1. A method of monitoring transmission of data in a network, comprising: receiving data packets for one or more data flows by at least one of a plurality of network switches; sampling queue data in each of the plurality of network switches receiving the one or more data packets, the sampled queue data taken from at least one queue attached to a switch port to which the one or more data packets have been forwarded; and transmitting the sampled queue data to a logically centralized network controller connected to the plurality of switches, the network controller configured to control the plurality of network switches, the network controller configured to determine whether a congestion, unfairness or Service Level Agreement (SLA) infringement condition exists in one or more switches based on the sampled queue data and send a control message to one or more data flow sources based on the congestion, unfairness or SLA infringement condition.