The Gx application is defined as a vendor specific Diameter application, where the vendor is 3GPP and the Application-ID for Gx application is 16777238. The vendor identifier assigned to 3GPP by IANA is 10415.
When a Diameter protocol defined over the Gx interface, the node (PCEF) acts as a Diameter Client and the PCRF acts as a Diameter server. The Gx Diameter application uses existing Diameter command codes from the Diameter Base Protocol (RFC 6733) and Diameter Credit Control Application (RFC 4006), both of which are already implemented on the node.
Gx is using Attribute-Value Pairs (AVPs) for data representation within its messaging structures (command codes). AVPs in Gx come from various sources:
Gx-specific AVPs defined in 3GPP Gx specification TS 29.212.
Reused AVPs from other existing Diameter applications (RFC 4006, RFC 4005, and so on), other 3GPP specifications, ETSI, and so on.
RADIUS reused attributes (AVP codes 0-255 are reserved for RADIUS re-used attributes)
Vendor-specific AVPs
The initialization and maintenance of the connection between the node (PCEF) and the PCRF is defined by the underlying Diameter protocol as defined in RFC 3588/6733.