1. A method of processing Accounting Request (ACR) messages in an IP Multimedia Subsystem (IMS) network, the method comprising: for an ACR [Interim] or ACR[Stop] message for a session received by a charging collection function node, if a corresponding ACR [Start] message is not stored at the charging collection function node, the ACR [Interim] or ACR[Stop] message is stored in an incomplete database associated with the charging collection function node; if the charging collection function node that hosts the correlation functionality (the “correlation host”) for the ACRs is in-service, the charging collection function node processes the ACRs as follows: if the charging collection function node is sending aggregated charging data records (CDRs), then the CDRs are written into a correlation database of the correlation host; if the charging collection function node is sending ACRs from the incomplete database, then the ACRs are written into the incomplete database of the correlation host; reviving an out of service charging collection function node; processing the revived charging collection function node's corresponding incomplete database as follows: for ACR messages wherein the revived charging collection function node is not the correlation host, determining the correlation host and writing the message to the incomplete database of the host; for pending messages in the incomplete database of the revived charging collection function node, for which the revived charging collection function node is the correlation host, broadcasting a message to the other nodes in the network indicating that the charging collection function node has been revived.