Cx/Dx (application id: 16777216)

The Cx and Dx interfaces are part of the IMS (IP Multimedia Subsystem) architecture, which is used for delivering IP multimedia services. These interfaces facilitate communication between the Call Session Control Functions (CSCFs) and the Home Subscriber Server (HSS) or Subscriber Location Function (SLF).
The Cx interface is used for communication between the Serving CSCF (S-CSCF) and the HSS. This interface handles:

• User Authentication: Verifies the identity of the user.
• User Authorization: Ensures the user has the right to access the requested services.
• Subscription Information: Retrieves user profiles and subscription data.

Cx Interface Messages:

• User-Authorization-Request (UAR): Requests authorization for a user.
• Location-Info-Request (LIR): Requests location information of the user.
• Multimedia-Authentication-Request (MAR): Requests multimedia authentication for the user.
• Server-Assignment-Request (SAR): Assigns the user to a particular server.

The Dx interface is used for communication between the Interrogating CSCF (I-CSCF) and the SLF.
This interface helps in locating the appropriate HSS: Queries the SLF to determine which HSS holds the subscriber's data.

Dx Interface Messages:

• Location-Info-Request (LIR): Requests location information of the user.
• User-Data-Request (UDR): Requests user data.

Cx/Dx interface workflow

1. Registration: 
   • The UE sends a registration request to the Proxy CSCF (P-CSCF).
   • The P-CSCF forwards this request to the I-CSCF.
   • The I-CSCF uses the Dx interface to query the SLF for the appropriate HSS.
   • The I-CSCF then forwards the request to the S-CSCF.
   • The S-CSCF uses the Cx interface to communicate with the HSS for user authentication and profile retrieval.
2. Session Establishment:
   • When the UE initiates a session, the S-CSCF queries the HSS using the Cx interface to retrieve user-related information necessary for session management.
3. Service Invocation:
   • During service invocation, the S-CSCF may interact with various application servers and use the Cx interface to bear user service profiles and policies from the HSS.

For complete technical specification of Cx/Dx interface in Diameter protocol please refer to: [3GPP TS 29.229]