failure or certain unexpected scenarios, the UE session has to be monitored for inactivity. If the device is idle for a long time, the device has to be deleted.

Handling of Failures

The TDF handles the following failures gracefully:

  • PDN GW failure—This condition can be detected when the PDN GW sends an Accounting On message which indicates that all the devices which were being served by this PDN GW has restarted. Upon receipt of this message, the TDF deletes all the UE contexts locally.

  • TDF failure—To gracefully handle the restart of TDF, the TDF moderates the CCR-I traffic towards the PCRF. While the traffic is flowing, local policies are applied for those sessions for which PCRF has not responded.

  • PCRF error scenario—If the PCRF has successfully authenticated the subscriber without sending any policies, local policies are applied. If the PCRF has responded with a temporary error, then local policies are applied for the subscriber. If the PCRF responds with a permanent error, then the packets for such UEs are dropped. A timer is started (aging), which deletes the user to avoid a DoS attack.