A device for determining oversubscription of a first virtualized network function in order to enable flow control in the virtualization of at least one node (62, 64, 66) in a communication network, the device comprising a first virtualized network function having packet handling resources comprising at least one queue (108, 112, 114, 120) and being a downstream network function located downstream from at least one second upstream network function (71, 72) in a packet flow between the network functions, the device comprising a processor acting on computer instructions whereby the device is operative to implement a flow control functionality set to: monitor said at least one queue (108, 112, 114, 120), track a queue occupancy measure of the queue, determine if the first virtualized network function is oversubscribed based on said monitoring comprising comparing the queue occupancy measure with a first threshold and determining that the first virtualized network function is oversubscribed in case the first threshold is crossed, determine a second upstream network function at least partly responsible for the oversubscription, wherein determine the second upstream network function comprises: keep, for every upstream network function, a number of related queue attributes associated with said at least one queue and the queue occupancy measure, investigate the queue attributes of the at least one queue on the downstream virtual network function corresponding to the different upstream network functions, wherein each upstream network function contributing packets to the queue has its own set of queue attributes, and determine the second upstream network function to be instructed based on the attribute investigation, determine a transmission rate of the determined upstream network function, and generate, in case the first virtualized network function is determined to be oversubscribed, an instruction for the determined upstream network function (71, 72) to reduce its transmission rate to the first virtualized network function in order to reduce the oversubscription. monitor said at least one queue (108, 112, 114, 120), track a queue occupancy measure of the queue, determine if the first virtualized network function is oversubscribed based on said monitoring comprising comparing the queue occupancy measure with a first threshold and determining that the first virtualized network function is oversubscribed in case the first threshold is crossed, determine a second upstream network function at least partly responsible for the oversubscription, wherein determine the second upstream network function comprises: keep, for every upstream network function, a number of related queue attributes associated with said at least one queue and the queue occupancy measure, investigate the queue attributes of the at least one queue on the downstream virtual network function corresponding to the different upstream network functions, wherein each upstream network function contributing packets to the queue has its own set of queue attributes, and determine the second upstream network function to be instructed based on the attribute investigation, determine a transmission rate of the determined upstream network function, and generate, in case the first virtualized network function is determined to be oversubscribed, an instruction for the determined upstream network function (71, 72) to reduce its transmission rate to the first virtualized network function in order to reduce the oversubscription.