Process of Setting up and Maintaining Links at the Base Layer

Figure 1 shows the process of setting up and maintaining links at the base layer over the Gx interface.

1. Link establishment at the transport layer

   The PCEF and PCRF establish a link at the transport layer by using a three-way handshake provided by the TCP.

2. Link establishment at the Diameter base layer

   It is a process of Gx capability negotiation. CER and CEA messages are used for Gx capability negotiation. Exchanging the two messages allows the PCEF and PCRF to discover the identity and capability, including protocol version, supported Gx applications, and security mechanism, of each other.

   1. After a link is established at the transport layer, the PCEF sends a CER message to the PCRF. The CER message indicates that Gx applications are used by the PCEF. Figure 2 shows the Auth-Application-Id AVP captured over the Gx interface. Application Identifier in the message header is set to 0, indicating a Diameter common message. In addition, a Vendor-Specific-Application-Id attribute value pair (AVP) is included in the message body. In the Vendor-Specific-Application-Id AVP of type grouped, the value of the Vendor-Id AVP is 10415 and the value of the Auth-Application-Id AVP is 16777238. Value 10415 indicates the compliance with the 3rd Generation Partnership Project (3GPP) protocol, and value 16777238 indicates the support of 3GPP Gx application. For details about CER messages, see Capabilities Exchange Request (CER) Message.

      A CER message may contain the following AVPs:

      • Origin-Host AVP and Origin-Realm AVP

        The Origin-Host AVP identifies the host name of the endpoint that sends the CER message, and the Origin-Realm AVP identifies the domain of the originator. Usually, CER messages are sent by the PCEF.

      • Host-IP-Address AVP

        The Host-IP-Address AVP indicates the IP address of the originating PCEF.

      • Vendor-ID AVP

        If the Vendor-ID AVP is set to 10415, the AVP complies with the 3GPP protocol.

      • Vendor-Specific-Application-Id AVP

        The Vendor-Specific-Application-Id AVP is of type grouped and includes the Vendor-Id and Auth-Application-Id AVPs. The Auth-Application-Id AVP indicates the applications of a Diameter message.

        Currently, the PCRF supports only 3GPP Gx application which is indicated by the value 16777238. In captured messages, all the Diameter capabilities that the PCEF supports are displayed regardless of the interface on which the message is captured. The PCEF determines the application used on the interface based on the CEA message returned by the PCRF.

        http://localhost:7904/pages/CEC1230A/01/CEC1230A/01/resources/public_sys-resources/icon-note.gif NOTE:

        The position and presence of AVPs vary according to the version but do not affect message resolution. For details about CER messages, see Capabilities Exchange Request (CER) Message.

        Figure 2 Auth-Application-Id AVP captured over the Gx interface
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   2. The PCRF sends a CEA message to confirm the Gx application negotiation with the PCEF. Figure 3 shows a CEA message captured over the Gx interface. In the CEA message, the Application-Identifier is set to 0, and the Vendor-Specific-Application-Id AVP is included in the message body. In the Vendor-Specific-Application-Id AVP, the Vendor-Id AVP is set to 10415 and the Auth-Application-Id AVP is set to 16777238. If any information element (IEs) contained in the CEA message is incorrect, the PCEF stops message processing at the transport layer.
      Figure 3 CEA message captured over the Gx interface
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      A CEA message may contain the following AVPs:

      • Result-Code AVP

        If the value of the Result-Code AVP is 2001, the PCRF sends the CEA message successfully.

      • Vendor-Specific-Application-Id AVP

        If the value of the Auth-Application-Id AVP contained in the Vendor-Specific-Application-Id AVP is 16777238, the PCRF supports the 3GPP Gx application. The capability negotiation is successful, and a link is established between the PCEF and the PCRF.

      http://localhost:7904/pages/CEC1230A/01/CEC1230A/01/resources/public_sys-resources/icon-note.gif NOTE:

      The position and presence of AVPs vary according to the version but do not affect message resolution. For details about CEA messages, see Capabilities Exchange Answer (CEA) Message.

3. Link maintenance

   The PCEF and PCRF periodically exchange DWR and DWA messages. The device watchdog (DW) messages monitor links at the Diameter base layer.

   http://localhost:7904/pages/CEC1230A/01/CEC1230A/01/resources/public_sys-resources/icon-note.gif NOTE:

   • For details about DWR messages, see Device Watchdog Request (DWR) Message.
   • For details about DWA messages, see Device Watchdog Answer (DWA) Message.

4. Subsequent message exchanges between the PCEF and the PCRF

   The Application-Id AVP in the message header is set to 16777238 because these messages are transmitted at the Gx application layer. The Auth-Application-Id AVP is also set to 16777238. In addition, all 3GPP-Specific-AVPs must contain the Vendor-Id AVP that is set to 10415.