A method and apparatus for controlling a present service charging session for charging service usage in a communication network. The method comprises receiving a first request for grant of a reservation service unit quota from a charging client, the first request including at least data identifying an account to charge for the service usage. A first granted service unit quota for the first request is determined and a first carry-over service units quota from a previous service charging session associated with the identified account is accessed. A first reservation service unit quota is determined by deducting the first carry-over service units quota from the first granted service unit quota. According to the method it is reserved for the first reservation service unit quota from the account and the first granted service unit quota is sent to the charging client.