There is included in one example a switch, including: an ingress port to receive data from a source host; a first egress port to send data to a destination host; a second egress port to send data to the source host; and a congestion notification generator (CNG) including: a congestion detector to compute bandwidth consumption of a flow associated with a packet received on the ingress port and assigned to the first egress port, and determine based on the computed bandwidth consumption that the flow is congested; a flow extractor to extract a flow identifier of the flow from the packet; a throttle calculator to calculate a quantitative flow throttle value for the flow; and a header builder to build a congestion notification packet to instruct the source host to throttle the flow.