Congestion management is a very critical problem tackled in interconnection networks for years but not solved yet. Although several mechanisms have been recently proposed for lossless Multistage Interconnection Networks (MINs), they either have drawbacks or are partial solutions. Some of them introduce penalty over packets not really addressed to the hot-spots, whereas others can cope only with congestion situations that last a short time. In this paper, we propose an effective and efficient congestion management mechanism for lossless interconnection networks based on explicit congestion notification. The mechanism uses two different flags in ACK packets, a Marking Bit (MB) and a Validation Bit (VB), to detect congestion and warn the origin hosts. In this way, packets belonging to "cold flows" but stopped because of head-of-line (HOL) blocking can be distinguished from "hot flow" packets which are really causing congestion. In response, origin hosts can apply corrective actions only to the "hot flows", minimizing the negative impact on "cold flows" performance. Evaluation results show that the proposed congestion management strategy is able to avoid the degradation of network performance, regardless of traffic load and the location of the congestion in the network.