A switch or network interface can detect congestion caused by a flow of packets. The switch or network interface can generate a congestion hint packet and send the congestion hint packet directly to a source transmitter of the flow of packets that caused the congestion. The congestion hint packet can include information that the source transmitter can use to determine a remedial action to attempt to alleviate or stop congestion at the switch or network interface. For example, the transmitter can reduce a transmit rate of the flow of packets and/or select another route for the flow of packets. Some or all switches or network interfaces between the source transmitter and a destination endpoint can employ flow differentiation whereby a queue is selected to accommodate for a flow's sensitivity to latency.