Methods for improving end-to-end congestion reaction using adaptive routing and congestion-hint based throttling for IP-routed datacenter networks and associated apparatus. In connection with forwarding packets between sending and receiving endpoints coupled to one or more networks, one or more network switches are configured to detect current or approaching congestion conditions, generate congestion notification packets (CNPs), and return the CNPs to sending endpoints. The CNPs may be routed using one or more adaptive routing mechanisms to forward the CNPs along non-congested paths or may be forwarded along a fastest path to a sender. The CNPs further may comprise meta-data including a flow identifier associated with a packet sent from an endpoint, a congestion level for the flow, and a timestamp. CNPs may comprise unreliable datagrams that may be received out-of-order, with the timestamps being used to determine whether CNPs should be disregarded at a switch along the forwarding path to the sending endpoint or at the sending endpoint. Endpoint network interfaces implement a congestion-CNP table storing entries associating congested flows with congestion levels.