PC2 (application id: 16777337)

The PC2 interface is an integral part of the Proximity Services (ProSe) architecture, which enables devices (UEs) to discover and communicate with each other when they are in close proximity. The PC2 reference point facilitates communication between the ProSe Application Server and the ProSe Function, allowing for the implementation of ProSe features such as direct discovery and EPC-level ProSe discovery.
The primary purpose of the PC2 interface is to manage the interactions required for ProSe service functions, including the registration of ProSe applications, the handling of ProSe User IDs, and the mapping of identifiers for discovery purposes.

ProSe Function: This logical network entity handles network-related actions necessary for ProSe, including storing and managing EPC-level discovery information and facilitating signaling with third-party application servers. It also generates and maintains the ProSe Discovery User ID (PDUID) for restricted ProSe direct discovery.
ProSe Application Server: This server stores relevant ProSe User IDs and Function IDs, manages the mapping of Application Layer User IDs with EPC ProSe User IDs, and oversees the permissions and metadata necessary for ProSe discovery. It supports both open and restricted discovery procedures.

PC2 interface workflow

1. Application Registration for ProSe:
   • A UE that wishes to participate in ProSe services initiates an application registration by sending a ProXimity-Action-Request (PXR) command to the ProSe Function. This request includes the EPC ProSe User ID (EPUID) and Application Layer User ID (ALUID) of the originating UE. 
   • The ProSe Function forwards the PXR command to the ProSe Application Server for processing.
   • If the registration is successful, the ProSe Application Server responds with a ProXimity-Action-Answer (PXA) command, indicating success and allowing the UE to proceed with ProSe services.
2. Proximity Map Request:
   • To enable ProSe discovery, the ProSe Function may send a PXR command to the ProSe Application Server, requesting a proximity map for a targeted UE. The request includes identifiers for both the originating and targeted UEs.
   • The ProSe Application Server evaluates the request and, if permissible, responds with a PXA command, providing the necessary identifiers for discovery.
3. Transport and Session Management:
   • The PC2 interface uses the Diameter base protocol for all its messaging, ensuring robust and secure communication between the ProSe Function and the ProSe Application Server. The transport protocols supported over the PC2 interface include TCP and SCTP.
   • The PC2 Diameter session is established as a peer-to-peer connection between the ProSe Application Server and the ProSe Function. The session is terminated after each request and answer pair interaction, with no session state maintained, as indicated by the Auth-Session-State AVP set to NO_STATE_MAINTAINED.

For complete technical specification of PC2 interface in Diameter protocol please refer to: [3GPP TS 29.343].