A network element includes multiple ports and packet processing circuitry. The ports are configured for exchanging packets with a communication network. The packet processing circuitry is configured to forward first packets over a forward path from a source node to a destination node, to forward second packets over a reverse path, which is opposite in direction to the forward path, from the destination node to the source node, and to mark one or more of the second packets that are forwarded over the reverse path, with an indication that notifies the source node that congestion is present on the forward path.