CCR (Credit Control Request) messages exist in the Diameter-based credit control architecture. Which are commonly exist in 2G/3G and 4G mobile networks for real-time online charging and policy control.

The main purpose of the CCR message is to request credit authorization and information from the Online Charging System (OCS) or the Policy and Charging Rules Function (PCRF) in the network.

The CCR message is the request sent from the network element to the OCS or PCRF. And the response to this request is the CCA message (Credit Control Answer) message.

So that the Credit Control Answer (CCA) message, is the response from the OCS or PCRF to the network element.