1. A method of processing Accounting Request (ACR) messages in an IP Multimedia Subsystem (IMS) network, the method comprising: for any ACR [Interim] or ACR[Stop] for a session received by a charging collection function node, if a corresponding ACR [Start] is not there with the charging collection function node, the ACR [Interim] or ACR[Stop] 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 CDRs, then those CDRs are written into the correlation database of the correlation host; if the charging collection function node is sending ACRs from the incomplete database, then those ACRs are written into the incomplete database of the correlation host; for each session in the charging collection function node and the correlation host's incomplete databases, the following applies: the charging collection function node in question is the correlation host for the session, but it is missing ACR [Start] and/or ACR [Stop] information for the session; the charging collection function node in question is not the correlation host, but it cannot send these records to the correct correlation host since that host is out of service.