A method and apparatus for controlling charging of a service in a communication network comprising in an Online Charging System OCS receiving a request message for charging of the service from a charging client where the request message is including at least data identifying a first communication network account to charge for the service. A system utilization indicator is accessed representing a computer resource utilization value of a node involved in providing the service. Based on at least the system utilization indicator a determined service usage quota is determined, including a determined service usage quota attribute, to be reserved for from the account. The service usage quota is reserved for from the account and an answer message is transmitted to the charging client including the determined service usage quota attribute of the service usage quota reserved for from the account to be applied in controlling the service.