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Call Flows and Protocol Details


This topic describes the call flows that TDF is required to support when interworking with pre Rel-11 PCRF. The PCRF support for ADC functionality and ability to interwork with PGW as well as standalone TDF is assumed. The pre Rel-11 PCRF and TDF interacts using proprietary implementation of Sd interface between them so only unsolicited reporting mode is being supported by the TDF in the first version. PCC rules can be used to control the traffic flow by pre Rel-11 PCRF and the stand alone TDF. The call flows also cover the details of TDF interaction with OCS. The following sections also cover the RADIUS and the Diameter protocol details as used by the TDF.

Subscriber Session Creation Triggered by RADIUS Accounting-Start

This section describes the session create call-flows when Gx interface is used by TDF towards PCRF. The subscriber session creation on TDF can be triggered by one of the following

  • Accounting-Request Start message from the packet data network (PDN) gateway

  • User plane packet arriving for a new IP address

The PCRF differentiates the interface with TDF from the interface with PGW by using the TDF IP address configuration. This differentiation is needed on PCRF because not all AVPs being used in the messages on the interface with PGW are used in the same messages on interface with TDF. To enable PGW to inform the TDF about session establishment and to trigger the session creation on TDF, the PGW needs to have the TDF configured as accounting server.

  • As part of session establishment, PGW creates Gx session with the PCRF.

  • PGW sends Accounting-Request Start to TDF. The TDF obtains the IMSI and the IPv4/v6 address information from the Accounting-Request Start message. TDF sends CCR-I for the session with the IMSI and the IP address information to the PCRF.

  • The PCRF responds with CCA-I.

  • If Rating-Group or Service-Identifier is provided within a PCC rule; once a flow matching the criteria of such rule is detected on the TDF, TDF sends CCR-I to OCS.

  • The OCS responds to the TDF with CCA-I message that contains the following elements:

    • Quota allocation to the rating group (RG) or service identifier (SI) indicated in the Multiple Services Credit Control (MSCC) AVP

    • No quota allocation with specific cause

    • Quota allocation with Final-Unit-Indication (containing specific actions to be performed at quota exhaust)

    • Subscription to triggers or notification of triggers to TDF (for example, related to rating conditions)

  • TDF responds to PGW with Accounting-Response Start.

Gx Session Update triggered by PCRF

The PCRF indicates the session update to TDF over Gx by sending RAR. It happens in the following cases

  1. PCRF initiated PCC rules (provisioning or modification of new PCC rules or activation of non active PCC or ADC rules, removal of previously installed PCC rules). The modification is either a deactivation-activation (for preconfigured rules) or removal-install (for dynamic rules) and a single command can contain both Charging/ADC-Rule-Install and Charging/ADC-rule-remove.

  2. PCRF initiated request for Usage Monitoring report on one or more UM keys.

  3. PCRF initiated request for subscription (or cancellation to subscription) to TDF generated event triggers (for example, Application-start).

  4. PCRF initiated modification of Revalidation-time.

  5. PCRF initiated session teardown (using Session-release-cause + Charging/ADC-Rule-Remove).

  6. PCRF initiated event reporting for events subscribed by the TDF. In this case the Event-report-indication contains the data, which has been modified for the session. The TDF is expected to report these to the OCS for the relevant RGs.

Session Termination Triggered by RADIUS, PCRF, and TDF

The subscriber session is deleted from the TDF in the following two cases:

Receipt of Accounting-Request Stop from PGW applies to the case when session creation was triggered by Accounting-Request (Start) message sent by subscriber edge network element. Receipt of the Accounting-Request (Stop) message indicates the termination of session on subscriber edge network element and the session can be detached from TDF.

Aging of the subscriber based on inactivity. This scenario occurs when session is created by Accounting-Request Start (in case the Accounting-Request Stop is lost) as well as when session was created by user plane packet.

The following sequence of events occur during the detachment of a subscriber session:

  1. Subscriber session detach is triggered on TDF by the Accounting-Request Stop message from GGSN/PGW or by subscriber session inactivity on the TDF.

  2. TDF indicates the session termination to PCRF by sending CCR-T to PCRF.

  3. PCRF responds with CCA-T.

  4. TDF indicates the subscriber session termination to OCS by sending CCR-T.

  5. OCS responds with CCA-T.

  6. TDF responds to GGSN/PGW by sending Accounting-Response Stop.